978142001516selüloz
Transcript of 978142001516selüloz
5 Cellulose and CelluloseDerivatives
Donald G. Coffey, David A. Bell, and Alan Henderson
CONTENTS
5.1 Introduction ...............................................................................................................148
5.2 Cellulose..................................................................................................................... 148
5.2.1 Occurrence and Isolation ................................................................................ 148
5.2.2 Structure..........................................................................................................149
5.2.3 Biosynthesis.....................................................................................................151
5.2.4 Chemical and Physical Properties ................................................................... 151
5.2.5 Physically Modified Celluloses ........................................................................ 152
5.2.5.1 Microfibrillated Cellulose ..................................................................152
5.2.5.2 Microcrystalline Cellulose..................................................................152
5.3 Chemically Modified Cellulose Derivatives ...............................................................152
5.3.1 The Manufacture of Cellulose Ethers .............................................................153
5.3.1.1 Preparation of Alkali Cellulose ......................................................... 153
5.3.1.2 Alkylation and Hydroxyalkylation of Alkali Cellulose .....................153
5.3.1.3 Product Purification .......................................................................... 154
5.3.2 Characterization of Cellulose Ethers...............................................................154
5.3.2.1 Degree of Substitution.......................................................................154
5.3.2.2 Molar Substitution ............................................................................154
5.3.3 Sodium Carboxymethylcellulose ..................................................................... 155
5.3.3.1 Effect of DS on Solubility ................................................................. 156
5.3.3.2 Solution Rheology ............................................................................. 156
5.3.4 Methylcelluloses ..............................................................................................159
5.3.4.1 Solution Rheology ............................................................................. 159
5.3.4.2 Thermal Gelation............................................................................... 159
5.3.4.3 Solute-Induced Gelation ....................................................................162
5.3.5 Ethylmethylcellulose........................................................................................163
5.3.6 Hydroxypropylcellulose...................................................................................163
5.4 Applications in Foods................................................................................................163
5.4.1 Fish/Meat ........................................................................................................164
5.4.2 Sauces, Gravies, Soups, and Syrups................................................................ 165
5.4.3 Emulsions........................................................................................................166
5.4.4 Baked Goods...................................................................................................167
5.4.5 Frozen Desserts ............................................................................................... 168
5.4.6 Emerging Technologies: Barrier Films............................................................ 168
5.5 Nutritional Effects of Cellulose Derivatives .............................................................. 169
5.5.1 Dietary Fiber...................................................................................................169
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5.5.2 Water-Holding Capacity ................................................................................. 170
5.5.3 Metabolism ..................................................................................................... 170
5.6 Conclusion .................................................................................................................171
Appendix 1 .........................................................................................................................171
Appendix 2 .........................................................................................................................172
Acknowledgment................................................................................................................ 174
References .......................................................................................................................... 174
5.1 INTRODUCTION
Cellulose is the world’s most abundant naturally occurring organic substance, rivaled only by
chitin. It has been estimated that nature synthesizes from 100 to 1000 billion (1011 to 1012)
metric tons of cellulose every year [1–3]. It is therefore not surprising that humans have made
use of cellulose on a vast scale in the paper, mining, building and allied industries, and as a
source of bioenergy. This applies to cellulose in its natural state, isolated, or as a source of raw
material for modification into products having different properties from those of pure
cellulose. Wood pulp is the main source of processed cellulose, the bulk of which is converted
to paper and cardboard, and about 2%, amounting to just over 3 million tons, into regener-
ated fiber and films or chemical derivatives [3].
(probably a small fraction of 1%) in comparison with that of the animal world, it is symptom-
atic of the importance attached to human existence and activity that the emphasis in this
chapter and book is upon human beings’ adaptation of natural food resources for their own
use and enjoyment. Regarding cellulose, the major carbohydrate nutrient for herbivores,
including insects, enzymatic approaches to modification could change radically the value and
applicability of this abundant polysaccharide to human nutrition [4]. As it is, cellulose performs
production of processed animal feed based on the normally inaccessible lignified cellulose
sources, whereby degrading enzymes are used to enrich the cellulose and related carbohydrate
polymers and render them acceptable to both ruminants and nonruminants [5–8]. Such pro-
cesses may continue to the sugar level, and it is of interest that sugarcane bagasse may serve not
only as a source of paper and industrial ethanol, but also as animal fodder [9,10].
Given that cellulose has been a natural part of the world’s diet since time immemorial, and the
excellent toxicological profile enjoyed by it and its ether derivatives (which constitute the only
food-allowed group of modified celluloses), it is not surprising that these materials have found
wide acceptance within the food industry. Most of this chapter is concerned with their applica-
tions. A brief account is given (considering the vast literature on the subject) about the occur-
rence, biosynthesis, analysis, and properties of cellulose. This leads to a description of the salient
to bring them into a state of solution or dispersion for practical use. An overview of regulatory
the basis of Section 5.4. The reader is referred to the first edition of this book for a list of key
manufacturers of cellulosics, and to additional references describing earlier work [11].
5.2 CELLULOSE
5.2.1 OCCURRENCE AND ISOLATION
Cellulose is the major building block of the cell wall structure of higher plants [12]. Cellulose
constitutes 40–50% of wood, 80% of flax, and 90% of cotton fiber. Green algae also have
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Although (as stated in Chapter 1) the total food consumption of humans is minuscule
a critical function as dietary fiber (see Chapter 18). Nevertheless, there has been a surge in the
aspects is given inAppendix 2,whereas details concerning the applications of these products form
features of physically and chemically modified celluloses, and in Appendix 1 to the methods used
cellulose in their cell walls, as do the membranes of fungi. Cyanobacteria [13] represent a
primitive source, and bacterial cellulose is well described [14]. Acetobacter xylinum can
synthesize extracellular pellicles of cellulose from glucose. Simple marine animals such as
tunicates deposit cellulose in their cell walls [15].
Commercial purification of cellulose is centered on cotton linters and wood pulp, in the
first case because of the high cellulose content and, in the second, the relative abundance and
ease of harvesting of wood and straw. Cellulose in the natural state is difficult to purify due to
its insolubility in commercial solvents. Because of their high polarity, amine oxides are used
successfully in a variety of applications, e.g., N-methylmorpholine-N-oxide hydrate as solvent
in the Lyocell regeneration process [3]; 1,3-dimethyl-2-imidazolidinone/LiCl as a system that
is used as solvent in a food product application [17]. Isolation of cellulose in a pure form
usually involves alkaline pulping to remove waxes, proteins, and — in particular from wood —
lignins. Pulps for cellulose ether production often undergo extra alkali extraction steps; these
are carried out to remove low-molecular-weight polysaccharides and so-called hemicelluloses,
as well as to raise the pure (alpha) cellulose content [2,18].
5.2.2 STRUCTURE
One of the certainties in cellulose science is that cellulose is an aggregate of linear polymers of
D-glucopyranosyl residues in the chain form, which are linked together entirely in the b-1,4
configuration [3,19] (Figure 5.1), chemical and enzymatic hydrolyses, acetolysis, methylation
studies, NMR and x-ray analysis affording ample proof of this. Cellulose is an isotactic b-1,4-
polyacetal of 4-O-b-D-glucopyranosyl-D-glucose (cellobiose), as the basic unit consists of two
units of glucose b-1,4 linked [2]. The b-1,4 diequatorial configuration results in a rigid and
linear structure for cellulose. The abundance of hydroxyl groups and concomitant tendency
to form intra- and intermolecular hydrogen bonds results in the formation of linear aggre-
gates. This contributes to the strength shown by cellulose-containing structures in plants and
also to the virtual insolubility of cellulose in common solvents, particularly water.
As a glucan, cellulose may be analyzed, after enzymatic removal of starch, by hydrolysis in
a concentrated aqueous solution of H2SO4, dilution with water, and anthrone assay [20].
Cellulose has a characteristic CP/MAS 13C NMR spectrum [21], and functional groups in
derivatives are readily identified by 1H NMR [22]. The last reference describes the first
application of NMR to the end-group determination of molecular weight of a cellulose
derivative. Other standard methods of spectroscopic analysis are readily applied [23–25].
Molecular size of polymer molecules can be conveniently described in terms of degree of
polymerization (DP), which is an average value of the number of monomer units. By various
physical techniques (intrinsic viscosity measurement [26,27], light scattering, etc.) the DP of
O
O
CH2
CH2
OH
OH
OH
OH
HO
HOHO
HH
H
HH
O
O
CH2
OH
OHHO
HO
H
HH
O
n−2
HH
H HHH
FIGURE 5.1 Cellulose.
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can be used in the analytical steric (size) exclusion chromatography (SEC) mode (see Section
5.2.2) instead of the more common dimethylacetamide (DMA)/LiCl [16]; and an amine oxide
cellulose from various sources can be estimated. A number of examples are given in Table 5.1.
Much attention has been given to molecular weight distribution measurements of cellulose and
derivatives using various adaptations of SEC [16,28–30], andby these techniques, informationwas
obtained about the semiflexible chain structure of cellulose in alkaline urea solution [31], and the
effects of oxidation [32] and heat [33]. Carboxymethylcellulose (CMC), hydroxymethylcellulose,
and more complex derivatives have been analyzed similarly, employing various stationary phases,
elution systems, and detectors devised to improve the reliability of the method [34–38].
In the solid state, highly ordered crystalline areas are interspersed between less-ordered
amorphous zones. These amorphous zones are regions in which the hydroxyl groups are more
readily available for reaction than in the more highly ordered crystalline areas, which are less
reactive [12,39–41]. Cellulose reactivity is thus dependent on the source of cellulose and the
conditions of isolation and purification.
In the native state cellulose is in the form known as Cellulose I, which has a unit cell
containing two cellulose molecules in line with the b-axis, but arranged countercurrently to
each other [42]. A lattice structure known as Cellulose II is obtained after mercerization [43],
i.e., treatment with sodium hydroxide, in which the c-axis is lengthened and the a-axis
shortened. Other forms of cellulose have been described, but a description of these is outside
the scope of this chapter. More specialized discussions are readily available [2,3,44].
Early work employing electron microscopy showed that the linear cellulose molecules are
bound together (through hydrogen bonding and other electronic forces) into long threadlike
bundles called microfibrils. In certain areas, these microfibrils have the chains arranged in
stacked layers. These areas are then sufficiently organized in regular fashion to form discrete
crystalline regions known as crystallites [45] and at a higher level of organization, micro-
tubules [46]. Microfibrils are not necessarily the only mode of organization of cellulose chains,
as in wheat straw the highly oriented crystalline lamellae are arranged perpendicularly to the
tangential direction with respect to the annual rings [47]. A great deal of work has neverthe-
less been done on the detailed structure of microfibrils, with early reviews by Delmer [48] and
of Marchessault and Sundararajan [12]. The article by Klemm et al. [3] admirably summarizes
the various structural levels of cellulose from data derived by crystal analysis using x-rays to
fiber morphology (microfibrils and microfibrillar bands), which includes the crystallites and
amorphous regions, and to the structural design of plant cell walls in which the cellulose is
accompanied by various lignin, hemicellulosic, and pectic components.
Different techniques have been used to establish many facets of cellulose formation
and structure apart from the biosynthetic routes discussed in Section 5.2.3. The effects of
glucomannan and xylan on the cellulosic structure of A. xylinum were defined using NMR
and electron diffraction [49], and the behavior of microfibrils when attacked by cellulases, by
x-ray [50]. Periodic disorder along ramie microfibrils was demonstrated by small-angle
neutron scattering [51], and the ultrastructure of iodine-treated wood (fluorescent markers
TABLE 5.1Degree of Polymerization of Cellulose
from Various Sources
Source DP
Purified cotton 1500–300
Cotton linter 6500
Spruce pulp 3300
Aspen pulp 2500
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are also used) by light microscopy [52] ). Improved activated carbon results from retaining the
microfibrillar structure of the pyrolyzed cellulose [53].
5.2.3 BIOSYNTHESIS
For a compound as abundant and important in nature as cellulose, the uncovering of the fine
details of its biosynthesis has proved remarkably difficult. A number of general articles
introduce the observations that follow [14,42,54–60].
The substrate uridine diphosphate glucose (UDP-glucose) splits off glucose to form a lipid–
pyrophosphate–glucose, and from this a lipid–pyrophosphate–cellobiose derivative. The cello-
biose unit is then detached and forms the end part of a water-soluble, short-lived polymer based
on glucose. The enzyme complex responsible for these transformations functions on plasma
membrane surfaces. As they are formed, the chains undergo hydrogen bonding to form the
partially crystalline microfibrils. The absorbing question as to how cellulose has become by far
the main source of organic terrestrial matter has been approached in the basic reviews by Delmer
and coworkers [61,62] on the roles of genes and catalytic proteins in the biosynthetic pathways
leading to cellulose. One aim of such fundamental studies has been to improve cellulose quality
and its suitability for applied purposes, and parallels work on starch in some respects (see
has led to successful cultivation of improved cotton [56,65], better pulp quality in Eucalyptus [66]
and poplar [67], and the modulation of cellulose content of tuber cell walls in transgenic potatoes
[68]. Numerous biochemical aspects have been pursued, such as the requirement of the N-glycan
processing enzyme a-glucosidase [69] for cellulose biosynthesis, and factors in the alignment
of cellulose in microfibrils, the patterns of which govern the direction of cell wall expansion
[70,71].
5.2.4 CHEMICAL AND PHYSICAL PROPERTIES
Cellulose is a hygroscopic material, insoluble but able to swell in water, dilute acid, and most
solvents. Solubility can be achieved in concentrated acids [72] but at the expense of consid-
erable degradation through acetal (glycosidic) hydrolysis. Alkali solutions lead to consider-
able swelling and dissolution of hemicelluloses present.
The chemical reactions of cellulose are dictated by its polymorphic nature. The less-
ordered amorphous regions are more reactive than the ordered crystallite regions, initial
chemical reaction takes place on the less-ordered surfaces of the fibrils. Little or no effect is
observed on the impenetrable crystalline structure.
Caustic alkaline solutions penetrate cellulose by swelling and by subsequent capillary
attraction, which allows entry into the regions between the crystallite zones. Consequently,
the crystallite zones are disrupted. This process, often termed mercerization [43], is used to
(>1508C) cellulose undergoes hydrolysis in basic media, and oxidation may also occur.
Microbiological degradation occurs via enzymatic hydrolytic cleavage of the b-1,4-glucosidic
link [72a]. Substituted cellulose ethers are for steric reasons less exposed to this process, and their
relative stabilities may be considerably greater. It is also known that, as expected, amorphous
cellulose is more susceptible to enzymatic hydrolysis than more highly crystalline cellulose [3,73].
Partial degradation of natural cellulose using multicomponent, degrading enzymes leads to
products having certain practical advantages, including enhanced solubility [73a].
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Chapter 1 and Chapter 2). The critical discovery of genes from Arabidopsis encoding for cellulose
activate cellulose for cellulose ether production (see Section 5.3.1.1). At higher temperatures
synthase in 1996, of which ten true and eight weaker subdivisions have been reported [63, cf. 64],
5.2.5 PHYSICALLY MODIFIED CELLULOSES
5.2.5.1 Microfibrillated Cellulose
The preparation of this material has been disclosed in a patent from the ITT Corporation [74].
Essentially, a slurry of cellulose is passed through a small orifice under conditions of
high shear and at a great pressure differential. This treatment disrupts the cellulose into
microfibrillar fragments [45]. The assay for microfibrillated cellulose (MFC) is described in
the Food Chemicals Codex (FCC) [75] and involves titration by ferrous ammonium sulfate of
a potassium dichromate and sulfuric acid-treated solution of the cellulose. The assay specifies
at least 97% cellulose.
MFC has considerably more water-retention capability than normal-grade materials and
is considerably less prone to precipitation in the cupriethylenediamine residue test (cuene
test). The change in DP with respect to the pulp feed stock is kept at a minimum during
microfibrillation. Suspensions of MFC are shear thinning (pseudoplastic), exhibit slight
thixotropic behavior, and apparently do not suffer a viscosity drop on heating. Electrolyte
tolerance is also on a par with that of other commonly used cellulosics.
5.2.5.2 Microcrystalline Cellulose
Microcrystalline cellulose (MCC) is produced by treating natural cellulose with hydrochloric
acid to dissolve the amorphous regions of the polysaccharide, leaving behind the less reactive
crystalline regions as fine crystals. As with MFC, the viscosity of dispersions of this product is
both pH and heat invariant. A number of varieties are available — powdered, bulk-dried
colloidal, spray-dried colloidal (with CMC), and spray-dried with sweet whey [76]. MCC
dispersions have been shown to exhibit both thixotropic and pseudoplastic behavior [77].
MCC can be assayed in a manner similar to MFC by employing the ferrous ammonium sulfate
titration technique; other determinations, including loss on drying and residue on ignition, can
be accomplished as described in the FCC [75]. The assay specifies not less than 97% cellulose.
Particles may be approximately 1 mm in length, and the technique used for dispersion is
in the commercial product. Purified cellulose powders for use as dietary fiber (Section 5.5.2), of
average fiber length varying from 25 to 120 mm, are essentially unmodified [78]. Many import-
ant industrial food applications have been listed [45,79] and patents registered [80–82].
5.3 CHEMICALLY MODIFIED CELLULOSE DERIVATIVES
Despite the wide variety of cellulose derivatives that have been made, notably acetate and
nitrate esters [3,22,83–86], only a few of the cellulose ethers find application (and are
approved for use) in foodstuffs [87,88]. The most widely used cellulose derivative is sodium
CMC [89,90]; other ethers have unique and interesting properties, however, which ensure
their inclusion in a widening array of products. Thus, for example, methylcellulose (MC) and
hydroxypropylmethylcellulose (HPMC) find uses as a result of interfacial activity and their
ability to form gels on heating.
Although a number of cellulose ethers are available, they are all made in essentially the
same manner. Naturally, individual suppliers have their own technologies, but the production
process can be broken down into the generation of alkali cellulose, alkylation or hydroxy-
alkylation, and finally product purification. Alkylation specifically at primary alcohol sites
requires a more elaborate synthetic procedure [91].
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important [77]; in the reference cited, some 10% of CMC (see Section 5.3.3) is incorporated
5.3.1 THE MANUFACTURE OF CELLULOSE ETHERS
5.3.1.1 Preparation of Alkali Cellulose
Alkali cellulose is most commonly produced by slurrying, spraying, or otherwise mixing
chemical cellulose chip, pulp, or sheet with aqueous sodium hydroxide solution (35–60%
w/v). Inert organic solvents may be used as slurry media. This mixture is then held for a
predetermined time at a controlled temperature and pressure to ensure complete reaction and
to control the viscosity of the final product through an ageing process. Alkali cellulose is quite
readily degraded by air oxidation; thus the quantity of oxygen present in the alkalization
reaction and the type of cellulose used have a crucial influence on the DP of the final product
and hence its viscosity in solution [2].
Although in theory it would be possible to produce alkylcelluloses by the use of potent
alkylating agents such as diazomethane, the heterogeneous nature of the cellulose leads to
variations in the availability and reactivity of the hydroxyl groups. Treating the cellulose with
caustic alkaline solution disrupts hydrogen bonding between and within the polymeric
strands, making the majority of the hydroxyl groups available for modification, with the
C2 and C6 hydroxyl sites typically more reactive than the C3 site [83]. Additionally, sodium
hydroxide acts as a catalyst in the Williamson etherification reaction.
For high-viscosity products, cotton linters are utilized under strictly controlled conditions
in order to minimize oxidation. These linters have an extremely high DP and a high
a-cellulose content (>99%). Other products with lower viscosity requirements are made
from a variety of wood pulps.
The preparation of alkali cellulose takes place as follows:
RcellOH����!NaOH
RcellOH �NaOH
RcellOH �NaOH ��!RcellONaþH2O
5.3.1.2 Alkylation and Hydroxyalkylation of Alkali Cellulose
As industrial production processes make use of alkyl chlorides (Williamson ether synthesis)
for the alkylation step and epoxides (oxiranes) for hydroxyalkylation, this chapter will deal
only with these routes.
5.3.1.2.1 AlkylationIn the Williamson ether synthesis, a nucleophilic alkoxide ion reacts with an alkyl halide to
give an ether and a salt. Thus, the alkali cellulose slurry reacts with methyl chloride to give
MC and sodium chloride. In a similar manner sodium monochloroacetate is used to produce
sodium CMC.
The preparation of MC and CMC takes place as follows:
RcellOHþ CH3Cl����!NaOH
RcellOCH3 þNaClþH2O
RcellOHþ ClCH2CO2Na����!NaOH
RcellOCH2CO2NaþNaClþH2O
5.3.1.2.2 HydroxyalkylationHydroxyalkylation of cellulose is carried out by treating alkali cellulose with an epoxide.
Propylene oxide is used to prepare hydroxypropylcelluloses (HPCs), ring opening at the
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primary carbon leading to a product containing a secondary alcohol function within the ether
moiety introduced.
The preparation of HPC proceeds as follows:
O OHNaOH
RcellOH + CH3CH CH2 RcellOCH2CHCH3 + NaOH
For the production of mixed alkylhydroxyalkylcelluloses, alkylation and hydroxyalkyl-
ation can be carried out either sequentially or concurrently. An important factor in the
hydroxylation reaction is that a new hydroxyl group is generated and is available for further
reaction. The effect of this will be considered later in the discussion.
The properties of the resulting cellulose ethers are a complex function of the molecular
weight average of the polymer and the types and level of substitution. It is here that the skill of
the manufacturer comes into play to produce useful products consistently.
5.3.1.3 Product Purification
Because byproducts are generated in these reactions, purification is required to meet the food
and other premium application standards. Byproducts include alcohols, alkoxides, ethers, and,
in the case of CMC production, glycolic acid and its salts. Generally, the thermal gelling
and hot water-insoluble products such as MC and HPC, respectively, are purified by hot
water washing and filtration procedures. Cold and hot water-soluble products such as CMC
are purified by washing with solvent systems such as aqueous ethanol or acetone. The purified
products are then dried, and their particle sizes are modified by suitable means. Finally, they
are analyzed for premium application compliance and are packaged.
5.3.2 CHARACTERIZATION OF CELLULOSE ETHERS
Besides the type of substituents carried by the cellulose backbone and the viscosity of the
cellulose ether in aqueous solution (normally quoted for a 1 or 2% solution w/v), these
products are described by the degree of substitution (DS) and the molar substitution (MS)
level. Molecular weights and distributions are now readily accessible [28,34–36,38].
5.3.2.1 Degree of Substitution
Each anhydroglucose unit in the cellulose molecule has three hydroxyl groups available for
derivatization. Thus, if all of these hydroxyl groups were substituted, the product would be
said to have a DS of 3. If an average two out of three of these groups were reacted, then the
block reactive hydroxyl groups; reagents that allow further chain growth are characterized
by MS.
5.3.2.2 Molar Substitution
Derivatization of a reactive hydroxyl group with propylene oxide generates on a one-for-one
basis a replacement hydroxyl site for further reaction. Thus, as the reaction continues, chain
extension occurs. Oxyalkyl substitution is thus described by the MS level, i.e., the number of
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DS would be 2, and so on (Figure 5.2). The term DS is reserved for those substituents that
moles of alkylating agent per mole of anhydroglucose in the chain (Figure 5.3). The ratio of
MS to DS gives the average chain length (DP) of these side-chain substituents.
5.3.3 SODIUM CARBOXYMETHYLCELLULOSE
acid or its sodium salt or mixtures thereof. The sodium salt is the most common for food use,
as the free acid form is insoluble in water. In this discussion, for convenience, the term CMC
will be used to refer sodium CMC. In the United States the term ‘‘cellulose gum’’ is often used
for food-grade CMC [45]. CMC is assayed by determining the sodium chloride and sodium
glycolate percentages [75] and subtracting these from 100%. The FCC specifies that the assay
be at least 99.5% CMC. The principle of SEC has been applied to the characterization of
CMCs [34,92–94].
CMC was initially developed in Germany as a gelatin substitute. The real drive for its
commercial usage was the discovery in 1935 that CMC could improve the efficacy of laundry
detergents. Food-grade CMC was introduced by the Hercules Company in 1946 [95] and since
then it has become the dominant cellulose ether in terms of its total usage. It was estimated
that in 1983 the consumption rates in the United States, Western Europe, and Japan were
22,200, 93,000, and 17,200 metric tons, respectively, with about 5% of CMC production used
by the food industry, or about 6,600 metric tons per annum globally [96].
O
O
O
OO
O
OH
HH
HHH
H HH H H H
H HHH
HO
HO
HO
HO CH2
CH2
CH3
CH3
CH3
H3C
O
O
CH2
CH3
O
O
CH3
O
n–2
FIGURE 5.2 Methylcellulose with a DS of 2.0.
O
O
O
OO
O
OH
HH
HHH
H
OH
HH H H H
H HHH
HO
HO
HO
HO CH2
CH2
CH2CHCH3
CH2CHCH3
CH3CHCH2
CH3CHCH2
O
O
CH2
CH2CHCH3
O
CH2CHCH3
OH
OHOH
OH
O
OOH
n−2
FIGURE 5.3 Hydroxypropylcellulose with a MS of 2.0.
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CMC (Figure 5.4) is an anionic, linear, water-soluble polymer that can exist either as the free
5.3.3.1 Effect of DS on Solubility
Commercially, CMC is available in the DS range 0.4 to 1.5. As the interchain association of
adjacent hydroxyl groups is the most important determinant of solubility, the DS of CMC
(and other cellulose ethers) has a profound effect on the physical properties of solutions. At
low DS the bulk of the etherification has occurred in the amorphous and the crystalline
surface regions of the cellulose. This gives a product with a high residual degree of chain:chain
association and resultant poor water solubility. Thus, CMC with a DS < 0.3 is only soluble in
alkali. At higher DS, the interchain associations are more disrupted, yielding partially soluble
material. As the DS approaches 0.7, the crystalline regions have been sufficiently disrupted to
yield highly water-soluble material. Above a DS of 1.0 the relative concentration of unreacted
hydroxyl sites is so low that little or no interchain association occurs. In the United States,
CMC for food use is limited to DS � 0.95 [97].
5.3.3.2 Solution Rheology
Because CMC and other cellulose ethers are produced by controllable reactions, it is possible
to tailor desirable property features into the polymer. Characteristics such as solution
viscosity, thixotropy, and pseudoplasticity can be controlled by varying the amount of
chain oxidation (molecular weight control), DS, and uniformity of chemical substitution.
As discussed earlier, the viscosity of the final product is a function of the molecular weight
of the polymer. Since the viscosity of a given solution and the concentration of the cellulosic
required to achieve this may be significant factors in the choice of product used, CMC is
offered in a range of viscosity grades. Typically, manufacturers of food-grade material offer
products giving 1% aqueous solution viscosities from 20 to 4000 mPa.
Solution viscosities are affected by temperature. As is generally true with cellulose ether
solutions (with the notable exceptions of MC and HPMC), the viscosity decreases with
As with most polyelectrolytes, the pH of the environment will also affect the viscosity of a
CMC solution, though in general little effect is observed between pH 5.0 and 9.0. Below pH
4.0 the free acid is substantially produced, which can cause precipitation of the polymer (see
The effect of solutes such as salts or polar nonsolvents on viscosity can be marked and is
dependent on order of addition. In general, if the solute is added to a prepared solution of
CMC then the viscosity drop is minimized compared to the situation in which the CMC is
added to the solute in solution. For brine this is probably due to an immediate ionic
interaction between the salt and the polymer carboxylate group, effectively shielding the
OO
O
O
OH
HH
HHH
H HH H H H
H HHH
HO
HO
HO
HO CH2
CH2
CH2
CH2
O
O
CH2
COO-Na+
COO-Na+
CH2
COO-Na+
OH
OH
O
OOH
n-2
FIGURE 5.4 Carboxymethylcellulose with a DS of 1.0.
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156 Food Polysaccharides and Their Applications, Second Edition
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increasing temperature (Figure 5.5) linearly on a semilog plot.
Figure 5.6). Above pH 10 viscosity decreases and cellulose degradation becomes important.
anionic charge and substantially inhibiting hydrogen bonding between the anionic site and
To avoid the problem of reduced viscosity in these situations, it is recommended that the
CMC be dispersed in water first and allowed to hydrate. Following polymer hydration, the
salt is added to the desired concentration. The probable reason this is effective is because as
the CMC hydrates, it collects water molecules around itself; these dipoles orient with respect
to the anionic carboxylate groups, partially satisfying and partially shielding them from the
cations that are added later. In general, monovalent salts form soluble salts of CMC;
therefore, the solution properties such as haze, viscosity, and clarity are relatively unaffected
CMC, and therefore the solution quality decreases. The effect of trivalent cations is generally
to precipitate the polymer. Examples of inorganic cations that are incompatible with CMC
solutions are aluminum, chromic, ferric, ferrous, silver, and zinc.
The interactions of this anionic polymer are not limited to inorganic salts. The carboxylate
can also interact with proteins, as long as the pH of the food system is greater than the
isoelectric point of the protein. In such a case, ionic interactions between the anionic CMC
chain and the cationic protein chains generate a higher viscosity than is otherwise expected.
displays very little interfacial activity except when ionically driven.
Another fundamental characteristic of many aqueous polymer solutions is pseudoplasti-
city: solutions that tend to lose viscosity with increasing shear rate are said to be pseudoplastic.
For CMC, pseudoplasticity generally decreases with increasing DS. At DS greater than or
10
1% 7HF
1% 9M31F
2% 7LF
2% 7MF
Temperature, �C
Vis
cosi
ty, m
Pa
s
100
1,000
10,000
20 40 60
FIGURE 5.5 Effect of temperature on viscosity of CMC solutions. Commercial grades of CMC
produced by Hercules.
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Cellulose and Cellulose Derivatives 157
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water molecules (see Figure 5.7).
at low to moderate salt levels [cf. 81]. Divalent cations generally form less soluble salts of
This synergistic activity is well-known and described in the literature [3,45, cf. 24]. CMC
equal to 1.0, CMC forms solutions with very little interchain association and that exhibit little
or no hysteresis loop during increasing and decreasing shear.
Thixotropy is a time-dependent change in viscosity at constant shear rate. For CMC,
thixotropic rheology is a result of the interchain hydrogen bonding between adjacent areas of
relatively unsubstituted anhydroglucose units. In CMC, which has relatively large regions of
unsubstituted hydroxyl groups on the rings, there is a strong tendency for the chains to
2
2% 7 M
2% 9M31
1% 7H
100
500
1,000
5,000
4 6
pH
8 10 12
App
aren
t vis
cosi
ty, m
Pa
s
FIGURE 5.6 Effect of pH on solution viscosity of CMC.
20
0.04 0.10 0.20 0.40 1.00
40
60
80
100
200
300
Vis
cosi
ty, m
Pa
s
Molal concentration NaCl
Solute addedafter CMC
Solute addedbefore CMC
FIGURE 5.7 Effects of NaCl and order of dissolution on viscosity of 1% CMC solutions.
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develop interchain hydrogen bonds, resulting in a gel-like network. Conditions that allow for
large regions of the chain to escape substitution favor products that will yield thixotropic
solutions. Generally, the tendency for this to happen is much greater at the lower DS.
Typically, a DS of 0.7 affords a substitution pattern heterogeneous enough to produce
carboxyl-rich and carboxyl-poor regions. As the substitution on the chain increases or
becomes more uniform, the tendency for interchain association to occur decreases. Commer-
cial manufactures take advantage of this to produce nonthixotropic CMC. Nonthixotropic
CMC is available today at DS as low as 0.7. This material has a high degree of uniformity in
substitution.
5.3.4 METHYLCELLULOSES
MCs comprise a family of cellulose ethers in which methyl substitution occurs with or without
additional functional substituents. Thus besides MC, this category includes methylhydroxy-
ethylcellulose (MHEC), which is allowed in the European Economic Community (EEC) (up
to 5% hydroxyethyl substitution permitted under the MC specification), and HPMC. MC, the
first cellulose ether to be made, was first described in 1905. Since then, intensive development
by The Dow Chemical Company, Hercules Inc., and Hoechst AG inter alia has led to the
availability of a wide range of the above cellulose ethers of food-grade quality. The MCs are
assayed by determining their substitution percentages, as described in the FCC [75].
The usefulness of these nonionic cellulose ethers is essentially based on four key attributes:
efficient thickening, surface activity, film-forming ability, and, probably of greatest interest to
the food technologist, the ability to form thermal gels that melt upon cooling.
5.3.4.1 Solution Rheology
The solution behavior of the nonionic MC family is markedly different from that of the ionic
CMC. The effect of pH is especially reduced, and the temperature-dependent rheology is
much more complex. When dissolved in water these gums give clear, smooth-flowing solu-
tions that are pseudoplastic and nonthixotropic [98,99]. The pseudoplastic behavior of MC is
a function of molecular weight, with MCs of higher molecular weight exhibiting greater
but is rather a factor of the molecular weight of the polymer. In the graph the four gums have
the same molecular weight but differing degrees of substitution.
The curves showing the effect of polymer concentration on solution viscosity are com-
5.3.4.2 Thermal Gelation
Aqueous solutions of MC and HPMC (at >1.5 wt%) form gels when heated, then on cooling
return to the solution state at their original viscosity. The temperature at which this gelation
begins and the texture of the gel formed are dependent on the type and substitution level of
the gum. For example, the commercially available MC and HPMC products have gel
temperatures ranging from 50 to 858C and gel strengths varying from firm to weak [100], as
with no anticipated loss of properties.
Thermal gelation is affected by a number of polymer-dependent characteristics. The most
important determinant of gel strength is the concentration of methyl groups and the methyl:
hydroxypropyl ratio. As the methyl concentration increases, the gel formed on heating
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shown in Table 5.2. MC gel becomes fluid upon heating and the solution that eventually
pseudoplasticity as shown in Figure 5.8.
DS does not affect the rheology of MC and HPMC solutions as exemplified in Figure 5.9,
parable to those for CMC, with the curves for HPMC (Figure 5.10) illustrative of the class.
forms regains its initial viscosity (see Figure 5.11). This process may be repeated continuously
becomes firmer. Conversely, as the hydroxypropyl substitution increases, the gel becomes
softer. The most probable explanation involves dehydration followed by hydrophobic asso-
ciation of the chains. At temperatures greater than the thermal gel point (TGP), the vibra-
tional and rotational energies of the water molecules increase, exceeding the ability of
the weak hydrogen bonding to orient the dipolar water molecules around the polymer
chain. The energized water molecules then tend to disengage from the fragile envelope of
ordered water surrounding the chain. The dewatered hydrophobic polymer segments then
begin to associate with each other. As the temperature or the time at which high temperature
increases, the hydrophobic interactions increase in number, producing an ever-firmer gel due
to the increasing formation of cross-links.
10
100.1 1
100
100
25
400
1,500
4,000
100
1,000
1,000
10,000
App
aren
t vis
cosi
ty, m
Pa
s
Shear rate, Sec−1
FIGURE 5.8 Effect of shear rate on apparent viscosity of 2% solutions of methylcellulose. Note:
Numbers on curves indicate viscosity types.
1010
Shear rate, Sec−1
0.1 1
MC (4,000 mPa s)
HPMC (4,000 mPa s)
HPMC (4,000 mPa s)
HPMC (4,000 mPa s)
100
100
1,000
1,000
10,000
10,000
App
aren
t vis
cosi
ty, m
Pa
s
FIGURE 5.9 Effect of shear rate on apparent viscosity of solutions of HPMC with different degrees of
substitution.
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The addition of hydroxypropyl groups to MC always tends to diminish the rigidity of the
gel and increase its critical thermal gelation temperature. Hydroxypropyl substituents are
more hydrophilic than methyl groups, and hence are better able to retain water of hydration
when exposed to heat. Because they hold on to their water more tightly, the temperature
needed to drive the substituent groups apart is correspondingly greater than with MC alone.
Furthermore, the equilibrium association between water and HP substituents tends to pro-
duce a more hydrophilic gel than is possible with MC.
10
400
0 1 2
Concentration, � mv−1
3 4 5 6 7
100
100
1,000
1,500
15,000
75,000
4,000
40,000
10,000
100,000
Vis
cosi
ty, m
Pa
s at
20�
C
FIGURE 5.10 Effect of concentration on viscosity of hydroxypropylmethylcellulose. Note: Numbers
on curves represent viscosity types.
TABLE 5.2Thermal Gelation Properties of MC and HPMC
Viscosity
Range (mPa)
Gel
Texture
Nominal
Gel Point (˚C)
Degree of Methyl
Substitution (DS)
Degree of Hydroxypropyl
Substitution (MS)
MCa 15–4000 Firm 50 1.6–1.8 —
HPMCb 3–4000 Semifirm 60 1.63–1.85 0.1–0.3
HPMCc 50–4000 Semifirm 65 1.0–1.8 0.1–0.2
HPMCd 3–100,000 Soft 85 1.1–1.4 0.1–0.3
aMethocel (trademark of Dow Chemical Company) A.bMethocel E.cMethocel F.dMethocel K.
Source: From Dow Chemical Co., Form No. 192-976-586, 1986.
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The strength of the gel increases with increasing molecular weight until a maximum
strength is reached at about Mn 40,000. This corresponds to a 2% solution viscosity of
about 400 cps as determined by the ASTM Methods [101]. Further increases in molecular
weight do not increase the gel strength. In addition, the molecular weight has no effect on the
thermal gel temperature. This indicates that gel formation is essentially dependent on polymer
chemistry and thermal kinetics and not on inherent viscosity contributions due to molecular
weight [87].
A final point of interest in this discussion is the tendency of MC and HPMC to concen-
trate at air–water and oil–water interfaces. Like thermal gelation, this pronounced surfactant
behavior is the result of substitutional heterogeneity in the polymer. The concentrations at the
interface of dilute solutions of MC can be as high as several weight percent. At the interface of
dilute solutions, one expects and can find very resilient gels [102]. This can lead to, among
other things, foam stabilization, emulsion stability, and positive effects on crumb structure,
dough rising, and baking, etc. These topics are discussed later in the text.
5.3.4.3 Solute-Induced Gelation
In addition to thermal gelation, MC and HPMC also gel upon the addition of sufficient
coagulative cosolute. Solutes in this category are phosphate, sulfate, and carbonate salts. The
action of these salts tends to strip water molecules away from the polymer via disruption of
the hydrogen-bonding forces. This is analogous to the effect of thermal energy as a hydrogen
bond disruptor. The net result is that certain polymer segments have an insufficient attraction
to the electrolyte solvent, partially dehydrating the chain and allowing the formation of the
hydrophobic interactions.
A number of salts (e.g., trisodium polyphosphate and sodium sulfate) have been found to
be most effective for lowering the gelation temperature. MC is more sensitive in this respect
100 20 30 40 50 60 70
Vis
cosi
ty, m
Pa
s
Cooling
Heating
Incipient gelationtemperature
Temperature, �C
FIGURE 5.11 Effect of temperature on viscosity of methylcellulose (gelation of a 2% aqueous solution
of methylcellulose; heating rate 0.258C/min). (Methocel A-type material courtesy Dow Chemical Co.)
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than are the various HPMCs. Firm gels can be formed at room temperature by adding 3%
trisodium polyphosphate to a 2% solution of MC. The use of salts to form gels with nonionic
cellulose ethers is presently investigated. The opportunities that room temperature gel for-
mation present to food formulators are obvious. A magnetic resonance probe has been
developed for the mapping of temperature profiles in gels [103].
5.3.5 ETHYLMETHYLCELLULOSE
Ethylmethylcellulose (EMC) is manufactured by only one supplier worldwide [11,104]. The
product has an ethyl DS of 0.3 and methyl DS of 0.7. Rheology, salt tolerance, pH stability,
etc. are in line with those of MC. Instead of gelling, however, EMC precipitates from aqueous
solution when heated above 608C; re-solution occurs on cooling.
5.3.6 HYDROXYPROPYLCELLULOSE
There are two manufacturers of HPC. Food-grade quality HPC is available in six viscosity
ranges, all at an MS of about 4 [11]. As is the case with MC and HPMC, this ether exhibits
considerable surface activity — for example a 0.1% aqueous solution of HPC has a surface
tension of 43.6 dyn/cm [105]. HPC differs from MC and HPMC in that it does not gel
thermally but, similarly to EMC, precipitates from aqueous solution above 458C. Addition-
ally, HPC shows a markedly lower tolerance to dissolved electrolytes; thus, while MC, CMC,
and HPMC are all soluble in 10% aqueous NaCl solution, HPC is insoluble. HPC has
long been known to show a greater degree of solubility than either MC or HPMC in polar
organic solvents [105]. HPC is assayed using the hydroxypropyl determination included in
the FCC [75].
5.4 APPLICATIONS IN FOODS
Cellulose and its physical and chemical derivatives have long been used in fabricating
formulated foods. The physically modified celluloses are useful in many products where
bulk properties are desirable [106]. This would include reduced- or low-calorie foods, flavor
oil imbibers, or flowable products such as artificial sweeteners and flavor packets. The use of
these cellulosics is generally due to their rheology, controlled water interaction, and textural
attributes, and not to solubility or other chemical properties. Hence, MCC and finely ground
cellulose perform a valuable bulking role in low-calorie foods. Five important roles for the
chemically modified cellulose derivatives in foods are the regulation of rheological properties,
emulsification, stabilization of foams, modification of ice crystal formation and growth, and
water-binding capacity.
The applicability of cellulose derivatives for specific food applications can be determined
from their physical and chemical properties. When a choice is to be made, a number of
parameters must be considered: (a) the chemical structure of the polymer; (b) the molecular
weight of the polymer; (c) the presence of other active ingredients in the food matrix; (d) the
processing operations to which the food will be subjected; and (e) the physical properties,
including fiber dimension of the polymer.
Arguably, the most important factor is the chemical structure of the cellulose derivative.
For physically modified celluloses [107], this generally refers to the crystalline or amorphous
nature of the product. For ethers, there are a number of substituent groups and a range of
substitution patterns allowed by regulation that affect the rheological and surface-active
properties of the derivative. For instance, addition of carboxylic acid moieties to the cellulose
chain increases the hydrophilicity, whereas addition of alkyl residues such as methyl or ethyl
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increases the hydrophobicity of the polymer chain. By increasing hydrophilicity, the polymer
is better able to hydrate in the presence of other water-soluble species such as sugars. By
increasing the hydrophobicity, a polymer may be produced that is also a surfactant, thus
conferring on the chain a host of interesting physicochemical characteristics.
The molecular weight of the polymer is readily manifested in solution viscosity; as
molecular weight decreases, solution viscosity decreases. For many of the derivatives dis-
cussed herein, the most important property is viscosity; for some, however, the most important
characteristic is film-forming or surface activity. In such cases products of low molecular
weight might better serve the application.
The inclusion of other active ingredients in the product also plays a crucial role in food
formulation. The presence of proteins, simple carbohydrates, and some starches is important
in the choice of cellulose derivatives. For example, in products with high salt concentrations,
CMC does not build up viscosity to the same extent as when salt is absent, and therefore
more CMC is required to overcome the effect of salt. Similarly, because CMC is ionic, it can
interact with certain proteins. Interfacially active MC and its derivatives can also interact with
proteins through hydrophobic–hydrophilic mechanisms.
Transient environmental modifications such as changing temperature can have a pro-
found influence on gum selection. Because the solubilities of some cellulose ethers are affected
by temperature, systems can be designed to yield widely divergent rheological profiles under
different processing regimes. For instance, MC interacts strongly with other MC chains at
elevated temperatures. This can sometimes cause firming in a product upon heating.
Finally, the physical properties of the derivatives are important in the context of the food
product and its processing. In some cases, it is necessary to deliver the polymer in a powdered
form by dry blending. In others it may be more efficient to use a granular material.
In this section, we shall describe classes of food products and include formulations that
specifically reference the various cellulose derivatives and provide chemical and physical
explanations for their functions. Typical food applications for cellulose and its derivatives
are shown in Table 5.3.
5.4.1 FISH/MEAT
Protein-based foods frequently need stabilizers for increased shelf life during ambient or frozen
storage. These products generally have sufficient binding capacity to preserve structural
TABLE 5.3Commercial Food Uses for Cellulose and Its Derivatives
Cellulose/Cellulose Derivative Food Applications
Hydroxypropylcellulose Whipped toppings, mousses, extruded foods
Hydroxypropylmethylcellulose Whipped toppings, mousses, baked goods, bakery fillings, icings, fried foods,
sauces, dressings, frozen desserts, reduced-fat foods
Methylcellulose Sauces, soups, breads, tortillas, fried foods, restructured (matrix) foods,
reduced-fat foods, foams
Methylethylcellulose Whipped toppings, mousses, egg white substitute
Microcrystalline cellulose Dressings, sauces, baked goods, beverages, whipped toppings, reduced-fat foods
Powdered cellulose Breads, cookies, pastries, pasta, imitation cheeses, cereals, canned meats
Sodium carboxymethylcellulose Frozen desserts, baked goods, dressings, sauces, syrups, beverages,
extruded foods, animal foods, reduced-calorie foods
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integrity during storage, but gums may be added to provide primary binding in special products
or secondary benefits for quality improvement.
Seafood processors can use cellulose derivatives to allow fabrication of novel, value-added
seafood products. Extruded shrimp or fish nuggets may be produced that take advantage of
fish pieces, shrimp bits, or whole shrimp that are too small to be otherwise useful. By
incorporating a small amount (0.6–2.0%) of MC, the products have good cold extrusion
and forming characteristics and also high-temperature stability. High-temperature stability is
crucial because these products are generally deep-fried and would lose their integrity in the
Unique red meat products can be formulated using cellulosics. For example, patties can be
formed from chopped corned beef, sauerkraut, and cheese using MC (0.6%) as a binder. This
formulation allows cold-forming of what is normally a nonbinding mixture, but also prevents
loss of product integrity during frying due to its thermally gelling nature.
Trends in industry toward reducing the fat content of meats to promote a more healthy
consumer diet have presented new opportunities for cellulosics. In particular, the film-
forming characteristic of the interfacially active MCs, combined with the variety of viscosities
that can be achieved with these products, offers the possibility of mimicking the texture of
lipids in these fat-reduced systems.
Batters for deep-frying are an important family of products associated with processed
meats. Cellulosics are an integral part of many batter formulations. These polymers contribute
viscosity and thermal gelation to improve processing control and batter quality. Cold viscos-
ity is a crucial quality-control indicator. Batters that are too fluid fail to enrobe a product
sufficiently; conversely, those that are too viscous cause inappropriately high coating levels. A
further benefit of using a thermally gelling cellulosic in a batter is the production of a
relatively oil-insoluble barrier during frying. By gelling, MC and HPMC generate a water-
holding gel that prevents oil ingress during frying, with a reduction of up to 50% oil
absorption in selected batters.
5.4.2 SAUCES, GRAVIES, SOUPS, AND SYRUPS
Sauces, gravies, soups, and syrups represent a broad range of fluid food products that are
generally stabilized using hydrocolloids, including cellulose derivatives. The physically modi-
fied celluloses can help to maintain structural integrity during freezing. In addition, they can
substantially reduce the caloric content of the food by replacing carbohydrates or fats.
Chemically modified derivatives are often used in these products to increase the efficiency
of water binding and reduce the problem of syneresis on thawing. They can provide fatlike
mouthfeel and viscosity in systems containing reduced levels of fats and oils. Additionally,
some cellulose ethers are used in conjunction with other stabilizers as emulsifiers in fluid
systems that contain fat. The ability of cellulose ethers such as HPC, HPMC, and MC to
accumulate at oil droplet interfaces and prevent oil droplet coalescence is important in many
of these products to prevent oiling-off during storage. In general, sauces and gravies also
include starches to provide bulk viscosity and desirable sensory characteristics.
Fruit fillings and table syrups, especially low- or reduced-calorie products, can make good
use of cellulosics. CMC is particularly effective in these applications as it is an efficient
thickener in systems where the concentration of soluble solids is quite high (45–60%) and,
like most chemically modified cellulosics, it produces transparent solutions, required in this
product category.
Sauces, gravies, fillings, and cream soups are similar in that most of them take advantage
of starches for viscosity control. However, a drawback to using starches exclusively for
rheology control is that they generally do not maintain viscosity over a wide temperature
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absence of a thermally gelling binder [cf. 108].
range and many of them are not freeze–thaw stable. Cellulosics have long been used to boost
the performance of these products, especially those that are held for long periods at elevated
temperature as in a food-service operation or those that are frozen. Recent work in our
laboratory has shown that low molecular weight MC interacts with modified starches to yield
systems with rheological properties that are quite stable over wide temperature ranges. This is
in contrast to starches alone, which lose viscosity as temperature increases, and MC alone,
which gels at temperatures in excess of 508C. The specific nature of this interaction is not
known but may be due to intergranule bridging or hydrophobic interactions between the MC
and the modifying groups on the starch granules.
5.4.3 EMULSIONS
Two important emulsified food categories are salad dressings and whipped toppings, the
former oil-in-water emulsions, the latter, foams of oil-in-water emulsions. Both types require
certain fundamental chemical and physical properties that are obtainable by the use of cellulose
derivatives. Pourable salad dressings are typically oil-and-water emulsions with oil concen-
trations ranging from 10 to 50%. In these systems it is important to prevent the flocculation
and coalescence of oil droplets that would lead to rapid phase separation. Cellulose ethers
contribute to emulsion stability by concentrating at the oil–water interface, imposing a barrier
of hydrated polymer around each droplet.
In low-calorie salad dressings, emulating the mouthfeel of higher-oil dressings can be a
greater challenge than stabilizing the low level of oil that is present. The surface-active
cellulose derivatives can provide the film-forming property and slip that is characteristic of
oils, and can also be selected to optimize the viscosity needed to achieve the appropriate
texture in these products.
Dry mix salad dressings pose unique challenges to food formulators. In these commod-
ities, a dry mixture of stabilizers, spices, and flavors is packaged for consumer use. The
consumer is the end processor of the emulsion and requires an easily formulated and easily
processed product for domestic use. The use of HPMC and MC is advantageous because they
offer rapid hydration and emulsification under the relatively low shear conditions that are
encountered in home preparation. The rapid interfacial migration of cellulose ethers contrib-
utes to rapid stabilization of the emulsion and ease of use for the consumer.
Nondairy whipped toppings, whipped desserts, and mousses are foams of oil-in-water
emulsions. These products pose special concerns and constraints because the bubble wall is
thin, relatively weak, and unsupported. This is in contrast to the surface of an oil droplet in a
dressing, which is supported from inside by a mass of oil. In these systems, two requirements
exist. The first is a physical stabilization of the liquid within the interstitial regions of the
foam. The second is a strengthening of the cell wall, accomplished by the use of gums and
other ingredients in the mix.
For whipped toppings, structural integrity between the foam cell walls and in the inter-
stitial areas can be achieved by using a physically modified cellulose derivative such as MCC.
MFC was a unique physically modified cellulose derivative, which also provided stability in
whipped toppings; its current commercial status is unclear. The rheological role played by
MFC in these applications is that of imparting yield-point properties that result in structuring.
Cell wall integrity can be achieved by using cellulose ethers that accumulate at interfacial
surfaces. Examples often used in whipped toppings are HPC, MC, HPMC, and MEC, and
MC is used to stabilize foams [109]. In the case of the methyl derivatives, the polymer
concentrates at the interfaces and undergoes gelation, as the concentration at the interfacial
boundary is substantially higher than that of the bulk phase (usually less than 1%). This
surface gel confers stability on the bubble cell wall, resulting in a more resilient and stable
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product. Furthermore, whipped products usually have an additional storage burden as they
must survive frozen storage. The use of chemically modified cellulose derivatives can reduce
syneresis, a severe quality defect, during repeated freeze–thaw cycling.
Cellulosics have been used frequently in emulsified food products. The most common
prepared emulsions are salad dressings, in which the two cellulosics most used are HPMC and
MCC. HPMC is incorporated in pourable and dry mix salad dressings for emulsification,
rheology control, and improved organoleptic properties. HPMC, since it is a surface-active
agent, migrates to the oil droplet–water interface and establishes a hydrated polymer barrier
at the interface. This prevents oil droplet coalescence and subsequent development of an oil
slick on the emulsion surface. In addition, since the additive accumulates at the droplet
interface and prevents large droplets from forming, the refractive index of the droplet changes
and the color is lighter than it would be otherwise. This is an advantage in creamy dressings.
Finally, HPMC increases the viscosity of the product and improves organoleptic properties,
ameliorating the otherwise ropy mouthfeel and gelatinous appearance of pourable dressings.
In contrast to their employment in pourable dressings as surfactants, cellulosics can also
be used in high-viscosity, spoonable dressings. MCC assists in the manufacture of full- and
reduced-calorie products by increasing the viscosity of the interstitial continuous phase and
contributing bulk to reduced-calorie formulations.
5.4.4 BAKED GOODS
There are a number of beneficial uses for cellulose and cellulose derivatives in baked products.
Breads require a certain strength-to-volume ratio to allow good formation, cell structure,
even texture, and high eating quality. Typical white pan breads achieve most of their volume
and textural quality from gluten, the most abundant protein in wheat flour. However,
variations in the quality and quantity of flours from various wheats affect product quality.
Additives have long been used to correct this.
High-fiber and variety of diet breads, buns, and rolls generally contain reduced quantities
of flour as a means of decreasing the caloric value; as a result, they require special reformu-
lation to provide loaf structure and baked quality comparable to products containing wheat
flour. High-fiber breads make use of several cellulose derivatives. Usually a physically
modified product such as MCC or cellulose flour is used as a partial replacement for wheat
flour and consequently some of the nutrient energy of the flour. However, physically modified
celluloses generally have little functionality in baked goods, and, therefore, certain compon-
ents must be modified to yield a product of suitable quality. Dough rheology is affected when
physically modified celluloses replace part of the flour because the water demand and mixing
qualities change. The baking qualities of reduced wheat flour breads are usually inferior and
must be compensated for by other functional ingredients.
The use of MC and HPMC returns to the bread the functionality lost due to the lowered
wheat flour concentration. These cellulose ethers provide an elastic mass during proofing and
baking that traps CO2 and allows the bread to rise and maintain adequate volume. Breads of
this type have been patented and are widely available in the U.S. retail trade.
In providing structure to breads and rolls, the thermally gelling derivatives are very useful.
Since MC and HPMC are interfacially active and form elastic gels at elevated temperatures,
they can impart added dough strength and uniformity through make-up, proofing, and
baking, and lead to an even, consistent crumb structure in the final baked product [110].
The properties of surface activity and gelation with heat also increase moisture retention and
tolerance to the changing environmental conditions of today’s automated bakeries. This
improves the production of bread from typical wheat flours as well as baked goods with
very low wheat protein levels or specialty breads made from other grains, such as rice,
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sorghum, and barley, which contain no gluten [111,112] and to which as much as 3% of the
cellulosic may be added.
As well as acting as a loaf volume enhancer, cellulosics can be used as bulking agents in
the production of high-fiber breads. Since cellulose is nondigestible, the physically modified
derivatives can easily be used at substantial concentrations as a supplementary fiber source.
Sweet baked goods, such as cakes, can also be improved by the inclusion of cellulosics
albeit at lower levels. Like breads, cakes need elasticity and some structural integrity to trap
the gases produced for leavening action. Thermally gelling cellulosics can perform well in
achieving these requirements. Increased cake volumes have been demonstrated with MC and
HPMC as well as with CMC, with the CMC-induced volume generated through a viscosity
quality and cake heights have been observed with the use of HPMC and MC [114]. In the
sensory evaluations associated with this study, textures were identified as moist without
excessively chewy.
Cellulosics also play a role in improving the texture of fat-reduced snack cakes by
providing batter thickening, moisture retaining, and film-forming properties in these products
[115,116]. In addition to a functional contribution, physically modified cellulosics can be used
as a bulking agent for production of cakes with reduced nutrient energy. This application is
expected to grow dramatically as heat-stable, low-calorie sweetening compounds (see Ref.
glazes sometimes use cellulosics, especially CMC and HPMC, at low levels to improve texture
[118]; CMC has been demonstrated [90] to enhance flavor perception and have some effect on
sweetness. HPMC improves low shear flow properties in icings, facilitating spreading, pro-
viding favorable mouthfeel, and improving resistance to icing runoff.
5.4.5 FROZEN DESSERTS
Frozen desserts frequently include hydrocolloids such as cellulosics, gelatins, starches, and
carrageenans. The cellulosics are often used to control ice crystal growth and to modify
rheology [119]. A number of formulations are available that include a variety of cellulosic
materials. This is especially true for some whipped frozen products that take advantage not
only of the ability of a gum to modify rheology but also to entrain air.
The trend toward replacing fat in food systems has a strong presence in the frozen dessert
industry. Cellulose derivatives can produce textures in low-fat or fat-free frozen desserts that
are similar to the mouthfeel associated with products containing higher fat levels.
5.4.6 EMERGING TECHNOLOGIES: BARRIER FILMS
Barrier films, such as those described by Fennema and coworkers [120–123], take advantage
of cellulose derivatives as film substrates in complex systems for water vapor transmission
control, the concept being expanded, in principle, to reduce oil uptake [124–127]; these films
are especially formulated for frozen prepared foods to prevent migration of water from areas
of high relative humidity to areas of low relative humidity and to preserve the textural
qualities associated with fresh prepared products. In this technology, films are produced by
casting in successive stages. First a film of HPMC is formed; this is overlaid with a sprayed
coating of a triglyceride, and then coated with a thin layer of beeswax. These films when
finished are 2–5 mm thick, unnoticeable in a fabricated product, and are excellent water vapor
barriers at temperatures below the melting point of the wax and fat. However, a unique
property of these films is that at temperatures in excess of 658C, the hydrated HPMC film gels
inhibit moisture migration. This reduces such migration from areas of relatively high water
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[117] ) are developed and formulated into processed baked goods. Finally, bakery icings and
contribution. In cake mixes formulated for microwave baking [cf. 113], improved eating
activity to areas of low water activity. As an illustration, the concept could be applied to
frozen pizza, with a film incorporated between the crust and the sauce and thus preventing
moisture migration during storage and home preparation. Other types of membrane, such as
developed [128].
5.5 NUTRITIONAL EFFECTS OF CELLULOSE DERIVATIVES
Any food component can affect the nutritional density or quality of the diet in a number of
ways. The most obvious is as a nutritious component of a food. An example of this is the use
of vitamins and minerals to fortify ready-to-eat cereals. A second way is as a functional
additive, without which food could not be produced, distributed, or accepted by consumers;
CMC added to ice cream improves the mouthfeel and reduces the growth of ice crystals
during frozen storage. A component could also have an impact on the nutrient density of
foods as a nonnutrient replacement, reducing the caloric load and making a wider variety of
products available to consumers; examples are the use of gums in the formulation of low-oil
salad dressings and cellulose in making low-calorie breads.
None of the cellulosics are of value as nutrient sources since humans lack the digestive
enzymes necessary to generate the b-1,4-linked glucose monomers. This fact is the basis for
much of the utility of these products in food, with cellulose and its derivatives playing
important functional and bulking agent roles. The use of CMC, for example, enables ice
cream to retain more of its initial quality during frozen storage than control samples. This
functional role is crucial to modern production and distribution systems and ultimately
permits more efficient use of raw materials, flexibility in handling and storage, and conser-
vation of food resources. Physically modified celluloses are especially important as bulking
agents in formulated foods, cellulose flours, and MCC are widely used as partial replacements
for flour and other nutrient materials in breads and some desserts.
5.5.1 DIETARY FIBER
Dietary fiber plays a significant role in gut physiology and nutrient absorption and has come
under very close scrutiny in recent years. In spite of its importance in our diet and for our
well-being, there is still much debate and confusion over the specific definition of dietary fiber,
fiber (crude) referred to the residue of plant material that was indigestible in acid and alkali.
This definition, based on a method of analysis, limited crude fiber to cellulose, hemicelluloses,
and lignin and is a dated term that has fallen out of favor because of its limited chemical
and inadequate biological usefulness. Today the term dietary fiber refers to a much broader
group of compounds. Most researchers and interested observers define dietary fiber as
ingested material that is resistant to digestion in the gastrointestinal tract of humans. The
components that make up dietary fiber are cellulose, hemicelluloses, lignins, pectins, gums,
mucilages, waxes, monopolysaccharides, and undigestible proteins. Chemically modified
cellulosics such as CMC, MC, HPMC, HPC, and EMC, being indigestible but soluble, fall
under the same umbrella [129–131].
There are two types of dietary fiber: soluble and insoluble. The insoluble materials form a
bulky mass and speed transit time through the gastrointestinal tract because of their bulk;
cellulose, hemicellulose, and lignin fall into this category, so that cellulose flour, MCC, and
MFC are included. Among the soluble dietary fibers are the pectins, gums, natural and
derived, and mucilages. All of these have the ability to hold water and thereby increase the
viscosity of the food bolus. Reference to gums as soluble dietary fiber was once confined to
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which has come to represent many things to many people (see Chapter 18). Originally,
those formed by cross-linking cellulose with diepoxides (cf. HPC manufacture), have been
those of vegetable origin, such as gum Arabic, guar gum, and locust bean gum; today,
however, there is wide acceptance of the inclusion of chemically modified celluloses in this
category. All gums increase the water content of the stool, but may do so in different ways.
The natural gums, not based on b-1,4-glucan backbones, are all fermented to a substantial
degree, thereby losing their innate water-binding ability and promoting high bacterial cell
contents in the stool. This increased bacterial cell mass carries with it roughly 80% water and
is resistant to dehydration [130]. The water-soluble cellulosics, on the other hand, are resistant
to digestion and retain their molecular integrity and water-binding ability even in the colon.
The increased moisture content in the stool associated with derivatized cellulosics is therefore
to a large extent due to the water of hydration of the polymers. There is a beneficial dietary
effect in that the rate of diffusion of glucose is lessened in the presence of such viscous fibers
as MC [132,133].
5.5.2 WATER-HOLDING CAPACITY
Cellulose and its derivatives vary widely in their water-holding capacity. Cellulose and
physically modified celluloses generally tend to imbibe very little water because of their
relative insolubility and tendency to aggregate together through strong hydrogen bonding.
Chemically modified celluloses, on the other hand, are water soluble as a result of the
inclusion of substituent groups (methyl, carboxymethyl, or hydroxypropyl) and when in a
food product tend to retain high moisture contents in the stools after ingestion.
5.5.3 METABOLISM
The metabolism of cellulose and its derivatives has been studied extensively; as dietary
fiber components, they are very unreactive and nonfermentable. Marthinsen and Fleming
[134] in early work evaluated the response of rats to feeding with xylan, pectin, cellulose,
and corn bran by investigating the excretion of gases following administration of the
purified substances as dietary components. Breath gases were monitored to determine the
extent of fermentation occurring in the large intestine. Increased fermentation in the colon
was indicated by elevated gas excretion levels. The authors found that diets containing cellulose
and corn bran generally caused gas excretion levels that were not significantly different from
those of the fiber-free controls. This indicates the relative nonfermentability of cellulose [135]
and is supported by the work of Fleming and coworkers [136–138], who studied the effect of
fiber on fecal excretion of volatile fatty acids (VFA). The concentration of VFA in excreted
feces was found to be less for cellulose-containing diets than for the control diet. Higher levels
of VFAs or SCFAs (short-chain fatty acids) in the colon are associated with health benefits, and
MCFAs (medium-chain fatty acids) that are more readily absorbed by the colon may be of
The chemical derivatives of cellulose are also known to be safe for use in foods. Like
cellulose, they are indigestible. No significant radioactivity accumulates in the organs of rats
fed with radiolabeled MC, HPMC, HPC, and CMC, and no chronic or subchronic toxicity to
test animals fed up to several percent of the diet was noted. One beneficial effect of water-
soluble cellulosics in the diet is their very efficient water retention, promoting large, bulky
stools. It is not necessary to review the safety and toxicology of these products in this chapter;
our purpose to state that the cellulose derivatives allowed for food use have been reviewed
Canada, Australia, and many others.
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assistance when small-bowel function is impaired [139,140] (see Chapter 18).
and approved by the food regulatory agencies in the United States, the countries of the EEC,
the interested reader may refer Refs. [141,142] for comprehensive reviews. It is sufficient for
5.6 CONCLUSION
Although cellulose is universally present in foodstuffs of plant origin and contributes to
human nutrition as a source of dietary fiber, relatively little is processed or submitted to
chemical modification in order to provide additives for use in the food industry. Powdered
a-cellulose and MCCs increase viscosity and provide bulk in the baking, dairy, and meat
industries, especially when fat reduction is required. The range of permitted ethers, of which
CMC in salt form predominates, adds the useful rheological properties of enhanced viscosity,
thixotropy, and pseudoplasticity, thereby extending the uses of cellulosics to include inter alia
confectionery and frozen desserts. The remarkable stability of cellulose and its derivatives
under physiological conditions, the diversity of chemical functionality of the polysaccharide,
and above all the abundance of the raw material will doubtless result in considerable
expansion of the technological development of food cellulosics.
The cultivation of transgenic plants is adding a new dimension to the production of
cellulose-derived substances aimed at improved technical performance in fibers, pulps, and
edible foodstuffs. Of passing interest to food aspects of cellulosics is the development of novel
derivatives to extend the important range of analytical uses of purified cellulose and of CMC
and diethylaminoethyl (DEAE)-cellulose, for example, as stationary phases in planar and
column chromatography [143,144], and for other purposes [145].
Technical appendices appended below offer information on the practical handling of
cellulosics and on the regulatory status of these products. The first edition of Food Polysac-
charides and Their Applications [11] gives more information on sources of supply, and includes
references to the earlier literature.
APPENDIX 1: DISPERSION–DISSOLUTION OF CELLULOSEAND CELLULOSE ETHERS
MICROFIBRILLATED CELLULOSE
ITT Rayonier Inc. has supplied MFC in three forms under the Nutracel trademark: a 4% paste
in water; a granular product 25% MFC, 75% water; and a powder 80% MFC, 50% sucrose. The
paste is readily dispersible in water or a number of water-miscible organic solvents. Medium
shear mixing is needed to obtain the maximum viscosity and water retention. The granulated
and powdered products require high shear mixing to allow the product to swell fully in aqueous
systems. Dry blending the MFC with other ingredients before adding to the wet mix is often
advantageous. The current commercial status of MFC is unclear.
MICROCRYSTALLINE CELLULOSE
MCC, as supplied by FMC Corporation under the Avicel and Micro-Quick trademarks, is
available in three basic forms: bulk dried, spray dried (both with the addition of CMC as a
dispersant), and dried with sweet whey. The bulk-dried material requires homogenization at
132 bar (2000 psi) after premixing to achieve full water uptake and viscosity. Spray-dried
material requires high-speed agitation, whereas the spray-dried whey-containing product
disperses and swells in water through the action of simple agitation.
CELLULOSE ETHERS (CMC, EMC, HPC, HPMC, MC)
Food-grade cellulose ethers have a tendency to lump if incorrectly added to water or aqueous
solutions. The key to successful solution preparations is to disperse the cellulose ether
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particles, then hydrate them in an even manner leading to lump-free, clear solutions. Four
basic methods are used:
1. Vortexing: The cellulose ether is added into the vortex of rapidly stirred water. A
balance has to be struck here between a rate of addition slow enough to allow the
particles to separate and wet-out, yet fast enough to prevent interference in this process
by viscosity build-up. Method 1 is most suitable for CMC and HPC.
2. Nonsolvent dispersion: The cellulose ether is dispersed in a water-miscible solvent.
Water is added (or vice versa) with stirring. The mixture will gradually clear and a
solution will be obtained. Method 2 is most suitable for HPMC and MC.
3. Dry blending: The cellulose ether is blended with a nonpolymeric, dry material at a
ratio approximately 1 part cellulosic to 7 parts of other ingredient. The mixture can
then be added to or mixed with water. Method 3 is suitable for all cellulosics.
4. Hot–cold aqueous dispersion: A number of cellulose ethers are not soluble in hot water.
Approximately one third of the total water is heated to approximately 908C for MC
and HPMC, 708C for EMC, or 508C for HPC. The cellulose ether is added with
stirring, then the balance of cold water is added — stirring is continued until a solution
is obtained. Method 4 is most suitable for MC, EMC, HPC, and HPMC.
The above four methods are generalizations, as certain cellulose ethers are more or less
soluble, depending on, for instance, DS for CMC and the ratio of hydroxypropoxyl to
methoxyl substitution for HPMC.
The manufacturers of celluloses and cellulose ethers produce a wide variety of literature
covering the properties of their products and copies are readily available.
APPENDIX 2: REGULATORY STATUS OF CELLULOSE AND CELLULOSEETHERS IN FOOD APPLICATIONS
A full discussion of this subject is beyond the scope of this chapter. Companies intending to
use cellulose or its approved derivatives are urged to check that, first, the product is allowed
within the foodstuff group in question and that, second, the grade of product to be used meets
in full the criteria of purity set either internationally or nationally. As a source of sound
advice and data, the authors highly recommend the excellent service provided to members by
the Leatherhead Food Research Association, Leatherhead, U.K., on regulatory issues and
other subjects throughout the spectrum of food science.
EUROPE — THE EUROPEAN ECONOMIC COMMUNITY
The EEC is striving to harmonize food regulations throughout its member states. Essentially
the EEC Scientific Committee for Food reviews the data or the various additives for food and
then assigns to them an E number and sets purity criteria. Member states then decide whether
to allow the use of these E derivatives and set limits on use quantity and application.
Confusingly, some additives only have numbers, but are under review for E status. It is a
real dichotomy within certain European states that E-numbered compounds, reviewed as to
their suitability for food use, are often viewed by the public and in particular by a number of
pressure groups as potentially harmful. The opposite opinion is often held about generally
impure and often totally untested natural ingredients, usually sold under the guise of health
foods.
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Celluloses and cellulose ethers fall under regulations stemming from EEC directive 74/329/
EEC and its amendments — covering emulsifiers, stabilizers, thickeners, and gelling agents.
Cellulose and cellulose ethers have the following E numbers:
E460 (i) Microcrystalline cellulose
E460 (ii) Powdered cellulose
E461 Methylcellulose
E463 Hydroxypropylcellulose
E464 Hydroxypropylmethylcellulose
E465 Ethylmethylcellulose
E466 Carboxymethylcellulose
NON-EEC COUNTRIES
Each of these has its own food regulations. In general, however, cellulose and its derivatives
are widely accepted. Those products having wide regulatory approval (FDA, USP, EEC,
etc.), such as MC and CMC, tend to be more frequently used.
United States
Food additives are regulated in the United States by the Food and Drug Administration and
the U.S. Department of Agriculture (USDA). Of the chemical derivatives, only MC and
sodium CMC are generally recognized as safe (GRAS). The specific approval is listed in the
U.S. Code of Federal Regulations (CFR) Title 21; MC under 21 CFR 182.1480 and CMC
under 21 CFR 182.1745. The other cellulose derivatives of importance — HPC, HPMC,
MEC, and EC — are approved under Part 172 of Title 21 of the CFR. The compounds and
their specific approvals are HPC, 172.870; HPMC, 172.874; MEC, 172.872; and EC, 172.868.
There are other specific approvals for cellulose derivatives such as those that define their use
in Adhesives and Coatings for Food Use (21 CFR 175.300) or specific food products such as
Artificially Sweetened Fruit Jellies (21 CFR 150.141).
The USDA has regulatory authority over meat products in the United States. The
approvals for cellulosics in meats are different than those for other food products. For red
meats, sodium CMC is approved as an extender or stabilizer in baked pies and MC is
approved as an extender or stabilizer in meat and vegetable patties, both according to
9 CFR 318.7. For poultry products, both CMC and MC are approved as extenders and
stabilizers according to 9 CFR 381.147. Finally, in addition to these approvals for CMC and
MC, HPMC is listed in the USDA Standards and Labeling policy book as an appropriate
ingredient when used in a manner consistent with the policy book’s regulations.
Canada
In Canada, CMC, MC, HPMC and HPC, and MEC are listed on the Food and Drugs Act
and Regulations as food additives that may be used as emulsifying, gelling, stabilizing, and
thickening agents.
Australia
In Australia, sodium CMC, HPMC, and MC have been approved for inclusion in the Food
Standards Regulations under Section A10, group 1, and referencing modifying agents.
Japan
In Japan, MC and sodium and calcium CMC have approval for food use. The materials must
meet specifications for the Japanese Pharmacopeia or the Japanese Food Codex.
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Other Areas
For all other areas not listed specifically, the formulator should determine the approvals and
regulated limits for various cellulosic derivatives for foods.
ACKNOWLEDGMENT
A.M. Stephen provided the new material for the second edition.
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