Characterization of Starch Based Edible

8/14/2019 Characterization of Starch Based Edible

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09603085/02/$23.50+0.00# Institution of Chemical Engineers

www.catchword.com=titles=09603085.htm Trans IChemE, Vol 80, Part C, September 2002

CHARACTERIZATION OF STARCH BASED EDIBLECOATINGS

C. PAGELLA, G. SPIGNO and D. M. DE FAVERIInstitute of Oenology and Food Engineering, Catholic University of Sacro Cuore, Piacenza, Italy

While market globalization and increasing use of minimally processed foods requireincreased product shelf life, environmental care constraints induce us to search fornatural materials to be used in effective packaging systems.

Both these issues can be addressed by using edible coatings based on naturally occurringpolymers. The characteristics of basic materials need to be modied in formulated coatingsto obtain satisfactory results. As edible coatings must act as a barrier, the correct measurementof transport properties is extremely important in product development and in assessingeffectiveness.

In the present work, different starch samples were tested to obtain edible lms or coatings.After optimizing concentration, composition, dispersion and gelatinization time and tempera-ture, a high amylose starch was selected for preparation of the lms. Films obtained by castingwere characterized for water and oxygen permeability developing suitable and simple testingmethods, nding a water vapour transfer rate of about 32 g m2h

1and an oxygen transfer rateof 5.6cm3O2m

2h1. They were also tested for protection from UV radiation by adding food-approved iron oxides.

Keywords: edible lms; starch-based lms; barrier properties; oxygen permeability; watervapour permeability.

INTRODUCTION

An ediblepackagingis dened as a thin layerof edible materialformed on a food as a coating or placed (pre-formed) on orbetween food components. Its purpose is to inhibit masstransfer of moisture, oxygen, carbon dioxide, aromas andlipids; it can be used to encapsulate aroma compounds, anti-oxidants,antimicrobialagents, pigments,ions that stop brown-ing reactions or nutritional substances such as vitamins1,2.

Furthermore, edible packagings contribute to the protec-

tion of the environment because they consist of natural andbiodegradable substances and so are non-pollutant products.Since edible lms are both a packaging and a food compo-nent, they have to fulll certain requirements, such as goodsensory qualities, high barrier and mechanical efciencies,enough biochemical, physicalchemical and microbial stabi-lity, to be free of toxicsubstances andsafe for health, of simpletechnology and have a low raw material and process cost3.

Sometimes, stand-alone edible lms with good mechan-ical properties can replace synthetic packaging lms. Manyliterature works have investigated physical and mechanicalproperties of edible lms412.

Starches can be used to form edible or biodegradablelms and to date high amylose starch and hydroxypropy-lated high amylose starch have been used to form self-supporting lms by casting from aqueous solution4,1315.

The purpose of the present research was to investigatedifferent starch raw materials in order to select those more

suitable for lm preparation and then to characterize the lmfor its principal transport properties (e.g. oxygen and watervapour permeability, water resistance and protection fromUV radiation).

METHODS AND MATERIALS

Materials

The following starch types were tested:

S1: maize starch standard;S2: waxy maize starch;S3: high amylose maize starch (approximately 70%

amylose);S4: maize starch modied by ottenyl succinate of starch;S5: pre-gelatinized maize starch with long spinning;S6: pre-gelatinized maize starch with short spinning.

As a comparison, lms were also formed from Bacto-Agar (Difco Laboratories) and Sodium Alginate (Carlo ErbaCodex). Samples 16 were kindly donated by RoquetteS.p.A.

Further substances were tested as adjunctive ingredients:

(a) Glycerin RPE-ALS Carlo Erba;(b) Sorbitol (Roquette Italia S.p.A);(c) Fe2O3H2O (Kingland Chemical Industrial Company);(d) Wax of operculum (Azienda Api Cremona).

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Film Forming Solution

Film forming solutions were prepared using three differ-ent starch concentrations (2, 5 and 8% weight=volume) indistilled water. All the raw materials were mixed with waterin different ratios (2, 5 and 8% w=v) in order to nd theoptimal value for starch solubilization and gelatinization. S5and S6 were not cooked, being pre-gelatinized samples and

soluble in cool water. In both of these cases the mixture wasstirred for 3 hours to give good dispersion. Differentmethods were tested to achieve dispersion and gelatinizationof starch. The rst method consisted of warming and stirringthe solution at 90C for 2 hours on a magnetic stirrer=hotplate. In the second method, the starch-solution waspreheated at 90C for 15 min under magnetic stirring andthen autoclaved twice with an intermediate stirring for besthomogenization.

Agar solution was prepared suspending 2.5 g in 100 mldistilled water, boiling to dissolve completely and thensterilizing at 121C for 15 minutes.

For alginate based lms, 3 g of sodium alginate were

added to 100ml distilled water under stirring and thenautoclaved. Then gelation was carried out externally orinternally. In the external gelation an amount of solutionwas poured onto a Petri plate and the plate was dipped intoCaCl 0.5M. In the internal gelation 67.36g of solution wasmixed with Ca3(C6H5O7)4H2O (1.14g in 10ml distilledwater), stirred and D-gluconic-1,5-lattone (1 g in 10 mldistilled water) added and stirred again.

Film Formation Method

Gelatinized solutions were allowed to cool (when agar

was used, the solution was used whilst still hot, due to rapidsolidication) and different amounts were spread by meansof a BYK-Gardner lm-applicator onto a support (MilliporeType AA 0.8mm) sitting over a leveled smooth surface.Alternatively, lms were cast by pipetting solutions ontoplastic Petri plates (8.5 cm diameter). Solutions wereallowed to dry at 23C, 60% and the lms were peeled off.

Film Thickness Measurement

Film thickness was determined using a magnetic induc-tion micrometer (DUO CHECK ST2 Namicon; measure-

ment range: 0 to 2000 mm). Reported thicknesses are themean values of 5 random measurements over the lm, takenat the centre of the sample and each half diagonal length.

Changes in Composition

Further ingredients besides starch and water were testedto improve physical and mechanical properties of the lms.In particular, sorbitol and glycerol were added as plastici-zers7,9,10while beeswax was used to decrease water vapourpermeability8,17.

Water Drop Test

A piece of the lm was placed over a support raised fromthe table and a water drop was poured onto the lm. Apass=fail judgement was given depending on whether or notthe drop passed through leaving a hole.

Water Vapour Transfer Rate

Water vapour transfer rate (WVTR) was determined usingthe DIN 52615 method16. A Payne cup (Braive), internaldiameter 3.6 cm, was lled with distilled water, sealed withthe lm and then placed into a desiccator at 23 C. This waya constant relative humidity difference (0100% RH) wascreated, since inside the cup there was saturated water

vapour while the silica gel in the desiccator absorbed allthe released moisture. The cup weight was periodicallyrecorded to measure weight loss. The method allows theWVTR in the steady state to be determined.

Water Permeability

For this test a similar procedure as for WVTR wasadopted, using a Payne cup upside-down with the waterphase in direct contact with the lm. The lm is wetthroughout the whole test and swelling of the materialmay occur. Also in this case weight loss was periodicallydetermined.

Oxygen Permeability

Oxygen permeability of lms was evaluated by monitor-ing the oxidation of a substance which is typically found infood and is readily oxidized: ascorbic acid (AA). 150 ml ofAA solution (200300 mg=150 ml water) was put into aplastic container and closed gas-tightly by a stopper with acircular hole, diameter 20 mm. The hole was sealed with thelm and the container placed in the dryer. The amount ofAA was determined periodically by iodometric titration. Theexperiment was performed in duplicate. In order to account

for the other factors which may affect the rate of ascorbicacid degradation, AA decrease values were calculated on thebasis of blank samples without starch lms.

UV Radiation Protection

To protect food from photocatalytic decomposition reac-tions, a UV absorbing substance was added to the lm.Films with 3% and 5% of Fe2O3, a transparent yellow oxidepigment (pigment yellow 42) were prepared and tested witha UV-VIS Perkin Elmer spectrophotometer. The analyseswere carried out in transmittance mode placing a piece oflm between two sample holders, with air as blank. Thetransmittance was monitored at different wavelengths in therange 100 to 400 nm.

RESULTS AND DISCUSSION

Film Forming Solution and Film Formation

The results obtained were similar for all the tested starchsamples: particularly the lower concentration did not allow acomplete lm formation even though the cooking processwas simpler. On the other hand, the higher value was toohigh, since the cooking was difcult and not complete. The

5% w=

v starch concentration tted well since the lmsobtained showed good characteristics and the cooking wasoptimal (viscosity, transparency and homogeneity werevisually assessed and results are reported in Table 1).

When the solution was heated on a magnetic stirrer=hotplate at 90C water evaporated causing a change in the

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starch concentration so that water was periodically added ora refrigerant condensation system installed. However, theglobal process required 23 h and the nal solution was notperfectly homogenous. Cooking, by means of autoclave at121C, was tested. This method allowed a good degree ofviscosity, loss of opacity, lack of lumps and homogeneity.

Furthermore, these properties were being kept even whensolution cooled so that it could be cooked and used later on.Autoclaving also gave sterility to the sample, which isimportant for food applications. This was not a goodmethod for standard starches because they neither comple-tely solubilized nor gelatinized, giving too viscous a solutionfor thin lms formation.

Agar and alginate gave bad results with both poor homo-geneity and poor transparency. Agar based lms appearedyellow even though they were rather transparent. Alginate-based lms from internal gelation showed many lumps and afriable and rough surface; those from external gelation

showed large blisters with sodium alginate only partiallygelled inside.The use of a lm applicator did not provide good lms

since the support used absorbed small amounts of solutionand tended to fold, making it difcult to peel off the driedlm. It was not possible to obtain either a homogenousthickness, or to control the thickness, particularly in thethinner lms. On the other hand, the use of Petri plates, alsosuggested in the literature, allowed homogeneous lms to becast in shape and to reliably control the thickness. The lmcould be easily removed from the plates.

A further test was carried out to select the more suitablesamples for food coating: the resistance to water leakage. A

summary of these preliminary tests is shown in Table 1.Finally, S3 was chosen and its lms casted thereof subjectedto further tests.

Changes in Composition

In accordance with previous results in the literature,additive ingredients were tested to modify lm performance,in particular transparency and plasticity. Sorbitol was usedas plasticizer and enhancer of transparency in different ratios(410 and 40% on dry starch). Glycerol (1020% on dry

starch) was also tested as plasticizer. A micro-emulsion ofwax, prepared as reported by Baker et al.17

, was added toimprove the barrier effect to water vapour. The purestfraction of bee wax, operculum, was used.

As reported in literature, sorbitol was added to improvetransparency of the nal lm. 4, 10 and 20% of sorbitol on

dry weight did not cause any meaningful difference intransparency. Sorbitol can also act as plasticizer, but inthis case it did not give any improvement in lm properties.The only difference observed, which agrees with literaturedata, was an increase in water vapour permeability. Glycerolwas added (1020% on dry weight) as a plasticizer, but this

did not have a positive effect and permeability and brittle-ness became worse.

Since lm from S3 showed a low resistance to watervapour permeability, the purest fraction of bee wax wasadded to try and reduce permeability, being both hydro-phobic and edible. 10% of wax (dry weight) was addedbefore cooking (so that it could melt). Unfortunately, duringlm drying the wax did not solidify uniformly. Thus, aliterature method applied on protein based lm was partiallyadopted: 10 g of minced wax was melted with 2.4 g of oleinat 100C under stirring. The mixture was added with 1.4 gNH330% and 21 g of water at 65

C, stirred and again added

to 40 g of water preheated at 90100

C. Different amountsof the resulting micro emulsion (5-2.5-1 g) were mixed with50 ml of starch solution. The lm obtained was not stableand tended to brake off during drying. For the followingtests S3 solution without other ingredients was employed.

Water Vapour Transfer Rate

The test was conducted over 5 lms of different thickness,obtained by pipetting on to Petri plates, different amounts of5% starch solution (5, 8, 11, 14, 17ml), the thickness ofeach lm was measured. The measurement was difcult insome cases due to the lms being too stiff. Therefore, a datasmoothing procedure was used as follows.

Assuming the lm volume equals the product of the Petriplate area (56.72 cm2) by the measured thickness, andsupposing that all the water was removed during lmformation so that the mass could be considered as the dryweight starch, an average lm density (m Vd) was calcu-lated by linear regression, plotting the volume versus dryweight. Result was 1.83 g cm3. From the average densitycalculated mean values of thickness were derived fromsample dry weight and plotted versus measured values(see Figure 1).

According to the DIN 52615 method:

mdLdF

1

wherem is the moisture resistance factor; dLis the moisturepermeability of air; dFthe moisture permeability of the

Table 1.Results of preliminary tests for the selection of starch sample.

Sample Cooking degree Viscosity Transparency Homogeneity Water drop test

S3 = S1 = S4 = S2 S5 not cooked S6 not cooked

AGAR 2.5% boiled before yellow ALGINATE 3% ext gelat lumps ALGINATE 3% int gelat lumps

excellent; very good; good; poor; very poor.


STARCH BASED EDIBLE COATINGS 195


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tested material. m indicates how many times the moistureresistance of the material is greater than the resistance of amotionless layer of air of the same thickness at the sametemperature. The water vapour diffusion-equivalent air layerthickness, sd, of a specimen indicates the thickness of amotionless air layer which has the same moisture resistance

as the specimen with the thickness s, given by:sdms 2

where s is the mean specimen thickness. The concept ofsdvalue can give an immediate indication of the barrierproperties of a material and the advantage of being anadditive property and a linear function of thickness.

When determining the moisture permeability, the WVTRmay be determined using the following equation:

WVTR dm

Adt 3

wheremis the sample weight, and A is the test surface areaof the specimen.

In the experimental specimen, surface area was1.018 103m2 and WVTR decreased as a function oflm thickness was expressed as water weight loss versustime (see Figure 2). The WVTR for the 68 mm thick lm(about 31.7 gm2h1) is in agreement with other valuesreported in literature18,19 for corn starch-based lms ofsimilar thickness.

In the case of homogenous materials:

m1

sdLA

p1 p2I

sL

4

where I is the moisture ow rate, p1, p2are partial watervapour pressures in the specimen; sLis the mean thicknessof the air layer in the test cup beneath the specimen.

Combining equations (2) and (4) results in (with WVTRin gm2d1):

sd dL24;000p1 p2

WVTR sL

5

The moisture permeability of air can be derived by:

dL 0:083RDT

p0p

T273

1:816

where RD is the gas constant of water vapour(462Nmkg

1K1); T the temperature in the controlled-

atmosphere chamber, in K; p is the mean pressure in thecontrolled-atmosphere chamber, in hPa; p0 the standardatmospheric pressure.

Plotting the values of sdversus s (see Figure 3), a m ofabout 230 is obtained.

Water Permeability

The results were expressed as gm2 and reported inFigure 4 versus time. The only lms which did not breakoff under the weight of water were those formed from 8 and11 ml of starch solution.

Water mass transfer rates are generally high and, as theWVTRs, reveal the hydrophilic nature of the starch poly-mers which constitutes one of the major drawbacks for theiruse in most commercial applications19, even though forsome, ambient-temperature fruit and vegetable packs, aWVTR of 500700gm2day1may be benecial.

Oxygen PermeabilityFor the experiments, a starting concentration of

3333mg1 of AA in distilled water and the thickest lm(83 mm thick) were used.

Figure 1. Experimental and calculated thickness depending on dry weight(the error bars show the scatter as a 95% condence interval).

Figure 2. WVTR as weight loss along the experimental time for lmthickness of () 24, (&) 39, (~) 54 and () 68 mm.

Figure 3. Calculation of the moisture resistance factor of starch-lms.

Figure 4.Water transfer rate of starch-based lms: (&) 39 and (~) 5 4 mm.


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The depletion in AA concentration increased with time(see Figure 5). The resultant rate of AA degradation waslower than those reported in literature for degradation of AAin orange juice and other food systems2022.

Assuming that 1 mole of oxygen can oxidize two molesof AA, an oxygen transfer rate (OTR) of 5.6 cm

3h1

m2

was obtained (see Figure 6). Some works19,23 on starch-based lms report oxygen permeability values which have aquite wide range of variation. In these works, gas perme-ability is estimated using diffusion cells, while in the presentresearch a simpler method was selected that can be usefulfor application in the food industry. Furthermore, AA isrecognized as an important indicator of food quality andsafety. It must be said that it was not possible to account forthe water dissolved O2 and numerous pathways for thedestruction of AA may be available involving temperature,pH of the media, anaerobic mechanism and substances otherthan O2, so that OTR could have been overestimated.

As for water vapour, a resistance factor mO2 can becalculated also for O2 from:

mO2 DO2 C

JO2 s 7

whereDO2

is the oxygen diffusivity in air (m2s1),Cis theconcentration gradient (kgm

3); JO

2the oxygen transfer

rate.DO

2can be calculated as:

DO2 0:0166 T3=2

PV

1=3

A V

1=3

B

2

1

MA

1

MB

8

where VA and VB are molecular volumes; MA and MB themolecular weights;Tis temperature (K) and P the pressure(atm)24. It resulted in a sdof 2.3 m and amO

2of 27,711.

Protection From UV Radiation

Some food products (especially fresh vegetable and fruits)are sensitive to UV radiation. In order to give some protec-tion from UV, an UV absorber was added to the starchsolution after cooking. FeO2 was chosen because of itsyellow transparent colour and because there are food-

approved iron oxides.While absorption in the UV range has to be maximized,

absorption in the visible range is not desirable since it wouldcolour the lm altering the natural colour of coated foods.The results showed an absorption of lm at about 200nm.When increasing Fe2O3H2O concentration the adsorption at200 nm is still present but also some adsorption at longerwave lengths is evident. The optimal concentration is, there-fore, about 3% w=w. The data are reported in Figure 7.

CONCLUSIONS

In the present work preparation and casting methods wereoptimized and different materials were tested for physicalmechanical properties and applicability. The best perform-ing sample, a high amylose maize starch, was fully tested forbarrier properties. The barrier properties of the derived lmswere measured to assess their effectiveness as edible coat-ings for food. Testing methods were selected to be simple,practical and feasible for industrial application. Experimen-tal results for mass transfer properties were expressed usingthe equivalent air-layer thickness concept which is usefulto predict the barrier properties of a material.

Data on UV-barrier properties were also obtained.

NOMENCLATURE

A the test surface area of the specimen, m2

DO2 oxygen diffusivity in air, m2h1

I the moisture ow rate, kgh1

m sample weight, kgMA, MB molecular weightsp the mean pressure in the controlled atmosphere chamber, hPap1, p2 partial water vapour pressures in the specimen, Pap0 the standard atmospheric pressure, hPaRD the gas constant of the water vapour Nmkg

1K1

s the mean specimen thickness, msd water vapour diffusion-equivalent air layer thickness, msL the mean thickness of the air layer in the test vessel, m

T absolute temperature, KVA, VB molecular volumesJO

2oxygen transfer rate, kgm2h1

dF moisture permeability of the tested material, kgm1h1Pa1

dL moisture permeability of air, kgm1h1Pa1

m moisture resistance factormO

2

oxygen resistance factor

Figure 5. Acid ascorbic degradation in the presence of 86 mm thick lms(the test was repeated twice).

Figure 6.Oxygen transfer rate through a starch-lm, 83 mm thick.

Figure 7.UV light absorption of lm added with 3 and 5% of Fe2O3H2O.


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ADDRESS

Correspondence concerning this paper should be addressed toDr G. Spigno, Institute of Oenology and Food Engineering, CatholicUniversity of Sacro Cuore, Via Emilia Parmeme, 84, 29 100 Piacenza, Italy.E-mail: [email protected]

The manuscript was received 26 November 2001 and accepted for

publication after revision 29 April 2002.


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