Rheological Characteristics of Goat and Sheep Milk

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Small Ruminant Research 68 (2007) 73–87

Rheological characteristics of goat and sheep milk�

Y.W. Park ∗Georgia Small Ruminant Research and Extension Center, College of Agriculture, Home Economics and Allied Programs,

Fort Valley State University, Fort Valley, GA 31030-4313, USA

Available online 25 October 2006

bstract

Rheology represents the properties of both solid and liquid foods, where texture is the rheology related to solid foods, andiscosity is the rheology of fluid foods. Three categories of tests measure textural characteristics of solids foods, empirical (ballompressor, penetrometer, curd tension meter), imitative (texturometer, texture profile analysis [TPA]), and fundamental tests (smallmplitude oscillatory shear analysis [SAOSA], torsion analysis [TA]). Regardless of animal species, milk casein gels are mainlyesponsible for the rheological properties of cheese and other dairy products. In normal fluid milk, the viscosity is affected by thetate and concentrations of fat, protein, temperature, pH, and age of the milk. Average milk viscosity has been determined for goatst 2.12 cPas, sheep 2.48 cPas, camels 2.8 cPas, buffaloes 2.2 cPas, and cows 1.7 cPas. Heating decreases the dynamic viscosity,ut it increases at the point of coagulation. Sheep and goat milk have the same proteins as cow milk, but their proportions andenetic polymorphs differ widely, which explains different rennetabilities and considerable rheological variations in cheesemaking.asein micelle structure is similar in goat, sheep, and cow milk, but differs in composition, size and hydration. Sheep milk hasaseins richer in calcium than cow caseins, it is also very sensitive to rennet, because of higher �/�s-casein ratio, and coagulationroceeds faster than in cow milk. Rheological studies with Monterey Jack cheese from goat milk found “knitting” with progressingging time, less hardness, less shear stress values, and more rubberiness. High correlations were noted between SAOSA scoresnd proteolysis in cheeses. Terrincho sheep cheese showed increased hardness, fracturability, gumminess, chewiness, yelloweness,ecreased adhesiveness, resilience, and cohesiveness during ripening. Frozen storage of soft goat cheeses had minimal effects onextural qualities, which has valuable market implications. Feta cheeses showed increased compactness and porosity, when goat milkad been added to sheep milk. Yogurt studies, including Labneh from the Middle East, found highest viscosity for sheep followed
y goats, cows, and camels, and viscosity increased with solids contents. Three different transient viscosity stages were describedathematically, and camel milk varied least in viscosity during yogurt gelation. Viscosity decreased with increasing angular velocity
f the inner cylinder, suggesting that yogurt behaved as a shear-thinning non-Newtonian fluid. Rheological properties are importantonitors of quality control in dairy processing and in scientific research.2006 Elsevier B.V. All rights reserved.

ture pro
eywords: Goat milk; Sheep milk; Rheology; Texture; Viscosity; Tex
. Introduction

Rheology is defined as the study of material deforma-ion and flow (Scott-Blair, 1969), and includes what is

� This paper is part of the special issue entitled “Goat and Sheepilk” Guest edited by George Haenlein, Young Park, Ketsia Raynal-

jutovac and Antonio Pirisi.∗ Tel.: +1 478 827 3089; fax: +1 478 825 6376.

E-mail address: [email protected].

921-4488/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.smallrumres.2006.09.015

file analysis

termed “small-strain” testing (deforming a small percentof that required to break the sample) and “large-strain”testing (deforming to the point of permanent structuralchange) (Hamann, 1988).

In food research, rheology is often used interchange-
ably with texture, which refers to the flow, deforma-tion, and disintegration of a sample under force (Tunick,2000). Since rheology represents the properties or char-acteristics of both solid and liquid foods, in strict terms,
mailto:[email protected]

dx.doi.org/10.1016/j.smallrumres.2006.09.015

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74 Y.W. Park / Small Rumin
texture is the rheology related to solid foods, while vis-cosity is the rheology related to fluid foods (Tunick,2000). Rheology of fluid milk can be measured withviscometers, while that of solid dairy foods is usuallyevaluated with texturometer, Instron Testing machineand torsion gelometry, etc.

Caprine milk differs from cow milk in severalphysico-chemical characteristics, which explain majordifferences in the technological behavior of the twomilks (Remeuf, 1992). The poorer cheesemaking abilityof goat milk is largely attributable to the lower caseincontent, and to specific properties of casein micellesin caprine milk such as their composition, size andhydration (Remeuf, 1992). Goat milk also has differentproportions of the four major caseins (�s1, �s2, �, �)compared to cow counterparts, and there are great vari-ations, especially between �s1-casein and �s2-caseincontents between individuals and breeds of goats andsheep, because of the occurrence of genetic polymor-phisms for all milk proteins, which influence greatly theircheesemaking properties (Remeuf, 1992). Sheep milkhas higher specific gravity, viscosity, refractive index,titratable acidity, but lower freezing point than averagecow milk (Haenlein and Wendorff, 2006).

Regardless of milk of any dairy species (i.e., cow,goats and sheep), casein gels are responsible for mostof the various rheological/textural properties of cheeseand other dairy products that gel, stretch and fracture(Tunick, 2000). Rheological properties are examined asa quality control method in dairy processing plants andas a scientific technique for food scientists and rheolo-gists to perform research on the structure/texture of foodproducts.

Although rheological characteristics of cow milk andits dairy products have been extensively studied, the doc-umentations on rheological properties of goat and sheepmilk products, especially their fluid milks have beenscarce. This review focuses on goat and sheep milk prod-ucts, but due to the paucity of reports on fluid milk ofgoats and sheep, this paper covers more research on fer-mented goat and sheep milk products.

2. Rheology of goat milk and sheep milk

Rheology of fluid milk is largely influenced by its vis-cosity. The dynamic viscosity η is a parameter related tothe inner friction of a liquid (Spreer, 1998). The viscos-ity of milk is twice as high as that of water due to the
friction of fat in milk (emulsified in milk) (Spreer, 1998).In normal fluid milk, viscosity is affected by state andconcentrations of fat and protein, temperature, pH andage of the milk (Jenness and Patton, 1976). Viscosity is
earch 68 (2007) 73–87

a physical property of a fluid to resist its flow or pour. Itdepends on internal friction within a liquid and the rela-tion between kinetic motion and free surface (Jennessand Patton, 1976). Under most conditions milk behavesas a Newtonian liquid, meaning that the shear stress isproportional to the shear rate (dv/dx) (Walstra et al.,1999).

2.1. Dimensions of viscosity

Viscosity of a fluid can be measured in absolute orrelative terms. The absolute unit of measurement forviscosity is the poise, named after Poiseuille, wherea centipoise represents one one-hundredth of a poise(Jenness and Patton, 1976). The viscosity of water at20 ◦C is equal to 1.005 centipoises, and the relative vis-cosity is ordinarily measured in terms of the rate of flowof a liquid.

The dynamic viscosity η is expressed as N s/m or Pa.s(Spreer, 1998). The centipoise cP (10−3 Pa.s) is an oldterm for viscosity. The viscosity value for cow milk at5 ◦C is a function of the fat content and ranges from2.96 × 10−3 Pa.s (skim milk) to 3.25 × 10−3 Pa.s (wholemilk), whereas at 20 ◦C the ranges for the skim and wholemilks are 1.79 × 10−3 Pa.s and 1.3 × 10−3 Pa.s (Spreer,1998).

The viscosity of sheep milk is reportedly much higherthan that of goat or cow milk, cow milk being the lowest.The viscosity of Egyptian camel milk was estimated at2.2 cPas (Hassan et al., 1987) and 2.35 cPas (El-Agamy,1983), which is higher than for cows (1.7 cPas) and goats(2.12 cPas), but less than for sheep (2.48 cPas), and sim-ilar to that of buffalo milk (2.2 cPas) (Mehaia, 1974).

2.2. Effect of temperature and incubation time onmilk viscosity

The dynamic viscosity η value decreases when tem-perature increases. The contribution of casein micellesappears to closely depend on temperature. Milk increasesin viscosity upon heating to the point of coagulation ofthe proteins, which is the basis for producing high viscos-ity in superheated condensed milk (Jenness and Patton,1976). The voluminosity of the micelles is markedlyincreased at low temperature, and part of the �-caseinbecomes dissociated from the micelles, resulting in asteep increase in viscosity (Walstra et al., 1999).

Temperature apparently affects viscosity in various
ways. When the serum proteins become insoluble inskim milk due to heat treatment, the viscosity of skimmilk increases by about 10%. This increase in viscos-ity may be explained by increase in voluminosity of

Y.W. Park / Small Ruminant Res

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camel milks to the viscosity-incubation time profiles(Fig. 2). No viscosity changes were noted with a drop inthe pH 6.4–5.4 range, where the casein micelles appearedto be uniform in size and distribution (Hassan et al.,

ig. 1. Effect of milk source on viscosity-incubation time curvesJumah et al., 2001).

he serum proteins (Walstra et al., 1999). Heat stabil-ty of goat milk is considerably lower than of bovine

ilk (Remeuf, 1992), which may be attributed to theigh ionic calcium content and low micellular solvationn caprine milk which would contribute to its heat insta-ility.

The effect of different species milks on viscosity-ncubation time was studied by Jumah et al. (2001).hey found that sheep milk reached the highest viscos-

ty value, followed by caprine and bovine milk (Fig. 1).he differences in viscosity between species appeared

o be due to the differences in total solids contents of theilks, which caused a significant effect on the firmness

f yogurt curd (Tamine and Robinson, 1985; Jumah etl., 2001). The higher viscosity may also be attributableo an increased water-binding capacity in the milk pro-eins (Labropoulos et al., 1984). On the other hand, camelilk did not show any elevation in viscosity during the

elation process which might be accounted for by theower protein content (Jumah et al., 2001).

.3. Effect of pH on milk viscosity

The viscosity of milk increases as the pH of milkncreases, which is presumably attributable to the addi-ional swelling of casein micelles (Walstra et al., 1999).

slight decrease in milk pH usually causes a smallecrease in viscosity, whereas a more drastic decrease
n pH leads to an increase in milk η value because ofggregation of casein (Walstra et al., 1999). The viscos-ty of milk is little affected by homogenization, whilehat of cream would be significantly increased.
earch 68 (2007) 73–87 75

Cheese curd clotting times for sheep milk are shorterat lower pHs. Renneting time decreased from 17 minto 7 min when the pH of sheep milk was lowered from6.65 to 6.16 (Bencini, 2002). Curd consistency in sheepmilk is largely unaffected by temperature, particularly atlower pH (Bencini, 2002), where the curd consistencywould be an important factor for viscoelastic propertiesof sheep milk cheese.

The role of pH in cheese texture is particularly impor-tant because changes in pH are related directly to chem-ical changes in the protein network of the cheese curd(Pinho et al., 2004). However, the influence of wateractivity and salt content on rheological properties ofcheese is indirect. A high level of salt increases theosmotic pressure, which diverts significant amounts ofwater from the structural bonds of the casein network(Prentice, 1991). On the other hand, decreased wateractivity would result in a reduced proteolytic activityin cheese.

The most important factors in the acid curd structureformation are casein content, pH and calcium contentof the milk. At low pH, calcium is progressively dis-sociated from the casein micelle. In addition to this, theneutralization of the negative charges of the casein favorsextensive aggregation and fusion between the micelles,which tend to form a gel (Parry, 1974). At pH 4.6, acasein network is formed and the other components areentrapped in it.

Jumah et al. (2001) observed similar curve trendsof viscosity–pH profiles of ovine, caprine, bovine and

Fig. 2. Effect of milk source on viscosity–pH curves (Jumah et al.,2001).

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1995). At pH 5.4–5.3, the maximum viscosity increasewas observed, and the casein became coarser at this stageindicating the initiation of aggregation, which formeda three-dimensional network composed of clusters andchains (Hassan et al., 1995) In the pH range of 5.1–4.6,a decrease in viscosity occurred.

2.4. Effect of chemical characteristics on rheologyof goat and sheep milk

Viscosity is elevated with protein coagulation andincreasing fat content (Spreer, 1998). Casein micellesare fairly voluminous, where dry casein may occupyabout 0.7 ml/g and the rest of the volume is water, whilemicelle voluminosity is about 4 ml/g of casein (Walstraet al., 1999). Some conditions and treatments that affectthe stability of casein can significantly influence the vis-cosity of milk, which include acidity, salt balance, heattreatment, and the action of various enzymes and bacteria(Jenness and Patton, 1976).

The effect of milk fat on the viscosity of whole milkmay not be as high as that of casein. However, the amountof fat, the fat globule size and the extent of clustering ofthe globules significantly affect milk viscosity (Jennessand Patton, 1976). Homogenization causes increases inthe amount of fat surface, the amount of protein bound bythe fat particles, and the degree of clustering and clump-ing of the fat contribute to the increasing viscosity ofmilk (Jenness and Patton, 1976).

Goat milk has poorer cheesemaking ability comparedto cow milk counterparts due to lower casein contentof caprine milk, and differences in casein micelle com-position, size and hydration between the two species(Remeuf, 1992). The lower casein content and lowercasein number of goat milk compared to cow milk(casein/total nitrogen: 75% versus 78%; Jenness, 1980;Grappin et al., 1981) are responsible for reduced cheeseyield (Ricordeau and Mocquot, 1967) and affect cheesetexture and rheology. Goat milk produces a more frag-ile rennet curd than bovine milk (Abou-Dawood andEl-Sawaf, 1977). Even when casein concentrations areequal, caprine rennet curd is softer than bovine counter-parts (Remeuf et al., 1989). In cheesemaking, a poorcoagulum strength can lead to more cheese particlelosses in whey, lower cheese yield and likely less textu-ral integrity. The uniqueness of renneting kinetic of goatmilk has also been characterized as a shorter coagulationtime and greater hardening rate (Puri and Parkash, 1962;
Remeuf et al., 1989).
The main physico-chemical factors affecting varia-tion in renneting properties of individual caprine milkinclude: casein content, total and colloidal calcium con-

earch 68 (2007) 73–87

centrations, average size of casein micelles, and �s/�-casein ratio (Remeuf et al., 1989). The difference ofstrength between bovine and caprine coagulum is essen-tially accounted for by the differences between caseinmicellular size and hydration of the two milks (Remeufet al., 1989).

Goat milk contains the same four casein fractions asbovine milk such as �s1, �s2, �, and � caseins. �s1-Casein has very large individual quantitative variationsdue to the occurrence of genetic polymorphisms (Addeoet al., 1987; Grosclaude et al., 1987). This polymorphismmakes great variations in milk �s1-casein levels fromapproximately 25% in certain milks to total lack in oth-ers. Reports have shown that �s1-casein polymorphismhas a significant influence on cheesemaking propertiesof goat milk (Remeuf, 1992), which would in turn havea significant impact on cheese textural quality.

In a study of cheese production potential of milk ofDahlem Cashmere (DC) goat in relation to rheologi-cal aspects, Dimassi et al. (2005) reported that cheeseproduction efficiency was directly proportional to aggre-gation rate and coagulation time. The DC breed hadsignificantly higher curd firmness and faster aggrega-tion rate, because the DC goats had much higher proteinand casein content than the German Fawn breed. Duringcoagulation, destabilized casein micelles and calcium-phosphate bonds form a network which entraps fat andother solids. The casein network is formed faster withhigher proteins mainly casein content, which results inhigher aggregation rate and the development of firmercurd.

Grosclaude et al. (1987) reported that ‘high type’ vari-ants A, B, and C were associated with higher amountsof �s1-casein (about 3.6 g/l), ‘intermediate type’ vari-ant E was associated with intermediate amounts (about1.6 g/l), and ‘low type’ variants D and F were associ-ated with low amounts (about 0.6 g/l) of �s1-casein ingoat milk. Clark and Sherbon (2000b) reported that �s1-casein genetic variants were not highly correlated withcoagulation properties, and they found that Nubian breedmilk was more likely to contain a high type genetic vari-ant than Alpine breed milk. Clark and Sherbon (2000a)also showed that Nubian and Nubian × Alpine crossescontained a higher amount of �s1-casein and other milkcomponents, and exhibited higher coagulation rate andcurd firmness than milk from Toggenburgs and Saa-nen × Alpine crosses. These differences in �s1-caseincontents between different breeds and crosses were
accountable for the variations in texture and rheologyof their corresponding goat milk cheese products.
Sheep milk is higher in fat, protein and total solidsthan cow or goat milk (Haenlein and Wendorff, 2006).

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he cheese yield is significantly greater per unit of sheepilk than that of cow and goat milk, because cheese

urd contains primarily the fat and casein from milkAnifantakis, 1986). Sheep milk has all major caseinractions of cow milk (Alichanidis and Polychroniadou,996). Casein micelle structure is similar in cow,oat and sheep milk, while sheep milk caseins areicher in calcium than cow casein (Alichanidis andolychroniadou, 1996). Since casein is the critical com-onent in milk that forms the primary structure of theurd of cheese, the clotting or coagulation of milk wouldffect the resultant cheese composition, texture and rhe-logy.

Sheep milk is very sensitive to rennet, because it hashigher �/�s-casein ratio, and also coagulation in sheepilk proceeds faster than in cow milk (Muir et al., 1993).his suggests that mode and amount of rennet addition

o sheep milk would definitely influence the final texturend rheological characteristics of the cheese. Sheep milklso requires less rennet than cow milk to obtain the sameoagulation time (Kalantzopoulos, 1993). Although theate of curd formation in sheep milk is faster than inow milk, the rate of syneresis in the former is slowerhan in the later (Muir et al., 1993), which is due to theigher casein and colloidal calcium content in sheep milkStorry and Ford, 1982).

. Rheological methods as a means of evaluatingunctionality of dairy foods

Texture plays an important role in the quality ofheese, while the textural measurements of cheese areomplicated and confusing (Chen et al., 1979). The tex-urometer can measure several textural properties suchs hardness, chewiness, elasticity, which was inventedy Friedman et al. (1963). Bourne (1968) discoveredhat the measurements from the texturometer can bebtained with the Instron Testing Machine. The correla-ion between textural measures of texturometer and sen-ory scores of a taste panel on several Cheddar cheesesevealed that only hardness was significantly correlated,ut adhesiveness was not (Brennan et al., 1970). In atudy using both the texturometer and the Instron by theame research group a few years later, the highest corre-ation coefficient was found between sensory evaluationnd the Instron data (Brennan et al., 1975).

There are three categories for measurement of textu-al or rheological characteristics, which are empirical,
mitative, and fundamental tests (Scott-Blair, 1969).
Empirical measurements involve test conditions thatannot usually be compared with those of more rigorousxperiments (Tunick and Van Hekken, 2002). Imitative

earch 68 (2007) 73–87 77

tests can be performed by texture profile analysis (TPA)and torsion gelometry, and fundamental tests can be con-ducted using small amplitude oscillatory shear analysis(SAOSA).

3.1. Empirical tests

Empirical tests can be as simple as manipulating curdor cheese with the fingers, and supply basic, single-point information (Tunick, 2000). Ball compressors,penetrometers, and curd tension meters have been usedas empirically measuring tools for rheological attributesof cheese in cheese plants for many years (Szczesniak,1963). These equipments are not regarded as preci-sion instruments because of the arbitrary test conditions(Voisey, 1976), and the test results are difficult to com-pare with those from more rigorous experiments (Raoand Skinner, 1986).

3.2. Imitative tests

Volodkevich (1938) demonstrated the first imitativetests, which consisted of two wedges exerting biting andsqueezing action on the sample held between the wedges.Later, these imitative tests were developed into motor-ized dentures with strain gauges by Proctor et al. (1955).For the next development, Friedman et al. (1963) intro-duced texturometers in the early 1960s. These machinescan compress bite-sized samples to produce force–timecurves, whereby the analysis succeeded several yearslater with universal testing machines, which are nowwidely used for texture profile analysis (TPA).

3.2.1. Texture profile analysis (TPA)Texture profile analysis utilizes the universal testing

machine which mimics chewing through the use of largedestructive shears, and the hardness, springiness, andcohesive nature of the food are calculated (Van Hekkenet al., 2004). However, these tests can also be regarded asempirical because there are no corrections for changesin the shape of the specimen. TPA is useful for makingcomparisons, while it does not measure true rheologicalproperties (Tunick and Van Hekken, 2002).

TPA is carried out by dropping a crosshead downa vertical column, which causes a flat plate to deforma cylindrical sample specimen placed on a lower plate(Tunick, 2000). The crosshead of the universal testing
machine returns at the same rate and repeats the proce-dure, closely mimicking the action of biting twice on apiece of cheese. A TPA curve with force versus time forMozzarella cheese is shown in Fig. 3 (Tunick, 2000).

78 Y.W. Park / Small Ruminant Research 68 (2007) 73–87

Fig. 3. Texture profile analysis curve for cow milk Mozzarella cheese(Tunick, 2000).

Likewise, Table 1 lists the textural parameters whichcan be derived from a TPA curve. Szczesniak (1995)clarified that chewiness and gumminess cannot be cal-culated for the same product, because chewiness is for asolid food and gumminess is for a semisolid food, wherea semisolid food undergoes permanent deformation andhas no springiness.

3.3. Fundamental tests

Fundamental tests on cheese include compression,dynamic and transient tests. Small amplitude oscillatoryshear analysis (SAOSA) and torsion analysis (TA) are
the two representative fundamental tests. Cheese spec-imens used in these tests are of a specific shape, anddeformed in a specific manner, which allows a rheol-ogist to analyze the results systematically (Tunick and
Table 1Definitions and calculations of texture terms (Szczesniak, 1963; Bourne, 196

Term Definition Obt

Fracturability Force with which food fractures Forccurv

Hardness Force needed to attain a given deformation Maxcycl

Adhesiveness Work needed to overcome attractive forcebetween food and other surface

Forcfollo

Cohesiveness Strength of food’s internal bonds Ratithat

Springiness Rate at which deformed food returns tooriginal condition after removal of force

Heigfirst

Gumminess Force needed to disintegrate a semisolid foodto a state ready for swallowing

Prod

Chewiness Work needed to masticate a solid food to astate ready for swallowing

Prodspri

Fig. 4. Force–strain curve for cow milk Mozzarella cheese in uniaxialcompression, showing points at which structure begins to break downand then fractures (Tunick, 2000).

Nolan, 1992). Tunick (2000) postulated that in the sim-plest fundamental test, uniaxial compression, a stress(force per unit area) is applied downward to a sample,and the resulting deformation is measured as “Couchy”or engineering strain (ratio of height change to originalheight). An example diagram is shown in Fig. 4, whichis a force–strain curve for Mozzarella obtained by a uni-versal testing machine. Tunick (2000) further delineatedthat an inflection point appears if the structure begins tobreak down, and the cheese sample is to be fractured at60% compression as the breakage of the internal bondsoccur.

3.3.1. Small amplitude oscillatory shear analysis(SAOSA)

It is a fundamental test for rheological properties,which uses specific specimen geometries and instru-

8; Tunick, 2000)

ained from Calculation (andunits) using Fig. 3

e at first significant break (if any) ine

Height of F (N)

imum force during first compressione

Height of H (N)

e area of negative peak (if any)wing first peak

Area of A (J)

o of positive force area of second peak toof first peak

Ratio of A2 area toA1 area (none)

ht specimen recovers between end ofcompression cycle and start of second

Length of S (mm)

uct of hardness and cohesiveness (N)

uct of hardness, cohesiveness, andnginess (J)

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ents, allowing systematic analysis of the resultsTunick and Van Hekken, 2002). SAOSA stretchesnternal bonds using small-strain oscillatory motionnd measures the viscoelastic characteristics (elasticnd viscous moduli, complex viscosity) of the cheeseurd (Van Hekken et al., 2004). In SAOSA tests, stressnd strain are linearly dependent on one another andhe sample does not fracture or change shape (Tunicknd Van Hekken, 2002). SAOSA tests provide data foriscoelastic properties including storage modulus (G′),oss modulus (G′′), and complex viscosity (η*) (Tunick,000). These tests are called dynamic tests because theyeasure dynamically, varying either stress or strain

armonically with time.

.3.2. Torsion analysis (TA)Torsion analysis or torsion gelometry is another fun-

amental test of rheology. In TA, specimens are twistedn a viscometer with the shear stress and shear straineing measured at the fracture point (Tunick and Vanekken, 2002). For cheese analysis, cheese specimens

re milled into a capstan shape so that the fractureakes place at the narrow center of the specimen. Underhese conditions, Hamann and Foegeding (1994) indi-ated that fracture can occur in compression, shear, orension mode, which are imposed at equal magnitudesn different directions. TA has recently received morettention in research because improvements in this tech-ique have reduced the difficulties previously involvedn sample preparation and analysis (Tunick and Vanekken, 2002). Since TA measures fundamental rhe-logical properties at the fracture point, it would bealuable in analyzing cheese samples as they are bro-en or fractured when scientists and consumers cut orite their cheeses.

.4. Cheese rheology studies using TPA, SAOSA, TAnd sensory scores

In a comparison study of TA, TPA and sensory tex-ure of various gels, Montejano et al. (1986) found thathe highest correlations among instrumental parametersere between shear stress and TPA hardness, and shear

train and TPA cohesiveness.In another study with fresh and aged Brick, Ched-

ar, Colby, Gouda, Havarti, Mozzarella, and Romanoow cheeses, Tunick and Van Hekken (2002) showedhat there were strong correlations (>0.8) between tor-
ion shear stress and TPA hardness, and between torsionhear strain and TPA cohesiveness. They also found thatAOSA did not correlate with TA nor TPA, while tor-ion data could be used to draw a map depicting trends
earch 68 (2007) 73–87 79

during aging toward brittle, mushy, rubbery, and toughtexture.

4. Recent research on rheological characteristicsof goat and sheep milk products

4.1. Rheological characteristics of cheeses

Tunick (2000) pointed out that the texture of a cheeseis as important as its flavor, and most of the rheologi-cal research on solid dairy foods has dealt with cheese(Lawrence et al., 1987). The texture is closely relatedto the specific proteins in the cheese. The quantity anddistribution of the caseins, as well as the manufacturingsteps, determine the structure of the cheese matrix (VanHekken et al., 2004). Attributes such as fracture willdepend on factors including time and rate of deforma-tion, type of deformation used (tension or shear), cheesecomposition, temperature, and repeatability of specimengeometry, whereby the interpretation of all rheologicalresults may need a degree of caution (Olson et al., 1996).Natural cheese is not entirely isotropic or homogeneous,and its rheological behavior is nonlinear outside of cer-tain limits (Tunick, 2000).

With some minor genetic variations, the chemicalmakeup of caseins in caprine and bovine milks is similarin amino acid sequence and structure, and responds toproteolytic enzymes in similar manners (Trujillo et al.,1997; Awad et al., 1998), whereas the ratio of caseins inthe milks are not the same.

4.1.1. Rheology of goat milk cheesesIn a study with young Monterey Jack (MJ) goat milk

cheeses, Park et al. (2000) characterized various rheo-logical properties of the MJ cheeses stored at 4 ◦C for 1and 4 weeks, and observed that the curds of the goat milkcheeses appeared to be knitting with time, as reflected inthe springiness, G′ as well as shear strain results (Fig. 5).The increased meltability and viscosity values of thecheeses also occurred in the elevated G′′, and η* datawith storage time (Fig. 5). As the cheeses aged, theybecame softer as seen by hardness and shear stress val-ues, and more rubbery as seen when plotting shear stressand strain on a texture map. The moisture contents ofcheeses A and B were 43.4% and 45.8%, respectively.The higher moisture cheese B was a softer, springier, andmore viscous cheese than cheese A, probably due to com-positional differences stemming from problems involved
in the pressing step of manufacture. It was concludedthat the young goat milk Monterey Jack cheeses becamemore elastic, cohesive, meltable, viscous and softer after4 weeks of aging presumably due to proteolysis.


Fig. 5. Comparison of meltability, springness and cohesivenessbetween 1 week and 4 weeks aged young Monterey Jack (MJ) goat
milk cheeses (Park et al., 2000). (a) Meltability of young MJ cheesesA and B; (b) springness of young MJ cheeses A and B; (c) cohesivenessof young MJ cheeses A and B.
Van Hekken et al. (2004) conducted an extensivestudy on rheological and proteolytic properties of Mon-terey Jack goat milk cheese stored for 6 month frozenand 4 weeks refrigeration conditions, and reported sev-eral rheological characteristics including small-straindynamic analysis results (Fig. 6). Similar trends wereobserved for all three indices of G′, G′′, and η* of the
cheeses over the 26 weeks of storage. Also, viscoelas-tic properties of the MJ cheese significantly increasedover the first 4–8 weeks of storage and then remainedconstant for the rest of 26 weeks of storage. The G′,
Fig. 6. SAOSA analysis on viscoelastic properties [(a) elastic modulus,G′; (b) viscous modulus, G′′; and (c) complex viscosity, η*] of goatmilk Monterey Jack cheese (Van Hekken et al., 2004).

which measures the energy stored or the elastic proper-ties of the cheeses, increased significantly from 13.8 kPato 35.1 kPa over the first 8 weeks. The G′′, which mea-sures the energy lost or the flow properties of the cheeses,increased significantly from 4.17 kPa to 12.7 kPa over thefirst 4 weeks. The η*, which measures the phase relation-ships (G′′/G′), increased significantly from 1.44 kPa sto 3.91 kPa s over the first 8 weeks. There was a highcorrelation between the SAOSA and proteolysis data(Table 2) as the elastic (G′) and complex viscosity (η*)properties of the cheese increased. The concentration
of peptides in the 22–18 kDa range also increased withr values of 0.92 and 0.90, respectively. This correla-tion was expected as proteolysis of caseins disrupts

Y.W. Park / Small Ruminant Research 68 (2007) 73–87 81

Table 2Summary of correlation factors (P < 0.05) among proteins and peptide concentrations and rheological properties of Monterey Jack goat cheese

Rheological properties Proteins Peptides (kDa) Protein:peptide

�s2-CN �-CN 22–18 18–15 <14

SAOSAElastic modulus, G′ −0.84 −0.81 0.92 0.01 0.29 −0.95Viscous modulus, G′′ −0.68 −0.65 0.82 −0.14 0.09 −0.86Complex viscosity, η* −0.81 −0.78 0.90 −0.02 0.25 −0.93

TPAHardness 0.99 0.97 −0.96 −0.26 −0.59 0.96Springiness −0.79 −0.91 0.70 0.62 0.71 −0.90Cohesiveness −0.83 −0.80 0.85 0.14 0.33 −0.83

TAShear stress 0.89 0.92 −0.86 −0.31 −0.60 0.95Shear strain −0.67 −0.64 0.83 −0.17 0.01 −0.84

−0

V analysis

tfl

fntnacos1noh�t

VaacdB22swsnrn(

Shear rigidity 0.87 0.88

an Hekken et al. (2004); SAOSA: small amplitude oscillatory shear

he protein bonds and the cheese matrix becomes moreexible.

In the same study, Van Hekken et al. (2004) foundrom the texture profile analysis (TPA) that the hard-ess of the cheese decreased significantly from 43.4 No 26.7 N over the first 8 weeks of storage, while no sig-ificant differences (P < 0.05) were observed between 8nd 26 weeks of storage. The springiness of the MJ goatheese increased significantly from 7.8 mm to 9.2 mmver the first 4 weeks of storage and then did not changeignificantly over the next 5 months of storage. After onlyweek of storage, the cheese had the lowest cohesive-

ess and was significantly lower than the cohesivenessf cheese aged for 16 weeks. They also observed thatardness of the cheese was correlated with �-CN (0.97),s2-CN (0.99), and negatively correlated with the pep-

ides in the 22–18 kDa range (−0.91) (Table 2).For torsion analysis of the same MJ cheese (Fig. 7),

an Hekken et al. (2004) showed that the shear stresst point of fracture and the shear rigidity (stress/strain)t the point of fracture had similar trends and signifi-antly decreased over the first 8 weeks of storage butid not change significantly over the rest of the study.etween weeks 1 and 8, shear stress decreased from8.4 kPa to 18.3 kPa and shear rigidity decreased from9.4 kPa to 14.9 kPa. The shear strain at point of fractureignificantly increased from 0.97 to 1.20 over the first 4eeks of storage, peaked at 1.28 at 16 weeks, and then

ignificantly decreased to 1.14 at week 26. They also
oted that shear stress at the point of fracture was cor-elated with �-CN (0.92) and shear rigidity correlatedegatively to the large peptides at 22–18 kDa (−0.90)Table 2).
.90 −0.17 −0.42 0.98

; TPA: texture profile analysis; TA: torsion analysis.

In a rheological study of Jack goat milk cheese, theforce required to compress cheese 20% and 75% duringthe first and second compression cycles was evaluated(Attaie, 2005). There was no major trend in the forcerequired to compress the cheese 20% or 75% during thesecond compression cycle. The slopes of the lines (forceversus ripening time) for the first and second compres-sions were same, indicating that the cheese matrix hadlost structural integrity at the second compression cycle.The intercept of data from the TPA is a measure of theforce needed for compression and fracture of curd.

Concerning of rheology of soft goat milk cheeses,Van Hekken et al. (2005) also studied the effect of frozenstorage on proteolytic and rheological properties of com-mercial soft caprine milk cheese produced in the US.They observed that the fresh control cheeses had a frag-ile texture. At day 1, the frozen control cheeses (stored−20 ◦C for 2 days before being thawed and aged) usuallyhad significantly lower values for rheological proper-ties relative to the fresh control cheeses, while 6 monthsfrozen cheeses tended to have the highest values. Theyalso postulated that the creation and removal of ice crys-tals in the cheese matrix and the limited proteolysis ofthe caseins have small, but measurable, impact on cheesetexture. However, since the rheological changes were sosmall, soft cheeses may be supplied throughout the yearby frozen-storage method with minimal loss of texturequality.

Using the commercial plain soft goat milk cheeses
produced in the US, Park and Drake (2005) examinedsensory characteristics of the fresh, 0 and 3 monthsfrozen cheeses in relation to organic acid contents andtextural properties. They observed that the differences


days and 60 days. These pH changes may indicate theimportant metabolic activity of lactic acid bacteria in thesheep cheese as shown in Fig. 9. Lawrence et al. (1987)suggested that the decrease in pH would be related to

Fig. 7. Torsion analysis properties [(a) shear stress, (b) shear strain,and (c) shear rigidity at point of fracture] of goat milk Monterey Jackcheese (Van Hekken et al., 2004).

of the sensory scores in the cheeses among the stor-age treatment groups at the initial stages (0 day refrig-eration aging after thaw) were not significant. How-ever, prolonged refrigerated storage at 4 ◦C, regard-less of frozen-storage treatment, caused some (P < 0.05)changes in most flavor scores including cooked/milky,diacetyl, milkfat flavors, brothy, waxy, sweetness, sour-ness, saltiness, freshness, yeasty, and oxidized flavorsacross all treatment groups. The rheological propertiesof the frozen goat cheeses had significantly lower valuescompared to the fresh control cheese (Van Hekken et al.,
2005).
4.1.2. Rheology of sheep milk cheesesTerrincho sheep cheese is a typical product of the

north-eastern region of Portugal and is manufactured

Fig. 8. Changes in moisture and water activity (aw) of Terrincho sheepmilk cheese (Pinho et al., 2004).

from raw “Churra da Terra Quente” ewe milk. Pinho etal. (2004) evaluated the changes in chemical parameters(moisture, acidity, pH, and water activity) and physicalparameters (color and texture) of Terrincho sheep milkcheese during 60 days ripening, and also determinedcorrelations between the changes in instrumental tex-ture and color parameters, and the ripening time of theproducts.

During the first 20 days of ripening, Pinho et al. (2004)observed an increase in hardness, fracturability, gummi-ness, chewiness, and yellowness of the Terrincho sheepmilk cheese, while there were concomitant decreases inadhesiveness, resilience, L* (inside cheese, “i” and exter-nal “e”), and cohesiveness in the sheep cheese.

Pinho et al. (2004) also found that moisture and wateractivity were significantly decreased during 60 days ofripening (Fig. 8), where these parameters would greatlyaffect proteolytic activity in cheese. The ripening of thesheep cheese caused increases in acidity, and decreasesin pH up to 30 days, but slight increases between 30

Fig. 9. Changes in acidity and pH during 60 days of ripening in Ter-rincho sheep milk cheese (Pinho et al., 2004).

Y.W. Park / Small Ruminant Res

Fig. 10. Categorical principal component biplot showing the relation-ship between mean values from texture profile analysis and colorpcp

tw

cstwfdsdfwlaa

crIappmccpctaco

in hardness.Each of the textural measurements for the 11 cow

cheeses was correlated with composition and pH by Mul-tiple Linear Regression Analysis (Chen et al., 1979). For

arameters and mean sensory scores from four texture attributes andolor intensity of sheep milk cheeses from B, V, R, T, and M dairylants (Pinho et al., 2004).

exture parameters because no fusion of curd particlesould occur until pH value of approximately 5.8.Pinho et al. (2004) noted that the grouping of Terrin-

ho sheep cheese according to TPA analysis, color andensory attributes, was similar to the grouping accordingo instrumental analysis (Fig. 10). The sensory panelistsere able to distinguish differences between cheeses

rom B and M dairy plants and cheeses from the otherairy plants (R, T, and V), in which the three cheeses hadimilar chemical and sensory characteristics. Fig. 10 alsoemonstrates that color intensity, hardness on the mouth,racturability when cutting with a knife, and elasticity,hen pressing with the fingers, were positively corre-

ated with b*(e), b(i), TPA hardness, TPA fracturability,nd TPA springiness. The authors found only sensorydhesiveness was not correlated with TPA adhesiveness.

Feta cheese belongs to the family of white-brinedheeses that are indigenous to Greece, the Mediter-anean, and Middle East regions (Tsigkros et al., 2003).t is a semihard, crumbly variety with a salty, slightlycid taste (Anifantakis, 1991). Feta cheese with a higherroportion of caprine milk had a more compact and lessorous appearance than feta produced from purely ovineilk (Tsigkros et al., 2003). The hardness of the feta

heese was increased with increasing caprine milk con-entration from a force of 2.45–3.65 N over a range fromure ovine cheese to 30% caprine milk addition. Pureaprine milk gave a harder cheese with a stronger flavor
han that produced using pure ovine milk (Tsigkros etl., 2003). These differences might be due to differentasein structures or concentrations in the milks, wherevine milk contains higher levels of casein than caprine
earch 68 (2007) 73–87 83

milk (Kalantzopoulos, 1993). Several factors can influ-ence the final feta cheese texture, such as fat, protein andmoisture content as well as salt content during brining. Ithas been shown that high acidity, protein, and total solidscontents generally make the cheese harder and less eas-ily deformed (Creamer and Olson, 1982; Kehagias et al.,1995).

4.1.3. Rheology of cow milk cheesesTextural characteristics of representative cow cheese

varieties were tested in sensory evaluation by Chen et al.(1979). A sensory panel determined: (1) hardness as theforce required to penetrate the cheese sample with themolar teeth, (2) cohesiveness as the degree to which thecheese sample deforms before rupturing, (3) adhesive-ness as the force required to remove the cheese samplethat adheres to the mouth surface, and (4) chewiness asthe time required to masticate the cheese sample at a con-stant rate of force application to reduce it to a consistencysuitable for swallowing.

The range of hardness was based on the amount offorce required to rupture the sample between the molarteeth for Parmesan cheese as 13 and cream cheese as 1scale, used as the reference standards (Chen et al., 1979).As shown in Fig. 11, Parmesan cheese was greatest, fol-lowed by Edam, Gouda, Swiss, Cheddar and Mozzarellacheeses, and the processed Cheddar cheese was the least

Fig. 11. Hardness of cow cheeses (Chen et al., 1979).


Table 3Multiple linear regression coefficients of textural properties on composition and pH (Chen et al., 1979)

Hardness Cohesiveness Gumminess Chewiness Adhesiveness Elasticity

Protein 0.216 0.017 0.116 0.065 0.963 0.002Water −0.056 0.023 0.023 0.019 0.272 0.011Fat −0.005 0.009 0.007 0.010 0.090 0.004NaCl −1.00 0.017 −0.259 −0.240 −2.67 −0.051pH 0.665 0.311 0.439 0.261 3.47 0.045

−5.30.9

Constant −3.25 −3.01Correlation coefficient 0.923 0.886

this regression analysis, the textural measure was thedependent variable, and protein, water, fat, NaCl con-tents, and pH were independent variables. The resultsof the stepwise regression analysis are summarized inTable 3. For example, a typical regression equation forhardness is expressed as:

hardness = −3.25 + 0.216(protein) − 0.0558(water)

−0.0054(fat) − 1.00(NaCl) + 0.665(pH)

As shown in Fig. 12, the elasticity was highest forMozzarella, followed by Brick, Swiss and ProcessedCheddar cheese, and Parmesan cheese was the lowest.
For cohesiveness measurements, the cheese sample wasplaced between the molar teeth, and the force was exertedslowly as the sample deformed. The degree of defor-mation prior to rupturing as determined by comparing
Fig. 12. Elasticity of cow cheeses (Chen et al., 1979).

−3.23 −44.0 −0.10377 0.965 0.953 0.873

each sample with the range of all the samples, with 5being low, 9 moderate and 13 high cohesiveness (Chenet al., 1979). The same authors also observed that theorder of strength in cohesiveness was Processed Cheddarcheese, Muenster, Mozzarella, Provolone, Brick, Swiss,Colby, Edam, Gouda, Parmesan and Cheddar cheese(Fig. 13).

Torsion gelometry was compared to vane rheometryin tests on Cheddar, Mozzarella, and processed cheeses(Truong and Daubert, 2001). They reported that Ched-dar, the hardest cheese they tested, exhibited the highestshear stress; and Mozzarella, the most elastic cheesein their experiments, exhibited the highest shear strain.Gwartney et al. (2002) conducted torsion gelometryto determine texture scores from a sensory panel for
low- and full-fat Monterey Jack, Cheddar and Ameri-can cheeses, and found that fracture stress and strainwere each significantly correlated with several sensorydescriptors.
Fig. 13. Cohesiveness of cow cheeses (Chen et al., 1979).

ant Research 68 (2007) 73–87 85

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.2. Rheological characteristics of yogurts

The rheology of yogurt can be measured or charac-erized by viscometer, curd strength devices, penetrom-ters, and the vane method (rotating the sample aroundfixed vane) (Tunick, 2000). The rheology of stirred

ogurt is partially attributable to the production of ropyxopolysaccharides by specific culture strains duringncubation, which prevents gel fracture and syneresisCerning, 1995).

.2.1. Rheology of goat milk yogurtThe effect of the gelation process on the rheologi-

al properties of yogurt curd made from sheep, goat,ow and camel milks was investigated by Jumah etl. (2001). They found that the highest value for vis-osity was exhibited by sheep milk, followed by goat,ow and camel milks. Three different transient viscos-ty stages were identified and described by mathematicalxpressions for cow, sheep and goat milk, whereas camelilk showed no significant variation in viscosity during

elation.The milk curd is formed with the acid produced by

actic acid bacteria, which is a consequence of removingalcium and neutralizing the negative charges of caseinicelles, causing destabilization of casein, which aggre-

ates and forms a curd (McMahon et al., 1984). Curdexture or firmness is an important property of yogurt,hich determines the quality and acceptability of theroduct. Adequate firmness without syneresis is essentialor the superior quality of yogurt (Kroger, 1973). Usingrotational viscometer, the yogurt curd profile analysis

echnique has become a valuable and extensively usedool in the research of the enzymatic and nonenzymatichases of milk coagulation (Berridge, 1952; Gassem andrank, 1991).

Hassan et al. (1995) postulated formation of yogurturd structure during acid gelation of milk may occur inthree-stage process: (a) induction period without any

hanges in viscosity, (b) flocculation stage with maxi-um increases in viscosity, and (c) decrease in viscosity

ue to contraction with rearrangement of casein micellesnd the syneresis of gel.

Jumah et al. (2001) determined the flow curves ofogurt curd during the gelation process and at the max-mum viscosity value for sheep, goat, cow and camel

ilks (Fig. 14). They found that the measured viscos-ty decreased with increasing angular velocity of the
nner cylinder, suggesting that the yogurt behaved as ahear-thinning non-Newtonian fluid. Jumah et al. (2001)howed that the rheological flow properties of shear-hinning materials can be most commonly described by
Fig. 14. Effect of milk source (bovine, ovine and caprine) on flowcurves of yogurt curds (Jumah et al., 2001).

a two-parameter power law model of the form as:

τ = mγn

where τ is the shear stress, γ the shear rate, m the con-sistency coefficient, and n is the flow behavior index.

4.2.2. Rheology of sheep milk yogurtIn the Middle East region, concentrated yogurt (lab-

neh) is highly appreciated and consumed with bread allyear round. Labneh has been an important supplemen-tary food to that region’s diet and provides vital nutri-tional elements for growth and good health (Mohameedet al., 2004). Labneh is a concentrated sheep milk yogurt,which is defined as a semisolid food derived from yogurtby draining away part of its water and water-solublecompounds (Lebanese Standards, 1965). There are twotypes of labneh containing 22 wt.% and 40 wt.% solidconcentrations, where the 22% one is manufactured tobe consumed within 2 weeks, which usually stored inrefrigerators, while the 40% product (labneh anbaris)is stored in vegetable oil at room temperature and con-sumed within 2 years (Keceli et al., 1999). Labneh madefrom sheep milk is less popular and produced in muchless amounts than cow labneh. The cow milk availabilityis not the only issue but also the organoleptic acceptanceof cow labneh is higher due to the sharp flavor of sheeplabneh (Mohameed et al., 2004).

Evaluating rheological properties of labneh, Ozer etal. (1998) found that it is a weak viscoelastic gel where G′was greater than G′′. Kelly and O’Donnell (1998) studiedquarg, which is an acid-precipitated product containing

ant Res
about 80% water, and observed that both proteolysis andpremanufacture hydrolysis decreased G′ values.

Mohameed et al. (2004) investigated the effect ofsolids concentration on the apparent viscosity of sheepmilk labneh using a rotary viscometer, and found thatsheep labneh with different solids concentration exhib-ited shear-thinning and thixotropic behavior. The powerlaws model fitted satisfactorily the apparent viscosity-shear rate experimental data, and both the consistencycoefficient and the flow behavior index were correlatedas a function of solids concentration.

References

Abou-Dawood, A.E., El-Sawaf, S., 1977. Curd tension of milk fromsome mammals in comparison to some infant milk powder. Egyp-tian J. Dairy Sci. 5 (2), 129–133.

Addeo, F., Masi, P., Rubino, R., 1987. The aptitude of caprine milk tocheesemaking Relationship between �s1-casein content and rhe-logical properties of curd and cheese. In: L’evaluation des oviset des caprins mediterraneens, Symposium Philoetios, Fonte-Boa,Portugal.

Alichanidis, E., Polychroniadou, A., 1996. Special features of dairyproducts from ewe and goat milk from the physico-chemical andorganoleptic point of view. In: Proceedings of the IDF-CIRVALSeminar Production and Utilization of Ewe and Goat milk, Crete,Greece, October 19–21, 1995. International Dairy Federation Pub-lication, Brussels, Belgium, pp. 21–43.

Anifantakis, E.M., 1986. Comparison of the physico-chemical proper-ties of ewe’s and cow’s milk. In: Proceedings of the IDF SeminarProduction and Utilization of Ewe’s and Goat’s Milk, Athens,Greece, September 23–25, 1985. International Dairy FederationPublication, Brussels, Belgium, pp. 42–53, Bulletin No. 202.

Anifantakis, E.M., 1991. Traditional feta cheese. In: Robinson, R.K.,Tamine, A.Y. (Eds.), Feta and Related Cheeses. Ellis Horwood,Chichester, UK, pp. 49–70.

Attaie, R., 2005. Effects of aging on rheological and proteolytic prop-erties of goat Jack cheese produced according to cow milk proce-dures. Small Rumin. Res. 57, 19–29.

Awad, S., Luthi-Peng, Q.Q., Puhan, Z., 1998. Proteolytic activities ofchymosin and porcine pepsin on buffalo, cow, and goat whole andbeta-casein. J. Agric. Food Chem. 46, 4997–5007.

Bencini, R., 2002. Factors affecting the clotting properties of sheepmilk. J. Sci. Food Agric. 82, 705–719.

Berridge, N.J., 1952. An improved method observing the clotting ofmilk containing rennin. J. Dairy Res. 19, 328–332.

Bourne, M.C., 1968. Texture profile of ripening pears. J. Food Sci. 33,223.

Brennan, J.G., Jowitt, R., Muglai, O.A., 1970. Some experiences withthe General Foods Texturometer. J. Texture Stud. 1, 167.

Brennan, J.G., Jowitt, R., Williams, A., 1975. An analysis of the actionof the General Foods Texturometer. J. Texture Stud. 6, 83.

Cerning, J., 1995. Production of exopolysaccharides by lactic acidbacteria and dairy propionibacteria. Lait 75, 463–472.

Chen, A.H., Larkin, J.W., Clark, C.J., Irwin, W.E., 1979. Textural anal-ysis of cheese. J. Dairy Sci. 62, 901–907.

Clark, S., Sherbon, J.W., 2000a. Alphas1-casein, milk compositionand coagulation properties of goat milk. Small Rumin. Res. 38,123–134.

earch 68 (2007) 73–87

Clark, S., Sherbon, J.W., 2000b. Genetic variants of �s1-casein in goatmilk: breed distribution and associations with milk compositionand coagulation properties. Small Rumin. Res. 38, 135–143.

Creamer, L.K., Olson, N.F., 1982. Rheological evaluation of maturingCheddar cheese. J. Food Sci. 47, 631–647.

Dimassi, O., Neidhart, S., Carle, R., Mertz, L., Migliore, G., Mane-Bielfeldt, A., Zarate, A.V., 2005. Cheese production potential ofmilk of Dahlem Cashmere goats from a rheological point of view.Small Rumin. Res. 57, 31–36.

El-Agamy, E.I., 1983. Studies on camel’s milk. M.Sc. Thesis. Alexan-dria University, Egypt.

Friedman, H., Whitney, J., Szczesniak, A.S., 1963. The texturo-meter—a new instrument for objective texture measurement. J.Food Sci. 28, 390.

Gassem, M.A., Frank, J.F., 1991. Physical properties of yogurt madefrom milk treated with proteolytic enzymes. J. Dairy Sci. 74,1503–1511.

Grappin, R., Jeunet, R., Pillet, R., Toquin, A., 1981. Etide des laits dechevre: teneur du lait de chevre en matiere grasse, matiere azoteeet fractions azotees (Studies of goat milk: contents of fat, proteinand protein fractions). Le Lait 61, 117–133.

Grosclaude, F., Mahe, M.F., Brignon, G., Di Stasio, L., Heunet,R., 1987. A Mendelian polymorphism underlying quantitativevariations of goat �s1-casein. Genet. Sel. Evol. 19 (4), 399–412.

Gwartney, E.A., Foegeding, E.A., Larick, D.K., 2002. The textureof commercial full-fat and reduced-fat cheese. J. Food Sci. 67,812–816.

Haenlein, G.F.W., Wendorff, W., 2006. Sheep milk. In: Park, Y.W.,Haenlein, G.F.W. (Eds.), Handbook of Milk of Non-bovine Mam-mals. Blackwell Publishing Professional, Oxford, England, pp.137–194.

Hamann, D.D., June 1988. Rheology as a means of evaluating musclefunctionality of processed foods. Food Technol., 66–71.

Hamann, D.D., Foegeding, E.A., 1994. Analysis of torsion, com-pression and tension for testing food gel fracture properties.http://www.gelconsultants.com/torsion.html.

Hassan, A.N., Frank, J.F., Farmer, M.A., Schmidt, K.A., Shalabi, S.I.,1995. Formation of yogurt microstructure and three-dimensionalvisualization as determined by confocal scanning laser microscopy.J. Dairy Sci. 78, 2629–2636.

Hassan, A.N., Hagrass, A.I., Soryal, K.A., El-Shabrawy, S.A., 1987.Physicochemical properties of camel milk during lactation periodin Egypt. Egypt. J. Food Sci. 15, 1–14.

Jenness, R., 1980. Composition and characteristics of goat milk: areview 1968–1979. J. Dairy Sci. 63, 1605–1630.

Jenness, R., Patton, S., 1976. Physical properties of milk. In: Principlesof Dairy Chemistry. R.E. Krieger Publishing Co., Huntington, NY,pp. 219–263.

Jumah, R.Y., Shaker, R.R., Abu-Jdayil, B., 2001. Effect of milk sourceon the rheological properties of yogurt during the gelation process.Int. J. Dairy Technol. 54, 89–93.

Kalantzopoulos, G.C., 1993. Cheeses from ewes’ and goats’ milk.In: Fox, P.F. (Ed.), Cheese: Chemistry Physics and Microbi-ology, vol. 2, 2nd ed. Chapman and Hall, London, pp. 507–543.

Keceli, T., Robinson, R.K., Gordon, M.H., 1999. The role of olive oil
in the preservation of yogurt cheese (labneh anbaris). Int. J. DairyTechnol. 52, 68–72.
Kehagias, C., Koulouris, S., Samona, A., Malliou, S., 1995. Effect ofvarious starters on the quality of cheese in brine. Food Microbiol.12, 413–420.

http://www.gelconsultants.com/torsion.html

ant Res

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K

L

L

L

M

M

M

M

M

O

O

P

P

P

P

P

P

P

R


elly, A.L., O’Donnell, H.J., 1998. Composition, gel properties andmicrostructure of quarg as affected by processing parameters andmilk quality. Int. Dairy J. 8, 295–301.

roger, M., 1973. Controlling the quality of yogurt. Dairy Ice CreamField 156, 38–39.

abropoulos, A., Collins, W.F., Stone, W.K., 1984. Effect of ultra hightemperature and vat process on heat-induced rheological propertiesof yogurt. J. Dairy Sci. 67, 405–409.

awrence, R.C., Creamer, L.K., Gelles, J., 1987. Texture developmentduring cheese ripening. J. Dairy Sci. 70, 1748–1760.

ebanese Standards, 1965. LS-24. Lebanese Standard Institute, POBox 2806, Beirut, Lebanon.

cMahon, D.J., Brown, R.J., Richardson, G.H., Ernstrom, C.A., 1984.Effect of calcium, phosphate and bulk culture media on milk coag-ulation properties. J. Dairy Sci. 67, 930–936.

ehaia, M.A., 1974. A comparative study of milk of different dairyanimals. Some physical properties. M.Sc. Thesis. Alexandria Uni-versity, Egypt.

ohameed, H.A., Abu-Jdayil, B., Al-Shawabkeh, A., 2004. Effectof solids concentration on the rheology of labneh (concentratedyogurt) produced from sheep milk. J. Food Eng. 61, 347–352.

ontejano, J.G., Hamann, D.D., Lanier, T.C., 1986. Comparison oftwo instrumental methods with sensory texture of protein gels. J.Texture Stud. 16, 403–424.

uir, D.D., Horne, D.S., Law, A.J.R., Sweetsur, A.W.M., 1993. Ovinemilk. 2. Seasonal changes in indices of stability. Milchwissensch48, 442–445.

lson, N.F., Gunasekaran, S., Bogenrief, D.D., 1996. Chemical andphysical properties of cheese and their interactions. Neth. MilkDairy J. 50, 279–294.

zer, B.J., Bell, A.E., Grandison, A.S., Robinson, R.K., 1998. Rhe-ological properties of concentrated yoghurt (labneh). J. TextureStud. 29, 67–79.

ark, Y.W., Drake, M.A., 2005. Effect of 3 months frozen-storage onorganic acid contents and sensory properties, and their correlationsin soft goat milk cheese. Small Rumin. Res. 58, 291–298.

ark, Y.W., Van Hekken, D.L., Tunick, M.H., Holsinger, V.H., 2000.Rheological characteristics of young Monterey Jack goat milkcheese. In: Proceedings of the Seventh International Conferenceon Goats, Tours, France, May 14–20, p. 601.

arry, R.M., 1974. Milk coagulation and protein denaturation. In:Webb, B.H, Johnson, A.H., Alford, J. (Eds.), Fundamentals ofDairy Chemistry, 2nd ed. The AVI Publishing Co. Inc., Westport,CT, pp. 603–661.

inho, O., Mendes, E., Alves, M.M., Ferreira, I.M., 2004. Chemical,physical, and sensorial characteristics of “Terrincho” ewe cheese:changes during ripening and intravarietal comparison. J. Dairy Sci.87, 249–257.

rentice, J.H., 1991. Cheese rheology. In: Hui, Y.H. (Ed.), Encyclope-dia of Food Science and Technology, vol. 1. John Wiley & SonsInc., New York, NY, pp. 348–369.

roctor, B.E., Davison, S., Malecki, G.J., Welch, M., 1955. A recordingstrain-gage denture tenderometer for foods. I. Instrumental evalu-ation and initial tests. Food Technol. 9, 471–477.

uri, B.R., Parkash, S., 1962. Studies in physico-chemical propertiesof milk. 12. The renneting time of milk of different species. IndianJ. Dairy Sci. 15, 114–122.

ao, V.N.M., Skinner, G.E., 1986. Rheological properties ofsolid foods. In: Rao, M.A., Rizvi, S.S.H. (Eds.), Engineer-

earch 68 (2007) 73–87 87

ing Properties of Foods. Marcel Dekker, New York, NY,pp. 215–254.

Remeuf, F., 1992. Physico-chemical properties of goat milk in rela-tion to processing characteristics. In: Proceedings of the NationalSymposium on Dairy Goat Production and Marketing, OklahomaCity, OK, pp. 98–110.

Remeuf, F., Lenoir, J., Duby, C., 1989. Etude des relatios entre lescaracteristiques physico-chemiques des laits de chevre et leur apti-tude a la coagulation par la pressure (Study of the relationshipsof physico-chemical characteristics of goat milk and aptitude forcoagulation under pressure). Le Lait 69 (6), 499–518.

Ricordeau, G., Mocquot, G., 1967. Influence des variations saison-nieres de la composition du lait de chevre sur le rendement enfromage (Influence of seasonal variations in the composition ofgoat milk and effect on cheese). Annales de Zootechnie 16 (2),165–181.

Scott-Blair, G.W., 1969. Elementary Rheology. Academic Press, NewYork.

Spreer, E., 1998. Milk and Dairy Product Technology. Marcel DekkerInc., New York, Basel, pp. 73–154, Translated by A. Mixa.

Storry, J.E., Ford, G.D., 1982. Some factors affecting the post clottingdevelopment of coagulum strength in renneted milk. J. Dairy Res.49, 469–477.

Szczesniak, A.S., 1963. Objective measurements of food texture. J.Food Sci. 28, 410–420.

Szczesniak, A.S., 1995. Texture profile analysis—methodology inter-pretation clarified. J. Food Sci. 60 (6), vii.

Tamine, A., Robinson, R.K., 1985. Yoghurt: Science and Technology.Pergamon Press Ltd., Oxford.

Trujillo, A.J., Guamis, B., Carretero, C., 1997. Hydrolysis ofcaprine:�-casein by plasmin. J. Dairy Sci. 80, 2258–2263.

Truong, V.D., Daubert, C.R., 2001. Textural characterization of cheesesusing vane rheometry and torsion analysis. J. Food Sci. 66,716–721.

Tsigkros, D., Folland, E., Moate, R., Brennan, C.S., 2003. Feta cheesetexture: the effect if caprine and ovine milk concentration. Int. J.Dairy Technol. 56, 233–236.

Tunick, M.H., 2000. Rheology of dairy foods that gel, stretch, andfracture. J. Dairy Sci. 83, 1892–1898.

Tunick, M.H., Nolan, E.J., 1992. Rheology of cheese. In: Baianu,I.C. (Ed.), Physical Chemistry of Food Processes. Fundamen-tal Aspects, vol. 1. Van Nostrand Reinhold, New York, NY, pp.273–297.

Tunick, M.H., Van Hekken, D.L., 2002. Torsion gelometry of cheese.J. Dairy Sci. 85, 2743–2749.

Van Hekken, D.L., Tunick, M.H., Park, Y.W., 2004. Rheological andproteolytic properties of Monterey Jack goat milk cheese during 6months aging. J. Agric. Food Chem. 52, 5372–5377.

Van Hekken, D.L., Tunick, M.H., Park, Y.W., 2005. Effect of frozenstorage on proteolytic and rheological properties of soft caprinemilk cheese. J. Dairy Sci. 88, 1966–1972.

Voisey, P.W., 1976. Instrumental measurements of food texture. In:deMan, J.M., Voisey, P.W., Rasper, V.F., Stanley, D.W. (Eds.), Rhe-ology and Texture in Food Quality. AVI, Westport, CT, pp. 79–141.

Volodkevich, N.N., 1938. Apparatus for measurements of chewingresistance or tenderness of foodstuffs. Food Res. 3, 221–225.

Walstra, P., Geurts, T.J., Noomen, A., Jellema, A., van Boekel,M.A.J.S., 1999. Dairy Technology: Principles of Milk Propertiesand Processes. Marcel Dekker Inc., New York, NY, pp. 27–147.

Rheological Characteristics of Goat and Sheep Milk

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