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 Abstract Rheology represents the properties of both solid and liquid foods, where texture is the rheology related to solid foods, and viscosity is the rheology of fluid foods. Three categories of tests measure textural characteristics of solids foods, empirical (ball compressor, penetrometer, curd tension meter), imitative (texturometer, texture profile analysis [TPA]), and fundamental tests (small amplitude oscillatory shear analysis [SAOSA], torsion analysis [TA]). Regardless of animal species, milk casein gels are mainly responsible for the rheological properties of cheese and other dairy products. In normal fluid milk, the viscosity is affected by the state and concentrations of fat, protein, temperature, pH, and age of the milk. Average milk viscosity has been determined for goats at 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, but it increases at the point of coagulation. Sheep and goat milk have the same proteins as cow milk, but their proportions and genetic polymorphs differ widely, which explains different rennetabilities and considerable rheological variations in cheesemaking. Casein micelle structure is similar in goat, sheep, and cow milk, but differs in composition, size and hydration. Sheep milk has caseins richer in calcium than cow caseins, it is also very sensitive to rennet, because of higher / s -casein ratio, and coagulation proceeds faster than in cow milk. Rheological studies with Monterey Jack cheese from goat milk found “knitting” with progressing aging time, less hardness, less shear stress values, and more rubberiness. High correlations were noted between SAOSA scores and proteolysis in cheeses. Terrincho sheep cheese showed increased hardness, fracturability, gumminess, chewiness, yelloweness, decreased adhesiveness, resilience, and cohesiveness during ripening. Frozen storage of soft goat cheeses had minimal effects on textural qualities, which has valuable market implications. Feta cheeses showed increased compactness and porosity, when goat milk had been added to sheep milk. Yogurt studies, including Labneh from the Middle East, found highest viscosity for sheep followed by goats, cows, and camels, and viscosity increased with solids contents. Three different transient viscosity stages were described mathematically, and camel milk varied least in viscosity during yogurt gelation. Viscosity decreased with increasing angular velocity of the inner cylinder, suggesting that yogurt behaved as a shear-thinning non-Newtonian fluid. Rheological properties are important monitors of quality control in dairy processing and in scientific research. © 2006 Elsevier B.V. All rights reserved. Keywords: Goat milk; Sheep milk; Rheology; Texture; Viscosity; Texture profile analysis 1. Introduction Rheology is defined as the study of material deforma- tion and flow (Scott-Blair, 1969), and includes what is This paper is part of the special issue entitled “Goat and Sheep Milk” Guest edited by George Haenlein, Young Park, Ketsia Raynal- Ljutovac and Antonio Pirisi. Tel.: +1 478 827 3089; fax: +1 478 825 6376. E-mail address: [email protected]. termed “small-strain” testing (deforming a small percent of that required to break the sample) and “large-strain” testing (deforming to the point of permanent structural change) (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, 0921-4488/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2006.09.015

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  • Small Ruminant Research 68 (2007) 7387

    Rheological characteristics of goaY.W. Park

    Georgia Small Ruminant Research and Extension Center, College of AgricultuFort Valley State University, Fort Valley, GA 310

    r 2006

    Abstract

    Rheology represents the properties of both solid and liquid foods, where texture is the rheology related to solid foods, andviscosity is the rheology of fluid foods. Three categories of tests measure textural characteristics of solids foods, empirical (ballcompressor, penetrometer, curd tension meter), imitative (texturometer, texture profile analysis [TPA]), and fundamental tests (smallamplitude oresponsiblestate and conat 2.12 cPasbut it increagenetic polyCasein micecaseins richproceeds fasaging time,and proteolydecreased adtextural qualhad been adby goats, comathematicaof the innermonitors of 2006 Else

    Keywords: G

    1. Introdu

    Rheologtion and flo

    This papeMilk Guest eLjutovac and

    Tel.: +1 4E-mail ad

    0921-4488/$doi:10.1016/jscillatory shear analysis [SAOSA], torsion analysis [TA]). Regardless of animal species, milk casein gels are mainlyfor the rheological properties of cheese and other dairy products. In normal fluid milk, the viscosity is affected by thecentrations of fat, protein, temperature, pH, and age of the milk. Average milk viscosity has been determined for goats

    , sheep 2.48 cPas, camels 2.8 cPas, buffaloes 2.2 cPas, and cows 1.7 cPas. Heating decreases the dynamic viscosity,ses at the point of coagulation. Sheep and goat milk have the same proteins as cow milk, but their proportions andmorphs differ widely, which explains different rennetabilities and considerable rheological variations in cheesemaking.lle structure is similar in goat, sheep, and cow milk, but differs in composition, size and hydration. Sheep milk has

    er in calcium than cow caseins, it is also very sensitive to rennet, because of higher /s-casein ratio, and coagulationter than in cow milk. Rheological studies with Monterey Jack cheese from goat milk found knitting with progressingless hardness, less shear stress values, and more rubberiness. High correlations were noted between SAOSA scoressis in cheeses. Terrincho sheep cheese showed increased hardness, fracturability, gumminess, chewiness, yelloweness,hesiveness, resilience, and cohesiveness during ripening. Frozen storage of soft goat cheeses had minimal effects onities, which has valuable market implications. Feta cheeses showed increased compactness and porosity, when goat milkded to sheep milk. Yogurt studies, including Labneh from the Middle East, found highest viscosity for sheep followedws, and camels, and viscosity increased with solids contents. Three different transient viscosity stages were describedlly, and camel milk varied least in viscosity during yogurt gelation. Viscosity decreased with increasing angular velocity

    cylinder, suggesting that yogurt behaved as a shear-thinning non-Newtonian fluid. Rheological properties are importantquality control in dairy processing and in scientific research.vier B.V. All rights reserved.

    oat milk; Sheep milk; Rheology; Texture; Viscosity; Texture profile analysis

    ction

    y is defined as the study of material deforma-w (Scott-Blair, 1969), and includes what is

    r is part of the special issue entitled Goat and Sheepdited by George Haenlein, Young Park, Ketsia Raynal-Antonio Pirisi.78 827 3089; fax: +1 478 825 6376.dress: [email protected]

    termed small-strain testing (deforming a small percentof that required to break the sample) and large-straintesting (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,

    see front matter 2006 Elsevier B.V. All rights reserved..smallrumres.2006.09.015Available online 25 Octobet and sheep milk

    re, Home Economics and Allied Programs,30-4313, USA

  • 74 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    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 usuallyevaluatedand torsion

    Caprinephysico-chdifferencesmilks (Remof goat micontent, anin caprinehydration (proportioncomparedations, espcontents bsheep, becphisms forcheesemakhas highertitratable acow milk (

    Regardlgoats andof the variand other(Tunick, 20a quality cas a scientgists to perproducts.

    Althougits dairy proumentationmilk produscarce. Thiucts, but dgoats and smented go

    2. Rheolo

    Rheologcosity. Thethe inner frity of milkfriction of fIn normalconcentratiage of the

    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 behaves

    Newtoortion).

    Dimen

    iscositive tersity itipoisess ais eq

    y is orliquid.he dyner, 19for viis a f 10), whes are 1).he viscthat ofviscosPas (H), whicPas)

    o that

    Effectviscos

    he dynure inars to cscosityrotein

    supe). Theased ames dincremper. Whemilkincreaay bewith texturometer, Instron Testing machinegelometry, etc.milk differs from cow milk in several

    emical characteristics, which explain majorin the technological behavior of the twoeuf, 1992). The poorer cheesemaking ability

    lk is largely attributable to the lower caseind to specific properties of casein micellesmilk such as their composition, size and

    Remeuf, 1992). Goat milk also has differents of the four major caseins (s1, s2, , )to cow counterparts, and there are great vari-ecially between s1-casein and s2-caseinetween individuals and breeds of goats andause of the occurrence of genetic polymor-all milk proteins, which influence greatly theiring properties (Remeuf, 1992). Sheep milkspecific gravity, viscosity, refractive index,

    cidity, but lower freezing point than averageHaenlein and Wendorff, 2006).ess of milk of any dairy species (i.e., cow,sheep), casein gels are responsible for mostous rheological/textural properties of cheesedairy products that gel, stretch and fracture00). Rheological properties are examined as

    ontrol method in dairy processing plants andific technique for food scientists and rheolo-form research on the structure/texture of food

    h rheological characteristics of cow milk andducts have been extensively studied, the doc-s on rheological properties of goat and sheepcts, especially their fluid milks have been

    s review focuses on goat and sheep milk prod-ue to the paucity of reports on fluid milk ofheep, this paper covers more research on fer-at and sheep milk products.

    gy of goat milk and sheep milk

    y of fluid milk is largely influenced by its vis-dynamic viscosity is a parameter related toiction of a liquid (Spreer, 1998). The viscos-is twice as high as that of water due to theat in milk (emulsified in milk) (Spreer, 1998).fluid milk, viscosity is affected by state andons of fat and protein, temperature, pH andmilk (Jenness and Patton, 1976). Viscosity is

    as a

    prop1999

    2.1.

    Vrelatviscoa cen

    (Jenn20 Ccositof a

    T(Spreterm5 C2.96milkmilk1998

    TthanThe2.2 c1983(2.12ilar t

    2.2.milk

    Tperatappein vithe pity in1976increbecosteep

    Tewaysskimmilkity mnian liquid, meaning that the shear stress isal to the shear rate (dv/dx) (Walstra et al.,

    sions of viscosity

    y of a fluid can be measured in absolute orms. The absolute unit of measurement fors the poise, named after Poiseuille, wheree represents one one-hundredth of a poise

    nd Patton, 1976). The viscosity of water atual to 1.005 centipoises, and the relative vis-dinarily measured in terms of the rate of flow

    amic viscosity is expressed as N s/m or Pa.s98). The centipoise cP (103 Pa.s) is an oldscosity. The viscosity value for cow milk atunction of the fat content and ranges from3 Pa.s (skim milk) to 3.25 103 Pa.s (wholereas at 20 C the ranges for the skim and whole.79 103 Pa.s and 1.3 103 Pa.s (Spreer,

    osity of sheep milk is reportedly much highergoat or cow milk, cow milk being the lowest.

    ity of Egyptian camel milk was estimated atassan et al., 1987) and 2.35 cPas (El-Agamy,

    ch is higher than for cows (1.7 cPas) and goats, but less than for sheep (2.48 cPas), and sim-of buffalo milk (2.2 cPas) (Mehaia, 1974).

    of temperature and incubation time onity

    amic viscosity value decreases when tem-creases. The contribution of casein micelleslosely depend on temperature. Milk increasesupon heating to the point of coagulation of

    s, which is the basis for producing high viscos-rheated condensed milk (Jenness and Patton,

    voluminosity of the micelles is markedlyt low temperature, and part of the -caseinissociated from the micelles, resulting in aase in viscosity (Walstra et al., 1999).ature apparently affects viscosity in variousn the serum proteins become insoluble indue to heat treatment, the viscosity of skimses by about 10%. This increase in viscos-explained by increase in voluminosity of

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 75

    Fig. 1. Effec(Jumah et al.,

    the serumity of goatmilk (Remhigh ionicin caprine mbility.

    The effincubationThey foundity value, fThe differeto be due tomilks, whiof yogurt cal., 2001).to an increteins (Labrmilk did nogelation prlower prote

    2.3. Effect

    The visincreases, wtional swelA slight ddecrease inin pH leadaggregationity of milkthat of crea

    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 from

    to 6.16 (Bencini, 2002). Curd consistency in sheepis largely unaffected by temperature, particularly atr pH (Bencini, 2002), where the curd consistencyd be an important factor for viscoelastic propertieseep milk cheese.he role of pH in cheese texture is particularly impor-ecause changes in pH are related directly to chem-hanges in the protein network of the cheese curdo et al., 2004). However, the influence of waterity and salt content on rheological properties ofse is indirect. A high level of salt increases thetic pressure, which diverts significant amounts of

    r from the structural bonds of the casein networktice, 1991). On the other hand, decreased waterity would result in a reduced proteolytic activityeese.he most important factors in the acid curd structureation are casein content, pH and calcium contente milkted froalizatsive ah tendn netwpped imahscositl milk2). No

    H 6.4unifot of milk source on viscosity-incubation time curves2001).

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

    euf, 1992), which may be attributed to thecalcium content and low micellular solvation

    ilk which would contribute to its heat insta-

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

    ollowed by caprine and bovine milk (Fig. 1).nces in viscosity between species appearedthe differences in total solids contents of the

    ch caused a significant effect on the firmnessurd (Tamine and Robinson, 1985; Jumah etThe higher viscosity may also be attributableased water-binding capacity in the milk pro-

    6.65milklowewoulof sh

    Ttant bical c(Pinhactivcheeosmo

    wate(Prenactivin ch

    Tformof thsocianeutrextenwhiccaseientra

    Juof vicame

    (Fig.the pto beopoulos et al., 1984). On the other hand, camelt show any elevation in viscosity during the

    ocess which might be accounted for by thein content (Jumah et al., 2001).

    of pH on milk viscosity

    cosity of milk increases as the pH of milkhich is presumably attributable to the addi-

    ling of casein micelles (Walstra et al., 1999).ecrease in milk pH usually causes a small

    viscosity, whereas a more drastic decreases to an increase in milk value because of

    of casein (Walstra et al., 1999). The viscos-is little affected by homogenization, while

    m would be significantly increased.Fig. 2. Effect2001).. At low pH, calcium is progressively dis-m the casein micelle. In addition to this, the

    ion of the negative charges of the casein favorsggregation and fusion between the micelles,to form a gel (Parry, 1974). At pH 4.6, aork is formed and the other components aren it.et al. (2001) observed similar curve trendsypH profiles of ovine, caprine, bovine ands to the viscosity-incubation time profilesviscosity changes were noted with a drop in

    5.4 range, where the casein micelles appearedrm in size and distribution (Hassan et al.,

    of milk source on viscositypH curves (Jumah et al.,

  • 76 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    1995). At pH 5.45.3, 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 (Haa decrease

    2.4. Effectof goat and

    Viscositincreasingare fairlyabout 0.7 mmicelle voet al., 1999the stabilitycosity of mtreatment, a(Jenness an

    The effemay not beof fat, the fthe globuleand Pattonthe amountthe fat parting of themilk (Jenn

    Goat mito cow miof caprineposition, s(Remeuf, 1casein num(casein/totaGrappin etyield (Ricotexture andile rennetEl-Sawaf,equal, caprparts (Remcoagulumlosses in wral integritymilk has altime and grRemeuf et

    The mation in reninclude: ca

    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 casein

    llular, 1989oat me mil

    in haso the o, 1987s gre

    oximaeportsigni

    at milnificana stu

    em Cspectsuctionn rateficantlrate, baseinulationphate

    solider proer agg.rosclaA, B,s1-case

    was

    /l), anwith lmilk.n genulationwas m

    an Ashoweined aonentfirmnAlpi

    nts buntableir coreep mcow ossan et al., 1995) In the pH range of 5.14.6,in viscosity occurred.

    of chemical characteristics on rheologysheep milk

    y is elevated with protein coagulation andfat content (Spreer, 1998). Casein micelles

    voluminous, where dry casein may occupyl/g and the rest of the volume is water, while

    luminosity is about 4 ml/g of casein (Walstra). Some conditions and treatments that affectof casein can significantly influence the vis-

    ilk, which include acidity, salt balance, heatnd the action of various enzymes and bacteriad Patton, 1976).ct of milk fat on the viscosity of whole milkas high as that of casein. However, the amountat globule size and the extent of clustering ofs significantly affect milk viscosity (Jenness, 1976). Homogenization causes increases inof fat surface, the amount of protein bound by

    icles, and the degree of clustering and clump-fat contribute to the increasing viscosity ofess and Patton, 1976).lk has poorer cheesemaking ability comparedlk counterparts due to lower casein contentmilk, and differences in casein micelle com-ize and hydration between the two species992). The lower casein content and lowerber of goat milk compared to cow milkl nitrogen: 75% versus 78%; Jenness, 1980;al., 1981) are responsible for reduced cheeserdeau and Mocquot, 1967) and affect cheeserheology. Goat milk produces a more frag-

    curd than bovine milk (Abou-Dawood and1977). Even when casein concentrations areine rennet curd is softer than bovine counter-euf et al., 1989). In cheesemaking, a poorstrength can lead to more cheese particlehey, lower cheese yield and likely less textu-. The uniqueness of renneting kinetic of goat

    so been characterized as a shorter coagulationeater hardening rate (Puri and Parkash, 1962;al., 1989).in physico-chemical factors affecting varia-neting properties of individual caprine milksein content, total and colloidal calcium con-

    miceet al.

    GbovinCasedue tet al.makeapprers. Rhas aof goa sig

    InDahlcal aprodgatiosignitionand ccoagphosotherhighhighcurd

    Gantsof ant E1.6 gatedgoatcaseicoagmilkant thalsocontacompcurdnen conteacco

    of thSh

    thansize and hydration of the two milks (Remeuf).

    ilk contains the same four casein fractions ask such as s1, s2, , and caseins. s1-very large individual quantitative variationsccurrence of genetic polymorphisms (Addeo

    ; Grosclaude et al., 1987). This polymorphismat variations in milk s1-casein levels fromtely 25% in certain milks to total lack in oth-s have shown that s1-casein polymorphismficant influence on cheesemaking propertiesk (Remeuf, 1992), which would in turn havet impact on cheese textural quality.

    dy of cheese production potential of milk ofashmere (DC) goat in relation to rheologi-, Dimassi et al. (2005) reported that cheeseefficiency was directly proportional to aggre-and coagulation time. The DC breed had

    y higher curd firmness and faster aggrega-ecause the DC goats had much higher proteincontent than the German Fawn breed. During, destabilized casein micelles and calcium-

    bonds form a network which entraps fat ands. The casein network is formed faster withteins mainly casein content, which results inregation rate and the development of firmer

    ude et al. (1987) reported that high type vari-and C were associated with higher amountsin (about 3.6 g/l), intermediate type vari-

    associated with intermediate amounts (aboutd low type variants D and F were associ-ow amounts (about 0.6 g/l) of s1-casein inClark and Sherbon (2000b) reported that s1-etic variants were not highly correlated with

    properties, and they found that Nubian breedore likely to contain a high type genetic vari-

    lpine breed milk. Clark and Sherbon (2000a)d that Nubian and Nubian Alpine crosseshigher amount of s1-casein and other milk

    s, and exhibited higher coagulation rate andess than milk from Toggenburgs and Saa-ne crosses. These differences in s1-caseinetween different breeds and crosses weree for the variations in texture and rheologyresponding goat milk cheese products.ilk is higher in fat, protein and total solids

    r goat milk (Haenlein and Wendorff, 2006).

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 77

    The cheese yield is significantly greater per unit of sheepmilk than that of cow and goat milk, because cheesecurd contains primarily the fat and casein from milk(Anifantakis, 1986). Sheep milk has all major caseinfractions o1996). Cagoat andricher in cPolychroniponent in mcurd of cheaffect the rology.

    Sheep ma higher /milk proceThis suggeto sheep mand rheologalso requirecoagulationrate of curcow milk,than in thehigher case(Storry and

    3. Rheolofunctional

    Texturecheese, whcomplicateturometer cas hardnesby Friedmthat the mobtained wtion betweesory scoresrevealed thbut adhesivstudy usingsame resea

    lation coeffiand the Ins

    There aral or rheoimitative, a

    Empiriccannot usuexperiment

    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).

    Empir

    mpiriceeseinfor

    tromepiricaeese i). Theinstrumey, 19with tkinne

    Imitat

    lodkewhich

    ezing a, thesdenturhe nexd textuompres, whwith

    ly use

    . Textxtureine wuctivesive n, 2004rical be shaparison

    ertiesPA istical cindricick, 2ine recloseof ch

    zarellaf cow milk (Alichanidis and Polychroniadou,sein micelle structure is similar in cow,sheep milk, while sheep milk caseins arealcium than cow casein (Alichanidis andadou, 1996). Since casein is the critical com-

    ilk that forms the primary structure of theese, the clotting or coagulation of milk wouldesultant cheese composition, texture and rhe-

    ilk is very sensitive to rennet, because it hass-casein ratio, and also coagulation in sheepeds faster than in cow milk (Muir et al., 1993).sts that mode and amount of rennet additionilk would definitely influence the final textureical characteristics of the cheese. Sheep milks less rennet than cow milk to obtain the sametime (Kalantzopoulos, 1993). Although the

    d formation in sheep milk is faster than inthe rate of syneresis in the former is slowerlater (Muir et al., 1993), which is due to thein and colloidal calcium content in sheep milkFord, 1982).

    gical methods as a means of evaluatingity of dairy foods

    plays an important role in the quality ofile the textural measurements of cheese ared and confusing (Chen et al., 1979). The tex-an measure several textural properties such

    s, chewiness, elasticity, which was inventedan et al. (1963). Bourne (1968) discoveredeasurements from the texturometer can beith the Instron Testing Machine. The correla-n textural measures of texturometer and sen-of a taste panel on several Cheddar cheeses

    at only hardness was significantly correlated,eness was not (Brennan et al., 1970). In aboth the texturometer and the Instron by the

    rch group a few years later, the highest corre-cient was found between sensory evaluation

    tron data (Brennan et al., 1975).re three categories for measurement of textu-logical characteristics, which are empirical,nd fundamental tests (Scott-Blair, 1969).al measurements involve test conditions thatally be compared with those of more rigorouss (Tunick and Van Hekken, 2002). Imitative

    3.1.

    Eor chpointpeneas em

    of ch1963sion(Voispareand S

    3.2.

    Votests,squeLaterizedFor tducecan c

    curve

    laterwide

    3.2.1Te

    machdestrcoheet al.empiin thcompprop

    Ta ver

    a cyl(Tunmachdure,pieceMozical tests

    al tests can be as simple as manipulating curdwith the fingers, and supply basic, single-mation (Tunick, 2000). Ball compressors,

    ters, and curd tension meters have been usedlly measuring tools for rheological attributesn cheese plants for many years (Szczesniak,se equipments are not regarded as preci-ents because of the arbitrary test conditions

    76), and the test results are difficult to com-hose from more rigorous experiments (Raor, 1986).

    ive tests

    vich (1938) demonstrated the first imitativeconsisted of two wedges exerting biting and

    ction on the sample held between the wedges.e imitative tests were developed into motor-es with strain gauges by Proctor et al. (1955).t development, Friedman et al. (1963) intro-rometers in the early 1960s. These machinesss bite-sized samples to produce forcetimeereby the analysis succeeded several yearsuniversal testing machines, which are nowd for texture profile analysis (TPA).

    ure prole analysis (TPA)profile analysis utilizes the universal testinghich mimics chewing through the use of largeshears, and the hardness, springiness, and

    ature of the food are calculated (Van Hekken). However, these tests can also be regarded asecause there are no corrections for changese of the specimen. TPA is useful for makings, while it does not measure true rheological

    (Tunick and Van Hekken, 2002).carried out by dropping a crosshead downolumn, which causes a flat plate to deform

    al sample specimen placed on a lower plate000). The crosshead of the universal testingturns at the same rate and repeats the proce-ly mimicking the action of biting twice on aeese. A TPA curve with force versus time forcheese is shown in Fig. 3 (Tunick, 2000).

  • 78 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    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 aa semisolidhas no spri

    3.3. Funda

    Fundamdynamic anshear analythe two repimens useddeformedogist to an

    Fig. 4. Forcestrain 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

    gineet). An

    orcesl testin infldow

    compr.

    . SmaSA)is a

    h use

    Table 1Definitions an ick, 20

    Term from

    Fracturability

    Hardness

    Adhesiveness

    Cohesiveness

    Springiness

    Gumminess

    Chewinessnd gumminess is for a semisolid food, wherefood undergoes permanent deformation and

    nginess.

    mental tests

    ental tests on cheese include compression,d transient tests. Small amplitude oscillatorysis (SAOSA) and torsion analysis (TA) areresentative fundamental tests. Cheese spec-in these tests are of a specific shape, and

    in a specific manner, which allows a rheol-alyze the results systematically (Tunick and

    or en

    heighis a fversa

    that abreak60%occu

    3.3.1(SAO

    Itwhic

    d calculations of texture terms (Szczesniak, 1963; Bourne, 1968; TunDefinition ObtainedForce with which food fractures Force at first signcurve

    Force needed to attain a given deformation Maximum force dcycle

    Work needed to overcome attractive forcebetween food and other surface

    Force area of negfollowing first pe

    Strength of foods internal bonds Ratio of positivethat of first peak

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

    Height specimenfirst compression

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

    Product of hardne

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

    Product of hardnespringiness (J)ring strain (ratio of height change to originalexample diagram is shown in Fig. 4, which

    train curve for Mozzarella obtained by a uni-ng machine. Tunick (2000) further delineatedection point appears if the structure begins ton, and the cheese sample is to be fractured atression as the breakage of the internal bonds

    ll amplitude oscillatory shear analysis

    fundamental test for rheological properties,s specific specimen geometries and instru-

    00)Calculation (and

    units) using Fig. 3

    ificant break (if any) in Height of F (N)

    uring first compression Height of H (N)

    ative peak (if any)ak

    Area of A (J)

    force area of second peak to Ratio of A2 area toA1 area (none)

    recovers between end ofcycle and start of second

    Length of S (mm)

    ss and cohesiveness (N)

    ss, cohesiveness, and

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 79

    ments, allowing systematic analysis of the results(Tunick and Van Hekken, 2002). SAOSA stretchesinternal bonds using small-strain oscillatory motionand measures the viscoelastic characteristics (elasticand viscoucurd (Vanand strainthe sampleand Van Hviscoelastiloss modul2000). Themeasure dharmonica

    3.3.2. TorsTorsion

    damental tein a viscombeing meaHekken, 20are milledtakes placethese condcated thattension moin differenattention innique havein sampleHekken, 2ological prvaluable inken or fracbite their c

    3.4. Cheesand sensor

    In a comture of varithe highestwere betwestrain and T

    In anothdar, Colbycow cheesthat there wsion shear sshear strainSAOSA dision data c

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

    4. Recent research on rheological characteristicsat an

    Rheol

    nick (impo

    esearc

    rence

    e specbution, deteren et

    nd ontype oositioetry,

    ts mayral chets rheoimitsith soup ofino a

    olytic; Awailks a

    . Rhea stud

    ses, Paal proweek

    ses apppringinincreases alsstorag

    me sond motrain

    ses Aighervisco

    ional de preshe youelasti

    eks ofs moduli, complex viscosity) of the cheeseHekken et al., 2004). In SAOSA tests, stressare linearly dependent on one another anddoes not fracture or change shape (Tunick

    ekken, 2002). SAOSA tests provide data forc properties including storage modulus (G),us (G), and complex viscosity (*) (Tunick,se tests are called dynamic tests because theyynamically, varying either stress or strainlly with time.

    ion analysis (TA)analysis or torsion gelometry is another fun-st of rheology. In TA, specimens are twistedeter with the shear stress and shear strain

    sured at the fracture point (Tunick and Van02). For cheese analysis, cheese specimensinto a capstan shape so that the fractureat the narrow center of the specimen. Under

    itions, Hamann and Foegeding (1994) indi-fracture can occur in compression, shear, orde, which are imposed at equal magnitudest directions. TA has recently received moreresearch because improvements in this tech-reduced the difficulties previously involvedpreparation and analysis (Tunick and Van

    002). Since TA measures fundamental rhe-operties at the fracture point, it would beanalyzing cheese samples as they are bro-

    tured when scientists and consumers cut orheeses.

    e rheology studies using TPA, SAOSA, TAy scores

    parison study of TA, TPA and sensory tex-ous gels, Montejano et al. (1986) found thatcorrelations among instrumental parametersen shear stress and TPA hardness, and shearPA cohesiveness.er study with fresh and aged Brick, Ched-

    , Gouda, Havarti, Mozzarella, and Romanoes, Tunick and Van Hekken (2002) showed

    ere strong correlations (>0.8) between tor-tress and TPA hardness, and between torsionand TPA cohesiveness. They also found that

    d not correlate with TA nor TPA, while tor-ould be used to draw a map depicting trends

    of go

    4.1.

    Tuis ascal r(Lawto thdistristepsHekkdepetion,compgeomresulNatuand itain l

    Wmakein amprote1997the m

    4.1.1In

    cheelogicand 4cheethe sThecheewithbecaues, a

    and scheeThe hmore

    positin ththat tmore

    4 wed sheep milk products

    ogical characteristics of cheeses

    2000) pointed out that the texture of a cheesertant as its flavor, and most of the rheologi-h on solid dairy foods has dealt with cheeseet al., 1987). The texture is closely relatedific proteins in the cheese. The quantity andof the caseins, as well as the manufacturing

    mine the structure of the cheese matrix (Vanal., 2004). Attributes such as fracture willfactors including time and rate of deforma-f deformation used (tension or shear), cheesen, temperature, and repeatability of specimenwhereby the interpretation of all rheologicalneed a degree of caution (Olson et al., 1996).ese is not entirely isotropic or homogeneous,logical behavior is nonlinear outside of cer-

    (Tunick, 2000).me minor genetic variations, the chemicalcaseins in caprine and bovine milks is similarcid sequence and structure, and responds toenzymes in similar manners (Trujillo et al.,

    d et al., 1998), whereas the ratio of caseins inre not the same.

    ology of goat milk cheesesy with young Monterey Jack (MJ) goat milkrk et al. (2000) characterized various rheo-

    perties of the MJ cheeses stored at 4 C for 1s, and observed that the curds of the goat milkeared to be knitting with time, as reflected iness, G as well as shear strain results (Fig. 5).

    sed meltability and viscosity values of theo occurred in the elevated G, and * datae time (Fig. 5). As the cheeses aged, they

    fter as seen by hardness and shear stress val-re rubbery as seen when plotting shear stress

    on a texture map. The moisture contents ofand B were 43.4% and 45.8%, respectively.moisture cheese B was a softer, springier, andus cheese than cheese A, probably due to com-ifferences stemming from problems involvedsing step of manufacture. It was concludedng goat milk Monterey Jack cheeses became

    c, cohesive, meltable, viscous and softer afteraging presumably due to proteolysis.

  • 80 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    Fig. 5. Combetween 1 wemilk cheesesA and B; (b) sof young MJ

    Van Hestudy on rhterey Jackand 4 weekeral rheolodynamic aobserved fcheeses ovtic propertover the ficonstant foparison of meltability, springness and cohesivenessek and 4 weeks aged young Monterey Jack (MJ) goat(Park et al., 2000). (a) Meltability of young MJ cheesespringness of young MJ cheeses A and B; (c) cohesivenesscheeses A and B.

    kken et al. (2004) conducted an extensiveeological and proteolytic properties of Mon-goat milk cheese stored for 6 month frozens refrigeration conditions, and reported sev-gical characteristics including small-strain

    nalysis results (Fig. 6). Similar trends wereor all three indices of G, G, and * of theer the 26 weeks of storage. Also, viscoelas-ies of the MJ cheese significantly increasedrst 48 weeks of storage and then remainedr the rest of 26 weeks of storage. The G,

    Fig. 6. SAOSG; (b) viscoumilk Montere

    which meaties of the cto 35.1 kPasures the enincreased sfirst 4 weekships (G/to 3.91 kPacorrelation(Table 2) apropertiesof peptidesr values otion was eA analysis on viscoelastic properties [(a) elastic modulus,s modulus, G; and (c) complex viscosity, *] of goaty Jack cheese (Van Hekken et al., 2004).

    sures the energy stored or the elastic proper-heeses, increased significantly from 13.8 kPaover the first 8 weeks. The G, which mea-ergy lost or the flow properties of the cheeses,ignificantly from 4.17 kPa to 12.7 kPa over thes. The *, which measures the phase relation-G), increased significantly from 1.44 kPa ss over the first 8 weeks. There was a highbetween the SAOSA and proteolysis data

    s the elastic (G) and complex viscosity (*)of the cheese increased. The concentrationin the 2218 kDa range also increased with

    f 0.92 and 0.90, respectively. This correla-xpected as proteolysis of caseins disrupts

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 81

    Table 2Summary of correlation factors (P < 0.05) among proteins and peptide concentrations and rheological properties of Monterey Jack goat cheeseRheological properties Proteins Peptides (kDa) Protein:peptide

    SAOSAElastic modViscous moComplex v

    TPAHardnessSpringinesCohesivene

    TAShear stresShear strainShear rigid

    Van Hekken e : textur

    the proteinflexible.

    In the sfrom the tness of theto 26.7 N onificant difand 26 weecheese incover the firsignificantl1 week ofness and wof cheese ahardness ofs2-CN (0.tides in the

    For torsVan Hekkeat point ofat the poincantly decrdid not chaBetween w28.4 kPa to29.4 kPa tosignificantlweeks of ssignificantlnoted thatrelated witnegatively(Table 2).

    a rherequirst anie, 20red tod coms ripewere

    tructuinterceneede

    oncernekke

    ge onial soobser

    xture.s2-CN -CN 2218

    ulus, G 0.84 0.81 0.92dulus, G 0.68 0.65 0.82

    iscosity, * 0.81 0.78 0.90

    0.99 0.97 0.96s 0.79 0.91 0.70ss 0.83 0.80 0.85

    s 0.89 0.92 0.860.67 0.64 0.83

    ity 0.87 0.88 0.90t al. (2004); SAOSA: small amplitude oscillatory shear analysis; TPA

    bonds and the cheese matrix becomes more

    ame study, Van Hekken et al. (2004) foundexture profile analysis (TPA) that the hard-

    cheese decreased significantly from 43.4 Nver the first 8 weeks of storage, while no sig-ferences (P < 0.05) were observed between 8ks of storage. The springiness of the MJ goatreased significantly from 7.8 mm to 9.2 mmst 4 weeks of storage and then did not changey over the next 5 months of storage. After onlystorage, the cheese had the lowest cohesive-as significantly lower than the cohesivenessged for 16 weeks. They also observed thatthe cheese was correlated with -CN (0.97),

    99), and negatively correlated with the pep-2218 kDa range (0.91) (Table 2).

    Inforcethe fi(Attarequisecon

    versu

    sionslost sTheforce

    CVan Hstoramerc

    Theyile teion analysis of the same MJ cheese (Fig. 7),n et al. (2004) showed that the shear stressfracture and the shear rigidity (stress/strain)t of fracture had similar trends and signifi-eased over the first 8 weeks of storage butnge significantly over the rest of the study.eeks 1 and 8, shear stress decreased from18.3 kPa and shear rigidity decreased from14.9 kPa. The shear strain at point of fracturey increased from 0.97 to 1.20 over the first 4torage, peaked at 1.28 at 16 weeks, and theny decreased to 1.14 at week 26. They alsoshear stress at the point of fracture was cor-h -CN (0.92) and shear rigidity correlatedto the large peptides at 2218 kDa (0.90)

    20 C forhad signifities relativefrozen chealso postultals in thethe caseinstexture. Hosmall, softby frozen-squality.

    Using tproduced isensory chfrozen chetextural pr1815

  • 82 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    Fig. 7. Torsioand (c) shearcheese (Van Hof the senage treatmeration agiever, prololess of frozchanges indiacetyl, mness, saltinacross all tof the frozecompared t2005).4.1.2. Rhe

    Terrinchnorth-easte

    . Changes in moisture and water activity (aw) of Terrincho sheepheese (Pinho et al., 2004).

    raw Churra da Terra Quente ewe milk. Pinho et004) evaluated the changes in chemical parameterssture, acidity, pH, and water activity) and physical

    eters (color and texture) of Terrincho sheep milkse durFig. 8milk c

    fromal. (2(moiparamcheen analysis properties [(a) shear stress, (b) shear strain,rigidity at point of fracture] of goat milk Monterey Jackekken et al., 2004).

    sory scores in the cheeses among the stor-ent groups at the initial stages (0 day refrig-ng after thaw) were not significant. How-nged refrigerated storage at 4 C, regard-en-storage treatment, caused some (P < 0.05)most flavor scores including cooked/milky,ilkfat flavors, brothy, waxy, sweetness, sour-ess, freshness, yeasty, and oxidized flavorsreatment groups. The rheological propertiesn goat cheeses had significantly lower valueso the fresh control cheese (Van Hekken et al.,

    ology of sheep milk cheeseso sheep cheese is a typical product of thern region of Portugal and is manufactured

    correlationture and coproducts.

    During tobserved aness, chewmilk cheesadhesivenenal e), an

    Pinho etactivity weripening (Faffect protesheep cheein pH up tdays and 6important msheep cheesuggested

    Fig. 9. Changrincho sheeping 60 days ripening, and also determineds between the changes in instrumental tex-lor parameters, and the ripening time of the

    he first 20 days of ripening, Pinho et al. (2004)n increase in hardness, fracturability, gummi-iness, and yellowness of the Terrincho sheepe, while there were concomitant decreases inss, resilience, L* (inside cheese, i and exter-d cohesiveness in the sheep cheese.al. (2004) also found that moisture and waterre significantly decreased during 60 days ofig. 8), where these parameters would greatlyolytic activity in cheese. The ripening of these caused increases in acidity, and decreaseso 30 days, but slight increases between 300 days. These pH changes may indicate theetabolic activity of lactic acid bacteria in the

    se as shown in Fig. 9. Lawrence et al. (1987)that the decrease in pH would be related toes in acidity and pH during 60 days of ripening in Ter-milk cheese (Pinho et al., 2004).

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 83

    Fig. 10. Cateship betweenparameters ancolor intensitplants (Pinho

    texture parwould occu

    Pinho etcho sheepsensory attto instrumewere ablefrom B anddairy plantsimilar chedemonstratfracturabiliwhen presslated withand TPA sadhesivene

    Feta chcheeses thranean, andIt is a semacid taste (proportionporous appmilk (Tsigcheese wascentration fpure ovinecaprine mithan that pal., 2003).casein struovine milk

    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. It

    een shown that high acidity, protein, and total solidsnts generally make the cheese harder and less eas-formed (Creamer and Olson, 1982; Kehagias et al.,).

    . Rheology of cow milk cheesesxtural characteristics of representative cow cheeseties were tested in sensory evaluation by Chen et al.9). A sensory panel determined: (1) hardness as the

    required to penetrate the cheese sample with ther teeth, (2) cohesiveness as the degree to which these sample deforms before rupturing, (3) adhesive-as the force required to remove the cheese sampledheres to the mouth surface, and (4) chewiness as

    me required to masticate the cheese sample at a con-rate of force application to reduce it to a consistencyble for swallowing.he range of hardness was based on the amount of

    required to rupture the sample between the molarfor Parmesan cheese as 13 and cream cheese as 1, usedown i

    d by Eses, anrdnessach ofses waLineagorical principal component biplot showing the relation-mean values from texture profile analysis and color

    d mean sensory scores from four texture attributes andy of sheep milk cheeses from B, V, R, T, and M dairyet al., 2004).

    ameters because no fusion of curd particlesr until pH value of approximately 5.8.al. (2004) noted that the grouping of Terrin-

    cheese according to TPA analysis, color andributes, was similar to the grouping accordingntal analysis (Fig. 10). The sensory paneliststo distinguish differences between cheeses

    M dairy plants and cheeses from the others (R, T, and V), in which the three cheeses hadmical and sensory characteristics. Fig. 10 alsoes that color intensity, hardness on the mouth,ty when cutting with a knife, and elasticity,ing with the fingers, were positively corre-b*(e), b(i), TPA hardness, TPA fracturability,pringiness. The authors found only sensory

    has bconteily de1995

    4.1.3Te

    varie(197forcemolacheeness

    that athe tistantsuita

    TforceteethscaleAs shlowecheein ha

    Echeetipless was not correlated with TPA adhesiveness.eese belongs to the family of white-brinedat are indigenous to Greece, the Mediter-

    Middle East regions (Tsigkros et al., 2003).ihard, crumbly variety with a salty, slightlyAnifantakis, 1991). Feta cheese with a higherof caprine milk had a more compact and lessearance than feta produced from purely ovinekros et al., 2003). The hardness of the fetaincreased with increasing caprine milk con-rom a force of 2.453.65 N over a range fromcheese to 30% caprine milk addition. Pure

    lk gave a harder cheese with a stronger flavorroduced using pure ovine milk (Tsigkros etThese differences might be due to differentctures or concentrations in the milks, wherecontains higher levels of casein than caprine Fig. 1as the reference standards (Chen et al., 1979).n Fig. 11, Parmesan cheese was greatest, fol-dam, Gouda, Swiss, Cheddar and Mozzarellad the processed Cheddar cheese was the least.the textural measurements for the 11 cow

    s correlated with composition and pH by Mul-r Regression Analysis (Chen et al., 1979). For

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

  • 84 Y.W. Park / Small Ruminant Research 68 (2007) 7387

    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 0.045Constant 3.25 3.01 5.3 3.23 44.0 0.103Correlation coefficient 0.923 0.886 0.977 0.965 0.953 0.873

    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 betwslowly asmation pri

    Fig. 1

    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.Gwartneyto determi

    and fcheeseeach

    riptorseen the molar teeth, and the force was exertedthe sample deformed. The degree of defor-or to rupturing as determined by comparing

    low-can

    were

    desc2. Elasticity of cow cheeses (Chen et al., 1979). Fig. 13.et al. (2002) conducted torsion gelometryne texture scores from a sensory panel forull-fat Monterey Jack, Cheddar and Ameri-s, and found that fracture stress and strainsignificantly correlated with several sensory.Cohesiveness of cow cheeses (Chen et al., 1979).

  • Y.W. Park / Small Ruminant Research 68 (2007) 7387 85

    4.2. Rheological characteristics of yogurts

    The rheology of yogurt can be measured or charac-terized by viscometer, curd strength devices, penetrom-eters, and ta fixed vanyogurt is pexopolysacincubation(Cerning, 1

    4.2.1. RheThe effe

    cal propertcow and cal. (2001).cosity wascow and caity stages wexpressionmilk showegelation.

    The millactic acidcalcium anmicelles, cagates and ftexture orwhich deteproduct. Adfor the supa rotationatechnique htool in thephases of mFrank, 199

    Hassancurd structa three-stagchanges inmum incredue to contand the syn

    Jumah eyogurt curdimum viscmilks (Figity decreasinner cylinshear-thinnshowed ththinning m

    4. Effeof yog

    -para

    mn

    e iscy co

    . Rhethe M

    is higroundfood tl elem, 2004h is defined as a semisolid food derived from yogurtraining away part of its water and water-solubleounds (Lebanese Standards, 1965). There are twoof labneh containing 22 wt.% and 40 wt.% solid

    entrations, where the 22% one is manufactured tonsumed within 2 weeks, which usually stored inerators, while the 40% product (labneh anbaris)red in vegetable oil at room temperature and con-d within 2 years (Keceli et al., 1999). Labneh madesheep milk is less popular and produced in muchmounts than cow labneh. The cow milk availability

    t the only issue but also the organoleptic acceptancew labneh is higher due to the sharp flavor of sheeph (Mohameed et al., 2004).

    valuating rheological properties of labneh, Ozer et998) found that it is a weak viscoelastic gel whereGreater thanG. Kelly and ODonnell (1998) studied, which is an acid-precipitated product containinghe vane method (rotating the sample arounde) (Tunick, 2000). The rheology of stirred

    artially attributable to the production of ropycharides by specific culture strains during

    , which prevents gel fracture and syneresis995).

    ology of goat milk yogurtct of the gelation process on the rheologi-ies of yogurt curd made from sheep, goat,amel milks was investigated by Jumah etThey found that the highest value for vis-exhibited by sheep milk, followed by goat,mel milks. Three different transient viscos-ere identified and described by mathematical

    s for cow, sheep and goat milk, whereas cameld no significant variation in viscosity during

    k curd is formed with the acid produced bybacteria, which is a consequence of removingd neutralizing the negative charges of caseinusing destabilization of casein, which aggre-orms a curd (McMahon et al., 1984). Curd

    firmness is an important property of yogurt,rmines the quality and acceptability of theequate firmness without syneresis is essential

    erior quality of yogurt (Kroger, 1973). Usingl viscometer, the yogurt curd profile analysisas become a valuable and extensively used

    research of the enzymatic and nonenzymaticilk coagulation (Berridge, 1952; Gassem and

    1).et al. (1995) postulated formation of yogurture during acid gelation of milk may occur ine process: (a) induction period without anyviscosity, (b) flocculation stage with maxi-

    ases in viscosity, and (c) decrease in viscosityraction with rearrangement of casein micelleseresis of gel.t al. (2001) determined the flow curves ofduring the gelation process and at the max-

    osity value for sheep, goat, cow and camel. 14). They found that the measured viscos-ed with increasing angular velocity of theder, suggesting that the yogurt behaved as aing non-Newtonian fluid. Jumah et al. (2001)

    at the rheological flow properties of shear-aterials can be most commonly described by

    Fig. 1curves

    a two

    =whersisten

    4.2.2In

    neh)yeartarytionaet al.whicby dcomptypesconc

    be corefrigis stosume

    fromless ais noof colabne

    Eal. (1was gquargct of milk source (bovine, ovine and caprine) on flowurt curds (Jumah et al., 2001).

    meter power law model of the form as:

    the shear stress, the shear rate, m the con-efficient, and n is the flow behavior index.

    ology of sheep milk yogurtiddle East region, concentrated yogurt (lab-

    hly appreciated and consumed with bread all. Labneh has been an important supplemen-o that regions diet and provides vital nutri-ents for growth and good health (Mohameed). Labneh is a concentrated sheep milk yogurt,

  • 86 Y.W. Park / Small Ruminant Research 68 (2007) 7387

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

    Mohameed et al. (2004) investigated the effect ofsolids concmilk labnesheep labnited shear-tlaws modeshear ratecoefficientas a functio

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    Rheological characteristics of goat and sheep milkIntroductionRheology of goat milk and sheep milkDimensions of viscosityEffect of temperature and incubation time on milk viscosityEffect of pH on milk viscosityEffect of chemical characteristics on rheology of goat and sheep milk

    Rheological methods as a means of evaluating functionality of dairy foodsEmpirical testsImitative testsTexture profile analysis (TPA)

    Fundamental testsSmall amplitude oscillatory shear analysis (SAOSA)Torsion analysis (TA)

    Cheese rheology studies using TPA, SAOSA, TA and sensory scores

    Recent research on rheological characteristics of goat and sheep milk productsRheological characteristics of cheesesRheology of goat milk cheesesRheology of sheep milk cheesesRheology of cow milk cheeses

    Rheological characteristics of yogurtsRheology of goat milk yogurtRheology of sheep milk yogurt

    References