Small Ruminant Rearch68(2007)
紧张的英文
207–220
Heat stability and enzymatic modifications of goat and sheep milkଝK.Raynal-Ljutovac a,Y.W.Park b,F.Gaucheron c,S.Bouhallab c,∗
a Institut Technique des Produits Laitiers Caprins,Avenue F.Mitterand,BP49,17700Surg`e res,France
b Georgia Small Ruminant Rearch and Extension Center,Fort Valley State University,Fort Valley,GA31030-4313,USA
c UMR1253Science et Technologie du Lait et de l’Œuf,Inra-Agrocampus Rennes,65rue de St-Brieuc,35042Rennes Cedex,France
Available online30October2006
Abstract
Heat treatments are important process that affect the quality of milk and especially the physico-chemical and renneting properties. Goat and sheep milk are less stable to heat treatments than cow milk.Although heat-induced reactions are similar between animal species,the changes relative to tem
perature differences are due to differences in micellar structure,partition of salts between colloidal and aqueous phas,and protein interactions.Goat milk contains less protein and cains but also less calcium and phosphate,approximately1200and900mg/kg,respectively,against2200and1300mg/kg for sheep milk,which affects their micellar systems noticeably.Caprine and ovine milk are also characterid by a lower colloidal stability than bovine milk.Change in pH,addition of salts(phosphate or citrate),or u of membrane ,change of whey protein to cain ratio)can decrea problems of heat instability.In addition to the instability of the milk itlf due to heat treatments,milk enzymes are also inactivated by thermal processing,which can be ud as a barometer of the effectiveness and for meeting legal requirements of, e.g.,milk pasteurization by testing activities ,alkaline phosphata(AP).However,there are considerable variations in AP contents between species,breed within species,and individual animals.The ranges of AP concentrations in goat,sheep and cow milk are115–1300,8300–17,300,and1800–4750g phenol/ml,respectively,and are not related to the fat content in milk apparently, but rather to species uniqueness.However,in the ca of incomplete inactivation of enzymes,proteolysis and lipolysis can occur and affect the quality of milk and dairy products.
©2006Elvier B.V.All rights rerved.
Keywords:Heat stability;Caprine milk;Ovine milk;Thermal processing;Enzymes;Minerals;Proteins;Proteolysis;Lipolysis
1.Introduction
six n
The thermal process is designed to eliminate poten-tial pathogenic and spoilage microorganisms from raw milk with minimal chemical,physical and organolep-tic changes in the milk.The higher the total bacterial counts,the greater the risk of dia outbreak or pro-
ଝThis 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.
∗Corresponding author.Tel.:+33223485742;
fax:+33223485350.
E-mail address:said.bouhallab@rennes.inra.fr(S.Bouhallab).cess impairment.Thus,many ways of heat treatments exist and are applied to milk processing according to its different transformations(Table1).The short-time treatment is usually ud in most industrial applications becau it is enough to remove harmful microorganisms and inactivate enzymes.
The ultra high temperature(UHT)process leads to organoleptic changes in milk without changing the nutri-tional value significantly.Sterilization is ud only,when filled products must be prerved for a period longer than 5months.Pasteurization is defined as:
“a process in which every particle of milk shall have been heated in properly operated equipment to不胜负荷
62.8◦C(145◦F)for30min(vat pasteurization)or
0921-4488/$–e front matter©2006Elvier B.V.All rights rerved. doi:10.1016/j.smallrumres.2006.09.006
208K.Raynal-Ljutovac et al./Small Ruminant Rearch 68(2007)207–220
Table 1
Objectives of the major heat treatments applied in dairy industry (Leur and Melik,1990)Treatments
下车的英文
Objectives
Time/temperature Thermization Destruction of psychrotrophs
15–20s;63–65◦C Pasteurization
Destruction of Mycobacterium tuberculosis 15–30s;75–85◦C High quality pasteurization Destruction of Mycobacterium tuberculosis
15–30s;72–75◦C Sterilization
Destruction of enzymes and pathogens (including spores)20min;100–120◦C UHT sterilization
Destruction of enzymes and pathogens (including spores)
2–3s;135–150◦C
71.7◦C (161◦F)for 15s (HTST pasteurization)and held continuously at or above that temperature for the specified time”(USFDA,2002).
The temperature and time combination methods are also ud for goat and sheep milk thermal treatments.Sheep milk can be effectively pasteurized by either the vat or HTST pasteurization process (Haenlein and Wendorff,2006).Sensory analysis of milk indicated that there was no significant flavour difference between the HTST pasteurized milk and raw milk,but vat pasteurized milk was sometimes described as “muttony”(Young,1986).Pasteurization can be applied before UHT treat-ment to prevent enzymatic degradation,which may occur during storage of this type of long shelf-life milk.The indirect process is the most frequent UHT treatment.However,the direct process,by steam infusion and injec-tion is also often applied.The effectiveness of each treat-ment can be tested either by determining the inactivation of enzymes (Fig.1),degree of whey protein denaturation,or occurrence of products such as furosine,
lactulo,
Fig.1.Rate of denaturation of an enzyme at various temperatures.The data are calculated for E a of 60,000cal/mol,where the first-order rate constant of denaturation k ,is 0.005,0.020,0.090,0.395,and 1.80min −1at 40,45,50,55,and 60◦C,respectively (calo-ries ×4.186=joules)(Whitaker,1994).
hydroxymethylfurfural for the most vere treatments.The primary test enzyme is alkaline phosphata,which has to be totally inactivated,if pasteurization was cor-rectly performed.An actual application of a lected process would be dependent mainly on the kinds of milk products as well as their legal requirements.In order to stabilize milk quality during longer storage time and extend the shelf-life of fresh milk,thermization is applied primarily as heat treatment of raw milk,inactivating psy-chrotrophic microorganisms (Le Jaouen,1989;Spreer,1998).The high heat process is ud for low quality raw milk,cream,chee milk,and milk concentrates for long shelf-life products.
Goat and sheep milk are very different in composi-tion,the contents of fat,protein,total solids being 4.0%versus 7.5%,3.5%versus 5.5%,and 13%versus 19%,respectively.Lacto content is about 5%for both milks.Concerning the protein fractions,goat milk contains less protein and cains than sheep milk.The major compo-nents of caprine and ovine cains are ␣s1-,␣s2-,-and -cains,like in cow milk.-Cain is,quantitatively,the major protein component of goat milk.Compared with ovine cains,caprine cains contain less ␣s -cain (␣s1-and ␣s2-)and more -and -cains.Goat
milk also has the highest proportion of non-protein nitrogen.Concerning minerals,goat milk contains approximately 1200and 900mg/kg of calcium and inorganic phosphate against 2200and 1300mg/kg for sheep milk,respec-tively (Mietton et al.,2004).As a conquence of the compositional differences,the micellar systems of the milk types differ noticeably from that of cow milk by v-eral properties,more specifically micelle composition,size,mineralization and hydration (Remeuf et al.,1989;Pellegrini et al.,1994).Furthermore,caprine and ovine milk are characterized by a lower colloidal stability than bovine milk.This is responsible for a different behaviour such as short clotting times during renneting coagulation (Remeuf et al.,1989;Pellegrini et al.,1994),low heat stability at high temperature (Zadow et al.,1983;Muir et al.,1993),and even more deposit formation during milder treatments such as pasteurization (De Raphael and Calvo,1996).
K.Raynal-Ljutovac et al./Small Ruminant Rearch68(2007)207–220209
It has been recognid for a long time,that heat treatment of cow milk modifies veral of its physico-chemical properties and impairs its rennetability,but goat and ewe milk has been studied far less than cow milk.There are some reasons,why results obtained with cow milk cannot be easily extrapolated to small ruminant counterparts.
This review provides an overview on the modifica-tions and destabilisations,that occur in goat and sheep milk after different heat treatments.Owing to the abun-dance of cow milk literature in this premi,references to cow milk processing have also been utilized.
2.Factors affecting milk stability
2.1.Composition
Heat stability(HS)of milk measured during1min of heat coagulation time(HCT)at140◦C depends on the pH of the specific species.Nevertheless,small ruminant milks are frequently not stable at their natural pH as is shown in Table2.
Concerning goat milk,a European study showed that heat stability of French and Portugue bulk goat milks was the highest at125–133◦C(1min)with pH 6.59–6.75,respectively.The low stability of Greek milk at92–110◦C(1min),associated with lower pH val-ues(6.51–6.61,respectively),could also be related to the physico-chemical composition(especially protein concentration),and to microbiological characteristics (Morgan et al.,2003).Moreover,for caprine milk,a great variability in heat stability has been noted(Fox and Hoynes,1976).Different patterns of heat stability versus pH were obrved between heat-stable and heat-unstable samples.Heat-stable milk samples had a max加班英语
imum sta-bility at pH clo to their natural ,pH6.8),while heat-unstable milk samples had a maximum stability at higher values(pH7–7.1)(Fig.2).The HS/pH
profiles Fig.2.Heat stability/pH profiles for two heat-stable individual caprine milk samples(solid lines)
and two heat-unstable individual caprine milk samples(broken lines).The natural pH is indicated by the clod symbol.The cross indicate that milk samples were coagu-lated(Morgan et al.,2000).
are similar to tho obrved by Tziboula(1997)con-cerning stable and unstable goat milks.
In another study,heat stability of71individual goat milks from a same herd(Poitou-Charentes,France)was evaluated.Half of the242milk samples were coagulated after1min at a temperature of135◦C.Analys of the physico-chemical composition of the samples revealed that,compared to unstable milks,heat-stable milk sam-ples were characterized by a higher pH(6.69versus 6.58),a lower soluble calcium concentration(0.37g/kg versus0.44g/kg),a higher phosphorus concentration (0.96g/kg versus0.81g/kg),and a lower total Ca/total P ratio(1.37versus1.75)(Morgan et al.,2000).The findings were confirmed in other reports(Zadow et al., 1983;Montilla and Calvo,1997).
All factors contributing to incread calcium activ-ity may thus decrea heat stability.For instance,the cold storage generally applied in dairy industry incread both-cain dissociation and colloidal calcium phos-phate solubilization,increasing thus calcium activity.
Table2伊利诺伊理工大学
Heat stability measured at140◦C for cow,ewe and goat milk(bulk milk and different␣s1-cain variants)
Natural milk pH Maximal stability References
apoloCow12–22min at pH6.65–6.730min at pH7.1–7.2Gallager and Mulvihill(1997) 15–18min at pH6.65–6.733min at pH7.1Rattray and Jelen(1997) Goat1min at pH6.6 6.8min at pH7.2Muir and Sweetsur(1978) 0–3s(at pH<6.9)38min at pH6.9(herd milk)Zadow et al.(1983)
2min at pH6.618min at pH6.75(AE and BE␣s1variants)Tziboula(1997)
45min at pH6.9(FF variants)
Ewe1min at pH6.39(September)18min at pH6.78(June)Muir et al.(1993) 10min at pH6.5(March)
210K.Raynal-Ljutovac et al./Small Ruminant Rearch68(2007)207–220
It has been shown that a72h−4◦C storage of goat milk decread dramatically its heat stability(Raynal-Ljutovac et al.,2004;Bouhallab and Raynal-Ljutovac, 2005).
A positive correlation was determined between heat stability of goat milk and cain content(Manfredi et al., 2002).In contrast,the same group found that the heat sta-bility ems to be negatively correlated with somatic cell counts of the milk as previously obrved for cow milk (Guthy,1979).Muir and Sweetsur(1978)and Tziboula (1997)reported a positive impact of urea content on the heat stability of goat milk,and Mukherjee et al.(1993) also found a positive relationship between non-protein nitrogen content and goat milk heat stability.
Owing to its high content in minerals and cains, sheep milk has a low natural pH compared to goat and cow milk.Maximum heat stability was in the pH range between6.73and6.84for sheep milk(Fox and Hoynes, 1976;Muir et al.,1993)compared to the higher pH value for goat milk(6.9)(Zadow et al.,1983).Muir et al.(1993) followed heat stability for a whole lactation period of ewes and obrved a decrea in both pH and heat sta-bility(measured at140◦C)from March to September with HS of about10min(at pH6.5)and1.0min(at pH 6.39),respectively.The same authors reported a nega-tive correlation between heat stability and non-nitrogen protein fraction for ovine milk.
2.2.Impact of genetic polymorphism
Concerning impact of relative heat treatments such as90◦C for5–10min on physico-chemical compos
i-tion and on renneting properties of goat milk,accord-ing to the␣s1-cain genotype,some differences were pointed out by Remeuf and Raynal(2000)on milk stan-dardized at pH6.5.Rate of whey protein denaturation emed to be higher for␣s1-cain AA milk,due to its higher protein concentration.Similarly,an impor-tant increa in micelle size(+75%)was obrved for this milk compared to FF milk(+30%)after10min at 90◦C.In parallel,micelle hydration decrea was more accentuated for A type milks,and the rennet coagulation time incread by30%,whatever the␣s1-cain geno-type was.Firmness of A type milk was more affected by such a treatment,which has to be connected with changes in micelle characteristics.
The␣s1-cain genotype,however,does not em to have any effect on the heat stability of goat milk(Remeuf, 1993;Tziboula et al.,1997;Morgan et al.,2000).The HS/pH profiles of AA and FF milks were found to be similar(Morgan et al.,2001).On the other hand,a most recent work on523milk samples from203goats showed a slightly higher heat stability for goat milk from high ␣s1-cain genotype goats(Manfredi et al.,2002),which disagrees with Tziboula(1997),who found higher heat stability for low genotype FF milk compared to“mild”genotype(AE/BE).Nevertheless,this latter study had been carried out only on12goats.Low type(FF)goat milk had a larger pH range(6.7–7.1)of heat stability at 140◦C than the mild variant(BE)type milk(6.75–6.85), and the latter being not stable at natural pH(pH6.7) contrary to the previous one.
牛的英文
jins
3.Methods for evaluating heat stability
3.1.Ramsdell test
In the dairy industry,addition of inorganic phosphate to milk from different species(cow,goat and buffalo) or differently procesd milk(raw,heated,concentrated and/or recombined milks)is a common practice for milk, that will be submitted to the UHT processing.It is also ud to follow the stability of UHT milk during storage. This test is named Ramsdell et al.(1931).Globally,it consists in adding different volumes of0.5M potassium dihydrophosphate(KH2PO4)solution to10ml milk.The pH of the milk is not readjusted.Then,milk is heated to 100◦C for10min and the operator notes the volume of phosphate solution,that is necessary to induce a desta-bilisation of milk.Stability of milk is in relation with the volume inducing destabilisation.If the volume is less ,45mM),milk is considered as potentially unstable and will not withstand UHT process.At the opposite,milk is considered as stable when the volume of phosphate inducing a milk destabilisation is more than 1ml.Despite this simplicity,the interpretation of the dif-ferent respons to this test is very difficult and this test does not em be ud for sheep milk.Three factors must be considered in this test:
(1)addition of phosphate,
(2)decrea in pH,and
(3)heat treatment of this milk.
3.2.Alcohol test
The alcohol(ethanol)test is widely ud as a simple and rapid indicator of milk freshness.It is also a practi-cal means of determining the propensity of milk for heat coagulation.It consists of mixing one volume of alco-hol solution at different percentage to one volume of milk.The respon of the test corresponds to the percent of alcohol solution,which induces a milk destabilisa-tion.Caprine skim milk exhibits markedly lower ethanol
K.Raynal-Ljutovac et al./Small Ruminant Rearch68(2007)207–220211
stability than bovine skim milk.Classically,goat milk precipitates upon addition of an equal volume of about 45%ethanol,whereas fresh cow milk precipitates at 70%ethanol.The differences in composition and micel-lar structure between goat and cow milk are probably responsible for the lower ethanol stability of goat milk and the instability that occurs when this milk is subjected to UHT processing.Horne and Parker(1982)indicated that the low ethanol stability of caprine milk as compar
ed with bovine milk is due to the different proportions of the individual cains,in particular a lack of␣s1-cain homologue in caprine milk.The higher ionic calcium content of goat milk compared to cow milk could be partly responsible for the greater instability of goat milk to alcohol test.Guo et al.(1998)suggested that the low ethanol stability of goat milk may be related to the ratio of sodium to potassium.
3.3.Direct capillary method
Another method to test heat stability is to measure the time of coagulation of milk at high temperature in a capillary.As goat milk is far from being always stable at 140◦C(classical temperature for measurement of heat coagulation time for cow milk),tests have been adapted from Fox(1982)for goat and sheep milk(Remeuf,1993). Milk samples(60l)are aled in glass-capillary tubes and heat treatment is performed in an oil bath at temper-atures ranging from120to150◦C for1min.The heat stability is defined as the maximum temperature within the range of120–150◦C,at which the sample is stable during a1-min treatment.
4.Heat-induced changes in milk
The modification of milk components varies accord-ing to the intensity of the applied treatments and
english interview
to the species(Raynal and Remeuf,2000).Main changes occur after mild heat treatments(possibly with following trans-formation)or ultra high temperature(UHT)treatments (forfluid milks).
4.1.Impact on pH
The main heat-induced changes that affect the acidity of milk as indicated by Jenness and Patton(1976)are: (1)loss of CO2caus a slight decrea in titer and
increa in pH;
(2)transfer of calcium and phosphate to the colloidal
state produces a slight increa in titer and decread
pH,where this change is slowly reverd after heat-ing;
(3)drastic heating caus the production of acids by the
degradation of lacto.
4.2.Impact on fat and lacto
Thermal processing has a minor effect on fat. Only the membrane of the fat globules with their heat-nsitive protein compounds undergo some modifi-cations,affecting the agglomeration of fat globules and their creaming(Spreer,1998).Longer residence times at temperature greater than100◦C can create complex compounds between cain and lacto by the Maillard reaction,which is a browning of such heat treated milk or bakery products.
4.3.Impact on whey proteins
Whey proteins are modified with increasing tempera-tures and residence times,where the solubility decreas to such an extent,that they coagulate with cain at a pH of4.6(Spreer,1998).Heat treatment of proteins results also in the liberation of SH groups,which increas the antioxidative properties of the milk and leads to the development of a cooked taste.
A complex is formed between-cain and-lactoglobulin during heat treatment(Dalgleish,1990). Unlike detailed studies on the formation and techno-logical conquences of heat-induced cain micelle/-lactoglobulin complex in bovine milk,there have been no investigations concerning the occurrence of such a complex between proteins in goat or sheep milk.In the ca of goat milk,Henry et al.(2002)reported identi-fication of a covalent complex between-lactoglobulin and purified cain
micelle.The authors ud a mul-tiple approach bad on ultracentrifugation of heated protein mixture,chromatographic fractionation,quen-tial enzyme digestion of disulfide-linked oligomers,and identification of disulfide-linked peptides by on-line liq-uid chromatography–electrospray ionization mass spec-trometry(LC–ESI/MS),and tandem MS.Three main covalent disulfide links were identified:
(1)the expected intermolecular bridges between-
lactoglobulin molecules;
(2)disulfide bond involving two-cain molecules;
and
(3)a disulfide bond between-lactoglobulin(Cys160)
and-cain(Cys88).
The results,obtained at the natural pH of caprine milk(6.7),demonstrated,as in the ca of cow milk,the
