牛奶的基本知識(shí)

1、第三篇 乳品工藝學(xué)第一章 乳的基礎(chǔ)知識(shí)1第一節(jié) 奶牛的基礎(chǔ)知識(shí)Milk production began 6 000 years ago or even earlier. The dairy animals of today have been developed from untamed animals which, through thousands of years, lived at different altitudes and latitudes exposed to natural and, many times, severe and extreme conditions.2T

2、able 1.1不同種類動(dòng)物乳汁的營(yíng)養(yǎng)組成表1中是不同種類動(dòng)物乳汁的營(yíng)養(yǎng)組成是平均數(shù)，影響因素還很多，包括：breed, feeding, climate, etc.3牛乳（Cow milk）乳是哺乳動(dòng)物生命之初唯一食物Milk is the only food of the young mammal during the first period of its life. 乳可以提供能量，并含有動(dòng)物生長(zhǎng)所需的物質(zhì)The substances in milk provide both energy and the building materials necessary for growth.

3、 4乳也含有抗體Milk also contains antibodies which protect the young mammal against infection.1頭犢牛大約需要1000L牛乳A calf needs about 1 000 litres of milk for growth, and that is the quantity which the primitive cow produces for each calf. 5 1頭奶牛1個(gè)泌乳周期大約分泌6000L牛乳，現(xiàn)在有些奶牛產(chǎn)奶量超過(guò)14000L。 Selective breeding has resulte

4、d in dairy cows which yield an average of more than 6 000 litres of milk per calf, i.e. six times as much as the primitive cow. Some cows can yield 14 000 litres or more. 6黃牛產(chǎn)乳量也可達(dá)600 kg左右乳用水牛則可達(dá)1000 kg2000 kg。 7小母牛7-8個(gè)月就可以達(dá)到性成熟，通常15-18個(gè)月才進(jìn)行配種。Before a cow can start to produce milk she must have cal

5、ved first. Heifers（小母牛） reach sexual maturity （性成熟）at the age of seven or eight months but are not usually bred until they are 15 18 months old. 8小母牛一般在2歲前進(jìn)行自然或人工配種 The heifer is bred (naturally or by insemination) before the age of 2 years. 奶牛的孕期為265-300天 The period of gestation is 265 300 days小母牛在

6、2-2.5歲產(chǎn)第一胎小牛 A heifer produces her first calf at the age of about 2 2.5 years.9奶牛的懷孕時(shí)間9個(gè)月 The gestation period is 9 months.奶牛產(chǎn)犢后泌乳期為10個(gè)月（通常按305天計(jì)算） After calving the cow gives milk for 10 months.奶牛產(chǎn)犢后1-2個(gè)月再進(jìn)行配種 1 2 months after calving the cow will again be bred. 奶牛一生大約產(chǎn)5胎后就宰殺。我國(guó)8-10胎 After having g

7、iven birth to some 5 calves, the cow is generally slaughtered.10Secretion of milk（乳的分泌）Milk is secreted in the cows udder （乳房）A sectional view of the udder is shown in Figure. 乳房主要由： 腺體組織約占75%80%， 結(jié)締和脂肪組織占20%25% 乳房?jī)?nèi)部又都是由乳腺泡、乳導(dǎo)管、乳池、乳頭管和間質(zhì)構(gòu)成。 11 整個(gè)乳房由 4 個(gè)相互獨(dú)立的乳區(qū)組成，它由乳房側(cè)韌帶和乳房懸韌帶將其隔開。 每個(gè)乳區(qū)都有獨(dú)立的分泌系統(tǒng)，互不相通

8、，每個(gè)乳區(qū)有一個(gè)乳頭。12 乳房由含有能夠產(chǎn)生乳的腺體The udder is composed of glandular（腺體） tissue which contains milk-producing cells. It is encased in muscular tissue, which gives cohesion to the body of the udder and protects it against injury from knocks and blows. The glandular tissue contains a very large number (about

9、 20 億) of tiny bladder scalled alveoli （腺胞）. 13每天大約有90 000 L血液流經(jīng)乳房Flow of blood through the udder approx. 90 000 l/day. 大約需要800 900 L血液方能產(chǎn)生1L乳Approx. 800 900 l of blood needed for formation of one litre of milk.14The lactation cycle（哺乳期） Secretion of milk in the cows udder begins shortly before calv

10、ing 奶牛產(chǎn)犢后連續(xù)300天的乳的分泌，這個(gè)期間叫哺乳期The cow then continues to give milk for about 300 days. This period is known as lactation. 產(chǎn)奶量在產(chǎn)犢后不斷上升，后又不斷下降。到300天時(shí)，產(chǎn)奶量比高峰時(shí)下降15-25%。因此開始停止榨乳，此時(shí)距下一個(gè)產(chǎn)犢有60天的時(shí)間，該階段為停乳期。 One to two months after calving the cow can be serviced again. During the lactation period milk producti

11、on decreases, and after approx. 300 days it may have dropped to some 15 25 % of its peak volume. At this stage milking is discontinued to give the cow a non-lactating period of up to 60 days prior to calving again. 15泌乳的發(fā)動(dòng) 母牛分娩時(shí)及分娩后，發(fā)育成熟的乳腺開始分泌乳汁的現(xiàn)象叫泌乳的發(fā)動(dòng)。 引起泌乳發(fā)動(dòng)的原因很多，目前一般認(rèn)為，分娩前后血液中激素濃度的變化是導(dǎo)致泌乳發(fā)動(dòng)的主要

12、誘因。妊娠期間，由于母體黃體分泌大量的孕激素，反饋抑制了垂體促乳素的分泌，并使乳腺對(duì)促乳素的敏感性有所降低；孕激素分泌量的增加會(huì)抑制雌激素對(duì)促乳素分泌的刺激作用，也引起促乳素分泌的減弱。而在妊娠后期，特別是臨近分娩時(shí)，由于在胎兒腎上腺糖皮質(zhì)激素的作用下，胎盤雌激素分泌增加，雌激素的分泌可刺激子宮內(nèi)膜PGF2的分泌，PGF2溶解黃體，使孕激素的水平迅速下降，減弱或解除了孕酮對(duì)促乳素分泌和釋放的抑制，相應(yīng)提高了乳腺對(duì)促乳素的敏感性。同時(shí)，雌激素也加速了促乳素的釋放，引起泌乳的發(fā)動(dòng)。在分娩當(dāng)天外周血液中促乳素的濃度可達(dá)到妊娠期的 25 倍, 此后也比妊娠期高 1 倍以上。分娩以后，雌激素水平隨之降低

13、。 16Colostrum（初乳）牛初乳是指乳牛產(chǎn)犢后34d內(nèi)所分泌的乳汁，乳汁色澤黃而濃稠，具有特殊的乳腥味和苦味。這些乳汁是母牛為了供給牛犢在新生環(huán)境下，低抗外來(lái)病毒及細(xì)菌而天然合成的。 The first milk that a cow produces after calving is called colostrum. 17初乳在組成和特性方面與常乳有很大區(qū)別，特別是乳清蛋白含量，因此初乳受熱很容易凝固It differs greatly from normal milk in composition and properties. One highly distinctive ch

14、aracteristic is the high content of whey proteins about 11% compared to about 0.65% in normal milk。This results in colostrum coagulating when heated. 18 A fairly large proportion of whey protein is immunoglobulins (Ig G, dominating in colostrum), which protect the calf from infection until its own i

15、mmunity system has been established. Colostrum has brownish-yellow colour, a peculiar smell and a rather salty taste. The content of catalase and peroxidase is high. Four to five days after calving the cow begins to produce milk of normal composition, which can be mixed with other milk.19牛初乳組成 根據(jù)大量研

16、究資料表明, 與常乳比較，牛初乳除了含有豐富的優(yōu)質(zhì)蛋白質(zhì)、維生素和礦物質(zhì)等營(yíng)養(yǎng)成分外，還富含免疫因子、生長(zhǎng)因子等活性功能組分。 2021(1) 免疫因子(Immune factor)：醫(yī)學(xué)和臨床研究表明，初乳中的免疫因子能高效地抵抗病毒、細(xì)菌、真菌及其它過(guò)敏原，中和毒素，調(diào)理身體免疫機(jī)能。 免疫球蛋白(抗體)： 乳鐵蛋白(Lactoferrin)： 乳過(guò)氧化酶(Lactoperoxidase)： 22乳過(guò)氧化酶(Lactoperoxidase)： 黃嘌呤氧化酶(Xanthine oxidase)： 富含脯氨酸的多肽(PRP)： 糖蛋白(Glycoprotein)： 溶菌酶(Lysozyme)：

17、 23生長(zhǎng)因子(Growth factor) 牛初乳中的各種生長(zhǎng)因子具有促進(jìn)組織正常生長(zhǎng)和加快傷口愈合的功效。主要包括：胰島素樣生長(zhǎng)因子(IGF-1)；神經(jīng)營(yíng)養(yǎng)生長(zhǎng)因子(NGF)；表皮生長(zhǎng)因子(EGF)；轉(zhuǎn)化生長(zhǎng)因子(TGF)；纖維細(xì)胞生長(zhǎng)因子(FGF)；促性腺激素-釋放激素(GnRH)及其締合肽(GAP)；生長(zhǎng)激素(GH)。 24Milking 排乳 乳汁生成后, 由腺泡上皮細(xì)胞分泌到腺泡腔, 當(dāng)腺泡腔和細(xì)小導(dǎo)管充盈時(shí), 通過(guò)腺泡腔周圍的肌上皮細(xì)胞和輸乳管的平滑肌反射性收縮, 將乳汁轉(zhuǎn)移到大乳導(dǎo)管和乳池內(nèi), 在哺乳或擠乳時(shí), 通過(guò)神經(jīng)激素的作用, 使乳汁從大乳導(dǎo)管和乳池中排出，這一過(guò)程稱為排

18、乳（或稱放乳）。25 排乳是一個(gè)復(fù)雜的反射過(guò)程, 由神經(jīng)一激素途徑調(diào)節(jié)。當(dāng)幼畜的吮乳或擠乳的刺激時(shí), 乳頭皮膚的神經(jīng)沖動(dòng)上行至下丘腦, 導(dǎo)致催產(chǎn)素分泌并經(jīng)垂體后葉釋放至血液中, 使腺泡和較小的乳導(dǎo)管周圍的肌上皮細(xì)胞收縮, 乳房?jī)?nèi)壓增加而迫使乳汁通過(guò)乳導(dǎo)管流入乳池。奶牛的排乳反射一般維持 4 min7min 后消失 A hormone called oxytocin must be released into the cows bloodstream in order to start the emptying of the udder. This hormone is secreted and

19、 stored in the pituitary gland. When the cow is prepared for milking by the correct stimuli, a signal is sent to the gland, which then releases its store of oxytocin into the blood stream. In the primitive cow the stimulus is provided by the calfs attempts to suck on the teat. The oxytocin is releas

20、ed when the cow feels the calf sucking. A modern dairy cow has no calf but is conditioned to react to other stimuli, i.e. to the sounds, smells and sensations associated with milking.26The oxytocin begins to take effect about one minute after preparation has begun and causes the muscle-like cells to

21、 compress the alveoli. This generates pressure in the udder and can be felt with the hand; it is known as the letdown reflex. The pressure forces the milk down into the teat cistern, from which it is sucked into the teat cup of a milking machine or pressed out by the fingers during hand milking. The

22、 effect of the letdown reflex gradually fades away as the oxytocin is diluted and decomposed in the bloodstream, disappearing after 5 8 minutes. Milking should therefore be completed within this period of time. If the milking procedure is prolonged in an attempt to “strip” the cow, this places an un

23、necessary strain upon the udder; the cow becomes irritated and may become difficult to milk.27Hand milking（人工榨乳） On many farms all over the world milking is still done by hand in the same way as it has been done for thousands of years. Cows are usually milked by the same people every day, and are qu

24、ickly stimulated to let down just by hearing the familiar sounds of the preparations for milking. Milking begins when the cow responds with the letdown reflex. The first lets of milk from the teats are rejected, as this milk often contains large amounts of bacteria. A careful, visual check of this f

25、irst milk enables the milker to detect changes that may indicate that the cow is ill.28Two diagonally opposed quarters are milked at a time: one hand presses the milk out of the teat cistern, after which the pressure is relaxed to allow more milk to run down into the teat from the cistern of the udd

26、er. At the same time milk is pressed out of the other teat, so that the two teats are milked alternately. When two quarters have been stripped this way, the milker then proceeds to milk the other two until the whole udder is empty. The milk is collected in pails and poured through a strainer, to rem

27、ove coarse impurities, into a churn holding 30 50 litres. The churns are then chilled and stored at low temperature to await transport to the dairy. Immersion or spray chillers are normally used for cooling.29Machine milking（機(jī)械榨乳）擠乳廳的形式 擠乳廳的形式比較多，如平面畜舍式、魚骨式、轉(zhuǎn)盤式等。301平面畜舍式擠乳廳擠乳欄位的排列與牛舍相似，奶牛從擠乳廳大門進(jìn)入廳內(nèi)的

28、擠乳欄里，由擠乳員套上擠乳器進(jìn)行擠乳。優(yōu)點(diǎn)是造價(jià)較低，缺點(diǎn)是擠乳員仍須彎腰操作，影響勞動(dòng)效率。這種擠乳廳一般只適于投資少并利用現(xiàn)有房舍改造成擠乳廳的牛場(chǎng)或小型奶牛場(chǎng)。312魚骨式擠乳廳 由于擠乳臺(tái)兩排擠乳機(jī)及被擠牛的排列形狀如魚骨狀而得名，其擠乳臺(tái)欄位一般按傾斜 30 設(shè)計(jì)，這樣就使得牛的乳房部位更接近擠乳員，有利于擠乳操作，減少走動(dòng)距離，提高勞動(dòng)效率，因此在奶牛場(chǎng)使用比較普遍。一般適于中等規(guī)模的奶牛場(chǎng)。 魚骨式擠乳廳棚高一般不低于2.5 m，中間設(shè)有擠乳員操作的坑道，坑道深0.85m1.07m，寬2.Om2.3m，長(zhǎng)度與擠乳欄的數(shù)量有關(guān)，一般為9m16 m。欄位根據(jù)需要可從 26 至216

29、不等。 323轉(zhuǎn)盤式擠乳臺(tái) 利用可轉(zhuǎn)動(dòng)的環(huán)形擠乳臺(tái)進(jìn)行擠乳流水作業(yè)。其優(yōu)點(diǎn)是奶?？蛇B續(xù)進(jìn)入擠乳臺(tái)，擠乳員在入口處沖洗乳房、套奶杯，不必來(lái)回走動(dòng)，操作方便，每轉(zhuǎn)一圈 7 min 10min，轉(zhuǎn)到出口處已擠完乳，勞動(dòng)效率高，適于較大規(guī)模奶牛場(chǎng)。但設(shè)備造價(jià)高。33343536第三節(jié) Composition of cowsmilk一、Definition of Milk1.Definition 乳是哺乳動(dòng)物產(chǎn)犢以后，由乳腺分泌的，白色或微黃色的不透明液體，是一種十分復(fù)雜的生物學(xué)液體37Normal milk（常乳）Abnormal milk （異常乳） 在泌乳期中，由于生理、病理或其他因素的影響，乳的

30、成分與性質(zhì)發(fā)生變化，這種成分與性質(zhì)發(fā)生了變化的乳，稱為異常乳。 一般情況下，異常乳是不宜于加工使用的。異常乳可分為生理異常乳、化學(xué)異常乳微生物污染乳及經(jīng)濟(jì)等異常乳幾大類。 38生理異常乳 初乳 末乳 其他39化學(xué)異常乳 酒精陽(yáng)性乳 高酸度酒精陽(yáng)性乳 低酸度酒精陽(yáng)性乳 冷凍乳 低成分乳 混入異物乳 風(fēng)味異常乳 40微生物污染乳 病理異常乳 乳房炎乳 其他病牛乳 經(jīng)濟(jì)異常乳41二、Excrete of milk（乳的分泌） ProteinLactose FatOther42The most widespread milking animal in the world is the cow, whi

31、ch is found on all continents and in nearly all countries.Table Composition of milk from different types of animals. （%）Animal total Pro Casein Whey protein Fat Carbohydrate Ash Human 1.2 0.5 0.7 3.8 7.0 0.2 Horse 2.2 1.3 0.9 1.7 6.2 0.5 Cow 3.5 2.8 0.7 3.7 4.8 0.7 Buffalo 4.0 3.5 0.5 7.5 4.8 0.7 Go

32、at 3.6 2.7 0.9 4.1 4.7 0.8 Sheep 5.8 4.9 0.9 7.9 4.5 0.8三、乳的組成43Table Quantitative composition of milk牛乳的主要組成Main constituent Limits of variation Mean valueWater 85.5 89.5 87.5 Total solids 10.5 14.5 13.0Fat 2.5 6.0 3.9Proteins 2.9 5.0 3.4Lactose 3.6 5.5 4.8Minerals 0.6 0.9 0.844 幾個(gè)概念：Total solids,

33、TS（總?cè)楣腆w）the term solids-non-fat (SNF) is used in discussing the composition of milk. SNF is the total solids content less the fat content. The mean SNF(非脂乳固體） content according to Table is consequently 13.0 3.9 = 9.1%. 45 The pH of normal milk generally lies between 6.5 and 6.7, with 6.6 as the most

34、 common value. (This value applies at temperature of measurement near 25C.)46Milk fat乳和稀奶油是油包水（水包油）的一個(gè)乳化形態(tài)。Milk and cream（稀奶油） are examples of fat-in-water (or oil-in-water) emulsions. 47乳脂肪以極其細(xì)小的顆粒分散在乳漿中。它們的直徑約為0.120um，平均大小為34um。每毫升牛乳中的脂肪球數(shù)量約為150億個(gè)脂肪球。 The milk fat exists as small globules or dropl

35、ets dispersed in the milk serum, figure 2.15. Their diameters range from 0.1 to 20 mm (1 mm = 0.001 mm). The average size is 3 4 mm and there are some 15 billion globules per ml. The emulsion is stabilized by a very thin membrane only 5 10 nm thick (1 nm = 10-9m ) which surrounds the globules and ha

36、s a complicated composition.484950乳脂肪的結(jié)構(gòu) 乳脂肪從外向內(nèi)大體分為上層： 脂肪球膜 過(guò)度層：由親水、親油成分組成 脂肪51The membrane consists 乳脂肪球膜中包含乳及細(xì)胞中的多種蛋白質(zhì)、磷脂、膽固醇、酶和無(wú)機(jī)物質(zhì)等 phospholipids, lipoproteins, cerebrosides（腦苷脂）, proteins, nucleic acids, enzymes, trace elements (metals) and bound water.52脂肪球不僅是乳中最大，而且是乳中最輕的 As the fat globules

37、 are not only the largest particles in the milk but also the lightest (density at 15.5C = 0.93 g/cm 3 ), they tend to rise to the surface when milk is left to stand in a vessel for a while, figure 2.16. 5354乳脂肪的上浮遵循Stokes LawThe rate of rise follows Stokes Law, but the small size of the fat globules

38、 makes creaming a slow process. Cream separation can however be accelerated by aggregation of fat globules under the influence of a protein called agglutinin（凝集素）. These aggregates rise much faster than individual fat globules. The aggregates are easily broken up by heating or mechanical treatment.

39、Agglutinin is denaturated at time-temperature combinations such as 65C/10 min or 75C/2 min.55Chemical structure of milk fat Milk fat is liquid when milk leaves the udder at 37C. This means that the fat globules can easily change their shape when exposed to moderate mechanical treatment pumping and f

40、lowing in pipes for instance without being released from their membranes. All fats belong to a group of chemical substances called esters, which are compounds of alcohols and acids. Milk fat is a mixture of different fatty-acid esters called triglycerides（三酰甘油）, which are composed of an alcohol call

41、ed glycerol and various fatty acids. Fatty acids make up about 90% of milk fat. 56Table lists the most important fatty acids in milk fat triglycerides. Milk fat is characterised by the presence of relatively large amounts of butyric and caproic acid. 57Table 2.4 Principal fatty acids in milk fatFatt

42、y acid % of Melting Number of atoms acid content Total fatty- point C H C OSaturated Butyric acid 3.0 4.5 7.9 8 4 2 Liquid at Caproic acid 1.3 2.2 1.5 12 6 2 room temperature Caprylic acid 0.8 2.5 +16.5 16 8 2 Capric acid 1.8 3.8 +31.4 20 10 2 Solid at Lauric acid 2.0 5.0 +43.6 24 12 2 room temperat

43、ure Myristic acid 7.0 11.0 +53.8 28 14 2 Palmitic acid 25.0 29.0 +62.6 32 16 2 Stearic acid 7.0 3.0 +69.3 36 18 258Unsaturated Oleic acid 30.0 40.0 +14.0 34 18 2 Liquid at - Linoleic acid 2.0 3.0 5.0 32 18 2 room tempLinolenic acid up to 1.0 5.0 30 18 2 eratureArachidonic acid up to 1.0 49.5 32 20 2

44、 59Melting point of fathigh-melting of fat:28.4-33.360Iodine value Fatty acids with the same numbers of C and H atoms but with different numbers of single and double bonds have completely different characteris-H tics. The most important and most widely used method of indicating their specific charac

45、teristics is to measure the iodine value (IV) of the fat. The iodine value of butterfat normally varies between 24 and 46.碘價(jià)系表示不飽和脂肪酸的數(shù)量。61Proteins in milkProteins are giant molecules built up of smaller units called amino acids. A protein molecule consists of one or more interlinked chains of amino

46、 acids, where the amino acids are arranged in a specific order. A protein molecule usually contains around 100 200 linked amino acids, but both smaller and much larger numbers are known to constitute a protein molecule.Proteins are built from a supply of approx. 20 amino acids, 18 of which are found

47、 in milk proteins. 6263The electrical status of milk proteins6465Classes of milk proteins乳中含有上百種蛋白質(zhì)，他們中的大多數(shù)含量較低，蛋白質(zhì)按照其化學(xué)、物理和生物功能進(jìn)行分類Milk contains hundreds of types of protein, most of them in very small amounts. The proteins can be classified in various ways according to： chemical properties physica

48、l properties biological functions. 66Table 2.5 Concentration of proteins in milkConc. in milk % of total protein g/kg w/wCasein （酪蛋白）a s1 -casein*) 10.0 30.6 a s2-casein*) 2.6 8.0 b-casein*) 10.1 30.8 k-casein 3.3 10.1 Total Casein 26.0 79.5*) Henceforth called a s -casein *) Including g-casein67Con

49、c. in milk % of total protein g/kg w/wWhey Proteins（乳清蛋白）a-lactalbumin 1.2 3.7 -lactoglobulin 3.2 9.8 Blood Serum Albumin 0.4 1.2 Immunoglobulins 0.7 2.1 Miscellaneous (including Proteose-Peptone) 0.8 2.4 Total Whey Proteins 6.3 19.3Fat Globule Membrane Proteins 0.4 1.2Total Protein 32.7 10068Casein

50、酪蛋白 Casein is a group name for the dominant class of proteins in milk. （乳中最主要的蛋白質(zhì)）Due to the abundance of ionisable groups and hydrophobic（疏水） and hydrophilic（親水） sites in the casein molecule, the molecular polymers formed by the caseins are very special. 69Precipitation of caseinOne characteristic

51、property of casein is its ability to precipitate. Precipitation can be caused by many different agents. Precipitation by acid The pH will drop if an acid is added to milk or if acid-producing bacteria are allowed to grow in milk. The isoelectric points of the casein components depend on the ions of

52、other kinds present in the solution. Theoretical values, valid under certain conditions, are pH 5.1 to 5.3. In salt solutions, similar to the condition of milk, the range for optimum precipitation is pH 4.5 to 4.9. A practical value for precipitation of casein from milk is pH 4.7. 70Precipitation by

53、 enzymes The amino-acid chain forming the k-casein molecule consists of 169 amino acids. From an enzymatic point of view the bond between amino acids 105 (phenylalanin) and 106 (methionin) is easily accessible to many proteolytic enzymes. Some proteolytic enzymes will attack this bond and split the

54、chain. The soluble amino end contains amino acids 106 to 169, which are dominated by polar amino acids and the carbohydrate, which give this sequence hydrophilic properties. This part of the k-casein molecule is called the glycomacro-peptide and is released into the whey in cheesemaking. 71The remai

55、ning part of the k-casein, consisting of amino acids 1 to 105, is insoluble and remains in the curd together with a s - and b-casein. This part is called para-k-casein（副-k-酪蛋白）.The splitting of the 105 106 bond in the k-casein molecule is often called the primary phase of the rennet action, while th

56、e phase of coagulation and syneresis is referred to as the secondary phase. There is also a tertiary phase of rennet action, where the rennet attacks the casein components in a more general way. This occurs during cheese ripening.72The durations of the three phases are determined mainly by pH and te

57、mperature. In addition the secondary phase is strongly affected by the calcium ion concentration and by the condition of micelles with regard to absence or presence of denatured milk serum proteins on the surfaces of the micelles.7374PrecipitationOthers75Whey proteins乳清蛋白Whey protein is the name com

58、monly applied to milk serum proteins.去除乳中酪蛋白后,其余蛋白質(zhì)為乳清蛋白 If the casein is removed from skim milk by some precipitation method, such as the addition of mineral acid, there remains in solution a group of proteins which are called milk serum proteins. 76When milk is heated, some of the whey proteins de

59、naturate and form complexes with casein, thereby decreasing the ability of the casein to be attacked by rennet and to bind calcium. Curd from milk heated to a high temperature will not release whey as ordinary cheese curd does, due to the smaller number of casein bridges within and between the casei

60、n molecules.77a-lactalbumin（乳白蛋白）This protein may be considered to be the typical whey protein. It is present in milk from all mammals and plays a significant part in the synthesis of lactose in the udder.78b-lactoglobulin（乳球蛋白）This protein is found only in ungulates(有蹄類動(dòng)物） and is the major whey pro