【正文】 C and kept at that temperature until analysed. The analysis should be carried out the same day, otherwise a phenomenon known as reactivation may occur, . an inactivated enzyme bees active again and gives a positive test reading. Cream is particularly susceptible in this respect. Lipase（脂肪酶） ? Lipase splits fat into glycerol and free fatty acids. ? Excess free fatty acids in milk and milk products result in a rancid taste（ 油腐敗味 ） . ? The action of this enzyme seems, in most cases, to be very weak, though the milk from certain cows may show strong lipase activity. ? The quantity of lipase in milk is believed to increase towards the end of the lactation cycle. ? Lipase is, to a great extent, inactivated by pasteurisation, but higher temperatures are required for total inactivation. Many microanisms produce lipase. This can cause serious problems, as the enzyme is very resistant to heat. Lactose（乳糖） ? Lactose ? Lactose is a sugar found only in milk。 C for 60 seconds. Phosphatase（磷酸酶） ? 乳中主要有兩種天然磷酸酶 ， 即堿性磷酸酶和酸性磷酸酶 ， 除此之外還有其他磷酸酶 ， 例如核糖核酸酶 。 ? 牛乳中 LPO相對較耐熱，在 HTST巴氏殺菌（ 72℃ ， 15S） 后損失 30％活力，也失去了足夠催化 SCN175。 在乳汁 、 唾液中以可溶形式存在 ， 但在細胞內(nèi)它可能鑲嵌在小細胞顆粒上 。 C. ? Enzymes also have their optimum pH ranges。 C. ? This means that the fat globules can easily change their shape when exposed to moderate mechanical treatment – pumping and flowing in pipes for instance – without being released from their membranes. ? All fats belong to a group of chemical substances called esters, which are pounds of alcohols and acids. Milk fat is a mixture of different fattyacid esters called triglycerides（ 三酰甘油 ） , which are posed of an alcohol called glycerol and various fatty acids. Fatty acids make up about 90% of milk fat. ? Table 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. Table Principal fatty acids in milk fat ? Fatty acid % of Melting Number of atoms acid content ? Total fatty point 176。 The milk fat exists as small globules or droplets dispersed in the milk serum, figure . Their diameters range from to 20 mm (1 mm = 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 = 109m ) which surrounds the globules and has a plicated position. ? 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 ? 過度層：由親水、親油成分組成 ? 脂肪 ? The membrane consists ? 乳脂肪球膜中包含乳及細胞中的多種蛋白質(zhì) 、 磷脂 、 膽固醇 、 酶和無機物質(zhì)等 ? phospholipids, lipoproteins, cerebrosides（ 腦苷脂 ） , proteins, nucleic acids, enzymes, trace elements (metals) and bound water. ? 脂肪球不僅是乳中最大 ， 而且是乳中最輕的 ? As the fat globules are not only the largest particles in the milk but also the lightest (density at 176。 C.) Milk fat 乳和稀奶油是油包水（水包油）的一個乳化形態(tài)。 第三節(jié) Composition of cows’milk 一、 Definition of Milk ? ? 乳是哺乳動物產(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 ? Normal 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。 欄位根據(jù)需要可從 2 6 至 2 16 不等。 ? 2．魚骨式擠乳廳 ? 由于擠乳臺兩排擠乳機及被擠牛的排列形狀如魚骨狀而得名 ， 其擠乳臺欄位一般按傾斜 30186。 the cow bees irritated and may bee difficult to milk. Hand 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 quickly 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 first milk enables the milker to detect changes that may indicate that the cow is ill. ? Two 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 udder. 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 remove 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. Machine milking（機械榨乳） ? 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。 ? 排乳是一個復(fù)雜的反射過程 , 由神經(jīng)一激素途徑調(diào)節(jié) 。 ? (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 、 細菌 、 真菌及其它過敏原 ， 中和毒素 ， 調(diào)理身體免疫機能 。 Colostrum（初乳） ? 牛初乳是指乳牛產(chǎn)犢后 3～ 4d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 。 ? 而在妊娠后期，特別是臨近分娩時，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 PGF2α的分泌，PGF2α溶解黃體，使孕激素的水平迅速下降，減弱或解除了孕酮對促乳素分泌和釋放的抑制，相應(yīng)提高了乳腺對促乳素的敏感性。 因此開始停止榨乳 ， 此時距下一個產(chǎn)犢有 60天的時間 ， 該階段為停乳期 。 ? 整個乳房由 4 個相互獨立的乳區(qū)組成，它由乳房側(cè)韌帶和乳房懸韌帶將其隔開。 ? Selective breeding has resulted in dairy cows which yield an average of more than 6 000 litres of milk per calf, . six times as much as the primitive cow. ? Some cows can yield 14 000 litres or more. ? 黃牛產(chǎn)乳量也可達 600 kg左右 ? 乳用水牛則可達 1000 kg～ 2022 kg。 ? 小母牛 78個月就可以達到性成熟 ， 通常 1518個月才進行配種 。 ? 每個乳區(qū)都有獨立的分泌系統(tǒng)，互不相通，每個乳區(qū)有一個乳頭。 ? One to two months after calving the cow can be serviced again. During the lactation period milk production 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 nonlactating peri