【正文】 100 Casein ? 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. Precipitation of casein ? One characteristic 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 acidproducing bacteria are allowed to grow in milk. ? The isoelectric points of the casein ponents depend on the ions of other kinds present in the solution. Theoretical values, valid under certain conditions, are pH to . In salt solutions, similar to the condition of milk, the range for optimum precipitation is pH to . A practical value for precipitation of casein from milk is pH . Precipitation by enzymes ? The aminoacid chain forming the kcasein 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 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 kcasein molecule is called the glyacropeptide and is released into the whey in cheesemaking. ? The remaining part of the kcasein, consisting of amino acids 1 to 105, is insoluble and remains in the curd together with a s and bcasein. This part is called parakcasein（ 副k酪蛋白 ） . ? The splitting of the 105 – 106 bond in the kcasein molecule is often called the primary phase of the ren action, while the phase of coagulation and syneresis is referred to as the secondary phase. There is also a tertiary phase of ren action, where the ren attacks the casein ponents in a more general way. This occurs during cheese ripening. ? The durations of the three phases are determined mainly by pH and temperature. 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. Precipitation ? Others Whey proteins ? 乳清蛋白 ? Whey protein is the name 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. ? When milk is heated, some of the whey proteins denaturate and form plexes with casein, thereby decreasing the ability of the casein to be attacked by ren 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 casein molecules. alactalbumin（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. blactoglobulin（乳球蛋白） ? This protein is found only in ungulates(有蹄類動(dòng)物 ） and is the major whey protein ponent of milk from cows. ? If milk is heated to over 60176。 C. ? Enzymes also have their optimum pH ranges。和 H2O2中有抑菌活性 。 在乳汁 、 唾液中以可溶形式存在 ， 但在細(xì)胞內(nèi)它可能鑲嵌在小細(xì)胞顆粒上 。 C for a few seconds, ? a fact which can be used to prove the presence or absence of peroxidase in milk and thereby check whether or not a pasteurisation temperature above 80 176。 ? 牛乳中 LPO相對(duì)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。 它能催化過氧化氫的降解 ， 反應(yīng)如下： ? 2H2O2 2H2O＋ O2 ? Catalase splits hydrogen peroxide into water and free oxygen. ? 通過測(cè)定 過氧化氫酶 ， 評(píng)價(jià)牛乳是否來自于健康奶牛 。 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 。 ? Phosphatase is destroyed by ordinary pasteurisation (72176。 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。 it is about 30 times less sweet than cane sugar, for example. ? 1/51/6 Vitamins in milk ? 牛乳中含有所有已知的維生素 ， ? Milk contains many vitamins. Among the best known are A, B 1 , B 2 , C and D. ? Vitamins A and D are soluble in fat, or fat solvents, while the others are soluble in water. Table Vitamins in milk and daily requirements ? Vitamin Amount in 1 L of milk Adult daily requirement ? ( mg) (mg) A – 2 1 – 2 ? B 1 1 – 2 ? B 2