牛奶的基本知识课件.ppt
151页
第三篇第三篇 乳品工艺学乳品工艺学第一章第一章 乳的基础知识乳的基础知识牛奶的基本知识 第一节第一节 奶牛的基础知识奶牛的基础知识wMilk production began 6 000 years ago or even earlier. wThe 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.牛奶的基本知识 Table 1.1不同种类动物乳汁的营养组成表1中是不同种类动物乳汁的营养组成是平均数，影响因素还很多，包括：breed, feeding, climate, etc.牛奶的基本知识 w牛乳（牛乳（Cow milk））w乳是哺乳动物生命之初唯一食物wMilk is the only food of the young mammal during the first period of its life. w乳可以提供能量，并含有动物生长所需的物质wThe substances in milk provide both energy and the building materials necessary for growth. 牛奶的基本知识 w乳也含有抗体wMilk also contains antibodies which protect the young mammal against infection.w1头犊牛大约需要1000L牛乳wA calf needs about 1 000 litres of milk for growth, and that is the quantity which the primitive cow produces for each calf. 牛奶的基本知识 w 1头奶牛1个泌乳周期大约分泌6000L牛乳，w现在有些奶牛产奶量超过14000L。
w Selective breeding has resulted 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. w Some cows can yield 14 000 litres or more. 牛奶的基本知识 w黄牛产乳量也可达600 kg左右w乳用水牛则可达1000 kg～2000 kg 牛奶的基本知识 w小母牛7-8个月就可以达到性成熟，通常15-18个月才进行配种wBefore a cow can start to produce milk she must have calved 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. w 牛奶的基本知识 w小母牛一般在2岁前进行自然或人工配种w The heifer is bred (naturally or by insemination) before the age of 2 years. w奶牛的孕期为265-300天w The period of gestation is 265 – 300 daysw小母牛在2-2.5岁产第一胎小牛w A heifer produces her first calf at the age of about 2 – 2.5 years.牛奶的基本知识 w奶牛的怀孕时间9个月w The gestation period is 9 months.w奶牛产犊后泌乳期为10个月（通常按305天计算）w After calving the cow gives milk for 10 months.w奶牛产犊后1-2个月再进行配种w 1 – 2 months after calving the cow will again be bred. w奶牛一生大约产5胎后就宰杀。
我国8-10胎w After having given birth to some 5 calves, the cow is generally slaughtered.牛奶的基本知识 Secretion of milk（乳的分泌）（乳的分泌）wMilk is secreted in the cow’s udder （乳房）wA sectional view of the udder is shown in Figure. 乳房主要由： 腺体组织约占75%～80%， 结缔和脂肪组织占20%～25% 乳房内部又都是由乳腺泡、乳导管、乳池、乳头管和间质构成 牛奶的基本知识 w 整个乳房由 4 个相互独立的乳区组成，它由乳房侧韧带和乳房悬韧带将其隔开w 每个乳区都有独立的分泌系统，互不相通，每个乳区有一个乳头牛奶的基本知识 w 乳房由含有能够产生乳的腺体wThe udder is composed of glandular（腺体） tissue which contains milk-producing cells. wIt is encased in muscular tissue, which gives cohesion to the body of the udder and protects it against injury from knocks and blows. wThe glandular tissue contains a very large number (about 20 亿) of tiny bladder scalled alveoli （腺胞）. 牛奶的基本知识 w每天大约有90 000 L血液流经乳房wFlow of blood through the udder approx. 90 000 l/day. w大约需要800 – 900 L血液方能产生1L乳wApprox. 800 – 900 l of blood needed for formation of one litre of milk.牛奶的基本知识 The lactation cycle（哺乳期）（哺乳期）w Secretion of milk in the cow’s udder begins shortly before calving w奶牛产犊后连续300天的乳的分泌，这个期间叫哺乳期wThe cow then continues to give milk for about 300 days. This period is known as lactation. w产奶量在产犊后不断上升，后又不断下降。
到300天时，产奶量比高峰时下降15-25%因此开始停止榨乳，此时距下一个产犊有60天的时间，该阶段为停乳期w 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 non-lactating period of up to 60 days prior to calving again. 牛奶的基本知识 泌乳的发动泌乳的发动w 母牛分娩时及分娩后，发育成熟的乳腺开始分泌乳汁的现象叫泌乳的发动w 引起泌乳发动的原因很多，目前一般认为，分娩前后血液中激素浓度的变化是导致泌乳发动的主要诱因。
w妊娠期间，由于母体黄体分泌大量的孕激素，反馈抑制了垂体促乳素的分泌，并使乳腺对促乳素的敏感性有所降低；孕激素分泌量的增加会抑制雌激素对促乳素分泌的刺激作用，也引起促乳素分泌的减弱w而在妊娠后期，特别是临近分娩时，由于在胎儿肾上腺糖皮质激素的作用下，胎盘雌激素分泌增加，雌激素的分泌可刺激子宫内膜PGF2α的分泌，PGF2α溶解黄体，使孕激素的水平迅速下降，减弱或解除了孕酮对促乳素分泌和释放的抑制，相应提高了乳腺对促乳素的敏感性同时，雌激素也加速了促乳素的释放，引起泌乳的发动在分娩当天外周血液中促乳素的浓度可达到妊娠期的 2～5 倍, 此后也比妊娠期高 1 倍以上分娩以后，雌激素水平随之降低 牛奶的基本知识 Colostrum（初乳）（初乳）w牛牛初初乳乳是是指指乳乳牛牛产产犊犊后后3～～4d内内所所分分泌泌的的乳乳汁汁，，乳乳汁汁色色泽泽黄黄而而浓浓稠稠，，具具有有特特殊殊的的乳乳腥腥味味和和苦苦味味这这些些乳乳汁汁是是母母牛牛为为了了供供给给牛牛犊犊在在新新生生环环境境下下，，低低抗抗外外来来病病毒毒及及细细菌菌而而天然合成的天然合成的 w The first milk that a cow produces after calving is called colostrum. w 牛奶的基本知识 w初乳在组成和特性方面与常乳有很大区别，特别是乳清蛋白含量，因此初乳受热很容易凝固wIt differs greatly from normal milk in composition and properties. One highly distinctive characteristic is the high content of whey proteins – about 11% compared to about 0.65% in normal milk。
This results in colostrum coagulating when heated. 牛奶的基本知识 w A fairly large proportion of whey protein is immunoglobulins (Ig G, dominating in colostrum), which protect the calf from infection until its own immunity 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. w Four to five days after calving the cow begins to produce milk of normal composition, which can be mixed with other milk.牛奶的基本知识 w牛初乳组成牛初乳组成w 根据大量研究资料表明, 与常乳比较，牛初乳除了含有丰富的优质蛋白质、维生素和矿物质等营养成分外，还富含免疫因子、生长因子等活性功能组分。
牛奶的基本知识 牛奶的基本知识 w(1) 免疫因子(Immune factor)：医学和临床研究表明，初乳中的免疫因子能高效地抵抗病毒、细菌、真菌及其它过敏原，中和毒素，调理身体免疫机能 w免疫球蛋白(抗体)： w乳铁蛋白(Lactoferrin)： w乳过氧化酶(Lactoperoxidase)： 牛奶的基本知识 w乳过氧化酶(Lactoperoxidase)： w黄嘌呤氧化酶(Xanthine oxidase)： w富含脯氨酸的多肽(PRP)： w糖蛋白(Glycoprotein)： w溶菌酶(Lysozyme)： 牛奶的基本知识 w生长因子(Growth factor)w 牛初乳中的各种生长因子具有促进组织正常生长和加快伤口愈合的功效主要包括：胰岛素样生长因子(IGF-1)；神经营养生长因子(NGF)；表皮生长因子(EGF)；转化生长因子(TGF)；纤维细胞生长因子(FGF)；促性腺激素-释放激素(GnRH)及其缔合肽(GAP)；生长激素(GH) 牛奶的基本知识 Milkingw 排乳w 乳汁生成后, 由腺泡上皮细胞分泌到腺泡腔, 当腺泡腔和细小导管充盈时, 通过腺泡腔周围的肌上皮细胞和输乳管的平滑肌反射性收缩, 将乳汁转移到大乳导管和乳池内, 在哺乳或挤乳时, 通过神经激素的作用, 使乳汁从大乳导管和乳池中排出，这一过程称为排乳（或称放乳）。
牛奶的基本知识 w 排乳是一个复杂的反射过程, 由神经一激素途径调节当幼畜的吮乳或挤乳的刺激时, 乳头皮肤的神经冲动上行至下丘脑, 导致催产素分泌并经垂体后叶释放至血液中, 使腺泡和较小的乳导管周围的肌上皮细胞收缩, 乳房内压增加而迫使乳汁通过乳导管流入乳池奶牛的排乳反射一般维持 4 min～7min 后消失 wA hormone called oxytocin must be released into the cow’s bloodstream in order to start the emptying of the udder. This hormone is secreted and 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 calf’s attempts to suck on the teat. The oxytocin is released 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.牛奶的基本知识 wThe oxytocin begins to take effect about one minute after preparation has begun and causes the muscle-like cells to 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 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 unnecessary strain upon the udder; the cow becomes irritated and may become difficult to milk.牛奶的基本知识 Hand milking（人工榨乳）（人工榨乳）w 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.牛奶的基本知识 wTwo 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.牛奶的基本知识 Machine milking（机械榨乳）（机械榨乳）w挤乳厅的形式挤乳厅的形式 w挤乳厅的形式比较多，如平面畜舍式、鱼骨式、转盘式等。
牛奶的基本知识 w1．．平面畜舍式挤乳厅平面畜舍式挤乳厅w挤乳栏位的排列与牛舍相似，奶牛从挤乳厅大门进入厅内的挤乳栏里，由挤乳员套上挤乳器进行挤乳优点是造价较低，缺点是挤乳员仍须弯腰操作，影响劳动效率这种挤乳厅一般只适于投资少并利用现有房舍改造成挤乳厅的牛场或小型奶牛场牛奶的基本知识 w2 2．鱼骨式挤乳厅．鱼骨式挤乳厅w 由于挤乳台两排挤乳机及被挤牛的排列形状如鱼骨状而得名，其挤乳台栏位一般按倾斜 30º 设计，这样就使得牛的乳房部位更接近挤乳员，有利于挤乳操作，减少走动距离，提高劳动效率，因此在奶牛场使用比较普遍一般适于中等规模的奶牛场w 鱼骨式挤乳厅棚高一般不低于2.5 m，中间设有挤乳员操作的坑道，坑道深0.85m～1.07m，宽2.Om～2.3m，长度与挤乳栏的数量有关，一般为9m～16 m栏位根据需要可从 2×6 至2×16 不等 牛奶的基本知识 w3．转盘式挤乳台．转盘式挤乳台w 利用可转动的环形挤乳台进行挤乳流水作业其优点是奶牛可连续进入挤乳台，挤乳员在入口处冲洗乳房、套奶杯，不必来回走动，操作方便，每转一圈 7 min ～10min，转到出口处已挤完乳，劳动效率高，适于较大规模奶牛场。
但设备造价高牛奶的基本知识 牛奶的基本知识 牛奶的基本知识 牛奶的基本知识 第三节第三节 Composition of cows’milk一、Definition of Milkw1.Definitionw 乳是哺乳动物产犊以后，由乳腺分泌的，白色或微黄色的不透明液体，是一种十分复杂的生物学液体牛奶的基本知识 wNormal milk（常乳）wAbnormal milk （异常乳）w 在泌乳期中，由于生理、病理或其他因素的影响，乳的成分与性质发生变化，这种成分与性质发生了变化的乳，称为异常乳w 一般情况下，异常乳是不宜于加工使用的异常乳可分为生理异常乳、化学异常乳微生物污染乳及经济等异常乳几大类 牛奶的基本知识 w生理异常乳 w 初乳 w 末乳 w 其他牛奶的基本知识 w化学异常乳 w 酒精阳性乳 w 高酸度酒精阳性乳 w 低酸度酒精阳性乳 w 冷冻乳 w 低成分乳 w 混入异物乳w 风味异常乳 牛奶的基本知识 w微生物污染乳 w病理异常乳 w 乳房炎乳 w 其他病牛乳 w经济异常乳牛奶的基本知识 二、Excrete of milk（乳的分泌） wProteinwLactose wFatwOther牛奶的基本知识 wThe most widespread milking animal in the world is the cow, which 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 Goat 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、乳的组成牛奶的基本知识 Table Quantitative composition of milkw牛乳的主要组成牛乳的主要组成wMain constituent Limits of variation Mean valuewWater 85.5 – 89.5 87.5 wTotal solids 10.5 – 14.5 13.0wFat 2.5 – 6.0 3.9wProteins 2.9 – 5.0 3.4wLactose 3.6 – 5.5 4.8wMinerals 0.6 – 0.9 0.8牛奶的基本知识 w 几个概念：wTotal solids, TS（总乳固体）wthe term solids-non-fat (SNF) is used in discussing the composition of milk. SNF is the total solids content less the fat content. w The mean SNF(非脂乳固体） content according to Table is consequently 13.0 – 3.9 = 9.1%. w 牛奶的基本知识 w The pH of normal milk generally lies between 6.5 and 6.7, with 6.6 as the most common value. (This value applies at temperature of measurement near 25°C.)牛奶的基本知识 Milk fat乳和稀奶油是油包水（水包油）的一个乳化形态。
Milk and cream（稀奶油） are examples of fat-in-water (or oil-in-water) emulsions. 牛奶的基本知识 乳脂肪以极其细小的颗粒分散在乳浆中它们的直径约为0.1~20um，平均大小为3~4um每毫升牛乳中的脂肪球数量约为150亿个脂肪球 The milk fat exists as small globules or droplets 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 has a complicated composition.牛奶的基本知识 牛奶的基本知识 牛奶的基本知识 w乳脂肪的结构w 乳脂肪从外向内大体分为上层：w 脂肪球膜w 过度层：由亲水、亲油成分组成w 脂肪牛奶的基本知识 wThe membrane consists w乳脂肪球膜中包含乳及细胞中的多种蛋白质、磷脂、胆固醇、酶和无机物质等 w phospholipids, lipoproteins, cerebrosides（脑苷脂）, proteins, nucleic acids, enzymes, trace elements (metals) and bound water.牛奶的基本知识 w脂肪球不仅是乳中最大，而且是乳中最轻的w As the fat globules are not only the largest particles in the milk but also the lightest (density at 15.5°C = 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. 牛奶的基本知识 牛奶的基本知识 w乳脂肪的上浮遵循Stokes’ LawwThe rate of rise follows Stokes’ Law, but the small size of the fat globules 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. wThe aggregates are easily broken up by heating or mechanical treatment. wAgglutinin is denaturated at time-temperature combinations such as 65°C/10 min or 75°C/2 min.牛奶的基本知识 Chemical structure of milk fatw Milk fat is liquid when milk leaves the udder at 37°C. wThis 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. wAll 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 called glycerol and various fatty acids. Fatty acids make up about 90% of milk fat. 牛奶的基本知识 牛奶的基本知识 wTable lists the most important fatty acids in milk fat triglycerides.w Milk fat is characterised by the presence of relatively large amounts of butyric and caproic acid. 牛奶的基本知识 Table 2.4 Principal fatty acids in milk fatwFatty acid % of Melting Number of atoms acid content w Total fatty- point °C H C OwSaturatedw Butyric acid 3.0 – 4.5 –7.9 8 4 2 Liquid at w Caproic acid 1.3 – 2.2 –1.5 12 6 2 room temperature w Caprylic acid 0.8 – 2.5 +16.5 16 8 2 wCapric acid 1.8 – 3.8 +31.4 20 10 2 Solid at w Lauric acid 2.0 – 5.0 +43.6 24 12 2 room temperaturew Myristic acid 7.0 – 11.0 +53.8 28 14 2 w Palmitic acid 25.0 – 29.0 +62.6 32 16 2 wStearic acid 7.0 – 3.0 +69.3 36 18 2牛奶的基本知识 wUnsaturated wOleic acid 30.0 – 40.0 +14.0 34 18 2 Liquid at - wLinoleic acid 2.0 – 3.0 –5.0 32 18 2 room tempwLinolenic acid up to 1.0 –5.0 30 18 2 eraturewArachidonic acid up to 1.0 –49.5 32 20 2 牛奶的基本知识 Melting point of fatwhigh-melting of fat:28.4-33.3牛奶的基本知识 wIodine valuew 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 characteristics is to measure the iodine value (IV) of the fat. The iodine value of butterfat normally varies between 24 and 46.w碘价系表示不饱和脂肪酸的数量。
牛奶的基本知识 Proteins in milkw Proteins are giant molecules built up of smaller units called amino acids. wA protein molecule consists of one or more interlinked chains of amino acids, where the amino acids are arranged in a specific order. wA protein molecule usually contains around 100 – 200 linked amino acids, but both smaller and much larger numbers are known to constitute a protein molecule.wProteins are built from a supply of approx. 20 amino acids, 18 of which are found in milk proteins. 牛奶的基本知识 牛奶的基本知识 wThe electrical status of milk proteins牛奶的基本知识 牛奶的基本知识 Classes of milk proteinsw乳中含有上百种蛋白质，他们中的大多数含量较低，蛋白质按照其化学、物理和生物功能进行分类wMilk contains hundreds of types of protein, most of them in very small amounts. The proteins can be classified in various ways according to： w chemical properties w physical properties w biological functions. 牛奶的基本知识 Table 2.5 Concentration of proteins in milkwConc. in milk % of total protein w g/kg w/wwCasein （酪蛋白）wa s1 -casein*) 10.0 30.6 wa s2-casein*) 2.6 8.0 wb-casein**) 10.1 30.8w k-casein 3.3 10.1 wTotal Casein 26.0 79.5w*) Henceforth called a s -casein **) Including g-casein牛奶的基本知识 wConc. in milk % of total protein w g/kg w/wwWhey Proteins（乳清蛋白）wa-lactalbumin 1.2 3.7 wβ-lactoglobulin 3.2 9.8 wBlood Serum Albumin 0.4 1.2 wImmunoglobulins 0.7 2.1 wMiscellaneousw (including Proteose-Peptone) 0.8 2.4 wTotal Whey Proteins 6.3 19.3wFat Globule Membrane Proteins 0.4 1.2wTotal Protein 32.7 100牛奶的基本知识 Caseinw酪蛋白酪蛋白 wCasein is a group name for the dominant class of proteins in milk. （乳中最主要的蛋白质）wDue to the abundance of ionisable groups and hydrophobic（疏水） and hydrophilic（亲水） sites in the casein molecule, the molecular polymers formed by the caseins are very special. 牛奶的基本知识 Precipitation of caseinw One characteristic property of casein is its ability to precipitate. Precipitation can be caused by many different agents. wPrecipitation by acid w The pH will drop if an acid is added to milk or if acid-producing bacteria are allowed to grow in milk. w The isoelectric points of the casein components depend on the ions of 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. 牛奶的基本知识 Precipitation by enzymesw The amino-acid chain forming the k-casein molecule consists of 169 amino acids. wFrom an enzymatic point of view the bond between amino acids 105 (phenylalanin) and 106 (methionin) is easily accessible to many proteolytic enzymes.w 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 k-casein molecule is called the glycomacro-peptide and is released into the whey in cheesemaking. 牛奶的基本知识 wThe remaining 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）.wThe splitting of the 105 – 106 bond in the k-casein molecule is often called the primary phase of the rennet action, while the 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.牛奶的基本知识 wThe 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.牛奶的基本知识 牛奶的基本知识 PrecipitationwOthers牛奶的基本知识 Whey proteinsw乳清蛋白wWhey protein is the name commonly applied to milk serum proteins.w去除乳中酪蛋白后,其余蛋白质为乳清蛋白 wIf 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. 牛奶的基本知识 wWhen milk is heated, some of the whey proteins denaturate 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 casein molecules.牛奶的基本知识 a-lactalbumin（乳白蛋白）（乳白蛋白）wThis 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.牛奶的基本知识 b-lactoglobulin（乳球蛋白）（乳球蛋白）wThis protein is found only in ungulates(有蹄类动物） and is the major whey protein component of milk from cows.w If milk is heated to over 60°C, denaturation is initiated where the reactivity of the sulphur-amino acid of b-lactoglobulin plays a prominent part. Sulphur bridges start to form between the b-lactoglobulin molecules, between one b-lactoglobulin molecule and a k-casein molecule and between b-lactoglobulin and a-lactalbumin. At high temperatures sulphurous compounds such as hydrogen sulphide are gradually released. These sulphurous compounds are responsible for the “cooked” flavour of heat treated milk.牛奶的基本知识 Immunoglobulins and related minor proteinswThis protein group is extremely heterogeneous, and few of its members have been studied in detail. In the future many substances of importance will probably be isolated on a commercial scale from milk serum or whey. wLactoferrin（乳铁蛋白） and lactoperoxidase（乳过氧化物酶） are substances of possible use in the pharmaceutical and food industries, and are now isolated from whey by a commercial process. 牛奶的基本知识 Membrane proteinsw Membrane proteins are a group of proteins that form a protective layer around fat globules to stabilise the emulsion. wTheir consistency ranges from soft and jelly-like in some of the membrane proteins to rather tough and firm in others. Some of the proteins contain lipid residues and are called lipoproteins. The lipids and the hydrophobic amino acids of those proteins make the molecules direct their hydrophobic sites towards the fat surface, while the less hydrophobic parts are oriented towards the water. 牛奶的基本知识 Denatured（变性）（变性） proteinsw As long as proteins exist in an environment with wa temperature wand pH within their limits of tolerance, wthey retain their biological functions. wBut if they are heated to temperatures above a certain maximum their structure is altered. They are said to be denatured. 牛奶的基本知识 Milk is a buffer solutionw Milk contains a large number of substances which can act either as weak acids or as weak bases, e.g. lactic acid, citric acid and phosphoric acid and their respective salts: lactates, citrates and phosphates. wIn chemistry such a system is called a buffer solution because, within certain limits, the pH value remains constant when acids or bases are added. This effect can be explained by the characteristic qualities of the proteins.牛奶的基本知识 wWhen milk is acidified, a large number of hydrogen ions (H + ) are added. These ions are almost all bound to the amino groups in the side chains of the amino acids, forming NH 3 + ions. The pH value, however, is hardly affected at all as the increase in the concentration of free hydrogen ions is very small. 牛奶的基本知识 牛奶的基本知识 Enzymes in milkwTwo factors which strongly influence enzymatic action are temperature and pH. woptimum temperature range between 25 and 50°C.wEnzymes also have their optimum pH ranges; some function best in acid solutions, others in an alkaline environment.wOthersw 牛奶的基本知识 Peroxidase(过氧化物酶）过氧化物酶）wLPO存在于多种哺乳动物的乳汁中，它在SCN¯和H2O2中有抑菌活性。
LPO也在哺乳动物的乳腺、唾液腺、泪腺及其分泌物中存在（如乳汁、唾液、眼泪等）在乳汁、唾液中以可溶形式存在，但在细胞内它可能镶嵌在小细胞颗粒上 wPeroxidase transfers oxygen from hydrogen peroxide (H 2 O 2 ) to other readily oxidisable substances. This enzyme is inactivated if the milk is heated to 80 °C for a few seconds, wa 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 °C has been reached. This test is called Storch’s peroxidase test.牛奶的基本知识 w过氧化物酶（LPO）是一种催化酶，它的原理：w底物＋H2O2 →氧化产物＋2 H2O w牛乳中LPO的浓度为30mg/L，且在泌乳期含量稳定。
w牛乳中LPO相对较耐热，在HTST巴氏杀菌（72℃，15S）后损失30％活力，也失去了足够催化SCN¯和H2O2间反应的活力 牛奶的基本知识 Catalase（过氧化氢酶）（过氧化氢酶）w过氧化氢酶，H2O2:H2O2氧化还原酶，是一种血蛋白它能催化过氧化氢的降解，反应如下：w2H2O2 2H2O＋O2 wCatalase splits hydrogen peroxide into water and free oxygen. w通过测定过氧化氢酶，评价牛乳是否来自于健康奶牛wBy determining the amount of oxygen that the enzyme can release in milk, it is possible to estimate the catalase content of the milk and learn whether or not the milk has come from an animal with a healthy udder. wMilk from diseased udders has a high catalase content, while fresh milk from a healthy udder contains only an insignificant amount. There are however many bacteria which produce this kind of enzyme. 牛奶的基本知识 wCatalase is destroyed by heating at 75°C for 60 seconds.牛奶的基本知识 Phosphatase（磷酸酶）（磷酸酶）w乳中主要有两种天然磷酸酶，即碱性磷酸酶和酸性磷酸酶，除此之外还有其他磷酸酶，例如核糖核酸酶。
wPhosphatase has the property of being able to split certain phosphoric- acid esters into phosphoric acid and the corresponding alcohols. 牛奶的基本知识 wALP是乳中一种重要的酶，液态奶及乳制品中的ALP活力都要求符合相关法规（例如：巴氏杀菌要小于1ug/ml，液态乳制品要小于350mu/L，其它乳制品也应小于500mu/ml），主要目的是来证明是否得到充分的巴氏杀菌和保障消费的安全性 wPhosphatase is destroyed by ordinary pasteurisation (72°C for 15 – 20 seconds), so the phosphatase test can be used to determine whether the pasteurization temperature has actually been attained. The routine test used in dairies is called the phosphatase test according to Scharer.牛奶的基本知识 w The phosphatase test should preferably be performed immediately after heat treatment. Failing that, the milk must be chilled to below + 5°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, i.e. an inactivated enzyme becomes active again and gives a positive test reading. Cream is particularly susceptible in this respect.牛奶的基本知识 Lipase（脂肪酶）（脂肪酶）w Lipase splits fat into glycerol and free fatty acids. wExcess free fatty acids in milk and milk products result in a rancid taste（油腐败味）. wThe action of this enzyme seems, in most cases, to be very weak, though the milk from certain cows may show strong lipase activity. wThe quantity of lipase in milk is believed to increase towards the end of the lactation cycle. wLipase is, to a great extent, inactivated by pasteurisation, but higher temperatures are required for total inactivation. Many microorganisms produce lipase. This can cause serious problems, as the enzyme is very resistant to heat.牛奶的基本知识 Lactose（乳糖）wLactosewLactose is a sugar found only in milk; wCarbohydrates also supply material for the synthesis of some important chemical compounds in the body. They are present in muscles as muscle glycogen and in the liver as liver glycogen（糖原）. Glycogen is an example of a carbohydrate with a very large molecular weight. wLactose (milk sugar) is a disaccharide, with a molecule containing the monosaccharides glucose and galactose（半乳糖）. 牛奶的基本知识 wThe lactose content of milk varies between 3.6 and 5.5%.wFigure shows what happens when lactose is attacked by lactic acid bacteria. wThese bacteria contain an enzyme called lactase（乳糖酶） which attacks lactose, splitting its molecules into glucose and galactose. 牛奶的基本知识 wLactic acid bacterial enzyme, lactasewLactose Glucose+ GalactosewFig Breakdown of lactose by enzymatic action and formation of lactic acid.牛奶的基本知识 wIf milk is heated to a high temperature, and is kept at that temperature, it turns brown and acquires a caramel taste. This process is called caramelization(焦糖反应） and is the result of a chemical reaction between lactose and proteins called the Maillard reaction. 牛奶的基本知识 wLactose is not as sweet as other sugars; it is about 30 times less sweet than cane sugar, for example.w1/5-1/6牛奶的基本知识 Vitamins in milkw牛乳中含有所有已知的维生素， w Milk contains many vitamins. Among the best known are A, B 1 , B 2 , C and D. wVitamins A and D are soluble in fat, or fat solvents, while the others are soluble in water. 牛奶的基本知识 Table Vitamins in milk and daily requirementswVitamin Amount in 1 L of milk Adult daily requirement w ( mg) (mg) A 0.2 – 2 1 – 2 wB 1 0.4 1 – 2 wB 2 1.7 2 – 4 wC 5 – 20 30 – 100wD 0.002 0.01牛奶的基本知识 Minerals and salts in milkwMilk contains a number of minerals. The total concentration is less than 1%. wMineral salts occur in solution in milk serum or in casein compounds. The most important salts are those of calcium, sodium, potassium and magnesium. They occur as phosphates, chlorides, citrates and caseinates. Potassium and calcium salts are the most abundant in normal milk. The amounts of salts present are not constant. wTowards the end of lactation, and even more so in the case of udder disease, the sodium chloride content increases and gives the milk a salty taste, while the amounts of other salts are correspondingly reduced.牛奶的基本知识 Other constituents of milkwMilk always contains somatic cells (white blood corpuscles or leucocytes)（体细胞）. The content is low in milk from a healthy udder, but increases if the udder is diseased, usually in proportion to the severity of the disease. The somatic cell content of milk from healthy animals is as a rule lower than 200 000 cells/ml, but counts of up to 400 000 cells/ml can be accepted. 牛奶的基本知识 wMilk also contains gases, some 5 – 6 % by volume in milk fresh from the udder, but on arrival at the dairy the gas content may be as high as 10 % by volume. The gases consist mostly of carbon dioxide, nitrogen and oxygen. They exist in the milk in three states: w1 、dissolved in the milk w2、 bound and non-separable from the milk w3、 dispersed in the milk wDispersed and dissolved gases are a serious problem in the processing of milk, which is liable to burn on to heating surfaces if it contains too much gas.牛奶的基本知识 Changes in milk and its constituentsw wChanges during storagew The fat and protein in milk may undergo chemical changes during storage. These changes are normally of two kinds: oxidation and lipolysis. The resulting reaction products can cause off-flavours, principally in milk and butter.wOxidation of fat wOxidation of protein wLipolysis 牛奶的基本知识 第三节第三节 Basic physical-chemical properties of cows’ milkwDefinitionswCows’ milk consists of about 87% water and 13% dry substance. The dry substance is suspended or dissolved in the water. Depending on the type of solids there are different distribution systems of them in the water phase.乳中干物质悬浮或溶解在水中，依据干物质的种类分散水中的不同相中牛奶的基本知识 Table Physical-chemical status of cows’ milkwtype.w Average Emulsion Colloidal Truew Composition（（%））Oil/Water solution/ suspension solutionwMoisture 87.0 wFat 4.0 X wProteins 3.5 Xw Lactose 4.7 X wAsh 0.8 Xw 乳浊液 胶体悬浮液 真溶液牛奶的基本知识 Table Relative sizes of particles in milkw.wSize (mm) Type of particlesw10 –2 to 10 –3 Fat globules w10 –4 to 10 –5 Casein-calcium phosphatesw 10 –5 to 10 –6 Whey proteins w10 –6 to 10 –7 Lactose, salts and otherw substances in true solutions牛奶的基本知识 w1、、 Emulsion（乳浊液）（乳浊液） wMilk is an emulsion of fat in water, butter an emulsion of water in fat. The finely divided liquid is known as the dispersed phase and the other as the continuous phase.w 2、、Colloidal solution（胶体溶液）（胶体溶液） wwhen matter exists in a state of division intermediate to true solution (e.g. sugar in water) and suspension (e.g. chalk in water) it is said to be in colloidal solution or colloidal suspension. The typical characteristics of a colloid are: w small particle sizew electrical charge w affinity of the particles for water molecules.牛奶的基本知识 wIn milk the whey proteins are in colloidal solution and the casein in colloidal suspension乳乳清清蛋蛋白白是是胶胶体体溶溶液，酪蛋白是胶体悬浮液液，酪蛋白是胶体悬浮液.wOrganic compounds contain mainly carbon, oxygen and hydrogen. Inorganic compounds contain mainly other atoms.牛奶的基本知识 w3、、True solutions（真溶液）（真溶液）: wMatter which, when mixed with water or other liquids, forms true solutions, is divided into:w non-ionic solutions. When lactose is dissolved in water, no important changes occur in the molecular structure of the lactose.w ionic solutions. When common salt is dissolved in water, captions ( Na + ) and anions (Cl – ) are dispersed in the water, forming an electrolyte.牛奶的基本知识 Appearance（外观）（外观）w The opacity of milk is due to its content of suspended particles of wfat, proteins and certain minerals.w乳的不透明性是由于乳中含有的悬浮物质，包括脂肪、蛋白质和其他矿物质wThe color varies from white to yellow according to the coloration (carotene content) of the fat. Skim milk is more transparent, with a slightly bluish tinge.w颜色从白色到淡黄色的变化，是由于脂肪中的有色物质（胡萝卜素含量）不同引起的牛奶的基本知识 Density（密度）（密度）w The density of cows’ milk normally varies between 1.028 and 1.038 g/cm 3 depending on the composition. wThe density of milk at 15.5 °C can be calculated according to following formula:牛奶的基本知识 比重： 15.5 °C 时与同体积的水的重量比密度： 20 °C时与同体积4 °C质量比 牛奶的基本知识 牛奶的基本知识 wD20/4=Dt/4+（t-20）*0.002wD比重=D20/4+0.002wBuffalo milk 1.030-1.032wSkim milk 1.035-1.037wTotal solids=0.25D+1.2F+CwC=0.14(如何获得）D：密度；F：脂肪 牛奶的基本知识 Flavour wChange:wType of feedwStage of lactationwCondition of udderwSanitation(卫生） during milkingwBreed牛奶的基本知识 wSweet taste of lactosewSalty of mineralswFatty acid ,fat, phospolipidswTemperaturewOtherwCu , sun, storage牛奶的基本知识 Osmosis渗透压w Osmosis is the term used to describe the spontaneous flow of pure water into an aqueous solution, or from a less to a more concentrated solution, when separated by a suitable membrane. wNow the water molecules will diffuse through the membrane into the sugar solution and dilute it to a lower concentration. This process is called osmosis. 牛奶的基本知识 w Osmotic pressure is controlled by the number of molecules or particles, not the weight of solute; thus 100 molecules of size 10 will have 10 times the osmotic pressure of 10 molecules of size 100. wIt follows that for a given weight, the smaller the molecules the higher the osmotic pressure. wMilk is formed from blood, the two being separated by a permeable membrane, hence they have the same osmotic pressure, or in other words, milk is isotonic with blood. w The same condition applies to milk, the total osmotic pressure being made up as in Table 2.8.牛奶的基本知识 牛奶的基本知识 牛奶的基本知识 Freezing pointw The freezing point of milk is the only reliable parameter to check for adultery-action with water. The freezing point of milk from individual cows has been found to vary from –0.54 to –0.59°C.w 牛奶的基本知识 Boiling point w1kg/cm2w100.17 °C w牛乳浓缩一倍可上升高0.5 °C 牛奶的基本知识 Viscosity wAt 20 °C , Viscosity of milk is 0.0015-0.002p=1.5-2.0cpwFactors:w Temperaturew NFS :nfs=c fat w Acidity w Enzymew Storage牛奶的基本知识 Acidityw The acidity of a solution depends on the concentration of hedonism ions [H + ] in it. w Normal milk is a slightly acid solution with a pH falling between 6.5 and 6.7 with 6.6 the most usual value. Temperature of measurement near 25°C. The pH is checked with a pH-meter.牛奶的基本知识 wNatural aciditywTitratable aciditywFermenting acidity 牛奶的基本知识 wSource of acidity:wProtein :3-4wCitrates and phosphates: 10-12 wCO2: 2牛奶的基本知识 wThe titratable acidity can be expressed in various values basically as a result of the strength of the sodium hydroxide (NaOH) needed at titration. w°SH = Soxhlet Henkel degrees, obtained by titrating 100 ml of milk with N/4 NaOH , using phenolphtalein as the indicator. Normal milks give values about 7. This method is mostly used in Central Europe. 牛奶的基本知识 w°Th = Thörner degrees, obtained by titrating 100 ml of milk, thinned with 2 parts of distilled water, with N/10 NaOH, using phenolphtalein as the indicator. Normal milks give values about 17. Mostly used in Sweden and the CIS. w°D = Dornic degrees, obtained by titrating 100 ml of milk with N/9 NaOH, using phenolphtalein as the indicator. Normal milks give values about 15. Mostly used in the Netherlands and France.w % l.a. = per cent lactic acid, obtained as °D with the result divided by 100. Frequently used in the UK, USA, Canada, Australia and New Zealand. In table 2.9 the various expressions for the titratable acidity are combined. The determination of acidity according to Thörner degrees is visualised in figure.牛奶的基本知识 wExample: 1.7 ml of N/10 NaOH are required for titration of a 10 ml sample of milk. 10 x 1.7 = 17 ml would therefore be needed for 100 ml, and the acidity of the milk is consequently 17 °Th.牛奶的基本知识 牛奶的基本知识 第四节第四节 Microorganismsw Bacteria in milk牛奶的基本知识 wWhen milk is secreted in the udder it is virtually sterile. But even before it leaves the udder it is infected by bacteria which enter through the teat channel(乳头导管）. wThese bacteria are normally harmless and few in number, only a few tens or hundreds per ml. wHowever, in cases of bacterial udder inflammation（发炎） (mastitis，乳房炎), the milk is heavily contaminated with bacteria and may even be unfit for consumption, not to mention the suffering of the cow. 牛奶的基本知识 Infection at the farmwIn the course of handling at the farm, milk is liable to be infected by various microorganisms, mainly bacteria. wThe degree of infection and the composition of the bacterial population depend on污染细菌的种类和数量取决于： wthe cleanliness of the cow’s environment奶牛环境的卫生状况w the cleanliness of the surfaces with which the milk comes into contact, e.g. the pail or milking machine, the strainer, the transport churn or the tank and agitator.挤奶过程以及与牛奶 接触的物体卫生状况牛奶的基本知识 Bacteria count in milkwFarm milk may contain anything from a few thousand bacteria per ml, if it comes from a hygienic farm, up to several millions if the standard of cleaning, disinfections and chilling is poor. w For milk to be classed as top quality, the bacteria count, the CFU (Colony Forming Units), should be less than 100 000 per ml.国际上规定细菌数量低于10万/mLw我国规定牛乳细菌数量低于50万/mL牛奶的基本知识 w Rapid chilling to below 4°C contributes greatly to the quality of the milk at the farm. This treatment slows down the growth of the bacteria in the milk, thereby greatly improving its keeping qualities. 迅速将牛乳降低至4°C以下，是保证原料乳质量的重要手段wThe influence of temperature on bacterial development in raw milk is shown in the graph in figure 4.14. Starting from 300 000 CFU/ml we can see the speed of development at higher temperatures and the effect of cooling to 4°C. Cooling to 4°C or even lower, 2°C, in conjunction with milk-1 in makes it possible to deliver milk at two-day intervals provided that the milk container/tanker is insulated.不同温度下细菌数量的变化牛奶的基本知识 牛奶的基本知识 Principal bacteria in milk（乳中主要细菌，乳酸菌）（乳中主要细菌，乳酸菌） Lactic acid bacteria （最适温度、发酵乳糖产生乳酸（最适温度、发酵乳糖产生乳酸等，发酵柠檬酸，产生酶及应用）等，发酵柠檬酸，产生酶及应用）牛奶的基本知识 wOviform bacteria（卵形细菌）（卵形细菌） wOviform bacteria are facultative anaerobic(兼性厌氧） with an optimum temperature of 30 – 37°C. They are found in intestines, in manure（肥料）, in soil, in contaminated water and on plants. wThey ferment lactose to lactic acid and other organic acids, carbon dioxide and hydrogen and they break down milk protein, resulting in an off flavor and smell. Some soil bacteria also cause mastitis（乳房炎）. 牛奶的基本知识 wOviform bacteria can cause serious trouble in cheese making.给干酪生产带来严重的问题 wBesides causing off flavor, 异味wthe relatively strong gas formation will result in an un-wanted texture at an early stage. 气体形成，在干酪成熟的早期阶段，不良组织质地wThe metabolism of oviform bacteria ceases at a pH just below 6. This explains their activity at an early stage of fermentation, before all lactose is broken down. 牛奶的基本知识 wOviform bacteria are killed by HTST pasteurization. 牛奶的基本知识 Butyric acid bacteria（丁酸菌）（丁酸菌）wButyric acid bacteria are very common in nature. They are found in the soil, on plants, in manure, etc. and easily find their way into milk.广泛存在于自然界，并以多种途径进入到牛乳中 wBadly stored silage and fodder, contaminated with soil, may have extremely high counts of spores of butyric acid bacteria. This results in the milk becoming heavily infected with these organisms.wButyric acid bacteria are anaerobic厌氧 spore-forming microorganisms with an optimum temperature of 37°C. They do not grow well in milk, which contains oxygen, but thrive in cheese where anaerobic conditions prevail. 牛奶的基本知识 Prop ionic acid bacteria（丙酸菌）（丙酸菌）w They do not form spores, their optimum temperature is about 30°C, and several species survive HTST pasteurization. wThey ferment lactate to prop ionic acid, carbon dioxide and other products. wPure cultures of prop ionic acid bacteria are used (together with certain lactobacilli and lactococci) in the manufacture of Menthol, Grayère, Carlsberg, Gregé and Macadam cheese, where they are responsible for the formation of eyes and contribute to the characteristic flavor.有些丙酸菌用于干酪生产，对气眼的形成和风味有贡献牛奶的基本知识 Fungi（真菌）（真菌）w wFungi are a group of micro-organisms which are frequently found in nature among plants, animals and human beings. wDifferent species of fungi vary a great deal in structure and method of reproduction. Fungi may be round, oval or threadlike. The threads may form a network, visible to the naked eye, in the form of mould on food, for example. Fungi are divided into yeasts and moulds.牛奶的基本知识 wYeasts wYeasts are single-cell organisms of spherical, elliptical or cylindrical shape. The size of yeast cells varies considerably. Brewer’s yeast, Saccharomyces cervical, has a diameter of the order of 2 – 8 mm and a length of 3 – 15 mm. Some yeast cells of other species may be as large as 100 mm.牛奶的基本知识 wMoulds wThe category of moulds comprises a fairly heterogeneous group of multi-cellular, threadlike fungi. Superficially they resemble each other very closely, but in fact they belong to quite different groups of fungi. wThe moulds consist of thread-like strands of cells called hyphen. The mass of hyphen which can be seen with the naked eye is called mycelium. The hyphen may or may not have crosswalks between the cells and are usually branched. The hyphen are the vegetative part of the mould, often colorless, and secrete enzymes by which they degrade food. 牛奶的基本知识 Bacteriophagesw wTort, an English scientist, discovered as early as 1915 that certain cultures of staphylococci were disrupted and broken down. A couple of years later d‘Harrell, a Canadian scientist, after having made similar observations, postulated that the phenomenon was caused by invisible organisms feeding on the bacteria. He called them “bacteriophages” (the last part from the Greek word phage in, meaning to eat). wBacteriophages are thus viruses, i.e. bacterial parasites. By themselves they can persist, but they cannot grow or replicate except within bacterial cells. They have very specific hosts, e.g. single species of strains of baste-rain.牛奶的基本知识 •Structure of bacteriophages牛奶的基本知识 牛奶的基本知识 。
