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Dairy Science & Technology

Dr. WANG Jianming

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References:www.foodsci.uoguelph.ca/dairyedu/home.htmlDairy processing handbook, 2003University elite course website赵晋府主编.食品工艺学.北京:中国轻工业出版社，2002 www.foodmate.comhttp://jds.fass.org/http://www.ales2.ualberta.ca/afns/courses/nufs403/PDFs/chapter17.pdfhttp://w16.easy-share.com/1700118657.htmlhttp://www.foodsci.uoguelph.ca/dairyedu/home.html

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Chapter 1 IntroductionThis course is about the study of milk and milk-derived

food products from a food science perspective. It focuses on the biological, chemical, physical, and microbiological aspects of milk itself, and on the technological (processing) aspects of the transformation of milk into its various consumer products, including beverages, fermented products, concentrated and dried products, butter and ice cream.

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World-wide Milk Consumption & Production

The total milk consumption (as fluid milk and processed products) per person varies widely from highs in Europe and North America to lows in Asia.Table 1 illustrates milk per capita consumption

information from various countries of the world. Table 2 shows the quantity of raw milk produced around the world.

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Per Capita Consumption of Milk and Milk Products in Various Countries, 2006 data.

3.712.2111.2UK (2005)5.622.2112.5Switzerland1.09.6119.1Spain (2005)4.316.0116.7Norway3.320.4122.9Netherlands2.910.5129.8Ireland1.018.5145.5Sweden5.319.1183.9Finland

Butter/kgCheeses /kgLiquid Milk Drinks /LCountry

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Per Capita Consumption of Milk and Milk Products in Various Countries, 2006 data

2.116.083.9USA6.37.190.0N Zealand7.323.992.2France6.422.492.3Germany4.218.492.6E U (25 c)3.312.294.7Canada3.711.7106.3Australia

Butter/kgCheeses /kgLiquid Milk Drinks /LCountry

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Per Capita Consumption of Milk and Milk Products in Various Countries, 2006 data.

N/AN/A8.8ChinaN/A2.140.7Mexico2.823.757.3Italy0.710.765.8Argentina0.728.969.0Greece

Butter/kgCheeses /kgLiquid Milk Drinks /LCountry

Source: International Dairy Federation, Bulletin 423/2007

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Cow milk production ('000 tons) in selected countries in the world (2006)

7854Canada14,359United Kingdom9550Australia15,000New Zealand

10,000Turkey24,195France10,250Argentina25,750Brazil10,352Mexico27,955Germany10,995Netherlands31,100Russia11,186Italy31,934China11,970Poland39,759India13,287Ukraine82,462United States

Source: International Dairy Federation, Bulletin 423/2007

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Chapter 2 Milk introduction Milk is the only food of the young mammal during the first period of its life.The substances in milk provide both energy and the building materials necessary for growth. Milk also contains antibodies which protect the young mammal against infection. A calf needs about 1 000 litres of milk for growth,Selective breeding has resulted in dairy cows which yield an average of more than 6 000 liters of milk per calf, i.e. six times as much as the primitive cow. Some cows can yield 14 000 liters or more.

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What’ milk?Milk is the first food of young mammals. It provides a high-quality protein, a source of energy, and vitamins and minerals. Worldwide, many mammalian species are used to produce milk and milk products. Some of these include goats, sheep, horses, and yaks. The focus of this chapter is milk from dairy cows.Most milk in the United States is produced by cows. The dairy industry produces milk as a fluid product and in a variety of manufactured products including butter, cheeses, condensed, dry, and cultured.

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Milk Composition The role of milk in nature is to nourish and provide immunological protection for the mammalian young.

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Composition of Milk from Different Mammalian Species (per 100 g fresh milk)

5020.149.410.2Seal4431.342.310.9Whale1713.013.19.0Mouse737.04.01.6Monkey855.35.04.0Elephant386.10.61.9Donkey674.73.82.9Goat1184.89.04.1Buffalo727.04.21.1Human664.63.73.2Cow

Energy (kc)Carbo- (g)Fat (g)Protein (g)

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Period of lactation The heifer is bred (naturally or by insemination) before the age of 2 years.The gestation period is 9 months.After calving the cow gives milk for 10 months. 1 –2 months after calving the cow will again be bred.After having given birth to some 5 calves, the cow is generally slaughtered.

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Cow milk The most widespread milking animal in the world is the cow. Heifers reach sexual maturity at the age of seven or eight months but are not usually bred until they are 15 – 18 months old. The period of gestation is 265 –300 days, varying according to the breed of cow, so a heifer produces her first calf at the age of about 2 –2.5 years.

（Holstein Friesian）

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Sheep (ewe) milkAmong the numerous breeds of sheep it is not easy to define dairy breeds, except by the purpose for which they are bred. Some breeds are mainly kept for production of meat and wool, but are occasionally also milked.The production per lactation

does not exceed 100 kg. On the other hand, the milk production of some meat breeds can be 150 to 200 kg

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Goat milkIn a well-managed milk production unit a goat can produce between 400 and 900 kg milk per lactation. The period of lactation varies from 200 to 300 days.

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Composition of milk from different animals

0.84.54.90.94.95.8sheep

0.84.74.10.92.73.6goat

0.74.87.50.53.54.0buffalo

0.74.83.70.72.83.5cow

0.56.21.70.91.32.2horse

0.27.0 3.8 0.70.5 1.2 human

Ash ％

Carbohy.％

Fat ％

Whey %

Casein％

Protein％Animal

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Chapter 3 Character of milk

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References:http://124.128.244.155/shengwu/uploadfile/200813182025355.ppthttp://nhjy.hzau.edu.cn/kech/spgc/http://cc.sjtu.edu.cn/courses/food/jxxg_jzcl.aspx

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The chemistry of milkThe principal constituents of milk are water, fat, proteins, lactose (milk sugar) and minerals (salts). Milk also contains trace amounts of other substances such as pigments, enzymes, vitamins, phospholipids (substances with fatlike properties), and gases.Cows’ milk consists of about 87% water and 13% dry substance. The dry substance is suspended or dissolved in the water.

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References:http://rapidshare.com/files/79000593/Principles_of_Food_Chemistry__3rd.pdfhttp://depositfiles.com/en/files/0rj309olghttp://depositfiles.com/en/files/0rj309olghttp://extension.usu.edu/AITC/teachers/pdf/experiments_foodscience.pdfhttp://www.ebook3000.com/Food-Additives-Databook_20956.htmlhttp://chemistry.about.com/od/chemistrylabexperiments/ig/Chemistry-Laboratory-Glassware/

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Physical-chemical status of cows’ milk

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Relative sizes of particles in milk.

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Quantitative composition of milk

The quantities vary considerably between cows of different breeds and between individual of the same breed.

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Milk fatMilk fat consists of

triglycerides (the dominating components), di- andmonoglycerides, fatty acids, sterols, carotenoids (the yellow color of the fat), vitamins (A, D, E, and K), and all the others, trace elements, are minorcomponents. http://www.youtube.com/watch?v=y6n6W8_Wvc0

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fat globulesAs the fat globules are not only the largest particles in the milk but also the lightest they tend to rise to the surface when milk is left to stand in a vessel for a while。Milk fat is liquid when milk leaves the udder at 37°C. This means that the fat globules

can easily change their shape when exposed to moderate mechanical treatment 。

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Chemical structure of milk fatComposition and thickness of the membrane are not constant because components are constantly being exchanged with the surrounding milk serum.

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Classes of milk proteinsThe proteins can be

classified in various ways according to their chemical or physical properties and their biological functions.

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Casein Casein is a group name for the dominant class of proteins in milk. The caseins easily form polymers containing several identical or different types of molecules.The three subgroups of casein, as-casein, k-casein and b-casein, are all heterogeneous and consist of 2 – 8 genetic variants.

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References:http://www.youtube.com/watch?v=lijQ3a8yUYQhttp://www.youtube.com/watch?v=T500B5yTy58&feature=relatedhttp://www.youtube.com/watch?v=3IL_Df5ouUc&feature=related

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The electrical status of milk proteins

The side chains of some amino acids in milk proteins carry an electric charge which is determined by the pH of the milk. When the pH of milk is changed by addition of an acid or a base, the charge distribution of the proteins is also changed.

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Precipitation by acidThe pH will drop if an acid is added to milk or if acid-producing bacteria are allowed to grow in milk. This will change the environment of the casein micelles in two ways.Both methods of action initiate a change within the micelles, starting with growth of the micelles through aggregation and ending with a more or less dense coagulum.

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Precipitation by acidIf a large excess of acid is added to a given coagulum theCasein will redissolve, forming a salt with the acid. If hydrochloric acid is used, the solution will contain casein hydrochloride, partly dissociated into ions.

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Precipitation by enzymesThe 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 chain.

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Precipitation by enzymesThe formation of the curd is due to the sudden removal of the hydrophilic macropeptides and the imbalance in intermolecular forces caused thereby.Bonds between hydrophobic sites start to develop and are enforced by calcium bonds which develop as the water molecules in the micelles start to leave the structure. This process is usually referred to as the phase of coagulation and syneresis.

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References:http://www.techfood.com.cn/passage/down/2006103060066033.pdfhttp://foodsci.sytu.edu.cn/sphx/http://nhjy.hzau.edu.cn/kech/sphx/http://jpkc.njau.edu.cn/spwswx/

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Whey proteinsWhey protein is the name commonly applied to milk

serum proteins.If the casein is removed from skimmilk 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.As long as they are not denatured by heat, they are not precipitated at their isoelectric points.

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Classification The whey proteins are: a-lactalbumin &b-lactoglobulin

Whey proteins in general, and a-lactalbumin in particular, have very high nutritional values. Their amino acid composition is very close to that which is regarded as a biological optimum. Whey protein derivatives are widely used in the food industry.

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Whey proteinsWhen 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.

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Stabilization of whey proteinsAs long as they are not denatured by heat, they are not precipitated at their isoelectric points. They are however usually precipitated by polyelectrolytes such as carboxymethyl cellulose.

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“Cooked” flavor of heat treated milk.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-lactoglobulinmolecule 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.

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Stability When 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.

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Physical-chemical status of cows’ milk

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Three simplified stages of influence on casein by an acid and alkali respectively.

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There are two ways to makecaseinate particles flocculateand coagulate: precipitation byacid and precipitation by enzymes

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“Cooked” flavor of heat treated milk.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-lactoglobulinmolecule 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.

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Enzymes in milkSeveral of the enzymes in milk are utilized for quality testing and control. Among the more important ones are peroxidase, catalase, phosphatase and lipase.

http://members.ift.org/IFT/Education/EduResources/eif.htm

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PeroxidaseThis enzyme is inactivated if the milk is heated to 80 °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 pasteurisationtemperature above 80 °C has been reached. This test is called Storch’s peroxidase test.

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CatalaseMilk from diseased udders has a high catalase content, while fresh milk from a healthy udder contains only an insignificant amount.By 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.

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PhosphatasePhosphatase is destroyed by ordinary pasteurisation(72°C for 15 – 20 seconds), so the phosphatase test can be used to determine whether the pasteurisationtemperature has actually been attained. The routine test used in dairies is called the phosphatase test.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 analyzed. The analysis should be carried out the same day, otherwise a phenomenon known as reactivation may occur.

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LipaseLipase splits fat into glycerol and free fatty acids. Excess free fatty acids in Milk and milk products result in a rancid taste.

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Milk is a buffer solutionMilk 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.

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pH 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. Other milk constituents also have this ability to bind or release ions, and the pH value therefore changes very slowly when acids or bases are added.

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Effects of heat treatmentMilk is heat treated at the dairy to kill any pathogenic microorganisms that may be present. Heat treatment also causes changes in the constituents of the milk. The higher the temperature and the longer the exposure to heat, the greater the changes. Within certain limits, time and temperature can be balanced against each other. Brief heating to a high temperature can have the same effect as longer exposure to a lower temperature.Both time and temperature must therefore always be considered in connection with heat treatment.

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FatAbove 135°C the proteins deposited on the fat globule membrane form a network which makes the membrane denser and less permeable. Homogenization downstream of the steriliser is therefore recommended in UHT treatment of products with a high fat content.

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ProteinThe major protein, casein, is not considered denaturable by heat within normal ranges of pH, salt and protein content.Whey proteins, on the other hand, particularly b-lactoglobulin which makes up about 50% of the whey proteins, are fairly heat sensitive. Denaturation begins at 65°C and is almost total when whey proteins are heated to 90°C for 5 minutes.

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Milk intended for cheesemaking should therefore not be pasteurized, or at any rate not at higher temperatures than 72°C for 15 – 20 seconds.In milk intended for cultured milk products (yoghurt, etc.), the whey protein denaturation and interaction with casein obtained at 90 – 95°C for 3 – 5 minutes will contribute to improved quality in the form of reduced syneresis and improved viscosity.Milk heated at 75°C for 20 – 60 seconds will start to smell and taste “cooked”.

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LactoseAt temperatures above 100°C a reaction takes place between lactose and protein, resulting in a brownish colour.

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VitaminsVitamin C is the vitamin most sensitive to heat, especially in the presence of air and certain metals.Pasteurization in a plate heat exchanger can however, be accomplished with virtually no loss of vitamin C. The other vitamins in milk suffer little or no harm from moderate heating.

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MineralsOf the minerals in milk only the important calcium hydroxyphosphate in the casein micelles is affected by heating. When heated above 75°C the substance loses water and forms insoluble calcium orthophosphate, which impairs the cheesemakingproperties of the milk. The degree of heat treatmentMust be carefully chosen.

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Chapter 4 Dairy Processing Milk provides high-quality protein, energy, vitamins, and minerals to human nutrition. Besides fluid milk the dairy industry produces a variety of milk products including butter, cheeses, condensed and dried products, and cultured products.

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References:http://www.chinappt.cn/moban/fajiaogongcheng.htmlhttp://rapidshare.com/files/49371604/MFM_xyp.rarhttp://jpkc.njau.edu.cn/spwswx/http://members.ift.org/IFT/Education/EduResources/mif.htm

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1 Fluid milkMilk is composed primarily of water (87 to 89 percent). Like other foods, if water is removed, the shelf life is extended. The term total milk solids describes the remaining 12 to 13 percent of milk. This includes the carbohydrates, lactose, fat, protein, and minerals of milk. The term milk solids-not-fat, excludes the fat and includes the lactose, caseins, whey, proteins, minerals (calcium, phosphorus, magnesium, potassium, sodium, chloride, and sulphur).

http://www.youtube.com/watch?v=2k8Puxz54FQ&feature=relatedhttp://www.youtube.com/watch?v=0SCaVRN3oKE

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1.1 Legal descriptionMilk is the lacteal secretion, practically free from colostrum, obtained by the complete milk milking of one or more healthy cows. Milk that is in the final package form for beverage use shall have been pasteurized or ultrapasteurized, an shall contain not less than 8.25% milk solids not fat and no less than 3.25% milk fat. Milk may have been adjusted by separating part of the milk fat therefrom, or by adding thereto cream, concentrated milk, dry whole milk, skim milk, concentrated skim milk or nonfat dry milk. Milk may be homogenized.

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1.2 Production practicesFew cows produce more milk in the United States, and the dairies are becoming larger. In major production areas, dairies of 1000 cows or more are common.Milk fresh from the cow is virtually a sterile product. All post milking handling must maintain the milk’s nutritional value and prevent deterioration caused by numerous physical and biological factors. In addition, equipment on the farm must be maintained to government and industry standards.

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1.3 Quality controlAt the dairy farm, inspectors monitor herd health, farm water supplies sanitation of milking equipment, milk temperature, holding times, and bacteria counts of milk. Violations of health standards result in heavy penalties up to and including suspension from business.All finished dairy products are tested regularly by state inspectors to ensure compliance with each of the following :

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(1) Standards of identity, which refers to such criteria as moisture, butterfat, and protein content

(2) Purity, which refers to such criteria as pathogens and residues. The Food and Drug Administration (FDA) sets standards of identity for beverage milk products.

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1.4 Processing When the milk arrives at the milk plant it is checked to make sure that it meets the standards for temperature, total acidity, flavor, odor, tanker cleanliness, and the absence of antibiotics. The butterfat and solids-not- fat contents of this raw milk are also analyzed. The amounts of butterfat (BF) and solids-not-fat (SNF) in the milk will vary according to time of year, breed of cow, and feed supply. Butterfat content, solids-not-fat content, and volume are used to determine the amount of money paid the producer.

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Once the load passes these receiving tests, it is then pumped into large refrigerated storage silos (nearly half-million pounds capacity) at processing plant.

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1.5 Pasteurizing & sterilizationAll raw milk must be processed within 72 hours of receipt at the plant. Milk is such a nutritious food that numerous naturally occurring bacteria are always present. The milk is pasteurized, which is a process of heating the raw milk to kill all pathogenic microorganisms that may be present. Pasteurization is not sterilization (sterilization eliminates all viable life-forms, whereas pasteurization does not).

http://www.agri.upm.edu.my/~szakaria/BIP3501/Lecture2.pdfwww.che.boun.edu.tr/che526/52610%20Heat%20Processing.PPT

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After pasteurization, some harmless bacteria may survive the heating process. These bacteria will cause milk to “go sour.” Keeping milk refrigerated is the best way to slow the growth of thee bacteria.

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Equipments

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ClassificationLow-Temperature Longer Time (LTLT): heat the milk to at least 63℃ and holds it at that temperature for at least 30 minutes.High Temperature/Short Time (HTST): heat the milk to at least 73 ℃ for at least 15 seconds. The milk is immediately cooled and packaged into plastic jugs or plastic-coated cartons.Ultrahigh temperature (UHT): sterilization of milk occurs when it is heated to 130 ℃ to 150 ℃ for 2 to 6 seconds.

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1.6 Butter fatButterfat content accounts for several different types of products. Whole milk, 2%, 1%, nonfat, and Half & Half are some examples. A machine called a separator separates the cream and skim portions of the milk. This usually comes off the separator with fat contents in the 35-45%.

http://wwwfst.ag.ohiostate.edu/min/Food%20Lipids%20Chemistry.htm

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1.7 HomogenizationMilk is homogenized to prevent the cream portion from rising to the top of the package. Cream is lighter in weight than milk. The cream portion of unhomogenized milk would form a cream layer at the top of the carton. A “homogenizer” forces the milk under high pressure through a valve that breaks up the butterfat globules to such small sizes they will not coalesce (stick together). Homogenization does not affect the nutrition or quality of the product.

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1.8 Beverage milkWhile the fat content of most raw milk is 4 percent or higher, the fat content in most beverage milks has been reduced to 3.4 percent. Lower-fat alternatives, such as 2 percent fat, 1 percent fat, or skim milk (less than 0.1 percent fat), are also available in most markets. These products are either produced by partially skimming the whole milk, or by completely skimming it and then adding an appropriate amount of cream back to achieve the desired final fat content.

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1.9 Nutritional qualitiesVitamins may be added to both full fat and reduced fat milks. Vitamins A and D (the fat-soluble ones) are often added to milk. Vitamin A is lost during fat separation and heating, and vitamin D is not present in milk. These are supplemented in the form of a water-soluble emulsion. Many states have milk standards that require the addition of milk solids. These solids represent the natural mineral (calcium and iron), protein (casein), and sugar (lactose) portion of nonfat dry milk.

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1.10 Quality controlQuality control personnel conduct numerous tests on the raw and pasteurized products to ensure optimum quality and nutrition. All milk products have a sell-by date printed on the package. This is the last day the item should be offered for sale.

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1.11 PackagingOnce the milk has been separated, standardized, homogenized, and pasteurized, it is held below 40℉ in insulated storage tanks, the packaged into gallon, half-gallon, quart, pint, and half-pint containers. The packaging machines are maintained under strict sanitation specifications to prevent bacteria from being introduced into the pasteurized product.

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2 Milk products and by-productsMilk products and by-products include butter, concentrated and dried products, cheese, whey products, yogurt and other fermented products, and ice cream.

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2.1 ButterButter is made by churning pasteurized cream. Churning breaks the fat globule membrane so the emulsion breaks, fat coalesces, and water (buttermilk) escapes. Federal law requires that it contain at least 80 percent milkfat. Salt and coloring may be added. Nutritionally, butter is a fat. Whipped butter is regular butter whipped for easier spreading. Whipping increases the amount of air in butter and increases the volume of butter per pound.

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References:http://www.youtube.com/watch?v=qwb2uZLSLhw&feature=relatedhttp://www.ebaumsworld.com/video/watch/490392/

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2.2 Concentrated and dried dairy products

Fluid milk contains approximately 88 percent water. Concentrated milk products are obtained through partial water removal. Dried dairy products have even greater amounts of water removed to usually less than 4 percent. The benefits of both these processes include an increased shelf life, convenience, product flexibility, decreased transportation costs, and storage.

http://www.skillsone.com.au/Industry/8/Rural Farming/Video/471/0/ The-wonderful-world-of-yoghurt/

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Concentrated dairy products include:

Evaporated skim or whole milkSweetened condensed milkCondensed buttermilkCondensed wheyDried dairy products include:Milk powder Whey powderWhey protein concentrates

http://www.niro.com/niro/cmsdoc.nsf/WebDoc/ndkw5y4brlLibrary

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2.3 cheeseTraditionally, cheese was made as a way of preserving the nutrients of milk. In a simple definition, cheese is the fresh or ripened product obtained after coagulation and whey separation of milk, cream, or partly skimmed milk, buttermilk, or a mixture of these products. It is essentially the product of selective concentration of milk. Thousands of varieties of cheeses have evolved that are characteristic of various regions of the world.

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References:http://www.youtube.com/watch?v=eofhmEvRtgs&feature=relatedhttp://www.youtube.com/watch?v=8VpliYoNh6E&feature=relatedhttp://www.youtube.com/watch?v=FPV8xEY8Qx0&feature=relatedhttp://www.metacafe.com/watch/yt-Jl09vriq3qI/goat_cheese_from_milking_goats_to_making_goat_cheese/

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2.3.1 cheese-making steps (1) ① Treatment of milk: Like most dairy products,

cheese milk must first be clarified, separated, and standardized. Homogenization is not usually done for most cheese milk.

② Coagulation: coagulation is essentially the formation of a gel by destabilizing the casein micelles causing them to aggregate and form a network that partially immobilizes the water and traps the fat globules in the newly formed matrix.

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Cheese Flow sheet:From Fresh cheese to Processed cheese

Quarg MozzarellaTwarog SemiHardcheese

Soft

cheese

Processed

cheese

Hard

cheese

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Additives in cheese makingCalcium chloride: added to replace calcium lost during pasteurization. Because milk color varies from season to season, color may added to standardize the color of the cheese throughout the year. The addition of lipases are common to ensure proper flavor development through fat hydrolysis.Enzyme: chymosin, or rennet, is most oftenusedfor enzyme coagulation.Acid: lowering the pH of the milk results in aggregation.

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Cheese making steps (2)① Inoculation and milk ripening: The basis of cheese

making relies on the fermentation of lactose by lactic acid bacteria (LAB). After inoculation with the starter culture, the milk is held for 45 to 60 minutes at suitable temperature to ensure that the bacteria are active, growing, and have developed acidity. This stage is called ripening the milk and is done prior to renneting.

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Cheese making steps (3)① Heat-acid: heat causes denaturation of the

whey proteins. The denatured proteins then interact with the caseins. With the addition of acid, the caseins precipitate with the whey proteins.

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Cheese making steps (4)(5) Curd treatment: after the milk gel has been

allowed to reach the desired firmness, it is carefully cut into small pieces with knife blades or wires. When the curds have reached the desired moisture and acidity, they are separated from the whey.

(6) Cheese ripening: except for fresh cheese, the curd is ripened, or matured, at various temperatures and times until the characteristic flavor, body, and texture profile is achieved.

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2.4 YogurtYogurt (also spelled yoghurt) is a semisolid fermented milk product that originated centuries ago in Bulgaria. Yogurt flavor and aroma varies from one region to another but, the basic ingredients and manufacturing are essentially consistent.The starter culture for most yogurt production in North America is a blend of Streptococcus salivarius thermophilus (ST) and Lactobacillus delbrueckii bulgaricus (LB). The yogurt mixture coagylates during fermentation due to the drop in pH.

http://www.ozscientific.com/Presentations/OzSC002%20Probiotic%20yogurt%20drink.pdf

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2.4.1 Other fermented milk beverages

Cultured buttermilk was originally the fermented by-product of butter manufacture, but today it is more common to produce cultured buttermilks from skim or whole milk.

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2.4.2 Acidophilus milkAcidophilus milk is a traditional milk fermented with Lactobacillus acidophilus (LA), which has been thought to have therapeutic benefits in the gastrointestinal tract.

2.4.3 Sour creamCultured cream usually has a fat content between 12 and 30 percent, depending on the required properties.

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2.4.4 Other fermented productsMany other fermented dairy products include kefir, koumiss, beverages based on bulgaricus or bifidusstrains, labneh, and a host of others.

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2.5 Ice creamIce cream is greater than 10 percent milkfat by legal definition, and as high as 16 percent gat in some premium ice creams, it contains 9 to 12 percent milk solid-not-fat, 12 to 16 percent sweeteners, 0.2 to 0.5 percent stabilizers and emulsifiers and 55 to 64 percent water. These percentages are by weight, either in the mix or in the frozen ice cream. However, when frozen, about one half of the volume of ice cream is air. All ice cream is made from a basic white mix.

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Basic steps in the manufacturing of ice cream

(1) Blending of the mix ingredients(2) Pasteurization(3) Homogenization(4) Aging the mix (5) Freezing(6) Packaging(7) Hardening

http://www.florenceicecream.it/http://www.youtube.com/watch?v=SiICaHAOrGE&NR=1

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3 Quality productsThe USDA establishes U.S. grade standards to describe different grades of quality in butter, cheese (Cheddar, Colby, Monterey, and Swiss), and instant nonfat dry milk. The FDA established the Grade A designation for fluid milk products, yogurt, and cottage cheese. Manufacturers use the grade standards to :

(1) Identify levels of quality(2) Provide a basis for establishing price at wholesale(3) Supply consumers with a choice of quality levels

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Storage of dairy productsOnce the consumer purchases milk and milk products, proper storage conditions and time maintain quality and safety. Guidelines of storage times for maintaining the quality of some products in the home refrigerator include the following:

(1) Fresh milk-5 days(2) Buttermilk-10 to 30 days(3) Condensed or evaporated milk-opened 4 to 5 days(4) Half & half, light cream, and heavy cream-10

days(5) Sour cream-2 to 4 weeks

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4 Milk substitutesOne well-known substitute for a milk product is margarine. It is made fro vegetable fat. Other substitutes include frozen desserts, coffee whiteners, whipped toppings, and imitation milk (soy milk). These substitutes are made by combining nondairy fats or oils with certain classes of milk components.

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5 Reduced fat productsIn an effort to reduce calories, saturated fat, and cholesterol in the diet, a number of low-fat, and no-fat products have appeared on the market. Where fat is replaced in a milk product, the replacement must perform the same function as the original fat. In other words, fat replacements must give he product the same texture (mouth feel). Fat substitutes have been made of proteins processed into very small particles and of carbohydrates that bind large amounts of water.

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Review questions(1) What heats the milk to at least 161℉ for at least

15 seconds? What eliminates all viable life-forms?(2) What is made by churning pasteurized cream?(3) List the steps in cheese making(4) After water, what are the components of milk

solids?(5) Name three dried milk products.(6) LAB or lactic acid bacteria aid in making ﹍?

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Student activities(1) Compare the taste of a dairy product to that of

one of the substitutes such as margarine, dairy creamer, or soy milk.

(2) Develop a taste test to identify various types of cheese.

(3) How is advertising used to encourage consumers to drink more milk?