And technology 主 编 蒋爱民 arth hill douglas goff carl lachat 副主编 樊明涛 李志成 马兆瑞 丁武 张静 西北农林科技大学 二零零三年八月




НазваниеAnd technology 主 编 蒋爱民 arth hill douglas goff carl lachat 副主编 樊明涛 李志成 马兆瑞 丁武 张静 西北农林科技大学 二零零三年八月
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《畜产食品工艺学》双语教材


DAIRY SCIENCE

AND TECHNOLOGY



主 编 蒋爱民 ARTH HILL DOUGLAS GOFF CARL LACHAT

副主编 樊明涛 李志成 马兆瑞 丁武 张静


西北农林科技大学

二零零三年八月


课程说明


随着教改的深入 “双语教材”建设成了教改的试点内容之一。

西北农林科技大学食品科学与工程学院“畜产食品工艺学”课程组6名主讲教师中4名到国外进行过合作研究或培训。从1990年开始课程组接合食品专业英语教学,介绍外国畜产食品加工贮藏领域中的最新研究成果。2002年开始,利用本课程组教师在国外合作研究和进修学习期间获得的英语专业资料,着手“畜产食品工艺学”双语教学工作。

根据西北农林科技大学“双语课程建设”要求,“双语课程”配套教材必须采用全英语教材。“畜产食品工艺学”双语教材建设计划分“乳、肉、蛋”三部分。目前,已经完成“乳品科学与技术”双语教材,并试用。“畜产食品工艺学”2004年被列入西北农林科技大学“双语教学”课程建设规划。

《乳品科学与技术》编写过程中,加拿大Guelph大学食品科学系执行主席ARTH HILL 博士、加拿大Guelph大学食品科学系DOUGLAS GOFF教授和比利时GUENT大学CARL LACHAT博士提供了大量的资料。书稿完成后,ARTH HILL博士对全稿进行了详细审阅、修改。

课程组计划继续与外国专家合作,在2年之内完成“肉品科学与技术”和“蛋品科学与技术”配套双语教材。

学科发展日新月异,不妥之处,希望兄弟院校及读者提出宝贵意见,更希望共同编写。


蒋爱民

电话029-87091664/879092940

Email:jiangaimin20000@163.com

陕西杨陵

2004-7-14


CONTENTS


INTRODUCTION……………………………………………………………………………1

CHAPTER 1 Milk Production and Biosynthesis …………………………………2

CHAPTER 2 Milk Grading and Defects………………………………………… 6

CHAPTER 3 Dairy Chemistry and Physics………………………………… ……10

CHAPTER 4 Dairy Microbiology …………………………………………………27

CHAPTER 5 Dairy Processing……………………………………………………… 36

Clarification, Separation, Standardization………………………………36

Pasteurization………………………………………………………………… 38

UHT Treatment …………………………………………………………… …46

Homogenization ……………………………………………………………… 49

Membrane Processing………………………………………………… …… 51

Evaporation and Dehydration……………………………………… …… 54

Production and Utilization of Steam and Refrigeration………………62

CHAPTER 6 Dairy Products………………………………………………………… 64

Overview and Fluid Milk Products……………………………………… 64

Concentrated and Dried Milk Products………………………………… 66

Cultured Dairy Products…………………………………………………… 70

Whipped Cream ……………………………………………… … …… … 77

Ice Cream ……………………………………………………………………… 78

Butter Manufacture………………………………………………………… 135

APPENDIX Glossary of Terms……………………………………………………146

Introduction


Milk is as ancient as mankind itself, as it is the substance created to feed the mammalian infant. All species of mammals, from man to whales, produce milk for this purpose. Many centuries ago, perhaps as early as 6000-8000 BC, ancient man learned to domesticate species of animals for the provision of milk to be consumed by them. These included cows (genus Bos), buffaloes, sheep, goats, and camels, all of which are still used in various parts of the world for the production of milk for human consumption.

Fermented products such as cheeses were dicovered by accident, but their history has also been documented for many centuries, as has the production of concentrated milks, butter, and even ice cream.

Technological advances have only come about very recently in the history of milk consumption, and our generations will be the ones credited for having turned milk processing from an art to a science. The availability and distribution of milk and milk products today in the modern world is a blend of the centuries old knowledge of traditional milk products with the application of modern science and technology.

The role of milk in the traditional diet has varied greatly in different regions of the world. The tropical countries have not been traditional milk consumers, whereas the more northern regions of the world, Europe (especially Scandinavia) and North America, have traditionally consumed far more milk and milk products in their diet. In tropical countries where high temperatures and lack of refrigeration has led to the inability to produce and store fresh milk, milk has traditionally been preserved through means other than refrigeration, including immediate consumption of warm milk after milking, by boiling milk, or by conversion into more stable products such as fermented milks.

World-wide Milk Consumption and 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. However, as the various regions of the world become more integrated through travel and migration, these trends are changing, a factor which needs to be considered by product developers and marketers of milk and milk products in various countries of the world.

Even within regions such as Europe, the custom of milk consumption has varied greatly. Table 1 illustrates milk per capita consumption information from various countries of the world. Several trends can be observed from these data. Consider for example the high consumption of fluid milk in countries like Ireland and Sweden compared to France and Italy where cheeses have tended to dominate milk consumption. When you also consider the climates of these regions, it would appear that the culture of producing more stable products (cheese) in hotter climates as a means of preservation is evident.

CHAPTER 1 Milk Production and Biosynthesis

Milk Production


Milk is the source of nutrients and immunological protection for the young cow. The gestation period for the female cow is 9 months. Shortly before calving, milk is secreted into the udder in preparation for the new born. At parturition, fluid from the mammary gland known as colostrum is secreted. This yellowish coloured, salty liquid has a very high serum protein content and provides antibodies to help protect the newborn until its own immune system is established. Within 72 hours, the composition of colostrum returns to that of fresh milk, allowing to be used in the food supply.

The period of lactation, or milk production, then continues for an average of 305 days, producing 7000 kg of milk. This is quite a large amount considering the calf only needs about 1000 kg for growth.

Within the lactation, the highest yield is 2-3 months post- parturition, yielding 40-50 L/day. Within the milking lifetime, a cow reaches a peak in production about her third lactation, but can be kept in production for 5-6 lactations if the yield is still good.


About 1-2 months after calving, the cow begins to come into heat again. She is usually inseminated about 3 months after calving so as to come into a yearly calving cycle. Heifers are normally first inseminated at 15 months so she's 2 when the first calf is born. About 60 days before the next calving, the cow is dried off. There is no milking during this stage for two reasons:

  1. milk has tapered off because of maternal needs of the fetus

  2. udder needs time to prepare for the next milking cycle

The life of a female cow can be summerized as follows:

Age


0 Calf born

15 mos Heifer inseminated for first calf

24 mos First calf born - starts milking

27 mos Inseminated for second calf

34 mos Dried off

36 mos Second calf born - starts milking

Cycle repeats for 5-6 lactations


Effects of Milk Handling on Quality and Hygiene

Cleanliness


The environment of production has a great effect on the quality of milk produced. From the food science perspective, the production of the highest quality milk should be the goal. However, this is sometimes not the greatest concern of those involved in milk production. Hygienic quality assessment tests include sensory tests, dye reduction tests for microbial activity, total bacterial count (standard plate count), sediment, titratable acidity, somatic cell count, antibiotic residues, and added water.

The two common dye reduction tests are methylene blue and resazurin. These are both synthetic compounds which accept electrons and change colour as a result of this reduction. As part of natural metabolism, active microorganisms transfer electrons, and thus rate at which dyes added to milk are reduced is an indication of the level of microbial activity. Methylene blue turns from blue to colorless, while resazurin turns from blue to violet to pink to colourless. The reduction time is inversely correlated to bacterial numbers. However, different species react differently. Mesophilics are favoured over pscchrotrophsa, but psychrotrophic organisms tend to be more numerous and active in cooled milk.

Temperature


Milk production and distribution in the tropical regions of the world is more challenging due to the requirements for low-temperature for milk stability. Consider the following chart illustraing the numbers of bacteria per millilitre of milk after 24 hours:

5°C 2,600

10°C 11,600

12.7°C 18,800

15.5°C 180,000

20°C 450,000


Traditionally, this has been overcome in tropical countries by stabilizing milk through means other than refrigeration, including immediate consumption of warm milk after milking, by boiling milk, or by conversion into more stable products such as fermented milks.

Mastitis and Antibiotics


Mastitis is a bacterial and yeast infection of the udder. Milk from mastitic cows is termed abnormal. Its SNF, especially lactose, content is decreased, while Na and Cl levels are increased, often giving mastitic milk a salty flavour. The presence of mastitis is also accompanied by increases in bacterial numbers, including the possibility of human pathogens, and by a dramatic increase in somatic cells. These are comprised of leukocytes (white blood cells) and epithelial cells from the udder lining. Increased somatic cell counts are therefore indicative of the presence of mastitis. Once the infection reaches the level known as "clinical' mastitis, pus can be observed in the teat canal just prior to milking, but at sub-clinical levels, the presence of mastitis is not obvious.

Somatic Cell Count (000's/ml) Daily Milk Yield (kg): 1st Lactation Older Lactations

0-17 23.1 29.3

18-34 23.0 28.7

35-70 22.6 28.0

71-140 22.4 27.4

141-282 22.1 27.0

282-565 21.9 26.3

566-1130 21.4 25.4

1131-2262 20.7 24.6

2263-4525 20.0 23.6

>4526 19.0 22.5

Antibiotics are frequently used to control mastitis in dairy cattle. However, the presence of antibiotic residues in milk is very problematic, for at least three reasons. In the production of fermented milks, antibiotic residues can slow or destry the growth of the fermentation bacteria. From a human health point of view, some people are allergic to specific antibiotics, and their presence in food consumed can have severe consequences. Also, frequent exposure to low level antibiotics can cause microorganisms to become resistant to them, through mutation, so that they are ineffective when needed to fight a human infection. For these reasons, it is extremely important that milk from cows being treated with antibiotics is withheld from the milk supply.

The withdrawal time after final treatment for various antibiotics is shown below:

Amoxcillin 60 hrs.
Cloxacillin 48 hrs.
Erythromicin 36 hrs.
Novobiocin 72 hrs.
Penicillin 84 hrs.
Sulfadimethozine 60 hrs.
Sulfabromomethozine 96 hrs.
Sulfaethoxypyridozine 72 hrs.

Anti-Microbial Systems in Raw Milk


There exists in milk a number of natural anti-microbial defense mechanisms. These include:

  • lysozyme - an enzyme that hydrolyses glycosidic bonds in gram positive cell walls. However, its effect as a bacteriostatic mechanism in milk is probably negligible.

  • lactoferrin - an iron binding protein that sequesters iron from microorganisms, thus taking away one of their growth factors. Its effect as a bacteriostatic mechanism in milk is also probably negligible.

  • lactoperoxidase - an enzyme naturally present in raw milk that catalyzes the conversion of hydrogen peroxide to water. When hydrogen peroxide and thiocyanate are added to raw milk, the thiocyanate is oxidized by the enzyme/ hydrogen peroxide complex producing bacteriostatic compounds that inhibit Gram negative bacteria, E. coli , Salmonella spp , and streptococci. This technique is being used in many parts of the world, especially where refrigeration for raw milk is not readily available, as a means of increasing the shelf life of raw milk.

Milk Biosynthesis


Milk is synthesized in the mammary gland. Within the mammary gland is the milk producing unit, the alveolus. It contains a single layer of epithelial secretory cells surrounding a central storage area called the lumen, which is connected to a duct system. The secretory cells are, in turn, surrounded by a layer of myoepithelial cells and blood capillaries.

The raw materials for milk production are transported via the bloodstream to the secretory cells. It takes 400-800 L of blood to deliver components for 1 L of milk.

  • Proteins: building blocks are amino acids in the blood. Casein submicelles may begin aggregation in Golgi vesicles within the secretory cell.

  • Lipids:

    • C4-C14 fatty acids are synthesized in the cells

    • C16 and greater fatty acids are preformed as a result of rumen hydrogenation and are transported directly in the blood

  • Lactose: milk is in osmotic equilibrium with the blood and is controlled by lactose, K, Na, Cl; lactose synthesis regulates the volume of milk secreted

The milk components are synthesized within the cells, mainly by the endoplasmic reticulum (ER) and its attached ribosomes. The energy for the ER is supplied by the mitochondria. The components are then passed along to the Golgi apparatus, which is responsible for their eventual movement out of the cell in the form of vesicles. Both vesicles containing aqueous non-fat components, as well as liquid droplets (synthesized by the ER) must pass through the cytoplasm and the apical plasma membrane to be deposited in the lumen. It is thought that the milk fat globule membrane is comprised of the apical plasma membrane of the secretory cell.

Milking stimuli, such as a sucking calf, a warm wash cloth, the regime of parlour etc., causes the release of a hormone called oxytocin. Oxytocin is relased from the pituitary gland, below the brain, to begin the process of milk let-down. As a result of this hormone stimulation, the muscles begin to compress the alveoli, causing a pressure in the udder known as letdown reflex, and the milk components stored in the lumen are released into the duct system. The milk is forced down into the teat cistern from which it is milked. The let-down reflex fades as the oxytocin is degraded, within 4-7 minutes. It is very difficult to milk after this time.

CHAPTER 2 Milk Grading and Defects


The importance of milk grading lies in the fact that dairy products are only as good as the raw materials from which they were made. It is important that dairy personnel have a knowledge of sensory perception and evaluation techniques. The identification of off-flavours and desirable flavours, as well as knowledge of their likely cause, should enable the production of high quality milk, and subsequently, high quality dairy products.

Milk Grading


  • Sense of Taste

  • Sense of Smell

  • Techniques

Milk Defects


  • Lipolyzed

  • Oxidiation

  • Sunlight

  • Cooked

  • Transmitted

  • Microbial

Milk Grading


An understanding of the principles of sensory evaluation are neccessary for grading. All five primary senses are used in the sensory evaluation of dairy products: sight, taste, smell, touch and sound. The greatest emphasis, however, is placed on taste and smell.

The Sense of Taste


Taste buds, or receptors, are chiefly on the upper surface of the tongue, but may also be present in the cheek and soft palates of young people. These buds, about 900 in number, must make contact with the flavouring agent before a taste sensation occurs. Saliva, of course, is essential in aiding this contact. There are four different types of nerve endings on the tongue which detect the four basic "mouth" flavours - sweet, salt, sour, and bitter. Samples must, therefore, be spread around in the mouth in order to make positive flavour identification. In addition to these basic tastes, the mouth also allows us to get such reactions as coolness, warmth, sweetness, astringency, etc.

The Sense of Smell


We are much more perceptive to the sense of smell than we are to taste. For instance, it is possible for an odouriferous material such as mercaptain to be detected in 20 billion parts of air. The centres of olfaction are located chiefly in the uppermost part of the nasal cavity. To be detectable by smell, a substance must dissolve at body temperature and be soluble in fat solvents.

Note: The sense of both taste and smell may become fatigued during steady use. A good judge does not try to examine more than one sample per minute. Rinsing the mouth with water between samples may help to restore sensitivity.

Milk Grading Techniques


Temperature should be between 60-70° F (15.5-21° C) so that any odour present may be detected readily by sniffing the container. Also, we want a temperature rise when taking the sample into the mouth; this serves to volatize any notable constituents.

Noting the odour by placing the nose directly over the container immediately after shaking and taking a full "whiff" of air. Any off odour present may be noted.

Need to make sure we have a representative sample; mixing and agitation are important.

Agitation leaves a thin film of milk on the inner surface which tends to evaporate giving off odour if present.

During sampling, take a generous sip, roll about the mouth, note flavour sensation, and expectorate. Swallowing milk is a poor practice.

Can enhance the after-taste by drawing a breath of fresh air slowly through the mouth and then exhale slowly through the nose. With this practice, even faint odours can be noted.

Milk has a flavour defect if it has an odour, a foretaste or an aftertaste, or does not leave the mouth in a clean, sweet, pleasant condition after tasting.

Characterization of Flavour Defects - ADSA

Lipolytic or Hydrolytic rancidity


Rancidity arises from the hydrolysis of milkfat by an enzyme called the lipoprotein lipase (LPL). The flavour is due to the short chain fatty acids produced, particularly butyric acid. LPL can be indigenous or bacterial. It is active at the fat/water interface but is ineffective unless the fat globule membrane is damaged or weakened. This may occur through agitation, and/or foaming, and pumping. For this reason, homogenized milk is subject to rapid lipolysis unless lipase is destroyed by heating first; the enzyme (protein) is denatured at 55-60° C. Therefore, always homogenize milk immediately before or after pasteurization and avoid mixing new and homogenized milk because it leads to rapid rancidity.

Some cows can produce spontaneous lipolysis from reacting to something indigenous to the milk. Late lactation, mastitis, hay and grain ratio diets (more so than fresh forage or silage), and low yielding cows are more suseptible.

Lipolysis can be detected by measuring the acid degree value which determines the presence of free fatty acids. Lipolytic or hydrolytic rancidity is distinct from oxidative rancidity, but frequently in other fat industries, rancid is used to mean oxidative rancidity; in dairy, rancidity means lipolysis.

Characterized: soapy, blue-cheese like aroma, slightly bitter, foul, pronounced aftertaste, does not clear up readily

Oxidation


Milk fat oxidation is catalysed by copper and certain other metals with oxygen and air. This leads to an autooxidation reaction consisting of initiation, propagation, termination.

RH --- R + H initiation - free radical

R + O2 ---- RO2 propagation

RO2 + RH --- ROOH + R

R + R --- R2 termination

R + RO2 --- RO2R

It is usually initiated in the phospholipid of the fat globule membrane. Propagation then occurs in triglycerides, primarily double bonds of unsaturated fatty acids. During propagation, peroxide derivatives of fatty acids accumulate. These undergo further reactions to form carbonyls, of which some, like aldehydes and ketones, have strong flavours. Dry feed, late lactation, added copper or other metals, lack of vit E (tocopherol) or selenium (natural antioxidates) in the diet all lead to spontaneous oxidation. It can be a real problem especially in winter. Exposure to metals during processing can also contribute.

Characterized: metallic, wet cardboard, oily, tallowy, chalky; mouth usually perceives a puckery or astringent feel

Sunlight


Often confused with oxidized, this defect is caused by UV-rays from sunlight or flourescent lighting catalyzing oxidation in unprotected milk. Photo-oxidation activates riboflavin which is responsible for catalyzing the conversion of methionine to methanal. It is, therefore, a protein reaction rather than a lipid reaction. However, the end product flavour notes are similar but tends to diminish after storage of several days.

Characterized: burnt-protein or burnt-feathers-like, "medicinal"-like flavour

Cooked


This defect is a function of the time-temperature of heating and especially the presence of any "burn-on" action of heat on certain proteins, particulary whey proteins. Whey proteins are a source of sulfide bonds which form sulfhydryl groups that contribute to the flavour. The defect is most obvious immediately after heating but dissipates within 1 or 2 days.

Characterized: slightly cooked or nutty-like to scorched or caramelized

Transmitted flavours


Cows are particulary bad for transmitting flavours through milk and milk is equally as susceptible to pick-up of off flavours in storage. Feed flavours and green grass can be problems so it is necessary to remove cows from feed 2-4 hrs before milking. Weeds, garlic/onion, and dandelions can tranfer flavours to the milk and even subsequent products such as butter. Barny flavours can be picked up in the milk if there is poor ventilation and the barn is not properly cleared and cows breathe the air. These flavours are volatile so can be driven off through vacuum de-aeration.

Characterization: hay/silage, cowy/barny

Microbial


There are many flavour defects of dairy products that may be caused by bacteria, yeasts, or moulds. In raw milk the high acid/sour flavour is caused by the growth of lactic acid bacteria which ferment lactose. It is less common today due to change in raw milk microflora. In both raw or processed milk, fruity flavours may arise due to psychrotrophs such as Pseudomonas fragi. Bitter or putrid flavours are caused by psychrotrophic bacteria which produce protease. It is the proteolytic action of protease that usually causes spoilage in milk. Malty flavours are caused by S.lactis var. maltigenes and is characterized by a corn flakes type flavour. Although more of a tactile defect, ropy milk is also caused by bacteria, specifically those which produce exopolysaccharides.

Miscellaneous Defects


  • astringent

  • chalky

  • chemical/medicinal - disease - associated or adulteration

  • flat - adulteration (water)

  • foreign

  • salty - disease associated

  • bitter - adulteration

More information on off-flavours in milk can be found in Bassette et al., and Shipe et al.

Milk flavour is graded on a score of one to 10. Some flavour defects, even if only slightly present, can decrease the score drastically. The following are suggested flavour scores for milk with designated intensities of flavour defects.

Flavour Intensity of Defect

Criticisms Slight Definite Pronounced

---------------------------------------------------------------------------------

Astringent 8 7 5

Barny 7 5 3

Bitter 7 5 3

Cooked 9 8 6

Cowy 6 4 1

Feed 9 7 5

Flat 9 8 7

Foreign 5 3 0

Garlic/onion 5 3 1

High acid 3 1 0

Bacterial 5 3 0

Lacks Freshness 7 5 3

Malty 7 5 3

Oxidized 7 5 3

Rancid 7 5 3

Salty 8 6 4

Unclean 7 5 3

CHAPTER 3 Dairy Chemistry and Physics

Composition and Structure


  • Overview

  • Milk Lipids

    • Chemical Properties

    • Physical Properties

    • Structure: The Milk Fat Globule

    • Functional Properties

  • Milk Proteins

    • Introduction

    • Caseins

    • Structure: The Casein Micelle

    • Whey Proteins

    • Enzymes

  • Lactose

  • Vitamins

  • Minerals

Physical Properties


  • Density

  • Viscosity

  • Freezing Point

  • Acid-Base Equilibria

  • Optical Properties

Composition and Structure: Overview


The role of milk in nature is to nourish and provide immunological protection for the mammalian young. Milk has been a food source for humans since prehistoric times; from human, goat, buffalo, sheep, yak, to the focus of this section - domesticated cow milk (genus Bos). Milk and honey are the only articles of diet whose sole function in nature is food. It is not surprising, therefore, that the nutritional value of milk is high. Milk is also a very complex food with over 100,000 different molecular species found. There are many factors that can affect milk composition such as breed variations (see introduction cow to cow variations, herd to herd variations - including management and feed considerations, seasonal variations, and geographic variations. With all this in mind, only an approximate composition of milk can be given:

  • 87.3% water (range of 85.5% - 88.7%)

  • 3.9 % milkfat (range of 2.4% - 5.5%)

  • 8.8% solids-not-fat (range of 7.9 - 10.0%):

    • protein 3.25% (3/4 casein)

    • lactose 4.6%

    • minerals 0.65% - Ca, P, citrate, Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate, many others

    • acids 0.18% - citrate, formate, acetate, lactate, oxalate

    • enzymes - peroxidase, catalase, phosphatase, lipase

    • gases - oxygen, nitrogen

    • vitamins - A, C, D, thiamine, riboflavin, others



The following terms are used to describe milk fractions:

  • Plasma = milk - fat (skim milk)

  • Serum = plasma - casein micelles (whey)

  • solids-not-fat (SNF) = proteins, lactose, minerals, acids, enzymes, vitamins

  • Total Milk Solids = fat + SNF



Not only is the composition important in determining the properties of milk, but the physical structure must also be examined. Due to its role in nature, milk is in a liquid form. This may seem curious if one takes into consideration the fact that milk has less water than most fruits and vegetables. Milk can be described as:

  • an oil-in-water emulsion with the fat globules dispersed in the continuous serum phase

  • a colloid suspension of casein micelles, globular proteins and lipoprotein partilcles

  • a solution of lactose, soluble proteins, minerals, vitamins other components.

Looking at milk under a microscope, at low magnification (5X) a uniform but turbid liquid is observed. At 500X magnification, spherical droplets of fat, known as fat globules, can be seen. At even higher magnification (50,000X), the casein micelles can be observed. The main structural components of milk, fat globules and casein micelles, will be examined in more detail later.

Milk Lipids - Chemical Properties


The fat content of milk is of economic importance because milk is sold on the basis of fat. Milk fatty acids originate either from microbial activity in the rumen, and transported to the secretory cells via the blood and lymph, or from synthesis in the secretory cells. The main milk lipids are a class called triglycerides which are comprised of a glycerol backbone binding up to three different fatty acids. The fatty acids are composed of a hydrocarbon chain and a carboxyl group. The major fatty acids found in milk are:

Long chain

  • C14 - myristic 11%

  • C16 - palmitic 26%

  • C18 - stearic 10%

  • C18:1 - oleic 20%

Short chain (11%)

  • C4 - butyric*

  • C6 - caproic

  • C8 - caprylic

  • C10 - capric

* butyric fatty acid is specific for milk fat of ruminant animals and is resposible for the rancid flavour when it is cleaved from glycerol by lipase action.

Saturated fatty acids (no double bonds), such as myristic, palmitic, and stearic make up two thirds of milk fatty acids. Oleic acid is the most abundant unsaturated fatty acid in milk with one double bond. While the cis form of geometric isomer is the most common found in nature, approximately 5% of all unsaturated bonds are in the trans position as a result of rumen hydrogenation.

Triglycerides account for 98.3% of milkfat. The distribution of fatty acids on the triglyceride chain, while there are hundreds of different combinations, is not random. The fatty acid pattern is important when determining the physical properties of the lipids. In general, the SN1 position binds mostly longer carbon length fatty acids, and the SN3 position binds mostly shorter carbon length and unsaturated fatty acids. For example:

  • C4 - 97% in SN3

  • C6 - 84% in SN3

  • C18 - 58% in SN1

The small amounts of mono- , diglycerides, and free fatty acids in fresh milk may be a product of early lipolysis or simply incomplete synthesis. Other classes of lipids include phospholipids (0.8%) which are mainly associated with the fat globule membrane, and cholesterol (0.3%) which is mostly located in the fat globule core.

Milk Lipids - Physical Properties


The physical properties of milkfat can be summerized as follows:

  • density at 20° C is 915 kg m(-3)*

  • refractive index (589 nm) is 1.462 which decreases with increasing temperature

  • solubility of water in fat is 0.14% (w/w) at 20° C and increases with increasing temperature

  • thermal conductivity is about 0.17 J m(-1) s(-1) K(-1) at 20° C

  • specific heat at 40° C is about 2.1kJ kg(-1) K(-1)

  • electrical conductivity is <10(-12) ohm(-1) cm(-1)

  • dielectric constant is about 3.1

*the brackets around numbers denote superscript

At room temperature, the lipids are solid, therefore, are correctly referred to as "fat" as opposed to "oil" which is liquid at room temperature. The melting points of individual triglycerides ranges from -75° C for tributyric glycerol to 72° C for tristearin. However, the final melting point of milkfat is at 37° C because higher melting triglycerides dissolve in the liquid fat. This temperature is significant because 37° C is the body temperature of the cow and the milk would need to be liquid at this temperature. The melting curves of milkfat are complicated by the diverse lipid composition:

  • trans unsaturation increases melting points

  • odd-numbered and branched chains decrease melting points

Crystallization of milkfat largely determines the physical stability of the fat globule and the consistency of high-fat dairy products, but crystal behaviour is also complicated by the wide range of different triglycerides. There are four forms that milkfat crystals can occur in; alpha, ß , ß ' 1, and ß ' 2, however, the alpha form is the least stable and is rarely observed in slowly cooled fat.

Milkfat Structure - Fat Globules


More than 95% of the total milk lipid is in the form of a globule ranging in size from 0.1 to 15 um in diameter. These liquid fat droplets are covered by a thin membrane, 8 to 10 nm in thickness, whose properties are completely different from both milkfat and plasma. The native fat globule membrane (FGM) is comprised of apical plasma membrane of the secretory cell which continually envelopes the lipid droplets as they pass into the lumen. The major components of the native FGM, therefore, is protein and phospholipids. The phospholipids are involved in the oxidation of milk. There may be some rearrangement of the membrane after release into the lumen as amphiphilic substances from the plasma adsorb onto the fat globule and parts of the membrane dissolve into either the globule core or the serum. The FGM decreases the lipid-serum interface to very low values, 1 to 2.5 mN/m, preventing the globules from immediate flocculation and coalescence, as well as protecting them from enzymatic action.

It is well known that if raw milk or cream is left to stand, it will separate. Stokes' Law predicts that fat globules will cream due to the differences in densities between the fat and plasma phases of milk. However, in cold raw milk, creaming takes place faster than is predicted from this fact alone. IgM, an immunoglobulin in milk, forms a complex with lipoproteins. This complex, known as cryoglobulin precipitates onto the fat globules and causes flocculation. This is known as cold agglutination. As fat globules cluster, the speed of rising increases and sweeps up the smaller globules with them. The cream layer forms very rapidly, within 20 to 30 min., in cold milk.

Homogenization of milk prevents this creaming by decreasing the diameter and size distribution of the fat globules, causing the speed of rise to be similar for the majority of globules. As well, homogenization causes the formation of a recombined membrane which is much similar in density to the continuous phase.

Recombined membranes are very different than native FGM. Processing steps such as homogenization, decreases the average diameter of fat globule and significantly increases the surface area. Some of the native FGM will remain adsorbed but there is no longer enough of it to cover all of the newly created surface area. Immediately after disruption of the fat globule, the surface tension raises to a high level of 15 mN/m and amphiphilic molecules in the plasma quickly adsorb to the lipid droplet to lower this value. The adsorbed layers consist mainly of serum proteins and casein micelles.

Fat Destabilization


While homogenization is the principal method for acheiving stabilization of the fat emulsion in milk, fat destabilization is necessary for structure formation in butter,whipping cream and ice cream. Fat destabilization refers to the process of clustering and clumping (partial coalescence) of the fat globules which leads to the development of a continuous internal fat network or matrix structure in the product. Fat destabilization (sometimes "fat agglomeration") is a general term that describes the summation of several different phenomena. These include:

Coalescence:

an irreversible increase in the size of fat globules and a loss of identity of the coalescing globules;

Flocculation:

a reversible (with minor energy input) agglomeration/clustering of fat globules with no loss of identity of the globules in the floc; the fat globules that flocculate ; they can be easily redispersed if they are held together by weak forces, or they might be harder to redisperse to they share part of their interfacial layers;

Partial coalescence:

an irreversible agglomeration/clustering of fat globules, held together by a combination of fat crystals and liquid fat, and a retention of identity of individual globules as long as the crystal structure is maintained (i.e., temperature dependent, once the crystals melt, the cluster coalesces). They usually come together in a shear field, as in whipping, and it is envisioned that the crystals at the surface of the droplets are responsible for causing colliding globules to stick together, while the liquid fat partially flows between they and acts as the "cement". Partial coalescence dominates structure formation in whipped, aerated dairy emulsions, and it should be emphasized that crystals within the emulsion droplets are responsible for its occurrence.

A good reference for more information on fat globules can be found in Mulder and Walstra.

Milk Lipids - Functional Properties


Like all fats, milkfat provides lubrication. They impart a creamy mouth feel as opposed to a dry texture. Butter flavour is unique and is derived from low levels of short chain fatty acids. If too many short chain fatty acids are hydrolyzed (separated) from the triglycerides, however, the product will taste rancid. Butter fat also acts as a reservoir for other flavours, especially in aged cheese. Fat globules produce a 'shortening' effect in cheese by keeping the protein matrix extended to give a soft texture. Fat substitutes are designed to mimic the globular property of milk fat. The spreadable range of butter fat is 16-24° C. Unfortunately butter is not spreadable at refrigeration temperatures. Milk fat provides energy (1g = 9 cal.), and nutrients (essential fatty acids, fat soluble vitamins).

Milk Proteins: Introduction and Review


The primary structure of proteins consists of a polypeptide chain of amino acids residues joined together by peptide linkages, which may also be cross-linked by disulphide bridges. Amino acids contain both a weakly basic amino group, and a weakly acid carboxyl group both connected to a hydrocarbon chain, which is unique to different amino acids. The three-dimensional organization of proteins, or conformation, also involves secondary, tertiary, and quaternary structures. The secondary structure refers to the spatial arrangement of amino acid residues that are near one another in the linear sequence. The alpha-helix and ß -pleated sheat are examples of secondary structures arising from regular and periodic steric relationships. The tertiary structure refers to the spatial arrangement of amino acid residues that are far apart in the linear sequence, giving rise to further coiling and folding. If the protein is tightly coiled and folded into a somewhat spherical shape, it is called a globular protein. If the protein consists of long polypeptide chains which are intermolecularly linked, they are called fibrous proteins. Quaternary structure occurs when proteins with two or more polypeptide chain subunits are associated.

Milk Protein Fractionation
The nitrogen content of milk is distributed among caseins (76%), whey proteins (18%), and non-protein nitrogen (NPN) (6%). This does not include the minor proteins that are associated with the FGM. This nitrogen distribution can be determined by the Rowland fractionation method:

  1. Precipitation at pH 4.6 - separates caseins from whey nitrogen

  2. Precipitation with sodium acetate and acetic acid (pH 5.0) - separates total proteins from whey NPN

The concentration of proteins in milk is as follows:


grams/ litre % of total protein

__________________________________________________________________________

Total Protein 33 100

Total Caseins 26 79.5

alpha s1 10 30.6

alpha s2 2.6 8.0

beta 9.3 28.4

kappa 3.3 10.1

Total Whey Proteins 6.3 19.3

alpha lactalbumin 1.2 3.7

beta lactoglobulin 3.2 9.8

BSA 0.4 1.2

Immunoglobulins 0.7 2.1

Proteose peptone 0.8 2.4

__________________________________________________________________________

Caseins, as well as their structural form - casein micelles, whey proteins, and milk enzymes will now be examined in further detail.
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