55-60%C. Therefore, always homogenize milk immediately before or after pasteurization and avoid mixing new and homogenized milk because it leads to rapid rancid ity 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 yield ing 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 rancid ity is distinct from oxidative rancid ity, but frequently in other fat industries, rancid is used to mean oxidative rancid ity; in dairy, rancid ity 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. Th leads to an autooxidation reaction consisting of initiation, propagation, termination RH-R+H -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 oxid ized 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
11 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 d iminish after storage of several days Characterized: burnt-protein or burnt-feathers-like, "medicinal"-like flavour 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 immed iately 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 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 corr flakes type flavour. Although more of a tactile defect, ropy milk is also caused by bacteria, specifically those which produce exopolysaccharides Miscellaneous defects ast trin gent chalk chemical/med icinal -disease-associated or adulteration flat- adulteration(water) oreign salty -disease associated
12 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 Cooked Cowy Flat 877969955 755847833 F 0 Garlic/onion High acid Bacterial Lacks freshness Malty Oxid ized Rancid 37778 S 6 Unclean 5 CHAPTER 3 Dairy Chemistry and Physics mposition and structure Overview Milk Lipids o Chemical Properties o Physical Properties o Structure: The Milk Fat Globule o Functional Properties Milk proteins o Introduction
13 • 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 o Chemical Properties o Physical Properties o Structure: The Milk Fat Globule o Functional Properties • Milk Proteins o Introduction
o Structure: The Casein Micelle o Whey proteins nzymes Lactose Ⅴ Itamin Minerals Physical Properties Density 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 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.9-10.0%) o protein 3.25%(3/4 casein) o lactose 4.6% o minerals 0.65%-Ca, P, citrate, Mg, K, Na, Zn, CL, Fe, Cu, sulfate, o acids 0.18%-citrate. formate acetate. lactate oxalate o enzymes-peroxidase, catalase, phosphatase, lipase o gases-oxygen, nitrogen o vitamins-AC.D thiamine riboflavin. others The following terms are used to describe milk fractions Plasma =milk- fat(skim milk)
14 o Caseins o Structure: The Casein Micelle o Whey Proteins o 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%): o protein 3.25% (3/4 casein) o lactose 4.6% o minerals 0.65% - Ca, P, citrate, Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate, many others o acids 0.18% - citrate, formate, acetate, lactate, oxalate o enzymes - peroxidase, catalase, phosphatase, lipase o gases - oxygen, nitrogen o 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 a colloid suspension of casein micelles, globular proteins and lipoprotein particles 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 case in 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 bind ing 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 C14-myristic 11% C16-palmitic 26% C18- stearic 10% C18: 1-oleic 20% Short chain(11%) C4-butyric* c6 lic C10-capric s butyric fatty acid is specific for milk fat of ruminant animals and is resposible for the
15 • 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