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 2.600 10°C 11,600 18800 15.5°C180,000 20°C 450.000 Traditionally, this has been overcome in tropical countries by stabil izing milk through means other than refrigeration, including immed iate consumption of warm milk after milk ing, by boil ing milk, or by conversion into more stable products such as fermented milk 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 d by increases in bacterial numbers, includ ing the possibility of human
6 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(000s/ml) Daily Milk Yield (kg) I st lactation Older 23.1 18-34 23.0 35-70 28.0 71-140 224 274 141-282 27.0 282-565 21.9 26.3 566-1130 214 25.4 1131-2262 20.7 24.6 2263-4525 20.0 19.0 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 ific 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 Erythromycin 36 hrs Novobiocin 72 hi Penicillin 84 hrs Sulfad imethozine 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 glycosid ic bonds in gram positive cell walls 7
7 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 bind ing protein that sequesters iron from microorganisms, thus taking away one of their growth factors. Its effect as a bacteriostatic mechanism in lactoperoxidase- an enzy me 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 oxid ized 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 means of increasing the shelf life of raw mif r raw milk is not readily available, as a of the world, especially where refrigeration fo 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 cell 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 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: build ing blocks are amino acids in the blood Casein submicelles may gin aggregation in golgi vesicles within the secretory cell Lipids o C4-C14 fatty acids are synthesized in the cells o 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 8
8 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: o C4-C14 fatty acids are synthesized in the cells o 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 knowled ge 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 aste Sense of smell Techniques Milk defects Lipolyzed Oxid iation Sunlight Cooked Transmitted Microbial Milk grading An understand ing 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 end ings 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
9 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 mercaptan 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 solvent Note: The sense of both taste and smell may become fatigued during steady use. A good udge 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(155-21 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 immed iately 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 Rancid ity 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
10 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