《畜产食品工艺学》双语教材 DAIRY SCIENCE AND TECHNOLOGY 主编蒋爱民 ARTHHILL 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
1 《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 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
2 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… CHAPTER 1 Milk Production and biosynthesis 126 CHAPTER2 Milk Grading and Defects...-. CHAPTER3 Dairy Chemistry and Physics"..-- ……10 CHAPTER 4 Dairy Microbiology"--.. ……27 CHAPTER5 Dairy Processing…… e Clarification, Separation, Standardization Pasteurization… O UHT Treatment …46 Homogenization… e Membrane processing…… Evaporation and dehydration…… o Production and Utilization of steam and Refrigeration.. CHAPTER 6 Dairy Products.-..--. Overview and fluid milk products Concentrated and dried milk products………… …………66 e Cultured Dairy Products.. e Whipped Cream Ice cream Butter manufacture…… 135 APPENDIX Glossary of Terms.-..-.-
3 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
Introducti 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 pecies 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 een 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 cred ited 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 trad itional diet has varied greatly in different regions of the world The tropical countries have not been trad itional milk consumers, whereas the more northern regions of the world, Europe(especially Scand navia)and North America, have trad itionally consumed far more milk and milk products in their diet. In tropical countries where high temperatures and lack of refrigeration has led to the inabil ity to produce and store fresh milk, milk has trad itionally 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
4 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 trend s 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 immunolog ical 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 milk has tapered off because of maternal needs of the fetus udder needs time to prepare for the next milking cycle The life of a female cow can be summerized as follows Age 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
5 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 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