PRODUCTION AND UTILIZATION OF MILK heated) and agitated to various degrees. These treatments will cause at some physical changes and permit some enzymatic and microbiolo changes hich may alter the processing properties of milk. Again, it may possible to counteract some of these changes 1.3 Classification of mammals The essential characteristic distinguishing mammals from other animal species is the ability de of specialized organs(mammary glands) for the nutrition of its newborn The class Mammalia is divided into three subclasses 1. Prototheria. This subclass contains only one order, Monotremes, species of which are egg-laying mammals, e.g. duck-billed platypus echidna, and are indigenous only to Australasia. They possess many (perhaps 200)mammary glands grouped in two areas of the abdomen the glands do not terminate in a teat and the secretion(milk )is licked by the young from the surface of the gland 2. Marsupials. The young of marsupials are born live(viviparous)after short gestation and are premature at birth to a greater or lesser degree, depending on the species. After birth, the young are transferred to a pouch where they reach maturity, e.g. kangaroo and wallaby. In marsu- pials, the mammary glands, which vary in number, are located within the pouch and terminate in a teat. The mother may nurse two offspring, differing widely in age, simultaneously from different mammary glands that secrete milk of very different composition, designed to meet the different specific requirements of each offspring 3. Eutherians. About 95% of all mammals belong to this subclass. The developing embryo in utero receives nourishment via the placental blood supply (they are referred to as placental mammals ) and is born at a high, but variable, species-related state of maturity. All eutherians secrete milk, which, depending on the species, is more or less essential for the development of the young; the young of some species are born sufficiently mature to survive and develop without milk The number and location of mammary glands varies with species from two, e.g. human, goat and sheep, to 14-16 for the pig. Each gland is anatomically and physiologically separate and is emptied via a teat. The wide interspecies variation in the composition(Table 1. 1)and the chemistry of the constituents of milk, as discussed elsewhere, renders milk species-specific, i.e., designed to meet the requirements of the young of that species. There is also a surprisingly good relationship between milk yield and maternal body weight( Figure 1. 1); species bred for commercial milk production, e.g. dairy cow and goat, fall above the linePRODUCTION AND UTILIZATION OF MILK 3 heated) and agitated to various degrees. These treatments will cause at least some physical changes and permit some enzymatic and microbiological changes which may alter the processing properties of milk. Again, it may be possible to counteract some of these changes. 1.3 Classification of mammals The essential characteristic distinguishing mammals from other animal species is the ability of the female of the species to produce milk in specialized organs (mammary glands) for the nutrition of its newborn. 1. Prototheria. This subclass contains only one order, Monotremes, the species of which are egg-laying mammals, e.g. duck-billed platypus and echidna, and are indigenous only to Australasia. They possess many (perhaps 200) mammary glands grouped in two areas of the abdomen; the glands do not terminate in a teat and the secretion (milk) is licked by the young from the surface of the gland. 2. Marsupials. The young of marsupials are born live (viviparous) after a short gestation and are ‘premature’ at birth to a greater or lesser degree, depending on the species. After birth, the young are transferred to a pouch where they reach maturity, e.g. kangaroo and wallaby. In marsu￾pials, the mammary glands, which vary in number, are located within the pouch and terminate in a teat. The mother may nurse two offspring, differing widely in age, simultaneously from different mammary glands that secrete milk of very different composition, designed to meet the different specific requirements of each offspring. 3. Eutherians. About 95% of all mammals belong to this subclass. The developing embryo in utero receives nourishment via the placental blood supply (they are referred to as placental mammals) and is born at a high, but variable, species-related state of maturity. All eutherians secrete milk, which, depending on the species, is more or less essential for the development of the young; the young of some species are born sufficiently mature to survive and develop without milk. The number and location of mammary glands varies with species from two, e.g. human, goat and sheep, to 14-16 for the pig. Each gland is anatomically and physiologically separate and is emptied via a teat. The wide interspecies variation in the composition (Table 1.1) and the chemistry of the constituents of milk, as discussed elsewhere, renders milk species-specific, i.e., designed to meet the requirements of the young of that species. There is also a surprisingly good relationship between milk yield and maternal body weight (Figure 1.1); species bred for commercial milk production, e.g. dairy cow and goat, fall above the line. The class Mammalia is divided into three subclasses: