01_Primary_production_of_milk

Dairy Processing Handbook/Chapter 1 1 Milk production began 6 000 years ago, or even earlier. The dairy animals of today have been developed from untamed animals which, over thousands of years, have lived at different altitudes and latitudes, at times exposed to natural and, many times, severe and extreme conditions. Practically everywhere on earth man started domesticating animals. As a rule, herbivorous, multipurpose animals were chosen to satisfy his need of milk, meat, clothing, etc. Herbivorous animals were chosen because they are less dangerous and easier to handle than carnivorous animals. The former did not compete directly with man for nourishment, since they ate plants which man could not use himself. Primary production of milk Dairy Processing Handbook/Chapter 12 The herbivorous animals used were all ruminants with the exception of the mare and ass. Ruminants can eat quickly and in great quantities, and later ruminate the feed. Today, the same animals are still kept for milk production, milk being one of the essential food components for man. The most widespread milking animal in the world is the cow, which is found on all continents and in nearly all countries. Table 1.1 Composition of milk from various animals. Animal Protein Casein Whey Fat Carbo- Ash total protein hydrate % % % % % % Human 1,0 0,5 0,5 4,5 7,0 0,2 Horse 2,2 1,3 0,9 1,7 6,2 0,5 Cow 3,5 2,8 0,7 3,7 4,8 0,7 Buffalo 4,0 3,5 0,5 7,5 4,8 0,7 Goat 3,6 2,7 0,9 4,1 4,7 0,8 Sheep 4,6 3,9 0,7 7,2 4,8 0,8 However, we should not forget the other milking animals, whose milk is of great importance to the local population, as a source of highly valuable animal protein and other constituents. Sheep are of exceptional importance among this group, especially in the Mediterranean countries and in large areas of Africa and Asia. The number of sheep in the world exceeds one billion, and they are thus the most numerous of all milk- and meat- producing domestic animals. Sheep are often accompanied by goats, whose contribution to milk and meat production in the poorest areas should not be overlooked. Both sheep and goats are a source of cheap, high-quality protein and are mainly kept in conditions where climatic, topographical, economic, technical or sociological factors limit the development of more sophisticated protein production systems. Table 1.1 shows the composition of milk from different species of animals. It should be noted that the figures given are only averages, as the composition for any species is influenced by a number of factors such as breed, feeding, climate, etc. Cow milk Milk is the only food of the young mammal during the first period of its life. The substances in milk provide both energy and the building materials necessary for growth. Milk also contains antibodies which protect the young mammal against infection. A calf needs about 1 000 litres of milk for growth, and that is the quantity which the primitive cow produces for each calf. There has been an enormous change since man took the cow into his service. Selective breeding has resulted in dairy cows which yield an average of more than 6 000 litres of milk per calf, i.e. six times as much as the primitive cow. Some cows can yield 14 000 litres or more. Before a cow can start to produce milk, she must first have a calf. Heifers reach sexual maturity at the age of seven or eight months but are not usually mated until they are 15 – 18 months old. The period of gestation is 265 – 300 days, varying according to the breed of the cow, so a heifer produces her first calf at the age of about 2 – 2,5 years. • The heifer is bred (naturally or by insemination) before the age of two years. • The gestation period is nine months and one week. • After calving, the cow gives milk for 10 months. • 1 – 2 months after calving the cow will again be bred. Dairy Processing Handbook/Chapter 1 3 Secretion of milk Milk is secreted in the cow’s udder, which is a hemispherical organ divided into right and left halves by a crease. Each half is divided into quarters by a shallower transverse crease. Because each quarter has one teat with its own separate mammary gland, it is theoretically possible to get milk of four different qualities from the same cow. A sectional view of the udder is shown in Figure 1.1. The udder is composed of glandular tissue which contains milk- producing cells. The external layer of this tissue is muscular, thus giving cohesion to the body of the udder and protecting it against injury from knocks and blows. The glandular tissue contains a very large number (about two billion) of tiny bladders called alveoli. The actual milk-producing cells are located on the inner walls of the alveoli, which occur in groups of between 8 and 120. Capillaries leading from the alveoli converge into progressively larger milk ducts which lead to a cavity above the teat. This cavity, known as the cistern of the udder, can hold up to 30 % of the total milk in the udder. 1 3 4 2 Large quantities of blood flow through the udder every day. Approx. 800 – 900 l of blood is needed for formation of one litre of milk. Fig. 1.1 Sectional view of the udder. 1 Cistern of the udder 2 Teat cistern 3 Teat channel 4 Alveolus The cistern of the udder has an extension reaching down into the teat; this is called the teat cistern. At the end of the teat there is a channel 1 – 1,5 cm long. Between milkings, the channel is closed by a sphincter muscle which prevents milk from leaking out, and bacteria from entering the udder. The whole udder is laced with blood and lymph vessels. These bring nutrient-rich blood from the heart to the udder, where it is distributed by capillaries surrounding the alveoli. In this way, the milk-producing cells are furnished with the necessary nutrients for the secretion of milk. “Spent” blood is carried away by the capillaries to veins and returned to the heart. The flow of blood through the udder very high. It takes between 800 and 900 litres of blood to make one litre of milk. As the alveoli secrete milk, their internal pressure rises. If the cow is not milked, secretion of milk stops when the pressure reaches a certain limit. Increase of pressure forces a small quantity of milk out into the larger ducts and down into the cistern. Most of the milk in the udder, however, is contained in the alveoli and the fine capillaries in the alveolar area. These capillaries are so fine that milk cannot flow through them of its own accord. It must be pressed out of the alveoli and through the capillaries into the larger ducts. Muscle-like cells surrounding each alveolus perform this duty during milking, see Figure 1.2. Fig. 1.2 Squeezing of milk from alveolus. In the Irish village of Blackwater, Big Bertha died on 31 December 1993. She was probably the oldest cow in the world when she died at an age of 49 years. The owner, Mr Jerome O’Leary, announced that Big Bertha would have been 50 years of age on 15 March 1994. Dairy Processing Handbook/Chapter 14 77 88 55 44 3399 1111 1010 1212 11 22 66llllll l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l lll Fig. 1.3 Milking takes 5 – 8 minutes. Fig. 1.4 The milk should be poured through a strainer and then chilled. The lactation cycle Secretion of milk in the cow’s udder begins shortly before calving, so that the calf can begin to feed almost immediately after birth. The cow then continues to give milk for about 300 days. This period is known as lactation. One to two months after calving the cow can be serviced again. During the lactation period, milk production decreases, and after approx. 300 days it may have dropped to only 25 – 50 % of its peak volume. At this stage milking is discontinued to give the cow a non-lactating period of up to 60 days prior to calving again. With the birth of the calf, a new lactation cycle begins. The first milk the cow produces after calving is called colostrum. It differs greatly from normal milk in composition and properties. See further in Chapter 2. Milk production is somewhat lower during the first lactation period. A cow is normally productive for 3 – 5 years. Milking A hormone called oxytocin must be released into the cow’s bloodstream in order to start the emptying of the udder. This hormone is secreted and stored in the pituitary gland. When the cow is prepared for milking by the correct stimuli, a signal is sent to the gland, which then releases its store of oxytocin into the bloodstream. In the primitive cow, the stimulus is provided by the calf’s attempts to suck on the teat. The oxytocin is released when the cow feels the calf sucking. A modern dairy cow has normally no calf present during milking. Stimulation of the milk let-down is done by the preparation of milking, i.e. the sounds, smells and sensations associated with milking. The oxytocin begins to take effect about one minute after preparation has begun and causes the muscle-like cells to compress the alveoli. This generates pressure in the udder and can be felt with the hand; it is known as the let-down reflex. The pressure forces the milk down into the teat cistern, from which it is sucked into the teat cup of a milking machine or pressed out by the fingers during hand milking. The effect of the let-down reflex gradually fades away as the oxytocin is diluted and decomposed in the bloodstream, disappearing after 5 – 8 minutes. Milking should therefore be completed within this period of time. If the milking procedure is prolonged in an attempt to “strip” the cow, this places an unnecessary strain upon the udder; the cow becomes irritated and may become difficult to milk. Hand-milking On many farms all around the world, milking is still done by hand in the same way as it has been done for thousands of years. Cows are usually milked by the same people every day, and are quickly stimulated to let- down just by hearing the familiar sounds of the preparations for milking. Milking begins when the cow responds with the let-down reflex. The first jets of milk from the teats are normally rejected. A careful, visual inspection of the first milk enables the milker to detect the status of the udder health. Two opposed quarters are milked at a time: one hand presses the milk out of the teat cistern, after which the pressure is relaxed to allow more milk to run down into the teat cistern from the udder cistern. At the same time milk is pressed out of the other teat. In this way the two teats are milked alternately. When two quarters have been emptied in this way, the milker then proceeds to milk the other two until the whole udder is empty. The milk is collected in pails and poured through a strainer, to remove coarse impurities, into a churn holding 30 – 50 litres. The churns are then chilled and stored at low temperature to await transport to the dairy. Immersion or spray chillers are commonly used for cooling. Dairy Processing Handbook/Chapter 1 5 Machine milking The basic principle of the milking machine is shown in Figure 1.6. The milking machine extracts the milk from the teat by vacuum. A vacuum pump, a vacuum vessel, a vessel for collecting milk, teat cups and a pulsator are essential parts of the milking machine. The teat cup unit consists of a teat cup containing an inner tube of rubber, called the teat cup liner. The inside of the liner, in contact with the teat, is subjected to a constant vacuum of about 50 kPa (50% vacuum) during milking. The pressure in the pulsation chamber (between the liner and teat cup) is regularly alternated by the pulsator between 50 kPa during the suction phase and atmospheric pressure during the massage phase. The result is that milk is sucked from the teat cistern during the suction phase. During the massage phase, the teat cup liner is pressed together allowing a period of teat massage. This is followed by another suction phase, and so on, as shown in Figure 1.7. Relief of the teat during the massage phase is necessary to avoid accumulation of blood and fluid in the teat. Such congestion in the teat can be painful to the cow, and milk let down and milking performance can be affected. Repeated congestion at successive milking sessions can even have an influence on the udder health. The pulsator alternates between suction and massage phases about 50 to 60 times per minute. The four teat cups, attached to a manifold called the milk claw, are held on the cow’s teats by suction and the friction between the teat and the teat cup liner. Vacuum is alternately (alternate pulsation) applied to the left and right teats or, in some instances, to the front teats and rear teats. The applying of vacuum to all four teats at the same time (simultaneous pulsation) is less common. The milk is drawn from the teats directly to the milk pail or via a vacuumised transport pipe to a receiver unit. An automatic Fig. 1.6 Machine milking equipment. – + + + + + + + – – – – – – – – – – – – – a a Fig. 1.7 The phases of machine milking. a Teat cup liner Fig. 1.5 Preparing the cow for milking by cleaning and massaging the udders before the teat cups are placed on the udders. Fig. 1.8 General design of pipeline milking system. 1 2 3 4 1 Vacuum pump 2 Vacuum pipeline 3 Milk cooling tank 4 Milk pipeline Dairy Processing Handbook/Chapter 16 shut-off valve operates to prevent dirt from being drawn into the system if a teat cup should fall off during milking. After the cow has been milked, the milk pail is taken to a milk room where it is emptied into a churn or a special milk tank for cooling. To eliminate the heavy and time-consuming work of carrying filled pails to the milk room, a pipeline system may be installed for direct transport of the milk to the milk room (Figure 1.8). Such systems are most common today. It allow milk to be conveyed in a closed system straight from the cow to a collecting tank in the milk room. This is a great advantage from a hygienic point of view. Regardless if the milking system is of bucket, pipeline or automatic type it is important that it is designed to prevent air leakage during milking. Excessive air leakage can influence the quality of the milk and cause elevated levels of free fatty acids. The machine milking plant is also provided with Cleaning-In-Place (CIP) facilities. Automatic milking systems Automatic milking systems, Figure 1.9, have been installed on commercial farms at an increasing rate in recent years. The potential benefits are reduced labour requirements, higher milk quality, improved animal health and increased yield. Figure 1.11 shows a typically dairy farm layout including an automatic milking system. Fig. 1.9 The heart of an automatic milking system. The cow goes when she wants into the milking station where the teats are cleaned and milked. Fig. 1.10 Teat-cup for cleaning, drying and pre-milking. The teat is flushed with tepid water for cleaning and finally dried with air. The pre-milk goes together with cleaning water to drain. In contrast to conventional milking, in which people bring the cows to be milked, automatic milking places emphasis on the cow’s inclination to be milked in a self-service manner several times a day. The idea that cows like being milked is very attractive, and one of the main financial benefits from automatic milking is the increase in milk yield from more frequent milking. When the cow wants to be milked, she walks to the milking station. A transponder on the cow identifies it, and if the cow was milked recently, she is directed back to the resting or feeding area. The cow enters the automatic milking station and an individual amount of concentrate is served. Dairy Processing Handbook/Chapter 1 7 In an automatic milking system the teats can be detected by a laser and vision camera. As an example, the teats can be cleaned separately by means of a teat-cup-like device, Figure 1.10, using tepid water applied intermittently at a certain pressure and turbulence to ensure efficient cleaning. Drying of the teats is carried out by compressed air in the same teat-cup. Foremilking is carried out by the cleaning teat-cup, which applies vacuum at the end of the cleaning cycle. The cleaning teat-cups are finally flushed with water. Sensors can detect whether foremilking has been carried out. Foremilking is applied for a few seconds to ensure that sufficient milk is evacuated and the let-down reflex is activated. The teat-cups for milking are automatically attached sequentially. Milk from the four teats is kept separate until the milk meter records the amount from each quarter. Spraying each individual teat with disinfectant is the final stage of milking. Milk yield, milking duration, milk flow rate, and certain characteristics of the milk are recorded during milking. In addition, data on cow movements, time of milking and time of concentrate feeding may also be available. Milk leaving the milking station can be divided into different categories and being collected separately from the normal milk. The categories can be: 1 Treated cow 2 Freshly calved cow (colostrum) 3 Cow with less than one milking in the last 24 hours 4 A cow which, although healthy, has cell counts above a certain level The fresh milk is forwarded to a buffer tank for cooling before being pumped to the storage tank. Cooling of milk Efficient cooling of the raw milk after milking is the best way to prevent bacterial growth. Various cooling systems are available; the choice depends on the produced volume of milk. An in-can cooler, shown in Figure 1.13, is suitable for small producers. It is much favoured by users of chilled water units and producers using direct- to-can milking equipment. An immersion cooler is designed for direct cooling of the milk in churns 7 5 6 4 3 2 1 Fig. 1.11 Layout of a modern dairy farm with an automatic milking system. 1 Automatic milking station 2 Control room 3 Milk cooling and storage 4 Smart gate for preselecting the cows attempting the milking station 5 Living area 6 Feeding station 7 Calf section Fig. 1.12 Milk must be cooled to 4 °C as soon as possible. 0 0 1 0 Dairy Processing Handbook/Chapter 18 as well as in tanks. The condensing unit is mounted on a wall, Figure 1.14. The evaporator is located at the lower end of the immersion unit. The immersion cooler can also be used for indirect cooling, i.e. for cooling water in insulated basins. The milk is then cooled in transport churns immersed in the chilled water. Insulated farm tanks for immersion coolers are available in both stationary and mobile types (Figure 1.15). When road conditions prevent access by tanker truck, a mobile tank can be used to bring the milk to a suitable collection point. Mobile tanks are easy to transport and thus suitable for milking in the fields. Direct expansion tanks as shown in Figure 1.16 can as well be used for cooling and storage of the milk. Cleaning and sanitising Manual cleaning with brushes is a common method where hand milking or bucket machine milking systems are used. Circulation cleaning is commonly performed in pipe line milking plants. The cleaning solution is circulated through the plant by vacuum and/or a pump. Detergents, sanitisers, liquid temperatures and other cleaning conditions recommended by the milking machine supplier should be applied. Cooling of milk on the farm Milk leaves the udder at a temperature of about 37 °C. Fresh milk from a healthy cow is practically free from bacteria. It must be protected from 1 Fig. 1.17 Milking equipment on a large farm with heat exchanger (1) for rapid cooling from 37 to 4 °C. Fig. 1.16 Direct expansion tank used for cooling and storage of milk. Fig. 1.14 The immersion cooler is placed directly on the transportation churn. Fig. 1.15 The insulated farm tank can be filled in the field and easily transported to the chilling unit. Fig. 1.13 An in-can cooler is place