Lec 1Milk (Chemical Composition) - Milk Composition Notes

Chemical Composition of Milk Milk Major Milk Constituents Minor Milk Constituents 1. Phospholipids (e.g. Lecithin)...

Chemical Composition of Milk Milk Major Milk Constituents Minor Milk Constituents 1. Phospholipids (e.g. Lecithin) Total Solids (TS) Water 2. Sterols (e.g. Cholesterol) 3. Pigments (e.g. Carotene, Riboflavin) 4. Vitamins (A, D, E, K, B complex) Solids Not Fat(SNF) Fat 5. Enzymes (phosphatase, lipase, Free (96%) Bound (4%) peroxidase, reductase, proteinase, …etc 6. Gases Protein Lactose Ash 7. Cells 8. Non protein nitrogenous substances. 1  Milk Milk is one of the most complete single foods available in nature for health and promotion of growth. The lactating animals which their milk used by man: The cow is the principle source of milk for human consumption in many parts of the world; other animals as source of milk for human beings are the buffalo, goat, sheep, camel and mare. Milk definitions: Biological: liquid secreted by female mammals for nourishment of young. Federal: the whole, clean, fresh lacteal secretion obtained by the complete milking of one or more healthy cows, properly fed and kept, excluding that obtained within 15 days before and 5 days after birth. Udder Anatomy and Milk Secretion The udder is composed of 4 distinct secretory glands termed “quarters.” The “quarter” consists of milk producing secretory tissue, which is referred to as alveoli (about 2 billion), a duct system to transport milk away from the alveoli, two storage areas termed “cisterns” and 1 teat. 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 20 % of the total milk in the udder. 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. An important component of the teat is the “streak canal,” a thick muscular tissue that is lined with antibacterial substances and closes the teat when milk is not being extracted. Each quarter is independent and is separated from the others by thick ligaments (Figure 1). Microorganisms cannot pass directly between the quarters but antibiotics (given either in 1-quarter or systemically) are absorbed and can spread throughout the entire udder.  Before the cow, (all cows are females, males are called bulls) can start to produce milk she must be serviced and drop a calf.  Heifers reach sexual maturity at the age of seven or eight months but are not usually serviced until they are 15-18 months old.  The period of gestation is 265–300 days, varying somewhat according to the breed of cow, so a heifer produces her first calf at the age of about 2 1/2 years. 2   Secretion of milk in the cow udder begins shortly before calving, so that the calf can begins to feed almost immediately after birth.  The first five days after calving, cow produces special milk known as colostrum.  The cow then continuous to give milk for about 300 days, this period is known as lactation.  The cow normally comes into heat again a month or two after calving.  Six to nine weeks before the new calf is born, milking is gradually tapered off. The cow dries up for 50– 70 days.  With the birth of the calf a new lactation cycle begins.  A cow can normally be expected to remain productive for about 5 to 8 years. Figure (1) Most of the udder is composed of alveoli and milk is stored in the following proportions: 60% in alveoli, 20% in ducts and 20% in the cisterns. The cells that line the alveoli actually produce the milk; as the alveoli fill with milk, pressure on the epithelial cells increases and milk production slows. Arteries that supply the nutrients for milk production support each alveolus. It is estimated each ml of milk requires between 500 and 1000 ml of blood to circulate through the udder and 8% of the total blood volume of the dairy cow is present in the udder. Muscle cells surround the alveoli. To extract milk, the muscles around the alveoli must 3 contract to move the milk into the ducts and cisterns. This process is termed “milk letdown”. The process of milk letdown is initiated by the environmental and physical stimuli that trigger a series of hormonal events. Positive stimulation signals the pituitary gland in the brain to produce oxytocin. The oxytocin travels to the udder in the bloodstream and causes the myoepithelial cells around the alveoli to contract and move the milk into the duct and cistern system where it can be extracted through the milking process. Negative stimulatory events (such as shouting at the cows, using dogs to chase the cows or striking the cows) stimulate the release of the hormone adrenaline. Adrenaline causes blood vessels to contract and reduces the effect of oxytocin. Milk Secretion Rate: Milk yield is dependent on (1) the amount of secretory tissue and (2) the rate of milk secretion (per unit of time). Secretion rate is affected by the accumulation of milk in the alveolar lumen. Accumulation of milk in the lumen increases the intra-mammary pressure (see figure below). Once, the intra-mammary pressure reaches a certain level (probably about 8 to 10 hours after the last milking in the dairy cow), secretion rate declines. If the pressure increases enough (in the cow at about 70 mm Hg), then secretion stops and milk starts to be reabsorbed. In the dairy cow, secretion rate stops (reaches zero) at about 35 hours after the last milking. Instruments have not been available to measure intra-alveolar pressure. Generally the intra-mammary pressure has been measured in the teat cistern using a teat cannula. So these intra-mammary pressure estimates reflect total gland pressure from accumulation of milk and not directly the intra-alveolar pressure. 4 Milking Interval: For 2X/day milking of dairy cattle, the optimum interval is 12 hr; milk accumulation has not significantly lowered the milk secretion rate, but by ~14 hr there is a decline in secretion rate. Compared with a 12 and 12 hr milking interval in a 24 hr period, 9 and15 hr interval results in 1.8% less milk yield, while a 8 and 16 hr interval results in 3.4% less milk. Secretion rate in higher producing cows is less affected by long milking intervals (such as 3 consecutive 16 or 20 hr intervals) than the secretion rate in lower producing cows. Stage of Lactation and Persistency A. Colostrum vs. Milk: Colostrum is produced by the udder immediately after parturition. The composition of colostrum is considerably different from the 5 composition of normal milk. Three to 5 days immediately postpartum is needed for the secretions to change to the composition of milk. During this period the total solids, especially the immunoglobulins, are elevated. Newborn calves are practically devoid of immunoglobulins, the antibodies against various disease organisms. Calves must ingest the immunoglobulins from colostrum to acquire a passive immunity against common calf-hood diseases. Feeding colostrum after birth is especially critical during the first 24 hours of a calf's life. After this time, enzymes in the digestive tract degrade the antibodies and the permeability of the gut to antibodies decreases. Lactose content is depressed in colostrum, whereas fat and casein percentage is rather variable. High lactose in the intestine can cause scours in calves, and presumably the reduced lactose content of colostrum helps to prevent this disease. Calcium, magnesium, phosphorus, and chloride are high in colostrum, potassium is low. Iron is 10 to 17 times greater in colostrum than in normal milk. This high level of iron is needed for the rapid increase in hemoglobin in the red blood cells of the newborn calf. Colostrum contains 10 times as much vitamin A and 3 times as much vitamin D as that found in normal milk. The newborn calf is practically devoid of vitamin A, and since it provides a degree of protection against various diseases every calf should be fed colostrum. B. Stage of Lactation and Persistency At parturition milk production commences at a relatively high rate, and the amount secreted continues to increase for about 3 to 6 weeks. Higher-producing cows usually take longer than low producing cows to achieve peak production. After the peak is attained, milk production 6 gradually declines. The rate of decline is referred to as persistency. After peak lactation, on average the decline in milk yield. Milk composition changes during lactation: Fat percentage in milk decreases slightly during the early lactation and then increases as total production decreases with advancing lactation. Milk protein content gradually increases with advancing lactation. Mineral concentration increases slightly during advancing lactation. 7 8 Average composition (in percent) of milk of certain mammals Mammals Water Total solids Fat Protein Lactose Ash Human 78.7 12.3 3.3 1.8 6.8 0.3 Cow 87.0 13.0 4.0 3.6 4.7 0.7 Buffalo 83.1 16.9 7.0 4.2 4.9 0.8 Ewe 80.1 19.9 7.9 6.4 4.7 0.9 Goat 87.5 12.5 4.0 3.3 4.4 0.8 She-Camel 86.5 13.5 3.1 4.0 5.6 0.8 (1)- Water  The principle constituent of milk, it constitutes at least about 80% and may reach about 90% as in case of mare.  Water holds up the constituents either in soluble form (e.g. lactose, minerals) or in colloidal form (casein) or in a form of emulsion (milk fat).  Water in this high percentage is important for secretion of milk, milking, suckling, drinking and digestion.  High water % also helps the technological processing during manufacturing of milk products as this liquid media is suitable for biochemical and biological reaction.  This high water percentage also facilitates the growth of both desirable and undesirable microorganisms in such suitable media. (2)- Milk Fat  Milk fat is the most valuable milk constituent, according to which the price is evaluated. The higher the fat % the more tasty the milk.  Milk fat is found in the form of stable emulsion of fat globules dispersed throughout milk. 9   These fat globules are very small each is surrounded by fat globule membrane that is composed of outer protein layer (euglobin) and inner phospholipid layer (mainly lecithin). This fat globule membrane helps to stabilize the fat globules in an emulsion within the aqueous environment of milk (remember that cow milk is about 87% water). Function of the fat globule membrane: 1- The fat globule membrane protects the fat from lipolysis by the action of lipolytic microorganisms (as demonstrated in the following diagram). 2- The membrane prevents the joining of the globules, without such a protective layer, the globules would unite and form large masses of fat. Enzymes The composition of milk fat, average size 3 – 4 m. Facts about milk fat:  The lightest component, they tend to rise to the surface when milk is left to stand in a vessel for a time (cream- line formation).  Milk fat is present in the form of triglycerides (about 98-99%) in which 3 fatty acid molecules are combined with one molecule of glycerol, Di and 10  monoglycerides contains two or one molecules of fatty acids linked to one molecule of glycerol and are present in milk in 0.5 and 0.04% respectively.  Also, phospholipids (0.6-1.0%); cholesterol (0.2-0.4%); glycolipids; and free fatty acids in milk (0.1-0.4%). by weight, milk fat is made of about 12.5% of glycerol and about 85.5% of fatty acids.  Each glycerol molecule can bind three fatty-acid molecules, and as the three need not necessarily be of the same kind, the number of different glycerides in milk is extremely large. Milk fat contains a greater number of different fatty acids than any other food fat, over 400 different fatty acids have been identified in bovine lipids*. 11  Classification of Milk Fat (butter fat) A fatty acid molecule is composed of a hydrocarbon chain and a carboxyl group (formula RCOOH). Fatty acids (Saturation) Saturated Unsaturated Polyunsaturated fatty Acids fatty acid fatty acids Constitutes 60 – 70% Constitutes 25 – 30% Constitutes about 4% Carbon atoms are linked together in a One double bond in hydrocarbon (Considered unsaturated) chain by single bonds chain More double bonds in hydrocarbon chain Examples: Butyric acid, Caproic acid, Examples: Oleic acid, Linoleic acid, Example: Arachidonic acid Caprylic acid (liquid at room Linolenic acid (Liquid at (Liquid at room temperature). room temperature) temperature). Capric acid, Lauric acid, Myristic acid, Palmitic acid, Stearic acid (Solid at room temperature). Butyric acid constitutes about 60-70% of the saturated fatty acids. 13  Milk Fat (Other classification) Volatile fatty acids Non Volatile fatty acids Examples: Examples: Butyric acid C4 Myristic acid C14 Caproic acid C6 Palmitic acid C16 Caprylic acid C8 Stearic acid C18 Capric acid C10 Oleic acid C18 This group is responsible for aroma This group is responsible for fat texture. Characteristics of milk Fat: - Presence of a number of short chain fatty acids (C 4, C6, C8, C10) as demonstrated before which are present in only very small amounts in other fats (e.g. butyric and caproic acids). 14 - Butyric acid (C4H8O2) in particular constitutes about 4% of fatty acids in cow and 5.8% in buffalo milk; this acid is not found in any other natural fat. The presence of butyric acid as well as short chain fatty acids is important in two respects: (a)- They have strong, characteristic odor which are important in determining the flavor of milk and its products. (b)- Presence of short chain fatty acids in milk fat provides a mean for indicating adulteration by foreign fats. - Hardness of milk fat refers to the relative amounts of the most abundant fatty acids, i.e. palmitic acid increases the hardness, while oleic acid decreases the hardness and so on. Factors affecting fat in milk: 1. Species, Breed of the animal. 2. Individual variation. 3. Feeding: Over feeding not increase the fat, but under feeding affect. 4. Seasonal variation. 5. Interval between milking. 6. Efficacy of milking. (3)- Milk Proteins (Found in colloidal form) They are complex organic constituents of milk, the primary structure of proteins consists of a polypeptide chain of amino acids formed from carbon, nitrogen, oxygen, hydrogen, sometimes, combines with phosphorous or sulpher. Amino acids contain both a weakly basic amino group, and a weakly acid carboxyl group, both connected to a hydrocarbon chain. Proteins are giant molecules built up of smaller units called amino acids. A protein molecule consists of one or more chains of amino acids. A protein molecule usually contains around 100 – 200 linked amino acids in average. 15  Milk contains all essential amino acids that must be supplied in adequate amount by the diet, they are: 1. Tryptophan 2. Leucine 3. Isoleucine 4. Lysine 5. Methionine 6. Phenylalanine 7. Therionine 8. Valine In addition milk supplies infants with histidine amino acid which is essential for them. Milk contains a number of protein components that differ in composition and properties.  Typically, true protein (TP) is 95 to 97% of crude Protein (CP), the difference between TP and CP represents the non-protein nitrogenous constituents, which are ammonia, urea, creatine, creatinine, uric acid. 16  Milk Protein Concentration of proteins in milk % of total protein w/w (a)- Casein (b)- Whey Protein (79.5% of total protein) (19.3% of total protein) -Lactalbumin (3.7%) - Casein - Casein - Casein -lactoglobulin (9.8%) Serum albumin (1.2%) Immunoglobulins (2.1%) S1 Casein S2 Casein 30.6% 8.0% Miscellaneous (including protease-peptone) 2.4% Total Casein = 79.5% Total Whey Protein = 19.3% + Fat globule membrane proteins = 1.2% All equals 100% 17  (a)- Casein Definition: Casein is defined as the protein that precipitated from skim-milk at pH 4.6 at 20C.  It constitutes about 79.5% of total milk protein.  It is rich in phosphorous (phosphoprotein)  It is precipitated by acidifiying to pH 4.6 (isoelectric point) using diluted acids; heavy metals, Rennin enzyme and rennin like enzymes produced by certain microorganisms.  Casein molecules aggregates in thousands (10000 – 25000) forming casein micelles.  A number of minerals are bound within casein micelles including mainly calcium, sodium, and magnesium.  Kappa casein acts as a stabilizing factor in that it holds the casein complex in colloidal suspension as most k-casein is at the outside of the micelle. Structure of a casein submicelle. Uses of Casein: 1. As a food: It is used as a stabilizer in ice cream, to improve the whipping properties of cream, it is the main constituent of cheese, butter milk and it is also used in manufacture of sausage. 18  2. Industrial uses: as a casein glue, as paper coating material, in manufacture of plastics, manufacture of textiles and as adhesive in some insecticides and fungicidal sprays. 3. Medical uses: Compounds of casein with arsenic, iron, mercury, iodine and silver have been used in medicine. 4. Dairy farm: Iodinated casein “thyroprotein” used for animals for increasing production and fat content in milk. (b)- Whey Protein (milk serum proteins or Non casein nitrogen) Definition: Milk proteins remains in solution after precipitation of casein from skim-milk using diluted acid. Characteristics of whey proteins: (1)- They are denaturated by heating at temperature more than 70C. (2)- They remain soluble at pH 4.6 when the casein precipitated (e.g. acidification). (3)- Not affected by renneting. (4)- Whey protein in general, and -lactalbumin in particular, have very high nutritional values. Their amino acid composition is very close to that which is regarded as a biological optimum. Whey protein derivatives are widely used in the food industry. 1. -lactalbumin: Constitutes about 3.7% of the total milk proteins, this protein may be considered to be the typical whey protein, it is present in milk from all mammals, it is synthesized in udder and plays a significant part in the synthesis of lactose in the udder. 19 2. - lactoglobulin: Fairly heat sensitive protein, constituent about 50% of whey proteins (9.8% of the total milk proteins), all its types (A, B and C) are formed in the udder, it contains the sulfhydrol group (sulphurous compounds) which appear to be associated with the development of a “cooked flavor” in milk heated over 75C. Denaturation begins at 65C and is almost total when whey proteins are heated to 90C for 5 minutes. Part of a whey protein in native (left) and denaturated state (right). Whey protein heat denaturation is an irreversible reaction. Milk heated at 75°C for 20 – 60 seconds will start to smell and taste “cooked”. This is due to release of sulphurous compounds from -lactoglobulin and other sulphur- containing proteins. 3. Immunoglobulins: Constitutes about 12% of the total whey proteins, occur in both blood and milk and serve as a carrier for the antibodies that protect calves against disease producing organisms. 4. Serum albumin: It is identical to blood serum albumin. 5. Proteose peptone: Not denaturated at 65C. Carbohydrate of milk (4)- Lactose Lactose (milk sugar) is the carbohydrate (CHO) of milk, which is not found naturally in any other foodstuff. It is disaccharide manufactured in udder of glucose and galactose. Properties of Lactose: The most important characters are: 1. Lactose is only about 1/6 as sweet as can sugar (sucrose) (3). 20  2. It has only limited solubility (approximately 21 grams/100 gram water), and therefore, less sweet. 3. It is fermented by lactic acid bacteria to produce lactic acid (property used for yogurt manufacture). 4. It is hydrolysed into its constituents monosaccharides, glucose and galactose, by the enzyme  - galactosidase (lactase). 5. If milk is heated to a high temperature, and is kept at that temperature, it turns brown and acquires a caramel taste. This process is called caramelisation and is the result of a chemical reaction between lactose and proteins called the Maillard reaction. Some uses of lactose: 1. It is used as a constituent of infant foods and medicinal products; also it is used for modification of cow’s milk for infant feeding in order to bring its composition closer to that of woman’s milk. 2. In pharmaceutical industry, it is used in manufacture of tablets and capsules. (5)- Minerals (milk salts = milk ash) 21  Milk contains about 0.8% minerals, mainly in the form of salts of Ca, Mg, K, Na, Cl, PO4, bicarbonates, and citrate, a number of other minerals are also present in small amounts, e.g. Copper, Zinc and Iron. Minerals are present in milk in TWO forms Soluble form (Ionic form) Colloidal form  90% of K and Na  The rest of K and Na bounded with  35% of Ca and Mg organic phosphates and casein  100% of chlorine  50% of Ca with phosphorous and N.B. Calcium in soluble form is casein in form of colloidal calcium found as monocalcium phosphate. phosphate  15% of Ca is bounded with citrate and bicarbonate. Calcium Milk contains more calcium per unit of dry matter than most other foodstuffs. With the exception of the leafy vegetables, no other food has sufficient calcium to meet the dietary requirements. In general children do not utilize the calcium of vegetables as well as well as that from milk. When milk is boiled or pasteurized, 10 to 20% of the calcium becomes less available to the body. The absorption of calcium from the intestinal tract is favored by an acid medium, such as is established by the lactose in milk. The presence of milk-fat also aids in the absorption of the calcium from milk, probably by favoring the use of lactose. Vitamin D also is essential for the utilization of calcium. Phosphorus As in case of calcium, the absorption of phosphorus is favored by the acid condition is the intestinal tract, about 85% of phosphorus in the body is combined with calcium in the formation of bone. 22  As a role, if the diet supplies sufficient calcium and protein, it most probably also supplies the phosphorus requirement, because such foods are good source of phosphorus. Minor milk constituents 1. Phospholipids: e.g. Lecithin 2. Sterols: e.g. Cholesterol 3. Non protein nitrogenous substances: About 5% of the total nitrogen in milk, present as non-protein nitrogenous substances and includes ammonia, urea, creatinine, creatine and uric acid. 4. Vitamins: (a)- Water soluble vitamins: B1, B2, B6, B12, nicin and pantothenic acid. There is also a small amount of vitamin C (ascorbic acid) present in raw milk but is very heat-labile and easily destroyed by pasteurization. (b)- Fat soluble vitamins: A, D, E and K. 5. Pigments: There are two pigments in milk, the pigment carotene and the pigment riboflavin (vitamin B2 ). 1- Carotene pigment It is the pro-vitamin A, it is fat-soluble pigment. It impart a yellowish color to the cow milk fat. Cows can transfer more carotene from their feed to the milk fat, while buffalos and goats transfer most of carotene into vitamin A. 2- Riboflavin pigment (Vitamin B2) It is water soluble pigment, responsible for bluish color of whey, but in whole milk this color is masked by the other constituents present in milk. 6. Enzymes: (details in the next page) (a)- Of economic importance (e.g. Proteinase, lipase). 23  (b)- Of heat treatment importance (e.g. Phosphatae, Peroxidase). (c)- Of sanitary importance (e.g. Reductase and catalase). 7. Gases: constitutes from 7 to 10% by volume as CO2, O2 and N2 from the surrounding atmosphere. 8. Cells:  Milk also contain cells “Milk Somatic Cells” “MSC” such as epithelial, leucocytes cells and cell debris.  Cellular content of milk found in the secretions of normal healthy animals.  The leucocytes increased in response to infection or injury while epithelial cells increased as result of infection or injury.  The cellular content is frequently a useful indication of udder health condition. Equal epithelial cells + leucocytes Enzymes in milk Definition: Enzymes are a group of proteins produced by living organisms. They have the ability to activate chemical reactions and to affect the course and speed of such reactions without being consumed. They are therefore sometimes called biocatalysts. The action of enzymes is specific (= each type of enzyme catalyses only one type of reaction). Factors influence the enzymatic action: Two factors which strongly influence enzymatic action are: (1)- Temperature: 24 As a rule enzymes are most active in an optimum temperature range between 25 and 50°C. Their activity drops if the temperature is increased beyond optimum, ceasing altogether somewhere between 50 and 120°C. At these temperatures the enzymes are more or less completely denaturated (inactivated). The temperature of inactivation varies from one type of enzyme to another. (2)- pH: Enzymes also have their optimum pH ranges; some function best in acid solutions, others in an alkaline environment. Sources of enzymes in milk: The enzymes in milk come either from: - The cow’s udder: Normal constituents of milk and are called original enzymes. - Bacteria: Bacterial enzymes vary in type and abundance according to the nature and size of the bacterial population. Classification of enzymes: Milk enzymes can be divided into two groups: Group (I): enzymes responsible for certain changes in milk constituents. Group (II): enzymes of sanitary importance. Group (I): enzymes responsible for certain changes in milk constituents: This group of may be responsible for desirable changes or undesirable changes. This group of enzymes include: 1. Protease enzyme: 25  It is a proteolytic enzyme as this enzyme splits proteins to peptones and amino acids. Protease enzyme digests cheese proteins during storage causing softness of its constituents. his process is known as ripening of cheese so it is considered as desirable changes. eating of milk at - inactivates this en yme.  Protein degradation can be undesirable and result in bitter off flavors )‫(مهمة‬, or it may provide a desirable texture to cheese ripening. 2. Lipase enzyme:  It is a fat splitting en yme which cause a rancid flavour of milk and it s products which is characteri ed by production of sharp pungent odour. But also may impart desirable flavors to some cheeses like parmesan. Lipase enzyme is present in milk in limited amount, and the amount of it increases in cases of :  Milk at late lactation period.  Milk comes from dairy animal with cystic ovaries.  Milk contains high number of lipolytic organisms as Pseudomonas fluorescence. The amount of lipase enzyme is inhibited by increasing acidity and it is destroyed by pasteurization. NB:  Raw milk contains enough lipase to hydrolyze all the fat in milk. But it does not happen. Why not? It cannot attack fat in intact milk fat globules (due to protection of the milk fat globule membrane) and milk contains some substances which inhibit lipase action. Group (II): enzymes of sanitary importance: This group of enzymes may be 26 (A)- Enzymes important in keeping quality of milk  Reductase enzyme  Catalase enzyme (B)- Enzymes important in heat treatment  Phosphatase enzyme  Peroxidase enzyme Catalase splits hydrogen peroxide into water and free oxygen. Milk from diseased udders has a high catalase content, while fresh milk from a healthy udder contains only an insignificant amount. However many bacteria produce this kind of enzyme. Catalase is destroyed by heating at 75°C for 60 seconds. Phosphatase enzyme: It is present in raw milk. It is destroyed by the time temperature combination required for efficient pasteurization. Therefore, phosphates enzyme is applied to detect error in pasteurization process. Peroxidase enzyme This enzyme is destroyed in milk heated above. Correlation between time and temperature used for destruction of peroxidase enzyme: Temperature Time ver At once. Some important definitions: 1. Milk plasma: All constituents of milk except the milk fat. 2. Milk serum: All the constituents except the fat and casein. 27  Nutritive value of milk Milk is the most nearly single perfect food; it is valuable in human nutrition and supplies significant quantities of all five groups of nutrients (Protein, Fat, CHO, Minerals and Vitamins). Milk Fat: The most valuable part of milk components concerning the price as well as the caloric value. 1. Milk fat is more easily digested and absorbed without producing digestive disturbances than any other common edible fat. As it has low melting point. About 97% of milk fat ingested is utilized by the body. 2. Milk fat increases the absorption of Calcium from the intestine in presence of Vitamin D as well as acidic medium. 3. Milk fat is a rich source of vitamins A, D, E and K (fat soluble vitamins). 4. 1 gram of milk fat produces about 8.8 calories. Milk Protein: 1. Proteins are built from approximately 20 amino acids, 18 of which are found in milk proteins. 2. The most valuable component of milk in human nutrition, as it provides an excellent balance of amino acids. 3. A milk protein contains all the essential amino acids (tryptophan, leucine, isoleucine, lysine, methionine, phenylalanine, therionine and valine) in sufficient amount as well as histidine which are essential for infants. 4. Milk proteins are broken down into simpler compounds in the digestive system and in the liver; these compounds are then conveyed to the cells of the body where they are used as construction material for building the body's own protein. 5. Milk proteins are rich in phosphorous as well as sulpher. 28 6. 1 gram milk protein produces 4.3 calories. Lactose: 1. As a source of energy, it splits into glucose and galactose, the former is used for providing energy while the latter is considered as indispensable sugar as it nourishes the brain, especially the infants. 2. Fermentation of lactose leads to formation of lactic acid which: 1. Kills the putrefactive bacteria. 2. Favors the absorption of calcium from intestine as well as phosphorous and magnesium. 3. 1 gram of lactose provides 3.9 calories. Minerals: 1. Milk is the best nutritional source of calcium; 250 ml of milk/day supplies the most daily requirement of most people except for pregnant and lactating woman. 2. 250 ml of milk provides the calcium required by the growing child, this amount may be also sufficient for adult persons while pregnant woman, infants, young and elderly needs more than 250 ml, lactating woman needs more calcium than that of pregnant one. 3. Also, phosphorus is an important element for all body cells. N. B.: The absorption of calcium from the intestinal tract is favored by an acid medium, such as is established by the lactose in milk. The presence of milk fat also aids in the absorption of the calcium from milk, probably by favoring the use of lactose. Vitamin D also is essential for the utilization of calcium. Vitamins: Milk is rich in fat soluble vitamins (A, D, E and K) and water soluble vitamins (B1, B2, B6, B12, nicotinic acid and pantothenic acid), but its content of vitamin C is little. 29  Basic physical-chemical properties of cows’ milk: ows’ milk consists of about % water and 13% dry substance. he dry substance is suspended or dissolved in the water. Depending on the type of solids there are different distribution systems of them in the water phase. Definitions: Emulsion: a suspension of droplets of one liquid in another. Milk is an emulsion of fat (oil) in water (O/W); butter an emulsion of water in fat (W/O). Colloidal solution: when matter exists in a state of division intermediate to true solution (e.g. sugar in water) and suspension (e.g. chalk in water) it is said to be in colloidal solution or colloidal suspension. True solutions: Matter which, when mixed with water or other liquids, forms true solutions, is divided into: Non-ionic solutions. When lactose is dissolved in water, no important changes occur in the molecular structure of the lactose. Ionic Solution Ionic solutions. When common salt is dissolved in water cations (Na+) and anions (Cl–) are dispersed in the water, forming an electrolyte. 30