MILK Food Material Science 2010/11 Inneke Hantoro Definition The normal secretion of the mammary glands of all mammals (Potter & Hotchiss, 1996). Milk is a complete food for the new born. High density of nutritious components. MILK COMPOSITION & STRUCTURE The average composition of milk Source: Walstra et al. (2006) Principal components Lactose or milk sugar is the distinctiveACIDS FATTY carbohydrate of milk. It is a disaccharide composed of glucose and galactose. The fat is largely made up of triglycerides, constituting a very complicated mixture. The component fatty acids vary widely in chain length (2 to 20 carbon atoms) and in saturation (0 to 4 double bonds). Other lipids that are present include phospholipids, cholesterol, free TRIGLISERIDA fatty acids, monoglycerides, and diglycerides. Principal components Protein About four fifths of the protein consists of casein, actually a mixture of four proteins: αS1-, αS2-, β-, and κ-casein. The caseins are typical for milk. The remainder consists, for the most part, of the milk serum proteins, the main one being β- lactoglobulin. Moreover, milk contains numerous minor proteins, including a wide range of enzymes. Principal components The mineral substances — primarily K, Na, Ca, Mg, Cl, and phosphate — are not equivalent to the salts. Milk contains numerous other elements in trace quantities. The salts are only partly ionized. The organic acids occur largely as ions or as salts; citrate is the principle one. Furthermore, milk has many miscellaneous components, often in trace amounts. Water Organic acids Proteins Carbohydrates citrate casein lactose formate -lactoglobuline glucose acetate -lactalbumine others lactate serum albumin SERUM Minerals oxalate immunoglobulines Ca, bound others proteose pepton Ca, ions Gases NPN Mg oxygen peptides K nitrogen amino acids Na Lipids urea Cl glycerides ammonia phosphate fatty acids Enzymes sulfate phospholipids acid phosphatase bicarbonate cerebrosides peroxidases Trace elements sterols many others Zn/Fe/Cu and Vitamins Phosphoric many B vitamines esthers others ascorbic acid Others Compotition and Structure (A) Uniform liquid. However, the liquid is turbid and thus cannot be homogeneous. (B) Spherical droplets, consisting of fat. These globules float in a liquid (plasma). (C) The plasma contains proteinaceous particles, which are casein micelles. The remaining liquid (serum) is still opalescent, so it must contain other particles. The fat globules have a thin outer layer (membrane) of different constitution. Fat Globules The surface layer or membrane of the fat globule is not an adsorption layer of one single substance but consists of many components; its structure is complicated. The dry mass of the membrane is about 2.5% of that of the fat. A small part of the lipids of milk is found outside the fat globules. At temperatures below 35°C, part of the fat in the globules can crystallize. Milk minus fat globules is called milk plasma, i.e., the liquid in which the fat globules float. Composition and structure of milk fat MEMBRAN Water FAT GLOBULE Protein Glycerides Phospholipids triglycerides Cerebrosides diglycerides Glycerides monoglycerides Fatty acids Fatty acids Sterols Sterols Other lipids Carotenoids Enzymes Vitamins A,D,E,K alkaline phosphatase Water xanthine oxidase many others Cu and Fe Casein Micelles Casein micelles consist of water, protein, and salts. The protein is casein. Casein is present as a caseinate, which means that it binds cations, primarily calcium and magnesium. The other salts in the micelles occur as a calcium phosphate, varying somewhat in composition and also containing a small amount of citrate. This is often called colloidal phosphate. The whole may be called calcium- caseinate/calcium-phosphate complex. Casein Micelles The casein micelles are just ‘small particles.’ The micelles have an open structure and, accordingly, contain much water, a few grams per gram of casein. Milk serum, i.e., the liquid in which the micelles are dispersed, is milk minus fat globules and casein micelles. CASEIN Protein MICELLE Casein Proteose pepton Salts Ca Phosphate Citrate K, Mg, Na Water Enzymes (lipase, plasmine) Other Milk Constituents Serum proteins are largely present in milk in molecular form or as very small aggregates. Lipoprotein particles, sometimes called milk microsomes, vary in quantity and shape. Presumably, they consist of remnants of mammary secretory cell membranes. Few definitive data on lipoprotein particles have been published. Other Milk Constituents Cells, i.e., leukocytes, are always present in milk. They account for about 0.01% of the volume of milk of healthy cows. Of course, the cells contain all cytoplasmic components such as enzymes. They are rich in catalase. Other milk constituents LEUKOCYTE LIPOPROTEIN PARTICLE Many enzymes Lipids e.g. katalase Protein Nucleic acids Enzymes Water Water Properties of the main structural elements of milk fat casein globular lipoprotein micelles proteins paricles Main component(s) Fat Casein, Serum Lipids, water salts proteins proteins To be considered as Emulsion Fine Colloidal Colloidal dispersion solution dispersion Content (% dry matter) 4 2.8 0.6 0.01 Volume fraction 0.04 0.1 0.006 10-4 Particle diameter 0.1 – 10 m 20 – 300 nm 3 – 6 nm 10 nm Number per ml 1010 1014 1017 1014 Surface area (cm2/ml milk) 700 40.000 50.000 100 Density (20 0C; kg/m3) 900 1100 1300 1100 Diffusion rate (mm in 1 h) 0.0 0.1 – 0.3 0.6 0.4 Isoelectric pH ~3.8 ~4.6 4–5 ~4 MILK FORMATION Digestion-1 Milk components are for the most part formed in the mammary gland (the udder) of a cow, from precursors that are the results of digestion. In ruminants like the cow, considerable predigestion occurs by means of microbial fermentation, which occurs for the most part in the first stomach or rumen. It contains numerous bacteria that can digest cellulose, thereby breaking down plant cell walls, providing energy and liberating the cell contents. Digestion-2 From cellulose and other carbohydrates, acetic, propionic, butyric and lactic acid are formed, which are taken up in the blood. The composition of the organic acid mixture depends on the composition of the feed. Proteins are broken down into amino acids. The rumen flora uses these to make proteins but can also synthesize amino acids from low- molar-mass nitrogenous components. Further on in the digestive tract the microbes are digested, liberating amino acids. Digestion-3 Also, food lipids are hydrolyzed in the rumen and partly metabolized by the microorganisms. All these precursors can reach the mammary gland. Milk Synthesis-1 The synthesis of milk components occurs for the greater part in the secretory cells of the mammary gland. Milk Synthesis-2 At the basal end precursors of milk components are taken up from the blood, and at the apical end milk components are secreted into the lumen. Proteins are formed in the endoplasmic reticulum and transported to the Golgi vesicles, in which most of the soluble milk components are collected. The vesicles grow in size while being transported through the cell and then open up to release their contents in the lumen. Milk Synthesis-3 Triglycerides are synthesized in the cytoplasm, forming small globules, which grow while they are transported to the apical end of the cell. They become enrobed by the outer cell membrane (or plasmalemma) while being pinched off into the lumen. This type of secretion is called merocrine, which means that the cell remains intact. Excretion-1 The glandular epithelium, consisting of layers of secretory cells, form spherical bodies called alveoli. Each of these has a central lumen into which the freshly formed milk is secreted. From there, the milk can flow through small ducts into larger and still larger ones until it reaches a cavity called the cistern. From the cistern, the milk can be released via the teat. Excretion-2 Excretion of the milk does not happen spontaneously. The alveoli have to contract, which can be achieved by the contraction of muscle tissue around the alveoli. Contraction is induced by the hormone oxytocin. This is released into the blood by stimulation of the teats of the animal, be it by the suckling young or by the milker. The udder is not fully emptied. Lactation When a calf is born, lactation — i.e., the formation and secretion of milk — starts. The first secretion greatly differs in composition from milk. Within a few days the milk has become normal and milk yield increases for some months, after which it declines. The yield greatly varies among cows and with the amount and the quality of the feed taken by the cow. Colostrum Colostrum is the secretion produced over the first few days after parturition. The components of colostrum are synthesised in the mammary gland over several days prior to parturition. Colostrum is rich in special nutrients for the newborn. Colostrum contains more mineral salts and protein and less ash than later milk. Ca, Na, Mg, P, and chloride are higher in colostrum but K is lower. The most remarkable difference between colostrum and milk is the high concentration of immunoglobulins (Ig’s) in colostrum. Ig’s are related to passive immunity against gut pathogens. Colostrum Colostrum has a higher level of -carotene, imparting an intense yellow colour, and a high level of somatic cells. Recently there has been a lot of commercial interest in colostrum because of its elevated levels of bioactives, especially growth factors, and there is a wide range of literature supporting the health benefits of colostrum Colostrum is 10 times more expensive than milk powder. MILK ATTRIBUTES Milk quality Factors that determine the quality of fresh milk (standard indicators) are: Total solid contents, including protein (min. 2.7%), fat (min. 3%), solid non fat (min. 8%). Raw milk is purchased by weight, but processed milk is sold by volume. Freezing point Density Milk quality Some factors can influence the quality of milk, including: Feed Genetic Climate The health status of cattle Milking process and storage Post harvest handling Fresh Milk Deterioration Milk can deteriorate fast since milk contains high nutrient contents such as carbohydrate, fat and protein which required by bacteria to grow. Moreover, pH of milk is close to neutral pH. This is very suitable for the growth of microorganisms. Lastly, since most of microorganism (mesophilic and psychotrophic bacteria) can grow very well at room temperature, fresh milk stored in room temperature is susceptible to microbial deterioration. Fresh Milk Deterioration Many of the psychrotrophic bacteria isolated from milk produce extracellular enzymes that degrade milk fat and protein (proteolysis and lypolysis). Bacterial lipase causes serious degradation of milk fat. Beside microbial degradation, fresh milk also susceptible to enzymatic degradation. Raw milk has an abundance of lipoprotein lipase, an enzyme that will rapidly hydrolyse milk fat to free fatty acids (FFAs). Some of these FFAs have low organoleptic thresholds and produce odors and flavors (rancid, bitter, soapy or unclean). UHT vs Pasteurized Milk Generally, there are two heat treatment given to fresh milk, i.e. pasteurization and sterilization using ultra high temperature (UHT). Pasteurization is done at 63oC for 30 min or 72- 75oC for 15-20 s (high temperature short time - HTST). Pasteurization is used mostly to kill Gram- negative psychrotrophs bacteria, but only has little effect on extracellular degradative enzymes. While UHT is done at 135 - 140oC for a few seconds. It can kill both pathogen and spoilage microorganisms. The most heat resistant pathogenic spore – C. botulinum and some enzymes also can be inactivated. UHT vs Pasteurized Milk UHT products are commonly stored at room (ambient) temperature and good quality products should be microbiologically stable. Nevertheless, chemical reactions and physical changes will take place which will change the quality of the product. These include oxidation reactions, Maillard browning and chemical & physical changes which may give rise to age- thickening and gelation. UHT vs Pasteurized Milk In pasteurization, thermoduric bacteria and spore forming bacteria can survive. Bacillus cereus spores are relevant here, being the main pathogen which will survive pasteurization and grow at low temperature. It will certainly cause spoilage in heat- treated milk. Enzymes in raw milk may give rise to problems in pasteurized milk. For example, indigenous lipases may give rise to soapy off-flavors. However, it is unlikely that bacterial lipases and proteases, which are very heat resistant, will cause problems in pasteurized milks because of their relatively short shelf-life and refrigerated storage conditions. Milk & Dairy Products Adulteration Watering of milk Milk of different species Addition of non-dairy protein Altering the casein/whey protein ratio Addition of buttermilk or whey powder to milk powder Addition of vegetable or animal fats to milk fat Addition of reconstituted milk to fluid milk Non-authorized preservatives. Milk Coagulation Desirable coagulation of milk can be seen in dairy products processing such as cheese, yoghurt, etc. Undesirable coagulation occur in liquid milk. It can caused by lactic acid (produced by bacteria) --- the reduction of pH or by physical separation (due to density difference) such as creaming, flocculation or coalescence --- see emulsion chapter). Milk Coagulation Milk protein, such as whey protein and casein have important role in coagulation. The example of desirable coagulation: Acidification forms the basis of production of all fermented milks. The gels of fermented milks, such as yoghurt, are formed by acidification of milk. As the pH is reduced, the casein precipitates selectively. The first signs of aggregation occur around pH 5 and once the pH falls to 4.6 all the casein becomes insoluble. Milk Coagulation Some factors influence coagulation, including: pH Temperature Heat treatment Casein concentration The presence of salt Emulsion Milk proteins have excellent emulsifying properties. Milk is categorized as o/w emulsion, since the oil part is dispersed in the water. Milk proteins, both caseinates and whey proteins, are surface active, they are absorbed rapidly to the oil-water interface, forming stable emulsions. Emulsion The primary processes leading to emulsion instability are: Creaming – refers to the gravitational separation of emulsified droplets to form a densely packed phase without change in droplet size. Flocculation – denotes the aggregation of droplets via interactions between adsorbed proteins. Coalescence – an increase in droplet size, gradually results in separation of the oil and aqueous phases. 2 layers formation Coalescence Creaming Flocculation Kinetically stable emulsion Creaming Since the specific gravity of lipids and skim milk is 0.9 and 1.036, respectively, the fat globules in milk held under quiescent conditions will rise to the surface under the influence of gravity, a process referred to as creaming. The rapid rate of creaming is due to the strong tendency of the fat globules to cluster due to the effect of indigenous immunoglobulin M which precipitates onto the fat globules when milk is cooled (cryoglobulins). Creaming Large globules rise faster than smaller ones, collide with them and form aggregates. The clusters of globules rise rapidly and therefore the creaming process is accelerated as the globules rise and clump. Creaming is inhibited by reduction of the fat globule size by homogenisation. The milk fat globules are reduced in size by pumping at very high pressure (up to 400 bar) through a small slit. The size reduction results in an increase in specific surface area . Whipping & Foaming As milk proteins are surface active, they have the ability to adsorb to the air-water interface during foam formation. Foams are most commonly formed by mechanically dispersing air into a solution containing surface-active agents. A rapid diffusion of the protein to the air-water interface to reduce surface tension, followed by partial unfolding of the protein is essential for the formation of protein-based foams. Whipping & Foaming Caseinates generally give higher foam overruns but produce less stable foams than whey protein concentrates (WPC). The foaming properties are influenced by many factors, including: protein concentration, level of denaturation, ionic strength, preheat treatment and presence of lipids. The Changes of Milk Flavor Deterioration of milk flavor can be caused by degradation milk fat and protein. Rancidity is a common indicator of the forming of undesirable flavor. Factors stimulating the off-flavor in fresh milk: Light Ion metals Transferred from cow to milk Microorganisms Enzymatic reactions References Walstra, P., J.T.M. Wouters & T. J. Geurts. 2006. Dairy Science and Technology 2nd Edition. Taylor and Francis Group. Boca Raton. Thank You….
Pages to are hidden for
"MILK - PowerPoint"Please download to view full document