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Water The nutritional value of milk as a whole is greater than the value of its individual nutrients because of its unique nutritional balance. The amount of water in milk reflects that balance. In all animals, water is the nutrient required in the greatest amount and milk does supply a great amount of water—it contains approximately 90% water. The amount of water in milk is regulated by the amount of lactose synthesized by the secretory cells of the mammary gland. The water that goes into the milk is delivered to the mammary gland by the blood.
Milk Proteins • There are several types of proteins in milk. • The major milk proteins are unique to milk. - not found in any other tissue • Milk proteins, particularly caseins, have an appropriate amino acid composition for growth and development of the young. • Other proteins in milk include an array of enzymes, proteins involved in transporting nutrients, proteins involved in disease resistance (antibodies and others), growth factors, etc.
Native Proteins: A “native” protein is a protein that is still in its original, natural structure. It has not been altered by heat, chemicals, enzyme action, or distress. Therefore, it is imperative to understand that a native protein represents the ideal biological function for which it was designed by nature.
Amino Acids: Amino acids are critical to life, and have a variety of roles in metabolism. One of their most important functions is to serve as the building blocks of proteins. All proteins in all species, from bacteria to humans, are constructed from the same set of twenty amino acids. A protein is formed by amino acid subunits linked together in a chain. The bond between two amino acids is called a peptide bond and the chain of amino acids is called a peptide (20 amino acids or less), or a polypeptide (more than 20).
Most of the nitrogen in the milk is found in the form of protein. The building blocks of all proteins are the amino acids. There are 20 amino acids that are commonly found in proteins. • The order of the amino acids in a protein, which is determined by the genetic code, gives the protein a unique conformation. In turn, the spatial conformation of the protein gives it a specific function.
Protein Structure Each protein consists of one or more unique polypeptide chains. These chains then undergo a folding process which results in a configuration that is the most stable for its particular chemical structure and environment. The final 3-D arrangement is called the protein’s conformation and typically assumes a globular appearance. To understand this very complex conformation, scientists describe four levels of how the amino acid peptide chains arrange themselves: Primary, Secondary, Tertiary, and Quaternary.
Primary Structure refers to the linear sequence of amino acids that make up the polypeptide chain. This sequence is determined by the genetic code. The bond between two amino acids (a peptide bond) is formed by the removal of a water molecule from the two different amino acids, forming a dipeptide. The sequence of amino acids determines the way that the protein folds into its final structure, or conformation. • Secondary Structure refers to the formation of a regular pattern of twists or kinks of the polypeptide chain. This pattern is due to hydrogen bonds forming between the atoms of the amino acid backbone. • Tertiary Structure refers to the three dimensional globular structure formed by the folding of these polypeptide chains. This typically results in a compact globular structure. The folding of the polypeptide chain is stabilized by multiple weak, non-covalent interactions. These interactions include: • Hydrogen bonds that form when a Hydrogen atom is shared by two other atoms. • Electrostatic interactions that occur between charged amino acid side chains. • Hydrophobic/Hydrophilic interactions: During the folding process, amino acids with a polar (water soluble) side chain are often found on the surface of the molecule while amino acids with non-polar (water insoluble) side chain are buried in the interior. This affects the water solubility of a protein.
Quaternary Structure refers to the fact that some proteins contain more than one polypeptide chain, adding an additional level of structural organization. Globular and fibrous proteins If the protein is tightly coiled and folded into a somewhat spherical shape, it is called a globular protein. If the protein consists of long polypeptide chains that are intermolecularly linked, they are called fibrous proteins.
Centrifugation of the skim milk in an ultracentrifuge (usually about 50,000 x g or greater) results in pelleting of the casein and in a supernatant called whey (also sometimes called the serum phase of milk) which contains the water, lactose and soluble non-casein proteins. Once casein is removed, then by definition every other protein left in the milk preparation is a whey protein. • Because the casein micelle is in suspension, it can be separated from the rest of milk by centrifugation at a very high speed. Generally the milk is first defatted (the cream is removed) from whole milk by low speed centrifugation (at about 5,000 to 10,000 x g), resulting in the cream layer at the top, the aqueous supernatant, and a small pellet of leukocytes and other debris. The aqueous supernatant is the skim milk (sometimes called the plasma phase of milk).
Casein Proteins (~80% of Total Milk Protein-TMP)1. Alpha s1 [30.6%]2. Alpha s2 [8.0%]3. Beta [28.4%]4. Kappa [10.1%] • 5. gamma [2.4%] • Whey Proteins (~20% of TMP) • 1. Alpha lactalbumin [3.7%]2. Beta lactoglobulin [9.8%]3. Bovine Serum Albumin (BSA) [1.2%]4. Immuoglobulins [2.1%]5. Proteose peptone [2.4%]
Milk Protein Fractionation The nitrogen content of milk is distributed among caseins (75%), whey proteins (18%), miscellaneous proteins (2%) and non-protein nitrogen (5%). This nitrogen distribution can be determined by the Rowland fractionation method: Precipitation at pH 4.6 - separates caseins from whey nitrogen Precipitation with sodium acetate and acetic acid (pH 5.0) - separates total proteins from whey NPN Ninety-five percent of the nitrogen is associated with protein.
Casein Proteins • Caseins are heterogeneous group of phosphoproteins precipitated from raw skim milk at pH 4.6 and 20C. • Caseins are highly digestible in the intestine and are a high quality source of amino acids. Most whey proteins are relatively less digestible in the intestine, although all of them are digested to some degree. When substantial whey protein is not digested fully in the intestine, some of the intact protein may stimulate a localized intestinal or a systemic immune response. This is sometimes referred to as milk protein allergy and is most often thought to be caused by ß-lactoglobulin. Milk protein allergy is only one type of food protein allergy.
Caseins is composed of several similar proteins which form a multi-molecular, granular structure called a casein micelle. In addition to casein molecules, the casein micelle contains water and salts (mainly calcium and phosphorous). Some enzymes are associated with casein micelles, too. The micellularstructure of casein in milk is an important part of the mode of digestion of milk in the stomach and intestine, the basis for many of the milk products industries (such as the cheese industry), and the basis for our ability to easily separate some proteins and other components from cow milk. Casein is one of the most abundant organic components of milk, in addition to the lactose and milk fat. Individual molecules of casein alone are not very soluble in the aqueous environment of milk. However, the casein micelle granules are maintained as a colloidal suspension in milk. If the micellularstructure is disturbed, the micelles may come apart and the casein may come out of solution, forming the gelatinous material of the curd. This is part of the basis for formation of all non-fluid milk products like cheese.
Alpha casein • Alpha s1 casein molecule contains 199 amino acids and 8 phosphate groups that are esterified to serine groups. The molecule has a net charge of about -24 at pH 6.7 and contains no cysteine residues. • Molecular weight of Alpha s1 casein is 23,500. • Alpha s1 casein is very sensitive to calcium ions. • Alpha s2 casein contain 8 more amino acids, have from 10 to 13 phosphates and also 2 cysteines. The a s 2 caseins have an average molecular weight of about 25,100. Beta casein • Beta-Casein comprises from 25-35% of the total casein. It is made up of 209 amino acids. The molecule contains 5 phosphates each as a serine phosphate ester. • Beta-casein contains no cysteine residues and contains 17% proline. The protein has a molecular weight of 23,980 and a net charge of -13 at pH 6.7. • Beta-casein is less sensitive to calcium ions. Kappa-casein • Kappa-casein consists of 169 amino acids and has a molecular weight of 19,005. • k-casein is soluble in the presence of Ca++. Gamma-casein • About 5% of the casein is made up of a heterogeneous group of proteins known as Gamma-casein. • Molecular weight of gamma casein is 20,000.
Whey Proteins The major whey proteins in cow milk are ß-lactoglobulinand a-lactalbumin. a-Lactalbumin is an important protein in the synthesis of lactose and its presence is central to the process of milk synthesis. ß-Lactoglobulin's function is not known. Other whey proteins are the immunoglobulins (antibodies; especially high in colostrum) and serum albumin (a serum protein). Whey proteins also include a long list of enzymes, hormones, growth factors, nutrient transporters, disease resistance factors, and others. Lactalbumin, alpha-, also known as LALBA, is a protein that in humans is encoded by the LALBA gene. α-Lactalbumin is an important whey protein in cow's milk (~1 g/l), and is also present in the milk of many other mammalian species. In primates, alpha-lactalbumin expression is upregulated in response to the hormone prolactin and increases the production of lactose.
β-Lactoglobulin is the major whey protein of cow and sheep's milk (~3 g/l), and is also present in many other mammalian species; a notable exception being humans. Unlike the other main whey protein, α-lactalbumin, no clear function has been identified for β-lactoglobulin, although it binds to several hydrophobic molecules, suggesting its role in their transport.
Other milk proteins In addition to the major protein fractions outlined, milk contains a number of enzymes. The main enzymes present are lipases, which cause rancidity, particularly in homogenised milk, and phosphatase enzymes, which catalyse the hydrolysis of organic phosphates. Measuring the inactivation of alkaline phosphatase is a method of testing the effectiveness of pasteurisation of milk. Peroxidase enzymes, which catalyse the breakdown of hydrogen peroxide to water and oxygen, are also present. Lactoperoxidase can be activated and use is made of this for milk preservation. Milk also contains protease enzymes, which catalyse the hydrolysis of proteins, and lactalbumin, bovine serum albumin, the immune globulins and lactoferrin, which protect the young calf against infection.
MilkFat Butterfat or milkfat is the fatty portion of milk. Milk and cream are often sold according to the amount of butterfat they contain. • The fatty acids of butterfat are typically composed as follows (by mass fraction): • Saturated fatty acids: • Palmitic acid: 31% • Myristic acid: 12% • Stearic acid: 11% • Lower (at most 12 carbon atoms) saturated fatty acids: 11% • pentadecanoic acid and heptadecanoic acid: traces • Unsaturated fatty acids: • Oleic acid: 24% • Palmitoleic acid: 4% • Linoleic acid: 3% • Linolenic acid: 1%
U.S. Standards for butterfat content of dairy products • Milks • skim milk contains less than 0.5% fat, typically 0.1% • Low fat milk contains between 0.5–2% fat; 1% and 2% varieties are widely marketed • whole milk contains at least 3.25% fat • Cheeses • dry curd and nonfat cottage cheese contain less than 0.5% fat • cottage cheese contains at least 4% fat • Low fat cottage cheese contains 0.5–2% fat • Cheddar cheese contains at least 50% fat relative to the total solids • Swiss cheese contains at least 43% fat relative to the total solids • Frozen desserts • ice cream contains at least 10% fat • Low fat ice cream, also called ice milk, contains no more than 2.6% fat • frozen custard, like ice cream, contains at least 10% fat, but it also must contain at least 1.4% egg yolk solids • sherbet contains 1–2% fat • Creams • half and half contains 10.5–18% fat • light cream and sour cream contain 18–30% fat • light whipping cream (often called simply "whipping cream") contains 30–36% fat • heavy cream contains a minimum of 36% fat • manufacturer's cream (not federally regulated) contains 40% fat • Butter (including whipped butter) contains at least 80% fat
Composition of milk fat Composition of fats in milk is usually discussed in terms of the fatty acids. Fatty acids do not occur as such in milk (and rarely in any food). Instead, they are incorporated into compounds called triglycerides. A triglyceride, also called triacylglycerol (TAG), is a chemical compound formed from one molecule of glycerol and three fatty acids. HO-CH2 HO-CH HO-CH2 CH3(CH2)n COOH R-COOH Fatty acid Glycerol or Glycerin or trihydric alcohol
Glycerol is a trihydric alcohol (containing three -OH hydroxyl groups) that can combine with up to three fatty acids to form monoglycerides, diglycerides, and triglycerides. Fatty acids may combine with any of the three hydroxyl groups to create a wide diversity of compounds. Monoglycerides, diglycerides, and triglycerides are classified as esters which are compounds created by the reaction between acids and alcohols that release water (H2O) as a by-product. Monoglycerides A monoglyceride, or monoacylglycerol (MAG), has only one fatty acid radical per molecule of glycerol. The fatty acid may be attached to carbon 1 or 2 of the glycerol molecule. HO-CH2 HO-CH R-COO-CH2 HO-CH2 HO-CH HO-CH2 H2O (water) + R-COOH + = 1-monoglyceride HO-CH2 R-COO-CH HO-CH2 2-monoglyceride
Diglycerides A diglyceride, or diacylglycerol (DAG), has two fatty acid radicals and exists in the 1,2 form and the 1,3 form depending on how the fatty acids are attached to the glycerol molecule. HO-CH2 R-COO-CH R-COO-CH2 HO-CH2 HO-CH HO-CH2 + 2H2O (water) = 2R-COOH + 1,2-diglyceride R-COO-CH2 OH-CH R-COO-CH2 1,3-diglyceride
Triglycerides R-COO-CH2 R-COO-CH R-COO-CH2 HO-CH2 HO-CH HO-CH2 + 3H2O (water) + = 3R-COOH Triglyceride Simple Triglyceride: When all Fatty acids are same(R) Complex/Compound Triglyceride: When Fatty acids are different(R1, R2, R3)
Fatty acids A fatty acid is a carboxylic acid with a long aliphatic tail (chain), which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as "free" fatty acids. • Fatty acid chains differ by length, often categorized as short to very long. • Short-chain fatty acids (SCFA) are fatty acids with aliphatic tails of fewer than six carbons (i.e. butyric acid). • Medium-chain fatty acids (MCFA) are fatty acids with aliphatic tails of 6–12 carbons, which can form medium-chain triglycerides. • Long-chain fatty acids (LCFA) are fatty acids with aliphatic tails 13 to 21 carbons. • Very long chain fatty acids (VLCFA) are fatty acids with aliphatic tails longer than 22 carbons. Milk fat consists of higher portion of short chain fatty acids.
Fatty acids that have carbon–carbon double bonds are known as unsaturated. Fatty acids without double bonds are known as saturated. They differ in length as well.
Unsaturated fatty Unsaturated fatty acids have one or more double bonds between carbon atoms. (Pairs of carbon atoms connected by double bonds can be saturated by adding hydrogen atoms to them, converting the double bonds to single bonds. Therefore, the double bonds are called unsaturated.) The two carbon atoms in the chain that are bound next to either side of the double bond can occur in a cis or trans configuration.
A cis configuration means that adjacent hydrogen atoms are on the same side of the double bond. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. A trans configuration, by contrast, means that the next two hydrogen atoms are bound to opposite sides of the double bond. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.
In most naturally occurring unsaturated fatty acids, each double bond has three n carbon atoms after it, for some n, and all are cis bonds. Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation). Nomenclature Fat Globule membrane (FGM) More than 95% of the total milk lipid is in the form of a globule ranging in size from 0.1 to 15 um in diameter. These liquid fat droplets are covered by a thin membrane, 8 to 10 nm in thickness, whose properties are completely different from both milkfat and plasma. The native fat globule membrane (FGM) is comprised of apical plasma membrane of the secretory cell which continually envelopes the lipid droplets as they pass into the lumen. The major components of the native FGM, therefore, is protein and phospholipids. The phospholipids are involved in the oxidation of milk.
Recombined membranes are very different than native FGM. Processing steps such as homogenization, decreases the average diameter of fat globule and significantly increases the surface area. Some of the native FGM will remain adsorbed but there is no longer enough of it to cover all of the newly created surface area. Immediately after disruption of the fat globule, the surface tension raises to a high level of 15 mN/m and amphiphilic molecules in the plasma quickly adsorb to the lipid droplet to lower this value. The adsorbed layers consist mainly of serum proteins and casein micelles.
Lactose C12H22O11 white solid substance Lactose is a disaccharide sugar derived from galactose and glucose that is found in milk. Lactose makes up around 0~8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from lac or lactis, the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11.
Lactose is a disaccharide derived from the condensation of galactose and glucose, which form a β-1→4 glycosidic linkage. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose can be in either the α-pyranose form or the β-pyranose form, whereas the galactose can only have the β-pyranose form: hence α-lactose and β-lactose refer to anomeric form of the glucopyranose ring alone. Lactose is hydrolysed to glucose and galactose, isomerised in alkaline solution to lactulose, and catalytically hydrogenated to the corresponding polyhydric alcohol, lactitol.
Lactose monohydrate crystals have a characteristic tomahawk shape that can be observed with a light microscope. Infant mammals nurse on their mothers to drink milk, which is rich in lactose. The intestinal villi secrete the enzyme called lactase (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simple sugarsglucose and galactose, which can be absorbed. Since lactose occurs mostly in milk, in most mammals, the production of lactase gradually decreases with maturity due to a lack of constant consumption. In people who are lactose intolerant, lactose is not broken down and provides food for gas-producing gut flora, which can lead to diarrhea, bloating, flatulence, and other gastrointestinal symptoms.
Lactose Physical Properties Lactose is dissolved in the serum (whey) phase of fluid milk. Lactose dissolved in solution is found in 2 forms, called the α-anomer and ß-anomer, that can convert back and forth between each other. The solubility of the 2 anomers is temperature dependent and therefore the equilibrium concentration of the 2 forms will be different at different temperatures. At room temperature (70°F, 20°C) the equilibrium ratio is approximately 37% α- and 63% ß-lactose. At temperatures above 200°F (93.5°C) the ß-anomer is less soluble so there is a higher ratio of α- to ß-lactose. The type of anomer present does not affect the nutritional properties of lactose.
Lactose crystallization occurs when the concentration of lactose exceeds its solubility. The physical properties of lactose crystals are dependent on the crystal type and can greatly influence their use in foods. Temperature affects the equilibrium ratio of the α- and ß-lactose anomers, as described above. Lactose crystals formed at temperatures below 70°F (20°C) are mainly α-lactose crystals. The α-monohydrate lactose crystals are very hard and form, for example, when ice cream goes through numerous warming and freezing cycles. This results in an undesirable gritty, sandy texture in the ice cream. Gums are often used in ice cream to inhibit lactose crystallization. The crystal form of ß-lactose is sweeter and more soluble than the α-monohydrate lactose and may be preferred in some bakery applications. When a lactose solution is rapidly dried it does not have time to crystallize and forms a type of glass. Lactose glass exists in milk powders and causes clumping. The clumping is desirable because it results in a milk powder that dissolves instantly in water.
Influence of Heat Treatments on Lactose Properties The normal pasteurization conditions used for fluid milk have no significant effect on lactose. The higher temperatures used for ultra high temperature (UHT) pasteurization of extended shelf life products and spray drying can cause browning and isomerization reactions, which may affect product quality and nutritional properties. The browning reaction, called the Maillard reaction, occurs between the lactose and protein in milk and produces undesirable flavors and color, and decreases the available content of the amino acid lysine in milk protein. The isomerization reaction is a molecular rearrangement of lactose to lactulose. Lactulose is produced for use by the pharmaceutical industry in pill production. When lactose is hydrolyzed by ß -D-galactosidase (lactase), an enzyme that splits these monosaccharides, the result is increased sweetness, and depressed freezing point. In addition to lactose, fresh milk contains other carbohydrates in small amounts, including glucose, galactose, and oligosaccharides.
Minerals in Milk Minerals have many roles in the body including enzyme functions, bone formation, water balance maintenance, and oxygen transport. Calcium Milk is a source of calcium, which is essential for the healthy growth and maintenance of teeth and bones and is a vital function in blood clotting and muscle contraction. Phosphorus It is the second most abundant mineral in the body and plays a vital role in calcium and protein metabolism. Phosphorus is also essential for healthy bones and teeth as well as cell membrane structure, tissue growth and regulation of pH levels in the body. Iodine Iodine forms part of the hormones thyroxine and triiodothyronine. These hormones are produced in the thyroid, a gland in the neck and regulate the body's rate of metabolism (how quickly the body burns energy and the rate of growth).
Magnesium Magnesium is abundant in bone and in all cells in the body. Magnesium is essential for skeletal development, protein synthesis, muscle contraction and nerve function. Zinc Zinc is a constituent of many enzymes in the body; its role is to fight infections, growth development, for sexual development, wound healing and for our sense of taste. Potassium Potassium is mainly present in the fluid of the cells in the body and is important for fluid balance, muscle contraction, nerve conduction as well as for the correct functioning of the heart. Some minerals, such as Zn, Mg, Fe, Cu, Mn, and Mo, are required by enzymes as cofactors.
Other minerals Sodium, selenium and iron are also found in milk in low levels. • Fe is low in milk of many species relative to the needs of the neonate. Fe is essential for the neonate as part of hemoglobin. In many species the neonate is born with some liver stores of Fe; however, the piglet does not and needs an Fe supplement (injection) soon after birth. Fe in milk is bound to lactoferrin, transferrin, xanthineoxidase, and some to caseins. • Zn in cow's milk is mostly bound to casein, but some is bound to lactoferrin. • Cu is bound to the caseins, to ß-lactoglobulin, to lactoferrin, and some to the milk fat membranes. • Mo is bound to xanthineoxidase, an enzyme associated with the cell membrane and on the inner surface of the milk fat globule membranes. • Mn is associated with the milk fat membranes. • Co is an essential part of vitamin B12. Minerals contribute to the buffering capacity of milk, the maintenance of milk pH, the ionic strength of milk, and milk's osmotic pressure.
Vitamins in Milk Vitamins have many roles in the body, including metabolism, co-factors, oxygen transport and antioxidants. They help the body to use carbohydrates, protein, and fat. Milk contains the water soluble vitamins thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), vitamin B6 (pyridoxine), vitamin B12 (cobalamin), vitamin C, and folate. Milk is a good source of thiamin, riboflavin and vitamin B12 . Milk contains small amounts of niacin, pantothenic acid, vitamin B6, vitamin C, and folate and is not considered a major source of these vitamins in the diet. Milk contains the fat soluble vitamins A, D, E, and K. The content level of fat soluble vitamins in dairy products depends on the fat content of the product. Reduced fat (2% fat), lowfat (1% fat), and skim milk must be fortified with vitamin A to be nutritionally equivalent to whole milk. Fortification of all milk with vitamin D is voluntary. Milk contains small amounts of vitamins E and K and is not considered a major source of these vitamins in the diet.
