MILK: a Food Source for Humans (3)

MILK: a Food Source for Humans (3) - Whey Proteins: The supernatant proteins contain milk after precipitation at pH 4.6 collectively called whey proteins. These proteins are water-soluble casein circular and are susceptible to thermal denaturation. Native whey proteins are good gelling and whipping properties. Denaturation increases their ability to retain water. In principle, the lines are the SS-lactoglobulin, alpha-lactalbumin, bovine serum albumin (BSA) and immunoglobulin (Ig).

ß -Lactoglobulins: (MW - 18,000; 162 residues) This group, including eight genetic variants, comprises approximately half the total whey proteins. ß -Lactoglobulin has two internal disulfide bonds and one free thiol group. The conformation includes considerable secondary structure and exists naturally as a noncovalent linked dimer. At the isoelectric point (pH 3.5 to 5.2), the dimers are further associated to octamers but at pH below 3.4, they are dissociated to monomers.

alpha-Lactalbumins: (MW - 14,000; 123 residues) These proteins contain eight cysteine groups, all involved in internal disulfide bonds, and four tryptophan residues. alpha-Lactalbumin has a highly ordered secondary structure, and a compact, spherical tertiary structure. Thermal denaturation and pH <4.0 results in the release of bound calcium.

Enzymes

Enzymes are a group of proteins that have the ability to catalyze chemical reactions and the speed of such reactions. The action of enzymes is very specific. Milk contains both indigenous and exogenous enzymes. Exogenous enzymes mainly consist of heat-stable enzymes produced by psychrotrophic bacteria: lipases, and proteinases. There are many indigenous enzymes that have been isolated from milk. The most significant group are the hydrolases:

• lipoprotein lipase
• plasmin
• alkaline phosphatase 

Lipoprotein lipase (LPL): A lipase enzyme splits fats into glycerol and free fatty acids. This enzyme is found mainly in the plasma in association with casein micelles. The milkfat is protected from its action by the FGM. If the FGM has been damaged, or if certain cofactors (blood serum lipoproteins) are present, the LPL is able to attack the lipoproteins of the FGM. Lipolysis may be caused in this way. 

Plasmin: Plasmin is a proteolytic enzyme; it splits proteins. Plasmin attacks both ß -casein and alpha(s2)-casein. It is very heat stable and responsible for the development of bitterness in pasteurized milk and UHT processed milk. It may also play a role in the ripening and flavour development of certain cheeses, such as Swiss cheese. 

Alkaline phosphatase: Phosphatase enzymes are able to split specific phosporic acid esters into phosphoric acid and the related alcohols. Unlike most milk enzymes, it has a pH and temperature optima differing from physiological values; pH of 9.8. The enzyme is destroyed by minimum pasteurization temperatures, therefore, a phosphatase test can be done to ensure proper pasteurization. 

Lactose 

Lactose is a disaccharide (2 sugars) made up of glucose and galactose (which are both monosaccharides). Because of the anomeric carbon on the right side of the structure below, lactose can exist as two isomers, alpha, as shown, or beta, in which the hydroxyl on the anomeric carbon would point up on the ring structure shown below.

It comprises 4.8 to 5.2% of milk, 52% of milk SNF, and 70% of whey solids. It is not as sweet as sucrose. When lactose is hydrolyzed by ß-D-galactosidase (lactase), an enzyme that splits these monosaccharides, the result is increased sweetness, and depressed freezing point.

One of its most important functions is its utilization as a fermentation substrate. Lactic acid bacteria produce lactic acid from lactose, which is the beginning of many fermented dairy products. Because of their ability to metabolize lactose, they have a competitive advantage over many pathogenic and spoilage organisms.

Some people suffer from lactose intolerance; they lack the lactase enzyme, hence they cannot digest lactose, or dairy products containing lactose. Crystallization of lactose occurs in an alpha form which commonly takes a tomahawk shape. This results in the defect called sandiness. Lactose is relatively insoluble which is a problem in many dairy products, ice cream, sweetened condensed milk. In addition to lactose, fresh milk contains other carbohydrates in small amounts, including glucose, galactose, and oligosaccharides. 

Vitamins 

Vitamins are organic substances essential for many life processes. Milk includes fat soluble vitamins A , D, E, and K. Vitamin A is derived from retinol and ß -carotene. Because milk is an important source of dietary vitamin A, fat reduced products which have lost vitamin A with the fat, are required to supplement the product with vitamin A.

Milk is also an important source of dietary water soluble vitamins: 

• B1 - thiamine
• B2 - riboflavin
• B6 - pyridoxine
• B12 - cyanocobalamin
• niacin
• pantothenic acid 

There is also a small amount of vitamin C (ascorbic acid) present in raw milk but it is an insignificant amount relative to human needs and is quite heat-labile: about 20% is destroyed by pasteurization. 

The vitamin content of fresh milk is given below: 

Minerals 

All 22 minerals considered to be essential to the human diet are present in milk. These include three families of salts:

1. Sodium (Na), Potassium (K) and Chloride (Cl):These free ions are negatively correlated to lactose to maintain osmotic equilibrium of milk with blood. 
2. Calcium (Ca), Magnesium (Mg), Inorganic Phosphorous (P(i)), and Citrate: This group consists of 2/3 of the Ca, 1/3 of the Mg, 1/2 of the P(i), and less than 1/10 of the citrate in colloidal (nondiffusible) form and present in the casein micelle.
3. Diffusible salts of Ca, Mg, citrate, and phosphate: These salts are very pH dependent and contribute to the overall acid-base equilibrium of milk. 

The mineral content of fresh milk is given below: 

Physical Properties 

Density 

The density of milk and milk products is used for the following: 

• to convert volume into mass and vice versa
• to estimate the solids content
• to calculate other physical properties (e.g. kinematic viscosity) 

Density, the mass of a certain quantity of material divided by its volume, is dependant on the following: 

• temperature at the time of measurement
• temperature history of the material
• composition of the material (especially the fat content)
• inclusion of air (a complication with more viscous products) 

With all of this in mind, the density of milk varies within the range of 1027 to 1033 kg m(-3) at 20° C. 

The following table gives the density of various fluid dairy products as a function of fat and solids-not-fat (SNF) composition:

Viscosity 

Viscosity of milk and milk products is important in determining the following: 

• the rate of creaming
• rates of mass and heat transfer
• the flow conditions in dairy processes 

Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in which the viscosity is independent of the rate of shear. 

The viscosity of these products depends on the following: 

• Temperature:
• cooler temperatures increase viscosity due to the increased voluminosity of casein micelles
• temperatures above 65° C increase viscosity due to the denaturation of whey proteins

• pH: an increase or decrease in pH of milk also causes an increase in casein micelle voluminosity 

Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependant on the shear rate. Agitation may cause partial coalescence of the fat globules (partial churning) which increases viscocity. Fat globules that have under gone cold agglutination, may be dispersed due to agitation, causing a decrease in viscosity. 

Freezing Point 

Freezing point depression is a colligative property which is determined by the molarity of solutes rather than by the percentage by weight or volume. In the dairy industry, freezing point of milk is mainly used to determine added water but it can also been used to determine lactose content in milk, estimate whey powder contents in skim milk powder, and to determine water activity of cheese. The freezing point of milk is usually in the range of -0.512 to -0.550° C with an average of about -0.522° C.

Correct interpretation of freezing point data with respect to added water depends on a good understanding of the factors affecting freezing point depression. With respect to interpretation of freezing points for added water determination, the most significant variables are the nutritional status of the herd and the access to water. Under feeding causes increased freezing points. Large temporary increases in freezing point occur after consumption of large amounts of water because milk is iso-osmotic with blood. The primary sources of non-intentional added water in milk are residual rinse water and condensation in the milking system. 

Acid-Base Equilibria 

Both titratable acidity and pH are used to measure milk acidity. The pH of milk at 25° C normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acidity of herd milks is 13 to 20 mmol/L. Because of the large inherent variation, the measure of titratable acidity has little practical value except to measure changes in acidity (eg., during lactic fermentation) and even for this purpose, pH is a better measurement.

There are many components in milk which provide a buffering action. The major buffering groups of milk are caseins and phosphate. 

Optical Properties 

Optical properties provide the basis for a rapid and indirect methods of analysis as the analysis of close infrared absorption or scattering of light. The optical properties also determine the appearance of dairy products. Scattering of light by fat globules and casein micelles causes milk to appear turbid and opaque. Light scattering occurs when the wavelength of light is close to the same order of magnitude of the particles. Thus, smaller particles scatter light wavelengths. Skim milk appears slightly blue because casein micelles scatter short wavelengths of visible light (blue) of a red. Carotenoid precursor of vitamin A, beta-carotene content in milk fat, is responsible for the color "cream" milk. Riboflavin gives it a greenish color of the serum.

Refractive index (RI) is normally determined at 20 ° C with the spectrum of the sodium D line. The refractive index of milk is 1.3440 to 1.3485 and can be used to estimate the total solids.

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