In this article we will discuss about the physico-chemical properties of milk and milk constituents.
Physico-Chemical Properties of Milk:
1. Physical State of Milk:
Water is the continuous phase in which other constituents are either dissolved or suspended. Lactose and a portion of the mineral salts are found in solution, proteins and the remainder of the minerals in colloidal suspension and fat as an emulsion.
2. Acidity and pH of Milk:
(a) Acidity:
Freshly-drawn milk is amphoteric to litmus, i.e. it turns red litmus blue and blue litmus red. However, it shows a certain acidity as determined by titration with an alkali (sodium hydroxide) in the presence of an indicator (phenolphthalein). This acidity, also called Titratable Acidity (T.A.) as it is determined by titration, is known as ‘natural’ or ‘apparent’ acidity and is caused by the presence of casein, acid-phosphates, citrates, etc., in milk.
The natural acidity of individual milk varies considerably depending on species, breed, individuality, stage of lactation, physiological condition of the udder, etc., but the natural acidity of fresh, herd milk is much more uniform. The higher the solids- not-fat content in milk, the higher the natural acidity (N.A.) and vice versa.
The titrable acidity of cow milk varies on an average from 0.13 to 0.14 per cent and buffalo milk from 0.14 to 0.15 per cent. ‘Developed’ or ‘real’ acidity is due to lactic acid, formed as a result of bacterial action on lactose in the milk. Hence the titrable acidity of stored milk is equal to the sum of natural acidity and developed acidity. The titrable acidity is usually expressed as a ‘percentage of lactic acid’.
(b) pH:
The pH of normal, fresh, sweet milk usually varies from 6.4 to 6.6 for cow milk and 6.7 to 6.8 for buffalo milk. Higher pH values for fresh milk indicate udder infection (mastitis) and lower values, bacterial action.
Note:
The acidity and pH of fresh milk vary with:
(i) Species;
(ii) Breed;
(iii) Individuality;
(iv) Stage of lactation;
(v) Health of the animal, etc.
3. Density and Specific Gravity:
Whereas density of a substance is its mass (weight) per unit volume, specific gravity is the ratio of density of the substance to density of a standard substance (water). Since the density of a substance varies with temperature, it is necessary to specify the temperature when reporting densities or specific gravities. The specific gravity of a substance (when referred to water at 4°C) is numerically equal to the density of that substance in the metric system. The specific gravity of milk is usually expressed at 60°F (15.6°C).
The density or specific gravity of milk may be determined by either determining the weight of a known volume or the volume of a known weight. The weight of a known volume may be determined either with a pycnometer or with a hydrostatic balance; while the volume of a known weight is determined by using lactometers, the scale of which is calibrated not in terms of volume but as a function of either density or specific gravity. The common types of lactometers are Zeal, Quevenne, etc.
Milk is heavier than water. The average specific gravity ranges (at 60°F) from 1.028 to 1.030 for cow milk, 1.030 to 1.032 for buffalo milk and 1.035 to 1.037 for skim milk. The specific gravity of milk is influenced by the proportion of its constituents (i.e., composition), each of which has a different specific gravity approximately as follows- water—1.000; fat—0.93; protein—1.346; lactose—1.666; and salts—4.12 (solids-not-fat—1.616).
As milk fat is the lightest constituent, the more there is of it, the lower the specific gravity will be, and vice versa. However, although buffalo milk contains more fat than cow milk, its specific gravity is higher than the latter’s; this is because buffalo milk contains more solids-not-fat as well, which ultimately results in a higher specific gravity.
The specific gravity of milk is lowered by addition of water and cream, and increased by addition of skim milk or removal of fat.
The percentage of total solids or solids-not-fat in milk is calculated by using the following formula, vide IS: 1183, 1965 (Revised)-
% TS = 0.25D + 1.22F + 0.72
% SNF = 0.25D + 0.22F + 0.72
where, D = 1000(d – 1)
d = density of sample of milk at 20°C (68°F)
F = fat percentage of sample.
Note:
The specific gravity of milk should not be determined for at least one hour after it is drawn from the animal; else a lower-than-normal value will be obtained (due to the Recknagel phenomenon).
4. Freezing Point of Milk:
Milk freezes at temperatures slightly lower than water due to the presence of soluble constituents such as lactose, soluble salts, etc., which lower or depress the freezing point. The average freezing point depression of Indian cow milk may be taken as 0.547°C (31.02°F) and buffalo milk 0.549°C (31.01°F).
Most bulk milk samples have a freezing point depression of 0.530°C (31.05°F); a freezing point depression lower than this value indicates added water. Mastitis milk shows a normal freezing point. The freezing point test of milk is a highly sensitive one and even up to 3 per cent of watering can be detected.
While the freezing point of normal fresh milk is remarkably constant and employed mainly for detection of adulteration of milk with water, souring results in a lowering of the freezing point due to increase in the amount of soluble molecules. Hence the freezing point should be determined on unsoured samples for greatest accuracy.
Boiling and sterilization increase the value of freezing point depression, but pasteurization has no effect. The fat and protein contents of milk have no direct effect on the freezing point of milk.
The drawbacks of the freezing point test are:
(i) It does not detect the addition of skim milk or removal of fat from the milk sample; and
(ii) Watered milk, which has subsequently soured, may pass the test.
5. Colour of Milk:
The colour is a blend of the individual effects produced by:
(i) The colloidal casein particles and the dispersed fat globules, both of which scatter light, and
(ii) The carotene (to some extent xanthophyll) which imparts a yellowish tint.
Milk ranges in colour from yellowish creamy white (cow milk) to creamy white (buffalo milk). The intensity of the yellow colour of cow milk depends on various factors such as breed, feeds, size of fat globules, fat percentage of milk, etc. Certain breeds of cow impart a deeper yellow tint to their milk than others.
The greater the intake of green feed, the deeper yellow the colour of cow milk. The larger the fat globules and the higher the fat percentage, the greater the intensity of the yellow colour. Skim milk has a bluish, and whey a greenish yellow colour (which in milk is masked by the other constituents present).
Note:
(i) The colour of foods is an important aspect of their marketability. Colour has three aspects, viz., tint, intensity and uniformity. Variation in intensity is tolerated as it occurs in practice,
(ii) The colour of an opaque object is the colour it reflects; the colours of the visible spectrum are absorbed. Thus an object is yellow because more yellow light is reflected to the eye than any other colour. (A white object reflects all the colours of light that fall on it, while a black object absorbs them all).
6. Flavour:
This is composed of smell (odour) and taste. The flavour of milk is a blend of the sweet taste of lactose and salty taste of minerals, both of which are damped down by proteins. The phospholipids, fatty acids and fat of milk also contribute to the flavour.
Changes in the flavour of milk occur due to type of feed, season, stage of lactation, condition of udder, sanitation during milking and subsequent handling of milk during storage. The sulfhydryl compounds significantly contribute to the cooked flavour of heated milks.
Note:
A pronounced flavour of any kind is considered abnormal to milk.
The sources of abnormal flavours may be:
(i) Bacterial growth;
(ii) Feed;
(iii) Absorbed;
(iv) Chemical composition;
(v) Processing and handling;
(vi) Chemical changes;
(vii) Addition of foreign material.
Physico-Chemical Properties of Milk Constituents:
1. Major Milk Constituents:
(a) Water:
Constitutes the medium in which the other milk constituents are either dissolved or suspended. Most of it is ‘free’, and only a very small portion is in the ‘bound’ form, being firmly bound by milk proteins, phospholipids, etc.
(b) Milk Fat (Lipid):
The bulk of the fat in milk exists in the form of small globules, which average approximately 2 to 5 microns, in size (range 0.1 to 22 microns). This is an oil-in-water type emulsion. The surface of these fat globules is coated with an adsorbed layer of material commonly known as the fat globule, membrane.
This membrane contains phospholipids and proteins in the form of a complex, and stabilizes the fat emulsion. In other words, the membrane prevents the fat globules from coalescing but keeps separated from one another. The emulsion may, however, be broken by agitation (at low temperatures), heating, freezing, etc.
When milk is held undisturbed, the fat globules tend to rise to the surface to form a cream layer. The thickest cream layer is secured from milks which have a higher fat content and relatively large fat globules (such as buffalo’s milk when compared with cow’s milk).
Chemically, milk fat is composed of a number of glyceride-esters of fatty acids; on hydrolysis, milk fat furnishes a mixture of fatty acids and glycerol. (That milk fat is a mixture of true fats is established from the fact that it has no sharp melting point.) The fatty acids are saturated or unsaturated. Saturated fatty acids are relatively stable. On the other hand, the unsaturated ones play an important role in the physico-chemical properties of milk fat.
(c) Milk Proteins:
Proteins are among the most complex of organic substances. They are vital for living organisms as they constitute an indispensable part of the individual body cell. Proteins are composed of a large number of amino-acids, some ‘essential’ and others ‘non-essential’. The essential amino-acids are necessary in the diet for the formation of body proteins.
On hydrolysis, proteins furnish a mixture of amino-acids. The proteins of milk consist mainly of casein, β-lactoglobulin, α-lactalbumin, etc. Casein exists only in milk and is found in the form of a calcium caseinate-phosphate complex. It is present in the colloidal state. It forms more than 8 per cent of the total protein in milk. It may be precipitated by acid, rennet, alcohol, heat and concentration.
Casein itself is composed of α, β, ϒ fractions. The heterogeneous nature of α -casein can be observed through electrophoresis. α-casein is the component in casein micelle that is responsible for the stabilization of the micelle in milk.
Later studies have also revealed that α-casein is composed of at least two sub-fractions, viz., αs-casein precipitable by calcium-ion under certain conditions and also called ‘calcium-sensitive casein’; and K-casein, also called ‘calcium-insensitive casein’, not precipitable by calcium-ion. K-casein is the richest repository of carbohydrates as against other casein fractions. It is also the site for rennin action.
β -lactoglobulins and α -lactalbumin are also known as whey or serum-proteins. They are also present in the colloidal state and are easily coagulable by heat.
(d) Milk Sugar or Lactose:
This exists only in milk. It is in true solution in the milk serum. On crystallization from water, it forms hard gritty crystals. It is one-sixth as sweet as sucrose. Lactose is responsible, under certain conditions, for the defect known as ‘sandiness’ in ice cream and sweetened condensed milk.
Chemically, lactose is composed of one molecule each of glucose and galactose. Lactose occurs in two forms, α and β, both of which occur either as the hydrate or the anhydride. It is fermented by bacteria to yield lactic acid and other organic acids and is important both in the production of cultured milk products and in the spoilage of milk and milk products by souring.
(e) Mineral Matter or Ash:
The mineral matter or salts of milk, although present in small quantities, exert considerable influence on the physico-chemical properties and nutritive value of milk. The major salt constituents, i.e. those present in appreciable quantities, include potassium, sodium, magnesium, calcium, phosphate, citrate, chloride, sulphate and bicarbonate; the trace elements include all other minerals and salt compounds.
The mineral salts of milk are usually determined after ashing. Although milk is acidic, ash is distinctly basic. Part of the mineral salts occur in true solution, while a part are in the colloidal state.
2. Minor Milk Constituents:
(a) Phospholipids:
In milk, there are three types of phospholipids, viz., lecithin, cephalin and sphingomylin. Lecithin, which forms an important constituent of the fat globule membrane, contributes to the ‘richness’ of flavour of milk and other dairy products. It is highly sensitive to oxidative changes, giving rise to oxidized/metallic flavours. Phospholipids are excellent emulsifying agents, and no doubt serve to stabilize the milk fat emulsion.
(b) Cholesterol:
This appears to be present in true solution in the fat, as part of the fat globule membrane complex and in complex formation with protein in the non-fat portion of milk.
(c) Pigments:
These are- (i) fat soluble, such as carotene and xanthophyll, and (ii) water soluble, such as riboflavin. Carotene is the colouring matter of all green leaves, where it is masked by chlorophyll. Carotene (the pure substance of which has a reddish- brown colour) is fat soluble and responsible for the yellow colour of milk, cream, butter, ghee and other fat-rich dairy products. Besides contributing to the colour of cow milk, carotene acts as an anti-oxidant and also as a precursor of vitamin A. One molecule of β-carotene yields two molecules of vitamin A, while α- carotene yields only one.
Dairy animals differ in their capacity to transfer carotene from feeds to milk fat; this varies with species, breed and individuality. Cows in general, and some breeds in particular (such as Guernsey and Jersey), can transfer more carotene from their feed to the milk fat than buffaloes, who do not seem to possess this capacity. Hence buffalo milk is white in colour. (The carotenoid content of buffalo milk varies from 0.25 to 0.48/ug/g, while that of cow milk may be as high as 30/ug/g.)
Riboflavin, besides being a vitamin, is a greenish-yellow pigment which gives the characteristic colour to whey. (Earlier, the terms ‘lactochrome’ and ‘lactoflavin’ were used instead of riboflavin.)
(d) Enzymes:
These are ‘biological catalysts’ which can hasten or retard chemical changes without themselves participating in the reactions. The enzymes are protein-like, specific in their actions, and inactivated by heat; each enzyme has its own in- activation temperature.
The important milk enzymes and their specific actions are as follows:
(i) Analase (diastase)—starch splitting;
(ii) Lipase—fat splitting, leading to rancid flavour;
(iii) Phosphate—capable of splitting certain phosphoric acid esters (basis of phosphatase test for checking pasteurization efficiency);
(iv) Protease—protein splitting;
(v) Peroxidase and Catalase—decomposes hydrogen peroxide.
(e) Vitamins:
Although present in foods in very minute quantities, these are vital for the health and growth of living organisms. As of today, over 25 vitamins have been reported. Those found in milk are- fat-soluble vitamins A, D, E and K; and water-soluble vitamins of the ‘B Complex’ group (such as thiamine or B1, riboflavin or B2, pantothenic acid, niacin, pyridoxine or B6, biotin. B12, folic acid, etc.) and vitamin C (ascorbic acid). Absence of vitamins in the diet over prolonged periods causes ‘deficiency diseases’.