CHEMISTRY OF MILK

Module 2. Structure of milk

Lesson 3
STRUCTURAL ELEMENTS OF MILK: FAT GLOBULES, CASEIN MICELLES, GLOBULAR PROTEINS, LIPOPROTEINS PARTICLES AND THEIR PROPERTIES AND GRADING OF MILK

3.1 Introduction

The multi cellular organisms organize the cells into tissues and further into specific organs with specific functions. Consequently the secretions of these cells will have specific composition with the arrangement of these components in a specific manner giving unique physical structure to it. Depending up on the requirements , the structure of milk is either maintained or being disturbed in order to prepare certain products. As such a knowledge of these elements will give better understanding of the structure of the milk.

3.2 Structure of Milk

Milk being a secretion of the epithelial cells of the mammary gland has specific interactive forces working between its components which help in maintaining the integrity of the whole system under different conditions and thereby maintaining its specific structure. The physical structure i.e., the arrangement of the components in space must also be known. The interactive forces between the components are important; they determine the integrity of the whole system under various conditions. The main structural elements are shown schematically in Figure 3.1. They are microscopicor sub-microscopic in scale. The structure of milk is very has simple and extensive studies are being conducted on this aspect. Since milk constituents exist in different physical state influencing milk properties in addition to the chemical nature of the constituents.

Fig. 3.1 Milk Viewed at Different Magnifications

(Source: Mulder and Walstra, The milk fat globule, 1974)

3.3 Structural Elements

The properties of various structural elements of milk are shown in the Table 3.1.

Table 3.1 Properties of the main structural elements of milk

3.3.1 Milk fat

In milk, fat exists in the form of fat globules. The unique feature of these fat globules is that these fat globules are surrounded with a membrane which is derived from the apicalmembrane of the mammary secretory cell. Milk minus fat globules is called “Milk plasma”.

3.3.2 Proteins

Casein is the major protein of bovine milk and exists mainly as micellar form, where as in human milk major proteins are whey proteins. The Casein micelle consists of water, casein, salts and some minor components including lipase and proteinase. Casein micelles are built of smaller particles called sub-units or sub-micelles. Milk plasma free from caseinmicelles is known as milk serum. The liquid that is obtained on clotting of milk either with rennet or by acidification to pH 4.6 (isoelectric point of casein) is known as whey. This whey differs in its composition from that of serumin having some of the polypeptides cleaved from casein by the action of rennet.The proteins present in whey are termed as whey proteins of milk. The whey proteins are mainly globular proteins. Lipoprotein particles sometimes called microsomes vary in their quantity, composition, and shape. They consists remnants of cell membranes, microvilli, etc.

3.3.3 Somatic cells

The somatic cells consist of mainly leukocytes of various types is considered as extraneous particles although they are always present. They are about 10 μ m in diameter; number is about 100,000 permilliliter and accounts for about 0.005% of the volume of milk. They contain all cytoplasmic components notable nucleic acids and enzymes. They are rich in catalase. In mastitic milk the cell counts are much higher

3.3.4 Physical structure of milk

If we try to see a drop of milk with only 5 times magnification it appears as uniform liquid, however, it cannot be homogeneous since it is turbid in nature.With an increase in the magnification by another 100 times one can observes pherical droplets of fat floating in plasma. A further increase of magnification by another 100 times the proteinaceous particles (casein micelles)become visible. Fat, casein micelles and other proteins form the major structural elements for milk, and these elements are microscopic or submicroscopic in their size. Although fat is considered to be present in the form of an emulsion but it is not just a simple emulsion, because of the fact that the fat globules are being covered with the fat globule membrane which is derived during its biosynthesis. In addition to the fat globules some of the fat in the milk is found outside the globules while the membrane material includes several enzymes and proteins and polar lipids. As such fat and fat globules are not identical.

3.3.5 Milk plasma

Milk gets its structure with the structural elements. The main structural element being the fat, all that portion of milk which is free from the fat is considered as milk plasma. It is often a practice to use cream separators for getting the skim milk, but all the fat present in milk will not be separated as such we cannot consider milk plasma to be synonymous to skim milk. Here we consider fat free -milk to be plasma but not skim milk. Although the words serum and plasma are used for fractionation of blood here in milk these words are denoting the physical fractions of milk.

3.3.6 Milk serum

All that portion of milk excluding the casein micelles is referred as milk serum. By using either rennet or dilute acids the casein micelles could be separated from the plasma to obtain milk serum. The globular proteins consists mainly the whey proteins. They also bind some cations and a little water. Lipoprotein particles present in milk are also referred as the milk microsomes.

3.3.7 Casein micelles

Water,casein and salts along with some minor components such as lipases and proteinases constitute this structural element of milk.Casein micelles do es not completely account for the casein content since some of the casein is also present in a solution form. As casein binds cations especially the Ca ^{- -} and Mg ^{+ +} ions at the normal pH of milk i.e ~6.6, it is referred as caseinate. Amorphous calcium phosphate and small amounts of citrates are the constituents associated with the casein micelle. Calcium caseinate phosphatecomplex is the term applicable to describe the casein micelles. Phosphatebeing a part of the colloidal particles, the term colloidal phosphate is also used.

3.3.8 Whey proteins

The non micellar proteins constitute the whey proteins, but it is often the globular proteins present as such are considered to be the whey proteins

3.4 Test for Grading of Milk

The milk procured at the dairy plants is not only sold as fluid milk but also used for the preparation of various dairy products. As such it is necessary to grade the milk procured at the dairy plants by conducting various tests. The tests that are commonly conducted for grading of raw milk are:

Sensory Tests

• Organoleptic tests
  

B) Physical tests

• Sediment test
• Clot on boiling test
• pH

C) Chemical Test

• Alcohol test
• Alizarin-alcohol test
  

3.4.1 Organoleptic tests

The first test that is expected to be conducted in all the dairies as soon as milk is received at the milk procurement section is organoleptic test. This test is primarily based on smell (odour) and appearance. An experienced person with an ability to detect the various abnormalities in the milk flavours should conduct this test. This will help in grading the good and bad quality milk. During this examination it is possible to detect the abnormalodours such as garlic, onion, cabbage etc., and sourness of the milk. Similarly,the abnormalities in the colour could be detected in these tests, apart from presence of various extraneous objects in the milk, such as hair, fibres, dung cakes etc.

3.4.2 Sediment test

This test reveals the extent to which visible insoluble matter has gained entry into milk. The sediment test presents a simple rapid and a quantitative measure of indicating the cleanliness of milk with respect to visible dirt. However it is necessary to give the interpretation judiciously since absence of visible dirt never ensures good quality milk.

3.4.3 Clot-On-Boiling (COB) test

This is a quick test to determine developed acidity and the suitability of milk for heat processing. Milk giving a positive test has acidity generally above0.17% lactic acid and is not suitable for distribution as liquid milk or for heat processing.

3.4.4 pH

The hydrogen ion concentration or the pH value is a measure of true acidity of milk. The pH of normal cow milk ranges from 6.6 to 6.8. Due to the development of acidity by the conversion of lactose into lactic acid this value would be reduced. Milk obtained form the animals suffering from mastitis will have pH above 7.0. The pH test is mainly used for detecting of abnormal mastitis milk.

3.4.5 Alcohol test

The alcohol test is used for rapid assessment of stability of milk to processing particularly for condensing and sterilization. The alcohol test is used as an indicator of disturbed salt balance in milk but not as an index for developed acidity. This test helps in detecting abnormal milk such as colostrums, late lactation milk, and mastitis milk.

3.4.6 Alizarin-alcohol test

This test is similar to the alcohol test and the addition of alizarin helps in detecting the acidity by judging of colour as well . The interpretation of the results is based on the size of the clot and the colour developed which in turn helps in grading of the milk.