INTRODUCTION:

Milk contains mainly two types of protein: whey and casein.

Both of which are high quality proteins according to science-based rating
scales. Both contain all the essential amino acids in amounts sufficient to
support the multiple roles of protein in the human body. So milk is
considered an ideal food. It is also a rich source of fats. It also contains
calcium and phosphorus in sufficient quantities. Therefore, milk is
considered to be an important diet for the growing children and pregnant
and lactating women.

According to the protein digestibility corrected amino acid

score (PDCAAS), a method of measuring protein quality based on
availability of essential amino acids as well as digestibility, milk provides
higher quality protein than meat, soy or wheat. The milk of cow, buffalo
and goat are commonly used for human consumption. Because of its high
quality, cow’s milk protein is used as a standard reference protein to
evaluate the nutritive value of food proteins.
USE AND COMPOSITION OF MILK:

Dairy products have been a part of the human diet for

thousands of years, with evidence of cheese being made as long as 7,500
years ago.

The Swedish consumption of cheese and fermented milk

products (e.g. yoghurt, sour milk ‘filmjolk’) is showing a positive trend,
whereas the consumption of drinking milk. Properties and yield of dairy
products are influenced to a great extent by the amounts and relative
proportions of each of the milk constituents. Consequently, as an
increasing part of the milk produced is utilized for processed dairy
products (Swedish Dairy Association, 2008) milk constituents such as
protein and fat have achieved higher economic significance.

The general process of coagulating liquid milk into dairy

products such as cheese and yoghurt/filmjolk is based on the formation of
an aggregated protein network, which mainly consists of a certain group of
proteins known as caseins. In this water, fat, and other milk constituents
are entrapped. The biochemical processes differ between cheese and
fermented milk products, where cheese making involves the separation of
casein from whey, whereas in fermented milk products the whole milk is
included in the final product.
MILK IS MILK IS MILK, ALL THE SAME?

During the past decades the focus of milk production has been
kg’s of milk protein, whereas the protein composition, i.e. relative
proportions of the different proteins, has not been addressed; a comparison
of the Swedish dairy milk 1970 and 1996 (Lindmark-Mansson et al,.
2003), showed that although there was non difference in total protein
concentration, the proportion of casein in total protein was significantly
decreased 1996. A direct effect of a decreased casein level is that a larger
quantity of milk is required to make a set amount of cheese.

PROTEIN COMPOSITION OF MILK:

Milk is a highly diverse fluid consisting of a vast number of

substances, the main ones being water, lactose, fat, protein, organic acids,
and minerals. Milk proteins are traditionally defined by their solubility at
pH 4.6. the precipitate formed when adjusting milk to pH 4.6 is casein,
whereas the protein remaining in solution is whey protein, or serum
protein. Bovine milk generally contains about 3.5%protein, of which
approximately 80% are caseins and 20% are whey proteins.
CASEINS:

Casein is a family of related phosphoproteins. These proteins

are commonly found in mammalian milk. The most common form of
casein is sodium caseinate. As a food source, casein supplies ammino
acids, carbohydrates, and two essential elements, calcium and phosphorus.

The first method to separate casein was described by Berzelius

in 1814. The three casein components found were called alpha-CN, beta-
CN, and gamma-CN in order of decreasing electrophoretic mobility.
Consequently, casein consists of alphas1-CN, alphas2-CN, beta-CN and k-
CN in approximate proportions 4: I:4: I. synthesized in the mammary
gland, post-translational modifications such as phosphorylation, cause
further diversity within the casein group.
 In milk about 95% of the caseins are aggregated in colloidal
structures, casein micelles, whose major function is to fluidize the casein
molecules and solubilize calcium and phosphate.

WHEY PROTEINS:

Whey protein is the, mixture of proteins isolated from whey.

The protein in milk is 20% whey and 80% casein. This protein is a
mixture of beta-lactoglobulin (65%), alpha-lactalbumin (25%), and bovine
serum albumin (8%) and immunoglobulins. Although whey proteins are
responsible for some milk allergies, the major allergies in milk are the
caseins.

Whey proteins, or serum proteins, share few common

characteristics other than being soluble at pH4.6. the three main proteins
are beta-LG, alpha-LA and blood serum albumin (BSA), representing
approximately 50, 20 and 10% of total whey proteins, respectively. The
remaining part comprises immunoglobulins (Ig) and trace amounts of
several other protein, including enzymes, most whey proteins are globular
with organized secondary and tertiary structures, which in contrast to the
caseins make them sensitive to heat denaturation at temperatures above
60C.
COAGULATION OF MILK:

The ability of casein micelles to stay in solution at natural

milk pH (6.7) relies on the net negative charge and hydrophilic character
of the C-terminal end of the micelle surface. There are two approaches to
induce micelle aggregation; by enzymatic action (cheese) or by
acidification (fermented milk products). The outcome of these reactions is
to a large extent determined by amounts and proportions of the various
components in milk, with the protein composition contributing
significantly in this regard.

PROTEIN TEST:

1. Xanthoproteic test
2. Millions test

There are several more test for finding protein i.e. casein
(cognizable) and whey (non-cognizable).
TEST 1:

1) XANTHOPROTEIC TEST:

Proteins on treatment with nitric acid give a yellow or orange

colour. Concentrated nitric acid is used for nitration. On the treatment of
nitric acid, proteins give yellow precipitate which turns to orange colour
on treatment with alkali.

Note: The appearance of a yellow colour solution confirms the presence of

proteins.

2) MILLION’S TEST:

Phenolic group of tyrosine of proteins reacts with mercuric

sulfate in the presence of sodium nitrite and sulfuric acid to give red color.
Millon’s test is given by proteins containing phenolic amino acids. Gelatin
does not give this test. First, a white precipitate is formed when proteins
are treated with millions reagent and then turns to brick-red color on
boiling, this confirms the presence of proteins.

Note: The appearance of brick red color solution confirms the presence of
proteins.
MATERIALS REQUIRED:

1) Stirrer
2) Dropper
3) Bunsen burner
4) Test tube holder
5) Clean and dried test tubes
6) Food sample – Milk, Apple, Paneer, Butter, Cheese, etc.
7) Sulfuric acid
8) Million’ reagent

PROCEDURE:

TEST 1:
1. Take a cleaned and dried test tube.
2. Add the food samples of your choice into the test tubes.
3. Add a few drops of concentrated sulfuric acid and shake the test
tube.
4. Heat the test tube gently on a Bunsen burner.
5. If there is a formation of yellow precipitate, then the presence of
protein is confirmed.
OBSERVATION:
The appearance of yellow colour confirms the presence of
proteins.

TEST 2:
1. Take a cleaned and dried test tube.
2. Add the food samples of your choice into the test tubes.
3. Add 2-3 drops of Millon’s reagent and shake well.
4. Observe the change.
5. If there is the formation of white precipitate or if the sample
changes to brick red on heating, then the presence of protein is
confirmed.
OBSERVATION:
The appearance of white precipitate confirms the presence of proteins.
REFERENCE:
1) www.sciencedirect.com
parameters for noncoagulating milk, milk coagulating
properties (Elin Hallen).
2) www.journalofdairyscience.com
published by Elsevier Inc. and Fass Inc.
 INDEX

1) INTRODUTION

2) USE AND COMPOSITION OF MILK

3) MILK IS MILK IS MILK, ALL THE SAME?

4) PROTEIN COMPOSTION OF MILK

5) CASEINS

6) WHEY PROTEIN

7) COAGULATION OF MILK

8) PROTEIN TEST

8.1) TEST 1 – XANTHOPROTEIC TEST

8.2) TEST 2 – MILLION’S TEST

9) MATERIALS REQUIRED

10) OBSERVATION

11) REFERENCE