The process of making a stable emulsion of milk fat and milk serum by mechanical treatment and rendering the mixture homogeneous is homogenisation. This is achieved by passing warm milk or cream through a small aperture under high pressure and velocity. Milk and cream have fat globules that vary from 0.1 to 20 µm in diameter. The fat globules have a tendency to gather into clumps and rise due to their lower density than skimmed milk. Homogenised milk fat globules size is 2 µm. The decrease in the size of the fat globules increases their number and surface area. A film of adsorbed protein or lipoprotein immediately surrounds each of the new globules, acting as an emulsifier and prevents them from reuniting. The newly formed fat droplets are no longer coated with the original membrane material. This brings about the stabilisation of milk emulsion and thus prevents the rising of the cream.
Homogenised milk has a creamier structure, bland flavour and whiter appearance. It has a greater Whitening power when added to coffee and tea. A soft curd is formed when coagulated and· is easily digested. In the manufacture of evaporated milk (condensed milk) and ice-cream homogenisation reduces the change of separation of fat resulting in a smoother texture of the finished product. Homogenisation accelerates the action of lipase and rancidity of fat takes place. Homogenisation is done before pasteurisation.
Freezing: When milk or cream is frozen at a relatively slow rate, the film of protein that acts as an emulsifying agent around the fat globules is weakened and ruptured. As a result, the fat globules tend to coalesce. The dispersion of protein and calcium phosphate is also disturbed by freezing. Both constituents tend to settle out on thawing and standing, thus reducing the whiteness of milk.
Sugar- protein mixtures: Non-enzymatic browning of Maillard type occurs in evaporated milk. Maillard (1912) was the first to describe the development of brown colour in mixtures containing amino acids and reducing sugars. The maximum effect is with lysine followed by tryptophan and arginine. Glucose reacts more strongly with lysine than lactose or fructose, while lactose reacts most readily with tryptophan.
The steps involved in the Maillard reaction between reducing sugars and amino acids or proteins are as follows:
Condensation of the aldehyde or ketone group with the amino group.
Sucrose does not react by itself as it has no reacting group but the hydrolytic products of sucrose, glucose and fructose react with amino acids. If proline is involved in the reaction the product may become bitter.
The rate of browning increases rapidly with a rise in temperature. The colour development increases with increasing pH above 6.8 for glucose – α amino acid solutions and above pH 6.0 with glucose – ω-amino acids. The involved amino acids are no longer biologically available.
The reaction rate being decreased at low is very high water levels although the dry materials will react. The optimal moisture levels for the reaction range from 10 to 15 % in a dehydrated product. The reaction is catalysed by the presence of metals such as iron and copper and by phosphate ions.Concentrated milk products such as evaporated milk and sweetened condensed milk contain substantial amounts of both protein and the sugar lactose and develop some brown colour on heating. This reaction may also occur in dried milk stored for long periods.
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Acid: When milk is heated, its acidity decreases at first owing to the release of dissolved carbon dioxide and then increases because hydrogen ions are liberated when calcium and phosphate forms insoluble compounds. A balance between these opposing factors prevents large changes of pH during heating.
Minerals: Iodine is a volatile substance and when heated tends to be lost from milk. The dispersion of calcium phosphate in milk is decreased by heating and part of it precipitated. Some of it collects on the bottom of the pan with coagulum of albumin and some is probably entangled in the scum on the top surface of the milk.
Colour, flavour and digestibility: Neither the flavour nor the general appearance of milk is appreciably changed by pasteurisation. Cooked flavour of boiled milk is due to loss of dissolved gases like carbon dioxide and oxygen and the changes that occur in protein. Digestibility may be slightly improved. Heated milk tends to form smaller and more tender curds in the stomach compared to raw milk.
Scum formation: Scum is formed when milk is heated in an uncovered pan on the surface due to drying out. The scum gets toughened as the temperature is increased. The insoluble Scum can be removed from the surface but another one forms. It contains a small amount of coagulated protein, minerals and fat globules. A tenacious layer of fat that forms on the milk when heated is due to the breaking of the film of the protein that surrounds the fat globules in unheated milk resulting in coalescence of fat globules. Scum formation can be prevented by heating or stirring the milk while heating it. Use of a milk boiler helps to prevent it. It can be prevented by covering the pan or by diluting the milk.
Boiling Over: The surface tension of milk is related to proteins, fat, phospholipids and few fatty acids present in milk. When milk is heated up to its boiling point, it gets agitated and its density also decreases which result in foaming. This can be prevented by pouring some water on its foaming surface which helps to increase the surface tension to some extent. Scum formation during the heating of milk is the principle reason for its behaviour of boiling over. A certain amount of pressure develops under the scum which laterforces this scum upwards and the milk flows over the sides of the pan. Stirring breaks the scum, releases the pressure and prevents spilling over.
Scorching of milk: Scorching is due in part to the film of coagulated albumin and other whey proteins that collect on the bottom and sides of the pan. Non-enzymatic browning may be responsible for the brown colour of scorched milk. Heating milk over hot water like milk cooker can prevent this. Stirring to some extent can also prevent this. Homogenised milk coagulates more readily than non-homogenised milk. This is due to increased amount of protein on the surface of the uniform fat globules and due to the variations in the temperatures and pressures used for homogenisation.
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