In this article we will discuss about:- 1. Definition of Homogenized Milk 2. Factors Influencing Homogenization 3. Sequence of Various Processes Involved in Production 4. Methods of Manufacture 5. Homogenizer 6. Merits and Demerits.
Definition of Homogenized Milk:
According to the United States Public Health Service, homogenized milk is milk which has been treated in such a manner as to insure breakup of the fat globules to such an extent that after 48 hours’ quiescent storage no visible cream separation occurs on the milk; and the fat percentage of the milk in the top 100 ml., of milk in a quart bottle, or of proportionate volumes in containers of other sizes, does not differ by more than 10 per cent of itself from the fat percentage of the remaining milk as determined after thorough mixing. (In efficiently homogenized milk, the fat globules are subdivided to 2 microns or less in diameter.)
Note:
Homogenization refers to the process of forcing the milk through a homogenizer with the object of sub-dividing the fat globules.
Factors Influencing Homogenization:
(a) Temperature of Homogenization:
The milk should, at the time of homogenization, be at a temperature above the melting point of fat, viz., above 33°C (91°F). This is because fat should be in the liquid state for proper sub-division. The enzyme lipase should be inactivated, preferably prior to homogenization or immediately afterwards. This can be achieved by heating the milk to a temperature of 55°C (131°F) or above. In routine practice, the milk is heated to 65-70°C (149-158°F) for homogenization. (The danger zone for lipase activity, viz., temperatures 38-49°C (100-120°F), should be avoided during or after homogenization.)
(b) Pressure of Homogenization:
In a single stage, upto 6 per cent fat milk, usually 2000-2500 psi pressure is sufficient. Higher pressures may increase the tendency for the milk to curdle when cooked, due to the increased destabilizing effect on milk-proteins. For liquid products with more than 6 per cent fat, two-stage homogenization is needed to prevent fat clumping- 2000 psi at the first stage and 500 psi at the second.
Sequence of Various Processes Involved in Production of Homogenized Milk:
The possible sequences are:
(i) Clarification, pre-heating, homogenization, pasteurization, cooling.
(ii) Clarification, pre-heating, pasteurization, homogenization, cooling.
(iii) Pre-heating, homogenization, clarification, pasteurization, cooling (Recommended under Indian conditions).
(iv) Pre-heating, clarification, homogenization, pasteurization, cooling.
(v) Pre-heating, clarification, pasteurization, homogenization, cooling.
Note:
The main considerations are:
(i) All homogenized milk must be pasteurized either prior to homogenization or immediately after;
(ii) The homogenized milk should preferably be clarified after homogenization;
(iii) Under Indian conditions, the sequence underlined above is desirable, so as to avoid any post-pasteurization contamination.
Methods of Manufacture of Homogenized Milk:
Flow Diagram of Manufacture:
Homogenization:
The pre-heated (60°C) milk is homogenized at 2500-3000 psi pressure in a single-stage homogenizer. This causes sub-division of the original fat globules to less than 2 micron size (diameter), which is considered satisfactory.
Homogenizer:
This is a machine which causes the sub-division of fat globules. It consists of a high pressure piston pump which forces the milk at high pressures (and velocity) through a narrow opening between the homogenizing valve and its seat; the fat globules in the milk are thereby sub-divided into smaller particles of more uniform size.
The homogenizing valve is held down by a heavy pressure spring against the seat of the valve. The valve and its seat are made of extremely hard material (e.g., stellite) and the contact faces are carefully ground so that the valve sits accurately on its seat. Homogenizers are either single stage or double stage.
Note:
(i) A viscolizer is a machine similar to a homogenizer, but usually operates at a lower pressure and has smaller openings,
(ii) A clarifixator is a machine, developed in Sweden, which not only clarifies but also homogenizes the milk.
Merits and Demerits of Homogenized Milk:
(a) Merits:
(i) No formation of cream layer/plug;
(ii) Fat in milk does not chum due to rough handling or excessive agitation;
(iii) Better adapted for bulk dispensing; mixing not necessary;
(iv) More palatable due perhaps to brighter appearance, heavier body and richer flavour;
(v) Produces soft curd and is better digested; hence recommended for infant feeding;
(vi) Less susceptible to oxidized flavour development.
(b) Demerits:
(i) Increased cost of production;
(ii) Returned homogenized milk difficult to salvage; fat recovery is a problem;
(iii) Sediment appears to a greater degree;
(iv) Curdling in cookery;
(v) More susceptible to production of activated or sunshine flavour defect;
(vi) Greater tendency for milk ‘seepage’ through bottle cap.