Heat Processing of Milk

There are several processes in which the main purpose of heat application is to make milk safe for human consumption and increase its keeping quality. Common process in which heat is applied are as follows:

i)Pasteurization: The main aim of pasteurization of milk is to kill all pathogenic microoganism and make it safe for human consumption. Pasteurization of milk is done either by High temperature short time (HTST) or Low temperature long time (LTLT) process. In Holder process milk is heated for not less than 30 minutes at 63 0 C while in HTST pasteurization heating not less than 71.7 0 C
for 15 seconds is used. Pasteurization of milk is done to meet the public health requirement. Pasteurization leads to the destruction of pathogenic bacteria, e.g.Mycobacterium tuberculosis and most of the non-pathogenic organisms and the enzymes present in milk. Both in Holder and HTST process alkaline phosphatase activity is taken as an index of destruction of Mycobacterium tuberculosis.

ii) Sterilization: The purpose of heat sterilization is to destroy all micro-organisms and their spores in milk. Sterilization is primarily employed for the preparation of sterilized milk. In the preparation of evaporated milk sterilization at temperatureof 116 0 C for 15 minutes is employed. Sterilized milk can be stored at room temperature for longer period.

ii) Forewarming or Preheating: Forewarming or preheating is applied for the manufacture of condensed or evaporated milk and dried milk. Temperatures between 88 0 C to 100 0 C are employed for forewarning. Present trend is to heat milk above 100 0 C to give maximum heat stability.

iv) Condensing: In condensing heating is carried out under vacuum to remove water. In a single effect vacuum evaporator the temperature ranges between 43 to 55 0 C. With the widespread use of multiple effect evaporators for condensing milk, it is not possible to specify precisely the degree of heat treatment which may be subjected in condensing operation.

v) Drying: The purpose of drying is to completely remove water with minimum physico-chemical changes in the dried product. The temperature employed in spray drying process ranges between 71 to 177 0 C in terms of air inlet temperatures. The precise temperature conditions vary with the particular drying equipment.

vi) UHT Process: In the ultra high temperature process for preparing UHT milk,the milk is heated at a high temperature of 135 0 C – 150 0 C with holding time of few (1 to 8) seconds. This is a microbially safe milk. UHT milk can be stored at room temperature for further use

vi) Homogenization: Homogenization is carried out to break down the fat globules into a smaller size, resulting in stable state of dispersion. Though homogenization can be carried out without heating but heating is generally required for proper and satisfactory homogenization. Before homogenization heating facilitates melting of fat and inactivation of the enzyme like lipase. If heating is not done it leads to the development of hydrolytic rancidity with the liberation of lower chain fatty acids. Heating destroys lipase and thus prevents lypolysis. Non-heating and homogenization will result in fast lypolysis by lipase resulting in rancid flavour by liberation of lower chain fatty acids from the glycerides. It is desirable to heat the milk before homogenization. Other due to heating and homogenization there are other physico-chemical changes which occur in milk.

These include easy digestibility of milk, soft curd formation and tasteful products.However, homogenization results in difficulty in cream separation. Homogenization is primarily employed in the preparation of flavoured milk, ice cream mix and evaporated milk.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.


Most Reading