Raw milk is milk in the natural form. It can be a source of bacteria that cause diseases like undulant fever salmonellosis, tuberculosis and dysentery. It also contains many disease producing organisms when in this form. Different heat and treatments are given to raw milk removes pathogenic organisms. This increase the shelve life of milk. Heating, if done inappropriately can cause changes in the constituents of milk, like affecting the solubility and the nutritional value of the milk (Morales 2000). There are heating criteria used on milk which must guarantee the correctness of heat treatment which comply with the correct standard and terms of processing milk like thermization, pasteurization and sterilization. Nowadays, there are two main heat treatments used on milk, namely, Ultra heat treatment (UHT) allowing longer periods of time like 2 – 3 months without refrigeration and pasteurization allowing a shorter period of less than 21 days with refrigeration.
However, there are technologies developed to obtain pasteurized and UHT milk using different temperature profiles and the effect of the quality of milk may vary depending on the process involved and temperatures applied (Birlouez – Aragon et al 2002).
Intense heating of milk can cause destruction on vitamins, like vitamin C which is heat labile. The two most stable pathogens in milk are Mycobacterium that causes tuberculosis and coxiella burnetti that causes Q fever. To kill this bacterial there has to be a minimal degree of temperature and duration. To ensure high quality of milk these temperature and time combinations are highly regulated. The higher the temperature utilized the shorter the time required destroying all pathogenic microbes. Heat treatment on milk involving heating it to 72C with a holding time of 15 seconds before it’s cooled is called high temperature short time treatment (HTST) mode of pasteurization.
This occurs when milk is brought to a temperature of 72C to 15 seconds; it is then cooled to 10 C or below. This temperature and time give a better margin of safety. Heating is usually done by hot water and heat exchange is done through stainless steel plates by passing the milk through the annular space which is between the concentric water heated tubes.
In HTST pasteurization, little piping is used to avoid superheating of milk. This is done by allowing milk to flow over a water heated surface which is larger than the amount of milk heated, and it is kept at a temperature not greater the 15 higher than required heating on milk. The hot surface is heated by contact with hot water; no live steams come into contact with the surface. A thermostat is used to keep the temperature uniform. The temperature is automatically controlled to ensure accurate heating on milk for accurate results.
This ensures that all micro organisms capable of causing deceases are killed. HTST pasteurization is effective in destroying various bacteria of pathogenic species causing diseases (Green et al 2002). More over the use of HTST allows total destruction of several microbial growths and inactivate the enzymatic activity. The shelf life of milk treated is from 2 days to 16 days. This is dependant to the quality of the raw milk and the degree of refrigeration. This method of pasteurization ensures that the original taste of the milk is maintained and also ensures minimal denaturarization of proteins and bioactivity reduction; maintaining the value and quality of milk.
However, mycobacterium avium paratuberculosis (MAP) survives the 72C and 15 seconds heating. This therefore lowers the quality of milk for it puts the consumers at risk.