The need to heat process milk

There are different methods of processing available to preserve the milk but heat treatment is still considered as a better alternative to preserve the milk. Depending on the degree of heat treatment, the heating process may be classified as follows.

The other side of heat treatment

Intense heat treatment of milk may be desirable from the microbiological point of view but it has its own disadvantage in adversely modifying the sensory characteristics of milk. The excess heat treatment has an effect on the appearance, taste and nutritional value of the milk. From the nutrients point of view, proteins in milk are denatured at high temperatures.

Hence it is difficult to make cheese from milk that is intensely heat treated. Excess heat treatment also affects the taste which is reflected as cooked flavor in the beginning while intense heat treatment results in burnt flavor. The temperature time combination has been carefully chosen to destroy the pathogenic and some times spoilage causing microorganisms without drastically affecting the nutrients in milk.


It is not possible to pasteurize and process all the milk immediately after reception  and this is more common in many large dairies. Under such circumstances, some of the milk must be stored in silo tanks for hours or days without heat treatment. Hence, it becomes necessary to preserve the milk without deterioration in its quality. Thermization comes in handy during such situations where even deep chilling of milk is not enough to prevent serious quality deterioration.

Name of the process

Temperature – Time combination


63 – 65°C 15 s

LTLT pasteurization of milk

63°C 30 min

HTST pasteurization of milk

72 – 75°C 15 – 20 s

HTST pasteurization of cream etc.

>80°C 1 – 5 s

Ultra pazteurisation

125 – 138°C 2 – 4 s

UHT (flow sterilization)

normally 135 – 140°C a few seconds

Sterilization in container

115 – 120°C 20 – 30 min

To define, “thermization” is a process of heat treating the milk which is definitely less than the temperature-time combination employed in the pasteurization process. It is actually a sub-pasteurization procedure to temporarily inhibit bacterial growth. The milk is heated to 63 – 65°C for about 15 seconds, a time-temperature combination that does not inactivate the phosphatase enzyme, which is a pre-requisite for pasteurization. Double pasteurization is forbidden by law in many countries and for this reason, the thermization procedure must stop short of pasteurization conditions.

To prevent aerobic spore-forming bacteria from multiplying after thermization, the milk must be rapidly chilled to 4°C or below and it must not be mixed with untreated milk. Many experts are of the opinion that thermization has a favorable effect on certain spore- forming bacteria. The heat treatment causes many spores to revert to the vegetative state, which means that they are destroyed when the milk is subsequently pasteurized. Thermization should be applied only in exceptional cases. The objective should be to pasteurize all the incoming milk within 24 hours of arrival at the dairy.

LTLT (Low Temperature Long Time) pasteurization

This type of heat treatment was originally a batch process in which the milk was heated to 63°C in open vats and held at that temperature for 30 minutes. This method is otherwise called as the “holder” method or “low temperature long time (LTLT)” method. Nowadays milk is almost always heat treated in continuous processes (like thermization, HTST pasteurization and UHT treatment) where bulk milk can be processed in a short span of time.

Learn more about Higher forms of heat treatment of milk