Theory of Pasteurization

We have understood that heating milk to selected time-temperature combination effectuates pasteurization to ensure destruction of all pathogenic microorganisms.Theoretically, aspect of pasteurization is “the heat treatment applied to the milk to destroy pathogenic organisms”. The process parameters of heat treatment or time-temperature combinations are elaborated below: (a) Limiting factors for heat treatment and (b) Types of heat treatment.

 

i. Limiting Factors for Heat Treatment

The upper and lower limits of temperature to pasteurization process are based on thermal death point of tubercle bacilli and beginning of reduction of the cream line.The thermal death time for tubercle bacilli provides the lower limit to heat treatment.The adverse effects on the commercial quality milk provide an upper limit for the possible time- temperature combinations used in pasteurization. As the cream line is the first quality to be affected, it is generally used as the standard indicator of changes in the chemical, biological and physical properties of milk caused by over heating.

In the early 1920’s, North and Park performed extensive tests by heating milk samples containing tubercle bacilli at different time-temperature combinations that destroyed all the tubercle bacilli present in them. The time-temperature combinations that destroy all tubercle bacilli are taken as thermal death points. Below Table shows a number of thermal death points for tubercle bacilli.

Thermal Death Points for Tubercle Bacilli

These thermal death points can also be plotted on a graph to give a thermal death line.

Safety margin: This is the additional amount of heat treatment (time and temperature above the thermal death point of the tubercle bacillus) so that, under no circumstances,will any tubercle bacilli be left alive after correct routine operation of a pasteurizer. Certainly, a more intense heat treatment would obtain more efficient antibacterial results than pasteurization. On the other hand, the milk is not inert to heating; over-heating adversely affects the appearance, taste, nutritional and technological value of milk. Combination of higher temperature and longer holding time-temperature are also recommended for HTST pasteurization of dairy products having higher contents of solids.

 

ii. Types of Heat Treatment

The heat treatment given in form of (i) holding and (ii) continuous correspondingly relate with two methods of pasteurization i.e.

• Batch, holding or Low Temperature Long Time (LTLT) method and
•  Continuous, High Temperature Short Time (HTST) method.

In the batch method, the milk is heated to 63°C in a tank or vat equipped with a hot water or steam jacket and agitators to keep the milk agitated; held for 30 minutes and then partly cooled in the batch pasteurizer. The further cooling is done by surface/plate cooler. This method is mostly used for processing of around 5000 liters of milk.

High Temperature-Short Time (HTST) pasteurization is the process, which is commonly used now a day all over the world. Plate Heat Exchanger (PHE) is used to heat, hold and cool the milk. Milk is heated to a temperature of at least 72°C and held at that temperature for not less than 15 seconds and then immediately cooled to a temperature not greater than 4°C.