The process of pasteurization was named after Louis Pasteur who discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point. The process was later applied to milk and remains the most important operation in the processing of milk.
Definition: The heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process.
Purpose There are two distinct purposes for the process of milk pasteurization:
1. Public Health Aspect - to make milk and milk products safe for human consumption by destroying all bacteria that may be harmful to health (pathogens)
2. Keeping Quality Aspect - to improve the keeping quality of milk and milk products. Pasteurization can destroy some undesirable enzymes and many spoilage bacteria. Shelf life can be 7, 10, 14 or up to 16 days.
The extent of microorganism inactivation depends on the combination of temperature and holding time. Minimum temperature and time requirements for milk pasteurization are based on thermal death time studies for the most heat resistant pathogen found in milk, Coxelliae burnettii. Thermal lethality determinations require the applications of microbiology to appropriate processing determinations. To ensure destruction of all pathogenic microorganisms, time and temperature combinations of the pasteurization process are highly regulated:
Ontario Pasteurization Regulations
Dairy with <10% mf:
63° C for not less than 30 min.; 72° C for not less than 16 sec.,
or equivalent destruction of pathogens and the enzyme phosphatase as permitted by Ontario Provincial Government authorities. Dairy is deemed pasteurized if it tests negative for alkaline phosphatase.
Dairy products with 10% mf or higher, or added sugar (cream, chocolate milk, etc)
66° C/30 min, 75° C/16 sec
There has also been some progress with low temperature pasteurization methods using membrane processing technology.
Methods of Pasteurization
There are two basic methods, batch or continuous.
Batch methodThe batch method uses a vat pasteurizer which consists of a jacketed vat surrounded by either circulating water, steam or heating coils of water or steam.
In the vat the dairy is heated and held throughout the holding period while being agitated. The dairy may be cooled in the vat or removed hot after the holding time is completed for every particle. As a modification, the dairy may be partially heated in tubular or plate heater before entering the vat. This method has very little use for milk but some use for milk by-products (e.g. creams, chocolate) and special batches. The vat is used extensivly in the ice cream industry for mix quality reasons other than microbial reasons.
Continuous process method has several advantages over the vat method, the most important being time and energy saving. For most continuous processing, a high temperature short time (HTST) pasteurizer is used. The heat treatment is accomplished using a plate heat exchanger. This piece of equipment consists of a stack of corrugated stainless steel plates clamped together in a frame. There are several flow patterns that can be used. Gaskets are used to define the boundaries of the channels and to prevent leakage. The heating medium can be vacuum steam or hot water.
Source: Professor Douglas Goff, Dairy Science and Technology Education, University of Guelph, Canada, www.foodsci.uoguelph.ca/dairyedu/home.html.