Heat treatment

Principle

Food is heated up or cooked.Heat kills micro-organisms and their spores, alters protein structure, destroys enzyme activity of micro-organisms in food .

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Methods of heating

Variation in temperature and treatment time

1. Blanching

a. Mild heat treatment, usually applied to fruits and vegetables to denature enzymes

b. Often used before freezing of fruits and vegetables

　

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2. Pasteurization

   The process is named after the French chemist Louis Pasteur, who devised it in 1865 to inhibit fermentation of wine and milk.

   a. Destroys pathogenic microorganisms and extends the shelf life of a food

   b. Pasteurized products still contain many viable organisms capable of growing and causing spoilage defects

    

   c. Usually pasteurization is combined with another means of preservation like refrigeration

   　

   d. Levels of pasteurization used to thermally process milk:

   (1) Low Temperature Long Time (LTLT): 63° C (145° F) for 30 min

   　

   (2) High Temperature Short Time (HTST): 72° C(161° F) for 15 s (LTLT and HTST are equivalent thermal processes)

   　

   (3) Ultra High Temperature(UHT): 138° C (280 ° F) (or higher) for 2 s

   　

   e. Concept of temperature-time processing conditions that give equal destruction of microorganisms

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3. Commercial sterilization

a. All pathogenic and toxin-forming organisms are destroyed, as well as other types of organisms, which if present could grow in the food and cause spoilage under normal handling and storage conditions

b. These foods may contain a small number of heat resistant bacterial spores, but they will not multiply under normal handling and storage conditions

　

c.Types of commercially sterile processes: canning, bottling, and aseptic processing

　

d. Most commercially sterile food products have a shelf life of 2 years or longer

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4. Sterilization

a. Complete destruction of all microorganisms, including both vegetative cells and spores

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Side effect

High temperatures can diminish product appearance, texture, and nutrient quality.

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Examples: All forms of cooked food, milk sterilized by UHT (ultra high temperature),beer, wine

Milk

  Milk is pasteurized by heating at a temperature of 63° C (145° F) for 30 minutes, rapidly cooling it, and then storing it at a temperature below 10° C (50° F). Beer and wine are pasteurized by being heated at about 60° C (about 140° F) for about 20 minutes; a newer method involves heating at 70° C (158° F) for about 30 seconds and filling the container under sterile conditions.

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