Thermal Processing of Food


  1. Introduction
  2. Blanching
    • Blanching and enzyme inactivation
    • Methods of blanching
    • Testing of the effectiveness of blanching
  3. 3. Pasteurization
    • Purpose of pasteurization
    • Method for pasteurizing
  4. 4. Sterilization
    • Canned foods
    • Conditions affecting the growth of micro-organisms
    • Micro-organisms in retorted foods
    • Microbial spoilage of canned foods
    • Sterilisation process and equipment
    • Containers for thermally treated products
    • Cleaning of containers prior to filling
    • Seaming of cans
    • Death rate curve (D value)
    • Thermal death time (TDT) curve
    • Some factors affecting heat resistance
    • Design of heat sterilization processes
    • The “F0 value”
    • The lethality factor “l”

1. Introduction

There are two main temperature categories employed in thermal processing: Pasteurization and Sterilisation. The basic purpose for the thermal processing of foods is to reduce or destroy microbial activity, reduce or destroy enzyme activity and to produce physical or chemical changes to make the food meet a certain quality standard. e.g. gelatenization of starch & denaturation of proteins to produce edible food. There are a number of types of heat processing employed by the food industry.

Mild processes

More Severe Processes

2. Blanching

The primary purpose of blanching is to destroy enzyme activity in fruit and vegetables. It is not intended as a sole method of preservation, but as a pre-treatment prior to freezing, drying and canning. Other functions of blanching include:

  • Reducing surface microbial contamination
  • Softening vegetable tissues to facilitate filling into containers
  • Removing air from intercellular spaces prior to canning

2.1 Blanching and enzyme inactivation

Freezing and dehydration are insufficient to inactivate enzymes and therefore blanching can be employed. Canning conditions may allow sufficient time for enzyme activity. Enzymes are proteins which are denatured at high temperatures and lose their activity. Enzymes which cause loss of quality include Lipoxygenase, Polyphenoloxidase, Polygaacturonase and Chlorophyllase. Heat resistant enzymes include Catalase and Peroxidase

2.2 Methods of Blanching

Blanching is carried out at up to 100°C using hot water or steam at or near atmospheric pressure.

Some use fluidised bed blanchers, utilising a mixture of air and steam, has been reported. Advantages include faster, more uniform heating, good mixing of the product, reduction in effluent, shorter processing time and hence reduced loss of soluble and heat sensitive components.

There is also some use of microwaves for blanching. Advantages include rapid heating and less loss of water soluble components. Disadvantages include high capital costs and potential difficulties in uniformity of heating.

2.3 Steam Blanchers

This is the preferred method for foods with large cut surface areas as lower leaching losses. Normally food material carried on a mesh belt or rotatory cylinder through a steam atmosphere, residence time controlled by speed of the conveyor or rotation. Often poor uniformity of heating in the multiple layers of food, so attaining the required time-temperature at the centre results in overheating of outside layers.

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