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 lling into containers
- Removing air from intercellular spaces prior to canning
Blanching and enzyme inactivation
Freezing and dehydration are insuffcient to inactivate enzymes and therefore blanching can be employed. Canning conditions may allow suffcient 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.
Methods of Blanching
Blanching is carried out at up to 100°C using hot water or steam at or near atmospheric pressure. Some use of 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 e uent, 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 diffculties in uniformity of heating.