Food Preservation Storage

Lesson 08: Preservation by Use of HeatPreservation by Use of Heat


Blanching is used for variety of purposes. It is defined as a mild heat treatment applied to tissue (usually plant) primarily to inactivate enzymes prior to freezing, drying or canning. It is also known as scalding.

Functions of blanching

  • Inactivate most enzymes
  • Some cleaning action
  • Removes substances in some products
  • Activates some enzymes (if controlled)
  • Removes undesirable odours/ flavours
  • Softens fibrous material and decreases volume
  • Expels air and respiratory gasses
  • Preheating of product prior to canning
  • Reduces number of microorganisms

Major function of blanching is inactivation of enzymes for frozen or dehydrated foods as enzymes can cause rapid changes in colour, flavour and nutritive value of such food products. Moreover freezing and dehydration processes involve temperatures which are insufficient to inactivate enzymes. Blanching as a pretreatment before drying has many advantages like it helps in cleaning the material and reducing the amount of microorganisms present on the surface; it preserves the natural colour in the dried products; for example, the carotenoid (orange and yellow) pigments dissolve in small intracellular oil drops during blanching and in this way they are protected from oxidative breakdown during drying; and it shortens the soaking and/ or cooking time during reconstitution. Blanching does not allow effective autoclaving, and stops the activity of autolytic enzymes.

For canned products, blanching removes gases, shrinks the food to correct fill weight in can and offers preheating, which are very important to provide vacuum in can and proper sterilization. Sometimes canning process may allow sufficient time for enzymatic activity and under blanching may increase the enzymatic activity.

Fruits are not blanched. As a thumb rule, all those vegetables which cannot be eaten raw are blanched. E.g. potatoes, greens green beans, carrots, okra, turnip and cabbage should always be blanched. On the other hand, blanching is not needed for onions, leeks, tomatoes and sweet peppers.

Using sodium bicarbonate with blanching water preserves the green colour of vegetables by preventing the conversion of chlorophyll into pheophytin, unattractive brownish-green colour compound.

Methods of blanching:
Blanching is a delicate processing step. It requires careful monitoring of time, temperature and the other conditions. Effective blanching time necessary to inactivate enzymes is dependent on various factors viz. type of food, method or type of heating, product size and temperature of heating medium etc. There are mainly two typical methods of blanching based on type of heating medium viz. hot water blanching and steam blanching use hot water and steam as heating medium, respectively. The former process involves temperatures below 100°C whereas the latter is carried out at temperatures above 100°C. A third type of blanching system exists which is a combination of hot water and steam blanching.

Blanching of green leafy vegetables especially spinach at boiling point causes loss of green colour but at lower temperature (77°C), it retains the natural green colour, even when heated at higher temperature (121°C) later during sterilization. At lower temperature, the enzyme chlorophyllase remains active for little time and converts chlorophyll to a phyllin, which retains green colour.l

Hot water blanching:
In this method, the cleaned food is subjected to hot water (85 to 100°C) until the enzymes are inactivated. Pot blanchers are used at home scale . Generally hot water blanching is done because of low capital costs and better energy efficiency. Disadvantages associated with hot water blanching include loss of water-soluble constituents, risk of contamination and higher cost of water and disposal of effluent.

Steam blanching:
In case of steam blanching, the food product is directly exposed hto steam, which avoids the loss of food soluble solids (flavours, vitamins, acids, sugars etc.) to blanching medium as well as solves the problem of disposing blanching medium after processing. Steam blanching is advantageous as it results in less loss of water-soluble constituents, less volume of waste, easy to clean and sterilize. But it has some disadvantages such as higher capital costs, uneven blanching, and low efficiency.

Central Institute of Agricultural Engineering (CIAE), Bhopal has developed the batch type steam blancher, (Figure 2) with 100 kg/ h capacity. It has been evaluated for blanching of cabbage, cauliflower, pea and okra. Results revealed satisfactory performance on account of colour retention in the dried product.
On commercial level, tunnel steam blanchers with product conveyers are used.

Blanchers with hot water and steam system:
This type involves three step process viz. product on conveyer belts is exposed to steam consecutively followed by contact between food and hot water and finally immersion in hot water.

Efficacy of blanching:
There are various types of enzymes such as lipoxygenase, polyphenolase, polygalacturonase and chlorophyllase, which cause loss of quality and therefore, must be inactivated. Normally, two heat resistant plant enzymes such as catalase and peroxidase, are used to evaluate blanching efficacy, as appropriate time and temperature is required to inactivate them.

Last modified: Saturday, 10 March 2012, 6:11 AM