Question

Explain the procedure for preserving foods by the use of chemical preservatives.

Answer 1

1. Pasteurised squashes and cordials spoil within a short period after they are opened, particularly in tropical climates.
2. In such cases, it is desirable to use chemical preservatives. However, the use of chemicals should be properly controlled since their indiscriminate use is likely to create harmful effects.
3. The preservatives used should not be injurious to health. They should also be non-irritant.
4. Two important preservatives permitted and popularly used are benzoic acid (or sodium benzoate) and sulphur dioxide (or potassium metabisulphite).
5. It is the salt of benzoic acid, called sodium benzoate, which is used as a preservative.
6. The use of benzoic acid increases the concentration of carbon dioxide in the medium.
7. Benzoic acid is more effective against yeasts than moulds. However, it does not stop the formation of lactic acid or vinegar fermentation.
8. Potassium metabisulphite is used as a salt of sulphur dioxide.
9. It is a crystalline salt and is fairly stable in a natural or alkaline medium.
10. It is decomposed by weak acids like carbonic, citric, tartaric, and malic acids.
11. When it is added to fruit juice or squash, the potassium radicals react with the juice's acid, forming the corresponding potassium salt and liberating sulphur dioxide.
12. The preservative effect of sulphurous acid depends not on its total quantity but the availability of sulphur dioxide.
13. It is because the pigment anthocyanin, which gives red or purple colour to fruits and vegetables, gets discoloured in the presence of sulphur dioxide.