FOOD CHEMISTRY

Module 8. Antinutritional factors and food contaminant

Lesson 21
TOXIC TRACE ELEMENTS, RADIONUCLIDES

21.1 Introduction

The unintentional incorporation of chemicals into food is as widespread as intentional addition and may pose health hazards. The sources of contamination are radioactive fall-out, chemicals used in agricultural production, animal food additives and accidental contaminants during food processing.

21.2 Anti-nutritional Factors

Many foods, particularly those of plant origin, contain a wide range of anti-nutritional factors which interfere with the assimilation of nutrients contained in them. The important anti-nutritional factors are trypsin inhibitors, phytates, oxalates, tannins, lectins and goitrogens. They interfere with the utilization of other nutrients like proteins, minerals like iron, zinc, calcium and iodine.

21.2.1 Trypsin inhibitors

Trypsin inhibitors are proteins distributed widely in plant foods like legumes (soyabean, lima and kidney bean) and certain animal foods like white of egg. They generally inhibit the activity of trypsin in the gut and interfere with digestibility of dietary proteins and reduce their utilization. They are heat labile; the extent and ease of heat inactivation varies from one trypsin inhbitor to another. However, autoclaving at 120^oC for 15-30 min inactivates almost all trypsin inhibitors. The heat treatment inactivates the trypsin inhibitors and improve considerably the utilization of protein present in these foods.

21.2.2 Phytate

Phytate is widely distributed in seeds. Unrefined cereals and millets are richest sources of phytates. Phytate is hexa phosphate of inositol. It acts as a source of bound phosphorus for the seeds during germination. These phytates bind iron, zinc, calcium and magnesium. In presence of calcium and magnesium, it forms insoluble complexes with iron and thus makes iron unavailable. Phytates present in cereals contribute significantly to poor absorption of iron from cereal based diets. On germination of the grains, the phytate content reduces due to enzymatic breakdown of phytate. Improved iron availability in germinated grains can be partly attributed to a reduction in phytate content.

21.2.3 Tannins

Tannins are condensed polyphenolic compounds which are widely distributed in plant kingdom. They are present in high amount in seed coat of most legumes, spices, tamarind, turmeric, in certain vegetables and fruits. Millets like bajra, ragi, sorghum also contain a fair amount of tannin. Tannins bind with iron irreversibly and interfere with iron absorption. Tannins are also known to bind proteins and reduce their availability.

21.2.4 Oxalates

Oxalic acids or its salts (oxalates) are widely distributed in plant foods. These oxalates are mostly calcium salts. Rich source of oxalates are green leafy vegetables and green vegetables and some legumes. Oxalates are known to interfere with calcium absorption by forming insoluble salts with calcium. Stone patients are advised to avoid high oxalate containing foods.

21.2.5 Goitrogens

Certain substances present in plant foods interfere with iodine uptake by thyroid gland and may contribute to development of iodine deficiency disorders when iodine intakes are marginal. Such compounds are termed as ‘goitrogens’. Thiocyanate, isothiocyanates and their derivatives etc. These compounds occur in leaves and vegetables like cabbage, cauliflower, rape leaves, radish, rapeseed, mustard, etc. soyabean, peanut, lentils also contain goitrogens.

21.3 Radionuclides ES

Certain chemical elements are found in unstable states which spontaneously decay as they change to more stable forms. During this change they emit radioactivity as