FNB News - Feature Anti-nutritional factors in food and plant crops

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Anti-nutritional factors in food and plant crops

Saturday, 18 August, 2012, 08 : 00 AM [IST]

Dr V K Joshi and Ghan Shyam Abrol

Introduction

Toxic compounds synthesised by plant microbes and animals cause serious problems in human and animal nutrition. The anti-nutritional factors are the undesirable substances present in food. These substances are naturally found in food but their activity as anti-nutritional factors depends on the digestion process.

Anti-nutritional factors (ANF) are compounds which reduce the nutrient utilisation or food intake of plants or plant products when used as human foods (Soetan and Oyewole, 2009). The presence of endogenous anti-nutritional factors within plant feedstuffs results in limiting their use in food or feed stuff. Although these factors vary in their individual effect, a large proportion of them can be destroyed or inactivated simply by heat treatment processes during cooking.

Unfortunately, detailed toxicological studies have not been performed on the majority of these anti-nutritional factors. Broadly speaking, their presence in untreated foodstuffs normally results in anorexia, reduced growth and poor food conversion efficiency when used at high dietary concentrations. Root crops, in common with most plants, contain small amounts of potential toxins and antinutritional factors such as trypsin inhibitors. Apart from cassava that has cyanogenic glucosides, cultivated varieties of most edible tubers and roots do not contain any serious toxins. Thus, wild species must be properly processed before their consumption. These wild species, however, are useful reserves in times of famine or food scarcity. Local people are aware of their potential risks in their use and have developed indigenous techniques to detoxify such crops before consumption.

Saponins

Saponins are glycosides with a distinctive foaming characteristic. They are found in many plants, but their name is derived from the Latin word sapo from soapwort plant (Saponaria). They are either a choline steroid or triterpenoid attached via C3 and an ether bond to a sugar side chain. Saponins are glycosides with a polycyclic aglycone moiety of either C27 steroid or C^ triterpenoid attached to a carbohydrate. They are widely distributed in the plant kingdom. Soybean, chickpea, faba bean, pea, lentil and peanuts are the different sources of saponins. Erythrocytes lyse in saponin solution, therefore these compounds are toxic when injected intravenously (Khalil and El-Adawy, 1994). The anti-nutritional effects of saponins have been mainly studied using alfalfa saponins. Saponins cause hypocholesterolaemia by binding cholesterol, making it unavailable for absorption. They also cause haemolysis of red blood cells and are toxic to rats (Johnson et al., 1986).

Table 1. Presence of total saponins content in commonly used spices (Sirohi et al., 2009)

Trypsin inhibitor

Legumes have beneficial nutritional effects in diets being low-cost source of protein (Borade et al., 1984). But most of legumes are under-used because of the presence of anti-nutritional compounds, such as enzyme (trypsin, chymotrypsin, alpha-amylase) inhibitors, phytic acid, flatulence factors, saponins and toxic factors (Lyimo et al., 1992). Trypsin inhibitors are present in soy protein. The tripsin inhibitor content of legumes increased during germination. The intake of trypsin inhibitor with the diet causes a rise of fecal nitrogen loss (Combs et al. 1967).

Table 2. Trypsin inhibitor activity of whole, kernels and shells from raw seeds of J. curcas. (Arab and Salem, 2010)

Trypsin inhibitor may have different modes of action, such as the antifibrinolytic or antithrombinogenic or antiproteolytic or a promoter of conversion of methionine to cysteine manifesting ultimately in growth retardation. Trypsin (protese inhibitor) causes pancreatic enlargement and growth depression (Aletor and Fetuga, 1987). Cooking and germination seem to be good procedures to improve the quality of lentil flour from the nutritional point of view, despite the fact that a large variation on the effects of processing, related to the different legume varieties, has been observed (Vidal et al., 1994).

Mimosine

Mimosine [ß-N-(3-hydroxy-4-pyridone)-a-aminopropionic acid] is a non-protein amino acid structurally similar to tyrosine found in the genera Leucaena and Mimosa. The mimosine contents in different parts varies differently in seeds such as 4 to 5% mimosine on a dry-weight basis, root contains 1 to 1.5 per cent mimosine and shoot contains 1 to 12 per cent while old stems contain the smallest and growing tips have the largest amounts (Jones and Lowry, 1984). The mechanism of action of mimosine in producing its effect, however, is not clear but it may act as an amino acid antagonist or may complex with pyridoxal phosphate, leading to disruption of catalytical action of B6-containing enzymes such as trans-aminases, or may complex with metals such as zinc (Hegarty, 1978). The main symptoms of toxicity in ruminants are poor growth, loss of hair and wool, swollen and raw coronets above the hooves, lameness, mouth and oesophageal lesions, depressed serum thyroxine level and goitre (Jones and Hegarty, 1984).

Cyanogens

Cyanogenic glucosides are phytoanticipins known to be present in more than 2,500 plant species (Conn, 1980). A cyanogenic food of particular economic importance is cassava, which is also known by the names manioc, yuca and tapioca. Amygdalin is the cyanogenic glycoside responsible for the toxicity of the seeds of many species of Rosaceae, such as bitter almonds, peaches and apricots. Sweet almonds are low in amygdalin as a result of breeding processes. The lethal dose of HCN for cattle and sheep is 2.0-4.0mg per kg body weight. The lethal dose for cyanogens would be 10-20 times greater because the HCN comprises 5-10 per cent of their molecular weight (Conn, 1979). For poisoning, forage containing this amount of cyanogens would have to be consumed within a few minutes and simultaneous HCN production would have to be rapid. Recorded accounts of livestock poisoning by cyanogenic plants show that such situations do arise. Cyanogens have also been suspected to have teratogenic effects (Keeler, 1984). Post-harvest wilting of cyanogenic leaves may reduce the risk of cyanide toxicity. Animals suffering from cyanide must be immediately treated by injecting a suitable dose of sodium nitrate and sodium thiosulphate.

Glycoalkaloids

Potatoes are good source of glycol-alkaloids alpha-solanine and alpha-chaconine, concentrated mainly in the flowers and sprouts (200 to 500mg/100g). For food safety purposes, an upper limit for glycoalkaloid content of 20mg per 100g of potato is generally accepted. Concentrations of glycoalkaloids are 3 to 10 times greater in the peel than in the flesh. In bitter varieties, the alkaloid concentration can go upto 80mg/100g in the tuber as a whole and up to 150-220mg/100g in the peel. At these concentrations of solanine and other potato glycoalkaloids are toxic. But because of higher temperature of decomposing (243 °C) they remain unaffected at normal cooking. To avoid toxic levels of glycoalkaloids, potato cultivar selection is very important. However, improper post-harvest handling conditions are the main cause of toxic levels in potatoes. To keep glycoalkaloid content low, store potatoes at lower temperatures, such as 7°C, keep potatoes away from light, market in opaque plastic films and paper bags, and rotate frequently on retail displays.