Though peanut is considered a wholesome food rich in proteins, fats and many nutrients vital to the body, it was shunned for some time by the international buyers, because of the presence of mycotoxins, the hazardous metabolic product of the mold belonging to Aspergillus species which contaminates the crop mostly in the field due to improper drying. Its good thermal stability at high temperatures make it hard to destroy during processing and mold infected raw material if used will taint the final product also. Non-availability of an acceptable process to decontaminate peanuts and bring down the aflatoxin content to safe levels is a constraint coming in the way of development of this agricultural crop, competing effectively against soybean, considered its rival in the international market. While decorticated nut is a versatile food material for humans both for direct consumption as well as for providing edible oil, the protein rich deoiled residues are much sought after as an ingredient for animal feeds. In contrast soybean is mostly used for oil extraction, soy flour and processed products based it and as a component in animal feeds.
Peanut seeds can be blanched to remove cuticles if white kernel is desired or can be roasted, boiled, fried or coated to get different consumer products. Peanut butter, a highly popular product in western countries is made by grinding decuticled and roasted kernels through a process that ensures no separation of oil during packing and storage. Nutritionally shelled peanut seeds contain good quality oil to the extent of 50% while its protein content is around 25%. Bold seeds are in good demand and hand picked bold seeds fetch premium price in the export market. In India peanuts are processed in oil mills using ghani type rotary presses, screw expellers and solvent extraction plants to recover as much oil as possible. Since oil recovery is the major goal, very little attention is paid to the quality of the deoiled residue which generally contains high levels of husk and cuticles besides other undesirable contaminants. Edible quality peanut cake can be made if the seeds are precleaned before pressing for oil and during nineteen fifties and sixties, peanut protein isolate made from the deoiled cake was touted as a concentrated protein source with 90-93% protein content. Deficiency of amino acid lysine is considered as a drawback when peanut protein isolate is compared with that from soybean. Even a milk like product was developed from the peanut 4 decades ago in India as a cheap substitute to dairy milk.
Aspergillus flavus and Aspergillus parasiticus, the two organisms that grow on moist peanut seeds containing more than 7% moisture, excrete aflatoxins of different types which if consumed have been found to cause acute hepatic necrosis, liver cirrhosis and eventually carcinoma. There are 13 different types of aflatoxins identified so far and the most potent is aflatoxin B1. Global standards do not permit aflatoxin presence in peanut or any preparations derived from it beyond 4-20 parts per billion (ppb) and unless very high precaution is taken it will not be possible to restrict the aflatoxin to the stringent standard that exists to day. However in a country like USA, aflatoxin up to 300 ppb is permitted in domestic corn for feeding beef cattle, swines and mature poultry birds. It was in 1960 that the first out break of aflatoxin poisoning was encountered in UK killing more than 1 lakh turkey poults and subsequently several such incidences were reported from different parts of the world due to feeding aflatoxin contaminated feeds.
Aflatoxins belong to the difurocoumarolactone group of chemicals with molecular weights ranging from 312 to 346 with melting points in the range of 190-299C. Neither heat processing as encountered in frying or baking nor any other known method can destroy the toxin very significantly. Toxins present in oil fraction are not affected by the conventional alkali refining process but special filtration system can remove the particulates of aflatoxin satisfactorily. Ammoniation (0.5-2% ammonia) at room temperature or at elevated temperature and pressure can bring down the toxin level considerably but whether it can be deployed commercially is not certain. Use of Flavobacterium auranatiacum bacteria to destroy aflatoxin has been found to be technically feasible but the limitation is that such biological system can work only in aqueous medium calling for expensive process technologies and long processing time. Gamma irradiation at a level of 5-20kGy dose has been reported to be effective in detoxification but limited facilities available for irradiation makes this technology impractical on a large scale.
In spite of 4 decades of scientific endeavor a satisfactory method of detoxification is still eluding the peanut industry. Preempting development of mold infection through prompt drying of the crop to less than 7% moisture, careful handling and storage at low temperature and humidity may be the best option. Use of electronic sorters to sort the seeds based on color development caused by the growth of the mold was explored without success. Considering the practical constraints in changing the prevalent agricultural practices, the best way to eliminate aflatoxin problem seems to be manual sorting of the seeds which has been accepted by many importing countries.