Effect of post-harvest handling on mycotoxin levels in soybeans from Rwanda and processing effects on nutritional value and acceptability of soymilk

Abstract

Soybean is considered to be a critical food and nutritional security crop in Rwanda. Soybean is largely consumed in such processed forms as soymilk and soy bean curd. One of the hindrances to increased soymilk consumption is the development of an unpleasant beany flavour due to oxidation of soybean lipids by soybean lipoxygenase. Several methods have been reported for processing soymilk, but their effects on nutritional and phytochemical quality are not well established. Other problems associated with soymilk consumption include the occurrence of flatulence in some individuals, due to the fermentation of soybean oligosaccharides by gut bacteria, and the potential susceptibility of soybeans to mycotoxin contamination. Thus, the objectives of this study were to determine (i) the level of mycotoxin contamination in soybeans from different production regions in Rwanda, and the influence of postharvest handling methods on the mycotoxin levels (ii) the effect of three common processing methods on nutrient and isoflavone content of soymilk (iii) the growth of different probiotic bacteria in soymilk and their effect on antioxidant activity and oligosaccharides content and (iv) compounds of five essential oils from cinnamon, basil, citronella, mint and eucalyptus by GC-MS and their effect on fungal growth and sensory acceptability of soymilk. Soybean samples were collected from farmers (n=300). The farmers also completed questionnaires about their preharvest farm practices, post-harvest farm practices, and aflatoxin awareness. Aflatoxin content in the soybean samples was determined by enzyme-linked immune sorbent assay (ELISA). Detection and quantification of other mycotoxins was done by liquid chromatography-mass spectrometry (LC-MS). Only 7.3 % of the respondents were found to have some knowledge of aflatoxin contamination, indicating a low level of awareness among the farmers. Despite the low levels of awareness on mycotoxins, the farmers were found to observe good postharvest practices including harvesting the crop when the pods were dry. Consistent with this, there was low contamination of the soybeans with aflatoxins and other mycotoxins. Only one sample (0.33%) had an aflatoxin concentration that exceeded the most stringent EU maximum permitted limit of 4 µg/kg. Moreover, apart from one sample contained 13µg/kg of sterigmatocystine, the rest of the samples had very low or undetectable levels of other mycotoxins. From these results, it can be concluded that, based on continued good pre- and postharvest practices, soybean can be promoted as a food product with low risk for mycotoxin contamination in Rwanda. The methods of processing involved (i) soaking soybeans (M1), (ii) blanching soybeans (M2) and (iii) soaking and cooking soybeans prior to soymilk extraction (M3). M1 was found to produce soymilk with significantly higher nutrient and isoflavone extraction than M2 and M3, and can be recommended for soymilk production. Sc. Squire variety has advantage of higher isoflavone content than the other varieties. The growth of seven probiotic lactic acid bacteria (LAB) attained around 8 log CFU/ml, which is sufficient for probiotic effects. However, only L. reuteri, L. brevis and L. plantarum induced a sufficient drop in pH and imparted a viscosity increase characteristic of a good fermented product. Fermentation with these LABS reduced the oligosaccharide content, and may therefore reduce the flatulence associated with unfermented soymilk. The volatile flavor components of the essential oils of five spices, namely mint, basil, cinnamon, eucalyptus and citronella were analyzed by gas chromatography-mass spectrometry(GC-MS).The Minimum inhibitory concentrations (MICs) of the essential oils were tested on the fungus. Cinnamon, with MIC of ≤0.1 µl/ml was the most effective at inhibiting fungal growth, while eucalyptus, with MIC >>10 µl/ml was the least effective. Sensory evaluation of soymilk flavored with the essential oils of citronella, basil and mint at different concentrations was done. The mint-flavored soymilk had the most preferred flavor, followed by citronella- and basil-flavored soymilk. Thus, essential oils can be used for improving acceptability of soymilk. In conclusion, soybean grown in Rwanda was found to be low in mycotoxin contamination. However, there is a need to educate farmers on mycotoxin contamination in food and feed. Soymilk extraction after soaking but not heating soybeans affords better nutrient and isoflavone extraction than methods involving heating before extraction, and soymilk quality can be improved by fermentation or the use of essential oils such as mint, citronella and basil. Breeders should develop varieties of high yield of soymilk and high nutrients contents for value-added soybean milk both at the household level and industrial level.