The dominant microflora and their role in the fermentation of 'agbelima' cassava dough

Abstract

Agbelima, a fermented cassava meal widely consumed in Ghana, Togo and Benin is produced by fermenting grated cassava with one of several types of traditional cassava dough inoculum. During fermentation a smooth textured sour dough is produced, the toxicity of cassava is reduced and there is a build up of volatile aroma compounds. The microflora of four different types of traditional inocula and inoculated fermenting cassava dough were examined in order to determine the microbial and enzymatic activities responsible for the breakdown of cassava tissue, acidification/souring of the dough, development of aroma and detoxification of the product during fermentation. Titratable acidity expressed as lactic acid increased from 0.31-0.38 'x, to 0.78-0.91 % (w/w) in all dough confirming the fermentation to be a process of acidification. The microflora of all inocula were dominated by lactic acid bacteria present at levels of log to 109 cfu/g. In two types of inocula the dominance was shared with Bacillus spp. present at levels of 107 - log cfu/g. In a third type of inoculum Bacillus spp. were present at lower levels of 106 - 107 cfu/g. In a forth type of inoculum, Bacillus spp. were either not isolated or present at extremely low levels but the dominance of the microflora by lactic acid bacteria was shared with moulds. Yeasts were found present in high numbers of about 107 cfu/g in all four types of inocula. Bacillus species mainly Bacillus subtiIis, the moulds Penicillium sclerotiorum, Penicillium citrinum, Geotrichum candidum and the yeasts Candida tropicalis and some strains of Zygosaccharomyces present in the different types of inocula were found to be responsible for the breakdown of cassava tissue during fermentation through cellulase activity. All these microorganisms demonstrated cellulase activity and were also able to disintegrate cassava tissue when plated directly unto sterile cassava slices. When cassava pieces were incubated in tissue degrading enzymes, amylase, cellulase, pectinase, pectin esterase and polygalacturonase solutions, only pieces in cellulase solution were dissolved. The lactic acid bacteria were found to be responsible for the souring of agbelima through the production of lactic acid and were present in all fermenting dough at levels of 10⁷-1O⁸,l0⁸ -10⁹ and 10⁹cfu/g at 0, 24 and 48 hr. Lactobacillus plantarum was the dominant lactic acid bacteria found in all inocula and fermenting dough accounting for 51% of 171 representative isolates taken from various stages of fermentation. Other major lactic acid bacteria found were Lactobacillus brevis, 16%, Leuconostoc mesenteroides, 15%, and some cocci including Streptococcus spp. whose numbers decreased with fermentation time. The dominant species of lactic acid bacteria, yeasts and moulds as well as Bacillus pumilus, B. licheniformis and B. amyloliquefaciens demonstrated Iinamarase activity, the enzyme required for the breakdown of cyanogenic glucosides during cassava detoxification. Wide variations were observed in the total cyanide levels of fresh cassava tubers examined, 56.8 to 369.2 mg/kg, and substantially reduced levels were found in the fermented products, 50.7 to 51.3 mg/kg. Lower levels of total cyanide were found in cassava dough fermented with inoculum than dough fermented spontaneously and the enhanced detoxification was attributed to the ability of inoculum to break down the texture of cassava resulting in more intimate contact between endogenous linamarase and the cyanogenic glucosides. The aroma profile of the product did not vary with the type of inoculum used and was attributed to the uniformity of the yeasts and lactic acid bacteria flora of all inocula which were considered to be responsible for the aroma compounds produced. The aroma profile of all four types of agbelima were dominated by a non-identified low molecular weight alcohol, I-propanol, isoarnylalcohol, ethylacetate, 3-methyl-l-butanol and acetoin