Microbial succession and potential cyanide-degrading starter culture isolated from retted cassava

Cassava-derived products are widely consumed across the globe. Cassava, in its natural state, consists of high cyanide content, which is usually degraded during fermentation. Therefore, consumers of inadequately processed cassava products are at risk of cyanide exposure, which is highly toxic to human health. Accordingly, this study investigated the microbial succession of spontaneously fermented/retted cassava tubers to select a promising starter culture with cyanide-degrading potential. Fermentation, isolation, characterization, and tentative identification were done using standard microbiological methods involving the use of Nutrient Agar (NA), MacConkey Agar (MAC), DeMan Rogosa and Sharpe Agar (MRS), and Yeast Extract Agar (YEA). Cyanide content was assessed using the alkaline picrate method. Microbial counts ranged from 3.9x10⁵ – 1.6x10⁷; 1.0x10⁷-8.3x10⁹; 1.4x10⁵-4.1x10⁷ were recorded for Enterobacteriaceae, lactic acid bacteria, and yeast, respectively. The pH and titratable acidity also ranged from 7.1-5.3 and 0.0042-0.019, respectively. Additionally, tentative identification of isolated microorganisms revealed Staphylococcus aureus, Bacillus sp., Corynebacterium sp., Shigella sp., Klebsiella sp., Pseudomonas sp., Lactobacillus sp., Lactococcus sp., Saccharomyces cerevisiae, Escherichia coli, Leuconostoc sp., Enterobacter sp., Geotrichum sp. and Candida sp. However, the highest survival rates throughout the spontaneous fermentation process were 18.10%, 10.95%, and 6.67% for organisms identified as Lactobacillus sp.2, Candida sp., and Saccharomyces cerevisiae, respectively. Furthermore, Lactobacillus sp. 2 survived throughout the fermentation process, and tested negative in pathogenicity assays.. Although, the spontaneous fermentation process was terminated after 72 hr, a 2.85 µg/mL of cyanide was recorded at 72 hr during the controlled fermentation, which was significantly reduced to 0.06 µg/mL at 120 hr. In conclusion, the findings suggest that Lactobacillus sp. 2 is a promising organism that may be employed for cyanide degradation, making it a potential starter culture for the fermentation of cassava.