Developing starter cultures for the optimisation of cassava (Manihot esculenta Crantz) fermentation
The overall objective of the study was to optimise natural fermentation of cassava through development of starter cultures of lactic acid cassava bacteria (LAB) and softening bacteria to address quality variations of the end product, in terms of acidification and softening inconsistencies. Several natural fermentations of cassava under various conditions were carried out to study the effects of time, temperature, substrate composition (cassava skin and leaves) on pH, cassava softening and microbial growth. Fermented cassava pieces were examined to detect reported bacteria (Bacillus, Clostridium spp.) associated with softening by conventional cultural and non-cultural methodology; to detect pectinolytic activity by viscometry and dinitrosalicylic acid reagent colorimetric methods; to evaluate a-glucuronase and ~-glucosidase activities by API Zym tests. For selection of starter cultures study: LAB (33), non- LAB (55), and odour producers (three) were isolated at different stages of natural fermentations. The isolates were identified by phenotyping and genotyping using PCR amplification of the 16S-23S rDNA intergenic transcribed spacer (ITS-PCR), repetitive sequence-based PCR (rep-PCR) and DNA sequencing. Out of 33 LAB isolates, 13 representative strains were selected and further characterised for the desirable properties of starch fermentation, initiation of growth at different pH values, rapid acidification, antimicrobial & linamarase activities, degradation of oligosaccharides and probiotic activities. Non lactic acid isolates (55) were individually tested for the desirable property of causing reproducible softening by discriminatory tests. Representative softening isolates (13) and odour producer (1) were selected and further characterised for sugar fermentation by API CHB tests, pectinolytic activity by viscometry and colorimetric methods & detection of metabolitic activities by High Performance Liquid Chromatography & Gas Chromatography. Fermentations using starter cultures of various combinations using 13 LAB, 12 softening and one odour producer bacteria were developed and assessed by studying their interactive effects on pH, softening, colour and odour of cassava. Thirteen LAB of nine species, 12 softening bacteria of three species and one odour producer were selected for starter culture development. The study highlighted technological issues and the significance of culture inoculation concentration when using mixed cultures. The study demonstrated possibilities to develop potential starter cultures of LAB, softening bacteria and odour producer strains, using various combinations to produce cassava products of variable desirable attributes.
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