Innocent-Ukachi, Adanma Chinedum
(2016)
Microbial population dynamics and impact on hydrolysis of phytate and phenolic compounds during fermentation of ogi-an indigenous fermented cereal product.
PhD thesis, University of Nottingham.
Abstract
Ogi is a fermented food made from maize, sorghum or millet which serves as complementary food for infants and breakfast for adults in Nigeria, West Africa. This study characterized the microbial diversity of maize and sorghum grains and ogi produced by their natural fermentation in an attempt to understand the roles of the key microbial species and the impact of the population dynamics and selected species on changes in nutritional composition and aroma notes of ogi during fermentation. A combined approach of culture dependent and culture independent methods of analysis was applied to investigate the microbial community of grains and ogi from two different sources. Microbial diversity and viable populations varied with the source of the grain. Bacterial and fungal genera identified with the partial 16S rRNA and 26S rRNA sequence analysis respectively in maize and sorghum were Bacillus, Enterobacter, Micrococcus, Kytococcus, Pantoea, Staphylococcus, Amycolatopsis, Methanoculleus, Aspergillus, Penicillium, Eupenicillium, Acremonium, Schizosaccharomyces, Meyerozyma, Hyphopichia, and Pichia in maize grains; Enterococcus, Enterobacter, Pantoea, Bifidobacterium, Aspergillus, Cladosporum, and Penicillium in maize ogi; Enterococcus, Enterobacter, Pantoea, Aeribacillus, Cyanobacterium, Acinetobacter, Fusarium and Trametes in sorghum grains; and Pediococcus, Lactobacillus, Enterococcus, Bacillus, Cladosporum and Penicillium in sorghum ogi. Similar species were observed in both sources of maize while those of sorghum differed slightly. Predominant microbes included species of Enterobacteriaceae and moulds. Acetic acid bacteria were not identified as part of the diverse community. Following the predominance of moulds during the natural fermentation, preliminary screening was performed by PCR using specific biosynthetic gene primers to test whether they are the mycotoxin producing species. None of the genes tested were detected by PCR thus they may not be the toxin producing species.
Starch, non-starch polysaccharide (NSP), phytate and phenolic compounds were determined in the grains and respective ogi to ascertain the levels of these nutritionally important components in the naturally fermented ogi and the impact of the varying microbial populations on the fate of these compounds during fermentation. In the grains, the average starch and NSP contents in each case were 80.35 g/100g and 9.40g/100g in maize and 93.12 g/100g and 8.14 g/100g in sorghum. Out of the total in grain the average percentage recovery of starch and NSP respectively in the ogi showed 63% and 42% in maize and 58% and 27% in sorghum. Maize showed good starch and fibre (NSP) retention than sorghum after fermentation. To further understand the types and levels of polymers in NSP hydrolysis in ogi fermentation, HPLC analysis of the hydrolysed extract was performed. Glucose was entirely present in maize and sorghum ogi which represents the beta-D-glucans while arabinose and xylose (in maize only), mostly lost with the pomace, signify the arabinoxylans. Overall variations in the microbial populations of sorghum seemed causal to the difference in starch and NSP recoveries.
Phytate was assessed based on release of total phosphorus in the samples by enzymatic and chemical methods. Recovery of phytate in the naturally fermented ogi ranged from 18-25% in maize and 40-48% in sorghum suggesting greater phytase activity and more nutrient bioavailability in maize ogi than in the sorghum. Greater activity in maize reflects the presence of phytate hydrolysing species such as Aspergillus in the grain. Total phenolic content (TPC) was assessed by Folin-Ciocalteu colorimetric method after direct extraction of samples by saponification. TPC in the original grains ranged from 410–437 mg GAE/100g in maize and 221–247 mg GAE/100g in sorghum. Due to the nutritional significance, the amount of phenolics that are either freely soluble or are covalently bound to the food matrix were assessed. Soluble phenolics in ogi ranged from 16-38% in maize and 32-49% in sorghum based on the total soluble fraction in the original grain. In all cases loss of soluble phenolics with the waste waters accounted for 12-25% and 31-39% with the pomace. Only the LAB population seemed to correlate with the release of phenolics in the natural fermentation. Given the higher value of soluble phenolics, naturally fermented sorghum ogi appeared to have higher antioxidant potential than the maize ogi.
Furthermore an attempt was made to ascertain whether the use of selected microbes would improve the antioxidant properties and aroma of ogi while minimizing the incidence of pathogens due to chance inoculation. Thus the impact of selected LAB (Pediococcus pentosaceus) and fungi (T. hirsuta and A. zeae previously shown to have phytase activity) on changes in phytate, phenolics and aroma of ogi was assessed following a parallel experiment to the previous study but using autoclaved grains. Five fermentation treatments of the pure and co-cultures were investigated. Cell populations in all culture fermentations varied and reached the average maximum of log 6-9 cfu/ml. Changes in the distribution of bound and soluble phenolics were observed showing esterase activity. Leaching of phenolics was evident in all cases but was higher in the sorghum fermentations. Higher levels of soluble phenolics were recovered in pure culture fermented ogi using T. hirsuta or P. pentosaceus than in the natural fermentation having 76% and 45% of the original soluble fraction in maize and sorghum respectively. This suggests greater antioxidant potentials than the naturally fermented ogi. Pure culture fermentations using T. hirsuta and co-culture of P. pentosaceus with A. zeae reduced phytate by 97% and 96% in maize and sorghum ogi respectively showing greater phytase activity and more nutrient bioavailabilty in the ogi than in the natural fermentation.
The aroma profile of ogi was analysed by solid-phase microextraction and gas chromatography-mass spectrophotometry (SPME GC-MS). Ethyl acetate, butyl acetate and ethyl hexanoate were observed as the key active aroma components in ogi. The ester, methyl thiobutanoate was found to be unique to the naturally fermented ogi suggesting that it may have been generated by species other than the selected starter organisms. Overall in both natural and starter culture fermentations, maize ogi showed high relative abundance of volatile components suggesting good substrate compatibility and utilization during fermentation. Thus compounds with high threshold values may be significant in the aroma notes of maize ogi. P. pentosaceus and T. hirsuta in pure and in co-culture fermentations produced ogi with aroma notes mostly related to the naturally fermented product.
In conclusion the diversity and levels of the initial microflora and the structural composition of grain could be major factors contributing to the nutritional compositional changes in ogi fermentation.
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