Isolation and Development of Yeast Strains as Baker’s Yeasts from Fermenting Cassava Liquor

Author: Nwachukwu, Ugumba Nwachukwu

Supervisor: Olatunji F. O.

Yeasts were isolated from fermenting cassava liquor, screened and developed as baker’s yeasts. Appropriate storage method which would maintain the availability and characteristics developed in the yeasts were also investigated. Identifications of the iasolates were carried out and these were later confirmed by the National Collection of Yeast Cultures (NCYC) as candida sake (NCYC D116), C. krusel (NCYC D119C), C. krusel (NCYC D9115), Rhodotorula rubra (NCYC D119p), and saccharomyces cerevisiae (NCYC C271). The non-pathogenic yeasts, c. sake and s. cerevisiae were found to ferment glucose, fructose, sucrose and maltose with reasonable co2 yield. These organisms also grew in a vitamin-free medium. Based on these properties which are expected of a baker’s yeast, the organisms were selected and further developed as baker’s yeasts. Of the five natural media experimentally formulated, viz; orange broth, banana broth, cane molasses, fortified cane molasses (FCM), and corn steep liquor extract, only the latter did not support good biomass productions of either the cassava yeasts or the standard baker’s yeast, when compared with the imported Oxide Sabouraud dextrose broth (SDB), a rich medium for yeast propagation. Therefore for commercial production of both cassava yeasts and standard baker’s yeast, FOM which supported optimum biomass production would serve as a good local substitute. The sugar utilization abilities of the yeasts were improved by ‘’training’’ prototypes from 6% or less optimum to 8% w/v. fermented products by the yeasts were analysed and theses included amongst others, carbon dioxide, alcohol, acids and biomass which consistently increased unlike the carbohydrate levels which decreased over the period during propagation of the yeasts in SDB or FCM. Furthermore, carbon dioxide level was highly and positively correlated with alcohol concentration at 95% level. The performances of the three yeasts in wheat or corn dough containing 7.5% w/v glucose, maltose, fructose, or granulated market sugar (GMS) were determined. Results showed that the ‘’trained’’ cassava yeasts competed favourably with the standard baker’s yeast in respect of the leavening profiles obtained. Further comparison was made of the leavening profiles obtained in wheat dough and in corn dough using the standard baker’s yeast or the ‘’trained’’ cassava yeasts. Results showed that there were significant differences at 95% level. When the leavening experiments were repeated with locally processed wheat flour for comparison with the imported wheat flour, results showed that there were no significant differences in the two wheat brands in their leavening profiles and nutritional compositions including gluten levels. Synchronous culture fermentations were carried out on the selected yeasts to determine any improvements on the yeasts for baking process. Results indicated some advantages over asynchronous culture fermentations and these included greater biomass and carbon dioxide yield, higher leavening profiles and reductions in lag phases. The effects of dough structure-disequilibrium experiments on retention of gas (carbon dioxide) evolved during wheat or corn dough fermentation revealed that the gas liberated as a fermentation product as well as the physical structure of the dough was responsible for the rising of the dough. Treatment of the ‘’trained’’ cassava yeasts with ethylmethyl sulphonic acid suppressed some morphological characteristics of the yeasts unlike exposure to uv radiation which resulted in exaggeration of some phenotypically manifested morphological characteristics. Therefore, treatment of the ‘’trained’’ cassava yeasts with ethylmethyl sulphonic acid suppressed some morphological characteristics of the yeasts unlike exposure to UV radiation which resulted in exaggeration of some phenotypically manifested morphological characteristics. Therefore, treatments of the ‘’trained’’ cassava yeasts with UV radiation produced mutant strains with improvement in respect of biomass and carbon dioxide yields hence optimisation of the leavening profiles. String of the ‘’trained’’ cassava yeasts on PDA containing 7% w/v dextrose at 50C maintained the stability and responsibility of the characteristics developed in the yeasts and also improved further the fermentative abilities of the yeasts unlike storage on 2% w/v or 10% w/v dextrose. The results of test production of bread samples on commercial plant using the developed cassava yeasts and the standard baker’s yeasts at reference showed lack of significant differences in the qualities and characteristics of the loaves such as ‘’oven spring’’, crust crumb structure, consistency index, appearance and other organoleptic properties, consistency index, appearance and other organoleptic properties. The implications of these observations were discussed.