Masters Degrees (Microbiology)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/3626

Browse

Browsing Masters Degrees (Microbiology) by Author "Blaauw, Domecia"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Crossbreeding of natural Saccharomyces cerevisiae strains for enhanced bio-ethanol production
    (Stellenbosch : Stellenbosch University, 2015-12) Blaauw, Domecia; Jansen, T.; Viljoen-Bloom, M.; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: The fluctuating fuel prices, possible future shortage of fossil fuels, the increasing demand and the negative impacts on the ecosystem have all contributed to the search and development of alternatives during the last two decades. Biofuels, bio-ethanol in particular, is a reliable substitute for fossil fuel (petroleum) and can be produced from inexpensive, non-edible feedstock such as lignocellulosic biomass. Lignocellulose is, however, a problematic substrate as the hydrolysis results in inhibitor formation that hinders the fermentation ability of the fermenting microorganism (Saccharomyces cerevisiae yeast strain). The problem can be circumvented by the construction of robust S. cerevisiae strains that can withstand the effect of inhibitors in addition to exhibiting fermentation vigour, ethanol tolerance, inhibitor tolerance, osmotolerance and thermotolerance. In this study, four natural strains of S. cerevisiae (HR14, YI64, YI2 and MF15) with different superior characteristics (fermentation vigour, inhibitor-, osmo-, thermo- and ethanol tolerance) were selected for mating experiments to generate hybrid progeny with superior traits. The HO gene of the diploid homothallic yeast strains was disrupted to produce haploid heterothallic strains. Haploid strains with the opposite mating-types and displaying different characteristics were mated to produce hybrid strains with combined / superior characteristics. Six hybrid strains (YH1, YH2, YH3, MY3, MY5 and MY7) were selected for the screening process. The parent and hybrid strains were screened for fermentation vigour, ethanol tolerance, inhibitor tolerance, growth at temperatures above 30°C and osmotolerance. The YH3 and MY5 hybrid strains displayed the highest fermentation vigour (productivity) of the hybrid strains and were able to consume all available glucose (200 g/L) and produce approximately 100 g/L and 81 g/L ethanol, respectively. These hybrid strains did however, not display superior fermentation abilities when compared to the parent YI64 and YI2 strains as these strains produced the same amount of ethanol during the fermentation trials. The MY5 hybrid exhibited an inhibitor tolerance, similar to the MF15 parental strain in the presence of 25% inhibitor cocktail. The HR14 and YI64 parental strains and their YH1, YH2 and YH3 hybrid strains were unable to grow and ferment in the presence of 25% inhibitor cocktail. None of the strains was able to grow and ferment in the presence of 10% ethanol. Some inherited characteristics (fermentation vigour and inhibitor tolerance) of the hybrid strains were not superior to that displayed by the parental strains. The inherited osmotolerance and thermotolerance were, however, superior to that displayed by the parent strains as the best performing hybrids managed to grow at 43°C and grew slightly faster than the parent strains in the presence of 65% glucose. The mating experiments yielded hybrid strains with combined characteristics such as fermentation vigour, inhibitor tolerance, osmotolerance and thermotolerance. Mating of yeast strains to combine and generate superior traits in the progeny is thought to be the best method to use. Hybrid strains generated during this method are produced through minimum gene manipulation. The use of these strains in the production of bio-ethanol should not cause public concerns nor should it infringe on legislation. The mating experiments can be followed by an adaptation to inhibitory compounds, as the hybrid strains in this study were slightly more tolerant to ethanol during the fermentation trials after adaptation.