Inhibition of the lactose to ethanol fermentation of Kluyveromyces marxianus Y113 and attempts at its alleviation through media improvement : a thesis presented to Massey University in fulfillment of the thesis requirement for the degree of Master of Technology
Inhibition of the lactose to ethanol fermentation of K.marxianus Y113 was investigated. The use of initial lactose concentrations of 150 g/litre or greater resulted in less biomass accumulation, lower ethanol productivity and incomplete substrate utilisation. Keeping the initial lactose concentration at 100 g/litre but increasing the medium osmolality by up to 5 times via the addition of non-utilised salt or maltose resulted in substantially reduced biomass accumulation and slightly lower ethanol productivity. This suggested that high medium osmolality inhibits the yeast in a non-specific way by increasing the energy required for cell maintenance at the expense of biomass production. Keeping the intial lactose concentration at 100 g/litre but adding up to 5% (by weight) ethanol reduced the amount and rate of biomass accumulation and led to incomplete substrate utilisation, as well as dramatically lowering the amount of ethanol produced by the least itself. The detrimental effects of added ethanol became significant only when more than 2 to 3% (by weight) was added. A maximum alcohol concentration of 4 to 5% (by weight) was observed in all cases, irrespective of the concentration of ethanol added initially. These results suggested that the ethanol inhibited the energy metabolism of the cell in some specific way and did not merely increase the requirement for cell maintenance energy. In the concentrations tried supplementation of the medium with yeast extract, magnesium, calcium and chitin all failed to produce any change in the performance of the fermentation. Supplementation with still bottoms was found to be quite strongly inhibitory to the fermentation. Demineralisation of the whey permeate medium reduced that the performance of the fermentation compared to that carried out on standard whey permeate medium. K.marxianus Y113 was able to ferment a medium of defined composition but the biomass growth, ethanol productivity and lactose utilisation were not as good as those achieved using complex media such as whey or lactose broth. increasing the concentration of nutrients in the defined medium was of small benefit but the performance was still well below that seen on complex media.