Lukas Marthinus Du Plooy (1), Khumisho Dithebe (1), Carolina Pohl-Albertyn (1), Chantel Swart-Pistor (1), Pieter Van Wyk (1); (1) University of the Free State, Bloemfontein, South Africa

Yeast, Fermentation, and Microbiology

Fermentation by yeasts of the genus Saccharomyces lies at the heart of beer brewing, as well as many other commercial processes. Two species are of particular commercial importance, namely S. cerevisiae and S. pastorianus. S. cerevisiae is used to brew ale beer and to leaven bread, while S. pastorianus is used to brew lager beer. Many researchers consider these two well-known yeasts to be heavily influenced by humankind, as brewers and bakers throughout history continuously selected for strains with higher fermentation efficiencies. It, therefore, is thought that strains traditionally used in commercial processes ferment with a higher efficiency than strains not used in such processes. It is well-known that members of this genus ferment hexose sugars to yield ethanol and carbon dioxide (CO2) as by-products. However, the path of CO2 from formation to release remained a mystery until gas bubbles were observed in the cytoplasm of S. cerevisiae and S. pastorianus. The discovery of these bubbles provided a possible explanation concerning the fate of CO2 after formation and before release from cells. The aim of this study was to determine the conserved status of gas bubble formation in the genus Saccharomyces and to correlate the number of these bubbles with gas production to determine if bubble formation could be ascribed to CO2 production. All known species were included in the study, including species traditionally used commercially, as well as species only recently isolated from nature. The aims were accomplished by growing cells in fermentable media and recording the number of bubbles with transmission electron microscopy (TEM). Further characterization of the bubbles was achieved by viewing with light microscopy (LM) and nano-scanning Auger microscopy (NanoSAM). Cells grown in non-fermentable media were included as a control. It was found that all of the strains not generally used in commercial processes yielded fewer gas bubbles after 48 hr of growth than strains used in commercial processes. A gas production assay was performed, which suggested a correlation between gas bubble formation and gas production. It was found that non-commercial strains are not necessarily slow fermenters, but that a different fermentation profile might render them unfit for certain commercial processes. However, discovering more about the fermentation profile of strains or species poorly represented in the literature might contribute to improvements in the fermentation industry.

Lukas Marthinus du Plooy received a B.S. degree (cum laude) and a B.S. honors degree (cum laude) in microbiology from the University of the Free State, Bloemfontein, South Africa, in 2014 and 2015, respectively. He is currently pursuing an M.S. degree in microbiology at the same university, during which he will focus on changes in the ultrastructure of yeast throughout the brewing process, as well as determining the effect of vigorous fermentation on cell health. In addition to doing research, he also functions as a tutor and teaching assistant for various undergraduate microbiology modules and as an assistant for electron microscopy sample preparation.