Panimopohjahiivojen maltoosin ja maltotrioosin kulutukseen vaikuttavat geneettiset ja ympäristötekijät: Diplomityö

The literature survey concentrated on uptake of wort sugars by brewer's lager yeast strains in brewery fermentations. The main focus was on transport of maltose and maltotriose (alfa-glucosides) into yeast across cell membrane. Discussed topics were the transport mechanism, diversity of different transporters and their properties in brewer's yeasts, and factors affecting uptake of fermentable sugars during primary fermentation of beer. Transport of alfa-glucosides is industrially relevant, because it is one of the rate limiting steps in metabolism of yeast, and fast and complete uptake is necessary for an efficient fermentation process. In the experimental part, two alfa-glucoside transporters, Malxl and Mttl of an industrial lager strain were characterized kinetically, using genetically defined laboratory strains, in which one of these is the only alfa-glucoside transporter. Affinities and maximum activities of maltose and maltotriose transport, and their temperature-dependencies, were determined by measuring initial uptake rate of radioactively labelled substrates. It was also studied, how stimulation of yeast cells with glucose prior to transport activity measurement affects the apparent transport activity. The transporter recently found in lager yeasts, Mttl, proved to be a better maltotriose than maltose transporter, and its maltose transport was less sensitive to low temperature than that of the studied Malxl transporter of the same industrial lager strain. Maltose transport of both Malxl and Mttl was clearly less temperature-dependent than that of Agtl transporter in a study Vidgren et al. (2007). The Malxl was primarily a high affinity maltose transporter in agreement with most literature reports concerning Malxl transporters of laboratory strains of S. cerevisiae. The Malxl also transported maltotriose, but with such a low affmity that it is not necessarily of importance regarding wort fermentation. Mttl is probably an important maltotriose transporter for lager yeasts in primary fermentation. Transport activity of growing laboratory yeast increased markedly with 2-10 minute incubation with glucose prior to transport activity assay, and the effect was greater with yeast suspension stored at starving conditions at 0°C. The activity decreased during storage, probably due to both inactivation of transporters and decrease in energy charge. Activity of industrially cropped lager yeast, taken from a recycled yeast storage tank, did not decrease during storage. The effect of glucose stimulation was smaller than for the laboratory yeasts studied.