Abstract
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.
Original language | English |
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Qualification | Master Degree |
Awarding Institution |
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Supervisors/Advisors |
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Place of Publication | Espoo |
Publisher | |
Publication status | Published - 2008 |
MoE publication type | G2 Master's thesis, polytechnic Master's thesis |