Dissertation: Intensified maltose and maltotriose uptake by yeast accelerates high-gravity wort fermentations
Maltose and maltotriose are the two most abundant sugars in brewer’s wort, and thus brewer’s yeast’s ability to utilize them efficiently is of major importance in the brewing process. VTT’s Research Scientist Virve Vidgren has demonstrated in her doctoral dissertation that significant savings in the main fermentation time can be obtained when modified strains are used. In high-gravity wort fermentations 8–20% and in very-high-gravity wort fermentations even 11–37% time savings were obtained. Vidgren will present her dissertation at the University of Helsinki (Address: Unioninkatu 34, Helsinki) on 10 December 2010.
The increasing tendency to utilize high and very-high-gravity worts containing
increased concentrations of maltose and maltotriose renders the need for
efficient transport of these sugars even more pronounced. Residual maltose and
especially maltotriose are quite often present after high and
very-high-gravity fermentations. Sugar uptake capacity has been shown to be
the rate limiting factor for maltose and maltotriose utilization. The main aim
of the present study was to find novel ways to improve maltose and maltotriose
utilization during the main fermentation. Maltose and maltotriose uptake
characteristics of several ale and lager strains were studied. Genotype
determination of the genes needed for maltose and maltotriose utilization was
performed. Maltose uptake inhibition studies were performed to reveal the
dominant transporter types actually functioning in each of the strains.
Temperature-dependence of maltose transport was studied for ale and for lager
strains as well as for each of the single sugar transporter proteins Agt1p,
Malx1p and Mtt1p.
The results showed that ale and lager strains predominantly use different maltose and maltotriose transporter types for maltose and maltotriose uptake. Agt1 transporter was found to be the dominant maltose/maltotriose transporter in the ale strains whereas Malx1 and Mtt- type transporters dominated in the lager strains. All lager strains studied were found to possess a non-functional Agt1 transporter. Mtt1 transporter was found to be specific for lager strains. The ale strains were observed to be more sensitive to temperature decrease in their maltose uptake compared to the lager strains. Single transporters were observed to differ in their sensitivity to temperature decrease and their temperature-dependence was shown to decrease in the order Agt1≥Malx1>Mtt1. The different temperature-dependence between the ale and lager strains was observed to be due to the different dominant maltose/maltotriose transporters ale and lager strains possessed. Increasing lager fermentation temperature from traditional 10°C close to 20°C would increase uptake of maltose and maltotriose and improve the efficiency of utilization of these sugars.
Improved maltose and maltotriose uptake capacity was obtained with a modified lager strain where the AGT1 gene was repaired and put under the control of a strong promoter. Modified strains fermented wort faster and more completely, producing beers containing more ethanol and less residual maltose and maltotriose. Significant savings in the main fermentation time were obtained when modified strains were used. In high-gravity wort fermentations 8–20 % and in very-high-gravity wort fermentations even 11–37% time savings were obtained. These are economically significant changes and would cause a marked increase in annual output from the same-size of brewhouse and fermentor facilities.
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