Improved Fermentation Performance of a Lager Yeast after Repair of Its AGT1 Maltose and Maltotriose Transporter Genes

Abstract

The use of more concentrated, so-called high-gravity and very-high-gravity (VHG) brewer's worts for the manufacture of beer has economic and environmental advantages. However, many current strains of brewer's yeasts ferment VHG worts slowly and incompletely, leaving undesirably large amounts of maltose and especially maltotriose in the final beers. ␣-Glucosides are transported into Saccharomyces yeasts by several transporters, including Agt1, which is a good carrier of both maltose and maltotriose. The AGT1 genes of brewer's ale yeast strains encode functional transporters, but the AGT1 genes of the lager strains studied contain a premature stop codon and do not encode functional transporters. In the present work, one or more copies of the AGT1 gene of a lager strain were repaired with DNA sequence from an ale strain and put under the control of a constitutive promoter. Compared to the untransformed strain, the transformants with repaired AGT1 had higher maltose transport activity, especially after growth on glucose (which represses endogenous ␣-glucoside transporter genes) and higher ratios of maltotriose transport activity to maltose transport activity. They fermented VHG (24°Plato) wort faster and more completely, producing beers containing more ethanol and less residual maltose and maltotriose. The growth and sedimentation behaviors of the transformants were similar to those of the untransformed strain, as were the profiles of yeast-derived volatile aroma compounds in the beers.The main fermentable sugars in brewer's wort are maltose (ca. 60% of the total), maltotriose (ca. 25%), and glucose (ca. 15%). In traditional brewery fermentations, worts of about 11°P lato (°P) are used, corresponding to a total fermentable sugar concentration of about 80 g ⅐ liter Ϫ1 . Many modern breweries ferment high-gravity worts (15 to 17°P), and there are efforts to raise the concentration to 25°P, corresponding to a total sugar concentration of about 200 g ⅐ liter Ϫ1 . Industrial use of such very-high-gravity (VHG) worts is attractive because it offers increased production capacity from the same-size brew house and fermentation facilities, decreased energy consumption, and decreased labor, cleaning, and effluent costs (34,35).Whereas glucose, which is used first, is transported into yeast cells by facilitated diffusion, the ␣-glucosides maltose and maltotriose are carried by proton symporters (2, 26, 39). Maltose transport seems to have a high level of control over the fermentation rate. Thus, during the early and middle stages of fermentation of brewer's wort by a lager yeast, the specific rate of maltose consumption was the same as the specific zero-trans maltose uptake rate measured off line with each day's yeast in each day's wort spiked with [ 14 C]maltose (27). Furthermore, introducing a constitutive MAL61 (maltose transporter) gene into a brewer's yeast on a multicopy plasmid accelerated the fermentation of high-gravity worts (17). Maltotriose is the last sugar to be used in brewing fermentations, and significant amoun...

Influence of valine and other amino acids on total diacetyl and 2,3-pentanedione levels during fermentation of brewer’s wort

“…The apparent attenuation (AA; %) of the samples was estimated from the apparent and original extract (i.e. the apparent extract of the wort at the time of pitching) as described in Vidgren et al (2009). The real extract (in degree Plato) of the samples was estimated from the AE (in degree Plato) and the ethanol content ( A ABW ; % ( w / w )) using an approximation proposed by Hackbarth (2009).…”

Lager Yeast Comes of Age

“…Interestingly, AGT1 encodes a low-affinity transporter that is mutated in lager yeast and nonfunctional due to a premature stop codon. Restoration of this transporter in lager yeast resulted in improved fermentation and increased alcohol production when high-gravity wort was used (76). Lager-specific Mtt1 preferably transports maltotriose over maltose and functions better at lower temperatures than Agt1 (77).…”

Promotion of maltose fermentation at extremely low temperatures using a cryotolerant Saccharomyces cerevisiae strain immobilized on porous cellulosic material

“…The fermentation rate of maltose was always markedly below that of glucose and research efforts to improve the fermentation efficiency of yeasts have revealed that maltose transport is the major control factor over the fermentation rate [1][2][3]. Understanding and regulating maltose metabolism in brewer's and baker's yeasts (Saccharomyces cerevisiae) is of great importance both for academia and industry, since maltose, sourced from starch hydrolysis by amylases, is the main sugar involved in processes such as beer, bread, whisky, vodka, fuel and potable alcohol production, etc.…”