Very high gravity sucrose fermentation by Brazilian industrial yeast strains: effect of nitrogen supplementation

ABSTRACT There are economic and other advantages if the fermentable sugar concentration in industrial brewery fermentations can be increased from that of currently used high-gravity (ca. 14 to 17ï¿½P [degrees Plato]) worts into the very-high-gravity (VHG; 18 to 25ï¿½P) range. Many industrial strains of brewer's yeast perform poorly in VHG worts, exhibiting decreased growth, slow and incomplete fermentations, and low viability of the yeast cropped for recycling into subsequent fermentations. A new and efficient method for selecting variant cells with improved performance in VHG worts is described. In this new method, mutagenized industrial yeast was put through a VHG wort fermentation and then incubated anaerobically in the resulting beer while maintaining the α-glucoside concentration at about 10 to 20 gï¿½liter−1 by slowly feeding the yeast maltose or maltotriose until most of the cells had died. When survival rates fell to 1 to 10 cells per 106 original cells, a high proportion (up to 30%) of survivors fermented VHG worts 10 to 30% faster and more completely (residual sugars lower by 2 to 8 gï¿½liter−1) than the parent strains, but the sedimentation behavior and profiles of yeast-derived flavor compounds of the survivors were similar to those of the parent strains.

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

Applied and Environmental Microbiology ◽

10.1128/aem.01558-08 ◽

2009 ◽

Vol 75 (8) ◽

pp. 2333-2345 ◽

Cited By ~ 48

Author(s):

Virve Vidgren ◽

Anne Huuskonen ◽

Hannele Virtanen ◽

Laura Ruohonen ◽

John Londesborough

Keyword(s):

Stop Codon ◽

Premature Stop Codon ◽

High Gravity ◽

Transport Activity ◽

Lager Yeast ◽

Very High Gravity ◽

Maltose Transport ◽

Yeast Strains ◽

Brewer’S Yeasts ◽

Very High

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. α-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 α-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.

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