Direct Evidence That Maltose Transport Activity Is Affected by the Lipid Composition of Brewer's Yeast

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o P), producing beers containing higher concentrations of ethanol, which are then diluted to drinking strength. These high-gravity worts often contain adjunct carbohydrates in addition to those from the barley malt. Because the adjunct carbohydrates contain a higher proportion of fermentable sugars, relatively more ethanol is produced from each degree Plato of extract. Fermenting at high gravity provides increased production capacity from the same size brewhouse and fermentation facilities and thus decreases capital investment costs. Other advantages are decreases in energy consumption and in labor, cleaning, and effluent costs (26). These advantages would be greater if very-high-gravity (VHG) worts could be used. Above about 18 o P, however, fermentation rates decrease disproportionately (i.e., it takes longer to ferment the same mass of sugar in the more concentrated wort), fermentations do not go to completion (i.e., unacceptably large concentrations of sugars, especially maltotriose, remain in the final beers, causing low ethanol yields and undesirable flavor), and the physiological condition of the yeast at the end of fermentation is poor (4,7,17,27,35). It is standard practice to crop (harvest) yeast at the end of brewery fermentations and to use this cropped yeast to pitch (inoculate) subsequent fermentations. Thus, it is important that the viability of the cropped yeast is high (Ͼ90%). Other problems in high-gravity and VHG wort fermentations include poor foam stability, possibly caused by the release of proteolytic enzymes from highly stressed yeast cells (6), haze or turbidity caused by release of glycogen from lysed yeast (18), and alterations in aroma profile. In particular, the concentration of some aroma esters in beers depends on the original gravity of the wort (20) as well as on the sugar composition (16, 33) and the ratio of total sugars to free amino nitrogen (FAN) (1, 14, 22).

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confidence: 85%

Section: Vol 76 2010 Selecting Brewer's Yeast Variants For Vhg Fermmentioning

confidence: 99%

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Selection from Industrial Lager Yeast Strains of Variants with Improved Fermentation Performance in Very-High-Gravity Worts

Huuskonen

Markkula

Vidgren

et al. 2010

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“…Zero-trans rates of ␣-glucoside uptake were determined with 5 mM substrate in 0.1 M tartrate-Tris (pH 4.2, 20°C) as described earlier (21,32), with minor modifications (12). One unit catalyzes the uptake of 1 mol of maltose (or maltotriose) per min under these conditions.…”

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confidence: 99%

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

Vidgren

Huuskonen

Virtanen

et al. 2009

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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...

“…The strongly flocculating strains T1 and T1-E were washed with a solution of ice-cold NaCl at 15 g ⅐ liter Ϫ1 (pH 3.0) instead of water. Zero-trans rates of lactose uptake were determined at 2 mM or 20 mM lactose and at 18, 20, or 30°C, essentially as described previously for maltose uptake (14,27,39 Ϫ1 for assays at 20 mM lactose). Reactions were stopped after 10 s by adding 10 ml of ice-cold water.…”

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confidence: 99%

Stimulation of Zero- trans Rates of Lactose and Maltose Uptake into Yeasts by Preincubation with Hexose To Increase the Adenylate Energy Charge

Guimarães

Multanen

Domingues

et al. 2008

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Initial rates of sugar uptake (zero-trans rates) are often measured by incubating yeast cells with radiolabeled sugars for 5 to 30 s and determining the radioactivity entering the cells. The yeast cells used are usually harvested from growth medium, washed, suspended in nutrient-free buffer, and stored on ice before they are assayed. With this method, the specific rates of zero-trans lactose uptake by Kluyveromyces lactis or recombinant Saccharomyces cerevisiae strains harvested from lactose fermentations were three-to eightfold lower than the specific rates of lactose consumption during fermentation. No significant extracellular ␤-galactosidase activity was detected. The ATP content and adenylate energy charge (EC) of the yeasts were relatively low before the [ 14 C]lactose uptake reactions were started. A short (1-to 7-min) preincubation of the yeasts with 10 to 30 mM glucose caused 1.5-to 5-fold increases in the specific rates of lactose uptake. These increases correlated with increases in EC (from 0.6 to 0.9) and ATP (from 4 to 8 mol ⅐ g dry yeast