Isolation and Characterization of Brewer's Yeast Variants with Improved Fermentation Performance under High-Gravity Conditions

Abstract

To save energy, space, and time, today's breweries make use of high-gravity brewing in which concentrated medium (wort) is fermented, resulting in a product with higher ethanol content. After fermentation, the product is diluted to obtain beer with the desired alcohol content. While economically desirable, the use of wort with an even higher sugar concentration is limited by the inability of brewer's yeast (Saccharomyces pastorianus) to efficiently ferment such concentrated medium. Here, we describe a successful strategy to obtain yeast variants with significantly improved fermentation capacity under high-gravity conditions. We isolated betterperforming variants of the industrial lager strain CMBS33 by subjecting a pool of UV-induced variants to consecutive rounds of fermentation in very-high-gravity wort (>22°Plato). Two variants (GT336 and GT344) showing faster fermentation rates and/or more-complete attenuation as well as improved viability under high ethanol conditions were identified. The variants displayed the same advantages in a pilot-scale stirred fermenter under high-gravity conditions at 11°C. Microarray analysis identified several genes whose altered expression may be responsible for the superior performance of the variants. The role of some of these candidate genes was confirmed by genetic transformation. Our study shows that proper selection conditions allow the isolation of variants of commercial brewer's yeast with superior fermentation characteristics. Moreover, it is the first study to identify genes that affect fermentation performance under high-gravity conditions. The results are of interest to the beer and bioethanol industries, where the use of more-concentrated medium is economically advantageous.Today's need to produce quality beers in a short time and in the least-expensive way has led most breweries to switch to high-gravity brewing. Traditionally, brewing worts of 12°Plato (12°P) (i.e., 12 g extract per 100 g liquid) are fermented to produce beers of 5% (vol/vol) ethanol. However, substantial cost savings can be realized by increasing the wort density (called "gravity") from 12°P to 18°P and diluting the 7.5% (vol/vol) ethanol-containing product in order to obtain beer with regular ethanol content (5%). The use of this technology increases the brewery capacity by 50% (for 18°P wort) without the need for any investments, reduces the costs in energy and labor (because liquid volumes are reduced), improves the stability of the beer, and enhances the recovery of ethanol per unit of fermentable sugar ("extract") (15, 44). However, the technology is limited by the stress resistance and fermentation performance of the brewer's lager yeast Saccharomyces pastorianus. S. cerevisiae (used for ale and wine fermentations) can cope much better with such stress conditions. Wine must typically contains up to 25% (wt/wt) sugar, but a major difference from beer production is that the yeast is only used once in wine fermentation whereas it is used multiple times in beer fermentation. In addition, wine m...

Effects of fermentation temperature on the composition of beer volatile compounds, organoleptic quality and spent yeast density

“…Previous studies have shown a decrease in cell density as a result of decrease in fermentable sugars present in the wort. The decrease in cell viability with time has also been attributed to nutrient depletion and early entry of the organisms into the death phase (Blieck et al 2007). …”

“…The 30ºC fermentation resulted in the lowest spent yeast density production of both strains with a 25.50% and 32.06% reduction in spent density produced compared to the room temperature fermentations for strain A and strain B, respectively. The reduction of spent yeast density and viable yeast population after fermentation at 30ºC could be attributed to increased metabolic rate at this higher temperature which could have led to faster utilization of sugars, and resulting in cell starvation, cell death and autolysis (Blieck et al 2007). …”

“…The spent yeast generated during the fermentative process is often used as an inoculum for subsequent fermentations (Blieck et al 2007). In addition, yeast cell wall fractions contain a large percentage of β-glucans, which is highly advantageous in improving the physical and functional properties of foods, as a thickening and waterholding agent (Thammakiti et al 2004) and for the gelatinization and retrogradation of starch (Satrapai and Suphantharika, 2007).…”

Stress responses in brewing yeast.

“…Houbové adenylátcyklázy fungují jako senzory CO 2 . Různé koncentrace CO 2 mohou ovlivnit fermentaci, protože existuje přímé propojení adenylátcyklázové aktivity a glykolýzy [39]. Vysoké hladiny CO 2 indukují u nerostoucích pivovarských kvasinek stresové odpovědi.…”

Estudio De Los Espectros Dielectricos en Alimentos Con Estructura Coloidal

“…However, its industry applies new technical, genetic and microbiological advances in order to improve this process. Among others, it can be cited the highgravity beer fermentation (Piddocke et al, 2009), immobilized yeast bioreactor (Tata et al, 1999) and genetic modifications of yeast strains to change some parameters of the final beer (Blieck et al, 2007;Nevoigt et al, 2002).…”