Study of Two Pools of Glycogen in Saccharomyces cerevisiae and their Role in Fermentation Performance

The Improvement of Reserve Polysaccharide Glycogen Level and Other Quality Parameters of S. cerevisiae Brewing Dry Yeasts by Their Rehydration in Water, Treated with Low-Temperature, Low-Pressure Glow Plasma (LPGP)

The increasing popularity of active dry yeast arises from its properties, such as ease of storage, and simplicity of preparation and dosing. Herein, we elaborate on the effect of plasma-treated water (PTW) under air atmosphere (PTWAir) and nitrogen (PTWN) on the improvement of the reserve polysaccharide glycogen level and other quality parameters of S. cerevisiae brewing dry yeast in comparison with the non plasma-treated water (CW). For this purpose, strains of top-fermenting (S. cerevisiae T58 (poor quality), S33 (poor quality)) and bottom-fermenting (S. pastorianus W30/70 (poor quality)) yeast stored one year after opening and S. cerevisiae US-05 (fresh strain) were selected to examine the influence of PTWs toward the quality parameters of yeast biomass after the rehydration and fermentation process. The obtained results showed that in the case of poor quality yeast strains, PTWAir increased glycogen content after the rehydration and fermentation process, which was a favorable trend. A similar increase was observed for the trehalose content. Results showed that PTWN significantly reduced the number of yeast cells in ale strains and the viability of all analyzed samples. The lowest viability was observed in Sc S33 strain for PTWAir (41.99%), PTWN (18.6%) and CW (22.86%). PTWAir did not contribute to reducing the analyzed parameter; in particular, the results of Sc T58 yeast strain’s viability are shown: PTWAir (58.83%), PTWN (32.28%) and CW (43.56%). The obtained results suggest that rehydration by PTWN of dry yeast with a weakened condition is not recommended for both qualitative and cost-related reasons, while PTWAir significantly contributed to the improvement of some yeast parameters after rehydration and fermentation (higher glycogen and trehalose content).

Screening of Non- Saccharomyces cerevisiae Strains for Tolerance to Formic Acid in Bioethanol Fermentation

Formic acid is one of the major inhibitory compounds present in hydrolysates derived from lignocellulosic materials, the presence of which can significantly hamper the efficiency of converting available sugars into bioethanol. This study investigated the potential for screening formic acid tolerance in non-Saccharomyces cerevisiae yeast strains, which could be used for the development of advanced generation bioethanol processes. Spot plate and phenotypic microarray methods were used to screen the formic acid tolerance of 7 non-Saccharomyces cerevisiae yeasts. S. kudriavzeii IFO1802 and S. arboricolus 2.3319 displayed a higher formic acid tolerance when compared to other strains in the study. Strain S. arboricolus 2.3319 was selected for further investigation due to its genetic variability among the Saccharomyces species as related to Saccharomyces cerevisiae and availability of two sibling strains: S. arboricolus 2.3317 and 2.3318 in the lab. The tolerance of S. arboricolus strains (2.3317, 2.3318 and 2.3319) to formic acid was further investigated by lab-scale fermentation analysis, and compared with S. cerevisiae NCYC2592. S. arboricolus 2.3319 demonstrated improved formic acid tolerance and a similar bioethanol synthesis capacity to S. cerevisiae NCYC2592, while S. arboricolus 2.3317 and 2.3318 exhibited an overall inferior performance. Metabolite analysis indicated that S. arboricolus strain 2.3319 accumulated comparatively high concentrations of glycerol and glycogen, which may have contributed to its ability to tolerate high levels of formic acid.

Valuation of brewers spent yeast polysaccharides: a structural characterization approach

Brewers spent yeast (BSY) is a by-product from beer industry that can be exploited as source of glucans and mannoproteins, with potential biological activities. In order to solubilize these carbohydrate-rich polymeric materials, a sequential extraction with hot water and alkaline solutions (0.1-8 M KOH) was performed. Mannoproteins were mainly (85%) extracted with 4 M KOH whereas glucans were extracted with 8 M KOH and in an amount that accounted only for 34% of total glucose. Final residue still accounted for 34% of the initial glucans and contained 98% of glucose. Cellulase and α-amylase treatments showed the presence of both α- and β-(1→4)-Glc linkages. To promote total solubilization of these insoluble glucans, the final residue was submitted to a partial acid hydrolysis. This work is the first report showing that the most abundant polysaccharides in BSY are polymers that contain structural features similar to cellulose, thus justifying their resistance to alkaline extractions, acid hydrolysis, and insolubility in water.