Selenium has a central role in antioxidant pathways as a cofactor to glutathione peroxidase. The present study evaluated the effects of four different preparations of inactivated yeast containing various concentrations of selenium and glutathione on a combined atherosclerosis and diabetes hamster model. The hamsters were supplemented with the yeast products for three months. The enriched yeast with the highest selenium and glutathione levels reduced the weight loss induced by diabetes, inhibited an increase in plasma cholesterol and triglyceride caused by a high-cholesterol and high-fat diet, increased the time taken for oxidation of lower density lipoproteins (lag time), and inhibited the formation of atherosclerosis better than low selenium/glutathione yeast supplementation. It was concluded that the yeast prepared to provide high selenium and high glutathione was the best for effecting beneficial changes in glutathione, cholesterol, atherosclerosis, and for demonstrating an antioxidant effect. The high selenium and low glutathione yeast was the best for improving selenium and glucose levels.

Yeast flocculation is the reversible aggregation of yeast cells promoted by the interaction between lectin‐like protein receptors with mannose side chains on adjacent cell walls. Flocculation is governed by several physiological factors, including the type of nutrient sugar available to yeast. We grew four industrial strains of Saccharomyces cerevisiae, representing applications in the brewing, winemaking and bioethanol sectors, to late stationary phase and quantified the cellular content of mannans, glucans and lectin‐like proteins on yeast cell surfaces. Results indicated that brewing and champagne strains showed moderate to high flocculation ability when grown with glucose, fructose, maltose or galactose, whereas winemaking and fuel alcohol strains only showed moderate flocculation when grown on maltose and galactose. All yeast strains studied were weakly flocculent when grown on mannose. With regard to lectin‐like receptors, their number played a more important role in governing yeast flocculation than the mannan and glucan contents in yeast cell walls. We conclude that all the industrial strains of S. cerevisiae belonged to New‐Flo type on the basis of their flocculation behaviour observed when cultured on different sugars. Quantification of yeast cell wall polysaccharides and receptor sites indicates that mannan and glucan levels remain almost constant, irrespective of the strain under investigation. The main difference in flocculation characteristics in industrial yeast strains appears to be due to variations in concentrations of lectin‐like cell surface receptors. Our findings may benefit brewers, winemakers and other yeast‐based technologies in design of media to prevent premature flocculation during fermentation. Copyright © 2017 The Institute of Brewing & Distilling

In the past few years, atomic force microscopy (AFM) has provided novel information on the ultrastructural and nanomechanical properties of yeast cell walls that play a major role in determining the flocculation characteristics of the yeasts. In this study, we used AFM to visualize at the nanoscale the cell surface topography and to determine cell wall nanomechanical properties (e.g. elasticity and adhesion) of different strains of S. cerevisiae employed for brewing, winemaking and fuel alcohol production. Cell surface topography was found to correlate with the flocculation behaviour of these strains during their late stationary phase, with the cell surface of flocculent cells being rougher than that of weakly flocculent cells. The elastic modulus of the yeast cell walls showed that weakly flocculent strains had a more rigid cell wall than highly flocculent strains. This difference in elasticity seemed to have an effect on the adhesive properties of the yeast cell walls, with weakly flocculent yeasts displaying lower adhesion energy than the highly flocculent strains. These findings seem to indicate that yeast cell surface nanomechanical properties play an important role in governing flocculation.

No abstract