The Effects of Osmotic Pressure and Ethanol on Yeast Viability and Morphology

Abstract

The selection of a brewing yeast strain with the required fermentation and recycling characteristics is critical. The yeast strain will influence the rate and extent of fermentation, the flavour characteristics and the overall quality and stability of the finished beer, and consequently, the economic viability of the brewery. Since high gravity worts can have a deleterious effect on yeast fermentation performance, it is imperative that the strain selected be suitable for this environment, which includes a capacity to withstand high osmotic pressures and elevated ethanol levels. Under controlled in vitro osmotic and ethanol induced stresses, there was a decline in mean cell volume in both lager and ale yeast strains. Whilst significant reductions in viability were observed in the lager strains, the ale strains studied were not affected. Cell surface investigations revealed shrinkage of the yeast cells and crenation of the outside envelope under both stresses, although exposure to ethanol had a more marked effect on the yeast cell surface than sorbitol-induced elevated osmotic pressure.Key words: Cell viability, cell volume, ethanol, high gravity wort, osmotic pressure. -2863(9'8-32In brewing, yeast is recycled and the fact that it is cropped and repitched into subsequent fermentations is one of the differentiating features between brewing and the production of many other alcoholic beverages including Scotch whisky 37 . Consequently, the quality of yeast cropped will not only affect the overall performance of most fermentations but also the quality and stability of the resulting beer. It is important therefore that the factors which influence yeast performance in the brewing process, and particularly in high gravity brewing, are considered, to ensure efficient fermentation and the production of a beer of a consistently high quality and stability. Breweries, the world-over, are continually seeking ways to reduce capital expenditure, labour, utilities, effluent and other operational costs and at the same time ensuring that the quality of their beers remains consistently high. As a result, many are employing the process of high gravity brewing due to its many advantages 39 . Similarly, some breweries have not implemented this process due to its disadvantages 40 . Of significance, are the deleterious effects of this process on yeast fermentation performance 41,43 . Fermentations have been reported to be sluggish due to elevated ethanol levels and high osmotic pressure, resulting in yeast viability and vitality reductions 6 . Osmotic pressure is the force that develops between two solutes of differing concentration separated by a semipermeable membrane 20 . When yeast is exposed to wort it is subjected to an osmotic pressure. Very high osmotic pressures such as those encountered in high gravity worts, may distort yeast metabolism or decrease yeast viability. The extent of the osmotic pressure will depend on the concentration of solutes surrounding the cell 20,23 . It has been shown that increases in wort osmotic pressu...

The Combined Effects of Oxygen Supply Strategy, Inoculum Size and Temperature Profile on Very-High-Gravity Beer Fermentation by Saccharomyces cerevisiae

“…increases specific gravity, viscosity, and interfacial surface tension) which can, in turn, influence mixing patterns, wort oxygen solubility, and rate of oxygen dissolution. Moreover, high osmotic pressure, 14,33,49,50,54,58 the formation of elevated levels of toxic fermentation by-products (e.g. ethanol), 11,65 and limiting yeast nutritional factors 9 have been implicated as key factors contributing to the deleterious results on yeast fermentation performance under very-high-gravity conditions.…”

The Horace Brown Medal Lecture: Forty Years of Brewing Research

“…Figures 26 and 27 illustrate experiments with 12 and 20°Plato worts fermented with a lager and an ale yeast strain. Cell viability decreased in both strains within the first 24 h of fermentation 99 . However, the decrease in viability was exacerbated with the 20°Plato compared to the 12°Plato wort.…”

“…However, the decrease in viability was exacerbated with the 20°Plato compared to the 12°Plato wort. However, with both yeast types (which are representative of a number of lager and ale strains studied 99 data not shown) the viability recovered later in the fermentation. In addition, for reasons that are unclear, ale strains maintained higher viability than lager strains 102 .…”

“…As the cultures acclimatised to the stress conditions imposed by this wort, the intracellular trehalose levels decreased. It is interesting to note that lager strains maintained higher trehalose levels than ale strains 99 .…”

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

“…The level of cell wall bound glycogen was found to increase to a maximum level by 36 h and the level was parallel with an increase in the ethanol level. An increase in ethanol is known to affect the growth and viability of yeast 15,23 . The correlating increase in cell wall bound glycogen suggests that this pool of glycogen probably plays a vital role in overcoming ethanol stress.…”