Yeast viability

Williams and Keeling 2002). We also suggest that this close association of the mitochondrion-like organelle and endomembrane system facilitates the assembly of cytosolic Rlil, the only known essential FeS protein for yeast viability (Iill et al. 2005 reviewed in Tachezy and Dolezal 2007). These data are consistent with previous hypotheses that the primary function of the mitochondrion-like organelle in C. parvum is the assembly of FeS clusters in order to provide mature FeS proteins to all cellular compartments, including the cytosol, mitochondrion, and nucleus. 

Presence of medium chain fatty acids The presence of MCFA can decrease yeast viability and even stop alcoholic fermentation. This problem is more prevalent in white winemaking because fermentation is usually carried out at low temperatures and without any aeration. Yeast hulls have been very useful for avoiding this problem. Yeast hulls adsorb MCFA from the media and provide sterols and UFA to the yeasts. Yeast hulls can be used as preventives (20 g/hl) or as curatives (40-50 g/hl) of stuck and sluggish fermentations. 

Patynowski, R.J., Jiranek, V., Markides, A.l. (2002). Yeast viability during fermentation and sur lie ageing of a defined medium and subsequent growth of Oenococcus oeni. Aust J Grape Wine Res., 8, 62-69. 

Palou, E., Lopez-Malo, A., Barbosa-Canovas, G.V., Welti-Chanes, J., Davidson, P.M., and Swanson, B.G. High hydrostatic pressure come-up time and yeast viability, /. Food Prot., 61,1657,1998. 

The rise in popularity of frozen breads has been driven mainly by the economic attraction of centralized manufacturing and distribution and the practicality of consumption (Best, 1995). Nevertheless, frozen doughs still have presented problems such as dough weakening and loss of yeast viability and consequently decrease in CO2 retention during proofing and reduction of loaf volume (Casey and Foy, 1995 Ribotta et al., 2003. These problems could make the frozen product less acceptable than fresh bread. 

Christen et al. (2004) developed an SLM system for the extraction of EtOH during semicontinuous fermentation of Saccharomyces bayanus. The membrane was a porous Teflon sheet as support, soaked with isotridecanol. The removal of EtOH from the cultures led to decreased inhibition and, thus, to gain in conversion of 452 g/1 glucose versus 293 g/1 glucose without extraction. At the same time, the EtOH volumetric productivity was enhanced 2.5 times, due to an improvement of yeast viability, while the substrate conversion yield was maintained above 95% of its theoretical value. In addition to these improvements in the fermentation performances, the process resulted in EtOH purification, since the separation was selective toward microbial cells and carbon substrate and likely selective to mineral ions present in the fermentation broth. For PV, a concentration of EtOH four times greater was obtained in the collected permeates. 

Among the most important carbonyls are the ketoacids, which are essential for amino acid synthesis and for the higher alcohols, since the carbonyls are intermediaries when the aldehydes are produced by the yeast (Nykanen. 1983). When the yeast viability or the cell activity is decreased, the yeast produces more aldehydes, because the metabolic reactions stop and fermentation cannot be completed (Nykanen. 1983, Moreno. 2009). 

Back, 1994). Automated yeast counting systems (outlined in Section 5.3.1), which offer a fluorescence measurement option, also find applications in automated yeast viability measurements. Flow cytometry can be used to measure viability as well. A standard fluorescent stain for this application is PI. 

Boyd, A. R., Gunasekera, T. S., Attfleld, P. V., Simic, K., Vincent, S. F., Veal, D. A. (2003). A flow-cytometric method for determination of yeast viability and cell number in a brewery. FEMS Yeast Research, 3, 11-16. 

Heggart, H. M., Margaritis, A., Stewart, R., PUdngton, J. H., Sohczak, J., Russell, I. (2000). Measurement of brewing yeast viability and vitality a review of methods. Tech Q Master Brewers Association of the Americas, 37, 409—430. 

Van Zandycke, S. M., Simal, O., Gualdoni, S., Smart, K. A. (2003). Determination of yeast viability using fluorophores. Journal of the American Society of Brewing Chemists, 61, 15-22. 

Bonix, M., Leveau, J. Y. (2001). Rapid assessment of yeast viability and yeast vitality during alcoholic fermentation. Journal of the Institute of Brewing, 107(4), 217-225. 

Lethal effects of a high fermentation temperature are often thought to result from the effect of temperature alone. However, inhibition is also the result of intracellular accumulations of ethanol. Temperature tolerance of yeast varies with species and strain and reflects intrinsic and extrinsic properties of the growth medium. Generally, yeast viability in alcoholic media subsides at temperatures near 35°C (95°F). 

Combs, N., C., Hatzis, and G. Philippidis. 1995. An epifluorescence staining procedure for monitoring yeast viability in the presence of lignocellulosic solids and for measuring toxicity of pretreatment hydrolysates. Annual Meeting of the Soc. for Ind. Microbiol. 

The weak organic acids such as acetic acid and formic acid both have positive and negative effects on the bioethanol produetion proeess. In fermentations using S. cerevisiae NCYC 2592, an addition of aeetie acid in a concentration of 20 mM increased the ethanol produetivity (unpublished data). The low acetic acid concentrations (lower than 20 mM) did not have an impact on the yeast viability. At fermentations with higher acid concentration, the intracellular pH decreases, requiring plasma membrane ATPase to pump protons out of the cell. The depletion of ATP affected the biomass formation. In comparison with acetic acid, formic acid has a more severe inhibitory effect, which has also been observed in other biosynthesis processes, e.g. succinic acid formation. ... 

Although traditionally used in brewing and winemaking industries, some researchers have argued against the application of methylene blue as a means to determine yeast viability. For example, O Connor-Cox et al. (1997) noted that the dye does not stain all dead cells in a sample, a fact that can lead to overestimating yeast viahUity. 

Methylene blue A differential stain that can be added to a liquid suspension to evaluate yeast viability. Live yeasts will reduce the dye to a colorless form, whereas dead cells appear blue/black as viewed using a microscope. 

Glycolysis and alcoholic fermentation enzymes (Chapter 2) as well as trehalase (an enzyme catalyzing the hydrolysis of trehalose) are present. Trehalose, a reserve disaccharide, also cytoplasmic, ensures yeast viability during the dehydration and rehydration phases by maintaining membrane integrity. 

Deere, D. Shen, J. Vesey, G. Bell, R Bissinger, R Veal, D. Flow cytometry and cell sorting for yeast viability assessment and cell selection. Yeast 1998, 14, 147-160. 

Smart, K. A. Chambers, K. M. Lambert, I. Jenkins, C. Use of methylene violet staining procedures to determine yeast viability and vitality. J. Am. Soc. Brewing Chem. 1999, 57, 18-23. 

---