Yeast populations

This type of autocatalytic reaction is a simplification of many biological reactions such as fermentation, where the reaction produces products (species B in the previous example), which accelerates the rate. In fermentation, yeast cells in the solution produce enzymes that catalyze the decomposition of sugar to produce ethanol as a byproduct of yeast reproduction. Since the yeast population increases as the reaction proceeds, the enzyme concentration increases, and the process appears to be autocatalytic. A highly simplified description of fermentation might be... 

Pure Yeast Culture. Recheck population periodically for purity. If a wild yeast population builds up significantly, discard the culture and begin again from a slant. Some wineries are using mass pitching techniques with dry or frozen yeast very successfully. This eliminates the need for any monitoring of culture population. Wild yeasts often cause unreliable and erratic fermentation rates, and this is often accompanied by off odors and off flavors in the wine. Erratic fermentations also often invite bacterial contamination. 

Fermentation Curves. Fermentation curves are an index of the general well being of the fermentation processes taking place in the fermentation room. Abnormal yeast populations or improper temperature control often can be seen first in the plotted data of sugar content vs. time during fermentation. 

Esteve-Zarzoso, B., Peris-Toran, M. J., Garcia-Maiquez, E., Uruburu, F., and Querol, A. (2001). Yeast population dynamics during the fermentation and biological aging of Sherry wines. Appl. Environ. Microbiol. 67, 2056-2061. 

Martinez, P., Codon, A. C., Perez, L., and Benitez, T. (1995). Physiological and molecular characterization of flor yeasts polymorphism of flor yeast populations. Yeast 11, 1399-1411. 

Romani et al. (2011) also evaluated the yeast population dynamics and fermentation kinetics, and their influences on the analytical profiles of Vin Santo obtained at industrial scale utilizing in separate trials two non-Saccharomyces yeasts, T. delbrueckii and Z. bailii. These results were compared with those obtained both with spontaneous fermentation and with an inoculum of a S. cerevisiae yeast strain. The standard kinetics of fermentations were observed in all of the trials, also if a higher fermentation rate was observed in the trials inoculated with S. cerevisiae compared to those inoculated with the two non-Saccharomyces yeasts, and in the spontaneous one. A rapid decrease in non-Saccharomyces yeast was observed in the trials inoculated with S. cerevisiae. In these last ones, after 6 months, 18.4% ethanol was reached versus 16% of the trials inoculated with the non-Saccharomyces strains. No substantial differences were seen for the higher alcohols or other byproducts assayed. 

With the aim to investigate yeast population dynamics during the production of Vin Santo under different fermentation conditions and to evaluate their impact on wine sensorial characteristics, Domizio et al. (2007) tested different fermentation conditions, such as yeast inoculum, madre addition, and temperature. The results indicate that the low temperatures that occurred during the initial stages of fermentation have different effects on the wine yeasts (Fig. 3.10). On the one hand,... 

Domizio, P., Lencioni, L., Ciani, M., Di Blasi, S., Pontremolesi, C., and Sabatelli, M. P. (2007). Spontaneous and inoculated yeast populations dynamics and their effect on organoleptic characters of Vinsanto wine under different process conditions, lnt. J. Food Microbiol. 115, 281-289. 

Egli, C. M., Edinger, W. D., Mitrakul, C. M., and Henick-Kling, T. (1998). Dynamics of indigenous and inoculated yeast populations and their effects on the sensory character of Riesling and Chardonnay wines. J. Appl. Microbiol. 85, 779-789. 

Naumov, G. I., Masneuf, I., Naumova, E. S., Aigle, M., and Dubourdieu, D. (2000). Association of Saccharomyces bayanus var. uvarum with some French wines Genetic analysis of yeast populations. Res. Microbiol. 151, 683-691. 

FIGURE 6.4 Course of alcoholic fermentation and evolution of the yeast populations during spontaneous fermentation of Tokaji Aszu. Botrytized berries were macerated with fermenting must (A) or dry wine (B) (Magyar, 2010). 

Wills, C. Three kinds of genetic variability in yeast populations. Proc. Natl. Acad. Sci. U.S.A. 61 937-944, 1968. 

Montrachet (University of California, 522), champagne (University of California, 505), Steinberg, and French White yeast are used most commonly in the North Coast, though other strains are available. The first two are widely available in dried form and the second two have come from Geisenheim Institute, Germany, and Pasteur Institute, Paris, respectively. The latter no longer supplies yeast cultures. Some wineries maintain their own cultures from year to year, and occasionally fermentations will be allowed to proceed with the grapes natural yeast population. 

Sweet white wines, however, are subject to refermentation in contact with yeast. Protection from refermentation includes a combination of three factors chilling (2°-10° C), S02 (25-35 mg/L free), and reduction of yeast population to a minimum level by pad or tight diatomaceous earth filtration. At bottling, yeasts are 100 percent removed from the wine to prevent refermentation in the bottle. 

Curves of cells of Saccharomyces cerevisiae yeast growth in suspension at various concentrations of nickel (II) sulfate may serve as an example (Figure 1) [14], These curves are the evidence of toxic action of nickel (II) ions. Increase of toxicant concentration leads to decrease of rate during the period of exponential growth (parameter p in equation (7)) and maximum size of population of yeast (Parameter p/a in equation(7)) at one and the same initial size of yeast population, N0=100 cells/ml. 

In none of listed cases equation (7) describes the growth of size of yeast populations in the whole time interval from inoculation up to reaching of maximum value. Calculated points of inflection on growth curves are also significantly differed from experimental points. 

Temperature extremes Yeasts have problems growing when the temperature is too low. Therefore, too low temperature at the beginning of the process can lead to a deficient yeast population. On the other hand, if the temperature is too high (more than 30 °C) the fermentation is at considerable risk of stopping. For that reason, thermic control of fermentations is nowadays indispensable. Another aspect to take into account is that sudden changes of temperature may provoke serious problems in fermentation. As previously mentioned, yeasts adapt their membrane composition to maintain the correct fluidity. A drastic temperature decrease can provoke excessive rigidity in the membranes before yeasts can adapt to it. 

Beltran, G., Torija, M.J., Novo, M., Ferrer, N., Poblet, M., Guillamdn, J.M., Rozes, N., Mas, A. (2002) Analysis of yeast populations during alcoholic fermentation a six year follow-up study. Syst. Appl. Microbiol., 25, 287-293. 

Constant , M., Reguant, C., Poblet, M., Zamora, F., Mas, A., Guillamon, J.M. (1998) Molecular analysis of yeast population dynamis Effect of sulphur dioxide and the inoculum in must fermentation. Int. J. Food Microbiol., 41, 169-175. 

Ibeas, J.I., Lozano, I., Perdigones, F, Jimenez, J. (1997). Dynamics of flor yeast populations during the biological aging of sherry wines. Am. J. Enol. Vitic., 48, 75-79. 

Sancho, E.D., Hernandez, E., Rodriguez-Navarro, A. (1986). Rresumed sexual isolation in yeast populations during production of sherrylike wine. App. Environ. Microbiol., 51, 395-397. 

Henick-Kling, T, Edinger, W., Daniel, P, Monk, P. (1998) Selective effects of sulfur dioxide and yeast starter culture addition on indigenous yeast populations and sensory characteristics of wine. Journal of Applied Microbiology, 84, 865-876. 

Few conclusive studies have identified the source or spread of Brettanomyces within the vineyard except for use of contaminated equipment. Contaminated and improperly sanitized crush equipment, drains, barrels, transfer hoses, valves, pumps, and bottling equipment can all act as sources for further infection (33). Harper (67) found the interior surface of plastic (polyvinyl chloride, polyethene, and plasticised nylon) pipes used commonly in the brewing industry maintained a variety of bacteria and yeast populations, including Brettanomyces. 

Solieri, L., Landi, S., De Vero, L., and Giudici, P. (2006). Molecular assessment of indigenous yeast population from traditional balsamic vinegar.. Appl. Microbiol. 101, 63-71. 

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