Use of non-saccharomyces yeasts to improve the quality and food safety of wine

Resumen

The doctoral thesis focus on two yeast denominated as no conventional becouse they do not belong to the classic Saccharomyces genus. This genus is the one used in most alcoholic fermentations. The studied non Saccharomyces yeast in the doctoral thesis are Lacchancea thermotolerans and Schizosaccharomyces pombe. The use of these species has as main objective to try to improve the quality of the Spanish wines, especially of those produced under specific conditions such as those of warm climate or affected by climate change. Under those conditions, the initial musts may get high probable alcohol degrees (over 14.5 º PAD) that forecast difficult fermentation endings for conventional fermentation protocols. The acidity generally is reduced on those cases, what generates high pH up to 4. The conventional alcoholic fermentation of musts of high probable alcohol degree and pH produces risks able to deteriorate the main quality parameters of wine such as volatile acidity, total acidity and residual sugar. Additionally, the difficult fermentation endings for yeast and the high pH, increase the risk of overlapping the alcoholic fermentation and malolactic fermentation. This fact may also influence over the former quality parameters, and even to produce hazardous compounds for human health such as biogenic amines and ethyl carbamate. The yeast species L. thermotolerans is able to improve the total acidity of the wines through its unique ability of generating L-lactic acid during the alcoholic fermentation without consuming significatlly malic acid or increasing the volatile acidity. What is very different from the lactic bacteria metabolism. However, L. thermotolerans possess a moderate fermentative power and it is not able to ferment more than 9 to 10 % acohol concentrations. This limitation avoid to make wines from conventional grapes without combining with another more fermentative yeast that guaranties the total fermentation of sugars. The yeast species S. pombe and the Schizosaccharomyces genus possess the unique ability of metabolizing malic acid to carbon dioxide and ethanol through the malo-alcoholic fermentation. That metabolism allow the malic deacidification of wines that may have occasionally high acidity due to the lack of maturity characterized by high contents of malic acid. This kind of wine usually are unbalanced from a sensory point of view. This situation is normal in wines produced in the north of Spain or Europe. However, S. pombe has been also used lately to remove small amounts of malic acid in red wines of high maturity characterized by low contents in malic acid, high ethanol levels and high pH. This way allow to avoid a possible malolactic fermentation that would took place at a high pH and possibly in presence of residual sugars due to a possible slow alcoholic fermentation ending under a high alcohol content that avoid the optimum performance of conventional yeasts. However, most S. pombe strains show as an important collateral effect the production of high acetic acid levels. This fact make it necessary to perform previous selection processes of S. pombe strains able to produce low concentrations of acetic acid. The doctoral thesis employs the combined use of L. thermotolerans and S. pombe to avoid the malolactic fermentation in wines of high alcohol concentration and pH. Where the risk of deviation during the malolactic fermentation is high and it even may overlap with the end of the alcoholic fermentation. To achieve that objective S. pombe consumes the malic acid and ends the alcoholic fermentation, while L. thermotolerans compensate the loss of acidity produced by S. pombe.