Yeast strains for producing alcohol

A method for obtaining stable yeast strains, said method comprising the steps of:

• crossing an unstable Saccharomyces strain being the I-4072 strain with a stable Saccharomyces strain selected from the group consisting of the I-4073 strain, the I-4074 strain and the I-4075 strain, so as to obtain at least one hybrid,
• wherein the crossing step comprises the following steps:
• sporulating said unstable Saccharomyces strain and said Saccharomyces strain selected from the group consisting of the I-4073 strain, the I-4074 strain and the I-4075 strain, so as to obtain segregants,
• culturing said segregants, so as to obtain a population R0 for each segregant,
• carrying out m successive subcultures of each segregant, so as to obtain a population Rm of each segregant, m being an integer greater than or equal to 6,
• measuring the alcohol production of the population R0 of each segregant, measuring the alcohol production of the population Rm of each segregant and measuring the alcohol production of population R0 of the I-4072 Saccharomyces strain,
• selecting the segregants for which:
• the alcohol production of the population Rm is greater than or equal to 70% of that of the population R0 of said segregant, and
• the alcohol production of the population R0 is greater than or equal to 85% of that of a population R0 of the I-4072 Saccharomyces strain, so as to obtain effective, stable segregants, and
• crossing effective, stable segregants stemming from said unstable strain with effective, stable segregants of opposite sign stemming from said unstable strain or from said other stable strain, so as to obtain at least one hybrid,
• culturing said hybrid, so as to obtain a yeast population R0,
• carrying out n successive subcultures of the yeast population R0, n being an integer greater than or equal to 6, so as to obtain a yeast population Rn,
• measuring the alcohol production of said population R0 and measuring the alcohol production of said population Rn, and
• selecting at least one hybrid for which the alcohol production of the population Rn is greater than or equal to 95% of that of the population R0, so as to obtain stable yeast strains,
• wherein the unstable yeast strain, being the I-4072 strain, is a strain which, over generations, produces decreasing amounts of alcohol, and
• wherein the stable yeast strains are strains for which the alcohol production of the population Rn is greater than or equal to 95% of that of the population R0,
• wherein the method comprises additional steps of:
• measuring the alcohol production of population R0 of the I-4072 Saccharomyces strain, and
• selecting at least one hybrid for which the alcohol production of the population R0 is greater than or equal to 95% of that of a population R0 of the I-4072 Saccharomyces strain.