Polyploid baker's yeasts having a low temperature inactivation property

A process for constructing a strain of baker's yeast comprising:

• sporulating a starting polyploid strain of Saccharomyces cerevisiae to obtain segregants having reduced ploidy;
• selecting at least one segregant having a CO.sub.2 production level of less than 30 ml per g pressed yeast after seven days in maltose medium refrigerated at from 3.degree. C. to 9.degree. C. and a CO.sub.2 production level of at least 60 ml per g pressed yeast after 6 days in maltose medium maintained at a temperature of at least 13.degree. C.;
• backcrossing a selected segregant at least once with another segregant having an opposite mating type;
• selecting at least two backcross segregants having opposite mating types and a CO.sub.2 production level of less than 30 ml per g pressed yeast after seven days in maltose medium refrigerated at from 3.degree. C. to 9.degree. C. and a CO.sub.2 production level of at least 60 ml per g pressed yeast after 6 days in maltose medium maintained at a temperature of at least 13.degree. C.;
• crossing the selected backcross segregants at least once to obtain polyploid strains;
• and
• selecting a polyploid strain having growth potential, ability to raise a dough, and a CO.sub.2 production level of less than 30 ml per g pressed yeast after seven days in maltose medium refrigerated at from 3.degree. C. to 9.degree. C. and a CO.sub.2 production level of at least 60 ml per g pressed yeast after 6 days in maltose medium maintained at a temperature of at least 13.degree. C.