Characterization of a new set of mutants deficient in fermentation-inducedloss of stress resistance for use in frozen dough applications

In frozen dough applications a prefermentation period during the preparatio n of the dough is unavoidable and might also be important to obtain bread w ith a good texture. A major disadvantage of the prefermentation period is t hat it is associated with a rapid loss of the freeze resistance of the yeas t cells. A major goal for the development of new baker's yeast strains for use in frozen dough applications is the availability of strains that mainta in a better freeze resistance during the prefermentation period. We have is olated mutants that retain a better stress resistance during the initiation of fermentation. Some of these showed the same growth rate and fermentatio n capacity as the wild type cells. These mutants are called 'fil', for defi cient in fermentation induced loss of stress resistance. First we used labo ratory strains and heat stress treatment, given shortly after the initiatio n of fermentation, as the selection protocol. The first two mutants isolate d in this way were affected in the glucose-activation mechanism of the Ras- cAMP pathway. The fil1 mutant had a partially inactivating point mutation i n CYR1, the gene encoding adenylate cyclase, while fil2 contained a nonsens e mutation in GPR1. GPR1 encodes a member of the G-protein coupled receptor family which acts as a putative glucose receptor for activation of the Ras -cAMP pathway. In a next step we isolated fil mutants directly in industria l strains using repetitive freeze treatment of doughs as selection protocol . Surviving yeast strains were tested individually for maintenance of ferme ntation capacity after freeze treatment in laboratory conditions and also f or the best performing strains in frozen doughs prepared with yeast cultiva ted on a pilot scale. The most promising mutant, AT25, displayed under all conditions a better maintenance of gassing power during freeze-storage. It was not affected in other commercially important properties and will now be characterised extensively at the biochemical and molecular level. (C) 2000 Elsevier Science B.V. All rights reserved.