A study was made of the importance of lipid composition, and particularly of the fatty-acyl composition of lipids, on the capacity of strains of Saccharomyces cerevisiae to retain fermentative power, when they were converted into an active dried form under conditions similar to those used on a commercial scale. Three strains of Saccharomyces cerevisiae were used. A strain isolated from commercial active dried yeast was desipnated the Parent strain. A mating strain obtained from an ascospore of this strain was also used and designated Mating Parent strain HP 92. Finally mutagenic treatment of the Mating Parent strain with N-Methyl-N'-Nitro-N-Nitroso-guanidine led to the isolation of a strain which is auxotrophic for unsaturated fatty acids. This v/as designated mating strain RP 108. Growth of the auxotrophic mutant strain RP 108 in aerobic batch-culture was promoted by supplementation of the medium with oleic acid (C18:19 cis) in pure form or as Tween 80. A preliminary investigation of the specificity of fatty-acid requirement of this strain indicated that other unsaturated fatty acids (notably linoleic acid, c18:29,12 all cis) of both different chain length and site and number of unsaturations could satisfy the growth-requirement. The requirement for an unsaturated fatty acid was not absolute, as some growth occurred in unsupplemented cultures. All three strains were grown aerobically in batch-culture under conditions of restricted growth rate, and both nitrogen and phosphate limitation. Yeasts were harvested and converted to active dried yeasts in a laboratory-scale tray drier. Following reconstitution in water at either 38 or 25°C the fermentative activies of the yeasts were compared with those of the yeasts prior to drying. Cultures of auxotrophic strain RP 108 were grown under various conditions of unsaturated fatty-acid supplementation. Recovery of fermentative activity from active dried yeasts prepared from such cultures was greatest when the medium had been supplemented with a high concentration of C18:19cis acid, and was comparable to that value obtained from cells of the Mating Parent strain. Subsequent analyses of fatty-acyl residues from whole-cell, phospholipid and isolated plasma membrane preparations from both the Mating Parent and auxotrophic RP 108 strains indicated that fatty-acid supplements were incorporated by the latter strain in a chemically unmodified form, primarily into the phospholipids and only into neutral lipids when supplied in excess. A close correlation existed between the fatty-acyl compositions of phospholipids and plasma membranes isolated from crops of both strains. Cells of the auxotrophic strain RP 108, containing various proportions of unsaturated fatty acid, contained similar amounts of total lipid, free fatty acid, total phospholipids and free sterols. However, difference were observed in the contents of individual phospholipids, individual sterols and esterified sterols isolated from such cells. The function of unsaturated fatty-acyl composition and membrane stability, particularly with regard to the stresses of drying and reconstitution in active dried yeast, is discussed.
|Date of Award||1976|