Saccharomyces cerevisiae cell fatty acid composition and release during fermentation without aeration and in absence of exogenous lipids

Medium-chain fatty-acids (MCFA) are among the main aroma compounds of ferme nted beverages. High concentrations of MCFA have been found in sluggish and stuck fermentations. It has been suggested that they arrest cell growth, a s they may be toxic, but the causes of sluggish and stuck fermentations are still unclear. The aim of this work was to see whether the production of M CFA is related to fatty acid synthesis in the absence of exogenous lipids a nd aeration, and whether their increase can be regarded as a consequence, i nstead of the cause, of sluggish and stuck fermentations. Two possibilities were considered: (i) MCFA are produced to replace unsaturated fatty acids (UFA) for cell membrane fluidity when the lack of oxygen makes desaturation of saturated fatty acids (SFA) impossible; or (ii) MCFA are produced follo wing the release of medium-chain acyl-CoA from the fatty acid synthetase co mplex (FAS) due to the accumulation of SFA, and their hydrolysis to recycle CoA-SH. In the first hypothesis, MCFA should be active in cell metabolism and be found in cell structures; in the second, MCFA should be a discard an d prevalently found outside the cell. We carried out a Saccharomyces cerevi siae fermentation experiment in a synthetic, lipid-free medium without aera tion. We measured the fatty acid composition of yeast cells and the amounts of MCFA and their ethyl esters in the medium throughout the fermentation. Cell growth and the oxygen content of the medium were also monitored. We fo und that MCFA are not immobilized in cell structures, but mainly released i nto the medium. Cell growth is arrested because fatty acid biosynthesis is prevented by the lack of oxygen. The higher MCFA concentrations found in sl uggish and stuck fermentations can be thus regarded as an effect, and not t he cause, of this arrest. Some suggestions for the prevention of these even ts are proposed. (C) 1999 Elsevier Science B.V. All rights reserved.