Characterization of spoilage yeasts isolated from fermented vegetables and Inhibition by lactic, acetic and propionic acids

Abstract

Two Saccharomyces sp. yeasts isolated from spoiled fermented vegetables were identified, and the effects of pH, temperature, initial yeast count and organic acids (lactic, acetic or propionic) on their growth in a vegetable juice medium (VJM) was examined. The VJM was fermented by a mixed lactic acid culture to a pH of 3·74, which represented the fermented VJM (VJM-F). A fraction of the VJM-F was neutralized to pH 6·0 with 5 N NaOH, which constituted the fermented and neutralized VJM (VJM-FN). Both yeast strains grew at 4°C in VJM and VJM-FN, and prefermentation by the lactic acid bacteria (LAB) did not affect their subsequent development. In VJM-F at pH 3·74, there were important drops in viable populations with the S. bayanus Y-43 strain during incubations at 4 and 30°C, but not with S. unisporus Y-42. In a broth containing 0·7% lactic acid and having a pH of 3·74, the addition of acetic acid or propionic acid generally inhibited growth of yeast. With S. bayanus Y-43, complete growth inhibition is observed with 0·4% of acetic acid, and propionic acid seemed twice as effective as acetic acid in inhibiting the growth of the Y-43 strain. The S. unisporus Y-42 strain was more resistant to organic acids than S. bayanus Y-43, and complete inhibition was only observed with a mixture of lactic, acetic and propionic acids of 0·7, 0·3 and 0·2%, respectively. Results suggest that acidification with lactic acid alone to pH 3·74 is insufficient to prevent the growth of spoilage yeast in fermented vegetables, and that the presence of other organic acids is desirable in this aim. Furthermore, the inoculation level of the yeast had a significant effect on the time at which visible gas production is observed.