Standardised methodology for testing malolactic bacteria and wine yeast compatibility

Metabolic interaction of wine yeast and malolactic (ML) bacteria may stimulate or inhibit malolactic fermentation (MLF). To study this interaction in the absence of extrinsic grape-derived and processing factors, a standardised methodology using a chemically defined medium was developed to reproducibly characterise the intrinsic metabolic interaction between any wine yeast and bacteria pair. The methodology can be adapted to study the modifying effects of extrinsic factors on yeast-bacteria interaction. The methodology involves measuring the growth characteristics of the chosen ML bacterium in the wine produced by the yeast of interest under standardised conditions. The methodology was applied to four Saccharomyces cerevisiae wine yeasts which have been characterised in terms of growth response of an Oenococcus oeni strain. Three yeasts gave compatible interactions, in which the level of bacterial growth stimulation was variable; the fourth yeast inhibited bacterial growth and produced a high concentration of sulfite. To test for the possibility that the bacterial growth response was inhibited by the depletion of nutrients by the fermentation yeast, supplementation of the wines with essential nutrients gave a strain-dependent growth response. A treatment designed to remove or inactivate yeast-derived inhibitory factors was also investigated. This treatment partly alleviated bacterial inhibition caused by the sulfite-producing yeast strain. To verify that the standardised methodology relates to grape juice and wine, the synthetic grape juice medium was replaced with a Chardonnay juice of comparable composition prepared from microbiologically sound grapes; similar results were obtained, suggesting that the grape-derived extrinsic factors in this case did not appreciably modify the intrinsic yeast-bacteria metabolic interaction.