Effects of SO2 on lactic acid bacteria physiology when used as a preservative compound in malolactic fermentation

SO₂ is a preservative compound widely used at an industrial scale to control the cider‐making processes. However, little is known about the effect that sulfur dioxide exerts on the cell physiology of lactic acid bacteria (LAB) during malolactic fermentation, taking into account the cellular heterogeneity of the microbial populations. Analysis of flow cytometry results has shown how increasing SO₂ concentrations were not related to a significant rise in the dead cell subpopulation. SO₂ affected malolactic activity and cell viability, giving rise to different levels of LAB tolerance to the fermentation conditions. Malate consumption in the bioreactor and the enzymatic potential of whole cells were monitored during the bioconversion process under the different assayed conditions. After SO₂ exposure, specific malolactic activity of whole cells measured by enzymatic assays in which SO₂ or other inhibitory compounds were absent showed higher values than the malolactic activity determined in the bioreactor, suggesting that inhibition was not irreversible, at least not at the concentrations tested. A correlation between total malolactic activity and viable and viable but nonculturable cell concentrations was established. Increasing amounts of SO₂ in the fermentation media also affected the malic acid/lactic acid ratio. Changes observed on LAB physiology at different sulfiting levels could be useful for practical cider‐making applications. Copyright © 2012 The Institute of Brewing & Distilling