Considerations for the Use of Yeast and Bacterial Starter Cultures: SO₂and Timing of Inoculation

With the increasing use of selected cultures for alcoholic and malolactic fermentation it is important to make sure that the yeast and bacteria used can work effectively either when inoculated together or when inoculated sequentially. In this study, we evaluated four commercial yeast cultures and three commercial bacterial starter cultures for their compatibility. When inoculated at the same time into grape must, the SO₂ added to the must before inoculation and that produced by the yeast during alcoholic fermentation mainly determines whether the bacteria survive and initiate malolactic fermentation. In grape must to which SO₂ had been added at pressing, the additional SO₂ produced by the yeast during fermentation caused a sharp decrease in the viability of the cultures and malolactic fermentation was inhibited until 15 to 40 days after completion of alcoholic fermentation; in some combinations of yeast and bacteria, malolactic fermentation was not completed within 40 days. The four yeast strains tested produced a maximum of between 13 and 42 mg/L SO₂ during fermentation, the larger amounts of SO₂ being strongly inhibitory to the growth of the bacteria. Concurrent malolactic and alcoholic fermentation was possible in must to which no SO₂ had been added. When inoculated after completion of alcoholic fermentation, growth of the bacterial starter was faster with two of the yeast cultures tested. This effect also seemed to be due to the small amount of SO₂ produced by these strains. Because of the high sensitivity of the lactic acid bacteria to SO₂, it is necessary to select a yeast which does not produce significant amounts of SO₂, and the must should receive only small additions of SO₂ before inoculation (<40 mg/L). When it is preferable to add some (20 - 70 mg/L) SO₂ to the must, then malolactic fermentation should not be induced until after completion of alcoholic fermentation.