Fact #1 – Various studies around the world have demonstrated that some of the more commonly found non-Sacch wine yeasts in grape musts, do have lower alcohol conversions than Saccharomyces cerevisiae (anywhere from 0.3 – 2% less) .
Practical advice – Combine a non-Sacch and S. cerevisiae yeast, both known to have a slightly lower than average alcohol yield. Suppliers of commercial yeasts can advise on this. Combine this with other low alcohol strategies such as green harvesting and blending, earlier overall harvest date, water addition (once approved in SA), increased aerated pump-overs (red) and open top fermenters (red). It’s usually a combination of two or more of these actions that delivers a desired lower alcohol outcome.
Uninoculated fermentations can also lead to lower final alcohol concentrations when compared to S. cerevisiae commercial yeast, due to the large initial non-Sacch population initiating the fermentation. NB: spontaneous fermentations are very risky and can have unpredictable outcomes. Proceed with extreme caution.
Fact #2 – Depending on the species, non-Sacch yeasts can rapidly consume most of the Yeast Assimilable Nitrogen (ammonia and amino acids) in a must before S. cerevisiae is inoculated (or start to ferment in the case of spontaneous fermentations). This can sometimes affect the aromatic outcome or the possibility of fermentations to complete.
Practical advice – Even though non-Sacch yeasts can release some nitrogenous compounds back into fermenting must as they die and autolyse, it is still advisable to add a complex yeast nutrient to the fermentation. Depending on the products used, suppliers can advise what and when. Addition will either be with S. cerevisiae inoculation 48 hours after non-Sacch inoculation or within the first third of the fermentation.
Fact #3 – Non-Sacch yeasts can rapidly assimilate thiamine leading to suboptimal levels for S. cerevisiae to function properly. Thiamine is a vitamin that is a co-factor in various enzymatic reactions in the yeast.
Practical advice – Add a yeast nutrient containing thiamine to fermentations initiated by non-Sacch yeasts, especially un-inoculated fermentations, since the species (and their thiamine uptake) conducting the fermentation are unknown. NB: use the correct dosage of thiamine as the amount that is allowed to be added to wine is regulated by the European Union.