It’s the time of year after the southern hemisphere harvest when the unfortunate with stuck fermentations seek advice. There are various causes of stuck fermentations with “natural” fermentations being one of the more common ones. Working for a commercial wine yeast producer I am often viewed as not being supportive of this practice. This is not entirely true. I have tasted some exquisite wines produced via spontaneous fermentations. The times that I am critical of the practice it is merely because I am familiar with the risks involved with it. At the end of each harvest I have to help various winemakers to re-inoculate their stuck “naturally fermented” wines with commercial yeast. So over time I have come up with a few guidelines as to how to somewhat make your natural ferments more “secure.” Unfortunately success can never be guaranteed.
Karien’s advice on more secure natural ferments:
1. Do not attempt natural fermentation when the initial grape sugar is above 24°Brix. This works for some people – they are the exception to the rule. Most naturally occurring wine yeasts are not very alcohol tolerant.
2. Add some complex yeast nutrients containing inorganic nitrogen (DAP) at the start of fermentation as to increase biomass formation. One of the main differences between inoculating with commercial yeast and letting nature takes it course is the size of the yeast population starting the fermentation. You need a critical mass to finish a fermentation.
3. Do not stress the yeast by fermenting at extreme fermentation temperatures, i.e. below 17°C or above 25°C. Your specific mix might not be cold tolerant or very alcohol tolerant – the higher the fermentation temperature, the higher the alcohol toxicity.
4. Add yeast cell walls to adsorb medium chain fatty acids produced by stressing yeasts, thereby making the environment less toxic.
5. If you ferment only some tanks natural and others using commercial yeasts, make sure that the commercial yeasts you use are very strong/alcohol tolerant fermenters. That way you can add that lees (once it has completed fermentation) to the natural tanks in case you develop sluggish fermentations.
This advice is not based on my experience of conducting natural ferments, seeing that I have never done any. It is based on my technical knowledge of what yeasts can and cannot do. Use it, don’t use it. Oh, that reminds me, we once did a yeast trial in our lab (at the yeast factory) and the control – with no added yeast – fermented as fast as the experiments. Yeah….so much for that theory…