Dan's Harvest Updates

The truth about wild fermentations

Fri 26 Mar 2010

Winemaking and brewing has been going on for thousands of years, but it is only in the last hundred or so that we have had a decent idea about what was really going on in that amazing transformation of grape juice into wine. The French genius Louis Pasteur identified yeast - single celled members of the fungi kingdom - as the organisms responsible for turning sugar into alcohol. That discovery ushered in a century of research and development and an explosion of knowledge about the role of yeast in winemaking. We now know the metabolic pathways involved, we understand what stresses yeast and what that will do to the fermentation, we marvel at the incredible diversity of yeast and are able to isolate, identify and culture individual strains from particularly good fermentations.

On a practical side, I now have hundreds of commercially prepared yeast strains to choose form, some have been identified as being tolerant to low temperatures, others require warm ferments but can handle high alcohols. Specialised yeast are able to carry out the second fermentation in Champagne style wines and then flocculate well to leave a clear wine, others naturally produce fruity characters to give extra lift to aromatic wines. All are expected to complete the fermentation with minimal fuss and no production of off flavours. This is all fantastic and a great tool for making consistently high quality wines, but great wines were and still are being made without recourse to cultured yeast.

Many top winemakers refuse to use cultured yeasts and instead rely on wild ferments - also called spontaneous or indigenous ferments. The fruit is simply brought in, crushed (and pressed if white) and left to kick-off of its own accord. The rationale is that the resulting wines are far more complex and are somehow more natural than those started artificially. Most will explain that the yeasts come in from the vineyard with the grapes and therefore are the true partners of that fruit. Unfortunately this is romantic hogwash - the truth is far more grubby and far more interesting.

Try as hard as you like to isolate wine yeast from a vineyard and you will come up empty handed. On the other hand, check any surface in a winery, even one that looks spotlessly clean, and yeast will be there in abundance. Yeast live, breed and evolve in wineries, just waiting for that yearly bonanza of fresh grape juice. In the thousands of years of winemaking and its gradual spread around the world, yeast have hitched a ride. As already mentioned there are hundreds of strains identified, and thousands more out there. This genetically malleable organism has slowly evolved into a semi-domesticated state, effectively an unseen winemaker's pet. Bring fruit in from the vineyard and it is these yeast that take over the ferments and get the job done. It takes a day or so for the numbers to build, but then they are off. By inoculating a ferment quickly with a cultured yeast, a winemaker is simply swamping the juice with a single strain already pumped up and ready to go. This strain then dominates the ferment to completion. In a wild ferment there may be ten or more, maybe hundreds of different strains all doing their bit. It is this chorus of yeasts that helps give wild ferments their complexity. This yeast ‘chorus' also prevents any singly strain from singing too loudly and dominating the flavours of the fruit. Ideally the result is both harmonious and restrained, allowing the true character of the wine to show.

There are dangers in allowing spontaneous ferments however. Some strains produce off flavours, others may have high requirements for certain micronutrients that are in short supply, sometimes the ferment just stops. So, it is a risky proposition, but the rewards can be enticing, resulting in wines of great depth and complexity.

See Also: Ask Dan