Wild yeast ferments: See  Yeast. | Yeast, wild is more complex.

Clive Dougall, told me (when he was the winemaker for Seresin Estate in Marlborough, New Zealand) that ‘wild ferment wines are seen as risky in terms of overall performance and of aroma or flavour profiles. In fact, they are easy to handle if you use your senses and experience rather than just relying on the science you learnt. We have learnt that a slightly warmer ferment and a considered amount of oxygen exposure is necessary for success in this area, so the modern winemaking techniques that exclude oxygen at cold temperatures will work against you in a natural ferment. It also goes without saying that the vineyard’s biological health is important as we rely on a healthy yeast population coming into the winery with the grapes.’

Allan (“Nash”) McWilliams, another winemaker based in Marlborough, New Zealand told me (perso communication) ‘organic viticulture encourages stronger native yeasts because levels of YANs (Yeast Available Nitrogen) are lower under organics, usually around 120-220 milligrams per litre (mg/L) compared to much higher levels of 350 mg/L with conventional fruit due to conventional farming’s use of nitrogen (N) rich soluble fertilizers. The wine-making rulebook says – and depending on how much sugar the grape juice contains – that grape juice with levels of YANs below 120 mg/L requires additions of yeast nutrients like diammonium phosphate or yeast hulls. However with the high levels of YANs found in conventional grapes the yeast get lazy. With the lower levels of YANS found in organics the yeast have to work harder to get the N out of the wine which is is a good thing. If the yeast find fermentation too easy they just go for the glucose sugar in the grape juice and only start to work on the harder fructose sugars at the end of fermentation. This can cause a stuck fermentation just at the moment when fermentation is hardest to restart. If yeast have to work harder right from the start of fermentation they must consume both fructose and glucose. Levels of YANs are especially high if spring is wet, because the grapes take up lots of nitrogen which later will make fermentations easier and generally quicker.’

Alessandro Ceretto told me (at Castello di Barolo, Sunday afternoon 16th July 2016) that alcoholic fermentation via wild yeast is slower than with cultured yeast, and allows steady fermentation which reduces stress on the yeast and obviates the need to add yeast food (DAP or diammonium phosphate and/or vitamin B or thiamin). The slower ferment means the wine is also naturally protected by carbon dioxide for longer than it would have been with a cultured yeast. Faster ferments are more likely to produce stress and thus off-flavours, typically eggy notes caused by hydrogen sulphide, which would then need fining out by adding copper sulfate. Cultured yeast need more nutrients compared to native yeast, too. And you have to add sulfur dioxide before fermentation starts to knock out the native yeasts so as to leave a clear run for the cultured yeasts.’ Diammonium phosphate additions may also increase the risk of brettanomyces.

Arnaud Immelè, an oenologist and consultant specialising in wines made using ancient winemaking practices told Demeter Austria’s Biodynamic wine conference in Vienna in February 2018 how barrels which had been used to ferment wines seeded with cultured yeast could carry those cultured yeasts to the barrel’s next owner, even after cleaning. A maker of natural wine, who had bought those second-hand barrels, and who fermented his/her wine in them without any yeast addition would feel justified in claiming the wine was ‘wild fermented’ when in fact it was a cultured yeast ferment because the residual population of cultured yeast is more aggressive and can outcompete any wild or ambient yeasts.

However, Immélé (perso comm. Dec 2018) added that ‘the microbiological situation in must is complex. For example, grapes harvested at low levels of maturity have few saccharomyces strains on them, and sometimes none at all, meaning the fermentation will be via the action of yeasts in the winery, possibly also including yeasts from old barrels. Grapes harvested at high ripeness levels, in warm conditions and possibly with some rot means the yeast population can attain up to 1 million cells/ml of must, and then the fermentation will be via the action of ambient yeasts on the grapes, nullifying any cultured yeasts present in the barrels. You can also change the situation by preparing your own self-made “pied de cuve”. Another example would be if the harvested grapes are kept in very cool containers, and kept at a cool temperature until the beginning of fermentation, you will also change the composition of the microbiological situation, promoting Hanseniaspora uvarum, and get a wine with higher levels of ethyl acetate. So each decision in the process of harvesting, the date, the time of day, and the weather at harvesting will shape the microbiological composition of the must, and therefore influence the microbiological typicity of wine.’