Fermentation is the process through which a yeast turns the sugar found in grape juice into alcohol and carbon dioxide (CO2). Typically, winemakers choose to either let their fermentations run spontaneously (driven by the natural yeast found on the grapes – this is also known as wild or natural ferment), or to inoculate the juice with a commercial yeast.
There are advantages to both methods, and winemakers make their decision based on the winemaking needs and the desired outcomes. Inoculating with commercial yeast strains (which are usually Saccharomyces cerevisiae – typically strong and efficient fermenters) bring certain predictable advantages to the fermentation process, such as higher alcohol tolerance, or known nitrogen requirements. Certain strains can also be reliably used to impart specific characteristics to the wine – for example you can choose a yeast which will impart more thiol characters (passion fruit, guava flavours), or one that will produce more esters (citrussy flavours) in a white wine.
Wild yeasts typically take longer to start fermenting and do so at a slower pace than their commercial counterparts. This is because the species present naturally on the grapes are not necessarily strong fermenters. Where an inoculated yeast will ferment a wine dry within 2-3 weeks, a spontaneous ferment can take up to 6 months. Natural ferments also need to be monitored very carefully, to ensure the yeasts’ nitrogen needs are seen to. This helps to avoid the dreaded stuck fermentation, which is a risk in spontaneous fermentations. Other ways to prevent stuck ferments include moving the barrels into warm areas or sunlight and stirring the lees regularly. Volatile Acidity (VA) in the wine also needs to be kept in check.
Despite the potential challenges of wild yeast fermentations, the benefits are worth the effort of managing them carefully. The unique microflora present on the grapes can be seen as part of the site’s terroir. The natural occurring yeasts also tend to bring more nuance and complexity to the wines, allowing a truer expression of terroir.
Nic chooses to use inoculation for fermenting our Bordeaux varietals, and uses a range of different commercial yeasts for that purpose. For our Rhone varietals and white wines, he lets the natural yeast present on the grapes do the work. The frequent mixing of the lees that natural fermentations require cause an increase in breakdown of the yeast cells. This leads to an increased release of glycerol, which in turn creates a richer, fuller mouthfeel on the wines. There are many advantages to wild ferments, and this method is considered by many to be key in expressing terroir, but a scientific approach is still required to ensure the fermentation completes without issues. Neither method is superior to the other – in the end, it is all about understanding your terroir, the grapes you are working with and the wine you want to make, and choosing the right path to create an exceptional wine.
