Winemakers traditionally allowed their juice/must to begin fermenting naturally. There are thousands of different species of yeast that can be present on grape skins and they will be transported into the winery during harvest. Over time, winemakers of the past began encouraging the 'best' yeast that would arrive with their grapes. Once the technology was available, these strains were isolated and have become today's active dry wine yeast.
Many wineries will attest to have an 'indigenous yeast'. Even with all the advances in winery sanitation, yeast can be surprisingly resilient and will stick around in wineries for years, ready to jump into the first sugary substance available. I like to refer to such cases as having a 'resident yeast' because more than likely it is a commercial yeast strain if a winery has used them. The reason is that most commercial strains are isolated specifically because they can overpower the other strains and microbes that may be present in juice/must.
Winemakers have varied opinions on the use of indigenous yeast in commercial wine production. Some will use it for 100% of their production, but plenty believe that commercial yeast strains are the only way to produce quality wine. A good number of these winemakers will still incorporate indigenous yeast into their winemaking programs: some allow only a portion of production to ferment spontaneously, while others will promote indigenous ferment for several days before inoculating with commercial strains.
Winemakers also have varied reasons behind their desire for indigenous fermentation. As marketing has become increasingly important in today's industry, some winemakers primarily want to claim their wine is produced 'naturally' or with 'minimal intervention.
Many winemakers desire the characteristics indigenous fermentation can provide. Proponents claim indigenous yeast:
- Ferment slower, helping retain volatile compounds that enhance aromas and flavors.
- Produce more textural wines compared to commercial counterparts.
- Lead to a more harmonious wine.
- Produce wines that better reflect the vineyard's terrior.
Those opposed to indigenous yeast will site the disadvantages.
- An extended lag phase. Indigenous yeast often takes several days to begin fermenting vigorously, giving other microorganisms such as lactic acid bacteria and Brettanomyces the opportunity to take hold.
- Higher susceptibility to alcohol toxicity. This is primarily due to problems with reproducing generations strongly throughout the ferment process.
- Easily stressed. This is due to several issues such as low population level for ferment and high nutrient demand.
I personally think the benefits outweigh the risks when it comes to indigenous yeast. The risks can be minimized with winemaking diligence, while wine quality can be increased greatly.
Ensuring the juice/must is prepared properly is essential for successful indigenous yeast fermentation. Nevertheless, I would be hard pressed to base my entire production off indigenous yeast due to the simple fact that such wines often lack many qualities that commercial yeasts provide. In my opinion, the complexity provided when blending wines produced by different yeast strains tend to make a more complex and well-balanced final product.