Cultured and natural yeasts

Michael Fridjhon wrote recently about the problems that can be associated with the use of ‘wild’ or ‘natural’ yeasts – as opposed to the cultured versions that most winemakers use these days. ‘When I read about natural yeasts, wild ferments, and a low/no sulphur regime’, he says, 'I brace myself for off-odours, taints, and the pong of bacterial spoilage.’ A Grape reader, Clive Sindelman, asked us to get a comment defending the use of ‘natural yeasts’ from a winemaker who uses them. So we asked Chris Williams of Meerlust and The Foundry for some explanations and thoughts (and would welcome any further contributions from others – either pro or con).

Chris says:

Firstly, let me state that I consider myself a ‘terroirist’ – the question is whether yeasts are a part of terroir.

Michael Fridjhon rightly states that there are no ‘un-natural’ yeasts: all the commercial yeasts used by winemakers all originated in nature, they are isolated from particularly successful fermentations from all over the world, usually for a specific positive attribute that they show such as fermentation rate, cold tolerance, short reproductive phase, particular ester formation, low SO₂ production, low foaming, ethanol resistance, high osmo-tolerance (able to ferment all those sugars from over-ripe fruit) and their ‘killer’ status (This means that these strains can produce a toxin which kills or inhibits other strains of yeasts).

The thing about these ‘inoculated’ strains is that they are a pure culture, one strain with known properties. Therefore a winemaker can use CKS or Vin 7 to accentuate varietal aroma on something like sauvignon blanc. All the yeasts which ferment grape juice are Saccaromyces cerevisiae, but within this group there are genetically different strains.

There are some winemakers who will use a ‘cocktail’ of yeast strains on a particular tank to give different characteristics and therefore possibly complexity, which is plausible but there are so many other factors at play that is difficult to prove.

What many winemakers call ‘wild’ or ‘natural’ fermentations should more correctly be termed un-inoculated fermentations. If a winemaker has used a cultured yeast in the winery before, especially a ‘killer’ yeast, that will be the dominant strain in the winery and will carry out any ‘un-inoculated’ fermentations, but not before a couple of other yeasts have fermented a little of the sugar before dying as a result of ethanol toxicity, maybe 2-3 percent alcohol. Most of these species (Hansanula spp, Pichia spp) are considered as spoilage yeasts which produce off -flavours.

Adding complexity?

Where ‘un-inoculated" fermentations can make a positive contribution to complexity it has more to do with fermentation kinetics than different species of ‘wild’ yeasts. A yeast will produce different by-products depending on the conditions of the fermentation. These are termed ‘low dosage’ fermentations because no actual yeast is added by the winemaker, but the dominant strain takes over the fermentation after a long ‘lag’ phase. It will not be a high population and will take longer to complete (if it does finish and not get stuck) the fermentation. In my experience, these fermentations do have a different organoleptic profile than inoculated fermentations – sometimes that profile is positive, sometimes not. Most commercial yeasts (apart from their selected traits of osmo-tolerance etc) are higher ester producers and thus accentuate ‘fruitiness’ in wines. This fades away relatively quickly. Un-inoculated fermentations tend to be more savoury, ‘oatmealy’ and richer on the palate, possibly because of glycerol formation. Such wines will almost always have a higher volatile acidity. I quite like playing with both techniques. Again, this is only my experience, I havn't convinced any wine biotechnologists to do the research yet.

Personally, I get good results on some batches of wines (chardonnay in barrel, syrah) by allowing the first half of the fermentation to be done by the resident yeast, then inoculating a strong fermenting yeast to ensure the wine ferments to dryness.

Winemakers who claim to have a ‘unique’, undiscovered strain coming from their vineyards which positively impacts the quality of their wines could make much more money by claiming ownership (if a person can own a genetic sequence) of this yeast and selling the rights to a biotech company than selling wine. Most of the wine yeasts have been isolated and are commercially available, in fact, one can purchase yeasts called ‘Bordeaux red’ and ‘Montrachet’, much cheaper than actually buying a bottle! You can make it yourself at home. That is a joke, of course the true character of a wine is mostly determined by the variety and geography and not by what us mere winemakers do.