On the right, you can see a fermentation carried out with a blend of fresh distillers’ yeast and brewers’ yeast (80% WHC Labs Ares and 20% Banana Split): foamy, vigorous, and highly active. On the left is a fermentation using only wild yeast. It fizzes gently, almost like a carbonated drink, and produces significantly less foam. The most interesting differences, however, are not visual, but microbiological.
Commercial yeast cultures are typically highly pure and consist of a single, well-characterised strain. There are many thousands of known yeast strains, each producing different flavour congeners and each with its own alcohol tolerance and fermentation kinetics.
Our wild yeast culture is the opposite. We have never formally analysed its composition, but it is almost certainly a mixed culture consisting of multiple yeast strains alongside a range of bacteria. It is also likely that some degree of cross-contamination from commercial strains has occurred over time, given that they coexist in the same production environment.
This culture has been continuously propagated for over 2.5 years, in a manner similar to a sourdough starter, and no commercial yeast has ever been added intentionally. When it was first established, it was simply allowed to sit until fermentation activity began.
For each fermentation, we reserve approximately 15% of the wort and inoculate it with wild yeast from the previous batch. This portion is taken from the late run-offs of the mash and is therefore significantly weaker than the wort used in our main fermenters. By comparison, our standard fermentations typically reach 6.5–7% ABV, whereas the wild fermentation generally reaches only 2.5–3% ABV.
The result? The wash has layers of acidity, funk, and complexity - almost cider like - and much of this carries over into the spirit. We believe that using natural yeasts and bacteria makes the whisky more distinctly Manx than when using commercial yeast alone.
These elevated acid levels also influence maturation. One of the key processes during cask ageing is esterification, in which organic acids react with alcohols to form fruity esters. A higher acid load in the new make increases the potential for ester formation over time, contributing to greater aromatic complexity in the mature spirit.
We have previously filled a single firkin with new make produced exclusively from wild fermentation. The cask was a refill Islay firkin, and when assessed at one year of age the whisky showed intense ferment-derived aromas: highly fruity, markedly funky, and multi-layered. These characteristics integrated well with the residual peat influence from the cask.
If the flavour is so compelling, why not use wild yeast exclusively?
The principal limitation is yield. Our wild culture typically ferments a maximum of around 75% of the available sugars, and often less, whereas commercial yeast achieves near-complete attenuation. This reduction in fermentative efficiency increases production cost. Nonetheless, we are actively developing methods to increase the contribution of wild yeast in our fermentations and to explore how these mixed-culture systems evolve over time.