By: Tim Vandergrift
Advanced beer brewers routinely make yeast ‘starters’, growing a culture of yeast up from a few hundred million live cells into a powerful, seething mass of hundreds of billions of cells, ready to launch a vigorous, thorough fermentation in their brews.
By contrast, people making wine from kits are instructed to simply rip open the packet of dried yeast and dump or sprinkle it over the top of the must. It almost seems like cheating, or at minimum, missing out on a step that would make better wine.
But is it really? To answer that, we need to understand the purpose behind a yeast culture and the difference between wine must and beer wort, and most importantly, how a yeast cell works.
How a Yeast Cell Works
For all that it’s a unicellular microorganism, yeast is an incredibly complex biological machine. Each cell is a hollow droplet filled with intercellular machinery. The outside ‘skin’ of the cell is two layers of fatty acid esters that forms passages that allow nutrients (water, trace elements, sugars) to pass into the cell, and waste products (carbon dioxide and alcohol) to pass out.
Yeast, however, don’t actually want to produce alcohol and carbon dioxide. As waste products, they’re actually toxic to yeast. What yeast really want is to settle down and raise a family, producing daughter cells and populating their environment. They only change from breeding to alcohol production when population numbers get so high that further reproduction would overwhelm the environment.
This number is somewhere between 10 and 20 million live yeast cells per millilitre of liquid, so it makes sense to get there as quickly as possible, to make sure nothing else happens to the fermenting medium in the time gap between yeast pitching and the onset of fermentation, and that requires a big culture. But does it benefit wine the same way as it does beer?
Beer benefits from a very vigorous starter culture for two reasons: first, the pH of beer is very high and the sugar concentration is moderate-low. This results in an environment that encourages growth of spoilage organisms and bacteria. It's important to get the right kind of yeast in there, in high quantity and in high vigor in order to exclude any spoilage organisms.
By contrast the pH of wine in Master Vintner kits is very low—usually below 3.2. There are a lot of potential spoilage organisms and competitive bacteria that simply can’t grow at that level. The wine just isn’t susceptible to being taken over by nasties.
Second, the YAN (Yeast Available Nitrogen) of beer is not usually as high as it is in the grape juice in our kits, so yeast tend to breed more slowly in beer than it does in Master Vintner kits.
On the yeast side, until recently dried wine yeast had more--a lot more--R&D done on it than beer had, because while commercial breweries handle their yeast in-house, wineries farm it out to specialty yeast companies. The strains commonly used for wine have been bred to the point where they are super-mutants, and even the 'traditional' strains have been carefully selected for:
- Breeding speed
- Competitive exclusion
- Broad range of nutrient tolerance
Wine yeast breeds to culture strength very quickly, which leads to competitive exclusion: the yeast quickly converts all available YAN into trans-fatty acid esters (the material that makes up the outer cell wall of the yeast), sequestering it before slower-breeding organisms can use it.
K-factor (Kill Factor) is the interesting part of yeast life. As they breed, yeast cells release a cytoplasmic toxin, in a type of chemical warfare, and that toxin actually kills bacteria and competing wild yeast. If you pitch equal amounts of EC1118 champagne yeast and 1056 American Ale yeast into a fermentation, in 24 hours there will only be live EC1118. Wine yeast actually sanitizes it's own must.
At Master Vintner we conducted a series of trials with starter cultures, proofed yeast (rehydrated in one of several ways) and simple sprinkling. While there are theoretical benefits from proofing, neither the rehydrated nor the starters made any difference to fermentation timing, clearing, or flavour and aroma to the finished product. Given that it is an extra set of steps that if not done correctly, could spoil the batch, we don’t encourage anything but dry sprinkling.
Liquid Strains and Ester profiles
There are liquid yeast strains available for both beer and wine, and some beer styles can only be made with specific, pure strains of specialty yeast. In fact, if you split your finished wort into two and ferment one with a neutral strain and another with a very assertive strain, such as a German Weisse yeast, they will seem like completely different beers, with the yeast character overwhelming the base material.
Wine’s final flavour profile is mostly derived from the character of the grapes used to make it, with an incredible load of aromatic and flavour compounds coming from the varietal, and from how (and where) the grapes were grown. As such, yeast can make a small difference, but this diminishes over time.
That’s because unlike beer, wine is usually aged for at least a few months before being consumed. Brewers will note that 90% of standard beers taste best when they're fresh. This is partly due to oxidation effects and a drop off in hop character, but it’s also due to the fact that many of the robust yeast flavors become muted over time.
This is true of wine--you can pitch a pure culture of a strain that will produce a significant ester profile, giving the wine a unique character, but after six months to a year that ester profile flags, and the wine is indistinguishable from one that got a simple dried strain. Of course, yeast manufacturers are always developing new strains and new propagation methods, so this could change in the future.