Slow Down (pt. 2)
Reviewing from last week, the following factors are known to accelerate fermentation - either in the winery or via controlled laboratory experiments: 1. the initial sugar concentration of the pre-fermentation juice ("must concentration")
2. nitrogen concentration
3. growth factors like biotin and thiamine
4. fatty sterols
6. yeast hulls
7. elevated fermentation temperatures
So down the checklist we go. Start with sugar. Our Anderson Valley grapes came in at 24 brix, which is just perfect. If the initial sugar is too low, yeast growth may be limited by inadequate nutrients. Conversely, and counterintuitive but true, if sugar is too high, fermentation actually slows down, and may even cease altogether (luckily we won't address that here). Next is nitrogen. Like bodybuilders, yeast require ample nitrogen, the building block of protein, in order to grow. The nitrogen in the grapes is measured in the laboratory as "YAN," the yeast available nitrogen. The value is highly variable across grape varietal and vineyard. Often low nitrogen is more pervasive in white wine making, when skins and seeds are absent from the fermentation potion. If nitrogen is deemed low, it can be added to the fermenting brew as diammonium sulfate. Luckily our pinot grapes came in at over 300 YAN ("300" being the magic number) so no additions were necessary. Next up is akin to Centrum Silver- A to Z. Yeast need vitamins and minerals too, and in lab experiments the addition of thiamine and biotin accelerates fermentation kinetics. But again, our pinot skin and seeds provided enough of both.
Now we get tricky. Obviously, yeast "eat" grape sugar, but clearly they lack the hands and mouths necessary to gorge themselves at will. So instead, the sugar "diffuses" across its yeast skin ("cell membrane") like a queue of Nordstrom shoppers floating through the revolving doors at the Anniversary Sale. Extrapolating back to our yeast, when the doors lock, sugar can't get inside, and fermentation stops. And their door is made up of fatty stuff called "sterols," and the amount and type of sterol in door either helps or stops sugar from getting inside. Some specific fatty things (like "C18") accelerate fermentation by opening the door even wider; others (short chain fatty acids like "C6, C8, C10") preclude membrane permeability and slam the door shut. Yeast hulls, essentially dried, dead yeast carcasses, mop up the bad sterols, essentially restoring the open door policy and re-establishing permeability. And remember when I told you that in the presence of oxygen yeast respire instead of ferment sugar? Well, I lied a little bit, since yeast actually need oxygen to make sterols. And so now you know that limited oxygen exposure, particularly during the active growth phase, accelerates fermentation considerably. We've fermented our grapes in open topped bins. While this seems like limitless oxygen exposure, in reality, the carbon dioxide by-products of vigorous fermentation condense into a heavy, invisible cloud over the tank, which limits ambient oxygen exposure. Our aeration is provided by twice daily punchdowns and cap management.
The last salient feature is temperature. Again, temperature requirements are fluid- too hot and yeast die but not hot enough, and they can't boogie at all. Our wine style favors a hotter fermentation to optimize color extraction. And elevated temperature, up to a limit, accelerates fermentation. Our hottest fermentation temperature, maxing out at 31.9 degrees C, hit just about the upper limit of fermentation manipulation. And remember too, effects are additive. So the addition of nitrogen or aeration will speed fermentation but not without an accompanying increase in temperature, as that exothermic process accelerates and blows off more heat. I believe the layered effects of ample nitrogen, sugar and elevated temperatures contributed to our vigorous and rapid fermentation. But sometimes all of the book learning in the world can't compensate for practical experience. So I just asked Chris for the answer.
He said this, "Once those yeast got started, it was like dropping a rowdy kid at college for the first time or giving an alcoholic keys to the liquor store. There was lots of nitrogen and sugar, and the yeast just went nuts." And if the yeast are happy, then we're happy, too.