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Tuesday, October 12, 2010

Wine Yeast

   Fermentations seem really exciting, like the first hour of dinner rush when everything is going well and everything is in the right place and all the dishes are up at the same time. Or that first dinner date full of your clever and witty comments for the first hour. Toward the end fermentations feel like the table that was seated 30 minutes after the kitchen has closed, and they want to do the full tasting menu, or when you made a joke about hating cats and you find out she has 14. Some fermentation are smooth and fairly linear. Some whites have been very nice and clean, just need a little bit of cooling during the peak activity, taper off and let them finish warm at the end and boom. Done. Some reds tends to be fast and hot and leave cab fare the next morning, like Barbera. Sometimes a fermentation is a problem every year, no matter how well you prepare.
   Our Dolcetto, which should be the quickest and easiest wine to make and drink is the biggest headache every year. For three years in a row everything looks fantastic - perfect temperatures, everything healthy, moving predictably, and then all of a sudden at 5 brix it slams on the brakes. That clean 7-day magic just became 12-13 days of nail biting drama, getting bitter and tannic and threatening to produce off flavors.
   Yeast are magical fungi that facilitate a lot of what is wonderful in the world. For example: alcohol (including favorites such as beer, wine, and Old Turkey), bready goodness like ciabatta and the amazing brioche (butter is another wonder), kefir, some stinky cheese rinds and MSG. They also have some evilness, like infections and food spoilage. They are quite a broad range of beasties. For winemaking we are interested in two basic types, and selection are propagated from those that have special features. Some are more voracious than others, some are more sensitive to heat or cold or their nutritional preferences. Some pump out extra goodness in their lifespan, like glycerol that imparts fullness and sweetness - this however is at the cost of greater nutritional fastidiousness and potential difficulty.
   More important though than just choosing yeast from a catalogue is that grapes can come with their own hurdles. High potassium in skins can actually shut down yeast metabolism. Different vineyards carry different microbial loads, and these can interfere with the yeast doing their job. Sometimes there are other organisms that are subtly part of a flavor profile that is accepted as part of "terroir."
   The problem is that yeast are particular, and when they are stressed, scared, or feeling insecure or unappreciated they create flavors and aromas that humans don't care for. Ever smelled rotten eggs, garlic, or farty wine? Yeast did it. One of the most widespread modern afflictions is a yeast called "brett" (Brettanomyces) that can feed off of wood sugar in barrels, residual sugar in wines (there is always some) and is highly resistant to sulfur. It is far more voracious than the yeast that conducts fermentation, and in fact in can ferment on its own as well. The problem is that it produces yucky smells like wet horse, sewage, or other fun descriptors. At a low level it can add complexity (this is common in some Malbecs from Argentina on purpose) and some beer styles actually inoculate for it - far away from the winery hopefully.
   Yeast actually start dying off about 1/3 of the way through a fermentation. They quickly build up a huge population, generate heat (we count on the yeast generally raising a tank's temperature from 68 degrees at the start to 84 at peak fermentation - all by their own kinetic activity), and start dying off and dwindling before they are even half done.  It is pretty amazing to think about. Four tons of grapes can be approximately one ton of sugar. And a few days later it is all ethanol, CO2 and some other stuff. All by a few horny fungi that generated millions of children in a couple of days - joylessly.
   Most winemakers develop a favorite yeast that behaves predictably and cleanly. Getting that wine dry smoothly is the first and most important task. Alice Fiering will tell you that winemakers just pick the yeast they want out of a catalog to give them the flavors and features they desire, destroying all authenticity in one quotidian blow, and to a very tiny limited extent this is somewhat true. You can get a little more passion fruit with one, a little more raspberry with another, but it quickly diminishes over time. Yeast choice is for Sauvignon Blanc, not Cabernet.  Getting the wine fermented cleanly is much more important. Even those wild/indigenous fermentations are in fact from the same family as cultured yeast - and they have been the most successful. Yeast don't make magic. They can catalyze some aromas that are already there yet undetectable to our limited senses (cue discussion of Hume's color theory), but so can "native" yeast. The issue often becomes what is historically correct and appropriate. There will be more said on this later...
   You can always press the wine while it is still sweet and allow it to ferment in barrels or in another tank - and this is common practice these days. People fear tannins and want fruit bombs, so they press the grapes early before they are dry and the ethanol extracts all the tannins from the seeds into your juice. Problem is that it can halt the fermentation by shocking the yeast with a temperature or turbidity change. It also introduces sugar into a barrel which raises the risk factor exponentially. We do not have the ability to keep barrels at a happily fermenting 70 degrees when it is 35 degrees at night outside, so for us it is a difficult option. Not to mention, the Italian historical paradigm that we try to follow is do not press until dryness, and you will notice that Italian reds are always very dry, and this is the reason.
  Anyway, yeast are magic. Stupid, lazy horny magic.

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