Monday, November 22, 2010

Yeast: Part I of XVI

During my time at the winery, I've written about most every step in the winemaking process except for the following:

- Adding yeast & nutrients for primary fermentation
- Malolactic (secondary) fermentation
- Fining and filtering (the steps that precede bottling)
- Blending
- Bottling

If there's one step that's more critical than all others, I'd have to say that it's adding yeast. Unfortunately, it's something that I didn't witness very often. We typically started working at 8:00 am, but the winemaker often added yeast and nutrients to the fermenting must around 7:00 am.

The process is not too complicated, but many things can go wrong. First, a package or bottle of dried yeast is mixed with warm water. The resulting mixture looks like a giant latte (coffee + milk colored, opaque) and smells like baking bread. There are different types of yeast, and choosing the right one(s) is what makes great winemakers great. We typically fermented different bins with different strains of yeast to obtain varying characteristics in the wine.

Syrah yeast.  There are hundreds of different types of yeast that can be used
to ferment must into wine.  We used about a dozen varieties this year.

The liquid yeast is poured into the tanks and bins, and this procedure is called "inoculation". The yeast cells then process the sugars in the must into CO2 and alcohol. And herein lies the difficulty in making wine: As the level of alcohol rises, it kills the yeast. As the sugar gets depleted, the yeast cells run out of food. If the temperature rises too much, the yeast dies. If the temperature is not high enough, the yeast doesn't process the sugar. If the pH isn't just right, the fermentation can be affected. And so on... It's a pretty delicate process, and getting it right isn't so easy.

The amount of alcohol that is produced by the yeast is directly proportional to the sugar content of the must, and since we crushed very ripe grapes we often had to add water to dilute the must. Too much sugar will create too much alcohol, which will kill the yeast before the fermentation is complete (but we have a secret weapon to fix this problem). Sometimes, the fermentation can be re-started once the wine has been transferred to barrels:

The master adds a little bit of WS yeast to some barrels of wine that are
still a little too sweet. This yeast will ferment the remaining sugar and
dry out the wine. It's not the preferred way to do things,
but sometimes you don't have much of a choice...

I'll post a few more things about yeast, most notably a little something about some very special yeast that we use in desperate cases. Stay tuned...

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