Fermentation 101: A Quick Primer

WINERY ALERT:  We're scheduled to press our Doctor's Vineyard pinot noir on Sunday 9/21 in the afternoon.  We have at least 4.5 Brigade members planning to attend.  If you want to join us, please drop us a line.  This won't be as work intensive or time consumming as the sorting, AND you will have the first opportunity to taste our wine!


Editor's Note:  This is a science intensive post.  As a reward for making it through the whole thing, there is an "informative" video at the end for those of you willing to brave a little biochemistry on a Monday morning.

The grapes from Doctor's Vineyard have been sorted and destemmed.  After 4 days in the cold room, we wheeled them into the winery on Saturday 9/13 to bring them up to room temperature.   They were innoculated with yeast on Sunday 9/14, and now the magic of winemaking begins.  Cue celestial chorus.

"Magic" is cute, unintimidating, Disney-speak for "mind numbing biochemistry."  And nobody really wants the resopnsibility of explaining it.  Codifying his reputation as the funny, Tuesday pun guy, Brian has kindly deferred this excruciatingly painful topic to my Monday blog.  Yes, it's "science," and my background is, well, science, but really, nary an autopsy doth an enologist make (Shakespeare, 1603).  If reading about pinot clones puts you to sleep, then this begs for No-Doz.  So grab a Starbucks and wear your thinking cap, because here we go.

Most simply, yeast consume the grapes' natural sugar and convert it to alcohol- done!

C6H12O6  è 2CH 3 CH2OH + 2CO2   56 kcal/mol; (-) rT

One unit of sugar (here known as "glucose," CHO) is converted to 2 units of wine (OK, it's really ethanol, COOH, but who's complaining) and carbon dioxide (same stuff you exhale with each breath).  Notice there's no oxygen here.  The yeast backstroking in their Olympic pool of grape juice quickly consume all of the available oxygen with their first sugar binge, leaving them in an anaerobic (i.e. oxygen-free) environment.  This is why we get wine.  Fermentation is an oxygen-free process that ceases in the presence of oxygen.  In contrast to fermentation, when oxygen is around, those pesky yeast "respire" instead and use oxygen to spin sugar into plain old water plus carbon dioxide.  And that's no fun, no disrespect to Evian.

So what's with the weird triangle?  It's a delta, my Greek frat brothers.  Delta T, change in temperature, signifying an exothermic reaction that releases heat.  Translation: swimming yeast train hard, and they just get hot.  Like a fat guy on a spin bike, the temperature in the room rises.  Each unit of sugar consumed releases 56 calories, and for approximately every percent of sugar lost, the temperature rises 2.30F.  This is fine.  We like some heat.  Heat helps extract the maximum color from those testy, thin-skinned pinot grapes.  Heat keeps fermentation brisk.  But alas, too hot and the yeast will crap out and die before their job is done.  So we do things to dissipate the heat.

Top Ways to Chill (if you're a microscopic wine fungus)

#1) Spend 4 days at the Four Seasons Hualalai with round the clock Mai Tais

(oops, wrong blog...)

#2) Hit the freezer.  I am being blithe, but for the lowest tech, bang for your buck, just move your fermentation bin into a cold room.

#3) Punch Downs.  The grape skins congeal into a gloopy, floating Pangea at the top of the fermentation tank.  Called a "cap," this amalgamation makes it hard for heat to escape.  Winemakers push the cap to the bottom of the tank with a giant, metal, rake-like contraption.  This is called a "punch down."  Juice can even be pumped from the bottom of the tank back over the top with a hose, as in a "pump over."  Both promote even skin/juice contact and dissipate heat.

#4) Be like Meb.  In Athens 2004, Olympic marathoner Meb Keflezighi donned a Nike-invented pre-race cooling vest to lower his core temperature.  Big fermentation tanks are also equipped with cooling jackets on their surface.  These so called refrigerator jackets are pretty popular in big wineries, as are other sundry heat exchange devices.

Being a small production gig, we're at an advantage since we ferment in short, plastic bins.  These containers have a HIGH surface to volume ratio, which is also useful for heat management.  Whooooshhh, physics.

"Now make it stop," you plead.  Well, the yeast stop automatically - oh you mean the lecture.  Almost there.  The yeast binge and chomp their way though Mary Kate Olsen's only-in-a-dream, total carb dinner, until all of the sugar is consumed.  At last we pronounce "fermentation is dry."  Thus a dry wine means no residual, natural sugar is present in the finished product.  Without an energy source, the yeast die, and their carcasses drop to the bottom of the bin.  Rather than "road kill," we call them "lees" and preach they add complexity and flavor to the wine (which they do).  Vegetarians, fear not; the dead critters are filtered out by the time your pristine and crystal clear wine is ready for bottling.

And there you have it - fermentation 101.

My brainiacs, as a reward for reading this far, Crushpad's own Kian Tavakoli (formerly of Opus One, ahem!) is going talk about harvest 2008 and our fermentation process in the video below.  If you can't see the video, please click here and scroll to the bottom of the page.