Fermentation Basics

Dear Brigade, we must backtrack.  With the frenzied push of harvest and press behind us, we now have time to examine grape fermentation in greater detail.  The hard working, harvest interns at CrushPad provide us with near daily e-mail updates detailing the progress of our fermenting musts (juice + grapeskins).  This way we can examine the data parameters, monitor fermentation progress and ameliorate any incipient problems, lest fermentation grinds to halt with unfermented, residual sugar.  A problematic or “stuck” fermentation is not uncommon, but it is certainly undesired.  To better understand the obstacles facing the fermentation of a high sugar Zinfandel must (i.e. Rockpile), the “normal” fermentation cycle first must be understood.  And so we turn to our Anderson Valley fruit, which has a near-perfect fermentation profile.




This chart displays a lot of information.  We won’t cover it all today, but we’ll slowly tease it apart until you’re all fermentation experts.  So the basics: the x axis has the date, which is “time,” while the white y axis conveniently tracks both degrees brix and temperature.  The green y axis is the number of punch downs, in other words how often the interns come by with a big metal plunger to push the tangle of floating grape skins back down to the bottom of the plastic fermentation bin.  Alas like poor Sisyphus re-hauling the cumbersome rock back up the rugged mountain each morn, the “cap” of grape skin is constantly nudged back to the surface by the force of the  incessant, percolating CO2 bubbles.  But no problem; interns work for free.  So when fermentation is fastest (see that really precipitous plunge in the slope of the blue line?) the intern get out of bed to punch down our cap three times a day instead of two.

Let’s start at time zero, where the red temperature line and blue sugar (“brix”) line are really far apart.  This represents the 5-day cold soak.  The fermentation bin is stored in the cold room (6.6 to 8°C) so fermentation won’t start on its own (known in wine parlance as a “spontaneous native fermentation”).  The cold sort of paralyzes the native yeast and any spoilage bacteria that piggy-backed in on our fruit from the vineyard.  Plus remember from the video that we added that SO2 at crush, which helps too.  We like a cold soak to help extract more color from the berries, especially since pinot is a thin skinned grape.  Cold soak is also reported to increase fruity aromas and flavors, enhance mouthfeel, and amp up the aromatic intensity.  Most studies show color extraction peaks between 3 and 5 days, so a 5 day cold soak is just fine for us.  Curious though, why did the sugar increase on days 2-4?  Well, that’s because the grape juice might rehydrate any raisins that slipped through (and raisins are super sweet) and as the must is better mixed, one gets a truer brix reading.  Thus the cold soak also allows us to fine tune our brix reading so we precisely know how sugary our juice really is.  This is important since yeast work harder when there is more sugar.  When they give up and die, fermentation stops, and this is not good.  Plus more sugar makes for higher alcohol, which itself is toxic to yeast. 

Next step, we (i.e. lowly interns) haul the plastic bin out of the cold room and into the main lobby to bring our must to room temperature.  You can’t just dump the yeast into a cold bin of juice, like drunken co-eds skinny dipping in a near-freezing lake.  The temperature shock is just too great to overcome; they’ll seize up and die.  So we wait and wait until the must warms up a bit.  Then we use the tried and true, highly scientific and rigorously tested “goldfish” technique.  Remember the county fair and the stupid 12 cent goldfish that cost you $35 in ring toss trials?  You kept the goldfish in the plastic bag and floated the bag in the fishbowl until the water in the bag reaches the same temperature as that tiny, clausterphobic fishbowl.  Then you let the critter free.  We do the same thing with the yeast.  You mix the yeast with water or grape juice and put the cup in the corner of the tank to slowly acclimate your single-celled buddies to their new environment.  You set the mood- light some candles, play some sexy Barry White music, and whisper your best, one-lined come ons.  You want those yeast to get ready for wild, fungal nookie, so they can multiply their way right up to a massive 108 biomass to support a fast, rapid fermentation to dryness.

So let’s talk about sex.  The yeast pass through 4 distinct phases during fermentation.  They are called lag, log, stationary and death.  Lag is the yeast adjusting to their new environment and starting foreplay.  Although the yeast are dividing, you don’t really see the results quite yet.  They multiply logrhythmically so there is a gap between when sex starts and when the fermentation really starts churning away.  Log, of course, is rapid yeast proliferation.  Yeast are actively dividing, and we can detect a steady increase in the cell number.  During the stationary phase, there is no growth and no death.  It’s a non-proliferative phase of non-dividing cells.  The bulk of fermentation happens here.  So the yeast are working hard to consume sugar and poop out carbon dioxide (for lack of a more delicate term), but cell division is undetectable.  Death is pretty self explanatory.  As fermentation progresses, sugar is depleted (no more food), alcohol concentration rises (which is toxic to yeast and screws up their cell membrane) and the temperature rises (too hot can be lethal too).  The fermentation curve is the mirror image of these phases.  If one were to plot yeast population against time, lag is a flat line, log is a steep climb as the population grows (and brix starts to drop), followed by a stationary phase (of fast fermentation) with a drop back to zero in cell death.  And in each phase, yeast need different “foods” to stay alive.  In log, the population exploding yeast orgy, the yeast require mostly sugar and nitrogen for proteins.  During the stationary phase, they need oxygen and fatty acids and sterols to armor their cell membranes to withstand the rising tide of alcohol.  Now check out the chart.  Just when the brix drop to 21 and the blue line gets really steep, those clever interns switched to 3 punchdowns a day to better aerate the yeast and ensure they’d have enough oxygen to ready themselves for a high-alcohol Jacuzzi soak.  Plus it dissipates heat to help keep 'em cool.

Let’s stop here for today.  Next week we’ll cover the different components of the fermentation profile as we trace the blue line from high sugar to no sugar and the red line from chilly to toasty.  As you can imagine, fermentation hiccups can happen anywhere along the curve, and the keenest enologist know how to spot trouble before fermentation arrests.