Fermentation Basics (Part II)

Blogging about how sensational our Doctor’s pinot tastes and smells is a joy.  Fermentation mechanics are not, but it is the essence of winemaking, so Brigade, we must soldier on with our science lesson.  Today we’re going to talk about the most exciting part of the fermentation curve in greater detail.  We’re going to focus on the maximum fermentation rate - those magical 48 hours where the sugar (°brix) dropped from 21.8 to about 0.8.  At the 0.8 mark, the slope of the curve decreases dramatically, and the °brix falls by a mere 0.4 over the next 24 hours.  This point where fermentation slows back down again is called the transition point.  It’s important to know about since it has predictive value.  For instance if the transition point hits when the must still measures more than 5° brix, your vat may be at risk for sluggish fermentation and possible arrest.  And check out the temperature, too.  While the sugar plummets from 21.6 to 0.8, the temperature of the juice sky rockets from 17.4 to 30.8°C. Like Paris Hilton says, “That’s hot.”




The maximum rate of fermentation corresponds to the greatest yeast biomass.  In other words, bedroom hanky panky in the early part of fermentation drives the faster stuff later.  Early on, the yeast must reproduce robustly enough to attain the maximum number of critters sustainable in this environment, which happens to be 108 cells/ml.  Imagine all of these single celled organisms consuming sugar and releasing alcohol.  It is a big job (that happens to be a biochemically exothermic process) so the yeast release heat.  That is why you see the temperature rise so dramatically.  And the heat is good.  It increases the yeasts’ metabolic rate and boosts the rate of fermentation overall (remember from high school chemistry that enzymes go faster with heat?).  Heat also discourages spoilage bugs by basically frying them to death.  Color extraction is maximized, as well.  The only downside is that a really hot fermentation may stick in the later phases, and of course, the yeast could boil themselves to death.  It’s a little like Bikram yoga.  It starts off steamy, which feels awesome, but after 45 minutes crammed into a room with 37 other sweaty people, sweat begets sweat, and the collective, accumulated heat becomes oppressive.  You need to take a bathroom break.  Instead of the restroom, the yeast enjoy a cooling punch down, two or three a day to be exact.  Then after that transition point, fermentation slows down and wraps up.  Concomitantly, the alcohol rises, sugar dwindles to zero, and the yeast die.  The process is self-limiting and runs its course.  But we’ll delve deeper still to grasp the details perfectly.

Yeast are surrounded by a plasma membrane.  It keeps their insides from mixing with the outside, like our skin but more fluid.  In some ways, it’s like one of those red bead-string doorways from an Austin Powers love lair.  It can sway, swish, and alter its configuration in different circumstances.  On the other hand, other membrane parts are more rigid, like a baby’s shape sorter toy.  The plastic cylinder can only slide through the circle hole but not through the triangular or butterfly-shaped orifice.  These different shaped holes are analogous to the transport proteins spanning the yeast plasma membrane.  They are doorways to the yeast innards, and only certain stuff can get through.  Sugar, for example, shoots through an exclusively-shaped tunnel and is dumped inside for fuel.  Only sugar can ride that passageway.  Luckily, there is more sugar in the fermenting must than inside the yeast, so sugar shimmies down the natural concentration gradient (from high to low), and the yeast doesn’t have to expend any energy at all to get it inside.  Just imagine if you could ride that doughnut conveyor belt in the Krispy Kreme factory, cruising under the cascading shower of sugary glaze, filling your mouth with delectable frosting without expending a single calorie.  It must be great to be a yeast…except when acid piggyback rides a sugar molecule and ends up inside the yeast, too (indigestion anyone?).  Remember our grape juice is acidic, way more acidic than the neutral yeast, meaning the acid (in the form of protons, H+) is also working a favorable gradient.  The only problem is that now the yeast must expend energy to push the proton back out the door.  This isn’t a big deal if the yeast is floating in a sugar bath, like grape juice, where ready meals abound.  But late in fermentation, when sugar is sparse, things might get dicey.  As the alcohol concentration rises, it messes with those rigidly shaped tunnels and screws them up.  Now the protons are flying into the yeast faster than they can pump them back out.  This acidifies their insides, which they don’t like very much.  In fact, high alcohol is toxic to yeast, and even the staunchest fighters can only survive 18%-19% alcohol at best (& 19% is rare).  This is one reason the yeast die in the presence of the rising tide of alcohol.  But they have ways to combat this, by fortifying their membranes, like wearing armor. 

We’ll talk about how they do that next time.