Stuck on You

Back when I was a resident, I dreaded my requirement at the Medical Examiner’s Office.  It just never seemed terribly intellectual.  One might query, “Just how did patient X die, doctor?”  To which I would reply, “That is pretty obvious, Officer.  His body is in three pieces on the I-5 freeway.”  Grotesque as it may sound, that’s sort of how it rolled for the whole month (including the guy who played Russian Roulette and lost, no joke).  Sure you could delve deeper; a decapitated guy isn’t getting blood flow to his brain, so we can talk about anoxia in neurons (a.k.a a guy not getting blood to his brain).  The story of our sticky 2009 zin is sort of the same dilemma.  Why did fermentation stick?  Well obviously because we picked the grapes way, way, way too ripe.  That wasn’t our intention, of course.  Now as you may recall, our Rockpile acquisition was Forrest Gump dumb luck.  After months of dead end phone calls, I made a final cold call that three more phone calls after that ultimately led us to Rockpile grower Chris Mauritson.  The fruit had fallen out of contract with another winery, so we swooped in as a last resort to anchor a mini-foothold in that amazing region.  Now that our inaugural Rockpile zin is finally bottled, it seems as good a time as any to review the ins and outs of stuck fermentation. A “stuck fermentation” is any fermentation that stops before you’d like it to.  Sometimes it is a winemaker’s preference to make a sweet dessert wine, a stylistic choice that necessitates residual sugar.  In that instance, the residual sugar isn’t a “stuck fermentation” at all; it’s intentional.  However our Rockpile zin - while indeed a tasty companion to a chocolate torte - is not intended to taste sweet.  It is dry, brawny, fruity, rich and tannic, strutting its Rockpile pedigree like Lindsay Lohan posing for paparazzi in stolen jewelry.  This is not yo mama’s zinfandel.  Needless to say I wasn’t surprised when the ’09 Rockpile fermentation stuck, and stuck again, and again.  Frankly I wasn’t sure the fermentation would ever end dry.  And the longer the yeast remained stagnant in their sugary, high alcohol bathtub, the harder it would be to restart their microscopic engines.  What makes high sugar so toxic to yeast anyway?  And what makes fermentation slow down to a sorry, lopsided crawl?  To find out, we’ll be taking a special imaginary journey, deep into the miniscule world of the yeast.  Let’s observe fermentation firsthand.

You know the basic drill.  The yeast intake grape sugars, convert them to alcohol, and spit them out.  At the most fundamental level, the rate of fermentation is controlled by the rate of sugar consumption.  The faster the yeast suck up sugar, the faster the juice ferments.  Yeast are decorated with different doors that allow stuff, like nutrients and food, to pass in and out of their bodies.  These doorways, called “transporters,” are shape specific.  Like a baby’s shape sorting toy, only sugar can fit through the sugar doorway; the conformation of the protein-specific gateway is entirely different.  If a sugar molecule sidles up to the doorway properly, it will bind to the yeast cell surface.  Once recognized as sugar, it is locked in place.  The “lock and load” initiates more shape shifting that ultimately flips the sugar inward where it’s metabolized as energy.  Once cargo is off loaded, the conveyor belt resumes its empty configuration on the yeast’s surface.  The transporter can only ferry one sugar molecule at a time; too much sugar trying to bind simultaneously jams the system.  Luckily the yeast can swap out one convoy channel for another at a moment’s notice.  This is because the transporters are governed by a gene family named “HXT,” which can up or down regulate sugar flow as necessary.  In a high sugar media (like our Rockpile juice), the binding is highly specific, so the sugar needs approach the loading dock from a specific angle to bind properly.  This curbs the tide of sugar and slows the rate of influx.  Conversely, if the yeast needs a 4 o’clock sugar fix, it can break out the low affinity hooks to nab any sugar molecule irrespective of its angle of approach.  In fact the yeast house a pretty complex and fine-tuned system, mainly because stockpiling sugar is toxic.  The yeast need to break down the sugars immediately, which means if sugar accumulates inside, then additional sugar intake slows down concurrently.  Loss of transporter activity is associated with stuck or sluggish fermentations but is also a survival mechanism to prevent death by massive sugar poisoning. 

Besides being dangerous in large amount (but what isn’t?), sugar is also a freeloader.  It glides down the yeast’s conveyor belt via “facilitated diffusion.”  This is scientific verbiage for “for free.”  In other words, the yeast doesn’t expend any energy to suck up the stuff, so it moves for free, at least in terms of metabolic energy costs.  Sugar flows down its natural concentration gradient, so long as yeast metabolize what they internalize.  This way there’s always more sugar outside than inside.  What’s expensive is pumping s*&^t out.  This is why high alcohol levels are so problematic.

“It has been suggested that ethanol impacts membrane function by polar, dielectric, and hydrogen bond interactions with polar head groups of the phospholipids and integral membrane proteins (Bisson).”  Right; let’s break it down.  This simply means alcohol screws up the unique shape of the loading docks.  Instead of the lock and key analogy where only sugar molecules can unlock and open the gated corridor, now we’re talking about unguarded, gaping tunnels.  Anything and everything can float inside, including acid.  Under normal circumstances, yeast use stored energy to pump acid out so they don’t fry to death.  Survival hinges on tight control of internal pH.  Compounding acid injury is a slow cytoplasmic leak.  Where protons flow in, goo oozes out.  Like water through a sieve, the yeast leak out their guts (called “cytoplasm”) which is probably more toxic to survival than death by acid.  So as you can imagine, fermenting that ripe Rockpile juice was a Herculean task.  Not only did we face high sugar concentrations from the onset but also the potential for profound alcohol toxicity.  Luckily, with some warmth (yeast like it snuggly) and some yeast hulls (yeast carcasses) to mop up the toxins, we got the job done.  At bottling, the wine tasted really delicious, and I know you’ll love it, too.  I’m also certain that everything you’ve retained about the physiology of stuck fermentation will be washed away with your first sip.

Cited: Linda F. Bisson, Stuckand Sluggish Fermentations, Am. J. Enol. Vitic., Mar 1999; 50: 107 - 119.