Hubris Part Deux: Meet The Playahs

Where did we leave off?  Ah - as I was reconstructing my decimated ego, two competing packs of foragers vied to dominate my harvest bounty.  The delinquent microbes were split amongst two rival gangs: the Bacteria Bruisers and the Yard Bird Yeast.  Today we’ll break them down in greater detail.  Without further adieu, let’s meet our first pugnacious pillagers, those rotten ravagers of ripe berries, the vile vandals of Vitis vinifera…heeeeeeeeere’s yeast! Despite the incomprehensible arsenal of microbial players infesting planet Earth, only 15 yeast species exist on grapes.  These select buggers both ferment grape juice to wine and incite wine disease.  And unfortunately, since yeast are ubiquitous, both helpful and detrimental vermin coat the surface of the grape berry from bud break straight through to harvest.  Before we even add our own proprietary “good” yeast to the fermentation bin, 103-105 “native” yeast already call each grape berry home.  Face it; it’s crowded out there.  Every berry is a microcosm of the now deflated, once combative housing market of yore.  Only the hungriest, cut-throat yeast will survive this fungus-eats-fungus sugar melee.   Only one species will devour those last molecules of sugar before the hexose buffet shuts down for the season.  The winner must successfully withstand a low oxygen environment, a rising tide of alcohol, ever dwindling sugar, and a hearty douse of sulfur at the crusher.  These four factors alone quash a majority of the competition.  Only Saccharomyces cerevisiae is inherently built to survive.  And he wasn’t even mentioned on my ETS lab report!

Whether “native” yeast or an exogenous inoculum completes alcoholic fermentation, it’s nearly always a member of the S. cerevisiae clan.  Yes even the “natives” are Saccharomyces kin that have infiltrated winery walls, equipment, and machinery.  But interestingly, S. cerevisiae rolls incognito in the vineyard.  Rare on grape berries, it is almost never identified in laboratory berry cultures.  That’s because Saccharomyces don’t usually start fermentation; they just finish it.  Bluntly, thugs like Hanseniaspora are fluffers; Saccharomyces is a closer. 

When berries are harvested, they host a hodgepodge amalgam of mixed microflora.  It’s like a Benetton ad circa 1986- a mélange of yeast colors, shapes, and sizes.  Saccharomyces gets poised to conquer about 20 hours into it.  After peacefully coexisting with their mixed microbial brethren (“lag” phase), Saccharomyces finally hunker down and outsex them (“log” phase).  Faster than the Dugger clan can pinch out another kid, Saccharomyces explode onto the scene, thriving in the low oxygen, high alcohol hot tub.  Any holdout fugitive species perish, as Saccharomyces dominate and finish the job.  At least that’s how it’s supposed to happen with native fermentation.

So how come there weren’t any Saccharomyces mentioned on the ETS report?  I suspect this mixed bag is pretty typical of early fermentation dynamics; remember this is a 48 hour juice sample.  Of the 4 yeast ETS identified, Hanseniaspora uvarium dominates by a long shot.  As it turns out, this bugger is the most common “native” yeast of all, comprising 99% of the yeast isolated from some samples (Ribereau-Gayon 41).  According to these experts, “in some rare cases…apiculated yeast [i.e. Hanseniaspora] began the fermentation.”  Clearly it’s not that rare since it happened in my bin.  The authors go on to state that “in unsulfited and non-inoculated must, contamination yeast can begin to develop within a few hours of filling the tank.  [Hanseniaspora] are the most frequently encountered…[Pichia] also develop” (83).  Fortunately Hanseniaspora is pretty alcohol sensitive so he wouldn’t have survived much longer anyway.  Yet even in unsulfited juice, S. cerevisiae is usually the only yeast identified by mid fermentation.  This suggests that even without my intervention, the “native” Saccharomyces embedded in the Mauritson Winery walls would have commandeered my bin and completed fermentation as best they could (hey can I go home now?).

The U.C. Davis enology staff cautions us ad nauseam that “off odors” are a very real consequence of native fermentations- one I experienced first hand.  My 48 hour juice sample documented the unintended onset of native fermentation.  But it took another 36 hours for the stench to truly crescendo.  Luckily, immediate inoculation with my yeast plus a couple of pump-overs did the trick.  After that, fermentation completed quickly and smoothly, and the malodorous vapors dissipated into the ether world. 

Whether I was stingy with the sulfur or parsimonious with the dry ice is a question for the ages.  But neither hypothesis accounts for the lactic acid, does it? 

Next week: bacteria!



Handbook of Enology, Pascal Ribéreau-Gayon (Editor) Wiley 2006