Between A Rock and A Hard Place
Like so many life experiences, I knew about it, I’d read on it extensively, but I’d never experienced it firsthand. After all, I never thought it could happen to me. In fact, just this past summer, I’d boasted in an online discussion that I’d never suffered a stuck fermentation (in my oh-so-extensive single year of harvest experience). Now let me eat crow and cower at the toes of the great karmic gods, for hubris followed me to Rockpile. “High Brix juices pose a fermentation problem for yeast” (Bisson and Butzke). We knew the Rockpile fruit would be challenging. Given the high Brix and the variable berry composition, which ranged form super ripe to full fledged raisining, we braced for an uphill battle against every conceivable handicap. Our grapes came in at over 30 °Brix. My UC Davis literature doesn’t even propose guidelines for the nutritional supplementation of grapes over 27 °Brix. We were in unchartered territory, for me at least. None of this was addressed in my neatly packaged, academic syllabus. Further complicating matters, the acid was high (yeah!) but so was the pH (boo!), meaning that if we added neutral water to dilute the sugar, it would just push the pH even higher. But we went for it, starting “au natural,” waiting for the must to start churning and bubbling on its own before supplementing with Clay’s cultured “Rockpile” yeast. Everything looked remarkably good. The sugar dropped, and the cap held firm. In fact, fermentation began so smoothly that we'd almost forgotten about those pesky raisins.
“Once cells lose viability or permanently adapt to the adverse conditions of the environment by reducing sugar consumption, it is very difficult to restore the rate of fermentation” (Bisson). Zinfandel is infamous for unevenly ripened clusters, and our super ripe fruit was no exception. We’d have to contend with raisins living in our must. Since we’re a mom and pop-sized operation, we ferment our wine in small, one ton plastic bins. I think you’ve seen them in video clips and photos. They are plastic boxes, devoid of fancy hoses, screens or pump attachments. As the must ferments, the raisins sink to the bottom of the box and stay there, rehydrating in the juice, leaching their potent, sugary venom into our percolating concoction. Contrast this with large production fermentation tanks. These bad boys employ sieve like grates so that during pumpovers, the raisins are ensnared in the mesh and removed from the vessel. This is a tremendous advantage which is lost to us. Instead, for every bit of sugar our yeast consumed, the raisins just spit out more. And the yeast got used to it, foreshadowing the problems ahead. Not only does this hinder any attempt to get a “true” sugar reading, but also for every step forward, we took two steps back. Amidst this chaos, the alcohol content was slowly rising. Unable to sustain a reasonable rate of fermentation against an unrelenting tide of sugar and stressed by the ascending alcohol, our yeast first grew sluggish, and then they gave up. Simmering quelled to the tranquil lull of the single, rare bubble. Brix chipped away in appalling 0.1 degree increments until it arrested altogether. And there was still substantial residual sugar in our half-fermented must.
“By the time the rate has dramatically slowed, it is often too late” (Bisson). It’s not like we hadn’t tried every trick in the book. As the tizzy of agitated turbulence slowed to a near simmer, we hurled life vests and buoys at our sluggish juice. We dumped in yeast hulls, ghostly silhouettes of deceased yeast, thought to sop up toxins and magically bring dead fermentations back to life. We moved the must into a small heated tank, hoping the warmth would stimulate the yeast to rev their engines for a final push. We aerated the juice and ultimately racked it, hoping to get some oxygen in there, too. None of it worked. Obviously, like Dr. Bisson warned, it was too late.
“Re-initiation of fermentations that contains a large population of nonviable cells is particularly challenging” (Bisson). Amen, Dr. Bisson. We were sitting on a box of dessert wine riddled with dead yeast, with no option other than to restart the entire process in earnest. So Clay’s awesome second in command mixed up some new yeast with fresh juice and added it back in small aliquots to our steadfastly stuck stuff. Behold the sugar dropped by a point and then stopped again. Everyday our morning e mails started with an excited, “What happened today? Is it dry yet? Is it dry yet? Is it dry yet?” only to be answered by a mournful, “Not yet. Down 0.1.” It was excruciating. Days rolled into weeks, and finally the cap began to collapse. When the once-firm raft of floating grape skins started to soften, disintegrate, and sink back down to the bottom, we knew it was time. We had to press our zin, even though it wasn’t dry. We had no choice.
We dropped into the winery on Halloween weekend (with pumpkin bread). Clay was all smiles, as that morning’s chemistries declared that our wine had ultimately fermented to dryness in the barrel. And it had been a sizeable drop, from about 1.3 °Brix into negative numbers (which indicates dryness). Nobody knows if this dumb luck was secondary to the aggressive racking, final aeration, or any of other Hail Mary gimmicks we employed in final desperation. None of this, by the way, is either sanctioned or condoned in my trusted textbook. But that doesn’t sway my interest in or appreciation for academic enology. I’m still that nerdy, literature searching, article reading kid with specs and braces. It’s just cool to have some renegade maneuvers in my winemaking armamentarium for the next time my yeast go rogue. But I promise you this, next year we’re gonna harvest that fruit a lot, lot sooner.
Linda F. Bisson Stuck and Sluggish Fermentations Am. J. Enol. Vitic., Mar 1999; 50: 107 - 119.
Linda F. Bisson and Christian E. Butzke Diagnosis and Rectification of Stuck and Sluggish Fermentations Am. J. Enol. Vitic., Jun 2000; 51: 168 - 177.