Fermentation Basics (Part III)
Herein, dear Brigade, we commence our final musing on normal fermentations for the 2009 harvest season. When we last convened, we’d abandoned our heroic yeast to fight a lonely battle against the rising tide of ethanol toxicity. But as I’d mentioned last week, those plucky yeast rely on some nifty tricks to fortify their body armor and better withstand the alcohol soak. As you know, high alcohol concentrations mess with yeasts’ cell membranes by forcing a tsunami of acid inside their insides faster than they can push it out. Alcohol also inhibits proteins like enzymes or sugar transporters and screws up their membrane fluidity (that is the Austin Powers beaded door curtain-like component of their outsides). This concept of “membrane fluidity” is paramount.
The plasma membrane is like a double layer wall surrounding the yeast, with fixed, unbending parts and fluid, flexible parts. The wall includes certain windows and docking ports that uniquely fit specific things, like sugar or nitrogen. When a fructose molecule floats through the grape juice to approach a hungry yeast, it looks for an open parking spot so it can “park” on the yeast’s surface. Like compact spots for mini Coopers and giant spots for Hummers, the yeast has different hexose transporters (sugar corridors) that recognize different substrates (like glucose versus fructose). A glucose molecule fits into a specially configured parking place like a puzzle piece in a puzzle. After the sugar docks and locks, the transporter becomes a Transformer©, changing its orientation and configuration. It will about-face and flip from facing the grape juice side to facing the inside of the cell, taking the sugar with it. The sugar is dumped inside the yeast and consumed for energy. The Transformer© transporter then undergoes a second shape shift to face back outwards again, ready to usher another sugar through its membrane wall. Obviously the Transformer© transporter is pretty rigid, since its shape is fixed to receive only sugar. However it sort of floats around in a lipid (i.e. fatty) bilayer. If the plasma membrane is too stiff, the transporters are straight-jacketed. The proteins can’t shape shift, and the sugar is stuck outside. If the membrane is too fluid, then the transporter gets all wiggly-wobbly, loses its shape and can’t do its job either. High alcohol concentrations disturb this perfect balance. But if the yeast can adapt its membrane composition before the must is too ethanol toxic, it will live long enough to take the wine to dryness (i.e. consume all of the sugar in the juice).
Yeast tolerate a higher alcohol environment by altering their plasma membranes in a number of technical ways. These include amping up the levels of sterols, swapping out saturated fatty acids for unsaturated ones (which means more double bonds), and increasing the relative protein content. What you need to understands is that #1) these membrane adjustments require oxygen and nitrogen (the building blocks of protein) and #2) the changes must be securely in place before the alcohol level gets too noxious. Adding more nutrients or nitrogen once the yeast are already petering out is like fastening your seat belt after you crashed; you’ve already missed the boat. In other words, unless the yeast have ready access to a protein source, sterols, and unsaturated fatty acids at the beginning of fermentation, they won’t have the necessary tools they need to bulwark their body armor. When a brisk fermentation grows sluggish and the rate of sugar consumption slows down, it’s usually an alcohol tolerance problem. You’ve got to understand this fundamental concept. The more sugary the grapes at harvest, the higher the potential alcohol of the finished wine. Those yeast are going to need a lot of protection and TLC. They’ve got to buckle down and secure their cell membrane or face death by acid-fried innards.
With our Rockpile lot, that’s exactly what happened; our inaugural zin fermentation stuck. What happened and how we overcame this potential disaster will be the subject of future posts.
Kudos to all readers who themselves endured three consecutive weeks of fermentation kinetics. Next week’s post will be a light hearted carrot to lure you into a final discussion of stuck fermentation.