Fermentation - Taaaaaaaake IV
The 2011 harvest ranks among my most challenging to date. Frost and late springs rains compromised yields, a cool summer stalled ripening and early rains tempted rot. But in spite of these overarching frustrations, there’s still something deeply gratifying about that first sizzling spurt of CO2 when fermentation ignites. The basic science of fermentation remains unchanged, despite Mother Nature’s archest smirk. To celebrate our fourth harvest at Bruliam Wines, let’s discuss fermentation from an entirely new perspective - redox balance. Yippee. We’re going to talk about NAD+/NADH and electron swapping. It’s like wife swapping but better. "Redox” is an abbreviation of reduction and oxidation. These are terms that describe the way organic compounds swap and exchange electrons. When you gain an electron, it’s called reduction. Your overall charge is more net negative; you’re reduced from no net charge to say -1. If someone steals your electron, you’re oxidized. Since oxygen is notorious for swiping electrons from unsuspecting compounds, electron loss is called oxidation. In fact any electron loss, whether by oxygen or another compound, is known as oxidation. There’s a nifty pneumonic for this: “OIL/RIG.” Oxidation is loss (of electrons), and reduction is gain (of electrons). This concept is paramount since electrons are energy. In fact, electron energy potential is stored in a compound called NADH. NADH can give away an electron to become NAD+. When this happens, NADH is itself oxidized, and the electron recipient is reduced since it takes that electron. NADH’s potential energy, in the form of electron donations, is known as “reducing power.” NADH energy drives cellular functions and metabolism. It’s energy currency, but it goes both ways (like wife swapping). Say you’ve got a lot to do so you burn through all of your NADH savings. Well you’re stuck with a stockpile of NAD+. From an energy perspective, it’s pretty useless. It’s like finding an empty Gu wrapper at mile 20 of a marathon. It’s a black fly in your chardonnay (just kidding, Alanis). Even so, you need some way to convert your NAD+ back to the good stuff, so you can proceed with your day. Enter fermentation.
Along the path of fermentation, grape sugars travel the trajectory from sticky, sweet grape juice to vino. The very first step of fermentation is a chemical conversion that initiates the magic, when grape sugars are transformed to pyruvate. Listen you don’t want to down a kegger of pyruvate, but it’s a start. But in order for glucose and fructose to become pyruvate, they need to give up an electron. What you say? You mean glucose and fructose are oxidized to pyruvate? Yes it’s true. And can you guess who sops up all of those freebie electrons? NAD+ of course. Fermentation actually helps recycle the NAD+ and renews the energy coffers. In fact, early in fermentation, during the yeast exponential growth phase (characterized by an orgy of asexual budding), yeast require lots and lots and lots of reducing power and harness the power of NADH to grow and reproduce. Luckily our insatiable appetite for wine can make that happen. I’ve got bins of glucose and fructose at the ready.
Perhaps you recall from previous posts the final step of fermentation is the conversion of acetaldehyde to ethanol. This is like the most important step since it makes wine, wine. In fact you may even remember my saying that ethanol is the final electron acceptor. I know. It’s a Hanukkah miracle come early. Acetaldehyde is reduced to ethanol. You know that I know that you know that that electron comes from NADH, the same stuff that was recycled when glucose and fructose were oxidized back at step #1. It’s a perfectly balanced cycle. The first step of fermentation generates the reducing power (NADH) that ultimately reduces acetaldehyde to ethanol. Ethanol captures that final electron, regenerating the NAD+ necessary to keep the machine churning. Yeast stockpile a pretty limited supply of NAD+, and without its regeneration, fermentation of grape sugars would grind to a halt. To keep fermentation moving, the redox balance must remain in check. The yeast adjust to the evolving flux in sugar, temperature, and alcohol by tinkering with and maintaining their redox balance.
Pasted below is one of my 2011 fermentation curves. As fermentation revs into overdrive, you’ll notice the temperature rising as the sugar drops. In fact, this curve looks pretty darned good.
