Biochemical Roulette

S. cerevisiae

budding, uni-cell fungus

happy to make wine.

 Yes - a haiku ode to our simplest wine making creature.  As you well know, S. cerevisiae is content to metabolize grape sugar into wine - pretty much every time.  Lacking the creativity, internal machinery or motivation to do otherwise, wine yeast render the biochemistry of wine relatively straightforward.  Conversely, lactic acid bacteria, of the sort responsible for malolactic fermentation, present a greater scientific challenge as they possess more tricks in their sugar consuming armamentarium.  For instance, both yeast and lactic acid bacteria disassemble glucose and fructose (collectively called hexoses because they have 6 carbons) into stuff called pyruvate, but then the lactic acid bacteria uniquely transform pyruvate into lactic acid.  It is this so called "lactic fermentation" that categorizes their kind.  And this is OK; we like lactic acid in our wine.  But what happens when they eat other junk, beyond simple grape sugars?  Thinking caps on; biochemistry ahead; Here we go.

Like cockroaches feasting on garbage, greedy lactic acid bacteria can also digest stuff like 5 carbon sugars, citric acid, and malic acid, especially when grape sugar concentrations are low.  You already know how they turn sour-apple malic acid into mellow, round lactic milk acid, so now let's tackle the other stuff.  First up: the 5 carbon pentoses (think of a stop sign- a 5 sided pentagon).  For this, lactic acid bacteria must engage their "pentose phosphate pathway" since 6 carbon grape sugars are chewed up differently than 5 carbon sugars.  Like the sixers (hexoses), sometimes the fivers (pentoses) end up as ethanol - i.e., wine.  Other times, the end product is acetate.  Clearly, mocking modern wine science, those one cell, mini-monsters spin pentoses into acetic acid-acetic acid as in vinegar - rotten, spoiled wine.  Vexing.

Citric acid metabolism is even crazier.  Imagine a candy factory with a conveyor belt that sends bulk chocolate down one path to get M&M's, another for Snickers, and a third for basic Hershey's bars.  Same starting material; different end products.  In the same way, lactic acid bacteria digest citric acid, a principal organic acid of wine, in different ways depending on their different environmental conditions.  Transported through one door, citric acid becomes a source of acetoin compounds - like the buttery popcorn smell - yum!  Or it can be transformed into lactate (that's OK) or even wine (better yet!).  Sometimes though, citric acid is converted into cellular material, bricks and mortar to fix bacterial cell walls.  Now I am all for home improvement, but in this case the by-product of Flip This Microbial House is acetic acid, again, meaning more wine spoilage.  Luckily for winemakers, we can predict the conditions that favor butter over vinegar, so we load the biochemical roulette wheel to our liking. 

Fermenting wine is a smorgasbord of bacterial food options, so much food and so many ways to eat it.  Perhaps this helps you understand why malolactic fermentation is so critical to winemakers.  Beyond the mega organoleptic qualities, it promotes microbiological stability.  In other words, through MLF, we control how and when wine bacteria consume all of their available nutrients - from grape sugar to the weird stuff like pentoses and citrate.  Last thing we'd want is for some rambunctious bacteria to go crazy in your bottle of Bruliam; I'd be mortified if you uncorked fizzy vinegar at your next dinner party.  You see, lactic acid bacteria aren't the only critters who eat this stuff.  Their bacterial brethren have similar tastes for citrate and ribulose.  With malo, we simply inoculate to ensure every potential particle of edible bacterial sustenance is consumed before we bottle.  And so we eagerly wait for e-mail updates like this one:

Vessel	Date	Meas/Add	Item	Amount	Units
Z2070119	December 9th, 2008	Measurement	Alcohol	14.08	%
Z2070119	December 9th, 2008	Measurement	Malic Acid	0.18	g/L

In our Anderson Valley pinot the malic acid is almost to zero; we're on our way.