MLF - No Relation to MILF

Dearest readers, today we turn our rosy cheeks from the comforts of slurping savory soup to that most nefarious and treacherous of all enology subjects: biochemistry.  This blog commences the first in a series of posts about secondary fermentation, also called malolactic fermentation.  Like primary fermentation with yeast, malolactic fermentation (hereafter dubbed MLF) is mediated by microscopic critters that imbibe one wine component (malic acid) and spit out another (lactic acid).  Today we'll meet its star player, Oenococcus oeni (O. oeni), a heterofermentive lactic acid bacterium, typecast by his ability to convert glucose (sugar) into lactic acid plus carbon dioxide, acetate, and ethanol (wine).  As with wine yeast, there are clones within the Oenococcus species itself and fancy DNA manipulations that distinguish Mr. Frank Oenococcus from Mr. Jack Oenococcus and Mr. Mark Oenococcus in a lineup of bacterial offenders.  And like with primary fermentation, MLF occurs either spontaneously or is forced by inoculation with lab grade bacteria.  However, quite unlike primary fermentation with yeast, this step is entirely optional.  Even without it, you still have drinkable wine. MLF, the conversion of malic acid to lactic acid via bacteria, is a process that ups the ante in premium wines.  In fact, most of the wines you drink have probably undergone MLF and you didn't even know it, since you're so accustomed to its sweet end-product.  MLF heightens wines' aromas, increases its complexity, and enhances all of its organoleptic qualities across the board, from creamy, mid-palate weight to fragrant bouquet.  By products of this chemical conversion include spooky names like ethyl lactate which embellishes a wine's fullness and body and diacetyl, the curious compound responsible for that fat, buttery popcorn aroma in some oaky chards.  After MLF, wines become softer, more approachable, and less aggressive (and no, it cannot be applied to your hyperactive, yippy puppy or terrorizing toddler).  MLF also imparts velvety softness, greater body, and a richer, fuller texture (and no, you can't apply it to your over processed hair either).  You see, MLF transforms malic acid, imagined as the biting tartness of unripe, green apples, into softer, mellower milk acid (lactic acid).  With MLF, a wine's overall acidity decreases via direct decarboxylation, so for every molecule of malic acid, one acid group evaporates- poof- with its resultant gustatory pleasures. 

 Now stay with me friends.

 This is the chemical structure that defines carboxylic acid (R-CO₂H):

 

Malic acid is a DIcarboxylic acid, HO₂C-CH₂-CH(OH)-CO₂H,  containing 2 of those CO₂H guys, one flanking either end of the molecule.  Compare this to lactic acid, a MONOcarboxylic acid, HO₂C- CH(OH)- CH₃, which has only one CO₂H.  (FYI: the other CO₂H had evaporated into the air as carbon dioxide, CO2).  And there it is!  You have literally LOST an acid group, molecule for molecule, transforming a chemical with two CO₂H guys into a chemical with only one CO₂H guy.  So not only do you have fewer "CO₂H guys" overall (less acid) but also you've replaced puckery, tart, green apple acid with softer, richer milk acid.  Volia, the magic of chemistry!  (Now don't you wish you'd stayed awake in that 7^th grade chemistry class instead of throwing spit balls in Amy's hair?)

 Why does this impressive creature, Oneococcus, use malic acid anyway?  Why can't he eat sugar like his BFF Mr. Yeast?  Lactic acid isn't buttery, is it?  So where's the butter from?  Hey, didn't you mention something about acetate up there?  Isn't that vinegar?  How do I know you won't screw up your own brew with this MLF garbage?  If you've already got wine, why bother?  The answers to these and other enticing chemical quandaries will be addressed in the upcoming weeks.  But fear not fortuitous friends; I'll be alternating these topics with awesome interview bites from super sommelier Jesse Rodriguez and star chef William Bradley, both of Addison Grand Del Mar.  Stay tuned!