For this Winery Update, we asked our Assistant Winemaker Keith for, in his own words, a “nerdy, long-winded explanation” of malolactic fermentation—the process of malic acid being converted to lactic acid as young wine ages in barrels.

The following is his gleefully provided response. 


The name malic acid is derived from the Lattin word “malus,” meaning apple in English. A perfect example of malic acid that we can all relate to is the tartness of a green apple. Lactic acid, on the other hand, is derived from the Latin word “lac” or milk, as it was first discovered in sour milk. Lactic acid is a much weaker acid, so this conversion in wine has a softening effect. It helps the wine feel richer and rounder on the palate.

The great Louis Pasteur discovered that a genus of bacteria dubbed Lactobacillus was responsible for this conversion, but many genus and species of bacteria can perform this action. We use the general term lactic acid bacteria or LAB, to group these. Throughout winemaking history we have generally found that many different LAB can produce off flavors in wine; specifically, Oenococcus oeni is best suited to produce the most desirable characteristics. 

Dozens if not hundreds of strains of Oe. oeni are commercially available, just like the yeast used in primary fermentation. Here at Emeritus, however, we have found that the naturally occurring strain on our estate grown fruit performs beautifully, so just like with primary fermentation we take advantage of the native population of Oe. oeni for malolactic (ML) conversion in our wines.  

ML conversion is generally a much slower process than alcoholic fermentation, and can take anywhere from a couple weeks to several months depending on wine conditions. Right now I test the malic and lactic acid content of each wine lot separately every 7-10 days to monitor the progress. 

After ML fermentation, all main chemical transformations in the winemaking process have occurred. The wine is “made,” and simply needs to age. It’s at this point David and I get to taste through each barrel, assessing the quality and identity of each lot and each barrel. From there, we’ll begin to think about blending individual barrels for each of the wines we make.


From the desk of

Keith Hammond,
Assistant Winemaker