More Butter? Wine Scientists May Have a Bacterium for That

Why do some Chardonnays taste like butter? That creaminess is the result of malolactic fermentation. Following the initial fermentation of sugar to alcohol by yeast, this process—actually a chemical conversion, not a fermentation—converts a wine’s malic acid (which gives green apples their tartness) to creamy lactic acid (like that found in milk) plus carbon dioxide. Almost all red wines undergo this conversion, but when it comes to white wines, whether to encourage malolactic fermentation is a major stylistic decision.

The process was not understood until the 20th century, and even today, relatively little is known about the strains of bacteria primarily responsible. Now, a new study from researchers in Spain and Italy promises to deepen our understanding of Oenococcus oeni, the most common malo bacterium, by producing a map that charts its proteins.

“This bacteria is very important for the wine industry,” Paloma López, of Madrid’s Centro de Investigaciones Biológicas and a study coauthor, told Wine Spectator. First of all, winemakers need to be sure that O. oeni will be able to complete its job. “The harsh wine environment represents a challenge to the survival of O. oeni,” López wrote, “and can strongly affect the successful outcome of the vinification.” Second, locating and identifying the components of O. oeni may help winemakers better understand which sensory qualities the bacterium contributes to finished wine.

López and her colleagues focused on proteins in the membrane and cytoplasm of cells, distinguishing them via two-dimensional gel electrophoresis, a protein-separation mechanism. The process revealed 152 distinct proteins—roughly 10 percent of O. oeni’s total composition, by the authors’ estimate. “In my opinion, this 10 percent characterization is meaningful,” said López, since many of the detected proteins play important roles in metabolic pathways, thus giving clues as to how the cells operate.

Specifically, the study discovered or confirmed the existence of several wine-quality determinants in O. oeni: buttery- and nutty-tasting diacetyl, acetoin and 2,3-butanediol; fruity-smelling esters; and enzymes responsible for the synthesis of various flavor and aroma compounds.

Winemakers already have a long menu of yeasts to choose from when they are inoculating for their primary fermentations. Similarly, a greater understanding of O. oeni could produce a variety of specialized bacterial strains that support different outcomes. For example, some O. oeni strains may lead to the production of putrecsine, a compound in wine that at high concentrations can cause nausea, headaches and other adverse reactions in people sensitive to it; to avoid such a bacterium strain, one could simply check its protein makeup. Additionally, López observed that some of the esters she and her colleagues found in O. oeni have been proposed as additives in wine to enhance fruitiness. Could it be possible to select for these fruity characteristics in certain strains of O. oeni? López thinks yes.