Gary’s Corner

Photo by Ian McCausland

Of Green Apples and Milk

By Gary Hewitt, DipWSET, CWE, FWS, Sommelier

Aromas of butter, cream, buttered toast, and popcorn kernels—yum, but is it wine? Is it Chardonnay? You bet, and it is all thanks to microscopic friends with a history.

In days past, as cold winter warmed into spring and sap rose in the vines, some casks and bottles of finished wine, warming gently in winery cellars, grew hazy and lightly sparkling. Producers in Portugal’s Vinho Verde region trapped the fizz to make fresh and lively lightly sparkling wines that they sold in opaque bottles—that is, until modern filtration techniques allowed them to make the wines perfectly clear. Other vignerons, especially of red wines, recognized that this was just a phase and, given time, wines would recover with improved mouthfeel and reduced vegetal character. However, the explanation of the phenomenon remained unknown.

Louis Pasteur’s discovery in the mid-1800s of yeast’s role in converting grape sugars into alcohol and carbon dioxide, a process known as alcoholic fermentation, was long thought to be the sole role of microorganisms. But microbiological work in the late 19 th century revealed that many microbes are active in wine, sometimes causing spoilage but sometimes to good effect.

Malolactic fermentation (MLF or la malo en français) turned out to be the reason for springtime fizz. MLF is a bacterial process that converts malic acid into lactic acid. Malic acid is the main acid in green apples, and, in fact, it derives its name from the Latin genus name for apples, Malus. Not surprisingly, malic acid in wine imparts a fresh green apple character. However, and especially in wines made from underripe grapes, malic acid levels can be tart and aggressive. Lactic acid, on the other hand, is softer and less aggressive. Lactic acid is named for the Latin word for milk, lac, and brings creamy, buttery notes to wines. A simple way to think about MLF is that it changes green apples to milk.

During the conversion to lactic acid, malic acid liberates carbon dioxide. The carbon dioxide dissipates or, if trapped, dissolves in the wine to become fizz. The conversion also reduces a wine’s total acidity (i.e., softens the wine) and changes the taste. MLF after bottling likewise creates gas, haze, and odd flavours so that the wine is commonly deemed spoiled. However, if the winemaker gets it right, MLF creates a more stable product.

A host of lactic acid bacteria (LABs) metabolize malic acid. LABs occur naturally on grapevines and on winery surfaces, such as the insides of barrels. Commercial LAB isolates are available that allow greater process control of MLF, but it remains a winemaker’s decision whether or not to use wild or inoculated strains. The key MLF protagonist is a bacterium called Oenococcus oeni, whose name is clearly inspired by its oenological (i.e., pertaining to wine and winemaking) origins.

O. oeni runs a clean house, producing mostly lactic acid, but other LABs run all over the place, metabolizing substrates other than malic acid, and produce a wide range of flavours. Acetic acid (of vinegar fame), for example, can reach moderate levels that “lift” wine aromatics or high levels that turn wine to vinegar.

A really distinctive product of MLF is diacetyl, a compound that imparts hazelnut and toasty flavours at low concentrations but at high concentrations imparts strong butter, buttered-popcorn or even rancid character. Diacetyl played a big role in the 1970s creation of Big Buttery Chardonnays, wines softened by MLF and adorned with the vanilla and spice of new oak. At the time, consumers were unfamiliar with the complex, nutty, toasty Chardonnays from Burgundy that inspired New World winemakers. But they sure loved the fat, overt, powerful Chardonnays coming out of California and Australia, so much so that the style persists today. However, the faithful must hunt for the now nearly mythical Big Buttery Chard from the few producers still willing to comply by throwing MLF restraint to the wind.

Almost all red wines pass through MLF, but only some white wines do so. Grapes with a strong varietal character such as Riesling and Sauvignon Blanc do not play well with MLF, but Chardonnay, “the winemaker’s grape,” has a great affinity for MLF’s nutty, creamy, buttery, butterscotch, yogurt, and caramel traits.

So now you know what the sap-watching old vignerons did not: that LABs perform MLF, change green apples to milk, and soften wine. Bring this up next time there is a lull in conversation! 