chris wrote this on March 21, 2009
I am taking a break from wines reviews this week to discuss a topic that I frequently refer to, malolactic fermentation (MLF), its process given by the equation:
Malic Acid + Bacteria = Lactic Acid + Carbon Dioxide
What follows is a simplified explanation of MLF and the role that acids play in wine and winemaking.
Acids in Wine
There are two main types of acid found in wines, tartaric acid and malic acid. Tartaric acid adds only a mildly sour flavor to wine but is the “acid” that critics refer to when describing the mouthfeel or texture of wines. It is an important element of the wine; too little and the wine can be thin and flat, too much and it can be harsh and offensive. When in harmony with the other components, it gives the wine structure, body, and “backbone”.
Malic acid is also responsible for adding texture to wines but is more involved in its taste and flavor. It is perceived on the palate as a sharp, tangy, tart, or sour sensation (depending upon its concentration) and is what gives wines made from Sauvignon Blanc their characteristic flavor and bite. In fact, many of the common fruit descriptors of wine; apple, cherry, blackberry, passion fruit, mango, peach, pear, and plum, all derive the majority of their flavor from their malic acid content (along with other flavor compounds called esters).
Tartaric and malic acid are found naturally in grapes and just like tannins, certain varieties of grapes have naturally higher acid levels than others. Climate also affects the amount of acid in the grapes; cooler climates promote the production of acids over sugars while warmer climates are the opposite. Note that the climate also causes the grapes to produce varying levels of tartaric and malic acid. As an example, the cooler climate chardonnay grown in France has a higher tartaric acid content but a lower malic acid content than the chardonnay grown in California. This overall malic acid content is a very important factor for MLF and its effect on the wine.
Lactic acid is not found naturally in grapes but, as its name suggests, is the primary acid found in milk and other dairy products. It adds nothing to the flavor of the wine though its effects are immediately apparent when evaluating the wine’s mouthfeel. Wines that undergo MLF have a smoother texture due to both the absence of harsh malic acid and the lactic acid giving the wine a creamy, softer feel.
Note that the buttery taste often associated with MLF is not due to lactic acid, but to the flavor compound diacetyl, which is produced by the yeasts that convert the grape sugars into alcohol. In fact many Californian winemakers promote the formation of diacetyl during fermentation and then undergo MLF in order to produce this rich creamy, buttery style of chardonnay.
MLF is actually a very important process in winemaking for reds and whites, not just because it makes the wines less harsh and sour, but more importantly it allows the wines to age without succumbing to bacterial spoilage. While both tartaric acid and lactic acid are naturally anti-microbial, malic acid is actually a food source for bacteria and other microorganisms.
There are many different types of wine bacteria that can initiate MLF, but only one type, oenococcus oeni, can perform the conversion of malic acid to lactic acid without producing undesirable compounds that can ruin the flavor of the wine (sauerkraut and sweaty socks being a few examples). Many times winemakers will filter their wines to remove the unwanted bacteria and then inoculate with the preferred species to better control the MLF process.
As to its effects on wine, the key factor is determined by how much malic acid is actually present in the grapes. Since some wines have less malic acid in them than others, the MLF is not as significant in shaping the wines as in those with a higher malic acid content. For example, a white Burgundy typically contains less malic acid than a Napa Valley Chardonnay, even though it has a higher total acid content. Therefore, when a white burgundy undergoes MLF, very little acidity is lost and the crisp, mineral character of the wine is preserved. On the other hand, a California Chardonnay contains more malic acid, producing more lactic acid after MLF, changing the flavor and character of the wine appreciably.
Winemakers who wish to retain the grape’s natural malic acid levels add sulfites to their wines after alcohol fermentation, which inhibits the growth of the bacteria responsible for MLF. It is also common for a winemaker to reserve a separate portion of his wine for MLF, adding it back to the tarter wine to produce a blend with the benefits of both malic and lactic acid.
MLF is thought to generally enhance the body and depth of wine, producing wines with better palate softness and roundness. Wines that typically undergo MLF, and are considered “improved” by the process, are full-bodied dry whites and medium to full bodied dry reds. Note that that not all wines benefit from MLF. All desert wines, from Port to Sauternes, require high levels of sour malic acid to balance their inherent sweetness; the same applies to Riesling (especially late harvest styles). Sauvignon Blanc, especially from the Loire Valley and New Zealand, is another classic example of a wine defined by its malic acid content.
Malolactic Fermentation & Oak
Many white wines, especially French wines such Chablis and other white Burgundies, generally spend little to no time in oak after MLF and therefore have little oak influence. Their fruit and mineral flavors aren’t masked by excessive oak and gain a softer texture from MLF, often with hints of hazelnuts, cream, honey, or butterscotch flavors as well.
Wines that undergo MLF and then spend appreciable time in oak often have a cooked or toasted quality to them, like buttered toast or buttered popcorn. As with most oak aging, the wines gain more depth at the expense of muted fruit qualities and if over-oaked, produces the one-dimensional “butter bombs” that have given California chardonnay a bad reputation. Many winemakers feel that a better integration of fruit and oak character can be achieved if MLF occurs while the wine is in oak. In winemaking it is all about balance and not allowing one aspect of the wine to dominate the style.