Malolactic fermentation - where buttery chardonnay comes from
You take a sip of a full chardonnay and feel a distinct, buttery, almost creamy note, as if someone had added a touch of butter to the wine. Where does it come from, since there is no dairy in wine? The answer is a fascinating, little-known process: malolactic fermentation, the second, hidden fermentation that takes place in many wines after the main one. It decides whether a wine will be sharp and crisp or soft and creamy, and in chardonnay it can give the characteristic buttery note that some adore and others avoid. Understanding this process opens a window onto how a winemaker consciously shapes the style of a wine. Here is a guide to malolactic fermentation: what it is, how it works, where the buttery taste of chardonnay comes from and why some wines avoid it, based on facts, not myths.
What malolactic fermentation is
Let us start with a definition, because the name sounds more menacing than it really is. Malolactic fermentation, MLF for short, is a winemaking process in which the sharp-tasting malic acid, naturally present in grape must, is converted into the softer-tasting lactic acid. It most often runs as a secondary fermentation, shortly after the end of the main, alcoholic fermentation, though it can sometimes run concurrently with it. Crucially, it is not a fermentation in the classic sense, that is turning sugar into alcohol, but the conversion of one acid into another, carried out by bacteria, not yeast. That is why it is spoken of as the second, hidden fermentation of wine. Its effect is simple but significant: the wine loses sharpness, gains softness and takes on new notes. Understanding that it is a conversion of acid, not sugar, is the key to everything else.
Malic versus lactic - two acids
The heart of the process is swapping one acid for another, and the difference between them is noticeable in the mouth. Malic acid, the same one you feel in a green, unripe apple, is sharp, hard and aggressive. Lactic acid, the same one that gives softness to yoghurt or buttermilk, is soft, smooth and far gentler. Malolactic fermentation turns the former into the latter, so a wine that before it was sharp and stabbing with acidity becomes round and velvety afterwards. This explains why MLF so strongly changes the perception of a wine: it is not about the amount of acid, but about its character. The wine keeps its freshness but loses its aggression. This swap is the essence of the process, and the vivid comparison - from green apple to yoghurt - best captures what happens in the glass. It is a natural route to softening a hard, acidic wine.
The role of Oenococcus oeni bacteria
If it is not yeast, then who carries out this conversion? The answer is lactic acid bacteria, and the main protagonist is the species Oenococcus oeni. It is these bacteria that metabolise malic acid and convert it into lactic acid and carbon dioxide. The winemaker can let the process happen spontaneously, thanks to bacteria naturally present in the cellar and on the equipment, or consciously inoculate the wine with a selected strain of Oenococcus oeni for more control. These bacteria work under specific conditions of temperature and pH, so the winemaker steers the process by regulating the environment. This shows that malolactic fermentation is not a matter of chance but a conscious tool in the winemaker hands. The choice of whether to carry it out, when and how, is one of the key stylistic decisions. Lactic acid bacteria are therefore a quiet but powerful co-author of a wine flavour, working in the background after the main fermentation.
Where the buttery note comes from - diacetyl
Let us get to the heart of it, that is where the buttery taste comes from. A specific compound is responsible: diacetyl. Diacetyl is a flavour compound with a distinctly buttery character, which accumulates during the alcoholic and malolactic fermentation of wine. When its concentration exceeds the sensory threshold, it gives the wine an aroma that can be described as buttery or nutty. It is the very same substance responsible for the buttery taste in many food products. In other words, the buttery note in wine comes not from butter or from the barrel, but from diacetyl produced during malolactic fermentation. It is a by-product of the bacteria work. Understanding that a concrete, measurable compound stands behind the creamy, buttery note demystifies the whole topic. Diacetyl is the chemical signature of malolactic fermentation, and its amount decides how buttery a wine will be.
Why chardonnay in particular
Since diacetyl appears in many wines, why do we associate the buttery note above all with chardonnay? The key is the sensory threshold, which differs dramatically between wines. The sensory threshold of diacetyl is about 0.2 milligrams per litre in chardonnay, 0.9 milligrams per litre in pinot noir and as much as 2.8 milligrams per litre in cabernet sauvignon. This means chardonnay is exceptionally sensitive to the buttery character of diacetyl - very little is needed for it to become noticeable. In cabernet sauvignon you need as much as fourteen times more to feel it at all, so the buttery note drowns there in other, strong flavours. That is why buttery chardonnay became an icon, and buttery cabernet practically does not exist in perception. The sensitivity of chardonnay to diacetyl makes it a natural showcase for the effect of malolactic fermentation. It is a matter of chemistry and threshold, not chance. We cover the grape itself more in chardonnay.
How the winemaker controls butteriness
Interestingly, the winemaker has real control over how buttery a wine will be, by steering the diacetyl level. The timing of malolactic fermentation decides it. A simultaneous alcoholic and malolactic fermentation favours lower diacetyl levels, because as it is produced, the present yeast and bacteria break it down. Inoculating malolactic fermentation only after the alcoholic fermentation has finished leads to higher diacetyl levels in the wine, because there are no longer active yeasts to reduce it. This shows that butteriness is not a matter of chance but a conscious choice by the winemaker. Wanting a more buttery wine, they run MLF separately, after the main fermentation; wanting a subtler one, they combine both processes. It is a level of control that surprises many drinkers. The buttery style of chardonnay is therefore designed in the cellar rather than accidental, and can be precisely dosed.
What else MLF gives besides butteriness
Malolactic fermentation is not only about the buttery note - it carries several other important effects. First and most important, it softens acidity, turning sharp malic acid into soft lactic, which makes the wine smoother and fuller. Second, it adds body and a creamy texture in the mouth, enriching the sensation. Third, it increases the microbial stability of the wine: since malic acid is converted in a controlled way, random bacteria will not do it later in the bottle, which could spoil the wine. Fourth, it enriches the aromatic profile with buttery, nutty and creamy notes. That is why MLF is not only cosmetic for flavour, but also a tool of stabilisation and building texture. These effects together make it one of the most important stages in producing many wines. It is far more than the buttery note alone - it is a comprehensive transformation of the wine character, connected to the role of acidity as the backbone of wine.
Which wines undergo MLF and which do not
Since we know the effects, it is worth knowing where malolactic fermentation is standard and where it is deliberately avoided. MLF is standard in the production of almost all red wines, softening their acidity and adding smoothness and complexity. Among whites it is common for some varieties, especially chardonnay, where it gives a buttery character. On the other hand, many wines, like sauvignon blanc and riesling, deliberately skip malolactic fermentation to preserve their crisp, fruity, acidic character. In these wines the sharp, fresh acidity is an asset and a hallmark of the style, so softening it would rob them of their identity. This shows that MLF is neither good nor bad - it is a tool matched to the style of the wine. Reds and full chardonnay gain from it, while crisp whites would lose. The decision to carry it out or skip it is a conscious stylistic choice, shaping the identity of a specific wine and its place on the shelf.
The essentials in brief
Let us gather it up. Malolactic fermentation is the second, hidden fermentation of wine, in which Oenococcus oeni bacteria turn sharp malic acid into soft lactic acid, softening acidity and adding smoothness. The buttery note, especially in chardonnay, comes from diacetyl, a by-product of this process. Chardonnay is exceptionally sensitive to it - the sensory threshold is about 0.2 mg per litre, while in cabernet it is as much as 2.8 mg. The winemaker controls butteriness through the timing of MLF: simultaneously with alcoholic fermentation gives less diacetyl, separately afterwards more. MLF softens acidity, adds body and stabilises the wine. It is standard for reds and common for chardonnay, but deliberately skipped in riesling and sauvignon blanc, to keep their crispness. Now you know where buttery chardonnay comes from and why it is a conscious choice by the winemaker, not chance.
Note every wine in GustoNote - the grape, style and the notes you sense. Over time you will start to recognise the buttery signature of malolactic fermentation and tell wines that underwent it from the crisp ones that avoided it.