Wine Fermentation Process: Yeast, Heat and Sugar

Fermentation is the point where yeast eats the sugar in grape juice and leaves alcohol and carbon dioxide behind. The primary, alcoholic stage of the wine fermentation process often takes between one and two weeks. The arithmetic under it does not move: for every gram of sugar converted, about half a gram of alcohol is produced. So a must needs about 24% sugars to reach a 12% alcohol concentration. Red wines ferment warm, at 22 to 25°C, with the skins in the vessel so they give up colour, flavour and tannin. Whites ferment cool, at 15 to 18°C, on juice alone. A second, bacterial stage called malolactic conversion then turns "crisp, green apple" malic acid into "soft, creamy" lactic acid. Texture, butter, freshness, vinegar: a taster meets all of them years later, but they were settled in those few weeks.

What happens during the wine fermentation process?

Yeast cells feed on the sugars in the must and multiply, producing carbon dioxide gas and alcohol. Red winemaking and white winemaking split at this moment, and the split is about skins.

Red wine is made from the must, the pulp including the juice. It ferments together with the grape skins, which impart colour, flavour and tannins through maceration. White wine is made by fermenting juice pressed from crushed grapes, with the skins removed so they play no further role. Carbon dioxide from the ferment floats the red skins to the surface as a layer called the cap. The cap has to be mixed through the liquid each day, or "punched", traditionally by stomping through the vat.

When the primary ferment finishes, the free run wine is pumped off and the skins are pressed for what remains. Free run juice is typically of higher quality than press juice, which carries more phenolic compounds and a herb-like taste. Modern presses usually ramp from 0 Bar to 2.0 Bar, and as pressure rises so does tannin extraction, often leaving the pressed fraction harsh. Pressed juice can represent 15% to 30% of the total juice volume from the grape, so blending press wine back into free run is a quality decision, not a rounding error. Our guide to how tannins behave in wine covers what that extraction does on the palate.

Which yeast does the work, wild or cultured?

Yeast is normally already on the grapes, often visible as a powdery appearance on the skins. An estate can ferment on that ambient population or add a cultured strain to the must, and what it is trading is predictability.

The guidance is blunt about the risk. Wild ferments give unpredictable results depending on the exact types of yeast present, and the main problem is a ferment that fails to go to completion, leaving unfermented sugar in a wine meant to be dry. Wild ferments also lead frequently to unpleasant acetic acid, which is vinegar, as a by-product. The Australian Wine Research Institute reports the numbers on that. Native yeasts such as Hansenula and Kloeckera can produce high concentrations of acetic acid before and during the early stages of fermentation. A sound wine sits at 0.1 to 0.4 g/L volatile acidity immediately after fermentation. The aroma threshold starts as low as 0.1 to 0.125 g/L, and the Institute treats the acid as detrimental above 0.7 g/L. Australia's legal maximum, excluding sulfur dioxide and expressed as acetic acid, is 1.5 g/L.

Strain choice also sets the alcohol ceiling. Anything above 12% needs a yeast that can withstand high alcohol, and some yeasts can produce 18%, though extra sugar is added to reach it. For the acid side of the same story, read how acidity works in wine.

How long does fermentation take, and in what vessel?

The primary ferment runs one to two weeks. The secondary fermentation and ageing period that follows takes three to six months. The wine sits under an airlock, which protects it from oxidation.

Through that slower phase, proteins from the grape break down, remaining yeast cells and fine particles settle out, potassium bitartrate precipitates, and the cloudy young wine turns clear. The wine can be racked to remove the lees while this happens. Vessels vary by intent: large stainless steel tanks of several cubic metres, oak barrels, or glass demijohns. Unoaked wine is fermented in a barrel made of stainless steel or another material with no influence on the final taste. Oak chips used with a non-wooden barrel are a shortcut we describe as mainly used in cheaper wine.

That choice reaches the glass. An oaked Chardonnay from a warm region such as Australia, typically heavier in body, sits against a stainless steel fermented Chardonnay from a cooler region such as Chablis.

What does malolactic fermentation change?

Malolactic fermentation is a bacterial process that converts "crisp, green apple" malic acid to "soft, creamy" lactic acid, softening the taste and reducing total acidity.

Most red wines go through it completely, both to lessen the acid and to remove the possibility of it happening later in the bottle. White wines vary. Lighter aromatic wines such as Riesling generally do not; fuller whites such as barrel-fermented Chardonnay commonly do, sometimes partially at less than 50%. It is more accepted in colder regions of production, because warmer regions run higher pH where the conversion is less necessary and can damage the final product. Winemakers watch pH through it and hold it below 3.55 for whites and 3.80 for reds.

The buttery note in some Chardonnay is diacetyl, produced by the bacteria from citric acid once the malic acid has gone. The AWRI reports that 1 to 4 mg/L adds "buttery" or "butterscotch" complexity. Above 5 mg/L the aroma is objectionable enough that the wine can be regarded as defective. Full detail sits in our guide to malolactic fermentation.

How does a second fermentation put bubbles in Champagne?

For Champagne and other wines made in that style, an additional secondary fermentation takes place inside the bottle, dissolving trapped carbon dioxide in the wine and creating the characteristic bubbles.

Those bottles then spend 6 months on a riddling rack before being disgorged to remove the sediment that has accrued. The bulk Charmat method does the same job in sealed tanks. For Prosecco, machinery adds the gas by force-carbonation. The route shows up on the table. We pair traditional method sparkling wine with bloomy rind cheeses such as Camembert or Brie. The acidity cuts the richness while the yeasty autolytic aromas complement it. Tank ferments carry a risk of their own, and the AWRI has observed indole off-flavours, described as "chemical", "plastic" and "mothballs", during the secondary fermentation of some tank-fermented sparkling wines. Regional context for the traditional method sits on our Champagne map.

How do winemakers stop fermentation to leave sugar behind?

Sweet and off-dry wines are made by arresting fermentation before all the sugar has been converted, so residual sugar remains in the finished wine.

There are two mechanical routes: chill the wine and add sulphur and other allowable additives to inhibit yeast activity, or sterile filter it to remove all yeast and bacteria. In the sweetest styles the ferment often stops without help, because the high sugar concentration and rising ethanol retard the yeast. Growers raise the starting sugar first by harvesting late, freezing the grapes for ice wine, encouraging Botrytis cinerea to dehydrate the fruit, or raisining the grapes on the vine, on racks or on straw mats. Fortified wines take the direct route: in port, high proof neutral grape spirit is added to arrest fermentation once the desired sugar level is reached. A German alternative, süssreserve, holds back sweet grape juice and adds it after the ferment is done.

This is what a label means by dry. A bone dry wine is one whose sugars are fully fermented into alcohol, with off-dry, semi-dry and dessert sweetness above it. See how sweet wine is made for those styles in full.

What goes wrong during fermentation?

Stress on the yeast is the usual root, and it arrives as smells you can name.

A ferment that stalls is the first problem. The AWRI notes that many wines carrying an indole fault had become stuck during primary fermentation, which points at yeast stress. It adds that acetaldehyde above 125 mg/L imparts odours of over-ripe bruised apples, sherry, or of a stuck ferment. Table wines generally sit below 75 mg/L immediately after fermentation, against a sensory threshold of 100-125 mg/L. Adding sulfur dioxide during fermentation raises acetaldehyde, as do higher pH and higher fermentation temperature.

Then the sulfur smells. Yeast excretes hydrogen sulfide, the rotten egg gas, when it is under stress, for instance when it starts to run out of nitrogen, which is why winemakers supplement juice with a soluble nitrogen source such as diammonium phosphate. Ethyl acetate, the nail polish remover note, is the major ester yeast produces, and it adds fruity complexity at low levels. Normal wines carry about 30 to 60 mg/L, defective wines 150 to 200 mg/L. Saccharomyces strains under stress make large amounts of acetic acid during low or high temperature ferments, in high sugar musts, when available nitrogen is low or when pH is low. Brettanomyces is a different yeast with a different signature, 4-ethylphenol, which the AWRI measured at an aroma threshold of 368 µg/L in a neutral red wine and describes as medicinal.

The OIV lists all of this under origins of defects in its competition standard: microbial volatile acidity and its esters, and an oxidation and reduction group covering thiols and sulphides described as rubber, cabbage and rotten eggs. If you are checking a bottle you already own, our guide to corked and oxidised wine covers what to do next.

How should fermentation change what you buy?

Most wine is not built to wait. Jancis Robinson records that "only about 10% of all red and 5% of white wine will taste better after five years than they will after just one year".

The spread is wide at both ends. Time from harvest to drinking runs from a few months for Beaujolais nouveau, made by carbonic maceration, to over twenty years for wine of good structure with high levels of acid, tannin or sugar. That is why the ferment matters to a buyer and not only to a winemaker: skin contact, malolactic conversion and vessel choice are what put the acid, tannin and texture there in the first place. Professional assessment treats them as evidence. A full quality assessment weighs, among other things, whether a wine "uses certain wine-making techniques, such as barrel fermentation or malolactic fermentation".

For what is ready on the shelf now rather than in five years, start with wines drinking well now.

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