Fermentation of Wine: Explained

What is wine fermentation? To many, it is a mysterious process that transforms vine-grown fruit into a heaven-sent beverage. But if you’re into wine investing and you’re using a wine investing app, you probably know that fermentation is key when it comes to the taste of wine. The oenophiles among us may also know that: 

  • The fermentation process of wine can last from just a few days to many weeks;
  • red wines and white wines have different fermentation processes; 
  • and that the fermentation of wine is what creates the alcohol.

But few contemplate the extent to which the specific conditions surrounding the fermentation process affect a wine and its value. And it’s essential knowledge for wine investors and connoisseurs alike. So read on to learn the must-know facts on wine fermentation.

What Does Wine Fermentation Do?

In winemaking, put simply, fermentation is the conversion of grapes into an alcoholic drink. Beyond this, fermentation is how a wine develops its key characteristics – its taste, its aroma, and its physical qualities. The world’s winemakers manipulate the process to create unique wines, with different types of wine fermentation accounting for thousands of distinctive tastes. It all begins with yeast. Once the yeast is added to the grape juice, or grape must, the fermentation process begins. Yeast converts the grapes’ natural sugars – glucose and fructose – into ethanol, as well as a series of compounds and the byproduct of carbon dioxide. After between two and three weeks – and two phases of fermentation – the wine is ready for stabilising, clarifying, and bottling.

Compounds Produced as an Effect of Fermentation:

Alcohol is not the only product of wine fermentation. Yeast also facilitates the production of complex compounds, which imbue the wine with its nuances. These compounds include:

  • Esters

As the yeast transforms the grape’s sugar into ethanol, it releases chemical compounds called esters, which are pleasant in their scent and give the wine a fruity or floral aroma. Each yeast strain forms different quantities of esters. In all, over 160 esters have been identified in wine, with multiple esters merging in any given wine to create an entirely unique aroma.

  • Tannins

Bitter in taste, tannins are a group of compounds that exist in natural produce, primarily to deter animals from eating a plant’s seeds and fruit before it is ripe. Grapes contain tannins in their skins, pips and stalks. Tannins give wine its dryness and mouthfeel, accounting for a wine’s body and structure. Tannin extraction increases during fermentation with the growth of alcohol levels. The other byproducts of the wine fermentation process react with tannins to impact on the drink’s structure and taste.

  • Acetaldehyde

Acetaldehyde is another scent-giving byproduct of the wine fermentation process, imbuing fruity aromas and tastes. Formed by the chemical oxidation of ethanol, acetaldehyde catalyses the polymerisation of tannins to stabilise red wines in terms of structure and mouthfeel. While, in moderation, acetaldehyde imbues sweet scents, levels that exceed 125 mg/l can lead to overbearing fragrances of bruised apples.

  • Anthocyanins

With antioxidant properties, anthocyanins are a type of flavanoid that account for the beneficial quality of wine. Odourless and astringent, this highly responsive family of compounds derives from grape skins and gives red wine its deep red hue, which is an indication of quality and value. During the ageing process, anthocyanins affect the reduction, oxidation, and polymerisation of the wine.

  • Sulfites

A group of chemical compounds, sulfites are a naturally occurring byproduct of fermentation that exist in organisms to deter competition – much the same as ethanol. Sulfites are preservatives that maintain taste and freshness, safeguarding the wine from microbial spoilage thanks to their antimicrobial properties. Winemakers rely on sulfites to minimise – and control – the wine’s oxidation. Avid wine fanatics may know that sulfites are responsible for a heavy head after an excess of wine. 

  • Amino Acids

Another important byproduct, amino acids contain nitrogen and are utilised by yeast to make proteins during fermentation. While, in grape must, protein levels account for less than 10% of nitrogen present, these levels reach up to 40% in wine. Nitrogen is critical for the growth of yeast and the fermentation of grapes, meaning amino acids are a catalyst in creating alcohol. Amino acids also affect the overall style of a wine and its aromas. 

What Helps the Wine Fermentation Process?


Yeasts are single-cell microorganisms that thrive with food, moisture, and warmth. In these conditions, yeast converts starch and sugars into alcohol and carbon dioxide. In the process of wine fermentation, yeast is the single most important ingredient, and its application determines the taste and characteristics of the wine. Winemakers use different yeast strains to distinguish their product, with naturally occurring yeasts and cultured yeasts each impacting on the finished product. 


Sugars occur naturally in grapes as fructose and glucose, with approximately 160 to 300 g/l of the two occurring in most grape must – approximately 15 to 20% of the entire grape. When it comes to the fermentation of wine, sugar levels are essential in determining the amount of alcohol produced by yeast. Winemakers must strike a fine balance. An over-abundance of sugar in the grape juice can stress the yeast, resulting in odd-tasting wine. When the yeast does not ferment all the natural sugars into alcohol, the wine is left with residual sugars, which gives it a sweet taste. If there is not enough glucose or fructose during fermentation, the process wains and the alcohol level is too low. As such, winemakers may add sugars to the grape must – a process known as chaptalisation – to increase the alcohol content of the wine.


The temperature at which the process of fermentation of wine is carried out also plays a key role on the outcome, with varying temperatures required for the production of both white and red wine. Winemakers generally ferment white wines over a longer period – over a period of months – at temperatures of between approximately 7°C and 20°C (between 45°F and 70°F) to form fruity flavours and to preserve volatile aromas. To maximise the extraction of tannin, wine producers ferment red wines at temperatures of up to 29°C (or 85°F). Temperatures that exceed 29°C may cause the damage of yeast, rendering them inactive and thus terminating the fermentation process, in what’s referred to as ‘stuck fermentation’. Ultimately, the temperature controls the rate of fermentation by controlling the chemical reactions of the wine.

The Vessel

Winemakers must also carefully select the appropriate vessel in which to house the grape must for the fermentation process, as the oxidation of the wine will differ according to the material of each. Porous oak barrels, for example, allow for the oxidation of the wine, making it the prime choice for the fermentation of red wine. Oak imbues red wines with notes of vanilla and subtle wood- or earth-like flavours. Most white wines and rosés, by contrast, are fermented in vast steel tanks, which prevents oxidation and allows for crisp flavours. In this instance, the material of the vessel does not impact the aromas and flavours of the wine.

Types of Wine Fermentation

As can be expected of an industry in which such a proliferation of produce exists, there are many different types and stages of wine fermentation, with nuanced takes on each. There are two main phases of fermentation in the production of wine: primary fermentation and secondary fermentation. Yet winemakers have found other ingenious ways to manipulate the production of wine.

Primary Fermentation

Primary fermentation begins from the moment in which yeast is added to the grape must. During this phase, the yeast count grows rapidly – feasting on the sugars – and the grape must shows visible signs of transformation. A layer of foam builds on top of the grape must, as carbon dioxide levels increase. The primary fermentation of wine lasts from just three to five days, culminating once the majority of the sugar is converted and the oxygen depletes. Up to 70% of the wine’s alcohol is created during the primary phase of fermentation. 

Secondary Fermentation

Not to be confused with a second fermentation, during which the excess sugars from the primary phased are consumed by yeast to restart fermentation, the secondary phase of wine fermentation lasts from one to two weeks and accounts for the final 30% of a wine’s alcohol content. The wine is transferred into a different vessel and the wine’s development is less discernible during this phase. Its completion depends on the fermentation conditions: the sugar content, the strain of yeast, and the temperature, etc.    

Malolactic Fermentation

Malolactic fermentation reduces the acidity – and impacts the taste – of wines by converting malic acid into lactic acid, raising a wine’s pH. This fermentation process begins at around 16°C, and so winemakers can choose to raise the temperature of a must or wine to begin the process. While malolactic fermentation is necessary in some wines, it is undesirable in others.

Other Types of Fermentation

Bottle Fermentation

In sparkling wine, winemakers use bottle fermentation to imbue the wine with carbon dioxide or fizz. Once a cuvée has completed its primary fermentation process, the wine is bottled and sugars and additional yeast are added, marking a second fermentation.

Carbonic Maceration

A rarer form of fermentation, carbonic maceration involves adding entire bunches of uncrushed grapes and carbon dioxide to a red wine to trigger what is known as intercellular fermentation. Wines produced under carbonic maceration include Beaujolais Nouveau and are best enjoyed young. 

Now that you know about the different types of fermentation, check out these wine tasting terms & tips for wine tasting!

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