Essential Pre-Fermentation Practices in Wine Production

Before a single yeast cell begins converting sugar into alcohol, winemakers invest considerable effort in preparing the grape juice – known as the must – for fermentation. These pre-fermentation practices are what separate a mediocre wine from a great one. From removing stems and crushing berries to clarifying the juice and adjusting its chemistry, every decision at this stage directly shapes the flavour, aroma, colour, and stability of the finished wine. Let’s walk through each of these critical steps.

What is the must, and why does it matter? 🔗

In winemaking, the term must refers to freshly crushed grape juice that includes the pulp, skins, seeds, and sometimes stems. It is the raw material that yeast will eventually transform into wine during fermentation. The condition of the must – its sugar content, acidity, clarity, and freedom from unwanted microbes – determines how smoothly fermentation proceeds and how good the final wine tastes.

Pre-fermentative practices are all the steps winemakers perform between harvesting grapes and the start of alcoholic fermentation. These steps typically take place within hours of harvest, and any delay or error during this phase can lead to oxidation, microbial spoilage, or off-flavours that persist throughout the wine’s life.

Stemming (destemming) 🔗

The first physical operation after grapes arrive at the winery is stemming, also called destemming. This step involves separating individual grape berries from their stems (also called the rachis or stalk) using a mechanical device known as a destemmer or a combined crusher-destemmer unit.

Why remove stems? 🔗

Grape stems are rich in tannins and can contribute harsh, bitter, and herbaceous flavours to the fermenting must. For red wines in particular, removing stems helps avoid excessive astringency in the final product. As noted by winemaking experts, suppressing the stalk prevents undesirable green flavours from leaching into the juice during maceration.

When are stems retained? 🔗

Interestingly, destemming is not always performed. In white wine production, stems are sometimes left intact because they facilitate the pressing process – they create channels within the press that allow juice to flow more easily. Some red wine producers also choose to include a portion of stems for added structural tannins and complexity, a technique known as whole-cluster fermentation. Additionally, in a process called carbonic maceration, whole grape clusters – stems included – are placed in closed tanks to undergo intracellular fermentation, producing lighter, fruit-forward wines such as Beaujolais Nouveau.

Crushing the grapes 🔗

After destemming, grapes move to the crushing stage. The goal here is to break open the grape skins so that the juice inside is released and exposed to yeast for fermentation. In modern wineries, mechanical crushers fitted with adjustable rollers or paddles perform this task efficiently.

How crushing works 🔗

A typical crusher-stemmer machine consists of a perforated cylinder with paddles rotating at high speed. The berries are broken and fall through the cylinder’s perforations, while stems are expelled separately. This equipment has largely replaced the ancient tradition of foot-treading, though some premium producers still practise it for gentle extraction.

The key challenge during crushing is gentleness. If grapes are crushed too aggressively, the seeds inside can break open, releasing bitter oils and harsh tannins into the must. Modern wineries control this by adjusting the gap between rollers. Some high-end producers use gravity-fed systems where grapes break under their own weight, minimising mechanical damage.

Timing is critical 🔗

Crushing must happen quickly after harvest. According to the Encyclopaedia Britannica, fresh, fully ripened grapes are preferred for winemaking, and any delay increases the risk of unwanted oxidation or wild yeast activity that could trigger premature, uncontrolled fermentation.

Juice separation 🔗

Once the grapes are crushed, the juice needs to be separated from the solid matter – skins, seeds, and pulp. This step differs significantly depending on whether a white wine or a red wine is being made.

White wine: immediate separation 🔗

For white wines, the juice is separated from the skins almost immediately after crushing. Prolonged contact between juice and skins would extract excessive tannins and colour, which is undesirable in white wines. Sometimes winemakers allow brief skin contact (3 to 24 hours) to enhance flavour extraction, but this must be done carefully to avoid picking up unwanted bitterness.

Red wine: fermentation on skins 🔗

Red wine production takes the opposite approach. The crushed grape must – including skins and seeds – goes directly into fermentation vessels. The skins remain in contact with the juice throughout fermentation because they provide the colour (anthocyanins), tannins, and flavour compounds that define red wine. Juice separation happens only after fermentation is complete.

Free-run juice vs. press juice 🔗

During juice separation, two types of juice are obtained. Free-run juice is the liquid that drains out of the crushed grapes under gravity, without any pressure applied. It tends to be lighter and more delicate. Press juice is extracted by mechanically pressing the remaining grape mass (called pomace). Press juice is typically deeper in colour, higher in tannins, and more robust. Winemakers can blend these two fractions in different proportions to achieve their desired wine style.

Common pressing equipment includes the traditional basket press, horizontal bladder presses (such as the Willmes press, which uses an inflatable tube inside a perforated cylinder), and continuous screw presses. Each method produces juice with different characteristics, giving winemakers further control over the final product.

Must clarification 🔗

After crushing and pressing, especially in white and rosé winemaking, the must contains a significant amount of suspended solids – tiny particles of grape skin, stem fragments, seeds, and colloidal matter such as pectins and proteins. If these solids are not removed before fermentation, they can produce off-flavours, bitterness, and reduce the fresh fruit character of the wine.

Clarification is the process of removing these suspended particles. However, it requires a careful balance. Removing too many solids deprives yeast of essential nutrients (minerals, vitamins, amino acids) needed during fermentation, which can lead to sluggish or stuck fermentation. Leaving too many solids can produce unpleasant hydrogen sulfide (rotten egg) aromas.

Settling (sedimentation) 🔗

The traditional and most widely used clarification method is static settling. The must is transferred to a tank and left undisturbed for 12 to 24 hours, typically at cool temperatures below 15°C (59°F). During this time, suspended particles slowly sink to the bottom under gravity. The clear juice is then racked – carefully siphoned or pumped off the sediment into a clean vessel for fermentation.

Cool temperatures serve a dual purpose: they slow down the settling process but, more importantly, they prevent premature fermentation and suppress microbial activity that could spoil the must.

Clarifying agents 🔗

To speed up settling, winemakers often add clarifying agents. Pectolytic enzymes are among the most common – they break down pectins (gel-like substances naturally present in grape juice) that increase viscosity and prevent particles from settling. By reducing viscosity, these enzymes yield a clearer must in less time and also help release aroma precursors that enhance the wine’s bouquet.

Fining agents such as bentonite (a type of clay), casein, and gelatine are also used. These substances bind to suspended particles and drag them to the bottom of the tank, accelerating clarification.

Advanced methods: centrifugation and flotation 🔗

Large-scale wineries often use more advanced techniques. Centrifugation uses centrifugal force to rapidly separate solids from the liquid, processing large volumes within a couple of hours. Flotation, adapted from the mining industry, works in the opposite direction – tiny gas bubbles (usually nitrogen) are injected from the bottom of the tank, and as they rise, they carry suspended particles to the surface where they can be skimmed off. Flotation is particularly popular in large Australian wineries processing over 1,000 tonnes of grapes.

Addition of sulfur dioxide 🔗

One of the most universal pre-fermentation treatments is the addition of sulfur dioxide (SO₂) to the must. SO₂ serves two essential functions: it acts as an antioxidant, preventing the juice from browning through oxidation, and as an antimicrobial agent, suppressing the growth of harmful wild yeasts, bacteria, and moulds that could spoil the wine.

The use of sulfur dioxide in winemaking has ancient roots. According to a study published through Springer Nature, the practice dates back to Egyptian and Roman times, though its full chemistry was understood only much later.

How much and when? 🔗

Research conducted at the Ohio Agricultural Research and Development Center found that wines where SO₂ was added early in the process – right after crushing and before pressing – scored higher in tasting evaluations than wines where SO₂ was added at later stages. This underlines the importance of early protection.

White and rosé musts require higher doses of SO₂ because they lack the natural phenolic antioxidants found in red grape skins. A typical initial dose ranges from 25 to 50 parts per million (ppm), though the exact amount depends on grape condition, pH, and the winemaker’s style. Musts with higher pH need more SO₂ to achieve the same level of microbial protection because the antimicrobial form of SO₂ (molecular SO₂) becomes less prevalent as pH rises.

Sugar adjustment (chaptalization) 🔗

The sugar content of grape must directly determines the potential alcohol level of the finished wine. When grapes do not reach full ripeness – common in cooler climates – the must may have insufficient sugar for a balanced wine. In such cases, winemakers perform chaptalization, the process of adding sugar to the must before or during early fermentation.

How chaptalization works 🔗

The term is named after the French chemist Jean-Antoine-Claude Chaptal, who developed the technique. The added sugar is not meant to sweeten the wine – yeast consumes it entirely and converts it into alcohol. As a general guideline, adding 17 to 19 grams of sugar per litre raises the alcohol content by approximately 1% ABV.

Common sugars used include sucrose (table sugar from cane or beet), dextrose (glucose), and rectified concentrated grape must (a solution of fructose and glucose derived from grapes). Pure white sugar is the most widely used option because it is inexpensive and leaves no residual flavour.

Regulations and limitations 🔗

Chaptalization is a regulated practice. It is permitted in cooler wine regions such as parts of France, Germany, Oregon, Canada, and New Zealand, where grapes routinely struggle to reach adequate sugar levels. However, it is prohibited in warmer regions like Argentina, Australia, California, Italy, Spain, and South Africa, where grapes easily achieve – or exceed – the required sugar content.

Excessive chaptalization dilutes flavour and complexity, and most regulations cap the permitted increase at around 1.5 to 3% ABV.

Alternative techniques 🔗

Besides direct sugar addition, winemakers can increase sugar concentration through more advanced methods like reverse osmosis (which removes water from the must), cryoextraction (freezing grapes to concentrate sugars), and vacuum distillation. These techniques are more expensive but avoid the addition of non-grape sugar.

Acidity adjustment 🔗

Acidity is just as important as sugar in determining wine quality. Wines with too little acid taste flat, flabby, and are chemically unstable. Wines with too much acid taste sharp and unpleasant. Winemakers aim for a balanced acidity that provides freshness and structure while complementing the fruit character of the wine.

Acidification 🔗

In warm climates where grapes ripen fully (or even over-ripen), the natural acid content may be too low. In these cases, tartaric acid – the primary acid found in grapes – is added to the must. This practice, known as acidification, is common in regions such as California, Australia, Argentina, and southern Italy. Adding acid before or during early fermentation ensures it integrates better with the wine.

Deacidification 🔗

Conversely, in cool climates where grapes struggle to ripen, the must may be excessively acidic. Deacidification can be achieved by adding calcium or potassium salts that neutralise excess acid, or through a technique called amelioration – diluting the juice with water and then adjusting the sugar level back up through chaptalization. The standard recommendation is to add about 6 grams of tartaric acid per litre of water used for dilution to maintain the acid balance.

Winemakers also consider malolactic fermentation (MLF), a secondary bacterial process that occurs after primary fermentation. During MLF, sharp malic acid is converted to softer lactic acid by lactic acid bacteria, effectively reducing total acidity and softening the wine’s taste. Knowing whether MLF will occur helps winemakers plan their initial acid adjustments accordingly.

Cold pre-fermentative maceration 🔗

One increasingly popular pre-fermentation technique is cold maceration (also called cold soaking or prefermentative cold maceration). In this process, the crushed must is cooled to temperatures below 15°C – and sometimes as low as 0-2°C with dry ice – and held for several hours to up to two weeks before fermentation is initiated.

Research published in Food Chemistry confirms that cold maceration promotes the extraction of water-soluble compounds such as anthocyanins (colour pigments), sugars, and proteins from grape skins without extracting the harsher, alcohol-soluble tannins. Studies on Merlot wines have shown that cold maceration can increase the concentration of desirable esters and aromatic compounds by more than 20% compared to traditional methods.

This technique is particularly valued in red winemaking, where deeper colour and enhanced aromatic complexity are prized.

Temperature control during pre-fermentation 🔗

Temperature management runs through every pre-fermentative practice. Keeping the must cool – typically between 10°C and 15°C – during the hours between crushing and the start of fermentation serves several purposes. It slows enzymatic browning, prevents premature fermentation by wild yeasts, preserves volatile aroma compounds, and gives the winemaker time to complete clarification and chemical adjustments.

Winemakers achieve this through refrigerated tanks, cooling jackets, and sometimes the addition of dry ice. In warm harvest climates, grapes may even be harvested at night or in the early morning to arrive at the winery as cool as possible.

Putting it all together 🔗

Pre-fermentation is not a single step but a series of carefully sequenced decisions. Each choice – whether to destem fully or partially, how aggressively to crush, how much to clarify, what level of SO₂ to add, and whether sugar or acid needs adjustment – depends on the grape variety, the vintage conditions, and the winemaker’s intended style. These decisions happen fast, often within 12 to 24 hours of harvest, and they collectively define the must’s readiness for fermentation.

A well-prepared must – with balanced sugar, appropriate acidity, minimal unwanted solids, adequate SO₂ protection, and a controlled temperature – gives yeast the best possible environment to perform a clean, complete fermentation. The result is a wine that faithfully expresses the character of its grapes and its terroir.

What do you think? How much of a wine’s final quality do you believe is determined before fermentation even begins? And should techniques like chaptalization be more strictly regulated worldwide, or is it simply a practical tool for managing climate challenges?

References

1. https://www.britannica.com/topic/wine/The-wine-making-process
2. https://finding.wine/blogs/blog-posts/basic-steps-of-the-winemaking-process
3. https://en.wikipedia.org/wiki/Winemaking
4. https://westgarthwines.com/blogs/news/grape-processing-must-treatments
5. https://www.hawaiibevguide.com/wine-prefermentation.html
6. https://link.springer.com/chapter/10.1007/978-1-4757-6255-6_12
7. https://winebusinessanalytics.com/features/article/86536/How-Much-SOSUB2-SUB-to-Add-and-When
8. https://en.wikipedia.org/wiki/Chaptalization
9. https://winefolly.com/tips/wine-additives-chaptalization-vs-acidification/
10. https://morewinemaking.com/articles/Dilution_chaptalization_must
11. https://www.sciencedirect.com/science/article/abs/pii/S0308814624031546