Grape processing: must treatments

Grape processing: must treatments

by Maurizio Broggi October 12, 2023

In the third part of our article series on pre-fermentation steps, Westgarth Wines wine specialist Maurizio Broggi delves into must treatments.

Before fermentation begins, the grape must may go through several treatments and adjustments that influence the style and quality of the wines. Sometimes adjustments are necessary due to less ideal climatic conditions such as in overly cool or hot growing seasons where the acid-sugar balance is not right for the style the winemaker wants to achieve. In general, additions to grape must are best made before or in the early stages of fermentation as fermentation promotes better integration of the adjustments as well as a more harmonious outcome.

Addition of sulfur dioxide

Prior to fermentation, the grape must is fragile and prone to oxidation and microbial spoilage. Sulfur dioxide is added to grape must to protect it from oxygen and harmful microorganisms. This is particularly critical in white and rosé winemaking to preserve the fresh fruit aromas that characterize these styles. Red musts also require the addition of sulfur dioxide; however, grape skins naturally contain phenolic compounds that act as antioxidants, thus red musts require less sulfur dioxide.

Pre-fermentation clarification in white wines

After crushing and pressing, the white grape must contains a significant amount of suspended solids in the form of grape skin particles, stem fragments, seeds, and colloidal matter. If left with the grape must, the suspended solid matter may produce off-flavors and bitterness and reduce the fruit character of the final wine. Clarification is the process of removing suspended solids in the must.

The degree of clarification and the optimum level of the remaining solids depends on the method adopted to clarify, the winemaker’s intention and expertise as well as the cultivar and the wine style. Removing too many solids negatively affects fermentation as solids provide nutrients for yeasts during fermentation in the form of minerals, vitamins, and amino acids. On the other hand, leaving too many suspended grape solids may result in increased levels of hydrogen sulfides which produce unpleasant reductive odors such as rotten eggs and cooked cabbage.


Various methods of clarification exist, among which, settling (or sedimentation) is the traditional technique adopted to clarify wine. The grape must is left to stand in a tank for 12 to 24 hours or sometimes even longer. Settling allows suspended solid particles to slowly sink by gravity at the bottom of the tank. Often, this is done overnight and at cool temperatures, normally below 15 °C/59 °F. Although cool temperatures make the process even slower, it is necessary to preserve the grape must and avoid microbial activity such as ill-timed onset of fermentation. Once the solid matter is deposited at the bottom of the tank, the clarified must is racked off the gross sediments of solids.

Clarifying agents

Clarification can also be promoted with the addition of clarifying agents that speed up the settling process and result in a more effective settling and thus a clearer grape must. Among the most common clarifying agents are pectolytic enzymes and fining agents. Pectins are gelatinous colloidal particles in grape juice that contribute to the must’s viscosity. The addition of pectolytic enzymes helps break down pectins, which reduces the viscosity in grape must, thus achieving greater extraction of juice and a clearer must in a shorter time. Interestingly, pectolytic enzymes also promote the extraction of aroma precursors that are converted into aroma compounds during fermentation. Fining agents such as casein, bentonite, and gelatine can also be used to facilitate and speed up settling, thus promoting clarification.

Centrifugation and flotation

Faster and highly effective clarification can also be achieved with more advanced and expensive processes such as centrifugation and flotation. In the former case, a centrifuge clarifies the grape must by centrifugal force, separating the suspended solids from the liquid. Clarification by centrifugation is significantly faster than traditional settling and it is typically adopted for large volumes of wine in large production facilities. Centrifugation allows clarification of large tanks within a couple of hours. Flotation is a technique originally adopted in the mining industry for refining. It works by injecting tiny bubbles of gas such as nitrogen through the grape must from the bottom of a tank. As the gas bubbles rise, they catch the suspended solids which are carried to the surface where they float until they are skimmed off, resulting in a clearer grape must. As with centrifugation, flotation shortens the clarification process significantly.

Grape must enrichment

In difficult vintages or in cool, marginal climates where full ripeness cannot be regularly achieved, the natural sugar levels in grapes may not be sufficient to achieve an adequate final alcohol content. In such conditions, the grape must can be adjusted by increasing the sugar levels, thereby increasing the potential alcohol.

The traditional way to boost sugar levels in grape must is known as enrichment (or chaptalization). It consists of adding sugar to the must before fermentation. Sometimes, sugar is added in the early stages of fermentation as the process of fermentation increases the temperature of the must which facilitates the dissolution of sugar. Sugars can be added in the form of sucrose (cane or beet sugar), or as rectified concentrated grape must – a solution of fructose and glucose obtained from grape must.

Too much sugar addition leads to diluted, unbalanced wines. To safeguard wine quality and avoid misuse of this practice, most wine-producing countries regulate enrichment so that permission is required to adopt the practice, and maximum enrichment levels are established by law. Although the sugar-alcohol-conversion degree varies depending on grapes, yeast strains, level of oxygen, and sugars already present, to increase the alcohol level by 1% ABV, between 17 to 19 grams of sugar per liter is required.

Sugars in grape must can also be increased by adopting more advanced (and often controversial) technologies that concentrate grape must by removing water such as vacuum distillation, reverse osmosis, and cryoextraction.

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