Wine Arcana

Concept

Carbonic maceration

Also known as Macération carbonique

Carbonic maceration is a way of beginning vinification with intact grapes in an oxygen-poor, carbon-dioxide-rich atmosphere. While the berries remain unbroken, living grape cells change their metabolism and transform part of their own contents. This intracellular phase can alter acidity, aroma, and the movement of phenolic compounds before ordinary yeast fermentation completes the conversion of sugar into alcohol.

Carbonic maceration can help make a supple wine for early drinking. Berry integrity, time, temperature, extraction, pressing, and the treatment of stems and press wine all shape the result.

How intact grapes ferment

A harvested grape is still living tissue. In the presence of oxygen its cells continue to respire; when carbon dioxide displaces that oxygen, they shift to anaerobic metabolism. Enzymes within the berry consume some sugar, form a limited amount of ethanol and other compounds, and break down some malic acid. Cell walls and membranes also change, allowing substances including pigments to move from the skin toward the pulp. This is called intracellular fermentation, although the defining reactions are performed by grape cells rather than by yeasts fermenting free juice.1. International Organisation of Vine and Wine, “Carbonic maceration,” International Code of Oenological Practices, II.1.7; Australian Wine Research Institute, “Winemaking treatment—carbonic maceration.”

Filling a vat breaks some fruit, and more berries split as the process continues. Yeasts ferment the released juice, while intact berries elsewhere in the same vessel continue their own anaerobic metabolism. Eventually the fruit is drained and pressed. The juice still contains substantial sugar, so yeast fermentation must then finish the wine, commonly away from the skins. Malolactic fermentation may follow; it is a separate microbial conversion from the malic-acid breakdown that occurred inside the grapes.

Full and semi-carbonic practice

In the strict or idealized form of full carbonic maceration, healthy, intact grapes enter a closed vessel that has been purged with externally supplied carbon dioxide. Avoiding free juice at the outset delays conventional yeast fermentation and gives more berries time in the intracellular phase. Perfect separation is difficult at cellar scale, however: bunches have weight, berries break, and a fermenting liquid phase usually develops.

Semi-carbonic maceration begins without first filling the vat with carbon dioxide. Whole bunches are loaded into the vessel, and grapes near the bottom are crushed under the mass above. Yeasts ferment their juice and generate the carbon dioxide that progressively makes intact fruit higher in the vat anaerobic. Conventional fermentation, extraction from grapes in juice, and intracellular metabolism therefore occur together. The International Organisation of Vine and Wine’s definition allows the gas to come from an external source, crushed berries, grape respiration, or a combination.1. International Organisation of Vine and Wine, “Carbonic maceration,” International Code of Oenological Practices, II.1.7; Australian Wine Research Institute, “Winemaking treatment—carbonic maceration.”

In whole-bunch fermentation, an open or actively worked vat may contain intact berries without the necessary atmosphere, while destemmed but unbroken berries can undergo intracellular fermentation if oxygen is excluded. Stems and carbonic conditions are thus separate variables, even though traditional whole-bunch practice often combines them.

Extraction, aroma, and structure

Keeping grapes intact initially limits the washing and mechanical working of skins and seeds that drive conventional red-wine extraction. If the fruit is pressed after a short carbonic phase and fermentation finishes off the skins, the wine will generally receive less skin and seed tannin. Longer vatting, higher temperature, pumping over, punching down, fermenting further on the solids can all increase extraction. Pressing regime and the decision to blend free-run and press fractions further shape the result. Retaining stems can add another source of phenolic material and aroma.

The free-run and press fractions also differ. In a controlled Tempranillo trial, wine drawn from the vat resembled the conventionally fermented wine in several phenolic measures, whereas the fraction pressed from grapes that had remained whole longer contained less tannin and anthocyanin but more of some fermentation-derived aroma compounds. The fractions are usually blended, so their proportions and the pressure applied help shape the result.2. Ana Rosa Gutiérrez et al., “Influence of microbial population on the characteristics of carbonic maceration wines,” LWT 166 (2022), article 113783, https://doi.org/10.1016/j.lwt.2022.113783.

Anaerobic grape metabolism changes the precursors available when yeast fermentation resumes, and the two phases together can emphasize distinctive fermentation-derived aromas. Their prominence depends on grape variety, fruit condition, temperature, duration, microbial activity, and ageing. Some can be conspicuous in a young wine and recede later; others may obscure rather than reveal varietal character. Likewise, partial loss of malic acid can soften the acid balance, alongside the effects of harvest ripeness and later malolactic fermentation.

Beaujolais

Traditional vinification in Beaujolais is semi-carbonic: whole Gamay bunches are loaded into a vat, yeast fermentation begins in the juice below, and intact berries above undergo intracellular fermentation. Regional accounts also describe pumping over, pressing, blending free-run and press wine, and completing fermentation together.3. Inter Beaujolais, “Beaujolais vinification: a process like no other” and “The Grower's Work,” sections on maceration, fermentation, and maturing.

For Beaujolais Nouveau, short vatting and rapid release can put supple structure and fermentation-derived aroma in the foreground. Longer maceration and more active extraction can give Beaujolais and Beaujolais-Villages firmer wines, while producers in the ten crus may use semi-carbonic practice for wines intended to develop in bottle. Other vinification methods are also used, and maturation may take place in vat, cask, or barrel.

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