© Best Rioja Wine

Whole bunch berries are carefully placed in a tank to begin the carbonic maceration process.

Have you ever wondered about carbonic maceration? Tom Jarvis is here to explain all.

Carbonic maceration is a process in winemaking which occurs when clusters of intact red grapes are placed in a sealed tank filled with carbon dioxide.

Fermentation occurs without the intervention of yeast or other microbial activity. In these anaerobic (oxygen-free) conditions carbon dioxide is absorbed by the grape, which converts from a respiratory to an anaerobic metabolism. A complex array of processes are conducted by enzymes, as an intracellular fermentation commences within the intact grapes.

This process produces alcohol plus various compounds which affect aroma and flavor. The effects of carbonic maceration (CM) on phenolic content may vary with different grape varieties. But most wines made with this method are light in color, low in tannin, soft and fruity. Wines are also usually less acidic.

History of controlled carbonic maceration

In 1934 French scientist Michel Flanzy (1902-92) was struggling to perfect the preservation of grapes using carbon dioxide. He noted the unique organoleptic qualities of the wines produced from the fruit used. This did not lead to any major uptake of the technique, but a CM working group formed by Michel in the 1960s would be more influential.

Based on his work from the same decade on, Jules Chauvet (d. 1989) Beaujolais négociant, taster and chemist, is often called the father of carbonic maceration. His studies initially focused on the technique's effect in raising pH. Chauvet identified Gamay and Grenache as varieties particularly suitable for full CM, plus others including Mourvèdre, Pinot Noir and Syrah which could benefit from semi-carbonic maceration. His studies have also strongly influenced winemaking without sulfur additions.

Aroma

The aromas associated with CM can produce polarized reactions in tasters. They are commonly described as fruity or musk-like, with red-berry and cherry/kirsch aromas (particularly in Beaujolais). Estery characters such as bubble gum are regularly mentioned as telltale signs. Vanilla, almond, cinnamon, and other spices are often detected, as are aromatic woods and oak-like characteristics.

The aromatic compounds that mark out CM are yet to be clearly identified. Confusingly, the method seems to enhance varietal characteristics in varieties such as Muscat and Shiraz. in others compounds deriving from the process itself come to the fore. This is particularly the case with more neutral varieties such as Carignan. Anaerobic metabolism does seem to develop specific aroma compounds. However whole berries sitting in must also absorb aroma compounds derived from yeast fermentation into their skins.

In very young wines, aromas based on esters such as ethyl cinnamate and isoamyl acetate tend to predominate, giving intense floral and fruit characters. These tend to moderate after a few months, when typical notes connected with aging begin to appear. As wines made using CM age, levels of volatile phenols often seem to increase. This is likely because the fermentations are particularly attractive to microbial contaminants.

© Cafa Formations

Berries gently crush each other creating carbon dioxide and natural yeasts are released.

The broader processes: full versus semi-carbonic maceration

For full CM, careful harvesting and handling are crucial; the bunches and berries must arrive at the winery unbroken. Berries detached from the stalk, even if otherwise intact, synthesize less alcohol. This means that mechanical harvesting is only an option with tough-skinned varieties. The cost of bunch handling was a major factor of the decline in popularity of the method after a period of rapid adoption in the 1970s.

The clusters are carefully placed in a sealable tank which was previously filled with carbon dioxide, either from a cylinder or another ferment. This tank must be supplied with carbon dioxide until the in-berry and yeast fermentation provide the gas in sufficient quantities.

Tanks can be made of wood, stainless steel or other typical materials. Short-sided vessels are preferred to minimize pressure – the size and shape of the vat has an impact upon pressure exerted at the bottom of the tank. In turn this impacts upon the volume of clusters which are covered in must, and therefore the proportion of intact berries within an anaerobic atmosphere. It is also possible to use sealable polythene bags. A variant of the process using bags to contain the fruit and dry ice to create carbon dioxide was patented in Australia in 1986.

But whatever the methodology employed, 100-percent CM is a highly technical, verging on theoretical goal. This is because of the difficulty in keeping berries at the bottom of the tank intact.

Even in a sealed, flooded tank, grapes often exist in three environments. Intact berries towards the top of the tank are immersed in a carbon dioxide-rich atmosphere. In the middle are intact berries immersed in the must released from the crushed fruit at the base. Yeasts, even in the absence of oxygen, can remain viable via fermentation.

In Beaujolais, the above method is used predominantly for Beaujolais Nouveau, which needs to be made and released quickly. The "maceration traditionelle" of Beaujolais is actually an example of semi-carbonic maceration, where crushing at the bottom of the vat is more explicitly expected, and to a higher degree. However an increasing number of producers in the region are turning from CM to "Burgundian" methods of crushed berry fermentation for top cru wines.

The semi-CM recipe can vary, but sealed vats, pre-flushing with, and additions of carbon dioxide, are not compulsory. The requirement for intact fruit is not absolute, as when the whole bunches enter the tank crushed berries at the bottom of the vat are crushed and start to ferment. The degree to which the tank is sealed dictates the level of CM in the upper section of the grape mass.

In the intact berry

Anaerobic metabolism only occurs within intact berries, but in both the carbon dioxide and the must, though with some variations in the precise processes. At the bottom of the tank, yeasts begin to ferment the must, which can in turn promote malolactic fermentation. Also, diffusion occurs between intact and crushed berries plus the stalks, and the fermenting must.

The shift to an anaerobic fermentatitive metabolism is related to the presence in a grape of the enzyme alcohol dehydrogenase, which is first detectable after veraison. It turns sugar into alcohol in a similar way to the alcohol dehydrogenase found in yeasts. ADH levels in the grape increases gradually with ripening. Though this development seems to occur with no restriction in exposure to oxygen, anaerobic metabolism function seems to be an indication of berry ripeness.

Intracellular fermentation produces small amounts of ethanol. In addition by-products such as glycerol and acetaldehyde are accumulated in the berry. There are changes in organic nitrogen content, and the diffusion of phenolic and some aromatic compounds from the skin to the pulp. Color compounds are more readily extracted than tannins. Another key process is the catabolization of malic acid. This forms additional alcohol plus succinic and other acids, but not lactic acid. Around 50 percent of the malic acid content can be converted in this way, and pH in the berry increase by roughly 0.25. The raising of pH in the ferment has various implications for other parts of the winemaking process, and along with the raised glycerol and modified phenolics, influences the mouthfeel of the final wine.

This intracellular fermentation does not persist throughout the entirety of alcoholic fermentation. Once alcohol production reaches around two percent , the enzymatic activity ceases, the berry usually splits and juice escapes. This may be because the ethanol produced disrupts grape cell integrity. Berries otherwise demonstrate tolerance to the absence of oxygen during this period.

Once berries burst (or carbonic maceration is halted) phenolics are extracted conventionally. This subsequent extraction also occurs at lower alcohol levels as the whole berries have lost around 20 percent of their sugar. Some berries will remain intact through the process as far as pressing; they will show a pinkish flesh to diffusion of skin phenolics.

Temperature of the fruit when harvested seems crucial; as it affects the rate at which the berries absorb carbon dioxide. Fruit at 35° will initially absorb around 50 percent of the volume of carbon dioxide in the tank and take around one week to reach berry death. Fruit at 15°C will absorb less carbon dioxide, and take up to 20 days and typically produce a wine much lower in aroma. 30 to 32°C probably produces the best-structured wines, though duration of the CM phase can also depend on the style of wine sought.

© Food & Wine Magazine

The fruit character of carbonic macerated wines often have bright berry notes of cherry, strawberry, raspberry and others.

Finishing the alcohol fermentations

Typically, completing the yeast fermentation takes just two to seven days, with malolactic fermentation completing a couple of days later. This speed facilitates the early release and drinkability of some CM wines such as Beaujolais Nouveau.

Low pH and plentiful residual sugar make CM juice vulnerable to microbial spoilage, so the wine may need to be pressed early to combat this. Free-run and press-wine components are usually kept separate for as long as possible to ensure microbial problems are isolated to a single batch. CM press wines have higher sugars, aromas and color so are deemed of higher quality, and top bottlings are often produced solely from press fractions.

Various maceration and maturation options can be employed to increase structure and age worthiness. Top Cru Beaujolais wines can age gracefully for 20 years, though some may show brettanomyces character. More usually such wines might peak with four to eight years of maturation.

Where else CM is used

Rioja – in particular the Alavesa subregion – also uses partial carbonic maceration in some of its wines aimed for more casual drinking, or younger/newer consumers. The centuries-old method used in Rioja is in essence to place whole bunches in an open vat or lagar and let those berries at the bottom be crushed, commencing fermentation. In Georgia, similar processes happen in the traditional kvevri jars, buried in the ground.

In Burgundy, carbonic maceration (as distinct from whole cluster fermentation as outlined above) is sometimes employed for wines at the Bourgogne AOC level. Pinot Noir growers worldwide have also tried fermenting at least a portion of their vintage using the technique. The technique is used with various varieties in the South of France. It has proved beneficial in making juicy, approachable everyday reds out of tough, rustic Carignan. However it is hard to clearly identify the extent of CM and semi-carbonic maceration, as it can be hard to divine whether producers are using these or other whole-bunch or whole-berry techniques (see below).

Carbonic maceration is also a process used by coffee producers.

Other whole bunch and whole berry techniques: brief notes

The term whole-bunch (or whole-cluster) fermentation is used sometimes to refer to full-on carbonic maceration. However it can also refer to a broader range of techniques centered on filling a fermenter with whole clusters with stems still attached.

Variations on this theme include mashing up a tank of whole bunches to crush some berries and start fermentation, depositing variable amounts of whole clusters in the tank then covering them with destemmed and crushed berries, or layering with crushed berries like a lasagne. The method was seen as old fashioned but is increasingly used to add elegance to a wine, especially when climate change provides more reliably ripe stems.

These fermentations are usually cooler and slower than crushed berry ferments. Proponents often say that they feel they ferment is more controllable. But whatever the whole bunch or berry method, some intracellular fermentation occurs. This provides some aromatic and taste effects, and raises pH, which can give a smoother texture but may also risk volatile aromas.

Whole berry fermentation can be conflated with whole bunch ferments. However it usually means that the harvest goes through a gentle destemming. The absence of stems removes a source of tannins, and diminishes the ability of the berry to produce ethanol.

Footnote: carbonic maceration in white winemaking

Experiments have been made with the technique in white winemaking. The rationale for this is hard to understand, given the results of carbonic maceration in red wines.

Many white grape varieties do contain astringent tannins, which are not made obvious when the juice is immediately run off the skins. But any more than 48 hours of carbonic maceration will likely extract some of these. Varietal aromatic character will also diminish in the final product.

The challenges do seem to vary depending on the variety used, and some cases will appear to be more counterintuitive than others. Riesling is an example where high phenolics are likely, and hallmark acidity and aroma is altered. Pinot Gris seems to be a better candidate for keeping phenolics under control.

Given this, the technique may make sense within the orange wine movement, as proponents look for variations on the theme of skin-contact, phenolic whites. A few winemakers have made and released white wines made entirely with carbonic maceration techniques, or in combination with components of skin-contact wine.