There have been lots of threads on this, but here’s a pretty simple explanation lifted from wineanorak.com:
The wine world is full of terms that people use without quite understanding just what they mean by them. Carbonic maceration is one such term, and in this short piece I’m going to attempt to get to grips with it, without becoming too technical and boring. It’s a method that’s employed to make lighter red wines with fruitier aromas, and it’s strongly associated with the Beaujolais wine region.
As with many topics in wine, the truth isn’t entirely black and white. It’s complicated. Pure carbonic maceration is quite rare, and there are a number of variations on the theme. But the simple version is thus:
Carbonic maceration is the process that occurs when intact bunches of red grapes are fermented in a sealed vessel that has first been filled with carbon dioxide. In the absence of oxygen, these intact berries begin an intracellular fermentation process, during which some alcohol is produced, along with a range of other compounds that can affect wine flavour.
Once the level of alcohol reaches 2%, which is after about a week at typical fermentation temperatures of 35 °C, the berries begin to die. They then either release their juice, or more typically are pressed before this happens. Then follows a normal fermentation (carried out by yeasts), resulting in a relatively pale coloured red wine with low tannin levels and enhanced fruity aromatics.
So let’s add some complexity to this story. First, we need to distinguish between aerobic and anaerobic respiration. The first is what happens in the presence of oxygen. Cells need energy, and to get this they break down sugar using oxygen to produce carbon dioxide, sugar and water. In the absence of oxygen, anaerobic respiration can also take place in some cells: yeasts do this preferentially even when oxygen is present, and the result is that sugar is broken down to alcohol and carbon dioxide. The cells in grapes can carry out anaerobic respiration, but they are less able to cope with the resulting alcohol than yeasts are, and if they do this for too long they die.
Carbonic maceration taking place at a winery in Spain
When whole bunches of grapes are placed in an atmosphere of carbon dioxide, they take it up and use it in anaerobic fermentation. In this process they break down sugars, but also malic acid, which is one of the main acids present in grapes. This malate degradation is the most significant step taking place during anaerobic fermentation, and it’s broken down sequentially to pyruvate, acetaldehyde and then ethanol. Typically, at a fermentation temperature of 35 °C, half of the malic acid is degraded in this way.
There is therefore a fall in acidity levels that can be quite significant, with titratable acidity (TA) declining by as much as 3.5 g/litre and pH increasing by up to 0.6 units. But bear in mind that there would be some loss of acidity during the malolactic fermentation that usually occurs after alcoholic fermentation in red wines. This is where bacteria break down malic acid to lactic acid.
During this process, polyphenols (such as tannins and anthocyanins) migrate from the skin to the pulp (the inside of the grapes), turning their flesh pink. Various compounds that are important for flavour (or which are flavour precursors) are produced. For example, extra amino acids are liberated from grape solids, which increases the nutrient status of the juice, and opens up the potential for these amino acids to act as flavour precursors. The ethanol produced can esterify some grape components, and one ester produced this way, ethyl cinnamate, gives strawberry and raspberry aromas. Another compound that increases is benzaldehyde, which adds cherry/kirsch aromas.
The berries eventually die when alcohol reaches a level of 1.5–2.5%.
Beaujolais is traditionally associated with carbonic maceration, but the traditional method used here (known as maceration traditionelle) is not a strict carbonic maceration. Here, the entire clusters are transported in 50 litre bins and dumped in wooden cuvees, or cement or steel tanks. Some of the berries on the bottom are crushed by the weight of those above them, they start fermenting, and the tank fills up with carbon dioxide. These intact berries begin internal fermentation and then when they die they release their juice, which still has quite a bit of sugar in it, keeping the fermentation process going. The higher pH that results from the intracellular degradation of malic acid means that malolactic fermentation can begin more easily after alcoholic fermentation finishes.
Even a stricter maceration carbonique, where the tank is filled with carbon dioxide before the clusters are added, will have a component of normal yeast-fulfilled alcoholic fermentation because some of the grapes will end up being crushed. The more yeast activity, the less of the distinctive carbonic maceration aroma the finished wine will have. Which can be a good thing.
There’s another variation on this theme that needs mentioning, and that is when whole bunches are used in a fermentation. In some cases, whole bunches are used, and then these are mashed up to release juice so that fermentation can start. In other cases, winemakers might seed the fermenting tank with a layer of whole bunches, and then add destemmed and crushed grapes on top. While much of the fermentation will be carried out by yeasts, the intact berries of the whole bunches will be in an anaerobic environment, and so some intracellular fermentation will take place. Even at pressing, there will still be some intact berries, and these will have pink flesh. One advantage is that in whole bunch fermentations sugar is more slowly released from the berries, keeping fermentation ticking along. Higher levels of glycerol are produced in these situations, which helps with the texture of the wine.
There are some risks associated with carbonic maceration. The first is that the rise in pH (and fall in acidity) can make the wine a more hopsitable environment for rogue microbes such as Brettanomyces. The second is that if oxygen isn’t excluded, volatile acidity can become a problem because of the growth of Acetobacter. But if these risks are controlled it can be a really useful technique for making lighter, approachable, fruitier wines, either as stand alone wines or as useful blending components.
The explanation is ‘geeky’ for sure but hopefully understandable . . .