Carbonic maceration

Intracellular fermentation in grapes#

The wine method rests on the metabolism of the intact fruit. Placed in CO₂ without crushing, the berries change from a respiratory to an anaerobic fermentative metabolism.^2,3 The intracellular fermentation is carried out by grape enzymes inside the whole berries, and its most important reaction is the decarboxylation of malic acid.² A small amount of sugar becomes ethanol, malic acid falls by about half, and secondary compounds form.¹ According to the AWRI, this happens only if the grapes remain intact with no free juice, in a closed vessel filled with CO₂.³

Temperature controls the intracellular phase. A temperature above 30 °C inside the whole grapes is essential for it to proceed optimally.² In a small-scale Spanish trial, the tanks never reached that temperature during 12 days of fermentation–maceration.²

The full process has four steps: vatting of intact berries, maceration–fermentation, pumping off and pressing, and a second fermentation.¹ Yeast fermentation of the released juice runs alongside the intracellular phase.^1,2 A carbonic maceration wine is therefore never purely intracellular.

What the coffee trials tested#

Catuaí, Brazil (2023). Coffea arabica Catuaí Vermelho from the 2018/19 harvest was grown at 1,040 m in Venda Nova do Imigrante, Espírito Santo.⁴ Lots of 2.6 kg of washed and sorted cherries were packed in polypropylene bags (40 × 50 cm). Air was removed with a vacuum sealer for 20 s, and CO₂ of 99.9% purity was injected.⁴ The bags were held at 18, 28 or 38 °C for 24–120 h.⁴ At 38 °C the sensory score rose significantly, by about 4 points between the start and end of fermentation.⁴ No unfermented control is described.⁴

Conilon, Brazil (2022 and 2025). C. canephora var. conilon from Jaguaré, Espírito Santo, was treated the same way in 4 kg bags, with four replications per treatment.⁵ The same temperatures and times were tested, again without an unfermented control.⁵ Scores ranged from 78.51 to 83.25 points, and the highest came from 38 °C for 120 h.⁵ The total score rose with temperature at 24, 72 and 96 h.⁵ Infrared spectra of the coffee showed no clear link with the sensory results.⁵ A 2025 paper from the group reports the same best treatment and score. It identified 92 volatile compounds and found temperature to play a significant role in the volatile profile.⁶

Ohmic-heated fermenter, Indonesia (2026). About 10 kg of whole arabica cherries from about 1,400 m in Tanah Toraja went into each fermenter.⁷ The fermenter was a PVC tube of 12.7 cm internal diameter, heated electrically, with potable water added as the electrical conductor.⁷ The paper calls the conditions CO₂-rich and anaerobic but does not say how CO₂ was supplied or checked.⁷ Cherries were held at 31 or 36 °C for 48, 144 or 240 h, then sun-dried to about 8% moisture (basis not stated).⁷ The control was depulped directly after harvest and not fermented; the paper also calls it untreated or honey processed.⁷ Two certified cuppers scored the control at 84.5 and most treatments at 85.0–85.75; the treatment at 36 °C for 48 h scored 83.0.⁷ Sugars generally fell as fermentation lengthened, and 3- and 4-caffeoylquinic acid generally decreased.⁷

“Semi-carbonic” maceration, Thailand (2023). Arabica was held at 15 or 20 °C for 10 days, or at 20 °C for 5 days with added pectinase, and compared with a pulped natural control.⁸ Fifty-one volatile compounds from 12 chemical classes were identified.⁸ The 15 and 20 °C treatments scored highest for fragrance, flavour, aftertaste, body, balance and overall impression; the enzyme treatments scored highest for acidity.⁸ The abstract opened for this entry does not define what “semi-carbonic” means.

Microbes rather than fruit metabolism#

The coffee evidence points to a microbial fermentation. In the Catuaí trial, lactic acid bacteria of the genera Leuconostoc and Lactobacillus predominated.⁴ Leuconostoc was relatively more abundant early in fermentation and at 18 and 28 °C.⁴ Microbial diversity was greatest at 38 °C after 72 h, and at that temperature Lactobacillus abundance correlated positively with the sensory score.⁴ The same bacterial groups dominate ordinary washed fermentation, as described under microorganisms.

What the coffee studies did not measure matters as much. None of the papers opened for this entry tracked malic acid breakdown or other markers of fruit metabolism, which define the wine process.^1,2 The conilon paper suggests that the fruit releases its juice into the fermenting environment, but does not measure it.⁵ Whether intracellular fermentation contributes anything in coffee is not established.

The temperatures also differ. Wine needs more than 30 °C inside the berry for the intracellular phase², while the coffee trials ran from 15 to 38 °C.^4,5,7,8

Reference protocol#

No agreed protocol exists. The Indonesian authors note that coffee carbonic maceration studies run under broad and variable conditions, which limits reproducibility.⁷ The Brazilian group suggests that research into microbial dynamics could lead towards standardisation.⁶ The table sets the four trial designs side by side; it is not a recipe.

Controls and measurement#

The trials controlled four things: temperature, time, the removal of air and the purity of the injected gas. The conilon trial used programmable incubators specified to ±0.1 °C.⁵ The Indonesian trial heated the fermenter electrically.⁷

Several basic readings are missing. The Catuaí and conilon papers report no pH during fermentation, and none of the three papers opened in full reports oxygen or CO₂ concentrations inside the vessel.^4,5,7 The Brazilian papers give the injection pressure as 20 kgf cm⁻² (about 2 MPa, our conversion), but not the pressure or gas composition in the bag during fermentation.^4,5 Without these readings, “CO₂-rich” cannot be checked or reproduced. The monitoring tools used in ordinary tank fermentation – pH, temperature and °Brix – are described under fermentation.

Risks#

• Asphyxiation. CO₂ is a colourless, odourless gas, heavier than air (relative gas density 1.53).¹² NIOSH lists 40,000 ppm as immediately dangerous to life or health; symptoms include headache, dizziness, breathing difficulty and, at worst, asphyxia.¹²
• Lower scores, not only higher ones. In the Indonesian trial, one CO₂ treatment scored below the unfermented control.⁷ In the conilon trial, 18 °C for 24 or 48 h gave scores below 79 points.⁵
• Loss of sugars. Sugar content tended to fall as fermentation lengthened, and was significantly lower after 10 days.⁷
• Drying whole fermented cherries. After fermentation the fruit is dried whole^4,7, so the drying risks of the natural process apply. No coffee carbonic maceration paper opened for this entry measured moulds or ochratoxin A.
• Unclear claims. Because no protocol is agreed, the same label can cover very different treatments, and consumers may expect novelty from it.^7,9

Open questions#

• Fruit metabolism in coffee. Whether coffee cherries carry out any intracellular fermentation under CO₂, as grapes do, has not been measured.^1,4
• CO₂ versus closed-tank anaerobiosis. No study opened for this entry compared injected CO₂ with self-induced anaerobic fermentation on the same lot.
• Controls. Two of the four trial designs had no unfermented control, so the effect of CO₂ cannot be separated from that of fermentation.^4,5
• Breadth. The evidence covers few varieties, one site per trial opened in full and panels of two to six cuppers.^4,5,7

Key studies#