Carbonic Maceration

Carbonic maceration: sure, I know what that is…

Carbonic maceration has been part of my wine vocabulary for years now. I have been able to tell you that it’s commonly used for Beaujolais, lends to fresh and fruity red wines, and involves fermenting whole grapes. I thought that I had a reasonable grasp of the concept. I wasn’t afraid to choose it as my variable in the experimental red wine making we’re doing in class this fall.

Last Thursday evening, I came home from a grape harvesting trip, looking forward to a Friday spent processing the merlot we had just unloaded into the student winery.  I thought it would be interesting to do a little internet research on the carbonic maceration (CM) technique. Anticipating a simple afternoon of gently tossing whole grape clusters into our fermenting tanks, I hoped to find a tip or two to differentiate our group’s wine from the rest.

Within about ten minutes, the scales fell off my eyes and I realized that I had blithely tossed “carbonic maceration” into conversations for years while remaining almost totally ignorant of its implications. And what might those be? That depends.

The Oxford Companion to Wine, Jancis Robinson:

” Carbonic maceration is a red wine-making process which transforms a small amount of sugar in grapes which are uncrushed into ethanol, without the intervention of yeasts. It is used typically to produce light-bodied, brightly coloured, fruity red wines for early consumption, most famously but by no means exclusively in the Beaujolais region of France.”

Concepts in Wine Chemistry, Yair Margalit (2nd ed):

“This is a special kind of fermentation which utilizes the ability of enzymes present naturally in grapes, to transform some small amount of sugar into ethanol. The process is eventually stopped by the accumulating alcohol which poisons the berry cells at about 2% ethanol.”

Some references make no mention of adding CO2, some insist that some of the grapes involved must be crushed by the weight of the grapes above, and some make a point of grapes remaining attached to the stems in whole clusters. Some say that, following some period (1-2 weeks) of CM, the juice is pressed off and final fermentation occurs without the skins, but not all make this point, either. Wikipedia seems to think that any wine that undergoes conventional yeast-driven fermentation following CM is properly only “semi-carbonic maceration.” Technically true, perhaps, but I don’t find the semantic distinction helpful.

In terms of practical commercial winemaking, removing grapes from the stems inevitably involves some degree of crushing, and any tank is big enough that grapes at the bottom will be crushed by grapes at the top. Then, of course, there is our little experimental wine making lab, the peculiar conditions of which may be the source of some of my confusion. Our “carbonic maceration”  ferments are entirely whole berry — even the grapes at the bottom — and perhaps only 50% remain attached to the stems. Then again, I doubt you will ever see a commercial winemaker spend three hours carefully shoving whole Merlot grapes into a 5-gallon glass carboy on a Friday afternoon.

Why bother?

Tannins: Ignoring the references for a moment, I can infer some likely consequences of CM on tannins. Tannins are found principally in grape skins, and different tannins are found in different layers of the skin. It is common, if mostly empirical and anecdotal knowledge, that tearing up red grape skins results in harsher tannins. This is a major motivating factor for gentler processing via gravity-flow rather than mechanical pump transfer, punch-downs versus pump-overs, and gentle crushing to release juice without pulverizing skins. If fermentation begins inside a whole grape, therefore:

1. Less total tannin will be transferred from the skin to the juice contained inside the grape per unit time. On the other hand, CM usually allows for a longer total contact time betwixt juice and skin. Since the kinetics of tannin-transfer are different, the type and feel of tannins should be different, too.

2. The tannins present in the inner layers of the grape skin will preferentially migrate into the fermenting grape interior, while minimal tannins will be extracted from the outer skin layers.

Aromatics:  Chemical analyses have shown that the “characteristic bouquet” of CM wines is related to higher levels of volatiles like vinylbenzene, benzaldehyde, ethyl cinnamate, and ethyl phenylacetate, to name just a few. A paper published in 1992 in the American Journal of Enology and Viticulture showed that a whole slew of free and bound monoterpenes increased in grapes treated to carbonic maceration for nine days. In this case, “carbonic maceration” meant storing whole grapes (Muscat canelli) in CO2 gas at 32°C (about 90°F.) Monoterpenes are a class of hydrocarbons related to floral-fruity aromas in grapes (and other fruits.) An overall higher concentration of monoterpenes, and a higher ratio of monoterpenes to other aromatic compounds, explains the “fresh and fruity” nose of wines processed by CM.

Acid: Simultaneous with fermentation of glucose, grape enzymes ferment malic acid — as much as 50% of the total concentration — to ethanol. Malic acid, a sharp-tasting acid found at high concentrations in grapes, is converted to lactic acid by bacteria during malolactic fermentation in most red wines. This “maloalcoholic fermentation” means that CM wines can forgo malolactic fermentation, avoid buttery-soft lactic flavors while decreasing sharp malic flavors, and maintain the perception of high acidity with lower actual acidity.

Start looking, and it seems as though CM is much more widespread than the classic Beaujolais example would lead one to believe. In Rioja (particularly the Alavesa sub-region), Spanish winemakers use CM to foster fruitier flavors in temperanillo wines intended for blending. A similar concept to Beajoulais nouveau; “temperanillo” literally means “young red wine” in Spanish. New world winemakers from California to Australia making “Nouveau”-style wines have taken up the technique with Gamay as well as other varietals.

Unanswered questions

1. Fermentation — the conversion of sugar into alcohol with release of CO2 — is usually catalyzed by microorganisms like the yeast Saccharomyces cerivisiae in winemaking. In CM, fermentation is catalyzed by native grape enzymes. (Side note: because the grapes retain active enzymatic activity, some folks classify the grapes involved in CM as still alive. “Help! I’m being fermented alive!”) There must be a difference in the products of non-microbial versus microbial fermentation. Yeast (and fermenting bacteria) produce all sorts of other, often flavorful and/or aromatic compounds as part of fermentative metabolism. Are the non-microbial enzymes so specific that they yield nothing but ethanol and CO2? Or do these enzymes yield their own unique set of fermentative byproducts? “An overview” of CM from 1989  says that succinic acid, shikimic acid, and glycerol are also formed, but methinks this is a gross oversimplification.

2. Wikipedia, amongst other sources, says that CM ferments “most of the juice while it is still inside the grape.” My handy wine chemistry textbook suggests that grape-derived fermentative enzymes are inhibited by ethanol concentrations above 2%. Does this mean that the inevitable juice spontanously released in a tank of whole grapes, in collusion with the virtually inevitable native yeasts that will find and ferment such juice, can and do raise CM ethanol levels substantially higher?  

And what about those 5-gallon carboys shoved full of whole merlot grapes and topped off with nitrogen gas (since I couldn’t find a CO2 tank but had N2 readily at hand)? Two days later, no visible change. I’ll take pictures and keep you posted.

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24 thoughts on “Carbonic Maceration

  1. If you’re ever in Woodinville, come swing by Page Cellars. This is our second year producing our Nouveau Merlot. I also had a hard time finding details on how to do a proper CM wine, but we think we have a pretty good handle on it now.

    It definately has a different taste, but it’s interesting to taste it side by side with the same grapes that are traditionally fermented. We’ll be releasing the 2010 vintage on Nov. 6th.

    Later,
    Tim Drake

  2. Tim, how exactly did you make your wine and what will the front label read?

    Does the TTB have a legal definition of Carbonic Maceration?

    Can or has CM be used for white grapes? If not, why?

    Beaujolais often smells like pink bubble gum and/or banana … is that from carbonic maceration or from a chosen strain of man-made yeast? The 1992 AJE&W paper used Muscat – already the fruitiest smelling grape – – a bad choice for their experiment – – Chardonnay aromas are more shy & neutral …

    In the real world, what typically is done after the CM and 2% alc has been acheived? (If CM only goes to 2%, then there is no wine made soley by CM, right?)

    What would be the differences between Whole Berry fermentation, Whole Cluster fermentation, and Carbonic Maceration?

    Rick Schofield, CWE
    Port Ewen, NY

    • Thanks for the invite, Tim. I’d love to hear more about the differences between your CM and your traditional ferments, and I’d love to stop by to taste the next time I find myself in Woodinville. I’m looking forward to a West-side visit soon.

      Rick, you’ve touched on many of the reasons I was frustrated in my search to properly define carbonic maceration. As with so many other winemaking techniques, precise methodology seems up to the individual.

      To the best of my knowledge and my ability to search websites, the TTB (Alcohol and Tobacco Tax and Trade Bureau in the US) doesn’t regulate the term “carbonic maceration.” Placing those words on a label doesn’t imply anything specific about winemaking technique from a legal standpoint.

      Again to the best of my knowledge, CM isn’t used for white grapes. Consider that for white wines, skin contact is usually minimal, while CM naturally involves prolonged, if gentle skin-juice contact. Interestingly, though, a major focus of the article I referenced on CM in the Rioja is the effects of cofermenting Temperanillo with white grapes. Legally, up to 15% of a Temperanillo can be comprised of white grapes — usually Viura — and it seems that white bunches are cofermented right along with the red grapes in this “non-strict” version of CM. (Incidentally, in this study, the sensory impact of Viura additions was relatively minor.)

      Let me do a bit more research and get back to you on the banana aroma and Muscat issues. There is always so much to consider that I forcibly restrict myself to a certain degree as I read up on a new topic. Without applying the stop button somewhere, I’d never write or converse about anything.

      You’re right about fermentation needing to finish via microbial fermentation after CM finishes. Whether by inoculation or by spontaneous fermentation, yeast activity is necessary to bring sugars down and alcohol levels up. In the gentlest of situations this might be almost contiguous with the end of (non-strict) CM as natural berry breakdown occurs and release of juice collaborates with permissive oxygen exposure to allow for yeast growth. More commonly, clusters are pressed off after several days to several weeks of CM or, occasionally, crushed and left to ferment on the skins for some additional period of time. In the case of Beajolais Nouveau, following completion of primary alcoholic fermentation, the wines may be pasteurized to inhibit malolactic fermentation and preserve the “fresh fruity” quality. This might also contribute to the bubble gum effect you discern.

      Whole berry fermentation and whole cluster fermentation seem fairly self-explanatory. Where confusion arises, I think, is when some folks use the term “carbonic maceration” to mean whole berry or whole cluster fermentation and nothing more, when most sources insist that CM also involves a carbon dioxide-rich, oxygen poor environment. The precise line between what constitutes CM and what is merely whole berry fermentation is fuzzy, as is suggested by the existence of references to “non-strict” CM.

      • I realize I am coming very late to this useful discussion but that could be a benefit. I am planning to use CM in the production of red wine from the Pais grape (Pais is the second largest in volume grown in Chile). Some outstanding wines are being made in Chile from this grape using CM (e.g., Louis Antoine Luyt’s Pais de Quenehuau being imported into the US by Louis Dressener). My question is whether you have come to any conclusion on what is the best way to proceed in producing high quality red wine using CM, or a CM hybrid process. I would be grateful for your guidance or suggestion for further reading. I am not an enologist.

      • Another late comment to a great discussion. I am an amateur winemaker who this year used a hardware-cloth screen to rub off whole berries into my BRUTE primary fermenters for two varieties that happened to have small berries–cab sauv and petit verdot. I ended up with must that contained both whole berries and some crushed–the level of the juice was below the level of the berries at the start of fermentation. During pressing, the free run juice was much less fruity and interesting than the pressed juice, which presumably expressed the juice that had fermented inside the berries. I assume that the latter was fermented both carbonically and in the usual way (once the commercial yeast got inside the berries via the stem-holes). Perhaps whole-berry fermentation, which is quite common in commercial wineries in Oregon, includes carbonic maceration as a natural by-process.

      • Thanks for sharing the story! Your observations about the free-run versus pressed juice here are interesting; while there have to be a number of variables here, it does get one thinking. I’ll be interested to hear about the results if you try experimenting with whole-berry ferments again.

  3. Yes, I must say szymanskiea (real name? whole name?) is very bright. She blogs in a way that is very scholarly but also logical & easy for regular people to understand.

  4. Hello
    I found your discussion fascinating and well beyond my knowlege level. However in an effort to understand better I searched out a great explanation in Ronald S. Jackson’s book “Wine Science” on pages 533 to 537 is a very indepth scientific explanation of the CM process.
    Thanks

  5. Boy, I must have an old Wine Science (1994), where the execllent CM coverage is from page 347 to 354 … I’m missing a couple hundred pages of new stuff!

    But I learned the difference between full carbonic maceration and semicarbonic maceration: They only do it to part of the crop! (And I was thinking that a full CM wine was a 2% wine with no subsequent microbial fermetation!)

    Whole berry fermentation and whole cluster fermentation … if the grapes are whole, how can you have a fermentation without juice?

    I assume that the little bit of juice at the bottom of the vat ferments normally and bubbles away hard enough to break up the rest of the berries? Then that, too, is only “Semi-Whole Berry-Fermentation, because it reverts to a normal fermentation? And there is no such thing as a full whole berry fermentation or are they talking about half of the crop again?

    Rick Schofield
    Port Ewen, NY

  6. Hmmm. My copy of Wine Science is the 2008 issue and upon further inspection, the section on CM is much longer ( pg.529 – 537 ). Here is what I have gleened from it.
    True CM ( anaerobic M. ) takes place in a CO2 rich enviroment where the whole berry (as oposed to juice from ruptured berries ) is subjected to enzymatic induced cellular decarboxylation reactions. As the amount of ethanol increases to 2% the enzymes die off and the production of CO2 from the process diminishes proportionately. A bi-product of cellullar distruction is the breakdown of pectins which weakens the cells attachment to one another and the pulp loses solid texture. “If the CO2 inside the intact fruit escapes, the berries become flacid”. At which time the resulting must is subject to microbial fermentation.
    On the subject of full or semi CM – Jackson makes reference to the suppression of the cabernet character of bordeaux wine if 85% of the crop is allowed to undergo CM so by regulating the uncrushed portion you can control the contribution of CM vs. varietal flavor. The significance of this is that if you are ignorant of the process of CM and you allow the crop to fully undergo CM the reulting wine will lack the desirable varietal qualities.
    These are some of the interesting concepts contained in the mentioned section of the book and I don’t know about you but I find it relatively easy to digest?
    Thanks for including me
    Albert

  7. Well, not everything is easy to digest, but it certainly is beneficial reading … did you see his sentence on the use of Nitrogen in the CM tank instead of CO2?

    (In his bio it says he tests the sensory ability of the MLCC Provincial wine buyers … pretty cool, I wish I had known … I was in Manitoba in August and could always use a palate check!)

    Rick

    • Rick and Albert, you two are making me sorry that I don’t have Jackson’s book! Albert, thanks for recapitulating the explanation of how fermentation occurs without juice. As you have said, the very idea behind carbonic maceration is that fermentation occurs inside the intact grape explicitly without juice. Jackson’s definition of semi-carbonic maceration as a wine made with some grapes subjected to carbonic maceration and some fermented conventionally is one of several I’ve encountered. Another definition of “semi-carbonic maceration” describes a single batch of grapes in which most berries are whole and undergoing enzymatic fermentation, but some grapes (usually at the bottom of the fermentation vessel) are crushed, forming a small amount of juice. That juice may or may not begin microbial fermentation depending on whether native yeasts begin a spontaneous fermentation, since yeast inoculation wouldn’t occur at this step.

      Albert, I’m not clear from your description what Jackson means by the desirability of about 85% CM. Does he recommend performing strict CM with most of the grapes, fermenting some must conventionally, and then blending the two, or does he recommend a single ferment in which most berries are whole but some crushed juice simultaneously begins microbial fermentation? In the latter case, this might also be described as a “non-strict CM.”

      I’ll have to dig up a copy of that book. Thanks for the reference!

  8. As far as I can understand, the section regarding the 85% CM is offered as an illustration of semi carbonic where the crop has not undergone crush and there is a percent ( whether 85% or other ) that may procede with CM concurrent with microbial fermentation ( non strict CM ). The resulting must and wine displays more of the “Beaujolais style” characteristics oposed to the varietal expression common to our expectations. There is additional explanations to this end, that goes into some science on the metabolization of malic acid ( 15 – 60% depending on grape variety )and to a lesser degree lactic and cirtic to other acids. If this is the case then combined with MLF it goes along way to explain the soft fuity easy quaffability of the resulting wine. In short exercise caution if you plan to not fully crush the crop or expect a simple, less structured result.
    Thanks Again
    PS
    Rick- is this “palate check” a scientific analysis or simply comparative/ component test? I’m always interested in anything that eliminates subjectivity in wine tasting.

  9. My Jackson book jacket cover just says: “He presently assesses the sensory skills and aptitudes of both current and prospective members of the MLCC Tasting Panel.” They buy 95% of all wine, beer & spirits for the whole of Manitoba. I have never heard of a scientist checking buyers palates/abilities before, for government employees or otherwise … usually your professonal credetials are sufficient … I assume he’s invented some scientific methods different from the common blind wine identification exams. I think most seasoned wine pros are objective but they do have different physiological sensitivities and thresholds for certain smells, tastes, flavors, etc.

    I was in Long Island this weekend … 2, days, 15 wineries, 100 wines, 1 wine (82% Merlot, 15% Cab Franc) smelled like some of it had undergone CM … so I asked – and the answer was – indeed that some fruit had undergone CM. One Semi-Carbonic wine out of 100 … which is how Jackson labels Beaujolais – Semi-Carbonic.

    At another winery, a veteran winemaker said that Whole Cluster Fermentation was a synonym for CM. And that Whole Berry Fermentation really meant that only about half the fruit was whole berries to begin with and about half the berries were crushed open so you have juice to ferment. And the whole berry fermentation had no stems on either the crushed berries or the whole berries. And that the whole berrys were technically broken open at least a tiny bit because of the hole in the skin where the stem used to be (and presumably microbiological / yeast fermentation could inluence such a grape). Those were his quick definitions of the 2 terms.

    But talk to another winemaker and who knows? For example he explained a couple of things this afternoon that were opposite of what an experienced winemaker in the morning said – one of which was about the best timing & length of ML fermentation. Another was about fining. Hey, both winemakers make great wines … at arguably the 2 best wineries on Long Island.

    Jacksons CM pages start off fairly clear before he goes off into technical detail. He says ” Fundamentally, CM differs from traditional vinification in that grape berry fermentation precedes yeast and ML fermentation. CM fermenation was initially proposed to refer to the anaerobic maceration of whole berries placed in an atmosphere of CO2. This has subsequently been extended to include grape cell alcoholic fermentation – whether or not air is initially removed from the fermentor by flushing with CO2. Typically, CM takes place in the presence of a small amount of must, released during loading of the fermentor. Thus berry fermentation typically occurs with limited yeast fermentation. In the absence of free juice, anaerobic maceration can occur.”

    Regarding the 85% bit … Jackson is quoting a Bordeaux study that stated that Cabernet character is suppressed if more than 85% of the fruit undergoes CM [and therefore full CM is not recommended there ... & the recommended amount of CM varies with the variety, intentions, etc.]

    More … “CM is even reported to enhance the varietal aroma of some white grapes (Benard et al., 1971).”

    Lastly, “In the absence of O2, grape cells change from respiratory to fermentive metabolism. This shift is more rapid if air is flushed out with CO2. CO2 is customarily preferred to Nitrogen as it has uniquely desirable properties. It directly induces leakage of ions from cells (Yurgalevitch and Janes, 1998) and shifts the equilibria of decarboxylation reactions (Isenberg, 1978.) CO2 may also accelerate the breakdown of pectins, by inducing the synthesis of grape pectinases.” I’m not sure if this relates to szymanskiea’s carboys …

    OK that last paragraph is over my head – I better sign off …

    Rick

    Rick

  10. Thanks! This has been great. I’ve learned more in the last week about CM than any other time and with the Nouveaus coming soon it will come in handy when tasting with freinds and associates. If you are ever on the canadian west coast ( Specifically Van. Isl. ) drop me a line.
    Thanks Again
    Albert

  11. Carbonic maceration is also used in the French Corbieres (quite a large area) and it produces wines very different from Beaujolais. No fruit-forward, banana-like aromas, but serious wines which could be compared to Rioja. As said in previous posts, the cm soften the tannins of the rather ferocious Carignan while keeping the delicate aromas of the other varietals. I always wonder what cm could do to other wines.

  12. Greetings all
    I am an Australian winemaker currently working in China and in vintage 2011 we are going to make a CM as a special wine for Chinese New Year in 2012.

    So you guessed it I am boning up on CM and came upon your discussion.
    The closest I have ever come to using CM is when I used to incorporate about 15% whole bunches in my Pinot Noir fermentations, looking for a bit of CM flavour to balance the hot fermentations I (read wild yeast) conducted.

    I was also aiming for just a little bit of stalky tannin.

    So I am interested in this semi/full CM tag. Seams to me that if 2% alc halts the enzymatic fermentation in the berry then ALL CM wines that are eg 12% alc have to be semi CM – the rest of the alc being contributed by the yeast fermentation.

    My query is how do you know when the enzymatic ferment is finished (Breath alc analyser?) so that you can start destemming and or pressing?

    Any comments pls.

    Ken

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  15. OK, if anyone is still reading these notes, is there a ‘simple’ explanation for the need for the CO2 environment? If all the CM is occuring inside the grape (no O2 or CO2 present) what impact is there from the external CO2? Further, if whole berries and crushed berries are fermented together, the whole berries enzymatic fermentation should proceed unimpeded regardless of CO2 or juice or whatever on the outside. (Rick Schofield touches this a bit in one of his last notes.) Thoughts?

    • The answer is in here …

      From the LinkedIn Thread at http://www.linkedin.com/groupItem?view=&gid=2695441&type=member&item=107835280&qid=4cbfd3e4-f8c2-4141-88ed-2210abf23971&trk=group_most_popular-0-b-ttl&goback=%2Egmp_2695441

      I don’t have information as to whether the CO2 stimulates fermentation, but it definitely permeates the berries and is utilised in the fermentation process

      The information I have is from Ribereau-Gayon – Handbook of Oenology Book 1 Section 12.9.2

      “the berry tissues absorb carbon dioxide. Metabolic pathways make use of this dissolved CO2. Using CO2 marked with 14C, it has been demonstrated that the gas is integrated not only into various substrates, malic acid and amino acids, but also into sugar and alcohol.”

      I think David Bird MW explains the process very well in his book, especially bearing in mind he is a chemist not a botanist.

      I may be wrong, but I thought the key was utilization of CO2 to push out O… as in you might be able to use other heavy gasses to obtain similar results… I am anxious to learn more!

      I thought CO2’s role is to allow grapes time for intracellular fermentation. I can’t see the logic of CO2 “permeating” grape skins and “stimulating” fermentation. Hoping a scientist will weigh in soon! Will check Bird shortly, Godrey. Does he address the permeate the skins issue?

      “The skins of the grapes do respire some and are permeable to gasses, but less so than other parts of the vine, especially leaves.” And he provided a link to Jamie Goode’s discussion of this topic: http://wineanorak.com/winescience/carbonic_maceration.htm

      Jamie Goode’s link looks mostly sound to me, though perhaps a bit detailed for the less scientific and there are comments that I’m not convinced by. I tend to explain carbonic maceration to students using the analogy of anaerobic respiration in people i.e. the shortcut that allows release of energy by respiration when there’s not enough oxygen, with lactic acid as a by-product ( I typically mention running for the bus, getting breathless and getting a stitch due to lactic acid build up so people recognise the concept) . Plant cells also have a “short cut” when there’s not enough oxygen for aerobic respiration and in plants it happens to be alcohol that’s the by-product. Must say I hate it when people talk about intracellular fermentation when it’s really anaerobic respiration ( I think it means people get confused between yeast cells and grape cells), and I thought the key role of CO2 was to ensure an anaerobic environment for the early part of the process to force the plant cells down this anaerobic route. So that’s the botanist’s view!

      The CO2 part of carbonic maceration is to allow the grapes to sit in an anerobic environment. This exclusion of oxygen prohibits normal fermentation. However natural pectolytic enzymes exist within the skin cells of the fruit that start to break apart the pectins in the cell walls. This releases the color from the skins as well as flavor compounds. It is not a true fermentation but an anerobic disentegration of the skins themselves. As this breakdown occurs the juice is released particuarly at the bottom of the vessel due to the weight of fruit above. This juice, once released from the broken down skins begins the primary fermentation process aided by yeast found on the skins. It is up to the winemaker to decided at what point it is appropriate to press the fermenting juice off the skins during this process. After that the juice/fermentation is treated as white wine juice would be and fermentation is allowed to continue in a vessel not in contact with skins.

      Issues: If not monitored properly the skins and stems can take on a silage like quality which is far more potent than the great fruity/bubble gum aromatics from the enzymatic reactions within the cells. It also can take a while and keeping the environment entirely oxygen free is challenging unless one is used to doing this process. I tried it about two harvests ago and while it was fun and I had great results with Valdigue it was VERY time and CO2 intensive.


      quoting from Jackson, is where things get truly interesting:

      “CARBONIC MACERATION (WHOLE GRAPE FERMENTATION): in the absence of oxygen, grape cells switch from respiratory [aerobic] to fermentative [anaerobic] metabolism. This shift is more rapid if air is flushed out with carbon dioxide. Because it is more dense than air, CO2 displaces air in the vat. CO2 is customarily preferred to nitrogen because of its uniquely desirable properties. CO2 is readily dissolved by cytoplasm, [the watery soup inside the cell walls that contains all the cell structures like the nucleus, the mytochondria, etc.] directly inducing ion leakage from cells. Furthermore, CO2 may accelerate the breakdown of pectins, by inducing the synthesis of grape pectinases.”

      In other words, not only does CO2 permeate the cell membrane, it is encouraged into the cell by the cytoplasm, since it can be dissolved by the cytoplasm so readily. And it accelerates the entire fermentation process by making more of the sugars inside the cell available to the fermentation agents by breaking down the pectin that acts as a wall holding the sugars and grape liquids apart. (That’s why you put pectinase in the must when making wine.)

      And he also states later in the para. I quoted that the anaerobic process does not produce anywhere near enough alcohol for wine, but it does encourage the aerobic fermentation at the bottom of the vat. He also implies, as Nova says, that it is up to the winemaker to decide what the “cutoff” is because he mentions that different durations of CO2 maceration have different effects on aroma, taste, ageing potential, etc. And he says that CO2 maceration is not exclusive to light, fruity wines like Beau Jo. Yet too much CO2 maceration is said to take away the “Cabernet flavor” of Bordeaux grapes.

      And Caroline’s right: it’s not fermentation, it’s anaerobic respiration.

      To answer Debra’s question, yes i believe CO2 does permeate skins, but it is not the reason that carbonic maceration creates wine.
      I agree with Caroline that carbonic maceration needs an anaerobic environment, provded by the blanket of CO2 gas.
      I’ve tasted from tank Alicante Bouschet that underwent carbonic maceration and that was awaiting pressing and the grapes themselves were effervescent, with intact skins. I can’t speak to whether the CO2 in those grapes came from the initial blanket of gas or the continued exposure due to CO2 production during the alcoholic fermentation.

      To Andrew’s point, yes: I think there is an initial reaction based on the CO2 in the tank, and follow-up reactions based on tank CO2 and the CO2 that results from the fermentation that inevitably starts from some free-run or exploded-grape juice at the bottom of the vat. Part of the art of the winemaker is then to decide how far to let the maceration go so as to produce fruitier or lighter wines, etc.


      According to “Carbonic Maceration Wines: Characteristics and Winemaking Process” – C.Tesniere and C.Flanzy

      “The marked absorption of ambient CO2 by berries (50% of their volume at 15°C) is accompanied by the incorporation of a small proportion of the CO2 by phosphoenolpyruvate (ℬ-carboxylation) and by malic acid synthesis. Malic acid is the key molecule during AM (anaerobic metabolism) since it is also catabloized to ethanol, Y-aminobutyric and succunic acids.”

      Debra, Yes the CO2 does permiate the skins but only a tiny fraction as the skins transpire before that chemical process shuts down post harvest.

      I tried to keep my earlier post a little less scientific for the layperson however I wanted to address Francois’s notes about diffusion.

      This is a correct physical process but not for this application. In this instance the area of higher concentration and density is actually within the cells themselves and the overall solution is what needs to come to equalibrium. For grapes, you start with a cell membrane with CO2 on the outside and a complex structure of liquid, sugar, acids, and phenols on the inside. The direction of the reaction will be for the more concentrated liquid side to move to the less concentrated CO2 side of the membrane (ie grape skin). This is facilitated by the breakdown of the membrane itself by pectinases. The skins will transpire a tiny bit bringing in small amounts of CO2 but the vast majority of the movement will be from the inside of the cells to the outside through the degridation of the membrane eventually leading to a burst berry and free running juice.

      During C.M. CO2 is ‘absorbed’ by the grape berries (Flanzy, 2000 and Chambroy, 1971 in Würdig and Woller, 1989). “The berry absorbs a fraction of volatile substances from the vinification tank atmosphere in which it is submersed: CO2, ethanol and other products of fermentation”

      C.M. happens due to the capacity of grape berries to evolve from respiratory metabolism to fermentative when they are put in an atmosphere enriched in CO2 and impoverished in O2.

      Rick Schofield
      Port Ewen, NY

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