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.
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.
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.