By Tom Mansell, Science Editor

Have you ever smelled grape juice? Not Concord or Niagara juice but grape must that's about to become wine? It doesn't smell like much. It's kind of like fresh mown grass. How does this relatively un-aromatic juice evolve into complex, aromatic wine? 

Most everybody's grapes are in (except for the ice wine grapes) and it's now time for winemakers to figure out how best to turn their grapes into quality wine. Surprisingly, magic is not involved. 

Now in the spirit of Harvest Winespeak last month, In this edition of Winespeak, we'll explore some terms and techniques that winemakers use before and during fermentation, giving us an inside look at the winemaking process.

Barrel-fermented:  Somewhat self-explanatory, fermentation occurs in oak barrels. Not so obvious: Barrel-fermented wines will actually have less oak character than wines fermented in stainless steel and transferred to barrels for the same amount of time. The reducing environment of fermentation converts aromatic oak aldehydes into relatively neutral alcohols, diminishing their overall impact.

Cap management: Red wines are fermented on the skins, since the anthocyanin (color) and tannin compounds are almost exclusively located in the skins. Therefore, good contact with the skins is essential for extraction of color and tannin. The problem is that carbon dioxide produced during fermentation makes the skins float, creating a "cap" of skins and keeping most of the skin out of contact with the fermenting juice.  Winemakers have come up with many ways to ensure good skin contact, most of which involve submerging the cap, and some cap management techniques are listed below

Carbonic maceration: It's an appropriate day to talk about this, since today is the official launch of Beaujolais Nouveau worldwide. Essentially, 'carbonic' involves sealing uncrushed grapes in a tank with carbon dioxide. This leads to anaerobic metabolic processes within the grapes themselves (no yeast involved yet), creating all kinds of interesting chemical compounds including benzaldehyde (almond/cherry), isoamyl acetate (banana) and a little bit of alcohol.  During carbonic, the cell walls of the grapes break down, releasing anthocyanins (color) and amino acids. The latter helps to kickstart alcoholic fermentation by yeast, leading to wine that can be quickly produced and shipped out just in time for Thanksgiving. (further reading: Ronald S. Jackson, Wine Science, p. 529-534).

Chaptalization: Illegal in some places, done in many places. It’s a fancy way of saying “adding sugar” to a must before fermentation with the intention of increasing potential alcohol.
Named after Jean-Antoine Chaptal, who discovered the relationship
between sugar and alcohol in wine. Not to be confused with
back-sweetening, which is adding sugar for sweetness.

Délestage: Often called "rack and return", this cap management technique involves draining a tank, then splashing the wine back onto the cap, breaking it up and redistributing the skins. This action also aerates the fermenting wine and can help blow off things like hydrogen sulfide (see YAN, below). Délestage can also involve the removal of seeds. 

Aside: It has traditionally been believed that seed tannins are much harsher and more astringent than skin tannins, so winemakers have encouraged skin contact over seed contact (using délestage, for example).  Anecdotally, this seems to make sense, since chewing on a skin is much nicer than chewing on a seed, even when the seed is crunchy and brown. However, new research from wine scientists concentrating on phenolic compounds seems to challenge this belief. They argue that the gallic acid found on seed tannins, which contributes to their harshness, is easily cleaved off during winemaking, making seed tannins less astringent than skin tannin (further reading: Tim Patterson, Wines and Vines, Oct. 2009). Tannin chemistry is very complex and more research is needed to make proper conclusions about the relative importance of seeds and skins in winemaking.

Esters: ESTER is not just a crossword clue, but a name for a series of chemical compounds, many of which are aromatic. In the case of wine, acetate and ethyl esters (descriptors of which include cherry, pineapple, pear, banana, etc., basically the whole fruit salad) are produced by yeast. It's thought that ester production is a reaction of yeast to toxic medium-chain fatty acids found in the grapes. Whatever the reason, if you smell fruit in your wine, there's a good chance you're smelling esters and you have the yeast to thank for that. Different strains of yeast can contribute different ester profiles (further reading: Soles et al., "Ester Concentration Differences in Wine
Fermented by Various Species and Strains of Yeast", AJEV, 1982) and companies like Lallemand and ScottLabs use this as a selling point for different commercial strains. Fermentation temperature can also influence ester production and cooler fermentation temperatures can lead to fruitier wines, and whites are generally fermented a little cooler for this reason.

Extraction: As I said above, extraction of color and tannin is important in red wine. Anthocyanins are more water-soluble, so much of the color of a wine is extracted in the first few days of fermentation. Tannin extraction occurs in the mid-to-later stages of fermentation, aided by the alcohol created in fermentation.  Some extended macerations can last 30-90 days. Here it's really a winemaking judgment call.

Malolactic fermentation:  A secondary fermentation in which lactic acid bacteria convert malic acid to lactic acid, softening the acidity of a wine. They also produce diacetyl, a buttery-smelling aroma compound. For full details, see my recent piece in Palate Press.

Pectinase: An enzyme that helps break up the cell walls of grapes, aiding in extraction. It can be purchased commercially and added to must or to fermenting wine. Pectinase also helps settle out particulate matter in the must, aiding in clarification (further reading here).

Potential alcohol: Fermentation converts sugar to alcohol. I think we all know that one, but how many degrees Brix correspond to %ABV?  Generally, the rule of thumb is about 0.55, so a juice harvested at 20 Brix would produce 11% ABV. It's highly dependent on yeast strain and fermentation conditions. Final alcohol can be adjusted by chaptalization.

Pumpover: Another cap management technique. Juice is pumped from the bottom of the tank to the top of the tank, mixing everything around. Lacks the aeration and seed removal components of délestage. Pumping of any kind can crack seeds, releasing seed tannins into wine.

Punchdown: Probably the most romantic cap management technique (after pigeage by actually climbing in the tank). Usually, it involves standing over the tank and manually using some kind of tool to break up the cap.  (Being careful not to be overcome by CO2 vapors and falling in. It's a serious problem.) If you volunteer or work at a winery around this time of year, be prepared to do some manual labor in the form of punchdowns.

YAN (Yeast Available Nitrogen): A huge component of fermentation that's often overlooked. Yeast need nitrogen to create biomass, and that nitrogen is usually in the form of amino acids. If the nitrogen levels in the must are low, yeast start to get stressed out. They will break down their own amino acids, like cysteine and methionine, to make more. When this happens, they release the gas hydrogen sulfide (H2S). You have smelled this one before, it smells like rotten eggs. Smelling it during fermentation means the yeast are not happy. Sometimes it makes its way into the wine itself, leading to the need to aerate some wines in order to perceive more fruit aroma.

Fermentations can be supplemented with diammonium phosphate (DAP) to bring YAN up, but DAP is widely regarded as yeast "junk food", leading to yeast favoring biomass production over alcohol production, and possibly exacerbating the H2S problem (further reading: Bell and Henschke "Implications of nitrogen nutrition for grapes, fermentation and wine", Aus. J. Wine Grape Res., 2008).  Better control of YAN is done in the vineyard.  Many wineries do not measure YAN themselves (the test requires some expensive equipment and nasty chemicals).