Inquiring Winemaker


The Skinny on Skin Contact for Whites

October 2013
by Tim Patterson
Some aspects of commercial winemaking, as Donald Rumsfeld might observe, are known-unknowns. We know, for example, that a lot of wineries use Velcorin to sterilize their wines, but we have no idea how many or how often, since neither winemakers nor the supplier will talk about it in public.

Then there are the unknown-unknowns, including practices that are out of official favor but still in some use anyway. It can be easy to assume that these practices fall into the “nobody does that anymore” category, yet they persist and even grow. There are, for example, far more “wild” fermentations conducted every harvest than you might imagine, and even though such uncontrolled microbial orgies are considered dangerous to your wine, most of them succeed anyway. However many that may be.

In that same Rumsfeldian space is the officially unfashionable technique of intentional skin contact for white grapes and juice. It’s high on the “not the best practices” list, but a lot of people do it anyway. It doesn’t show up on the label, it shows up in the wine instead; when it works, it means intensified aromatics and fuller body, and possibly more aging potential. 

    1. Available online here.

    2. Ronald S. Jackson, Wine Science, Academic Press, 2008, 336-7.

Now you might ask, if this is an unknown-unknown, how do I know about it? Not from talking to winemakers, though I did some of that, but from talking to enzyme suppliers and looking through their catalogs. Turns out they do a brisk business in products explicitly packaged for use with white wine skin contact, which suggests somebody out there must be soaking their exocarps.

Fear of phenolics
Modern white winemaking protocols just say no to skin contact. Skin shunning is a core element of the reigning textbook approach, along with commercial yeast inoculation, cool fermentation temperatures, reductive strategies, squeaky-clean sanitation, heat and cold stabilization and sterile filtration. While that package may sound somewhere between conservative and paranoid, it has, to be fair, resulted in a massive upgrade in the quality of the world’s white wines. As recently as a couple decades ago, plenty of white wines were flat, oxidized, murky, funky or some combination of the above. Today even low-end box wines are fresh, fruity, sparkling clear and fault-free. Clearly, the modern approach works.

Inevitably, successful innovations in winemaking go from experimental to trendy to dominant to something more like dogmatic. Somewhere along this continuum is the advice offered in this excerpt from the lecture notes to Enology 124, Introduction to Wine Production, at the University of California, Davis:

“The goals for white wines differ from red wine production in several respects. Generally little to no skin contact is desired. This is because the principal flavor and aroma compounds are located in the pulp of the grape with the skin providing little other than bitterness and astringency. Many white wine styles are designed to be consumed relatively young (less than five years of age), which is insufficient time to allow polymerization and softening of the phenolic content. In addition to bitterness, phenolic compounds lead to off-color production under oxidizing conditions. This color change is generally undesirable in white wines…

“Skin contact refers to the length of time the juice is left in contact with the skins and seeds. The longer the time of contact, the greater the extraction of the components of the skins into the juice. In contrast to red wine production, the majority of the important sensory components of white grapes are in the pulp, not in the skins. Since the microbial flora of the grapes is located on the skins, skin contact also increases the contact of these organisms with the juice. If the skins are separated from the juice quickly, the microbes also are separated, minimizing their numbers in the primary fermentation.”1

I don’t mean to pick on Davis here; I’m sure the lecture notes at Montpelier or the guidelines at the AWRI say pretty much the same thing. But this advice, however well intentioned, seems a bit one-sided on at least two counts. First, it suggests that all you get from white skin contact is phenolics, though in fact the skins of at least some varieties are home to a wide range of aromatic goodies and goodie precursors. And second, it suggests that all the phenolics can add is bitterness and astringency, when in fact they can also add positively to mouthfeel and perceived body. Surely there can be excessive phenolic extraction with white grapes; but then, that can happen with reds, too, and that doesn’t stop us, does it?

So why the ban on showing some skin? In Wine Science, Canadian researcher/writer Ronald Jackson provides some intriguing historical context for the emergence of grape skin phobia as the introduction to a section about white grape maceration.

“The shift to light, fruity white wines in the 1970s resulted in minimizing skin contact. This trend was encouraged by the widespread adoption of mechanical harvesting. However, depending on the tendency of the grapes to rupture, some inevitable maceration occurred on the way to the winery—its extent depending on the duration separating harvest and crushing/pressing and the temperature of the grapes. Reduced maceration also diminished the uptake of heat-unstable proteins, decreasing the need for protein stabilization products.

“Unfortunately, minimizing or eliminating maceration simultaneously reduces the uptake of varietal flavorants located in the skins, such as S-cysteine conjugates in Sauvignon Blanc. For wines dependent on aromatics extracted from the grapes, this became increasingly important with the adoption of gentler pressing, such as provided by pneumatic presses or whole-grape pressing. To offset this deficiency, use of the first and second press-run fractions increased. This option is often easier to manipulate than maceration, due to the complexities of temperature and duration on extraction, precipitation and degeneration of compounds during macera tion. Nevertheless, the addition of press fractions augments the wine’s phenolic content.”2

Jackson nicely captures what got lost in the transition to hyper-gentle, fruit-fixated white winemaking, and he is on the mark about how pressing techniques help to make up for what maceration doesn’t extract. Along the way he also touches on the Catch-22 of rampant skin avoidance: It’s impossible. Machine harvesting guarantees some level of juice ooze on the way to the press; hand-harvested grapes piled atop each other in bins do the same, and even whole-cluster-pressed grapes mix skins and sauce during the press cycle. (Notice the color in cluster-pressed pink wines.)

In fact, Wines & Vines hereby offers a cash prize (amount to be determined) to any winemaker who can extract the juice from a ton of grapes without incurring any skin contact. In other words, the right question about skin contact is not whether, but how much, and under what conditions and toward what stylistic end?

Extraction roulette
Jackson’s next sentence is, “Like most choices in winemaking, each decision has its pros and cons.” True enough: Intentional white skin maceration yields a more complex bundle of the good, the bad and the hard-to-tell than the simple strategy of avoidance. Chances are any skin soak will mine some combination of the following:

Aromatic compounds: Fully formed aromatic compounds are scarce in grapes, with most of them requiring fermentation to develop. But the skins of certain grape varieties can be home to substantial concentrations of monoterpenes (linalool, geraniol) and grassy methoxypyrazines, things you might want (or not want) in your wines.

Aromatic precursors: This is the bigger treasure trove for skin contacters. These include carotenoids (which can morph into rosy damascenone), cysteine conjugates (which lead to the thiols in Sauvignon Blanc) and glycoconjugates (sugar-bound potential volatiles). Just to keep things interesting, many of these compounds take their time in maturing, potentially creating an aromatic “bloom” well after fermentation is finished or even later in the bottle. 

Phenolics: Here we get from white skins flavonoid-type phenols, catechins and the like. Unlike the case of red skins, anthocyanins are not to be found, except in small concentrations in pinkish/gris grape varieties. (Color modification in skin-contact whites comes from carotenoids and other non-anthocyanin compounds.) Catechin monomers can produce bitterness, the factor that gives skin contact its bad reputation. More important, the absence of anthocyanins changes the normal rules for how tannins polymerize, how those polymers react with oxygen, how they precipitate and how astringent the tannins are—much less so in whites than reds.

White skins also contain some amount of hydroxycinnamic esters (coutaric, caftaric and fertaric acids), which function as precursors for later volatile phenols. Elevated phenolic content may also help with the ageability of white wines made with skin contact.

Nitrogen: Skins are the biggest repositories of nitrogen in grapes, and by extracting amino acids, skin contact increases the pool of available nutrients during yeast fermentation.

Skins are rich in this substance as well, and skin extraction can raise pH and lower acidity—a good reason not to try skin contact on already high-pH juice.

Lipids: These fatty acids can be friend or foe, depending on their concentration and composition. The good news is that they become less prominent and less problematic as grapes approach full maturity.

Microbes: As the Davis discussion notes, skins bring all manner of bugs into the mix—and not just earwigs or fruit flies. Since the general practice with skin contact is to postpone sulfur dioxide additions until after pressing—the presence of SO2 increases phenolic extraction—feral critters that come in on the skins will have a few more hours to play. Winemakers have to judge whether that’s a plus, potentially adding a touch of complexity, or a minus, potentially adding a whiff of yuck.

How much of what comes out depends, of course, on grape variety, composition and health. Skin contact is generally practiced on already aromatic varieties like Muscat, Riesling, Gewürztraminer and Sauvignon Blanc, since those are the grapes that have noseworthy stuff to extract. For more neutral varieties, the value isn’t so clear. Veteran winemaker John Buechsenstein remembers a fad for skin contact with Chardonnay in the 1980s, an approach that yielded no aromatic boost but did extract enough phenolic material to require fining for drinkability.

Fruit also needs to be fully ripe, but not overripe. For underripe fruit beset with green flavors, the addition of bitter phenols is no help. For overripe fruit, already headed for high pH and high alcohol, reverse osmosis may be more appropriate than skin contact.

Once skin contact is under way, the drivers of extraction are time and temperature. Contact time varies from a few hours to overnight to maybe 24 hours, with most practitioners on the low side. Enzyme-aided skin contact times are even shorter, since extraction rates are higher. Temperatures are generally held quite cool, well under 60ºF, which slows down the pace of extraction and keeps spontaneous fermentation activity at a minimum.

Most of the major additive suppliers offer an extractive enzyme specifically recommended for use with skin contact regimens. The universe of such products is all derived one way or another from the versatile fungus Aspergillus niger, and all of them do a mix of things besides pulling goodies out of the skins, including increasing juice volume and speeding clarification. In a sense, they do what natural grape enzymes do, only more so, which can mean more lovely aromatics, more troublesome phenolics, or both.

All of this is a different path from the road to so-called orange wines (some of which are not orange), which hold onto their skins all the way through fermentation. This is, for the record, how white wines were made for thousands of years. Orange winemaking offers a comprehensive alternative to the conventional white wine wisdom; the skin contact techniques discussed here are simply a variation on the modern theme, but one that can have definite impact on flavor and texture. Think of it as a Pinot Noir cold soak, only in this case we can actually measure what it accomplishes.

Worth a trial
Winemakers are all over the map regarding the value of skin contact. Skin contact is common for the aromatic whites of Alsace, and it’s one reason why those heady wines are so heady and why many of them deliver a trace of bitterness on the finish, sometimes masked by residual sugar. It’s common as well in the trendier precincts of Bordeaux for both Sauvignon Blanc and Sémillon. Jackson notes that the categorization of German whites according to their harvest ripeness also happens to correlate nicely with their phenolic content.

In the Finger Lakes, Morten Hallgren at Ravines Wine Cellars includes a portion of skin-contacted fruit in most of his white wines, the percentage is higher in warmer/riper years and lower in cooler ones. His favorite method is to leave crushed grapes in the press, with the outlet valves turned off, for eight to 12 hours. Santa Barbara Viognier queen Morgan Clendenen, on the other hand, gets rid of the skins ASAP, since Viognier can so easily get out of balance and become overbearing. At the Scholium Project, winemaker Abe Schoener does full-on fermentations with the skins on some whites, sometimes producing wines that are undrinkably coarse when young but come around nicely after four years or so in barrel.

Talking with Shirley Molinari of Lallemand and Peter Salamone of Laffort, skin contact is alive and well out there in winemaking land. And not just in the more experimental small wineries but now and then with the big boys, too. Salamone describes the practice as “not mainstream, but known and increasing.”

One more thing that makes white skin contact an unknown-unknown is that there’s not much research done about it. Ever since the method earned the status of “nobody does that any more,” it gets less attention, and a lot of puzzles are left unsolved. Key among the gaps is a full understanding of how white skin phenolics behave and transmogrify in the absence of anthocyanins, particularly over time.

By the time you read this, white grapes may have been harvested in your area, but there is plenty of time to rig up a trial on some promising batch of grapes next year. 

Tim Patterson is the author of “Home Winemaking for Dummies.” He writes about wine and makes his own in Berkeley, Calif. Years of experience as a journalist, combined with a contrarian streak, make him interested in getting to the bottom of wine stories, casting a critical eye on conventional wisdom in the process.

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