The Science of Grape Co-Fermentation

Researchers and winemakers weigh in on how the centuries-old practice of fermenting white and red grapes together affects a wine’s color, flavor, and texture

Close up of grapes of different varieties
Despite its longtime use in some of the world’s best-known wine regions, co-fermentation of red and white grapes is not widely practiced. Photo courtesy of Fossil and Fawn.

In a wine industry driven by an increasing fixation on single-origin and single-variety expressions, some long-standing winemaking practices would be implausible if introduced today, such as Porto’s unspecified field blends or Champagne’s intra-regional cuvées. The co-fermentation of white and red grapes is similarly unusual by today’s standards (although co-fermenting varieties of the same color is a more common practice). It has comparable historical value to the production methods of Porto or Champagne, yet perhaps lesser renown.

Much of today’s red wine is made exclusively with red-skinned grapes, but co-fermenting varieties of different colors is still allowed by a number of European denominations, and is occasionally employed in the New World, too. It is widely practiced across the Rhône Valley, including in Côte-Rôtie, where up to 20 percent Viognier can be added to Syrah, and Châteauneuf-du-Pape. In Burgundy, a portion of white grapes can still be used in red wines of any quality level, though this doesn’t happen very frequently. In Rioja’s Basque enclave of Alavesa, winemakers still regularly use Viura or Malvasia to round off their younger reds. Portugal even has a distinct term for this style of wines—palhete—whose history can be traced back hundreds of years, as 18th-century wine merchant John Croft documents in his Treatise on the Wines of Portugal.

According to Gonzalo Sáenz de Samaniego, the owner and winemaker of Rioja Alavesa’s Ostatu winery, the practice emerged in response to practical—rather than enological—needs. “Our ancestors were harvesting a single plot in one go, which often involved grapes of different colors,” he says. “That was the reason for co-fermenting then. Nowadays … a small contribution of white grapes gives red wines some freshness, tannic stability, and elegance.”

Pierre Fabre, the manager of Château Mont Redon, still uses Clairette, Bourboulenc, Picpoul, and a little Grenache Blanc as part of his Châteauneuf-du-Pape field blend for no other reason than upholding tradition. “Historically, most vineyards here had both white and red grapes, though whites might be just three percent. Maybe people thought it would help cross-fertilization?” he speculates.

Despite several academic studies on the subject, the effects of co-fermenting different grape varieties is still debated. Some researchers—and winemakers—find that co-fermentations result in noticeable differences in the final wine, while others are less convinced. SevenFifty Daily spoke with several experts to gain a better understanding of the science behind this ancient winemaking technique.

The Question of Color

Fabre dismisses the prevailing myth that white grapes aid in fixing a red wine’s color, a notion that finds support in recent academic research. “It has been anecdotally said that white grapes were added to stabilize color … We never found that,” says Federico Casassa, an associate professor of enology, wine, and viticulture at California Polytechnic State University. “Quite the opposite, that is, dilution of color.”

Casassa’s research involved studying Viogner and Syrah co-ferments in varying proportions. He found that additions of up to five percent Viognier showed no perceivable color differences compared to a pure Syrah wine; 10 percent and 20 percent Viognier lowered most of the chromatic parameters, while “20 percent Viognier wines had lower concentrations of anthocyanins and flavonols, suggesting possible dilution of these phenolics,” writes Casassa. “These results suggest that additions of Viognier at the rates studied here neither improve the phenolic composition nor enhance the color stability of the final wines.”

A study published in the Australian Journal of Wine and Grape Research on the co-fermentation of Tempranillo with Viura found similar results. It also highlighted that Viura additions do not significantly change any of the wine’s aroma or flavor attributes, but, when made via carbonic maceration, the addition of Viura may lead to increased perceivable acidity.

Some studies, however, found diverging results. A study from 2013 demonstrated increased color in one-year-old Chianti made with Sangiovese, Malvasia, and Trebbiano. A 2014 paper published in the Journal of Agricultural and Food Chemistry confirmed that larger additions of Pedro Ximénez pomace (20 percent) lead to color dilution, but argues that smaller ratios (10 percent) can benefit the phenolic potential of warm-climate reds in their youth.

Grapes being collected in a vineyard.
There are many prevailing theories about the impact of co-fermentation on a wine’s color. Photo courtesy of Rioja Alavesa Ostatu Winery.

Meanwhile, an experiment with Chambourcin, the lower-tannin, red hybrid variety, found that the addition of 20 percent Viognier diluted the wine’s color, but it benefited from significantly increased concentrations of phenolics such as catechin (seed tannin) and total tannins.

A 2018 Canada-based research paper published in Food Chemistry noted that dilution can actually be beneficial when working with hybrid varieties, particularly in regions that are not subject to strict regulations on blending. According to the study, “Red hybrid cultivars such as Frontenac carrying high anthocyanin content are known to produce dark coloured reds as well as highly coloured rosés that are unsuited for customer preferences. Addition of [white pomace] proves to be an effective tool to modulate the phenolic, volatile, and color profile of wine, which may result in a positive impact on wine mouthfeel, aroma, and appearance.”

The Question of Flavor

According to Fabre, picking times are the main challenge when it comes to co-fermentations. To ensure simultaneous fermentation, all grapes must be harvested together, which occasionally requires picking white grapes slightly overripe. “We have late-ripening whites [in order] to have everything ripe roughly at the same time,” says Fabre. “But last year ripening was [out of sync], so for the very first time we didn’t use the whites. I didn’t see much of an effect on the wine. It’s such a small amount of white grapes, no more than five percent. If you would co-ferment more white grapes, those would of course lower the alcohol but also dilute the structure, the tannin, and fruit concentration.”

Nicole Rolet, the CEO of Provence winery Chêne Bleu, argues that even small additions of white grapes have an effect on a red wine’s structure and flavors. “Our Syrah is very similar to that of the northern Rhône. Nice aromas, good tannin and aging potential, but also a bit austere,” she says, hinting at her Syrah, Grenache and Roussanne blend, Héloïse. According to Rolet, co-fermenting Roussanne lends the wine a certain fleshiness and pleasant aromatics. 

Casassa’s research partly corroborates Rolet’s claims: “We found that the dilution effect occurs across the board with additions of [white grapes],” says Casassa. “But this is a dilution effect on color, not on flavor. Flavor is sometimes lifted by these additions, depending upon the variety. Viognier seems to work best, and I suspect this is because of the addition of Viognier’s terpenes to the red ferments.”

Following years of experiments, Rolet found that blending white and red wines after fermentation yielded significantly different results as the white variety’s aromas were “getting lost in the blend”—a claim that this time clashes with Casassa’s research. “We never found that co-fermentation actually results in better liquids than simply blending the finished wines,” says Casassa. “This applies both to red-red or red-white co-ferments … chemically and sensorially, the difference between co-fermenting and blending seems to be relatively minor.”

In this instance, Casassa’s research focused on Syrah blends with white Rhône varieties, including Viognier, Marsanne, Roussanne, Picpoul, and Grenache Blanc. The study reports that, of all white varieties investigated, Viognier does appear to provide more aromatics to the wine. This outcome, however, is experienced both by co-fermenting the grapes as well as by blending post-malolactic conversion. In fact, the paper notes that “co-fermentation had no effect on wine composition … [while] post-[malolactic] blending gave increased aromatics and color, while offering greater flexibility and fewer logistical issues than co-fermentation.”

Old Tradition, New Wines

Beyond the confines of appellation rules, unconventional winemakers are experimenting with co-fermentation’s potential to craft vibrant and captivatingly aromatic red wines that show unique and unusual flavor combinations and textures, and whose style transcends conventional red and white categories. New Zealand’s The Hermit Ram, for instance, regularly produces multi-skin-color field blends, its latest featuring Riesling, Chardonnay, Pinot Noir, Cabernet Sauvignon, and Gewürztraminer from “one of North Canterbury’s earliest planted vineyards.”

Jenny Mosbacher at Fossil and Fawn adds pied de cuve to a Pinot Noir and Chardonnay co-fermentation
Jenny Mosbacher at Fossil and Fawn adds pied de cuve to a Pinot Noir and Chardonnay co-fermentation. Photo courtesy of Fossil and Fawn.

Meanwhile, Oregon’s Fossil & Fawn applies the carbonic maceration process to an ever-changing mix of white and red varieties to craft its Do Nothing cuvée. “In some vintages, it is a dark rosé in appearance, and in other years looks like a typical red wine … For the sake of brevity, we call it a red wine, but in truth, it’s kind of its own thing,” says co-owner Jim Fischer. “Thankfully, it seems like a lot of consumers don’t really get too hung up on needing to categorize things. And, for that matter, we’ve also stopped worrying about it. Our wine usually ends up in the light/chillable red section of menus and bottle shops if that category exists there.”

Co-fermentation of white and red grapes was once a customary technique across Italy’s wider Chianti area. While the practice is still permitted in the Chianti DOCG, it was prohibited in 2006 from the regulations governing the more prestigious Chianti Classico DOCG. Some local winegrowers are eager to revive this historical practice, and are prepared to declassify their wine to Toscana IGT in order to do so (in much the same way Tuscan winemakers did when white and red co-ferments were the DOCG norm and Super Tuscan blends were the IGT outliers).

L’erta di Radda owner and winemaker Diego Finocchi uses the traditional local recipe of Sangiovese, Canaiolo, Trebbiano, and Malvasia on his Due & Due Toscana IGT. “Blending red and white grapes together to make a unique wine is something that has always fascinated me,” he says. “I use white grapes to slim the wine down, soften the Sangiovese’s tannins, while at the same time creating a more complex aromatic profile.”

Despite the contradictory nature of the research, winemakers like Finocchi continue to experiment with co-fermentation. They are defying traditional classifications based on grape skin color in order to explore the value of this somewhat misperceived and neglected historical practice, and the uncharted textures and flavor profiles it has the potential to generate.


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Dr. Jacopo Mazzeo is a U.K.-based freelance drinks journalist, consultant, and photographer. He contributes to leading trade and consumer publications including Decanter, Wine Enthusiast, Whisky Magazine, and Good Beer Hunting. Jacopo consults on consumer trends and marketing strategies, is a former sommelier, and judges international wine, beer, and spirits competitions. Before he embraced full-time journalism, he studied musicology at the University of Bologna and took a PhD at the University of Southampton. Follow Jacopo on Instagram @jacopomazzeophoto

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