Co-Fermentation: Fermenting Different Grapes Together

What Is Co-Fermentation?

Co-fermentation is the practice of fermenting two or more grape varieties together in the same vessel at the same time. Rather than vinifying each variety separately and blending the finished wines later, co-fermentation combines the grapes -- as whole clusters, crushed fruit, or juice -- before or at the start of fermentation. The yeasts, bacteria, and enzymes then act on the mixed must simultaneously, producing interactions and flavor compounds that post-fermentation blending simply cannot replicate.

This technique predates modern winemaking by millennia. Before the era of varietal purity and controlled vinification, most vineyards were field blends -- mixed plantings of multiple grape varieties harvested and fermented together. The practice was not a matter of sophistication but of survival and pragmatism. Planting multiple varieties spread risk across different ripening windows, disease susceptibilities, and climatic events. If one variety failed, others might succeed, ensuring that something went into the fermenter every vintage.

Today, co-fermentation is experiencing a renaissance among winemakers who recognize that this ancient approach produces wines with a unique character unattainable through other means. The chemical interactions that occur when different grapes ferment together create synergistic complexity -- the whole becomes genuinely greater than the sum of its parts.

The History of Field Blends

Ancient and Medieval Vineyards

In the ancient Mediterranean, vineyards were planted with whatever cuttings were available. Greek, Roman, and early medieval vintners routinely grew multiple varieties interplanted in the same rows. Columella, the Roman agricultural writer, documented vineyards containing dozens of grape types. These vines were harvested simultaneously regardless of individual ripeness, crushed together, and fermented as a unified must.

The Rhone Valley Tradition

The northern Rhone's Cote-Rotie appellation preserves one of the most famous co-fermentation traditions. Syrah, a bold red grape, is co-fermented with up to 20% Viognier, an aromatic white variety. The Viognier is not added as finished wine after fermentation -- it is crushed and fermented alongside the Syrah from the beginning. This practice arose from interplanted vineyards and continues today because winemakers discovered that Viognier's anthocyanin-stabilizing compounds and floral aromatics integrate into Syrah in ways that post-fermentation blending cannot achieve.

The Port Tradition

Traditional Port production in Portugal's Douro Valley is perhaps the purest expression of co-fermentation still practiced at scale. Old vineyards contain dozens of indigenous varieties -- Touriga Nacional, Touriga Franca, Tinta Roriz, Tinta Barroca, Tinta Cao, and many others -- all interplanted and harvested together. The grapes go into the lagar (stone fermentation trough) as a mixed lot and are foot-trodden together. This field blend co-fermentation produces the layered complexity that defines great vintage Port.

Alsatian Edelzwicker and Austrian Gemischter Satz

In Alsace, the traditional Edelzwicker wines were co-fermented blends of Riesling, Pinot Blanc, Pinot Gris, Gewurztraminer, and other permitted varieties. Vienna's Gemischter Satz tradition similarly involves co-fermenting multiple white varieties grown together in the same vineyard. Both traditions fell out of favor during the varietal era of the late 20th century but have returned as winemakers rediscover the complexity they offer.

Co-Fermentation vs Post-Fermentation Blending

Understanding the difference between these two approaches is essential for deciding which to use.

Post-Fermentation Blending

In conventional blending, each variety is fermented separately under conditions optimized for that grape. The winemaker then tastes the individual wines and combines them in proportions that achieve the desired flavor, structure, and balance. This approach offers maximum control -- each component can be adjusted independently before blending.

Co-Fermentation Advantages

Co-fermentation sacrifices some of that control in exchange for unique benefits.

• Chemical synergy: When different grapes ferment together, their respective compounds interact during the most chemically active phase of winemaking. Anthocyanins from one variety combine with tannins from another. Aromatic precursors from one grape are liberated by enzymes produced during the fermentation of another. These interactions create novel compounds that exist only in the co-fermented wine.
• Enhanced color stability: Co-fermentation of red and white grapes (as in Cote-Rotie) produces more stable anthocyanin-tannin polymers than blending the finished wines. The white grape's copigmentation compounds integrate during formation rather than being added after the fact.
• Aromatic integration: Flavors and aromas in co-fermented wines are more seamlessly woven together than in post-fermentation blends, where individual components often remain perceptible as distinct layers. Co-fermentation produces a unified aromatic profile rather than a composite one.
• Textural cohesion: The tannin structures formed during co-fermentation tend to be more integrated and harmonious. Different tannin types polymerize together from the start, creating a smoother mouthfeel than blending wines with independently formed tannin structures.
• Complexity from unpredictability: The interactions between varieties during fermentation produce results that the winemaker could not have predicted or replicated through blending. This element of surprise often yields wines of remarkable individuality.

Co-Fermentation Limitations

• Less control over individual variety expression
• Commitment required -- you cannot separate the varieties after they are combined
• Harvest logistics can be challenging when varieties ripen at different times
• If the blend does not work, you cannot unbuild it

Grape Combinations That Work Well

Successful co-fermentation pairs varieties whose strengths complement each other's weaknesses.

Red and White Co-Fermentations

• Syrah + Viognier (3-10% Viognier): The classic pairing. Viognier lifts Syrah's aromatics with floral notes and stabilizes its color. Start with 5% Viognier for your first attempt.
• Grenache + Roussanne (5-15% Roussanne): Roussanne adds waxy texture and white pepper aromatics to Grenache's generous fruit.
• Sangiovese + Trebbiano (5-10% Trebbiano): A traditional Tuscan practice that adds freshness and aromatic complexity to Sangiovese.
• Zinfandel + Muscat (2-5% Muscat): A small proportion of Muscat enhances Zinfandel's already aromatic profile with exotic floral notes.

Red and Red Co-Fermentations

• Cabernet Sauvignon + Merlot: The foundational Bordeaux combination. Cabernet provides structure and cassis intensity while Merlot contributes plush fruit and mid-palate roundness.
• Grenache + Syrah + Mourvedre (GSM): The great southern Rhone and Australian triumvirate. Grenache provides generous fruit and alcohol, Syrah adds color, spice, and backbone, and Mourvedre contributes earthy complexity and tannic grip.
• Tempranillo + Garnacha: The backbone of Rioja, where these varieties have been co-fermented for centuries. Tempranillo offers structure and acidity while Garnacha brings warmth and fruit sweetness.
• Nebbiolo + Barbera: A traditional Piedmontese combination. Barbera's deep color and bright acidity complement Nebbiolo's pale color and firm tannin.

White and White Co-Fermentations

• Riesling + Gewurztraminer: Riesling's laser-like acidity balances Gewurztraminer's opulent body and exotic aromatics.
• Chardonnay + Viognier: Viognier adds floral lift and apricot richness to Chardonnay's structural frame.
• Semillon + Sauvignon Blanc: The classic white Bordeaux blend works even better as a co-fermentation, with Sauvignon Blanc's herbaceous aromatics weaving through Semillon's waxy richness.

Planning Your Ratios

The ratio between varieties in a co-fermentation dramatically affects the result. Unlike post-fermentation blending, where you can adjust by the percentage point, co-fermentation ratios are locked in at the crush.

The Dominant Variety

Choose one variety as the base -- this grape defines the wine's fundamental character. It should comprise 60 to 90% of the total volume. The supporting variety or varieties contribute accents and enhancements rather than competing for dominance.

Starting Conservative

For your first co-fermentation, err on the side of less supporting variety rather than more. You can always increase the proportion in future vintages, but you cannot reduce it once fermentation begins. A good starting point for a two-variety co-fermentation is 80% base variety and 20% supporting variety.

Accounting for Potency

Some varieties are aromatically potent -- a small amount has an outsized impact. Viognier, Muscat, and Gewurztraminer fall into this category. When using these as supporting varieties, start at 5% or less until you understand how forcefully they express themselves in your specific blend.

Managing Different Ripeness Levels

One of the greatest practical challenges of co-fermentation is that different varieties ripen at different times. In a commercial field blend, this is accepted as part of the character -- some grapes are slightly underripe, some are perfectly ripe, and some are slightly overripe, contributing breadth and complexity.

For the home winemaker, several strategies can address ripeness divergence.

Sequential Harvesting

Harvest each variety at its optimal ripeness and add it to the fermenter as it becomes ready. Start fermentation with the first-ripening variety, then add the later-ripening grapes when they reach maturity. This is technically a staggered co-fermentation, but the varieties still undergo the majority of fermentation together.

Compromised Harvest Date

Choose a single harvest date that represents the best compromise across all varieties. Early-ripening grapes may be slightly overripe, and late-ripening grapes may be slightly underripe. The diversity of ripeness levels can actually add complexity.

Cold Soaking the Early Variety

Harvest the early-ripening variety and hold it in a cold soak (35-45 degrees Fahrenheit) with sulfite additions to prevent fermentation. When the later variety is ready, harvest it, combine the lots, and allow fermentation to begin naturally as the must warms.

Frozen Must Addition

Harvest the early-ripening variety, crush and freeze the must in sanitized containers, and add the frozen must to the later variety at crush. The frozen must will thaw during the crush process and ferment together with the fresh grapes.

Fermentation Kinetics With Mixed Varieties

When multiple grape varieties ferment together, the fermentation dynamics differ from single-variety fermentation in several important ways.

Sugar Level Averaging

Combining grapes at different Brix levels creates a blended sugar concentration that is the weighted average of the individual contributions. A must of 80% Syrah at 25 Brix and 20% Viognier at 23 Brix will start at approximately 24.6 Brix.

pH and Acidity Interactions

Mixed musts often show non-linear pH behavior. The buffering capacity of one variety can interact with the acidity of another in ways that produce a pH different from what simple averaging would predict. Always measure the actual pH of the blended must rather than calculating it from individual measurements.

Nutrient Profiles

Different grape varieties have different nutrient profiles. A mixed must may have a more complete nutrient profile than any individual variety, potentially supporting healthier fermentation. However, if one variety is nutrient-deficient, it can drag down the overall nutrient availability. Measure YAN (Yeast Assimilable Nitrogen) on the blended must and supplement if necessary.

Yeast Selection

Choose a yeast strain that performs well across the range of conditions present in your mixed must. If the blended must has a high sugar level, select a strain with high alcohol tolerance. If one variety contributes high acidity, choose a strain that performs well at lower pH. For many co-fermentations, a robust, neutral strain like EC-1118 or a Rhone-specific strain like RP15 is a safe choice. Alternatively, consider a native yeast fermentation, which aligns philosophically with the field-blend tradition.

Temperature Considerations

Co-fermentations benefit from thoughtful temperature management, particularly when combining red and white varieties.

Red and White Co-Fermentations

When co-fermenting reds with a proportion of white grapes, ferment at red wine temperatures (75-85 degrees Fahrenheit). The white grapes will contribute their aromatic and structural elements within this range. If you ferment too cool (as you might for a pure white wine), you risk inadequate extraction from the red grape skins.

All-Red Co-Fermentations

Standard red fermentation temperatures (75-85 degrees Fahrenheit) work well. If the varieties have significantly different optimal fermentation temperatures, aim for the middle of the range. The thermal mass of a mixed must moderates temperature fluctuations effectively.

All-White Co-Fermentations

Ferment at cool to moderate white wine temperatures (55-65 degrees Fahrenheit) to preserve the aromatic contributions of each variety. If one variety is highly aromatic (Muscat, Gewurztraminer), lean toward the cooler end of the range to retain those volatile aromatics.

Famous Co-Fermented Wines

Studying wines that exemplify co-fermentation helps you understand the potential of this technique.

• E. Guigal Cote-Rotie La Mouline: Up to 11% Viognier co-fermented with Syrah. Floral, silky, and profoundly complex.
• Ridge Vineyards Geyserville: A field blend of Zinfandel, Carignane, Petite Sirah, and other varieties, co-fermented in the traditional California way.
• Chateau Musar Red (Lebanon): Cabernet Sauvignon, Cinsault, and Carignan co-fermented, producing one of the most unique and long-lived wines in the world.
• Traditional Vintage Port: Dozens of varieties co-fermented in lagares, producing the extraordinary layered complexity that defines great Port.
• Gut Oggau (Austria): Gemischter Satz wines from co-fermented field blends of indigenous Austrian varieties.
• Domaine Ganevat Vin Jaune (Jura): Traditional field-blend whites that include multiple varieties co-fermented for exceptional depth.

Step-by-Step Home Co-Fermentation Process

Step 1: Select Your Varieties and Ratios

Choose a dominant variety and one or two supporting varieties. For a first attempt, try Syrah with 5-8% Viognier or Cabernet Sauvignon with 15-20% Merlot. Calculate the weight of grapes needed for each variety based on your target volume.

Step 2: Source and Evaluate Your Grapes

Obtain the freshest, highest-quality grapes you can find. Measure the Brix, pH, and titratable acidity of each variety independently. Record these numbers -- they will help you predict the blended must's characteristics.

Step 3: Crush and Combine

For red co-fermentations, destem and crush all varieties into the same fermentation vessel. If you are co-fermenting red and white grapes, crush the white grapes and add them -- skins and all -- to the red must. For white co-fermentations, crush and press all varieties together, then combine the juice in the fermentation vessel.

Step 4: Measure the Blended Must

Test the combined must for Brix, pH, titratable acidity, and YAN. Make any necessary adjustments. Acidify if pH is above 3.6 for reds or 3.4 for whites. Add nutrients if YAN is below 200 ppm.

Step 5: Add Sulfite and Allow a Cold Soak (Optional)

Add 30-50 ppm SO2 to the blended must. For red co-fermentations, consider a 24-48 hour cold soak at 50-55 degrees Fahrenheit before inoculating. This allows color and flavor extraction from the mixed skins before fermentation begins.

Step 6: Inoculate and Ferment

Pitch your chosen yeast according to the manufacturer's instructions. Rehydrate with Go-Ferm or a similar rehydration nutrient. For red co-fermentations, punch down the cap 2-3 times daily. Monitor temperature and keep it within your target range.

Step 7: Monitor Fermentation Progress

Track Brix daily with a hydrometer. A co-fermented must may show a slightly different fermentation curve than a single-variety must because of the blended nutrient and sugar profiles. Add Fermaid-O or Fermaid-K at the one-third sugar depletion point to support healthy fermentation.

Step 8: Press

When fermentation is complete or nearly complete (below 2 Brix for reds), press the must. Co-fermented pomace may behave differently under the press than single-variety pomace -- the mixture of skin types, seed sizes, and pulp textures creates a unique pressing dynamic. Press gently and taste the free-run and press fractions separately before combining them.

Step 9: Complete Fermentation and Settle

Transfer the wine to a clean vessel, top off, and allow malolactic fermentation to proceed if desired. Rack off gross lees after 2-3 weeks.

Step 10: Age and Evaluate

Age the co-fermented wine as you would any wine of its type -- in barrel, carboy, or tank. Taste regularly and compare the result to wines you have made from the individual varieties. The differences will illustrate the unique contributions of the co-fermentation process.

Practical Tips for Success

• Keep detailed records of variety ratios, harvest dates, Brix levels, and fermentation parameters. These notes are invaluable for refining your co-fermentation program over future vintages.
• Start with proven combinations before experimenting with unconventional pairings. The classic pairings are classic for a reason.
• Be willing to accept surprises. Co-fermentation is inherently less predictable than single-variety winemaking. Embrace the unexpected -- it is often where the magic lies.
• Evaluate the co-fermented wine against the same varieties blended post-fermentation if possible. This side-by-side comparison is the most powerful way to understand what co-fermentation brings to the table.
• Scale up gradually. Start with a small batch -- 5 gallons of co-fermented wine alongside your regular single-variety production -- before committing your entire harvest to this technique.

Co-fermentation connects you to the oldest traditions in winemaking while opening a door to flavors and textures that modern single-variety vinification cannot access. It is a technique that rewards curiosity, careful planning, and a willingness to let the grapes speak as a chorus rather than as soloists.