How Fermentation Works
Fermentation is a biological process: yeast cells consume sugar (glucose and fructose) and produce ethanol (alcohol) and carbon dioxide (CO2). The equation is simple:
Sugar + Yeast → Alcohol + CO2 + Heat
One gram of sugar produces roughly 0.5 grams of alcohol and 0.5 grams of CO2. The heat generated is significant — a vigorously fermenting batch can raise its own temperature by 10-15°F (5-8°C), which is why temperature control matters.
Before You Start: Must Preparation
- Crush the grapes. For reds, crush and destem into your primary fermenter. Skins, juice, and pulp all go in. For whites, crush and press immediately — you ferment the juice only.
- Measure sugar (specific gravity). Take a hydrometer reading. Target SG of 1.085-1.095 for ~12-13% alcohol. If sugar is low, you can add cane sugar (chaptalization) — but only to reach your target, not beyond.
- Test and adjust acidity. Target pH of 3.4-3.6 for reds, 3.1-3.4 for whites. If pH is too high (not acidic enough), add tartaric acid. If too low (too acidic), add calcium carbonate sparingly.
- Add sulfite. Add 50 ppm potassium metabisulfite (1 Campden tablet per gallon) to suppress wild yeast and bacteria. Wait 12-24 hours before adding your cultured yeast.
- Add pectic enzyme (optional). Breaks down fruit pectin for better juice extraction and clearer wine. Add at the same time as sulfite.
Yeast Selection
The yeast you choose influences flavor, aroma, alcohol tolerance, and fermentation speed. Don't use bread yeast — it produces off-flavors and dies at low alcohol levels.
| Yeast Strain | Type | Alcohol Tolerance | Best For | Character |
|---|---|---|---|---|
| Lalvin EC-1118 | All-purpose | 18% | Any wine, restarts stuck fermentation | Clean, neutral, reliable. The workhorse yeast. |
| Lalvin RC-212 | Red | 14% | Pinot Noir, Burgundy styles | Enhances fruit, color, and mouthfeel. |
| Lalvin BM 4x4 | Red | 16% | Bold reds (Cab Sauv, Syrah) | Intense color extraction, full body. |
| Lalvin D-47 | White | 14% | Chardonnay, rosé | Enhances mouthfeel. Needs cool temps (<68°F). |
| Lalvin QA23 | White | 16% | Sauvignon Blanc, Riesling, aromatics | Preserves tropical and citrus aromas. |
| Red Star Premier Cuvée | All-purpose | 18% | Any wine, high-sugar musts | Very strong fermenter, clean profile. |
| Lalvin 71B | White/rosé | 14% | Semi-sweet wines, Nouveau style | Metabolizes malic acid, softening young wines. |
💡 When in Doubt: EC-1118
Lalvin EC-1118 (Champagne yeast) is the most reliable yeast in home winemaking. It ferments cleanly across a wide temperature range, tolerates high alcohol, and virtually never gets stuck. It won't add the character that specialty strains do, but it will finish the job. For your first few batches, EC-1118 is a safe default.
How to Rehydrate Yeast
- Heat water to 104°F (40°C). Use 50ml of clean, chlorine-free water per 5 grams of yeast.
- Sprinkle yeast on the water surface. Do not stir. Let it sit for 15 minutes.
- Stir gently, then let it sit another 5 minutes. The yeast should be creamy and slightly foamy.
- Temper. Add a small amount of must to the yeast solution to bring it within 10°F of the must temperature. Wait 5 minutes.
- Pitch. Pour the yeast into the must and stir gently to distribute.
Primary Fermentation
Primary fermentation is the vigorous, active phase. It happens in an open or loosely covered fermenter (a bucket with a lid set on top, not sealed) and lasts 5-10 days for reds, 10-14 days for whites.
Red Wine Primary
| Day | What Happens | What You Do |
|---|---|---|
| Day 1 | Yeast pitched. Quiet. | Cover loosely. Monitor temperature. |
| Day 2-3 | Bubbling begins. Cap forms on surface. | Punch down the cap 2x daily. Measure SG. |
| Day 4-6 | Vigorous fermentation. Heavy CO2. Cap rises quickly after punchdown. | Punch down 2-3x daily. Monitor temp — peak heat occurs now. SG dropping rapidly. |
| Day 7-10 | Fermentation slowing. SG approaching 1.010-1.000. | Decide when to press based on taste and SG target. Press and transfer to carboy. |
Punching Down the Cap
During red wine fermentation, grape skins float to the surface and form a dense layer called the "cap." Punching down means pushing this cap back into the liquid using a sanitized punch-down tool, large spoon, or clean hands.
Why it matters:
- Extracts color, tannin, and flavor from the skins
- Prevents the cap from drying out and developing acetobacter (vinegar bacteria)
- Distributes heat evenly through the must
- Keeps yeast in contact with sugar
White Wine Primary
White wine ferments as juice only (no skins). After crushing, press the grapes immediately and transfer the juice to a carboy with an airlock. Fermentation is slower and cooler — 55-65°F (13-18°C) to preserve delicate aromas.
Temperature Control
Temperature is the single most controllable factor affecting wine quality during fermentation.
| Wine Type | Ideal Range | Too Cold (<) | Too Hot (>) |
|---|---|---|---|
| Red wine | 70-85°F (21-29°C) | 65°F — sluggish, poor extraction | 90°F — yeast stress, harsh flavors, potential death |
| White wine | 55-65°F (13-18°C) | 50°F — very slow or stuck | 70°F — loss of delicate aromatics |
| Rosé | 58-68°F (14-20°C) | 55°F — sluggish | 72°F — loss of freshness |
Cooling a Hot Fermentation
- Wet t-shirt method: Wrap a damp towel around the fermenter and point a fan at it. Evaporative cooling drops temperature 5-10°F.
- Ice bottles: Freeze sanitized water bottles and float them in the must. Replace as needed.
- Move to a cooler location: Basement, garage (in cool weather), or air-conditioned room.
- Smaller batches: Smaller volumes have more surface area relative to volume and dissipate heat faster.
Pressing
For red wine, pressing separates the liquid wine from the solid grape skins, seeds, and pulp after primary fermentation. Timing depends on your style goals:
| Press When | SG Range | Result |
|---|---|---|
| Early (short maceration) | 1.020-1.010 | Lighter color, softer tannins, fruitier. Good for Pinot Noir, lighter styles. |
| At dryness | 1.000-0.998 | Standard extraction. Good balance of color, tannin, and fruit. Most common approach. |
| Extended maceration | Below 0.998, wait additional days | Maximum extraction. Big, tannic, age-worthy wines. Risk of bitterness if overdone. |
⚠️ Free-Run vs. Press Wine
The wine that drains freely from the skins without pressure (free-run) is generally smoother and finer. The wine squeezed from the skins under pressure (press wine) is more tannic and coarse. For your first batch, combine them. As you gain experience, you can keep them separate and blend to taste — or use press wine to add structure to a thin batch.
Secondary Fermentation
After pressing (reds) or when primary fermentation slows (whites), transfer the wine to a carboy and fit an airlock. Secondary fermentation is the slow, quiet phase where:
- Remaining sugar is consumed (SG drops to 0.995-0.998)
- Yeast and solids settle to the bottom (lees)
- The wine begins to clarify
- Harsh flavors start to soften
- CO2 slowly escapes through the airlock
Racking
Racking is siphoning wine off the sediment (lees) into a clean container. This removes dead yeast and grape solids that can produce off-flavors if left in contact too long.
| Racking | When | Purpose |
|---|---|---|
| First rack | 2-3 weeks after pressing | Remove heavy gross lees. Add sulfite (25 ppm). Top up carboy. |
| Second rack | 6-8 weeks later | Remove fine lees. Check clarity. Add sulfite if needed. |
| Third rack (optional) | 2-3 months later | Final clarification before bottling. Only if sediment is still forming. |
🍇 Minimize Headspace
Every time you rack, you lose a little volume to the lees left behind. The carboy must stay full — air space above the wine (headspace) exposes it to oxygen, causing oxidation and eventual spoilage. Top up with a similar wine (buy a bottle of the same variety) or use sanitized glass marbles to raise the level. Never leave more than an inch of headspace in a carboy.
Malolactic Fermentation (MLF)
MLF is a secondary bacterial process (not yeast) where Oenococcus oeni bacteria convert sharp malic acid (think green apple) into softer lactic acid (think milk). It happens naturally in most red wines and is deliberately encouraged.
Should You Do MLF?
| Wine Type | MLF Recommended? | Why |
|---|---|---|
| Full-bodied reds | Yes — almost always | Softens acidity, adds complexity, increases stability. |
| Light reds (Pinot Noir) | Usually yes | Rounds out the wine. Some winemakers skip it for a brighter style. |
| Oaked Chardonnay | Yes | Creates the classic buttery, creamy character. |
| Crisp whites (Sauv Blanc, Riesling) | No | You want the sharp acidity. MLF would make them flabby and bland. |
| Rosé | Usually no | Preserve freshness and acidity. Rosé should be bright. |
How to Initiate MLF
- Inoculate with a commercial MLF culture (like CH16 or VP41) after primary fermentation is complete or nearly complete.
- Keep the wine warm — 65-75°F (18-24°C). MLF bacteria are slow or inactive below 60°F.
- Do not add sulfite until MLF is complete. SO2 inhibits the bacteria.
- Test for completion using chromatography paper (available from homebrew shops). MLF is complete when malic acid is no longer detected.
- After MLF completes, add sulfite (25-30 ppm) to stabilize the wine and prevent further bacterial activity.
Fermentation Timeline Summary
| Phase | Duration | Key Actions |
|---|---|---|
| Must preparation | Day 0 | Crush, measure, sulfite, add enzyme |
| Yeast pitch | Day 1 | Rehydrate and add yeast |
| Primary fermentation | Days 2-10 | Punch down (reds), monitor temp & SG daily |
| Press & transfer | Day 7-14 | Press skins (reds), rack to carboy, fit airlock |
| Secondary fermentation | Weeks 2-6 | Monitor airlock, let wine settle |
| First racking | Week 3-4 | Siphon off lees, add sulfite, top up |
| MLF (if desired) | Weeks 4-12 | Inoculate, keep warm, test for completion |
| Aging & racking | Months 2-12 | Rack every 2-3 months, maintain sulfite levels |
| Bottling | Month 3-12+ | When clear, stable, and tasting good |