Secondary Fermentation and Racking Explained

What Is Secondary Fermentation?

Secondary fermentation is the quieter, slower phase that follows the vigorous primary fermentation. During this stage, the remaining sugar is gradually consumed, the wine clarifies as suspended particles settle, and complex flavors develop through subtle chemical changes. While primary fermentation is dramatic and fast, secondary fermentation is a patient process that transforms rough young wine into something smooth and drinkable.

The term "secondary fermentation" can be slightly misleading because the actual fermentation activity during this phase is minimal. A more accurate description might be conditioning or maturation. The wine is moved from the primary fermenter into a glass carboy or Better Bottle (a process called racking), where it sits under an airlock for weeks to months. During this time, yeast cells slowly finish processing the last traces of sugar, then die and settle to the bottom as lees.

This stage is where patience becomes your greatest asset. Rushing through secondary fermentation is one of the most common mistakes beginners make, and it leads to cloudy, yeast-heavy wines that lack polish and balance.

Why Racking Matters

Racking is the process of siphoning wine from one vessel to another, leaving behind the sediment that has collected at the bottom. The sediment from primary fermentation — called gross lees — consists of dead yeast cells, grape solids, proteins, and other particles. If wine sits on thick gross lees for too long (more than 2-3 weeks), the dead yeast cells begin to break down in a process called autolysis, which can release unpleasant sulfur compounds and bitter flavors into the wine.

By racking the wine off the gross lees and into a clean carboy, you separate the developing wine from this decomposing material. The wine continues to settle in secondary, producing a thinner layer of fine lees that is far less problematic. Some winemakers actually choose to age wine on fine lees for extended periods to add body and complexity, a technique called sur lie aging, common in Chardonnay production.

When and How to Rack

Timing your racking correctly and executing the transfer with minimal oxygen exposure are the two key skills to master during secondary fermentation.

Timing the First Rack

The first rack (from primary fermenter to secondary carboy) should happen when the specific gravity drops to approximately 1.010 or below. At this point:

The most vigorous fermentation is over
A thick layer of sediment has accumulated in the primary
The risk of autolysis increases if you wait too long
The wine is producing less CO2, meaning it has less natural protection from oxygen

For most batches, this first rack occurs 7-14 days after pitching yeast. Do not rely on time alone — always confirm with a hydrometer reading. Some fermentations finish primary in 5 days, while others take 3 weeks.

Step-by-Step Racking Technique

Sanitize everything: The receiving carboy, auto-siphon, racking cane, tubing, bung, and airlock must all be thoroughly sanitized.
Position the vessels: Place the full primary fermenter on a table or countertop at least 2-3 feet above the empty carboy on the floor. Gravity drives the siphon, so height difference matters.
Let sediment settle: If the primary has been disturbed recently, wait several hours for sediment to compact before racking.
Start the siphon: Insert the racking cane into the fermenter, keeping the tip 1-2 inches above the sediment layer. Pump the auto-siphon smoothly to start the flow. Avoid splashing in the receiving carboy — submerge the outlet end of the tubing so wine flows in below the surface.
Minimize oxygen contact: Work steadily and avoid interrupting the siphon. Every time you restart, you introduce air. Tilt the primary fermenter gently as the level drops to maximize the wine you can recover without disturbing the sediment.
Fill to the neck: The carboy should be filled to within 1-2 inches of the bung. Minimal headspace is critical because the low CO2 production during secondary offers limited protection against oxidation.
Seal and store: Insert the bung and airlock (filled halfway with sanitizer or water). Place the carboy in a cool, dark location at 60-68degF (15-20degC).

The Second Rack (and Beyond)

After 4-6 weeks in secondary, check the carboy. If more than 1/2 inch of sediment has accumulated, rack again into a clean, sanitized carboy. This second racking further improves clarity and separates the wine from fine lees before bottling.

Some wines — particularly reds intended for longer aging — may benefit from a third racking after an additional 4-8 weeks. Each racking carries a small risk of oxygen exposure, so rack only when the sediment buildup justifies it. If the sediment layer is thin, leave the wine undisturbed.

Topping Off to Minimize Headspace

After each racking, you will inevitably lose some wine to the sediment left behind. This creates headspace in the carboy — a pocket of air that promotes oxidation. Always top off the carboy after racking to eliminate headspace.

The best options for topping off, in order of preference:

The same wine from a smaller container you filled during the initial racking
A similar commercial wine (same grape variety and style)
Sanitized glass marbles added to the carboy to raise the liquid level
A neutral grape juice (only if a small amount is needed, as it may restart minor fermentation)

Many experienced winemakers plan ahead by fermenting a slightly larger volume than their carboy holds, storing the excess in a sanitized wine bottle with an airlock, specifically for topping off after each racking.

What Happens During Secondary Fermentation

While the wine sits quietly in the carboy, several important processes are occurring simultaneously, even though you may see little visible activity.

Finishing Fermentation

The remaining 1-2% of fermentable sugar is slowly consumed by the dwindling yeast population. The specific gravity will drop from around 1.010 to its final reading of 0.990-0.998 over the course of several weeks. You may see occasional bubbles in the airlock — perhaps one every few minutes — which is normal and indicates that trace fermentation continues.

Clarification and Settling

Suspended yeast cells, grape solids, proteins, and tannin particles gradually settle to the bottom under gravity. This natural clarification process turns hazy, opaque wine into clear or nearly clear wine over 4-8 weeks. Cold temperatures accelerate settling — if you can maintain the carboy at 50-55degF (10-13degC) during this phase, you will see dramatically improved clarity. This technique is called cold stabilization.

Malolactic Fermentation (Optional)

Some winemakers, particularly those making red wines or full-bodied whites like Chardonnay, introduce malolactic bacteria (such as Oenococcus oeni) during secondary. These bacteria convert sharp malic acid into softer lactic acid, reducing perceived acidity and adding a buttery, creamy quality. Malolactic fermentation (MLF) takes 4-8 weeks at 64-72degF (18-22degC) and is confirmed complete with a chromatography test kit.

MLF is entirely optional and a matter of personal taste. Most fruit wines and crisp white wines are better without it.

Flavor Development

Chemical reactions between acids, alcohols, and phenolic compounds continue throughout secondary fermentation and beyond. Esterification produces fruity and floral aromas. Polymerization of tannins in red wines softens harsh, astringent flavors into smoother, rounder ones. These changes happen slowly and are one of the primary reasons that wines improve with time in the carboy.

Troubleshooting Secondary Fermentation

Secondary fermentation is generally uneventful, but a few issues can arise that require attention.

Excessive Headspace and Oxidation

If you notice the wine developing a brownish tinge, a sherry-like or nutty smell, or a flat, lifeless taste, oxidation may be occurring. Check that the airlock is properly sealed and contains liquid. Top off the carboy immediately to reduce headspace. Adding 1/4 crushed Campden tablet per gallon provides sulfite protection against further oxidation.

Prevention is far easier than correction. Always minimize headspace, keep airlocks filled, and avoid unnecessary racking.

Wine That Refuses to Clear

If your wine remains stubbornly hazy after 8 weeks in secondary, consider these interventions:

Cold crash: Move the carboy to a location at 35-45degF (2-7degC) for 2-3 weeks. The cold accelerates particle settling.
Fining agents: Bentonite (1-2 tablespoons per 6 gallons, mixed in warm water) is effective for protein haze in white wines. Kieselsol/chitosan (two-part fining) works well for both reds and whites. Gelatin (1/2 teaspoon per 6 gallons dissolved in warm water) removes excess tannins and clarifies reds.
Pectic haze: If caused by residual pectin (common in fruit wines), add pectic enzyme and wait 1-2 weeks.

After adding any fining agent, allow the wine to settle for 2-3 weeks before racking off the precipitated material.

Unexpected Renewed Fermentation

If you notice increased airlock activity after weeks of quiet, the most likely cause is a temperature increase that reawakened dormant yeast. This is harmless and will resolve as the yeast finishes consuming any remaining trace sugars. Maintain stable temperatures to prevent this from occurring repeatedly, as each resumption of fermentation stirs up settled sediment.

A more concerning cause is contamination by wild yeast or bacteria. If the renewed activity is accompanied by off-smells (vinegar, nail polish, or barnyard), inspect the surface for a film or white spots. If contamination is confirmed, rack immediately, add 1 Campden tablet per gallon, and consider that the batch may be compromised.

How Long Should Wine Stay in Secondary?

The duration of secondary fermentation depends on the wine type, your patience, and your quality goals.

Minimum and Recommended Times

Wine Type	Minimum Secondary	Recommended Secondary
Light white wines	4 weeks	6-8 weeks
Full-bodied whites	6 weeks	8-12 weeks
Light red wines	6 weeks	8-12 weeks
Full-bodied reds	8 weeks	3-6 months
Fruit wines	4 weeks	6-8 weeks

These are guidelines, not rigid rules. The wine is ready to move toward bottling when it is clear, stable (consistent SG readings), and tastes good to you. Some complex red wines benefit from 6-12 months in a carboy before bottling, developing remarkable depth over time.

Signs That Secondary Is Complete

The specific gravity reads 0.998 or below and has not changed across two readings taken 3 days apart
The wine is visibly clear — you can read print through the carboy
The airlock shows no regular bubble activity
The wine tastes clean, balanced, and free of off-flavors

Once these conditions are met, your wine is ready for stabilization and eventual bottling.

Frequently Asked Questions

Can I skip secondary fermentation and bottle directly from primary?

This is strongly discouraged. Wine bottled directly from primary will contain excessive sediment, unresolved yeast, and potentially residual CO2 that could push corks out. The wine will also lack the clarity and flavor refinement that secondary conditioning provides. At minimum, allow 4 weeks in secondary before bottling.

How many times should I rack my wine?

Most wines require 2-3 rackings total. The first rack is from primary to secondary. The second is from secondary to a clean carboy after 4-6 weeks if significant sediment has accumulated. A third racking before bottling removes any remaining fine sediment. Rack only when needed — each transfer introduces a small amount of oxygen.

Is it normal to see no bubbles in the airlock during secondary?

Yes. During secondary, fermentation is very slow and may produce only occasional bubbles. Seeing one bubble every few minutes, or even no visible bubbles, is perfectly normal. The lack of bubbling does not mean something is wrong. Use hydrometer readings to confirm fermentation progress rather than relying on airlock activity.

What do I do if my carboy is not completely full after racking?

Top off the carboy to minimize headspace. Use the same wine if you set some aside, a similar commercial wine, or sanitized glass marbles to raise the level. In a pinch, a small amount of grape juice can work but may trigger minor refermentation. Never leave more than 1-2 inches of headspace, as oxygen exposure during secondary leads to oxidation.

Can I age wine in a plastic carboy for secondary?

Better Bottles (PET plastic carboys) are acceptable for secondary fermentation lasting up to 3-4 months. For longer aging, glass carboys are preferred because glass is completely impermeable to oxygen. Standard food-grade plastic buckets are not suitable for secondary because they allow too much oxygen transfer through the walls.

Should I add sulfite during secondary fermentation?

Add 1/4 crushed Campden tablet per gallon (or 1/4 teaspoon potassium metabisulfite per 6 gallons) at each racking to maintain sulfite protection against oxidation and microbial growth. This small addition will not affect the residual yeast that is finishing fermentation but provides an important safety net for wine quality.

How do I know if my wine has gone bad during secondary?

Warning signs include a vinegar smell (acetic acid contamination), white film or spots on the wine surface (surface yeast or mold), nail polish or solvent smells (ethyl acetate from bacterial contamination), or a mousy or barnyard flavor (Brettanomyces). Slight off-aromas that develop during secondary sometimes resolve with time and sulfite additions, but persistent or worsening problems usually indicate a contaminated batch.