Wine Stability: Protein, Tartrate, and Microbial Stability Tests

Why Wine Stability Matters

Wine stability refers to the ability of a finished wine to remain clear, sound, and consistent in the bottle over time. An unstable wine may develop haze, throw sediment, referment, or develop off-flavors after bottling, all of which are disappointing at best and dangerous at worst (in the case of refermentation producing enough CO₂ pressure to burst bottles).

Before bottling, winemakers should assess three main types of stability: protein stability (will proteins cause haze?), tartrate stability (will tartrate crystals form?), and microbial stability (will yeast or bacteria cause refermentation or spoilage?). Each type of instability has different causes, tests, and solutions.

Understanding these stability issues is especially important for home winemakers, who may not have the commercial equipment for sterile filtration or flash pasteurization. Fortunately, the basic stability tests are straightforward and the corrective treatments are accessible.

What Makes Wine Unstable?

Wine is a complex solution containing hundreds of dissolved compounds, many of which are in metastable equilibrium. Changes in temperature, light exposure, time, or chemical composition can shift these equilibria, causing previously dissolved substances to precipitate, aggregate, or react. The goal of stability testing is to identify these potential problems before bottling, when corrective action is still possible.

Protein Stability

The Science of Protein Haze

Protein haze is a cloudiness that develops in white and rose wines when heat-unstable proteins denature and aggregate into visible particles. The offending proteins are primarily pathogenesis-related (PR) proteins, specifically thaumatin-like proteins and chitinases, which grapes produce as part of their defense against fungal pathogens.

These proteins are soluble in wine at cellar temperature but can unfold (denature) when the wine is warmed, particularly above 25-30°C. Once denatured, they aggregate with each other and with other wine components to form a visible haze. This process is irreversible: once protein haze forms, it cannot be re-dissolved.

The risk of protein haze depends on the protein concentration in the wine (which varies with grape variety, vineyard conditions, and processing), the wine pH (proteins are less soluble at their isoelectric point), and the exposure conditions during storage and transport.

Testing for Protein Stability

The standard test for protein stability is the heat test. Place a sample of wine in a sealed container and heat it to 80°C for 2 hours (or 60°C for 4 hours). After cooling to room temperature, compare the heated sample to an unheated control. If the heated sample is hazy or cloudy while the control is clear, the wine is protein-unstable.

A quicker alternative is the bentonite test: add increasing doses of bentonite to small samples, wait 24-48 hours, and then perform the heat test on each. The lowest bentonite dose that produces a clear wine after heating is the minimum required for stability.

Achieving Protein Stability with Bentonite

Bentonite is a naturally occurring clay mineral with a strong negative charge that attracts and adsorbs positively charged proteins. When mixed into wine, bentonite particles bind to proteins, forming complexes that settle out as sediment (lees).

Typical bentonite dosages range from 0.25-1.0 g/L for most white wines, though some varieties (particularly Muscat and Sauvignon Blanc) may require higher doses. The exact dosage should always be determined by bench trials rather than guesswork.

Bentonite is not without drawbacks. It can strip desirable aromas and reduce wine body, particularly at high doses. It also generates substantial lees volume, resulting in wine loss. For these reasons, winemakers use the minimum effective dose and may combine bentonite with other strategies like cold settling, intermediate fining during fermentation, or the use of alternative protein-removal agents.

Tartrate Stability

Potassium Bitartrate Crystallization

Tartrate instability occurs when potassium bitartrate (KHC₄H₄O₆, cream of tartar) crystallizes in the bottle, forming clear, diamond-like crystals on the cork or in the sediment. While completely harmless and tasteless, these "wine diamonds" are often mistaken for glass shards by consumers and are considered a visual defect.

Crystallization occurs when the concentration of potassium and bitartrate ions exceeds their solubility product at a given temperature, alcohol level, and pH. Cold temperatures dramatically reduce solubility, which is why tartrate crystals often appear when wines are chilled in the refrigerator.

Testing for Tartrate Stability

The simplest tartrate stability test is the cold test (also called the freeze test). Chill a sample of wine to -4°C (25°F) for 48-72 hours and observe whether crystals form. If crystals appear, the wine is tartrate-unstable.

A more quantitative approach is to measure the wine's conductivity before and after chilling. Crystal formation removes ions from solution, reducing conductivity. A decrease in conductivity of more than 5% after cold treatment indicates instability.

Cold Stabilization

The most common treatment for tartrate instability is cold stabilization. The wine is chilled to near its freezing point (typically -4°C to -2°C for table wines) and held for 1-3 weeks. During this period, potassium bitartrate crystals form and settle to the bottom of the vessel. The clear, stable wine is then racked off the crystal sediment.

For home winemakers, cold stabilization can be achieved by placing the wine in a cold garage, unheated shed, or chest freezer during winter. Monitor the temperature to ensure it stays cold enough for crystallization but does not actually freeze the wine (which occurs at approximately -5°C to -6°C for most table wines, depending on alcohol content).

Cold stabilization also affects pH and TA: removing potassium bitartrate raises pH slightly and lowers TA. Re-measure both after racking off the crystal sediment and before making final SO₂ adjustments.

Alternatives to Cold Stabilization

Carboxymethyl cellulose (CMC) is a crystal inhibitor that prevents tartrate crystal formation without requiring cold treatment. It is added at rates of 1-2 mL/L just before bottling. CMC works well in white and rose wines but is not recommended for red wines because it can interact with tannins and cause a different type of instability.

Metatartaric acid is another crystal inhibitor, but it degrades over time (especially at warm storage temperatures) and provides only temporary protection (6-18 months). It is falling out of favor as CMC becomes more widely available.

Microbial Stability

The Risks of Microbial Instability

Microbial instability occurs when viable yeast or bacteria in bottled wine resume metabolic activity, causing refermentation, off-flavor development, or CO₂ production. The risks are highest in wines with residual sugar (which provides a food source for yeast), incomplete MLF (which leaves malic acid for bacteria to ferment), or insufficient SO₂ (which fails to suppress microbial activity).

The worst-case scenario is refermentation in the bottle: residual yeast consume residual sugar, producing CO₂ that builds pressure inside the sealed bottle. This can blow corks, cause excessive fizz when opened, or in extreme cases, shatter bottles.

Assessing Microbial Stability

For dry wines (RS below 2 g/L) that have completed MLF and have adequate free SO₂ for their pH, the risk of microbial instability is low. The combination of high alcohol, low pH, absence of fermentable substrate, and SO₂ protection creates an inhospitable environment for most microorganisms.

For wines with residual sugar, the risk is much higher. Stability can be assessed by warming a sample to 25-30°C for 2-4 weeks and observing for signs of fermentation (bubbles, haze, pressure buildup). If activity occurs, the wine needs stabilization before bottling.

Plating on selective media (YEPD agar for yeast, MRS agar for LAB) provides the most definitive assessment of microbial populations, but this requires microbiological equipment beyond most home setups.

Achieving Microbial Stability

For dry wines, maintaining adequate free SO₂ for the wine's pH is usually sufficient. For wines with residual sugar, additional measures are needed:

Sterile filtration through a 0.45-micron membrane removes all yeast and most bacteria. This is the gold standard for microbial stability in sweet wines.

Potassium sorbate (200 mg/L) inhibits yeast reproduction (but does not kill existing cells) and is commonly used in conjunction with SO₂ for off-dry and sweet wines. Sorbate must never be used in wines that have not completed MLF, because lactic acid bacteria can metabolize sorbate to produce geraniol (a powerful geranium-like off-aroma called "geranium taint").

Pasteurization (heating to 60-65°C for 1-2 minutes) kills all microorganisms but may affect wine quality. It is used primarily in high-volume commercial production.

Practical Stability Testing for Home Winemakers

Before bottling any wine, conduct a simple stability assessment. For white and rose wines: perform a heat test for protein stability and a cold test for tartrate stability. For all wines: confirm that MLF is either complete or has been deliberately prevented, verify that free SO₂ is at the appropriate level for your pH, and if the wine has residual sugar, ensure it has been stabilized with sorbate, sterile filtration, or both.

Keep records of your stability testing results and any treatments applied. Over time, this data helps you anticipate which of your wines need which treatments and allows you to streamline your pre-bottling process.

Frequently Asked Questions

What are the crystals in my wine bottle?

The crystals are potassium bitartrate (cream of tartar), a natural compound formed from tartaric acid and potassium in wine. They are completely harmless, odorless, and tasteless. They form when wine is chilled below the temperature at which tartrate remains dissolved. Cold stabilization before bottling prevents their formation, but even unstabilized crystals are not a wine defect, just a cosmetic issue.

How do I know if my wine is ready to bottle?

Your wine is ready to bottle when it has achieved clarity (no visible haze), stability (passes protein, tartrate, and microbial tests), correct SO₂ levels for its pH, completed any desired MLF, and tastes the way you want it. Rushing to bottle before confirming stability is the single most common cause of post-bottling problems for home winemakers.

Do red wines need protein stability testing?

Red wines rarely develop protein haze because their tannins react with and precipitate most unstable proteins during maceration and aging. The protein-tannin interaction that causes astringency also serves as a natural clarification mechanism. Protein stability testing is primarily a concern for white and rose wines that have minimal tannin content.

Can I skip cold stabilization?

You can skip cold stabilization if you accept the possibility of tartrate crystals forming in the bottle (many winemakers and consumers consider this a non-issue). Alternatively, you can use CMC (carboxymethyl cellulose) as a crystal inhibitor for white and rose wines. For red wines, tartrate crystals are less common because tannins inhibit crystal growth.

What free SO₂ level should I target before bottling?

The target depends on your wine's pH. For dry wines: pH 3.0-3.2 requires about 20-25 mg/L free SO₂; pH 3.2-3.4 requires 25-30 mg/L; pH 3.4-3.6 requires 30-40 mg/L; pH 3.6-3.8 requires 40-50 mg/L. For wines with residual sugar, add 5-10 mg/L above these levels. These targets provide approximately 0.5-0.8 mg/L of molecular SO₂, which is sufficient for microbial inhibition.