In winemaking, clarification and stabilisation are critical processes that remove insoluble matter from the wine before bottling. This matter includes dead yeast cells (lees), bacteria, tartrates, proteins, pectins, tannins, grape skin, pulp, stems, and gums. These processes may involve fining, filtration, centrifugation, flotation, refrigeration, pasteurization, and barrel maturation and racking.
Clarification and Wine Clarity
A wine is considered “clear” when there are no visible particles suspended in the liquid and, particularly for white wines, when it has some degree of transparency. Cloudy wines appear dull even if their aroma and flavor are unaffected, necessitating clarification to remove suspended matter.
Before fermentation, pectin-splitting enzymes and fining agents such as bentonite may be added to the must to promote the agglomeration and settling of colloids. Pectins, which bind plant cells together, increase in grapes as they ripen. Large pectin molecules can impact juice yield, filtration ease, and tannin extraction. Since natural pectolytic enzymes in grapes are inactive under winemaking conditions, fungal pectolytic enzymes are often added to white must to break up pectins, reduce juice viscosity, and speed up settling. In red musts, these enzymes also enhance color and tannin extraction.
Natural Settling and Racking
After fermentation, gravity naturally causes wine to clarify as larger particles settle. The wine can be siphoned or “racked” off the compact solids into a new container. However, this process can take months or years and may require several rackings to produce perfectly clear wine. Producers can accelerate clarification using fining agents, filtration, and flotation.
Fining
Fining involves adding a substance (fining agent) to the wine to bind suspended particles into larger molecules that precipitate out more readily.
Unlike filtration, which only removes particulates, fining can remove soluble substances like polymerized tannins, coloring phenols, and proteins. Reducing tannins can decrease astringency in red wines.
Common fining agents include:
- Organic compounds: egg whites, casein (milk), gelatin, and isinglass (fish bladders).
- Mineral materials: bentonite clay, activated carbon, silica, and kaolin.
Some fining agents are animal-based, which can be a concern for vegans. Laws in countries like Australia and New Zealand require allergenic fining agents to be listed on wine labels. Studies have found that only trace quantities of these agents remain in the wine.
Filtration
Filtration passes wine through a filter medium that captures particles larger than the medium’s holes. Complete filtration may require progressively finer filters. Many white wines need the removal of active yeast and lactic acid bacteria for stability, often achieved through fine filtration.
Types of filtration:
- Depth filtration: Uses pads made from cellulose fibers, diatomaceous earth, or perlite, often after fermentation.
- Surface filtration: Passes wine through a thin membrane. Cross-flow filtration minimizes filter clogging. Microfiltration, which uses an absolute rated filter of 0.45 μm, can sterilize wine by trapping all yeast and bacteria, usually done before bottling.
Flotation
Flotation, adapted from mining, injects small air or nitrogen bubbles into the must. As bubbles rise, they collect grape solids, including phenolic compounds, creating a froth that can be removed. This process must be done before fermentation, as yeast inhibits flocculation.
Temperature Instability
Tartaric acid is the most prominent acid in wine, with most of it present as potassium bitartrate. During fermentation, these tartrates bind with lees, pulp debris, and precipitated tannins and pigments. While this varies by grape variety and climate, about half of the deposits are usually soluble in wine. However, exposure to low temperatures can cause these tartrates to crystallise unpredictably. Although harmless, these crystals can be mistaken for broken glass or deemed unattractive by consumers. To prevent this, wine may undergo “cold stabilisation,” where it is cooled to near its freezing point to provoke crystallisation before bottling. Additionally, certain white wines contain proteins that are “heat-unstable” and can coagulate with temperature fluctuations. Fining agents like bentonite can prevent the haze caused by this coagulation.
Microbiological Instability
Dead yeast cells can leave wine cloudy, while active yeast may trigger further fermentation. A wine not sterilised by filtration might still contain live yeast cells and bacteria. If both alcoholic and malolactic fermentation have run to completion, and neither excessive oxygen nor Brettanomyces yeast are present, this usually causes no problems. Modern hygiene has largely eliminated spoilage by bacteria such as acetobacter, which turns wine into vinegar. However, residual sugar may undergo secondary fermentation, producing dissolved carbon dioxide as a by-product. When the wine is opened, it may be spritzy or sparkling, a serious fault in still wines, potentially causing bottles to explode.
Similarly, a wine that hasn’t undergone complete malolactic fermentation may do so in the bottle, reducing acidity, generating carbon dioxide, and adding a diacetyl butterscotch aroma. Brettanomyces yeasts add horse-sweat aromas like 4-ethylphenol, 4-ethylguaiacol, and isovaleric acid. These phenomena can be prevented by sterile filtration, adding substantial quantities of sulphur dioxide and sometimes sorbic acid, mixing in alcoholic spirit to fortify the wine, or pasteurisation.
Pasteurisation
Pasteurisation produces kosher wine, called mevushal, meaning “cooked” or “boiled”. Typically, wine is heated to 85°C (185°F) for a minute, then cooled to 50°C (122°F) for up to three days to kill all yeast and bacteria. It may then be allowed to cool or be bottled hot and cooled by water sprays. Since pasteurisation affects a wine’s flavour and ageing potential, it is not used for premium wines. Flash pasteurisation is a gentler procedure involving heating to 96°C (205°F) for a few seconds, followed by rapid cooling.
Other Stabilisation Methods
Clarification tends to stabilise wine by removing particles that promote instability. The gradual oxidation occurring during barrel ageing also has a naturally stabilising effect.
