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- Ascorbic Acid
- Citric Acid
- Egg White
- Fining Agents
- Metatartaric Acid
- Milk Protein
- Pectinolytic Enzymes
- Protein Fining Agents
- Sulphur Dioxide (SO2)
- Tartaric Acid
Ascorbic acid (vitamin c,) whilst not an alternative to sulphur dioxide, is a powerful antioxidant that is often added to white wines alongside sulphur dioxide. Many wine makers consider it most effective when added just prior to bottling. Its permitted EU limit is 250 mg/l and its use invariably reduces the amount of sulphur dioxide required.
Bentonite is a clay-like substance, often called Montmorillonite clay. It is mined all over the world though principally in Wyoming, U.S.A.. As with most clays, bentonite is a complex hydrated aluminium silicate. Bentonite is highly absorbent and negatively charged. It reacts quickly with positively charged particles, adsorping them onto its large surface area. Gravity then precipitates the bentonite to the bottom of the vessel.
In everyday white wines bentonite is principally employed to remove proteins in the clarification of the must before and during fermentation. It lightens and improves their colour, removes coarse flavours, accelerates settling of the lees for more frequent racking, and prevents cloudiness. Over-fining with bentonite results in the adsorption of flavour molecules and a corresponding reduction in flavour. Therefore, with top quality white wines bentonite is used judiciously prior to bottling to prevent any deleterious effect on flavour. It is not widely used on red wines as their much higher concentration of tannins naturally deposits potentially cloud forming proteins.
Unlike most other fruits, grapes contain very little citric acid but instead possess large concentrations of tartaric and malic acid.
Citric acid may be used for acidifying inexpensive wine, but only outside of the European Union. It is more commonly employed as a cleaning agent in wineries and is sometimes added to wines in order to prevent iron casse, but only when blue fining is impossible. If added before fermentation it possesses the unfortunate potential to being converted into acetic acid by yeasts, thereby producing aromas and flavours associated with volatile acidity.
Albumen, found in egg whites, is another protein fining agent and one that is commonly used in red wine making. Albumen possesses a positively charged surface that attracts negatively charged tannins. Just a few egg whites per 225 litre barrel of red wine is usually sufficient to soften any bitter or harsh tannins.
PVPP (Polyvinyl polypyrrolidone)
This is a synthetic material that complexes with phenolic compounds in wine by adsorption, attracting those precursors that cause bitterness and browning in red and white wine.
After fining, wines must be filtered to remove the PVPP.
In wine-making fining means a process that adds a reactive or adsorptive substance in order to remove one or more undesirable components. In essence fining serves to rapidly polymerise and precipitate microscopic particles suspended in the wine. Fining agents are also used to improve a wine’s flavour, texture, colour and stability.
A number of fining agents are employed in wine-making today, with specific mineral and organic compounds being the main materials used. These compounds assist in removing those particles that might readily precipitate with time, as well as those potential unstable soluble components of the wine that could later polymerise, causing cloudiness. Most fining agents react immediately and completely settle within a week or two. The wine can then be drawn off these and other deposits or filtered if desired.
An increasing number of wine producers are moving away from fining agents that are derived from animals.
Isinglass is another positively charged proteinaceous fining agent derived from the swim bladder of a fish and most often that of a Sturgeon. It is used mainly in the reduction of harsh tannins in red wines though it is sometimes used in the clarification of sparking and still white wines that are unlikely to be filtered.
Tartrates separate from young wine because the potassium acid tartrate is less soluble in wine than in grape juice. Metatartaric acid is produced by heating tartaric acid and is used at a maximum level of 100 mg/l in finished wine. It dissolves and inhibits the formation of tartrate crystals. It is unstable however, reverting to tartaric acid within 3 – 24 months, (more rapidly with higher storage temperatures,) with the risk of further tartrate crystals appearing in the wine.
Casein, the principal milk protein, is used as a fining agent that is effective at removing browning of white wines. Casein is generally used in the form of a salt, sodium or potassium caseinate, that when added to a cloudy wine will adsorb and remove suspended particles as it settles, including those pigments causing discolouration. Potassium caseinate is water soluble and is preferred over Sodium caseinate as the latter increases the sodium content of the wine.
Enzymes are used throughout the wine making process but their main application is during the clarification musts in the pre-fermentation stages of white wine making. Pectinase is the principle enzyme employed in order to speed clarification, optimise juice extraction and generally improve the quality of post-pressing musts.
Protein Fining Agents
Protein fining agents posses a positive charge at wine pH. They have an affinity for polyphenols and react with their protein component to form larger polyphenols and polymerised anthocyanins that more readily precipitate from the wine.
Sulphur Dioxide (SO2)
Whilst unpleasant to smell or taste, SO2 is a fundamental means of controlling oxidation. Oxidation must be kept to a minimum in order to conserve a wine’s fruit character, colour and prevent the growth of undesirable compounds that proliferate in an oxidative environment. SO2 will readily react with oxygen in an aqueous environment, binding it and rendering it harmless before it can cause damage to the grape must or wine.
SO2 is also employed as an anti-microbial additive that poisons bacteria as well binding the oxygen in which they thrive. Yeasts are also deactivated by varying levels of SO2, and their actions can be controlled at various stages throughout winemaking process.
As an anti-oxidasic, SO2 poisons oxidising enzymes such as laccase. This reduces the rate of oxidation, thereby strengthening a wine’s anti-oxidant properties.
SO2 is used as a corrective after oxidation as it combines with acetaldehyde, thereby limiting the effects of oxidation and restoring a wine’s character. This is particularly valid in preventing and reversing browning in white wines and brightening the hue of reds. In addition to this, when aceltaldehyde or pyruvic acid combine with anthocyanins in red wines, pigments resistant to the colour bleaching effects of SO2 are formed.
The most important acid found in grapes and wine and critical to winemakers because of its role in the taste and mouth feel of wine. Tartaric acid is the principal acid that constitutes a wine’s buffer capacity; its ability to resist fluctuations in pH that thereby help maintain colour stability and acidity in the wine.
Tartaric acid exists in wine as an intact acid and partly as a bitartrate ion. Its potassium acid salt, (potassium acid tartrate otherwise known as cream of tartar,) is only partly soluble in wine’s alcoholic solution and can crystalise out in the wine. Most winemakers attempt to remove any tartrate crystals remaining in the wine, or prevent their coming out of solution, by employing a variety of means at their disposal including ion exchange, electrodialysis, cold stabilisation and filtration or the addition of metatartaric acid.
These microscopic single celled fungi have been used by people throughout history in baking as well as the fermentation of alcoholic beverages. They reproduce by budding and are vital to the alcoholic fermentation process in so much as when they are starved of oxygen during the fermentation process, they transform grape juice to wine. The principal energy sources for wine yeasts are sugars whilst their main by-products during the fermentation process are ethanol and carbon dioxide.
The yeast employed in wine making is Saccharomyes cerevisiae, though within this species exist several hundred strains. Indigenous strains of this yeast are ubiquitous in vineyards and wineries throughout the world though, for a variety of reasons, many wines are fermented as a result of deliberate cultured yeast inoculations.
Many other genera of yeasts, often termed wild yeasts, exist in and around vineyards and winery buildings in varying populations and are often part of the wine-making process. These include Klöckera, Candida and Pichia. They generally die-off once the wine reaches around 5% alcohol and are very much part of the ‘complexing’ process that creates diversity of flavour in wine during the opening stages of fermentation by natural yeasts. Most yeasts employed in the productions of dry wines can tolerate alcohol concentrations of up to 15% to 16% in The impact of wild yeasts is restricted, though not entirely suppressed, by inoculated yeasts in the early stages of fermentation.
Brettanomyces yeasts can contribute to off-flavours in red wines because of their ability to convert certain phenolic compounds, resulting in medicinal or even farmyard aromas in red wine.
Once the yeasts have converted all of the sugar, they gradually die off and precipitate to the bottom of the wine making vessel, forming sediment know as the gross lees. In turn, this substance plays a crucial part in the flavour and stability of wines.