Introduction
Wine, a beloved beverage enjoyed worldwide, is more than just fermented grape juice; it’s a complex mixture of various components that contribute to its flavor, aroma, and overall appeal. Among these components, organic acids play a pivotal role in defining wine’s sensory characteristics. Tartaric acid, the most abundant acid in wine, is crucial in maintaining acidity, enhancing flavor, and protecting wine color. However, excessive tartaric acid can form insoluble salts, leading to the precipitation of crystals known as tartrates. These crystals, although harmless, can detract from the wine’s visual appeal and influence consumer perception. Therefore, stabilizing tartaric acid to prevent tartrate formation is essential for maintaining wine quality.
The Role of Tartaric Acid in Wine
Tartaric acid is a naturally occurring acid in grapes, present in the highest concentration compared to other fruits. It not only imparts a desirable sourness that balances wine’s sweetness but also plays critical roles in lowering pH, inhibiting microbial growth, and preserving wine freshness during storage. The acid’s stability during fermentation means it largely remains unaltered, thus directly influencing the final wine product.
However, tartaric acid has a tendency to bind with potassium (K+) and calcium (Ca2+) ions, forming potassium hydrogen tartrate (KHT) and calcium tartrate (CaT), respectively. These compounds can precipitate, creating unsightly crystals that may settle at the bottom of wine bottles or adhere to corks. Although these crystals do not affect the wine’s taste, their presence can be off-putting to consumers, making tartaric stabilization a critical step in the winemaking process.
Fig. 1. Three optical isomers of tartaric acid.
Methods for Stabilizing Tartaric Acid
Several methods are employed to stabilize tartaric acid in wine, each with distinct advantages and potential impacts on wine quality. These methods can be broadly categorized into two strategies: the “subtraction” strategy, which involves removing the components that form tartrates, and the “addition” strategy, which involves adding stabilizing agents to inhibit tartrate formation.
1. Subtraction Strategy
This approach focuses on reducing the concentration of ions that contribute to tartrate formation, primarily through cold stabilization, ion exchange, and electrodialysis.
- Cold Stabilization: This is the most traditional and widely used method, where wine is cooled just above its freezing point to induce the crystallization of KHT, which is then removed by filtration. Although effective, cold stabilization can be energy-intensive and time-consuming, and it mainly targets potassium tartrate, often leaving calcium tartrate untreated.
- Ion Exchange Treatment: This technique removes K+ and Ca2+ ions by passing the wine through ion exchange resins. Cation exchange resins are particularly effective, as they replace metal ions in the wine with hydrogen ions, thereby reducing the likelihood of tartrate formation. While this method is efficient and has a low energy requirement, it can alter the wine’s pH and potentially affect its overall balance.
- Electrodialysis: Electrodialysis uses ion exchange membranes to selectively remove ions under an electric field. This process efficiently removes both potassium and calcium ions, stabilizing wine without significantly affecting its flavor. However, the initial cost of equipment and the relatively low throughput make it more suitable for larger wineries.
Fig. 2. Biosynthesis pathways of tartaric acid in grapes. GME, GDP-D-mannose-3′, 5′-epimerase; GGP, GDP-L-galactose phosphorylase; GPP, L-galactose-1-phosphate phosphatase; GalDH, L-galactose dehydrogenase; GalLDH, L-galactono-1,4-lactone dehydrogenase; 2-KGR, 2-keto-L-gulonic acid reductase; L-IdnDH, L-idonate dehydrogenase; TK, Transketolase; TSAD, Tartaric semialdehyde dehydrogenase; GalUR, D-galacturonate reductase; Alase, Aldolactonase. The dotted line indicates the unconfirmed route.
2. Addition Strategy
The addition strategy involves introducing stabilizers that bind with tartrate ions, thereby preventing crystal growth and precipitation. The main stabilizers used in this approach include gum arabic, metatartaric acid, carboxymethyl cellulose (CMC), mannoprotein, and potassium polyaspartate.
- Gum Arabic: Derived from the sap of Acacia trees, gum arabic stabilizes wine by forming a protective layer around tartrate crystals, preventing them from growing. It’s often added just before bottling but is not recommended for wines intended for long-term aging as it can emulsify natural precipitates.
- Metatartaric Acid: This polyester of tartaric acid is effective at preventing both potassium and calcium tartrate formation. However, it has limited durability; high temperatures or prolonged storage can hydrolyze it back into tartaric acid, reducing its effectiveness over time.
- Carboxymethyl Cellulose (CMC): CMC works by binding with potassium ions, thus inhibiting the formation of KHT crystals. It is stable over time and provides a lasting stabilization effect, making it suitable for white and sparkling wines. However, it is not recommended for red wines due to potential interactions with pigments that can affect color.
- Mannoprotein: A natural stabilizer extracted from yeast cell walls, mannoprotein adsorbs onto crystal surfaces, preventing further growth. It also enhances the organoleptic properties of wine, improving mouthfeel and perceived aroma.
- Potassium Polyaspartate: This relatively new stabilizer is highly effective at preventing tartrate precipitation and can be used in both red and white wines. It maintains stability even at higher temperatures, making it particularly useful for wines stored in warmer climates.
Factors Influencing Tartaric Stability
Several factors influence the formation of tartrates in wine, including pH, temperature, and the concentration of K+, Ca2+, and tartaric acid. A higher pH increases the concentration of ionic tartrates, promoting crystal growth, while lower temperatures reduce tartrate solubility, enhancing crystallization. Additionally, the presence of other colloidal substances such as proteins, tannins, and saccharides can impact tartrate stability by either promoting or inhibiting crystal formation.
Impact of Stabilization Methods on Wine Quality
While tartaric stabilization is essential for preventing unsightly crystals, it can also affect other sensory attributes of wine, such as aroma, taste, color, and turbidity.
- Aroma and Flavor: Stabilization methods can alter the wine’s aroma profile. For example, mannoproteins can enhance fruity and floral notes, whereas ion exchange might affect the wine’s pH, subtly altering the perception of acidity and aroma intensity.
- Color: Stabilization treatments, particularly those involving cold stabilization and CMC, can impact the color of red wines by interacting with anthocyanins and other phenolic compounds. It’s crucial to balance the degree of stabilization to minimize negative effects on wine appearance.
- Turbidity: Stabilizers like potassium polyaspartate and gum arabic can affect the clarity of wine. Proper dosing is essential to avoid excessive turbidity, which could otherwise diminish the wine’s visual appeal.
Fig. 3. Schematic diagram of the combination of protein, tannin and tartar crystal (Lambri et al., 2014).
Future Directions in Tartaric Acid Stabilization
Innovations in stabilization techniques continue to evolve, with new methods focusing on energy efficiency and minimizing sensory impact. One promising approach is the combination of cold treatment with stabilizers like mannoprotein, which has been shown to reduce the amount of stabilizer needed and shorten stabilization time. Additionally, plasma surface modification technology, which applies nanoscale coatings to remove KHT crystals, offers a novel, energy-saving alternative with minimal impact on wine quality.
Conclusion
Tartaric acid stabilization is a critical aspect of winemaking, essential for maintaining wine’s aesthetic and sensory qualities. Understanding the different stabilization methods and their impacts on wine can help winemakers choose the best approach for their products. As the industry continues to innovate, new technologies and combined strategies promise to offer more efficient and less intrusive ways to keep wines stable, clear, and visually appealing.
Read more about it at: https://doi.org/10.1016/j.fochx.2024.101728