Protein is a natural component of wine, originating primarily from the grapes themselves and the yeast used during fermentation. While often invisible in a finished, clear wine, these proteins play a significant role in a wine's stability, clarity, and sometimes even its sensory profile.
Primary Sources of Wine Protein
The presence of protein in wine stems from two main biological contributors:
1. Grape-Derived Proteins
Grapes naturally contain various proteins that are extracted into the juice during crushing and pressing. These include pathogenesis-related (PR) proteins, which are inherent to the grape's physiology and contribute to its defense mechanisms.
- Role in Haze: A significant portion—about half—of the total protein found in wine readily binds to natural compounds called phenols, which are also derived from grapes. This interaction between grape proteins and phenols is a primary cause of protein haze formation, making the wine appear cloudy or turbid. This haze is generally undesirable in many wines, particularly whites and rosés, as it affects consumer perception of quality and shelf stability.
2. Yeast-Derived Proteins
Yeast, predominantly Saccharomyces cerevisiae, is essential for converting grape sugars into alcohol during fermentation. Yeast also contributes proteins to wine through two main mechanisms:
- During Fermentation: Yeast cells excrete small amounts of protein into the wine during the active fermentation process.
- Post-Fermentation (Autolysis): A much larger quantity of protein is released into the wine upon completion of fermentation, primarily due to yeast autolysis. This is a process where yeast cells break down and release their intracellular components, including proteins, into the surrounding wine. This phenomenon is particularly significant in wines aged sur lie (on the lees), contributing not only proteins but also complex flavors and textures.
Impact and Management of Wine Proteins
While certain proteins can positively influence wine by enhancing mouthfeel or contributing to complexity, unstable or excessive proteins can lead to visual defects.
- Protein Haze: The most common issue caused by unstable proteins is the formation of protein haze, especially in white and rosé wines. This occurs when proteins denature (unfold) and aggregate, often triggered by changes in temperature (e.g., chilling), leading to visible cloudiness or sediment.
- Sensory Contributions: In some specific wine styles, particularly those intentionally aged on lees, yeast-derived proteins and other released compounds can enhance the wine's mouthfeel, contributing to a richer, creamier texture and adding complexity to the wine's aroma and flavor profile.
Winemakers actively manage protein levels to ensure wine clarity and long-term stability.
Common Protein Management Techniques:
- Bentonite Fining: This is the most widespread method. Bentonite, a natural clay, has a negative charge that attracts and binds to positively charged proteins. The bentonite-protein complexes then settle out of the wine and are removed through racking or filtration.
- Enzyme Treatment: Proteolytic enzymes can be used to break down proteins into smaller, more stable peptides, reducing their potential for haze formation.
- Temperature Control: Storing wine at stable, cooler temperatures helps prevent protein denaturation and aggregation, thus reducing the risk of haze.
- Early Removal of Lees: For wines where clarity is paramount and no sur lie character is desired, removing yeast lees promptly after fermentation can minimize protein contribution from autolysis.
Key Protein Sources in Wine
| Source | Primary Contribution | Impact on Wine |
|---|---|---|
| Grapes | Natural grape proteins (e.g., PR proteins) | Primary contributor to potential protein haze formation |
| Yeast (Excretion) | Small amounts of excreted proteins | Minor contribution to overall protein content |
| Yeast (Autolysis) | Large amounts from cell breakdown | Major contributor to proteins; can enhance complexity |
By understanding these diverse sources, winemakers can effectively manage protein levels, ensuring both the aesthetic appeal and long-term stability of their wines.
