Getting Ready for Harvest – Part 4: Color Stability of Red Wines
By: Denise M. Gardner
The “Getting Ready for Harvest” seminar featuring José Santos from Enartis Vinquiry (California) included a detailed discussion about the technical aspects of several key processes that are currently of importance to Pennsylvania winemaking. Some of these topics included:
- White wine oxidation protection
- Yeast nutrition and its importance during fermentation
- Reducing “green” aromas and flavors in red wines
- Color stability in red wines
In this four part series, I will summarize some of the discussions led by José during the 2014 “Getting Ready for Harvest” workshop.
Stabilization of Color in Red Wines
The color of red wines exists in three forms:
- Free Anthocynanins
- Co-pigmented Anthocyanins
- Condensed Anthocyanins
Figure 1 describes these chemical differences and their influence on color stability. Free anthocyanins are most susceptible to color loss. This form is chemically unstable and sensitive to oxygen and sulfur dioxide. Co-pigmented anthocyanins are semi-stable forms of color pigments, which are anthocynanins that are weakly bound to tannins or polysaccharides (long-chain sugars). They are partially bleachable by sulfur dioxide. Condensed anthocyanins consist of covalently bonded anthocyanins to tannins, which elicits a strong bond. These forms of anthocyanins are color stable and are not bleachable by sulfur dioxide.
There are two time points within fermentation where a winemaker can focus on stabilizing red color:
- Until 2-4% alcohol is generated by primary fermentation
- In between primary fermentation and malolactic fermentation
The aqueous (i.e. water-based) environment prior to fermentation allows for tannin and anthocyanin extraction. Both components are needed to stabilize free anthocyanins. However, it is important to remember that more extraction does not guarantee better color stability. Tannin binding is non-specific, which means that tannins can bond to many different components found in wine including proteins or polysaccharides. All of these components are extracted during maceration. Once bound, the tannins are unavailable for anthocyanin bonding.
In this situation, the addition of sacrificial tannins may be required. Here the winemaker is making tannin additions prior to primary fermentation. Some of these added tannins will bind to other components within the fermenting must and precipitate out of solution during primary fermentation. As binding is nonspecific, some may also contribute to anthocyanin bonding interactions.
The last opportunity to stabilize color in red wines is in between primary fermentation and malolactic fermentation. At this stage, acetaldehyde generated during primary fermentation is the driving force to bind tannins and free anthocyanins. Oxygen is also needed to facilitate this reaction.
Therefore, winemakers are encouraged to use micro-oxygenation, or some incorporation of oxygen if micro-ox is unavailable, prior to the addition of malolactic bacteria to help facilitate condensation of anthocyanins. Additionally, winemakers should consider adding tannins, which increases the tannin concentration that could bind to anthocyanins. Regardless, winemakers should be incorporating oxygen to facilitate the anthocyanin-tannin bond.
Practicality in Production
The addition of tannins into the juice or must may seem counter-intuitive in cool climate regions that generate high-tannic red grapes on a regular basis. However, the basic building blocks to form condensed anthocyanins are needed to better stabilize red wine color, and increasing the starting pool of tannins allows for better bonding potential. Winemakers should acknowledge that not all tannins are equal, and some forms are better suited for specific functions. It is best if the winemaker contact their tannin supplier to determine which tannins are recommended for color stability purposes.
Color stabilization between primary and malolactic fermentation is important because after this period of time, sulfur dioxide is added to the wine. After malolactic fermentation has completed, most winemakers stabilize the microbial component of wine with sulfur dioxide. Sulfur dioxide binds readily with acetaldehyde, and once bound, the acetaldehyde cannot be used to fuel color stabilization reactions. Color stabilization reactions between tannins and anthocyanins occur at a much slower rate than sulfur dioxide binding with acetaldehyde.
Acetaldehyde is used as the connecting bridge between tannins and anthocyanins, which is why it is needed. Again, having more available tannins, which can be attributed by wine tannin additions, provides more potential building blocks for free anthocyanins to bind to. Using some sort of processing aid to integrate oxygen into the wine between primary and malolactic fermentation will help facilitate the binding reaction between tannins and anthocynains. Typically, oxygen addition is recommended using micro-oxygenation technology in which controlled rates of oxygen can be added to the wine. For those wineries that cannot afford a micro-ox investment, they can incorporate oxygen with pump-overs or splashing.