Getting Ready for Harvest – Part 2: Juice Oxidation
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.
White Wine Oxidation Protection
There are several ways that winemakers can improve upon current white wine processing operations. In the past three years, I’ve heard several key discussions on white juice clarity, prevalence of astringent tannins, loss of aromas/flavors essential to the variety, and early onset (i.e. within 1 year of harvest) of white wine oxidation. While mistakes in the winery can happen, there are various schools of thought that directly address white winemaking and ways to improve upon it. This summary will discuss the use of antioxidants during pre-fermentation juice settling, and their potential influence on wine quality post-fermentation.
One key topic that José addressed during his seminar was the importance of antioxidant protection in white wine juice. “Oxidation” and “reduction” are general chemical terms used to describe chemical processes that transfer electrons; oxidation involves the loss of electrons within a chemical species, while reduction results in a gain of electrons for a chemical species. Typically, many oxidative processes involve the use of oxygen. Antioxidants, therefore, are chemical species that prevent or inhibit oxidation from occurring. Generally speaking, wine contains many compounds that can be oxidized, or reduced, although not necessarily all at the same time or within the same chemical pathway.
Currently, most U.S. winemakers add sulfur dioxide (SO2) in the juice phase during settling (i.e. addition of a pectinase enzyme) to inhibit the oxidative browning reaction facilitated by the enzyme polyphenol oxidase (PPO). Although this is one way winemakers can address white wine oxidation pre-fermentation, many winemaking regions outside of the U.S. have other solutions.
Grape and juice oxidation begins the moment the grape is broken. For those that utilize mechanical harvesting, oxidation management in wine grapes should be a consideration. Oxidation can affect many different chemical aspects of juice and wine, including wine color, aroma and flavor development, and tannic stability. In the worst case scenario grape and juice oxidation can lead to a lack of varietal aroma and flavor, potential development of off-flavors, and a loss of color (for reds) or browning of color (for whites and reds). Botrytis, a fungal disease that is prevalent in tight-clustered varieties like Pinot Grigio or Pinot Noir and may be more common in wet or humid years, produces laccase, an enzyme that contributes to juice oxidation more efficiently than polyphenol oxidase (PPO). Additionally, laccase is heat tolerant, which is not the case for PPO, and may continue oxidative processes post-fermentation.
Figure 1 shows the effects of various antioxidants when used in combination with juice settling enzymes. The control (“without antiox.”) has the darkest associated color. The sample treated with SO2, however, lacks the amber color associated with PPO activity. Other samples treated with ascorbic acid or glutathione showed a reduction in the amber color, but not comparable to the SO2 treatment. However, the treatment with AST addition, a combination of ascorbic acid, SO2, and tannin was capable of inhibiting oxidation to the same degree of SO2 alone.
I know what you’re thinking: What’s the point of using a different product if SO2 gets the job done?
Well, first, I should note that allowing the juice to brown prior to fermentation may not be a bad thing for some varieties. Non-aromatic varieties, such as Chardonnay, Pinot Grigio, or Cayuga, may benefit from the browning of juice. This oxidative process will also contribute to oxidizing some phenolics that may otherwise be retained in the wine, and contribute to post-fermentation astringency, which may be undesirable for whites wines.
However, ascorbic acid converts oxygen to hydrogen peroxide 1700x faster than SO2 alone. For all those wine chemists out there, this sounds like a nightmare because hydrogen peroxide is a very reactive species that does contribute to oxidation. However, when used in combination with SO2, ascorbic acid can be a powerful tool to utilize the dissolved oxygen available in the juice, and inhibit aromatic oxidation.
Tannins also offer some antioxidant properties. They can bind to oxygen directly, block enzymatic oxidases (prevent them from working), or inhibit free radicals that contribute to many oxidative processes.
The combination of ascorbic acid, SO2, and tannin acts as an aggressive antioxidant treatment for white juice of aromatic (i.e. Riesling, Vidal Blanc, Gewürztraminer, etc.) varieties, in which the preservation of those aromatics is essential for wine quality. In this case, the lack of oxidation is also preserving the aromatic compounds from their own oxidative processes. Enartis’s product, AST (see above), offers this solution.
For those aromatic varieties that may develop a latent astringency, José recommended additionally fining the juice with PVPP or a combination of PVPP and potassium caseinate for high-phenolic varieties. This treatment should help remove some of those astringent phenolics that are typically more perceptible about 6 months post-fermentation.
It is essential to remember, however, that winemakers should not add ascorbic acid alone. Due to the fact it does quickly convert oxygen to hydrogen peroxide, generation of hydrogen peroxide alone will constitute more oxidation problems than no antioxidant addition. The SO2 is needed to quickly scavenge the hydrogen peroxide developed by ascorbic acid.
Practicality in Production
For those using mechanical harvesting the use of antioxidants during harvest time may be a consideration for preservation of aromatics in addition to managing other oxidative processes. Harvesting should take place during cooler hours of the day in order to slow degradative processes and help protect aroma and flavor compounds that have developed in grape berries.
Additionally, for those that are receiving grapes or juice from long distances, oxidation should be a concern. While SO2 provides antioxidant capabilities in the juice, it does not come without consequences. If grape growers or juice processors are shipping product with a high SO2 content, it may inhibit yeast growth for fermentation. Furthermore, depending on the variety, some winemakers may prefer no SO2 addition at this stage. It is essential that growers, processors, shippers, and winemakers work together to determine that the quality of the product upon receiving is well understood.
It is not uncommon to add SO2 to the crusher/destemmer during processing. Nevertheless, ascorbic acid – SO2 or products like AST, offer alternative solutions for winemakers that are looking to provide antioxidants to white juice.