Enzymes Key to White Wine Longevity

The role of the lipoxygenase pathway in shaping the aromatic profile of white wines during pre-fermentation is a crucial but often overlooked element of modern winemaking. This intricate process was the focus of a detailed presentation by Franco Battistutta from the University of Udine and María Manara, from the R&D department of DAL CIN Gildo spa, during the Enoforum conference. Their lecture, titled "Longevity of White Wines: Pursuing Quality from Vineyard to Glass," shed light on how biochemical reactions in grapes play a pivotal role in determining the sensory qualities and oxidative stability of white wines.

One of the key points in their discussion was the lipoxygenase pathway, which becomes active when the cellular structure of the grapes is disrupted. This enzyme-mediated process leads to the release of six-carbon aldehydes such as hexenal and trans-2-hexenal. These aldehydes are responsible for the characteristic green, vegetal notes found in certain white wines.

In the early stages of grape processing, particularly during crushing and pressing, two major biochemical pathways come into play: the polyphenol oxidase pathway and the lipoxygenase pathway. These pathways influence both the stability and aromatic profile of the wine, and winemakers must carefully manage these processes to achieve the desired result.

The Role of Polyphenol Oxidases

Polyphenol oxidases (PPO) are enzymes that catalyze oxidation reactions, often leading to the browning of the must, a phenomenon winemakers aim to avoid in white wine production. Enzymes such as polyphenol oxidase and tyrosinase oxidize cinnamic-tartaric acids, producing quinones that trigger further oxidative reactions. These reactions not only deplete key antioxidants like ascorbic acid and glutathione but also target catechins, compounds that play a significant role in the oxidative stability of white wines.

The depletion of these antioxidants weakens the wine's resistance to oxidation, ultimately affecting both its shelf life and its aromatic quality. Managing the activity of polyphenol oxidase is critical in maintaining the freshness and color stability that consumers expect from white wines.

From Vegetal to Fruity

While polyphenol oxidases are associated with undesirable outcomes, the lipoxygenase pathway is a double-edged sword. Early in the maturation process, lipoxygenases generate aldehydes responsible for green aromas, like hexenal. However, as grapes ripen, the activity of these enzymes diminishes, reducing the production of aldehydes. This natural decline in lipoxygenase activity aligns with a transition in the wine's aromatic profile, moving from vegetal to more desirable fruity notes.

Interestingly, new research presented by Battistutta and Manara has revealed that trans-2-hexenal can undergo further reactions with glutathione, forming precursors to compounds like 3-mercaptohexanol. This transformation introduces attractive fruity aromas, enhancing the complexity of white wines. This discovery has led winemakers to rethink their strategies, emphasizing the importance of timing and control in managing enzymatic activity during pre-fermentation.

Hyperoxygenation vs. Reductive Winemaking

The presentation also explored two primary strategies for managing oxidation in white winemaking: hyperoxygenation and reductive winemaking. These two techniques offer contrasting approaches to oxygen exposure, each with its advantages and trade-offs.

In hyperoxygenation, the must is deliberately exposed to oxygen. This approach can stabilize the wine by eliminating catechins that contribute to oxidative instability. By promoting early oxidation, winemakers can produce wines that are more stable over time, though some aromatic compounds may be lost in the process. The goal here is to create a more robust wine that is less susceptible to oxidation in the bottle, at the cost of sacrificing some of the delicate aromatics that are preserved in a reductive approach.

On the other hand, reductive winemaking aims to minimize oxygen exposure throughout the production process. By protecting the must and wine from oxygen, winemakers preserve aromatic compounds and antioxidants, producing wines that are fresher and more aromatic. However, the downside is that these wines can be more prone to oxidation later on, especially once the bottle is opened. This approach demands careful handling to maintain wine quality over time.

Sulfur Dioxide and its Role in Reductive Winemaking

A crucial tool in the reductive winemaking process is sulfur dioxide (SO₂). Battistutta and Manara emphasized the importance of adding controlled amounts of SO₂ early in the winemaking process to inhibit tyrosinase activity and protect the wine from oxidation. Sulfur dioxide acts as a preservative by binding with oxygen, preventing oxidative reactions, and preserving the wine's freshness and aromatic profile. In the right quantities, it can extend the wine's longevity, ensuring that it remains vibrant and fresh as it ages.

However, winemakers must strike a careful balance when using SO₂. Too much can negatively impact the wine's sensory qualities, while too little can leave the wine vulnerable to oxidation. The careful management of SO₂, in conjunction with oxygen exposure strategies, forms the backbone of modern white wine vinification.

Ultimately, the presentation highlighted that achieving the desired balance between hyperoxygenation and reductive winemaking is key to crafting long-lived white wines. The right approach depends on the type of wine being produced and the winemaker's goals for its aging potential and sensory profile. Understanding the roles of both the polyphenol oxidase and lipoxygenase pathways allows winemakers to better manage the complex interplay of factors that influence oxidation, aroma, and stability in white wines.

By mastering these biochemical processes, winemakers can optimize their production methods to create white wines that are not only aromatic and fresh upon release but also capable of maturing gracefully over time. This nuanced understanding of the enzymatic activities in grapes and their interaction with oxygen is vital in the pursuit of high-quality white wines that stand the test of time.