Oxygen and SO₂ Management in the Pre-Bottling Process

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Prof. Gerbi analyzes the management of oxygen and SO₂ in the stages leading up to the bottling of red wines. He begins with the main risk factors (residual sugars, high pH, Brettanomyces) and then illustrates the protective role of yeasts, aging on the lees, the influence of container porosity (wood, concrete, stoneware, steel), and micro-oxygenation. The presentation also delves into glutathione, the measurement of dissolved oxygen via luminescence, TPO in the bottle, and the relationship between free and combined SO₂ and pH. The goal is to minimize SO₂ while maintaining antioxidant efficacy and microbiological stability.

Why Wine Stability Can Never Be Taken for Granted

In the phase leading up to bottling, there is only one central question: Will the wine we consider excellent today remain so in the consumer’s glass? Climate change has made this question even more delicate, because it has altered the balance between two key parameters.

The Two Main Enemies

• Residual sugars: even a few grams per liter—below the detection threshold of laboratories—serve as food for microorganisms.
• High pH: This increases the aggressiveness of lactic acid bacteria and drastically reduces the percentage of active molecular SO₂, paving the way for Brettanomyces contamination.

The basic rule therefore remains: dry wines with a controlled pH, even before considering containers and dosages.

Yeast as a Natural Antioxidant

Viable Saccharomyces are capable of rapidly consuming dissolved oxygen (reducing it to zero in just a few minutes after a controlled addition), providing an important antioxidant defense during aging on the lees. However, the release of compounds from within the cell (mannoproteins, glutathione) only becomes significant after cell lysis, which takes at least 6–9 months.

Containers and Porosity: From Wood to Steel

A practical indicator of color stability in red wines is the percentage of free anthocyanins: the goal is to get below 20%. The result depends on the porosity of the container, which decreases in the following order:

• wood
• terracotta
• stoneware
• steel
• glass

In wood, the reaction occurs deep within the first centimeter of the staves, where ellagic tannins transport oxygen to the phenolic compounds. In steel containers, which lack natural porosity, micro-oxygenation combined with ellagic tannins can be used to achieve similar results.

Glutathione: An Ally with a Regulatory Limit

Luminescence has made it easier to measure dissolved oxygen even at very low levels, a decisive advantage during bottling. Glutathione, a natural tripeptide produced by yeasts, is not, however, included in the list of additives authorized by EU Regulation 2019/1934: it cannot therefore be added directly, but only promoted through prolonged contact with yeasts and their derivatives.

Bottling: The Most Critical Phase

TPO (Total Package Oxygen) is the sum of three components:

• oxygen already present in the wine;
• oxygen in the headspace;
• oxygen introduced during corking.

Every milligram of oxygen consumes 4 mg of SO₂; for this reason, an error on the bottling line can deplete a supply of free sulfur dioxide that initially seemed adequate in just a few weeks.

The Final Strategy: Less SO₂, More Efficiency

The goal is not to eliminate SO₂, but to maximize its effectiveness by reducing the proportion of bound sulfur dioxide in favor of free sulfur dioxide. Since the effectiveness of molecular SO₂ depends heavily on pH, a wine with a pH that is too high (e.g., 3.7) renders even a dose of 30 mg/l of free sulfur dioxide ineffective. It is therefore necessary to focus on pH control as well—including potential biotechnological solutions (such as yeasts that produce lactic or malic acid)—to address the challenges posed by climate change.

The report featured in this video was presented during one of the “Vinidea On The Road” 2026 in-person courses.