Gluconic and Galacturonic Acids – Critical Indicators for Wine Quality and Stability

1. Introduction: The Acid Architecture of Wine and the Role of Oxidizing Acids

The very matrix of wine is, in its essence, a complex solution with an acidic pH (typically between 3.2 and 3.4). In this environment, the delicate balance between the different chemical species not only defines the sensory profile but also guarantees the stability and longevity of the product. It is therefore crucial to distinguish the structure of “fixed” acids (such as tartaric and malic), intrinsic to the physiology of the grape, from acids resulting from oxidation or degradation processes. Underestimating the impact of the latter, especially in years marked by severe climatic instability, represents a serious technical error.

Gluconic acid and galacturonic acid are not simply structural components; they act as true “biochemical sensors” of the vineyard’s health. Their presence signals oxidative or degradative processes (enzymatic and fungal) unrelated to natural ripening. In particular, these compounds are molecular indicators of the activity of Botrytis cinerea, capable of compromising the entire oenological potential of the grape batch before the must even enters the tank.

2. Gluconic Acid: The Strategic Marker of Grape Health

In an era of climate change resulting in extreme water stress or delayed rainfall, gluconic acid is the most reliable parameter for quantifying fungal damage. Chemically, it derives from the oxidation of glucose by the glucose oxidase enzyme secreted by Botrytis.

Biochemistry and Fermentation Impact: Gluconic acid is a nonvolatile acid that yeasts cannot metabolize into alcohol. Its concentration is directly proportional to the severity of the attack:

• Healthy Grapes (No Damage): < 0.2 g/L
• Alert Threshold (Onset of Alteration): 0.2 – 0.5 g/L. This already requires adjustment of SO₂ dosages and nutrition
• Compromised Grapes (High Risk): 0.5 – 1.5 g/L. Requires drastic corrective measures on sulfur dioxide, enzymes and fining agents
• Severe Damage (Severe Impairment): > 1.5 g/L. The quality of the final product is irreparably compromised

For the producer, a high gluconic acid level is a warning sign requiring the utmost attention: it indicates not only a lack of Yeast Assimilable Nitrogen (YAN), consumed by the fungus, but also the presence of substances that inactivate sulfur dioxide, making the must vulnerable. Botrytis infection also involves the secretion of laccase, an oxidative enzyme that rapidly degrades anthocyanins and tannins. If gluconic acid is high, the colour stability of red wines is already critically endangered.

3. Uronic Acids: From Pectins to Bottled Instabilities

While gluconic acid reflects the oxidation of sugars, the accumulation of galacturonic acid and glucuronic acid (uronic acids) indicates the structural failure of the grape. These compounds are the direct byproduct of the breakdown of pectins by fungal enzymes. From a technological standpoint, a must rich in uronic acids is colloidal, viscous and hostile to any clarification or filtration process. The greatest risk is subtle and manifests itself months later: galacturonic acid, in oxidative environments, easily evolves into mucic acid. Having a very high affinity for calcium, this compound generates calcium mucate crystals, evading normal pre-bottling controls and then inexorably precipitating directly into the consumer’s bottle.

4. Technological Analysis: The CDR WineLab System vs. Traditional Methods

During the hectic grape harvesting phases, timely analytical data is the true differentiator between actively monitoring a process and tracking an imminent defect. Relying on traditional reference methods, which due to their complexity often require outsourcing or the preparation of complex reagents, means accepting latencies incompatible with the winery’s rhythms. In the presence of health stress, obtaining gluconic acid measurements hours or days later risks providing a delayed picture, returning data when oxidation or microbial activity has already triggered damage that is difficult to manage. Rapid and simple systems, even for use in the winery, such as the CDR WineLab allow for timely responses for actively monitoring the process.

5. Conclusions

Facing the challenges of increasingly unpredictable vintages today requires unprecedented technical clarity. The presence of fungal acids such as gluconic and galacturonic acids, with their devastating impact on redox balance and colour stability, demonstrates how wine quality often depends on the mere tiniest fraction of a gram.
Monitoring these health stress markers means protecting the entire winemaking process from fermentation crises and visual defects. This is where the time factor becomes crucial: freeing oneself from the latencies of outsourced enzymatic methods and embracing real-time reading systems transforms analytical data into immediate corrective action. Rapid analysis thus confirms itself as the fundamental technical procedure for preventing irreparable microbiological deviations and standardizing quality control in the winery.

Dr. Francesca Bruni
Researcher at the CDR Chemical Lab “Francesco Bonicolini”

16/4/2026