The fascinating journey of sugar: from vine to wine

Wine, an intriguing beverage that graces special occasions or simply enhances a peaceful lunch, is the product of an intricate and rich series of processes. One crucial ingredient lies at the heart of its production: sugar. This article delves into the intricate journey of sugar in wine production, the importance of sugar in determining the taste and quality of the wine, and how winemakers and consumers alike can appreciate the subtleties this brings to their glass.

The role of sugar in wine production

The two main sugars found in grapes are glucose (also known as dextrose) and fructose (or levulose). These sugars, during the fermentation process, are transformed into that exquisite liquid we so appreciate: wine. Besides these, other sugars that are non-fermentable by yeasts and do not impact the sensory characteristics of the wine can be found in small quantities (approximately 2g/l). These non-fermentable pentoses are measured as a component of residual sugar in the finished wine.

The journey of sugar: from plant to grape

Once the stage of véraison is reached – the moment when the grape changes color and begins its natural ripening process – photosynthates, in their form of sucrose, are sent to the grape. Inside the grape, the sucrose splits into its components, glucose and fructose. After reaching optimal maturity, the sugar concentration is increased due to grape dehydration, not by an increased amount of sucrose.

The sugar concentration is higher in the pulp. In the skin, the sugar quantity is reduced to around 80% of the sugar concentration of the pulp. During red wine fermentations, the sugars from the skin are released into the wine more than in white wines, resulting in an increase in the amount of residual sugar after the pressing process.

Measuring grape sugar

Grape sugars are quantified as the concentration of soluble solid compounds in the must using one of the available density scales, for example, the Brix scale, which equates to the density of distilled water at 20°C. As sugar is denser than water, it provides a positive Brix reading, while alcohol, being less dense than water, provides a negative Brix reading. Other methods of measuring density include the Balling, Density, Oechsle, and Baumé scales.

Regarding the methods to measure these scales, there are two main ones: hydrometry and refractometry. Hydrometry is based on the principle that an object will displace a volume of liquid equivalent to its relative weight. Therefore, a higher sugar concentration in the must will imply a higher weight and, consequently, the volume of displaced liquid will be less and the reading will be higher. On the other hand, refractometry measures the degree to which light refracts when it passes through a sample, a deviation that occurs depending on the sugar concentration in the sample.

Hydrometry is typically preferred for tracking fermentations, as the presence of alcohol significantly influences refractometer readings. However, refractometers are primarily used when handling small volumes and are also particularly handy when collecting samples in the vineyard.

Temperature and types of sugar

Both methods for measuring Brix degrees (or others) require a temperature correction if the sample is not at 20°C. For this purpose, there are algorithms with suitable corrections depending on the temperature. When using a handheld refractometer, an additional correction to the reading should be made after the temperature correction.

In addition to measurement variables, the peculiarities of the sugars in question need to be considered. During véraison, glucose concentrations can be up to five times higher than those of fructose. In the maturity period, glucose and fructose are found in similar proportions, with fructose even being present in higher concentrations in ripe and overripe grapes. Some grape varieties have a high concentration of fructose, while others are characterized by a high concentration of glucose. In taste terms, fructose is approximately twice as sweet as glucose.

Controlling sugar during fermentation

A key challenge during fermentation is the prevention of stops. This issue can arise because all Saccharomyces yeasts are glucophilic, meaning they prefer feeding on glucose and reject fructose. Therefore, a relatively high concentration of fructose in the must could increase the risk of fermentation stops. The grape variety, the degree of maturity, and the temperature during the development stage (higher temperatures lead to a reduction in glucose concentrations) could influence the glucose/fructose ratio, which should always be higher than 1.

If the glucose/fructose ratio is relatively low (< 1), the following factors should be carefully controlled before and during fermentation: the nutrients of the must (YAN) and yeast nutrition, yeast selection - opting for a less glucophilic yeast, for example, one that prefers feeding on fructose more than other yeasts - the temperature - avoiding sudden temperature changes during yeast preparation, inoculation, and fermentation - and the interaction with non-Saccharomyces yeasts and malolactic fermentation bacteria during fermentation, avoiding spontaneous or uninoculated fermentations.

The alcohol potential

Yeasts convert sugar into alcohol, meaning they feed on sugar and their waste products are alcohol and gas. The degree of this conversion depends on the yeast strain, fermentation temperature, and other fermentation conditions. This conversion rate can vary between 0.55 and 0.61 of the original Brix value. There are Brix-Alcohol conversion algorithms, or any other indicator of the degree of sugar. This degree of conversion is referred to as Alcohol Potential.

As we have seen, the journey that sugar undertakes in its different forms during wine production is much more profound than most imagine. The science of sugar from the plant to winemaking is as complex as it is fascinating. Understanding these principles can help both wine producers and consumers appreciate the subtleties of wine and the depth of the art and science behind it. The appreciation of wine is not just about the flavor or the aroma; it is also about the complex processes and the skillful craftsmanship that bring the final product to our tables. So, the next time you enjoy a glass of wine, remember the journey of the sugar from the vine to your glass.