The final process in the development of grapes leads to a remarkable change, both physical and metabolic, which is very important to monitor to ensure the optimal time for harvesting.

The berries are small and green in the early stages of development. Progressively, the water and sugar content increases and the grapes increase in weight and volume until the starting point of the physiological maturity process is reached, the winter. At this time the growth stops and a series of metabolic changes occur which will provide the characteristics sought in wine production.

The first change is that of the color of the grapes, which goes from green, associated with a high concentration of chlorophyll, to a purple-blue tone (for the red varieties) or yellowish green (in the white varieties) due to the increase of polyphenols (flavonoids, anthocyanins and tannins) in the skin and decrease in the pulp. This change occurs quickly for each berry (in one or two days), but not uniformly in the bunch or in the vineyard (which will completely develop the color in about 10-15 days, depending on the variety of climatic characteristics). The bloom, a whitish and waxy layer that acts as a protector and fixative of yeasts, covers the grapes. From this moment, the grapes will need between 35 and 55 days to complete their maturation and reach the optimal harvest time.

The disappearance of chlorophyll is also accompanied by significant metabolic changes in grapes which, in general, we can interpret as a process of alcoholic maturation, in which fermentable sugars are accumulated (which therefore influence the alcoholic degree that the wine will reach after fermentation), and a process of phenolic maturation, in which anthocyanins and tannins are fixed on the inner part of the peel (and will have an effect on aromas and astringency). It must be considered that both processes take place simultaneously, but not in parallel, so an optimal level of sugar and acidity may not correspond to the optimal level of polyphenols and vice versa. It is based on the point of equilibrium between these two processes, called technological or industrial maturity, that the oenologist decides the most suitable harvest time for each variety and growing conditions.

Phenolic maturation involves the activation of the phenylalanine-aminoliasis enzyme (PAL) by heat, light and the action of abscissic acid. This enzyme is involved in the synthesis of phenolic compounds starting from the degradation of phenylalanine and tyrosine to cinnamic acid and is present exclusively in the skin cells and in some parts of the pulp. The process takes place through three phases: a rapid accumulation in the skin cells, which causes the color of the grapes to change; a phase of stagnation, in which they reach their maximum concentration; and finally, a decreasing phase, in which the anthocyanin concentration begins to decrease. It is at the moment when this decrease begins that the complete phenolic maturation is reached and, from this moment, over-ripeness would provide notes of cooked or candied fruit.

The most common procedure to determine the level of phenolic maturation is based on the acid extraction of the content of polyphenols (or anthocyanins) at pH 3.2, simulating the conditions that occur during fermentation while respecting the integrity of the peel, and pH 1.0, very much more aggressive, in which the entire phenolic content is released (Glories method), so that the smaller the difference between the two values, the greater the degree of phenolic maturation. Measurement of polyphenols in both cases uses a colorimetric method (Folin-Cicalteau).

At the beginning of the alcoholic maturation, the grapes contain about 10-15 g of sugars per liter of must, mainly in the form of glucose (85% of the total). During alcoholic maturation, the concentration of sugar increases to 150-200 g per liter of must. This increase is also accompanied by isomerization of glucose to fructose, which leads to a concentration of fructose in the mature grapes of more than 95% of the total. This accumulation is not a product of the photosynthetic activity of grapes, since, as mentioned, chlorophyll disappears, but is a consequence of the contribution of the rest of the plant that mobilizes sucrose (the disaccharide formed by glucose and fructose, which is sugar main in the leaves) in the berry, where it is dissociated into fructose and glucose. Maturity is reached when the total concentration of sugar is sufficient to guarantee the desired alcoholic strength in the finished wine. At this point, the glucose / fructose ratio is practically equivalent ([Glu]/[Fru] = 1), but glucose will be consumed in cellular respiration, lowering the ratio to values ​​of about 0.92-0.95 at the time optimal harvest. Also the acid content of the grapes evolves considerably at this time, both in its concentration and in its composition. At the beginning of winter it has its maximum value, around 10-8 g/L and consists mainly of malic and tartaric acid, which represent more than 90% of the acids. From this moment, the acidity is reduced by the metabolism of the berry itself, above all due to the consumption of malic acid which is transformed into glucose. At the beginning of the harvest, the total acid content was reduced to 4-6 g/L with the prevalence of tartaric acid.

Given the importance of sugars and acids, the winemaker’s main decision is to establish the right relationship between them. One of the most used technological maturity indices, the Cillis-Odifredi index, relates the sugar content and total acidity (total sugars in 100 mL of must divided by grams of tartaric acid per liter of must). It is considered an appropriate time for harvesting if the index is between 3 and 5, depending on the grape variety and the type of wine desired.

The quality of the wine begins in the vineyard and choosing the right time for the harvest is the first critical decision that the winemaker must make. Sugars, acids and aromas will be, among others, fundamental elements of this decision. Having practical and reliable tools as a guide is undoubtedly essential for achieving the intended goal.

Kit for evaluating the technological maturity of grapes
SY2404 · Glucose+Fructose
SY2428 · Total Sugar
SY2402 · L-Malic-Acid
SY2412 · Tartaric Acid
SY2430 · Total Acidity
SY2424 · Polyphenols
SY2414 · Anthocyanins
SY2416 · Catechins

For more than 10 years, Sinatech’s commitment to the winemaker has always been working side by side to provide the most appropriate analytical solutions to control and monitor the winemaking process. Automatic methods easily adaptable to any work routine, with a personalized consulting team to help you implement it quickly and without problems.

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