Development of Grape Sugar
Wine Grape Sugar Development and it’s Role in Wine
Sugar development in grapes is an important event in the reproductive lifecycle of the vine. The seeds inside the grape contain the genetic material of the vine and its reproductive success is dependent on the survival of those seeds. Sugars are transported from the vine into the grape to make sure the seed has enough energy to live.
This phenomenon is seen in a wide variety of fruits, and is one of the reasons why animals love fruit so much. The reason why humans love grapes is because they make wine. And the development of sugar is a critical step in the process.
For the most part, sugars exist in the vine as sucrose, which is a glucose and fructose molecule bonded together. Keeping carbohydrates in the form of sucrose is beneficial for the plant’s osmolarity and transport.
As the berries are developing, sucrose is transported into the grape to be metabolized. But as development progresses, the surrounding leaves begin to supply the berries with carbohydrates and sucrose coming in from the vine slows.
Glucose and fructose are mainly stored in the flesh of the grape, while smaller amounts are stored in the skin. It was found that the highest concentration of sugar is in the flesh adjacent to the skin. The availability of grape sugar makes it easy to extract, and provides the building blocks for alcoholic fermentation.
The sugar that is stored in the grape begins to accumulate after veraison. This in part is due to the decrease in sugar consumption by the berry. The drop in sugar consumption is associated with an increase in malic acid respiration.
Malic acid accumulates in the developing berries in the early stages, but then drops off considerably after veraison. Another phenomenon associated with the drop in malic acid and rise in sugars is gluconeogenesis (making new sugar). In this process, malic acid is broken down, then reconstituted as sugars.
Upon reaching the berry, the sucrose is enzymatically split into glucose and fructose. In young berries, there is more glucose than fructose. As development progresses, the levels begin to shift, and by the time the berry is ripe, it has considerably more fructose than glucose.
The reasons behind this are not very well understood. But some viticulturalists hypothesize the difference lies in the different rates of metabolism; indicating that the grape will preferentially use glucose as fuel over fructose.
The total sugar content of a fully ripened grape is dependent on viticultural practices and varying environmental conditions. Depending on these things, the sugar content at maturity can range from 12% to 28%. Ripening past maturity will change the concentration of the sugar, but not total sugar content. This is mainly because of evaporation.
In some species, a very small proportion of sugar is stored as sucrose. Also, other sugars are also present, but there are in insignificant amounts. These include; melibose, raffinose, maltose, arabinose, xylose, galactose and stachyose. These sugars are not used by the vine for metabolic purposes and don’t not contribute to the wines sensory characteristics.
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