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Grape Chemical Components and Problems

by Staff Writer - B. Shaughnessy | February 08, 2012

The juice produced from grapes and ultimately used for winemaking is a complex medley of sugars, acids, minerals, polyphenols, proteins and flavor components. Grape juice is the raw material required from the vineyard crop and as such, must contain the proper balance of components to make an acceptable wine. Problems attributed to oxidation can result in the breakdown of the constituents within the juice. Therefore, an understanding of the components found within grapes as well as the role oxygen plays in the winemaking process is absolutely necessary for quality winemaking. Understanding and monitoring the constituents of the grapes through the growing season is a must for vintners.

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Out of the various components found within grape juice, sugar has the highest concentration. Sugar is present within grape juice in two main forms, glucose and fructose. The sugar form of sucrose is absent from grape juice. As with all plant life, photosynthesis is responsible for the production of sugar. With respect to the vine, the grape acts as a repository for the storage of sugar. Also, as sunlight directly controls the process of photosynthesis, the more abundant the sunlight the more sugar is synthesized. Sugars can be monitored with the use of a device known as a pocket refractometer. A pocket refractometer scatters light through a small juice sample to indicate the sugar level found in the juice sample. This information can be useful for vintners tracking the development of a harvest. The level of sugar combined with other vineyard information can assist the vintner in making management decisions concerning the grape harvest.

Aside from sugar, acids are the second most abundant component found within grape juice. Two main types of acids are found within grapes, tartaric acid and malic acid. It should also be noted that acids are not produced by photosynthesis but rather by the grape vine itself. Concerning the acids found in grapes, tartaric acid is unique to the grape and is fairly stable. Malic acid is not stable and is influenced by the life processes of the grapevine and by bacterium found in subsequent winemaking processes. It is for this reason that when balancing the constituents of wine, most vintners prefer to use tartaric acid.

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Other components of grape juice and eventually wine are mineral salts, polyphenols, flavor components and proteins. Mineral salts such as potassium and calcium are collected by the root structure of the vine and transported throughout the grape vine. Mineral salts add a distinction of place to wines and are a very important part of the final product. Excessive mineral salt concentration can lead to wine defects such as the presence of tartrate crystals. The importance of the proper concentration of mineral salts cannot be overemphasized, as the minerals are directly responsible for controlling the pH of the resulting juice. pH is important as it is directly responsible for the acid taste when a wine is consumed.

Polyphenols include the compounds of tannins and anthocyanins. The tannins found within grapes are contained mostly to the skins of the grape and are a good antioxidant. The presence of tannins is a desirable trait for wine, specifically red wine. However too much of a tannic presence can be overpowering for a wine. Also, growing climates affect the nature of the tannic characteristics. Grapes grown in warm climates have a soft and refined tannic taste while grapes from cool climates have a harsher tannic taste. Anthocyanins are compounds largely responsible for the color of red wine and are found within the cells of the grape skins. The resulting color of the wine is directly influenced by the manner in which anthocyanins are extracted during the fermenting process.

Flavor components are the minute substances found within wine that are responsible for the uniqueness of a particular wine. Due to this fact, flavor components are responsible for providing a specific aroma for the wine as well as defining the varietal character of the wine. Flavor components exist as esters, higher alcohols, aldehydes, terpenols and hydrocarbons. Proteins and colloids are larger molecules that are also found within wine. The presence of proteins and colloids can be positive, as proteins and colloids are a form of nutrition for yeasts. However, proteins and colloids do have a drawback of causing wines to become cloudy or hazy after a short period of time.

Perhaps one of the greatest challenges to vintners is how much of a part oxygen will play in the development of the final wine. With modern wine production and indeed modern chemistry applications, individuals have recognized the damaging effect that oxygen has concerning the production of wine. As with any substance in an oxygen-based environment, wine is subject to breakdown by oxidation. In the case of winemaking, oxygen causes the components of wine to breakdown and thus the resulting wine will inevitably loose some of its character. This breakdown begins the second a grape is detached from its vine. It is therefore the procedure of modern winemaking practices to eliminate the threat of oxygen from every winemaking step and practice a process known as anaerobic winemaking.

Anaerobic winemaking is a process where winemaking occurs in an oxygen free environment. The process of anaerobic winemaking is simpler than it sounds as a gas other than oxygen is substituted at different phases of winemaking. Inert gasses such as carbon dioxide, nitrogen and argon are popular choices among vintners for oxygen substitution. Each gas has its own set of positive and negative characteristics and in some cases, combinations of gasses are used in an anaerobic winemaking environment.

Regardless of the damaging effects oxidation has on wine, oxygen is still required in some shape or form to aid in the development of wine. Common practices include saturating the must (crushed grape juice) with oxygen during the fermentation stage. This is important as oxygen introduced at this stage helps yeasts reproduce more rapidly. Also, a process known as micro-oxygenation is gaining ground. In the process of micro-oxygenation, tiny microscopic bubbles of oxygen are introduced into fermented wine. This results in a change to the polyphenolic structure of the wine where the wine itself is smoother and has better integration of the tannins.

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