The chemical composition of grapes is wealthy. Grapes contain various ingredients that belong to many chemical groups. At maturity, the most important components of grapes are sugars, acids, minerals, nitrogen, pectin, vitamins, fragrant and aromatic ingredients, and tannins and colors.
Chemical Composition of Grapes–Sugars
They are mainly hexoses (basic formula – C6H12O6), glucose, and fructose, also called dextrose and levulose, depending on the property of behavior in the polarization of light right or left. Refractometers for determining the sugar level in strawberry juice have been developed precisely on the phenomenon of sugars that can “polarize” light.
The total sugar content ranges from 150 to 250 g / kg, mostly between 160 and 180 grams.
In addition to hexoses, there are also pentoses (arabinose, xylose, rhamnose…) that do not ferment. Their content ranges from 0.5 to 2.5 g / kg..
2. Next Chemical Composition of Grapes Is Acids
There are few mineral acids (less than 1 g / l of the must), and they are mainly in the form of salts: sulfates, chlorides, phosphates (calcium, potassium, magnesium, iron). There are many more organic acids (carboxylic) (between 5 and 15 g / l of the must). The largest share is represented by tartaric, malic, citric, succinic, and α-methyl-malic acid. “These acids form the total acids of grapes and together with the lactic“ total acids of wine ”(Foulonneau, 2009, 25).
In addition to the mentioned carboxylic acids, there is another group of organic (carboxylic) acids, which has less effect on total acids and more on grapes’ aroma. These are: methanoic (ant), ethanoic (acetic), propanoic, butanoic, pentanoic (valerian), hexanoic, decanoic (capric), nonanoic (pelargonium), dodecanoic (laurel), etc.
3. Chemical Composition of Grapes-Minerals
The content ranges between 3 to 6% by weight of the grapes and is found mainly in the hard parts. Due to red wine production (maceration), there are more of them in red than white wines.
They are mainly anions of sulfate, chloride, phosphate, etc., and cations of potassium, calcium, magnesium, etc. The most important salt is potassium bitartrate, known as tartar. It is poorly soluble in alcoholic solution, so it accumulates on the containers’ walls from which it must be removed.
4. Chemical Composition of Grapes-Nitrogen Compounds
Under this code, there are different classes of nitrogen compounds in grapes:
– amino acids (proline, glutamic, asparagine, alanine, arginine, thiamine, serine…),
– polypeptides and peptones,
various proteins and
– various ammoniacal substances.
The amino acid nitrogen represents 60 to 90% of the nitrogen in the must. Amino acids and ammoniacal substances are essential food for yeasts during alcoholic fermentation.
5. Chemical Composition of Grapes-Pectic Substances
This group consists mainly of gums, pectins, and mucus. These are compound glucosides that contribute to cell walls’ structure and are insoluble until the grapes ripen. They are then broken down by enzymes (proto-pectinases, pectin-methyl-esterases, polygalacturonases…).
Grapes contain a modest amount of vitamins. Thiamine (B1), riboflavin (B2), pyridoxine (B6), nicotinamide (PP), etc., are mainly found in the strawberry skin.
7. Fragrant and Aromatic Ingredients
Aromatic ingredients contribute a lot to the quality of the wine. They make a great impression on the wine drinker and thus contribute to whether to accept or reject the wine. Over the last three decades, remarkable progress has been made in aroma research. The new method for determining the quality of grapes (glycosyl-glucose) determines the aromatic potential, which is a very welcome piece of information for choosing the best harvest time. This analysis determines the total amount of glycosidically bound aromatic substances in grapes, must, or wine.
Varietal typical flavors are called primary because they include many groups, including methoxypyrazine. The most important group are terpenes and especially monoterpenes. They are in high concentrations, have a pleasant effect on the senses, and can be detected even at low contents. They play a significant role in certain aromatic varieties: especially in Muscats, but they are also contained in Rizvanec, Rhine Riesling, Traminer, White Pinot, and even Silvanec. “A large proportion of monoterpenes in grapes are bound in the form of glycosides” (Thell and Eder, 2009, 59).
Only volatile substances are detected by smell. According to the human threshold of perception, they are perceived and recognized according to the knowledge and knowledge of aromas.
Fragrant substances are varietal characteristics. We know non-aromatic and aromatic grape varieties. The most well-known aromatic types are Muscat, Sauvignon, Traminer, Scheurebe… Are varietal aromas equally perceived in grapes and wine? Grape berries contain aromas that are immediately detected (free) and aromas that are not seen when the grapes are bitten (hidden) but are released during alcoholic fermentation.
Substances that hide aromas are called aroma precursors. Flavors are numerous (hundreds identified), but they belong to alcohol, aldehyde, ketone, ester, fatty acids, various hydrocarbons, free amino acids, etc.
Research has shown that the variety and amount of aromas depend on the course of synthesis in the grapes. Each type has its own requirements. Varietal characteristics are most pronounced in a climatic environment that is borderline for the variety but is “optimal” to develop varietal aromas. “Hot” positions are not suitable for delicate aromas. The most valuable varietal aromas of Pinot Noir have developed in wine-growing areas where there are considerable differences between day and night temperatures. That is why this variety has proven itself in Burgundy and Champagne.
8. Tannins and Dyes
Like aromatics, tannins and color substances are essential for the wine’s character and quality. These phenolic substances (7 to 10 g / kg or g / L) plays several roles in the wine. Because many phenolic substances possess tannins’ properties, the concept of “tannin substances” has become established for them. The term “polyphenols” has also become established because many substances among them have multiple phenol functions due to their molecular structure. Polyphenols include dyes, tannins, and flavors. This simple division helps to understand their versatile role in wines.
Polyphenols protect the vine from diseases and pests. They also act as radical scavengers in chemical, thermal, and light damage. They increase in grapes two to three weeks before the onset of ripening. As the grapes ripen, the monomeric phenols in the berry skins responsible for the wine’s bitterness and bitterness combine into longer polymers. This reduces the bitterness and improves the sensory characteristics of the wine. Changes in phenolic maturity represent minimal changes in the sugar level of grapes. Polymerized tannins have a high molecular weight in strawberry skin and are softer in the mouth than low molecular weight tannins.
Tannins are quickly released from strawberries’ skin as a small amount of alcohol begins to melt them. The tannins of green stalks and seeds are coarser. By coarse processing, total phenols grow in the wine, which is most undesirable for white wines.
Professor Singleton (University of Davis, California) divided the polyphenols into two groups.
- non-flavonoids and
Non-flavonoids are mainly:
- hydroxyl derivatives of benzoic acids (gallic, vanillic, protocatechuic…) and their esters,
- hydroxycinnamic acid derivatives such as trans-Caftaric, and trans-Fertaric and their esters,
- other non-fluvial phenols, often brought from other substances (from a wooden container), eg: vanillin, coumarin, syringaldehyde, ellagic acid.
With its antioxidant properties in red wines, the popular non-flavonoid “resveratrol,” which in grapes as a phytoalexin, protects the grape berry from the intrusion of gray mold. It is important from the health point of view of wine drinkers, as it prevents cardiovascular diseases, lowers cholesterol, prevents cancer, reduces Alzheimer’s disease, and stimulates brain cells’ division.
Different varieties of resveratrol have been found in red varieties: Blue Franconian – 12.6 mg / l, Pinot Noir – 12.2 mg / l, Merlot – 8.23 mg / l, Cabernet sauvignon – 5.48 mg / l
Flavonoids are mainly catechins, flavonols, anthocyanins, flavan 3,4-diols, condensed tannins… Catechins are found in several isomeric forms of flavanols. D-catechin is most visible but in wine. There are also epicatechin, gallocatechin, epigallocatechin, derived from a wooden container.