Grape as raw material for wine

As reported earlier the grape used for wine production is generally Vitis vinifera L.; over forty species are known but only some are used for winemaking.

Wine grapes grow almost exclusively between thirty and fifty degrees north or south of the equator. The world's most southerly vineyards are in the South Island of New Zealand near the 45^th parallel and the most northerly is in Sweden, just above the 59^th parallel. As a rule, grape vines prefer a relatively long growing season of 100 days or more with warm daytime temperatures (not above 95°F/35 °C) and cool nights (a difference of 40°F/23 °C or more).

It is the grape quality more than any other factor that determines the quality of the wine. This quality is defined by different parameters that can be described as 1) permanent (variety, soil, climate, exposure), 2) variable (the weather during the growing season), modifiable (the time of harvest, pruning) and 3) accidental (hail storm, diseases). The combination of these effects is often referred to as terroir.

The grape berry in its early weeks after flowering is a tiny, green-coloured, very acidic pellet. During ripening the berry increases in volume so that after a few weeks the tiny berry becomes a plump, sweet, coloured fruit. When the berry changes its colour the veraison phase is reached. From an oenological perspective, veraison can be divided into different sub-stages based upon berry metabolism and the continued transport of substances to the vine. During veraison, water, sugars, and nitrogen compounds are transported to the berry via the phloem. Sucrose is hydrolyzed to glucose and fructose and flavour and aroma compounds are synthesized within the berry. The fruit does not increase in volume indefinitely. When the fruit has reached normal maturity there is a cessation of cell enlargement and a decrease in volume can occur under warm conditions. During these physical changes other chemical changes also occur.

When ripening starts there is a continuous increase in sugars in the fruits while the titratable acidity decreases as shown in figure 1.

Figure 1. Changes in sugar concentration and titrateable acidity during ripening of grapes.  Sugar,  Acidity.

Sugar concentration increases until a maximum value defined by climate and variety is reached. This value stays stable for a small time and then the sugar concentration increases again due to the over-ripening of grapes and the shrinkage of the berry. The determination of grape maturity is very important because high quality wines can only be produced when the grapes are harvested at their optimum maturity. Generally for white wines harvesting is performed early to preserve acidity that is very important for sensory characteristics (freshness) of wine while for red wines harvesting takes place later when the sugar concentration is higher. It is not possible to fully define this stage because it is correlated not only to the chemical composition of grapes but also to their health and the characteristics required in the wine. Optimum maturity is then defined for each vineyard and can be different each year depending on climate, grape maturity and above all the characteristics of the wine that will be produced. This evaluation is performed generally with analytical methods (determination of sugar, polyphenol and acid concentration) and harvest is conducted when the grapes have reached the correct chemical composition.

Harvesting is generally done by cutting the cluster from the vines by hand with picking knives or scissors. The grapes are moved rapidly to the vinery for winemaking since delay can result in product deterioration. Mechanical harvesting is used when possible but the quality of the wine obtained is generally lower due to some crushing of the grapes.