The development of the cluster begins with the fertilization of the ovules of the flowers of the vine. At the end of spring or beginning of summer, flowering or “flinching” occurs. Flowering occurs when average daily temperatures reach 18ºC, which happens 6 to 9 weeks after bud break, for a period of 2 to 7 days. Warm temperatures increase the flowering speed, but if they are excessively high, above 35°, they delay or prevent it.
Most cultivated vines are hermaphrodites. The fertilization process occurs naturally, but when they are unisexual female vines, the viticulturist must resort to artificial pollination to achieve a sufficient harvest.
As for the climate, the optimal conditions for the development of the clusters are found with temperatures around 20ºC, without rain and without humidity, with dry weather and little wind. On the contrary, cold temperatures, below 15°C, accompanied by rain or humidity (fog, dew,…), retard the development of the bunch, causing uneven ripening and consequently the levels of sugar and acidity in the fruits.
In addition to climatic factors, the possible defects in the conformation of the bunch can originate due to flowering with little vitality, as well as a deficient feeding of the bunch during this stage, which can determine the abortion of the flower, the absence of fertilization or the production of small and immature grape berries.
These defects in flowering, prior to the birth of the bunches of grapes, are called “millerandage”.
However, it should be noted that most of the time it is not possible to get all the flowers to transform (or set) into grapes, there is a percentage of flowers that do not develop and that depends on a large number of factors. On average it is accepted that in general only 100 to 200 berries per bunch develop, which corresponds to a fruit set rate of between 60% and 80%.
In addition, after flowering and fruit set, there is also a significant number of grape berries that stop growing and detach from the recently formed bunch. This phenomenon is known by the name of “corrimiento”. The bleeding occurs mainly due to unfavorable weather conditions, basically high temperatures and lack of water when the grapes need to grow and hydrate.
The shifting varies greatly depending on the varieties, since some have a specific sensitivity, while other types of grapes are much more resistant.
After the initial growth of the grape, in the subsequent development of the cluster there are four very marked and differentiated stages: the herbaceous or sour period, veraison, the ripening or translucent period and overripeness.
Herbaceous or agraz period
This phase has a variable duration between 45 to 65 days, depending on the variety of vine and the environmental conditions. During this stage the grape berries increase in size.
During this period, the grape berries behave like one more part of the plant, as if they were leaves rather than grapes, they remain green and carry out photosynthesis thanks to the chlorophyll they contain.
At the end of this period, the grape contains only about 20 grams of sugar per kilo and almost the same amount of acidity. The berries have an activity where they accumulate many acids.
The duration of this popular phase is only one or two days for each grape, however it does not happen simultaneously, so in a vineyard it can last for a couple of weeks.
In this phase, the growth of the grape grain stops, and the pigments typical of each grape variety appear: The white varieties acquire a yellowish color, and the inks can vary from a pinkish tone to a very dark blue, almost black. Although the grapes can get a little fatter, the size of the bunch (stem) in this period reaches its final size. The skin becomes thinner and consequently the grape berry becomes translucent in appearance, with a softer and more elastic consistency, covering itself externally with pruína, that kind of dust that is especially visible in red varieties.
Ripening or translucent period
As its name indicates, this is the maturation stage itself, it lasts from 35 to 55 days, depending on the varieties, climatic and environmental conditions, during which the grape berries continue to increase in size, accumulating water and sugar. However, the skin barely grows at this stage, so there is a progressive increase in skin tension, becoming thinner and more translucent, and in some cases cracks or breaks may occur due to excessive growth of the pulp.
This phase ends when the winemaker, winemaker or viticulturist decides to start the “industrial maturity” harvest, or the moment at which the grape must be used for the industrial purposes for which it is intended, in our case winemaking.
Industrial maturity, or harvest time, is reached when the sugar/acidity ratio is maximum, or in other words, when the weight of the bunch is the highest for the highest possible concentration of sugars, unless due to the type of processing decide otherwise.
The overripeness or “natural drying” of grapes cannot be considered as a period of the vegetative cycle itself, since the grapes are in decline and can be damaged. However, more and more many wines require this technique to make special wines, or even coupages.
During this phase, the grape berries evaporate water, which causes a concentration of sugar, as well as a decrease in weight and size. On the other hand, the berries continue to live, with the consequent production of substances, especially malic acid.
In addition, for overripeness to occur, precise climatic conditions are required. The overripening is carried out in those regions where the insolation allows it, with sunny, warm autumns, and above all very dry, with almost no rain.
The bunches are left unharvested on the vines themselves and even twisting or cutting the vine shoots that contain them, so that they dry on the trellis wires. Another technique consists of spreading the bunches on the ground, in small, thin layers, which are left to raisin.
The time required for raisining depends on each grape variety, degree of maturation and, above all, the weather conditions, which can range from one or two days to one to three weeks.
Partial raisining by exposure to the sun, is carried out as a main example, in the area of Jerez in Spain, where traditionally the harvest, generally white of the Palomino and Pedro Ximénez varieties, is subjected to an over-ripening process known as name of “soleo”. The harvested bunches are placed on esparto grass discs from 1.5 to 2.0 meters in diameter called “redores”, where, exposed to the sun for one or two days, they undergo partial drying, concentrating the sugars by 10 to 20%, which reduces the harvest by around 15%.
In some cold countries, such as Germany, Switzerland, Austria or Canada, where sunny conditions do not allow harvests with a high sugar content to be obtained, harvesting is delayed long enough for the first cold weather in winter to literally freeze grape berries, causing in these cases an over-ripening by freezing. Part of the water contained in the must is frozen in a solid state, remaining in the press together with the skins and seeds of the grapes, obtaining a concentrated must from which the famous ice wines or Eiswein are made. The exceptional climatic conditions mean that every year it is not possible to obtain this type of wine naturally, which means that they are highly valued wines.
Over-ripening or natural drying in the shade or under cover is practiced when it is not possible to carry it out under the above conditions. For this, constructions of two or three floors can be used, provided with numerous large windows distributed on two opposite sides (similar to granaries).
Another overripening technique consists in the use of stoves, hot air dryers or chambers.
Finally, another possibility of obtaining overripe crops, directly from the plant, is through the development of the ‘botrytis cinerea’ fungus which, under certain conditions, can develop a ripening process known as ‘noble rot’, which under other climatic conditions, although under the same fungus, can cause another type of ripening called ‘grey rot’ where the berries are highly altered. With this grape ripening technique, the world-renowned French Sauternes or Hungarian Tokaji, among others, are made.