The Berry Ripening Phase of the Grapevine

The sixth phase of the grapevine vegetative period spans the time interval from the initiation of berry ripening to the onset of full (physiological) maturity. The duration of this phase fluctuates extremely widely among different grape cultivars, as they comprise very early-ripening as well as very late-ripening genotypes.

Although the calendar dates for the onset of physiological maturity of each grape cultivar vary substantially depending on the meteorological conditions of a given year and, in particular, on the climatic features of each viticultural region, a distinct sequence in the ripening of grape cultivars remains relatively constant. Pulliat utilized this circumstance to develop an ingenious classification of grape cultivars by ripening "epochs." The underlying principle of this classification dictates that one widely distributed cultivar, Chasselas Dore (Chasselas doré), is selected as a benchmark, and all other cultivars are categorized into five groups based on how many days earlier or later they ripen relative to this benchmark cultivar:

• Epoch I: very early cultivars; ripening earlier than Chasselas.
• Epoch II: early cultivars; ripening concurrently with Chasselas.
• Epoch III: mid-season cultivars; ripening 15 days later than Chasselas.
• Epoch IV: late cultivars; ripening 30 days later than Chasselas.
• Epoch V: very late cultivars; ripening 45 days later than Chasselas.

Pulliat's classification has become widely adopted in viticulture and retains global recognition to the present day, although subsequent data indicated that under drastic shifts in climatic conditions, grape cultivars can shift from one epoch to another.

A general perception of how the calendar dates for the onset of physiological maturity vary across different years in the same cultivar within the same vineyard, and how the duration of the ripening period in days changes accordingly, is provided by phenological observations at the Magarach vineyards (southern coast of Crimea).

The variation in calendar dates for the onset of physiological maturity in different years for the same cultivar can reach 17 days, whereas the gap between early and late cultivars (e.g., Pinot Gris and Mourvedre) within the same year can reach 43 days (it should be noted that the provided table contains no very early cultivars).

The total duration of the ripening period in early-ripening cultivars is significantly shorter than in late-ripening ones (for example, 31 days for Pinot Gris compared to 50 days for Mourvedre). The inter-cultivar variation in this regard remains relatively constant despite the annual fluctuations observed across individual years.

By the initiation of ripening, the grape berry attains its definitive size, and thereafter its volume and weight change very insignificantly. Consequently, berry ripening is characterized by the progressive cessation of growth processes, while complex chemical transformations occurring within the cells of its tissues assume primary importance.

These processes are clearly reflected in the external appearance of the berry: the bright green chlorophyll pigmentation gradually fades, and the berry skin in white cultivars becomes more or less translucent, whereas in black cultivars, the first signs of dark pigmentation appear on the skin, initially as isolated spots that progressively expand over the entire surface of the berry. Concurrently, the berry begins to soften, as the cell walls of the pulp become thinner and more elastic. No less drastic changes occur in the flavor of the ripening berries: it shifts from sharply acidic to increasingly sweet. The sum of all these listed characteristics is utilized during phenological observations to establish the initiation of berry ripening in grape cultivars.

From the moment berry ripening initiates, the sugar content within the cell sap begins to rise rapidly and continuously until it eventually reaches a certain maximum value, after which further sugar accumulation in the berries terminates; this state of the grape berries is conventionally termed their full or physiological maturity. Simultaneously with sugar accumulation, the total content of organic acids in the sap of the ripening berries progressively decreases. Both of these processes represent two continuously diverging curves that characterize the progression of grape berry ripening.

The physiological rationale for these processes fundamentally reduces to the following: the sugar flowing into the green berry is almost entirely consumed for the vigorous growth and respiration of the cells. Cells in a state of growth cannot accumulate any perceptible amount of sugar, whereas the quantity of organic acids in the cell sap, functioning as intermediate products of respiration (i.e., incomplete oxidation of sugars), increases continuously. Only from the moment when berry growth ceases, and concurrently the respiratory energy of the cells declines, does the sugar entering the berry begin to accumulate in the cell sap, while the neoformation of acids nearly terminates. The decline in the acidity of ripening berries is explained both by the partial binding of acids into insoluble salts and by their further oxidation during cell respiration into carbon dioxide and water.

The sap of the grape berry contains two monosaccharides: glucose and fructose; sucrose, according to most researchers, is almost entirely absent in the berries of European grape cultivars (Vitis vinifera), with only traces of this disaccharide detected in unripe berries. At the initiation of berry ripening, glucose predominates in the sap, but the quantity of fructose progressively increases, so that by the time of full berry maturity, the concentrations of glucose and fructose in the sap become nearly equal.

Among other carbohydrates, mature grape berries also contain pectic substances (from 1 to 3 g per 1 kg of berries), pentosans, inositol, gums, and many others. Pectic substances appear in the berry at the end of its ripening; their quantity rises sharply in overripe and withered berries.

Among the numerous organic acids of the grape berry, tartaric and malic acids are the most critical. Variations in the quantity of these acids during berry growth and ripening proceed almost parallel, as demonstrated by the analyses of Mach. In addition to these two organic acids, citric, oxalic, succinic, glucossuccinic, glycolic, formic, and other acids have been detected in very small amounts within the grape berry; however, quantitative determinations of these acids are of low reliability due to their negligible content. The question of whether these acids are permanent constituents of the grape berry remains unresolved to date.

Nitrogenous compounds of the grape berry are currently poorly understood. A minor portion of them is of a proteinaceous nature, coagulating upon heating and precipitating with alcohol (albuminoids); the remaining nitrogenous substances of the grape berry belong primarily to amide and ammonium compounds (Windisch, Böhm), the precise nature of which remains unknown. The content of nitrogenous substances varies across different parts of the berry, being highest in the cell sap of the skin and lowest in the cell sap of the pulp. According to analyses by Mach and Portele, nitrogenous substances within the berry are distributed as follows (mean figures for 9 cultivars): in the sap expressed from the skin — 0.590%; in the sap expressed from the pulp — 0.418%; in the sap expressed from the central part of the berries surrounding the seeds — 0.544%. The total nitrogen content in grape juice, according to numerous analyses by G. von der Heide, ranges from 0.18 to 1.37 g per 1 liter.

Tannic substances, collectively designated as "enotannin," are contained in perceptible amounts only in the sap of unripe grape berries; they are entirely absent in the sap of mature berries. Significantly larger quantities of enotannin are present in the berry skins, as well as in the stems and seed coats. As an example, the following data by Mach and Portele regarding the quantity of tannic substances in the berry skins of various grape cultivars can be cited:

It should be borne in mind that existing methods for determining tannic substances do not yield sufficiently reliable results, so all the digital data presented above possess only relative significance. The chemical nature of the tannic substances contained in the berries remains obscure to the present day.

The green coloration of unripe grape berries, which is entirely identical across all grape cultivars, depends on the presence of chlorophyll. During berry ripening, chlorophyll progressively disappears from them. The golden-yellow coloration that appears in white grape cultivars by the time of their full maturity is due to the presence of specific substances closely related to (or perhaps identical with) carotene and xanthophyll; it is possible that these pigments are derivatives of chlorophyll. The coloration of mature berries in black and red grape cultivars results from the accumulation of a specific pigment within the cells of their skins, which, according to investigations by Willstätter, belongs to the anthocyanin group and represents a cyanidin monoglucoside. This author designates the anthocyanin of red grape cultivars as "enin." In the vast majority of grape cultivars, both white and black (excluding the so-called "dyers" — Teinturiers), the sap contained within the cells of the berry pulp is colorless and entirely translucent.

Recently, several enzymes, such as highly active invertase and oxidase, have been discovered in the sap of grape berries by numerous researchers. Undoubtedly, the biochemical processes occurring within the grape berry are largely mediated by the presence of various enzymes, but this subject remains completely uninvestigated at present.

During berry ripening, the content of ash elements in their sap increases, particularly potassium and phosphoric acid. However, a certain portion of potassium can precipitate in the form of insoluble salts, resulting in a slight decrease in the content of this element in the sap of completely mature grape berries. In general, the content of ash elements in grape juice fluctuates extremely sharply depending on the most diverse conditions. Thus, according to data by Kroemer, the total quantity of mineral substances in grape juice can range from 1.6 g to 10.2 g per 1 liter (averaging approximately 3.5 g per 1 liter); the content of individual ash elements in grape juice varies no less strongly. The primary ash elements in this case are: potassium (51–72% of pure ash), phosphoric acid (8–26%), calcium (2.9–12.7%), and magnesium (0.1–4.8%); sodium, iron, aluminum, manganese, chlorine, silicic acid, and traces of boric acid are also present in very small quantities.

All the physiological and biochemical processes reviewed above, which mediate grape berry ripening, can proceed with greater or lesser rapidity depending on external factors and the condition of the grapevine itself. The primary role in this regard undoubtedly belongs to temperature. It has long been observed that berry ripening initiates earlier and proceeds more rapidly in vineyards situated on southern slopes than on northern ones, and that the clusters located closest to the soil surface or generally to objects strongly heated by solar radiation (such as stone walls) always initiate ripening first; however, more precise experimental data on this matter are unavailable.

A very extensive study by Prof. Frolov-Bagreev, dedicated to elucidating the dependence of juice sugar and acid content in more than 200 grape cultivars on various meteorological elements, conducted by him over seven years on the grape collection of Magarach (southern coast of Crimea), demonstrated that the highest sugar content and the lowest acidity across all investigated cultivars were observed in those years when:

1. the mean daily air and soil surface temperatures were highest;
2. the mean daily number of hours of sunshine was greatest;
3. cloudiness and atmospheric precipitation were lowest.

However, in this study, the author did not aim to isolate from the general vegetative period of the grapevine that specific time interval corresponding strictly to the "ripening" of grape berries (from the initiation of ripening to their full maturity).

The influence of light on grape berry ripening is even less investigated. Certain experiments involving the artificial shading of clusters suggest that light exerts no noticeable influence on the progression of processes associated with grape berry ripening, since the latter attain nearly identical sugar and acid contents in the light and in darkness, provided that all other conditions (gas exchange, temperature, moisture, etc.) remain unchanged (Müller-Thurgau, Portele).

Since ripening berries accumulate very substantial quantities of sugar in their sap, they are naturally directly dependent on the assimilatory activity of the entire green leaf surface of the grapevine. All kinds of leaf damage that disrupt their normal functioning, and even more so the complete removal of a significant portion of leaves from the grapevine, affect berry sugar content. In northern viticultural regions, in years with unfavorable conditions for photosynthesis, the berries, although acquiring external signs of "maturity" — such as tissue softening and normal coloration — accumulate an insignificant amount of sugar and retain a very high acidity.

It is necessary to mention that during phenological observations, the mark of full (physiological) berry maturity is frequently made not on the basis of analytical data, but by the signs of seed ripening (lignification and coloration of their coats). In these cases, it is considered that seed ripening coincides chronologically with the maximum absolute sugar content in grape berries.

As investigations by Alexandrov and Makarevskaya indicate, during the period of berry ripening, vigorous accumulation of starch is observed within the tissues of the shoots, particularly in their lower zone. All medullary rays and the phloem are completely filled with starch. Starch grains attain their largest dimensions within the cells of the medullary rays; these grains are smaller in size and less numerous within the cells of the phloem.