For instance, seasonal variation accounts for an almost two-fold range in sugar concentration, a two- to threefold variation in acid and anthocyanin content, and at least ten-fold range in aroma components.2,3,8,35,42 The differences in anthocyanin profiles -- that is the relative proportion of individual pigments for a given cultivar -- varies more among years than from veraison to harvest in the same year.27 These differences hold true even at the genetic level: differences in gene expression in grape berries between growing seasons can be greater than those between pre- and post-veraison berries within a season.26

Cultural practices can, at best, be used to fine-tune what nature imposes in any given season. The same applies to clones of a cultivar and the rootstock used as grafting partner: season and site still vastly outweigh the influence of these on fruit composition, although rootstocks may lead to considerable differences in yield.33 Even grapes harvested at identical concentrations of soluble solids in different years can have very different amounts of anthocyanins and aroma-active compounds.

For a specific genotype, variation in grape composition occurs within one vine, between vines, between vineyards, and between growing seasons due to, among others (e.g. some effects of pathogen infection are discussed in Chapter 7.5), the factors discussed in the following sections (also see Jackson, 2008).

As argued in Chapter 2.3, the accumulation of color and aroma volatiles serves as “advertisement” to signal to potential seed vectors the availability of food material of high nutritional and health value. In fact, the majority of pigments and volatiles are manufactured from nutritionally valuable components such as sugars, amino and fatty acids, and carotenoids, and thus serve as positive nutritional signals.13

Many of the associated changes during grape ripening were discussed previously for individual compounds or specific groups of compounds. In addition to the well-known increase in the concentration of sugars, amino acids, and phenolic compounds (especially anthocyanins in red grapes) and the decrease in acidity (with a corresponding rise in pH), flavor and aroma components also undergo changes as grapes ripen.

For instance, methoxypyrazines decline most rapidly during the early ripening phase,22,31 while norisoprenoids and most other terpenes, or their glycosylated aroma precursors, appear to continue to increase even at very advanced stages of maturity (above 30° Brix).11,14 Such increases during extended ripening or “hang time” of the grapes form the basis for delaying harvest to maximize terpenoid- based aroma compounds.39 While the concentration of some volatile phenols (guaiacol) increases strongly during maturation, perhaps due to conversion of hydroxycinnamic acids, others (eugenol) seem to remain constant or even decrease.11

The precursors of some aroma-active esters also appear to decrease over time (resulting in wines with less “fruity” aroma), but the majority show no consistent trend. It is not clear which changes late in the growing season are indeed due to the continued production or modification of these quality-relevant compounds in the grape berries or simply brought about by the slow dehydration of the berries (concentration effect, weather permitting).

Moreover, physical and chemical changes in fruit composition also lead to changes in the number and composition of microorganisms that call the fruit home. As the sugar concentration and the pH increase, both the number and the diversity of this microflora increase. Yet the consequences of these population dynamics for grape juice and wine composition are not well understood and little appreciated.

Clearly, some of the alterations in fruit composition with increasing grape maturity are desirable (e.g. more sugar, less acid and bitter tannin, more “good” and less “bad” flavors). Whereas others are not (e.g. even more sugar, high pH, and less “good” and more “bad” flavors). Moreover, the desired level of fruit maturity depends on the intended use of the grapes.

Taking soluble solids as a simple measure of maturity, table and juice grapes may be harvested when they have accumulated just 16° to 18° Brix, which is immediately after they have fully turned color. Grapes destined for sparkling wine production are considered optimally ripe at approximately 18° to 20° Brix.

While the typical harvest “window” for many white wine grapes ranges from 20° to 24° Brix, red wine grapes are often left on the vines well beyond the physiological sugar maximum of 24° to 25° Brix – weather permitting. The factors described in the following sections exert much of their influence on fruit composition by accelerating or retarding grape ripening and hence maturity.

In a study conducted over four years with 80 individual Riesling vines, the between-vine variation in yield was about five-fold higher than that in either juice soluble solids or titratable acidity.12 A meta-analysis of Riesling clonal trials conducted at 16 locations over 37 years found that yield and soluble solids both increased over time, whereas titratable acidity decreased.23 Much of the variation in yield was attributed to site effects, whereas the change in fruit composition was clearly linked to the rise in average temperature over the same period.

Although yields and mean temperatures continued to vary considerably from year to year, the variation in fruit composition declined over time, indicating that composition was not greatly affected by yield and that improved vineyard management contributed to the change in fruit composition.

It is not so much the crop size or yield per se that is important but, rather, the crop load, which is a reflection of a vine’s sink:source ratio,1,18,32 (also see Chapter 6.1). For instance, an increase in vine spacing is typically associated with a higher yield per vine, but vine size, and thus leaf area per vine, also increases, so fruit composition and wine quality may be completely unaffected.41

Conversely, when a higher planting density, which is associated with lower yield per vine, leads to competition among shoots for sunlight, the fruit may have higher titratable acidity while, at the same time, the juice pH is also higher,10 possibly due to recirculation of potassium from shaded, aging leaves to the clusters.

As a general rule, a leaf area of 10 to 15 cm2 is required to fully ripen one gram of fruit, and this normally results in a yield:pruning weight ratio of approximately 5 to 10.21 If the crop load is lower than this (i.e. undercropping or sink limitation), then the vine will invest comparatively more resources in vegetative growth, which can, in extreme cases, reduce fruit quality via the follow-on effects of a dense canopy. Moreover, berry size may increase to compensate for the low number of berries relative to leaf area.20,32

Conversely, if there is insufficient leaf area to ripen the fruit (i.e. high crop load, overcropping, or source limitation), then the rate of ripening will decline. This is why vines of medium vigor often produce both higher yield and better-quality fruit than vines at either end of the vigor spectrum. Such vines are said to be balanced – that is, the crop size matches their vegetative growth and leaf area development.