BERRY SIZE AND YIELD PARADIGMS ON GRAPES AND WINES QUALITY

A key objective of this review is to open a discussion on what we really know about the effect of berry size and yield on grape and wine quality. Even if berry size and crop yield are widely recognized as important factors in the quality of resultant wines, most prior research has shown no effect of yield on wine quality, and the direct effect of berry size has not been evaluated. Recently, some pruning, thinning and irrigation experiments were done to test independently for relationships due to size and yield compared with those due to the normal cultural practices used to control yield (thinning, pruning and irrigation). Data from these experiments have clearly shown that these considerated practices affected in an independent way berry size and yield producing grape and wine of different quality. This renders the generalizations asserted in both of the paradigms unfounded. We draw these conclusions about the dependence of composition on yield and berry size: the viticultural practices used to control yield in a vineyard may be more important than the yield or berry size values per se in determining the quality of the resulting grapes and wines. REVISITING PARADIGMS OF WINEGRAPE QUALITY There is good evidence that viticulture and winemaking go back almost as far as civilization itself, perhaps earlier than 5,500 B.C. (McGovern, 2004). Regardless of the specific era of their emergence, it is clear that these endeavors have been with us for a very long time. From that history, much remains as tradition and belief. Indeed, most of what we do as viticulturists and winemakers is attributable to historical and traditional causes. Mixed up in there are probably some mistaken explanations for why certain traditions or practices are in place. This is inevitable. Humans (their minds) make things up to explain to themselves what they experience and ‘the nature of things’. Some long standing viticulture practices and explanations may have arisen out of superstition, mysticism, trial and error, the authority of others, or tradition. Tradition is comfortable, comforting, and effective for marketing. But, tradition is the lack of change. Science is, by contrast, modern – originating from Galileo or Copernicus, about 400 years ago – and direct to change or new understanding. Science, which necessarily involves measurement and usually experimentation, is by any account a remarkably successful way of understanding the natural world. The history, from the Age of Enlightenment, of science in agriculture reflects much progress in our understanding of how to cultivate plants for our sustenance and satisfaction. Yet, several paradigms of viticulture appear to have little basis in a science of experimentation and measurement. This paper addresses two aspects of what we consider received knowledge in viticulture: berry size and crop yield. In particular, it was assumed that small berries and/or lower yield produce higher quality wines and quasi-science has added putative explanations for the putative observations: Exhibit A: Small berries make better wine – due to higher surface:volume; Exhibit B: High yield means low quality as flavor compounds get diluted. Can science address these received beliefs? Surprisingly little data have been generated. It is easy to change and measure berry size or yield. It is less easy difficult to Proc. Intl. WS on Grapevine Eds. V. Nuzzo et al. Acta Hort. 754, ISHS 2007 424 evaluate their contributions to wine composition and wine sensory attributes. Thus, the truth about wine quality is difficult to discover. It is of up most importance to note that this difficulty is just as present for the traditional explanation as it is for any new relationships developed by experimentation. Can science asses how much yield or berry size affects quality? This quantitative information is fundamentally important to production decisions: if the change in wine quality with increasing yield (or berry size) is in fact negative but measurably so only at yields (or at berry sizes) above some threshold (Fig. 1 curve c), producers can exploit this knowledge to produce high quality wines over a range of yields (or berry sizes) not to exceed some value. Alternatively, if the yield (berry size) response is as shown in curves a or b of Figure 1, yields (berry sizes) above some intermediate value will all produce low quality wine, and only the wine with the lowest yield (berry size) will be best. Moreover, sensory science is a new science, among sciences. It is difficult because the subjects are humans, who make things up to explain their sensory inputs. Wine quality scales are necessarily dependent upon the ‘judges' interpretation of quality. Nevertheless, there has been much progress in developing objective metrics of the sensory experience. A more fundamental and scientifically accessible question is: what sensory attributes are reproducibly detectable as a consequence of variation in berry size and yield? BERRY SIZE Berry size is widely recognized as an important factor determining winegrape quality. However, this concept has gained acceptance primarily on the basis of intuition and implicit assumptions with little experimental evidence. Singleton (1972) estimated the role of berry size by removing and adding juice at crushing. However, there were no measurements conducted to indicate that the juice/solids was greater in larger berries. In its most simplistic form, the relationship, in which larger berries are less desirable, is described as a consequence of water diluting solutes of importance for wine flavor (Fig. 2a). This explanation misses important physiological factors of berry size. First, berries do not grow by ‘pumping’ water into a vessel (berry) of flavor solutes. Berries attain size via a double sigmoid growth habit. Growth is a complex process in which dry weight (cell wall, membrane, solutes, etc.) increases as well as fresh weight (Fig. 2b). Water flux into cells occurs because the cell sap accumulates solutes creating a water potential gradient down which water flows. The solutes in berry cells are translocated into the berry or synthesized from translocated solutes. The total amount of solute present in the berry is dynamic and generally increasing during growth and development. For the solutes to be diluted would require that water influx exceed solute influx. It is clear that plant tissues, and perhaps fleshy fruit like berries more than others like woody stems, act as capacitors for water on a daily basis. From this it does naturally follow that fruit harvested in the afternoon will contain less solvent water than the same fruit harvested at dawn. However, in the grape berry, this capacitance is dramatically reduced after veraison (Greenspan et al., 1994, 1996). FLESH SOLUTES: ACCUMULATION OF SUGARS The sugars in ripe berries are present at high concentrations in the flesh, and are not localized in the skin (Coombe et al., 1987; Possner and Kliewer, 1985). Thus, if there were a constant amount of sugar per berry, the concentration of sugars would decrease in a linear fashion as suggested in Figure 1, curve a. Matthews and colleagues collected about 1,800 Cabernet Sauvignon berries, which were sorted into 6 size categories, separated into skin, seed, and flesh, and analyzed for composition. Berry mass varied from about 0.4 to 1.6 g. The amount of sugar per berry was not constant, but increased linearly with berry size (Fig. 3). The strong correlation (r=0.99) indicates that the appropriate generalization is that sugar content is proportional to size; not that sugar content is constant (Roby et al., 2004). When the concentration (Brix) was plotted vs. size, a negative relationship was obtained, although the slope was rather shallow with a 10% decrease in Brix over the 4