In search of optimal grape maturity | Practical Winery & Vineyard Magazine

July/August 2001

By Linda Bisson
Department of Enology & Viticulture, UC Davis

Grape maturity can be defined as the physiological age of the berry on the vine. The berry functions to attract animals for dispersal of grape seeds. Dispersal is always critical, but even more so if the vine is under limiting growth conditions. This is the main reason that a certain amount of vine stress is beneficial for development of grape flavorants. Berry ripening is therefore tightly coordinated with seed development. Viticulturists have identified three stages of berry development following flowering: green berry; arrest of green berry development and pause before the onset of ripening; and veraison or ripening (Figure 1).[1]

From an enological perspective, veraison should be subdivided into different sub-stages based upon berry metabolism and the continued transport of substances to the vine (Figure 1). During veraison, water, sugars, and nitrogen compounds are transported to the berry via the phloem. Sucrose is hydrolyzed to glucose and fructose in the berry. Berry flavor and aroma compounds are synthesized within the berry. It is not clear whether the synthesis of these compounds is controlled by hormonal signals from the rest of the vine or occurs in the berry independently of other vine influences. Arrest of phloem transport and onset of dehydration are both expected to dramatically influence berry metabolic activities and composition. At some point, the synthesis of desirable enological characteristics ceases in the grape berry. This is the optimum time at which to harvest the fruit, prior to deterioration of berry characters. It is not clear what factors direct loss of berry flavorants and when this degradation is initiated. It is therefore important to define the optimal grape maturity for wine production and to develop clear chemical or biochemical traits that can be used to define the peak of ripeness. This article will survey our current understanding of maturity assessment. Clearly, further work is needed to define the parameters best associated with optimal ripeness of grapes for wine production. The definition of optimal maturity will vary depending upon the style of wine being made; the working definition of quality; varietal; rootstock; site; interaction of varietal, rootstock and site; seasonal specific factors; viticultural practices; and downstream processing events and goals. If a clear descriptive analysis of the quality target exists, then the time of harvest can be optimized to meet those goals. Several grape and cluster characteristics have been used to assess ripeness (Figure 2). There are, of course, other non-compositional factors that influence the decision to harvest, including labor availability; seasonal changes such as rainfall; heat waves; tank space limitations; and other factors beyond the winemaker’s control. But these will not be considered here.
Sugar is a component often used to assess ripeness. Sugar content increases during ripening and is therefore a function of berry age. Sugar is also relatively easy to assess, adding to its value as an index of ripeness. Sugar levels appear to be fairly uniform across the population of berries, meaning that the coefficient of variance is low (less than 10%), and thus the value at the press pan can be predicted with reasonable accuracy if appropriate vineyard sampling protocols are followed.[2] Variance is much greater if the fruit is not uniform across the clusters (as found with Zinfandel) or if the variation in cluster microenvironment is not correctly accounted for in the vineyard sampling protocol. However, several studies have shown no relationship between sugar levels and accumulation of grape berry flavorants. Thus, while sugar can indicate maturity level, it is not clear whether it is the best index of optimal maturity. Figure 3 shows a typical profile of changes in berry sugar composition during ripening. There is an initial rapid phase of sugar accumulation, but at some point during berry development and aging, the vine ceases transport of sugar to the fruit. Further increases in sugar concentration are due to dehydration.

Sometimes sugar accumulation will cease due to unfavorable environmental conditions — such as very high or low vineyard temperatures — but resume once the conditions have changed. It is important to be able to distinguish a transient effect from a permanent cessation of transport of materials via the phloem. Once phloem transport has ended, any further increases in sugar level will be due to loss of water, not continued synthesis and translocation of sugar. Assessing changes in berry weight, and noting the point at which average berry weight starts to decrease significantly while sugar content increases, can indicate the onset of dehydration. However, this can be quite difficult to monitor where fruit maturity is not uniform across the clusters, or where great differences in berry weight already exist across the population. That is, the variation in “normal” berry weight would obscure early detection of dehydration onset. Assessments of acidity are also used to define the optimal time of harvest. This can be evaluated as either pH or titratable acidity or both.[3] Changes in pH are complex and not necessarily a function of “berry age.” However, a historical index of ripeness suggests that optimal sugar / acidity balance is achieved if the product of the Brix value times the square of the pH is in the range of 220 to 260. For example, a 22� Brix juice at pH 3.2 would yield a value of 225.3. Late harvest fruit at a higher pH (24� Brix at pH 3.6, for example) would yield a value (311) outside of this range. Another scale relates titratable acidity and sugar level. In this case, the Brix value divided by the TA (g/100 ml tartaric acid equivalents) should yield a number around 30–32 for table wine production.[4] For a juice at 22� Brix with a TA value of 0.8, the number obtained would be 27.5. For a 24� Brix juice or must, the TA value could not drop much below 0.8 to meet this standard. Other authors suggest that this value can be higher (37–38) for late harvest fruit.[5,6] However, the sugar:acidity ratio is quite variable across different varieties and growing conditions, and these kinds of universal rules of thumb may be of little general predictive value for wine quality — especially if indiscriminately applied.[3] Further, it is not clear whether the optimal sugar (ethanol):acidity balance always coincides with optimal maturity of grape flavorants. From a survey of the literature, only the level of norisoprenoid flavorants appears to be correlated with sugar.[7] The norisopreniods are 13 carbon terpenoid molecules thought to be derived from the degradation of carotenoids.[7] In general, the 13 carbon norisoprenoid characters (grassy, tobacco, smoky, kerosene, tea, honey) are far more stable than the fruity components (red fruit notes). Changes in acidity level, as they reflect berry metabolic activities, may be useful to assess. It is well known that malate is consumed as an energy source in the berry during veraison, so malate levels decrease relative to tartrate (Figure 4).[1,3,5,6] Tartrate levels generally remain constant during veraison, but may rise slightly during grape dehydration. Malate levels decrease as the acid is consumed by the fruit, and seem to plateau at a low level, roughly 2 to 3 g/L.[5,6]

The grape may catabolize sugar if malate levels decline too much, but this varies dramatically by varietal. Malate levels may be quite high in some red varietals post alcoholic fermentation. The synthesis of many grape flavor and aroma compounds requires energy, but the factors leading to cessation of synthesis of berry flavorants have not been well defined. Attempts to use the ratio of malate to tartrate as an index of ripeness also suffer from a lack of correlation of this ratio with grape flavorant development. Further, the ratio itself is quite variable across different varieties and growing conditions and seasons, and is of little true predictive value.[3,5,6] One factor that has been reported in the literature as a useful indicator of berry status is arginine levels.[1] In theory, a decline in arginine content signals a deterioration of the fruit. However, arginine levels are so variable and subject to varietal and seasonal modification that this parameter is not always reliable.[1] The glutathione content of grapes also increases at the onset of veraison and during ripening, but it is not clear whether the level of this compound is correlated with flavor development.[8] A chemical marker of the onset of fruit (specifically flavorant) deterioration would be ideal from an enological perspective, however. Several studies have evaluated the potential use of various berry metabolites associated with varietal character. Increases in total phenolic content have been associated with maturity. In a principal component analysis of various ripening indices, phenolic content emerged as a key defining factor of grape maturity.[9] Anthocyanin levels have also been associated with maturity,[10] but dramatic effects of environmental and cultural conditions on anthocyanin pigment accumulation have been reported.[11] Malvidin-3-glucoside appeared to be unresponsive to growing conditions, with levels increasing as a function of maturity only.[11] An optical fiber probe capable of providing a rapid assessment of both total phenolic and anthocyanin content is being developed that might prove useful in the assessment of optimal maturity.[12] With all these factors making correlaton to maturity flavors and other compounds that are easily measurable difficult, it is generally accepted that optimal maturity can be assessed only by monitoring levels of grape flavorants themselves. However, all of these components are dramatically affected by growing conditions and viticultural practices. For example, bunch shading decreases the content of norisoprenoid glycoside conjugates (as does vine shading), but the effect of cluster microclimate exerts more of an influence than the vine environment.[14] Light exposure increases the levels of 2-methoxy-3-isopropyl and 2-methoxy-3-isobutyl pyrazines in unripe grapes, but it also catalyzes photodecomposition of these compounds in mature grapes.[15] Nitrogen and water availability also exert a strong impact on grape flavorant composition and timing of ripening.[16,17] Free and bound terpene levels have also been used to assess berry flavorant development and potential, but this relies on assessment of a single family of components. Many types of flavorants are present in the form of glycosidic precursors. Analysis of the total precursor level by assessment of the glycoside glucose (GG) content of the grapes may yield a more complete picture of the flavorant potential.[13] Pruning practices do not appear to affect free terpene content, but they dramatically impact levels of the glycoside terpene precursors.[18] In one study, total GG precursors were higher in the skins of minimally-pruned vines, but this did not correlate with overall wine quality.[19] Thus, any index of ripeness needs to be modified depending upon site-specific factors and cultural practices. Total protein content has been evaluated as a function of ripening, but found to be too variable. Specific proteins or their enzymatic activities have also been investigated, and some appear to be promising indices of berry physiological status. But this work is in its infancy.[8,20] Molecular tools are being developed at UC Davis and elsewhere that are defining critical protein changes in the berry during ripening. This work should yield a collection of proteins or enzymatic activities that are correlated with grape maturity. All of the factors noted above (sugar levels, acidity, pH, specific flavorants, metabolite levels, and proteins) change in the berry with time of ripening and are relatively easy to monitor. However, it is not clear whether they are truly related to each other or to any other types of fruit quality assessment. Tasting the fruit is obviously important in assessing its flavor and aroma components. A typical progression of character qualities described for red grapes is presented in Figure 5. This temporal pattern is consistent with my own experience as to the order in which different types of characters appear in the fruit, but not with the order in which they disappear.

In Cabernet, for example, sometimes at first appearance of jam there is no herbaceousness, while at other times it is still strongly present. This suggests that rates of deterioration of specific flavorants are not coupled to rates of appearance of others. It can be as important to taste for the absence of negative characters as it is to focus only on the appearance and nature of positive components. The loss of vegetation and unripe characters can be observed from tasting of the fruit or skins. Although berry taste is often regarded as the most accurate assessment of flavorant status, this can, in practice, be quite difficult to perform in an unbiased manner. The phenomena of flavors masking other flavors and the fact that many aroma compounds are present as glycosyl-glucose (GG) precursors released during fermentation and aging make tasting only a rough approximation of the flavor potential of the wine. Also, humans can differ markedly in their thresholds for detection of many compounds, so one can never be certain whether grapes will taste the same to different individuals. Rather than assess the presence of fruit complexity, it may be more advantageous to taste for the absence of characters associated with unripe fruit. Grape maturation can also be evaluated by assessing physical properties of the berry such as firmness and deformability.[21] Berry softening is due to changes in composition of cell walls of the fruit, particularly due to pectin and xyloglucan depolymerization [22,23] which accompanies arrest of xylem flow to the fruit.[24] Some winemakers taste seeds in order to assess grape maturity. However, seed bitterness may be overpowering, especially to super tasters. Many individuals may not be able to accurately discriminate levels of seed bitterness. Physical characteristics of the seeds — color, texture, and brittleness — may be more important indicators of seed and therefore berry maturity. It is common to look at seeds, waiting for them to turn from bright green to tan-brown and begin to dry or become woodier-looking and feeling as an assessment of maturity. This has some enological value also, since more mature seeds in a maceration tank will yield less bitter and harsh tannins if they happen to be broken on pumpover, or during pressing. Cluster stems can also be evaluated to assess berry ripeness. Stems undergo a change from green unripe to brown or ripe stems to overripe or brittle stems. These changes are varietal-specific. In some varieties, the stems never ripen beyond the green stage. Figure 6 presents the characters associated with different degrees of stem ripeness. It is believed that stem ripeness parallels berry maturity, but this has not been rigorously demonstrated. Stems can be tasted, as is the case with seeds. Depending on processing conditions, the presence of unripe stems may lead to extraction of undesirable components.
Tasting of berries can be fatiguing as well as subject to taster bias. Clearly the ideal method to assess optimal maturity would be numerical, and dependent upon the level of key or signature compounds of the varietal / style. Toward that end, assessments of phenolic compounds, anthocyanin content, and terpene (free and bound) levels have been proposed.[13] However, it is not clear how any of these individual characters correlates with overall grape quality. Most berry flavorants are likely synthesized independently of each other in the berry, and high levels of one are not necessarily correlated with high levels of another. Synthesis of most flavorant molecules varies dramatically with the season and vineyard practices. There are several techniques that allow for global compositional profiling of the fruit. Techniques such as solid phase microextraction (SPME) have great potential for quantitative assessment of a variety of chemical aroma compounds in wine,[20] particularly if coupled to enzymatic treatments to release the aroma potential of wine from precursor compounds. This technique is more direct and easier to perform than many other analyses. It does not require extensive sample processing or extraction or modification of components to be analyzed.[20] If the optimal aroma composition of fruit at harvest can be defined, SPME can be a useful tool for routine assessment of optimal maturity. It is equally imperative to have an index of the cessation of flavor and aroma development in the fruit. Grape flavorants display different rates of loss in the fruit while on the vine. Loss of the red fruit characters in Grenache can be quite dramatic following rainfall late in berry ripening, while other characters are more stable. An understanding of factors leading to rapid loss of optimal maturity is consequently critical as well. In addition to effects of on-the-vine berry aging on flavor and aroma compounds, it is clear that the microbial flora of the fruit also changes during maturation. Our experience with Grenache indicates an increase in problems associated with “bad” lactics (off-flavors, arrest of yeast fermentation) at higher maturity, as assessed by sugar levels. Since the grape microbial flora has a strong impact on wine composition, it is important to develop rapid and reliable tools for the assessment of berry flora. This may be partly due to higher pH values at higher levels of sugar maturity, so undesirable bacteria are encouraged.
Conclusion The criteria for optimal maturity are multi-faceted. Several important classes of compounds change during ripening and maturation of the fruit on the vine. These characters do not change in a highly coordinated fashion, and instead suggest a series of independently regulated pathways of synthesis. Each pathway is impacted by seasonal factors and vineyard practices, and the effect varies by varietal. Simultaneous analysis of all pertinent quality factors may be prohibitive both time-wise and economically. Vineyard or site-specific indices of optimal ripeness may need to be developed. It is also important to correlate grape composition with finished wine composition. Many flavor and aroma components are present in a precursor or undetectable form. These compounds can be hydrolyzed, becoming detectable during fermentation and aging. It is critical to achieve an understanding of the relationship between grape flavorants and wine quality. New analytical techniques are being developed that should provide great assistance in future assessment of optimal maturity. However, it is unlikely that any single index of maturity will be discovered that can be indiscriminately applied in all growing conditions and to all varietals. Historical experience with specific vineyards and growing regions will continue to be a critical factor in determining the optimal maturity of the fruit. References 1. Jackson, D., and P.B. Lombard. “Environmental and management practices affecting grape composition and wine quality: A review.” Am. J. Enol. Vitic. 44: 409-430 (1993). 2. Boulton, R.B., personal communication. 3. Boulton, R.B., V.L. Singleton, L.F. Bisson, and R. E. Kunkee. Principles and Practices of Winemaking, Chapman and Hall, New York, 604 pp. (1996). 4. Gallander, J.F. “Effect of grape maturity on the composition and quality of Ohio Vidal blanc wines.” Am. J. Enol. Vitic. 34: 139-141 (1983). 5. Amerine, M.A., H.W. Berg, R.E. Kunkee, C.S. Ough, V.L. Singleton, and A.D. Webb. The Technology of Wine Making, 4th Edition, AVI Publishing Company, Inc., Westport, Conn. 794 pp. (1980). 6. Amerine, M.A., and M.A. Joslyn. Table Wines, 2nd Edition, University of California Press, Berkeley, Calif. 997 pp. (1970). 7. Strauss, C.R., B. Wilson, R. Anderson, and P.J. Williams. “Development of precursors of 13-carbon norisoprenoid flavorants in Riesling grapes.” Am. J. Enol. Vitic. 38: 23-27 (1987). 8. Okuda, T., and K. Yokotsuka. “Levels of glutathione and activities of related enzymes during ripening of Koshu and Cabernet Sauvignon grapes during winemaking.” Am. J. Enol. Vitic. 50: 264-270 (1999). 9. Gonzalez-San Jose, M.L., L.J.R. Barron, B. Junquera, and M. Robredo. “Application of principal component analysis to ripening indices for wine grapes.” J. Food Comp. Anal. 4: 245-255 (1991). 10. Gonzalez-San Jose, M.L., L.J.R. Barron, and C. Diez. “Evolution of anthocyanins during maturation of Tempranillo grape cultivar (Vitis vinifera) using polynomial regression models.” J. Sci. Food Agric. 51: 337-344 (1990). 11. Keller, M., and G. Hrazdina. “Interaction of nitrogen availability during bloom and light intensity during veraison: II. Effects on anthocyanin and phenolic development during grape ripening.” Am. J. Enol. Vitic. 49: 341-349 (1998). 12. Celotti, E., G. Carcereri de Prati, N. Macri, M. Trevisi, and R. Zironi. “A new objective evaluation system of the red grape pheonolic quality by color measurement.” Proceedings of the 6th International Symposium on Innovations in Wine Technology, Stuttgart, Germany. pp 152-163 (2001). 13. Williams, P.J., and I.L. Francis. “Wine flavor research: Experiences from the past offer a guide to the future.” Proceedings of the ASEV 50th Anniversary Annual Meeting, Seattle, American Society for Enology & Viticulture, Davis, CA. pp. 191-195 (2000). 14. Bureau, S.M., R.L. Baumes, and A. Razungles. “Effects of vine or bunch shading on the glycosylated flavor precursors in grapes of Vitic vinifera L. cv. Syrah.” J. Agric. Food Chem. 48: 1290-1297 (2000). 15. Hashizume, K., and T. Samuta. “Grape maturity and light exposure affect berry methoxypyrazine concentration.” Am. J. Enol. Vitic. 50: 194-198 (1999). 16. Keller, M., K. Arnink, and G. Hrazdina. “Interaction of nitrogen availability during bloom and light intensity during veraison: I. Effects on grapevine growth, fruit development and ripening.” Am. J. Enol. Vitic. 49: 333-340 (1998). 17. Sipiora, M., and M.-J.G. Granda. “Effects of pre-veraison irrigation cutoff and skin contact time on the composition, color and phenolic content of young Cabernet Sauvignon wines in Spain.” Am. J. Enol. Vitic. 49: 152-162 (1998). 18. McCarthy, M.G. “Clonal and pruning effects on Muscat a petite grains blanc yield and terpene concentration.” Am. J. Enol. Vitic. 43: 149-152 (1992). 19. Werwitzke, U., S. Kraml, D. Rauhut, O. Lohnertz, W. Bettner, and H. R. Schultz. “Effect of canopy systems on the concentration and distribution of glycosyl-glucose (GG) in Riesling berries (Vitis vinifera L.)” Proceedings of the 6th International Symposium on Innovations in Wine Technology, Stuttgart, Germany. pp 67-75 (2001). 20. Ebeler, S. “Analytical chemistry: Unlocking the secrets of wine flavor.” Food Rev. Internat. 17: 1-20 (2001).