Vol. 11, No. 5 Sept. - Oct., 1996
Department of Food Science & Technology
VPI & SU
Blacksburg, VA 24061-0418
Table of Contents
I. Optimizing Quality 1
II. Our Research 2
I. Optimizing Quality
The blend of variety, climate, exposure, soil and time from bloom to maturity produces distinctive wine character and quality. Only in exceptional years do these factors aline to produce truly great wines. Grapes with desirable concentrations of secondary metabolites (aroma, flavor and phenols) at harvest creates note worthy vintages and represents the influence of reproductive and viticultural factors. Important viticulture elements include: site selection, steps to influence berry size including regulation to the water supply, canopy management practices which increase the light exposure of both the leaves and fruit, and the regulation of the leaf area to crop ratio (Hardie et al., 1996).
High quality wine is the result of the confluence of important fruit attributes (particular secondary grape metabolites) depicted in figure 1. Picking decisions must be based on an understanding of these factors and how they affect wine quality and style. For reds, it is important to evaluate grape maturity based upon the following (see Zoecklein et al., 1995 for additional information):
1. Aromas and flavors-green herbaceous to fruit jam
2. Texture, skin tannins (phenols)-polymerization to increase suppleness
3. Skin extractablity-degree of berry softness
4. Seed ripeness-ripeness of seeds and degrees Brix development are not the same. Unripe seeds increase the 'load' of hard or harsh tannins.
Considerable research has been conducted on the effects of canopy management on grape composition (including secondary metabolites) , the majority of which suggests that excessive shading may produce unbalanced musts resulting in poor wine quality.
While reports have indicated that sun-exposed clusters produce wines with higher sensory scores, excessive exposure of fruit produces undesirable aromas. Therefore, practices to increase wine quality by enhanced bunch exposure due to leaf removal must be considered vineyard by vineyard, depending on the history of canopy exposure and wine quality (Jackson and Lombard, 1993).
As discussed in previous issues, grape aroma compounds are present as free volatiles, which may contribute directly to odor, or as bound sugar conjugates, which are nonvolatile aroma precursors. Conjugates (mainly glycosides) can undergo acid or enzyme hydrolysis releasing free volatiles and potentially enhancing aroma. Thus, the pool of conjugates represents, in part, the potential aroma of a grape variety. Glycosidically bound volatile compounds have been shown to make significant contributions to varietal wine flavor. Indeed, it appears that the higher the concentration of these bound aroma/flavor precursors the greater is the quality of the resultant wine.
II. Our Research
We are involved in several studies evaluating the effects of vineyard management on aroma and flavor. The following is a summary of one study titled-Effects of Fruit Zone Leaf Removal on glycoconjugates and selected aglycones of Cabernet Sauvignon (Vitis vinifera L.) grapes grown on a divided and non-divided training system.
Selective fruit zone leaf removal was evaluated for two seasons as a means of influencing glycoconjugates. Two to four leaves per shoot were removed three weeks post-bloom from around fruit clusters on vines grown on a divided canopy (Lyre) and a mid-wire bilateral cordon system. Leaf removal increased fruit zone porosity as measured by the percentage of sunlight penetration into the canopy and by point quadrant analysis. The pool of total glycosides and the glycoside per gram of fruit weight were generally higher in the fruit from leaf-pulled vines. The data for one season is discussed below.
Leaf removal influenced oBrix at only 1 of 8 sampling dates. Several other defoliation studies have reported reductions in oBrix while increases have also been noted. Increases in oBrix have also been reported due to a lower water content of sun exposed berries.
Fruit pH response appears to be a function of leaf vs. cluster shading. In this study, fruit pH was unaffected by leaf removal as was titratable acidity. Clusters exposed to solar radiation have low malic acid concentrations compared to shaded clusters. The tartaric/ malic acid ratio was influenced by leaf removal in only three of eight sampling dates (one for the divided canopy and two for the non divided).
The total glycosides per gram of fruit weight increased with maturity (Brix) for both the leaf pulled and control fruit (Figure 2 and 3). The increase in glycosides as a result of leaf pulling was greater in the non divided vs the divided canopy. This illustrates an important point. Despite the widespread use of selective leaf removal, not all studies have reported improvements in grape composition. Basal leaf removal of canopies with low fruit zone light exposure increases the photosynthetic activity of the remaining leaves and fruit sugar. However, these changes are not found if the initial canopy interior sunlight level is high, such as occurred with the divided canopy.
(Fig. 2 & 3 here)
Berry weight was generally not influenced by treatment. Therefore, differences reflect mainly variation in glycoside content. Grape glycosides are in greater concentration in the skin than pulp, a factor which makes the analysis on a per gram basis significant. An ideal berry for most purposes is one of small to medium size (less than 1 g), which could provide an adequate content of secondary metabolites in the skin (Hardie et al., 1996). Fruit weight per vine generally did not differ between treatments, although there was a difference in weight per vine and leaf area per fruit weight ratio between the divided and non-divided systems. In previous research we demonstrated that crop level can effect glycoside concentration, possibly as a result of delayed maturity. Therefore, direct comparisons of the two training systems is difficult.
Grapevine modification studies which increase solar radiation into the canopy also report to increase photosynthetic efficiency of the remaining leaves possibly influencing secondary metabolites. Possible increases in photosynthesis were not consistently reflected in increased oBrix or sugar per berry in this study. Leaf removal generally increased the average sunlight of canopy fruiting zones, although these differences were not large. There is ample evidence that direct stimulus of the fruit by light can affect the content of secondary metabolites such as glycosides.
Glycoconjugates may also be influenced by changes in the pattern of assimilate movement. The increase in bound aroma components noted in this study may be the result of reducing both cluster shading and intra-plant competition through removal of parasitic sinks. Older basal leaves, which were removed, have photosynthesis rates 1/3 of recently expanded leaves. Shaded leaves on the basal portion of grapevine shoots do not contribute to either yield or fruit soluble solids. Research has demonstrated that various treatments, including the removal of basal leaves alters the pattern of assimilate movement toward the clusters. It seems apparent from this research and the research of others that there exists a relationship among the viticultural parameters of canopy manipulation, photosynthesis and canopy light radiation and the production of grape glycosides. Region and macroclimate will dictate which vineyard practices to follow to enhance wine quality.