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Grape ripeness and wine style of Shiraz
Kobus Hunter, Montse Nadal & Neels Volschenk
Key words: Shiraz, ripeness, phenolics, tannins, grape composition, wine quality, wine style
Introduction
Initial long-term cultivation choices and technological interventions by growers during the lifespan of a vineyard are steered by wine quality/style requirements, as dictated by the market (Hunter et al. 2004) (Fig. 1). The physiological functioning of the vine and the extent to which requirements are met, is an integrated expression of the terroir conditions and cultivation practices (Fig. 2). The vegetative and reproductive growth patterns of the grapevine may have differential and determining effects on grape and wine quality per se, but also on the length of the ripening period (harvesting window), thereby impacting on the level of ripeness achieved and the potential for different styles of wine within a particular terroir (Ojeda et al. 2002; Hunter et al. 2004; Hunter & Deloire 2005).
The uniformity of a vineyard sets the baseline for controlling ripening and harvesting windows for wine styles and will always be displayed in canopy quality (Fig. 3). The key words for judging canopy quality are uniformity, sufficiency and efficiency of both leaf mass and grapes (Fig. 4). A simplified scheme showing the importance of the canopy (via photosynthesis and the production of primary compounds) for the formation of various groups of secondary compounds inside the berries, is shown in Fig. 5. The microclimate and composition of the canopy (Fig. 6) are extremely important to increase the output of the canopy (sucrose production, water use efficiency) as a whole and to realise the full potential of the grapes (Figs. 7 & 8) (Hunter et al., 2004). The canopy is generally required to continue to satisfy the demands from the ripening berries undergoing physical and biochemical changes during the ripening period (Figs. 9 & 10) (Hunter et al., 2004). If this requirement is not met and stress factors (e.g. originating from soil type, soil variation, soil preparation, plant material quality, planting, cultivation practices and/or climate) result in non-uniform growth and premature senescence of the canopy, leading to non-synchronised progressive development of the canopy and grapes, grape composition will be one-dimensional and unbalanced and inferior wine quality will result.
Phenols and related compounds can affect the appearance, taste, mouth feel, flavour and antimicrobial properties of wine. The total berry phenolic concentration slowly increased during ripening, the level of which depended on variety and climatic conditions (Vivas de Gaulejac et al. 2001; Habertson & Adams 2002). Tannin concentration mainly decreased during ripening, but on a whole berry basis the tannin content increased at first and stabilised there after (Kennedy et al. 2001; Ojeda et al. 2002; Valls 2004). Bunch and berry characteristics, amount and molecular structure of the phenolics and anthocyanins, must composition, and extraction conditions, affect the concentration and stability of the colour, and the astringency and tannin structure of the wine as well as its ageing potential (Zoecklein 1991; Di Stefano et al. 1994; Mazza 1995; Vivas de Gaulejac et al. 2001). Wine colour is mainly due to copigmentation of anthocyanins with, amongst others, catechins and procyanidins, having a stabilizing effect on the colour by protecting anthocyanins from oxidation and modification. Longer maceration periods with longer seed and skin contact, pH, SO2, and ethanol content of the must, as well as temperature and duration of fermentation, may all affect phenol extraction and expression.
Our research was, and still is, aimed at determining grape ripeness levels/ranges for the making of top quality, but different style, wines with unique characteristics; this necessitates the quantification of changes during the grape ripening process and an integration of canopy quality, grape quality, and grape and wine style. Some results from collaborative studies done in South Africa and Spain (Catalonia) are presented. This forms part of a more extensive study (Hunter et al., 2004) as well as other related studies (Hunter & Deloire 2005; Cloete et al. 2005; Nadal et al. 2005; Pisciotta et al. 2005; Deloire & Hunter 2005; Varvaro et al. 2005; Deloire et al. 2005a; Deloire et al. 2005b; Hunter & Deloire 2006; Nadal & Hunter 2007).
Vineyard (South Africa): A seven-year-old Shiraz/R99 vineyard, situated in the Stellenbosch region (South Africa), was used during the 2002 - 2004 growth seasons. Vines are cordon trained (7-wire) and spaced 2.75 x 1.5 m in north-south orientated rows on a medium potential, clayey, Glenrosa soil and a west-facing slope of approximately 26 degrees. Microsprinkler-irrigation was applied at pea berry size and at vèraison stages (12 hours @ 32L/hour). Canopies were vertically trellised, suckered, shoot-positioned and topped, whereas leaves were randomly removed in the lower half of the canopy at two stages, i.e. berry set and pea size (Hunter 2000). Fortnightly sampling was done from berry set up to two weeks post-véraison, after which harvesting for grape analyses, wine making and wine analyses was done approximately every four days (from 17 February to 24 March), obtaining eight levels of ripeness.
Vineyard (Spain): Shiraz/R110 vineyards, situated in the high humidity, maritime influenced Tarragona (14 years old on a loamy-clay soil) and the dry, low humidity Priorat (18 years old on a Schist soil) regions were used during the 2004 growth season. Vineyards were non-irrigated, cordon trained and vertically shoot positioned. Berry sampling started two weeks after véraison. Grapes at two levels of ripeness were vinified for each terroir.
Analyses and winemaking (South Africa): Whole berries, skins, seeds, pomace and wine were analysed at each ripeness level. Grapes of all harvests were cooled overnight to 20 ºC before processing. Grapes were destemmed, crushed and the pomace inoculated with commercial yeast (VIN 13) in 60 L tanks. Alcoholic fermentation took place at a controlled temperature of 24 ºC (di-ammonium phosphate and SO2 were added). Skins were pushed through three times per day. Fermentation on the skins averaged five days, after which the pomace was pressed. Skins and juice were analysed for anthocyanins, tannins, total phenolics (A280) and colour density (A520 + 420) (Ribéreau-Gayon et al. 2000) on the first, second and fourth day during fermentation. On the fifth day after crushing (at pressing), seeds were analysed for proanthocyanidins by the DMAC method (that determines catechins and oligomers) (Vivas et al. 1994). The same analysis was performed on the skins and juice (wine). Proanthocyanidin content was also determined in the seeds from intact berries. Total must soluble solids, titratable acidity, and pH were analysed according to standard methods, whereas anthocyanins, tannins and phenolics were analysed in whole berries and wines according to Ribéreau-Gayon et al. (2000). The degree of alcohol, total phenolics (A280) and colour density (A520 + 420) were also determined in the different wines after bottling.
Analyses and winemaking (Spain): Whole berries, skins and pulp were analysed at each ripeness level. During the ripening process, berry mass, total must soluble solids, titratable acidity, and pH were determined according to standard methods, whereas anthocyanins, tannins and phenolics were analysed in the whole berries (total and extractable anthocyanins) and wines according to Ribéreau-Gayon et al. (2000). Grapes were crushed and the pomace inoculated with commercial yeast in 100 L tanks. Skins were pushed through two times per day. Alcoholic fermentation on the skins averaged 10 days, after which the pomace was pressed. The degree of alcohol, total phenolics (A280), colour density (A520 + A420), proanthocyanidin content (DMAC - Vivas et al. 1994), and total anthocyanins, total tannins, and different indexes (HCL index, Ethanol index, Gelatine index - according to Ribéreau-Gayon et al. 2000) were determined in the different wines.
Results and Discussion
South Africa
The °Balling of the berries reached a high at approximately 11 March (178 days after bud burst), coinciding with the reduction in berry size due to water loss (Fig. 11). The reduction in berry size corresponded with phenol changes in the skins (after 5 days of fermentation) whereas the soluble solid content pattern corresponded to changes in anthocyanin, tannin and total phenolic contents of the whole berry (whole berry extraction) (Fig. 12). Skins lost less colour at the beginning of the maturation period than towards the end. During the last three harvests (after 11 March), extraction of the different phenolic compounds seemed not to be favoured by a higher skin:pulp ratio as a result of the decrease in berry size. Tannins were increasingly extracted from the seeds (values after pressing) with progressive ripening of the berries, coinciding with an increase in the degree of tannin polymerisation and in wine tannin (Fig. 13). This was also clear from the change in wine tannin during fermentation, resulting in two clearly distinguishable groups according to ripeness level of the grapes (Fig. 14). The decrease in phenolic content at the end of fermentation (for all ripeness levels) may be due to different combinations or polymerisations which stabilise the wine and which are not readily analysed (Singleton & Trousdale 1992; Di Stefano et al. 1994; Mayen et al. 1994). The colour density and total phenolics in the bottled wine clearly corresponded with the trends found when whole berries were extracted, but were opposite to the trends found in skins (Fig. 15). Both berry size and release from skins therefore contributed to wine colour and phenolic content.
Wine tasting showed at least four main wine styles: Style 1: Wines with light colour, herbaceous flavours, high acidity, and astringency with little structure, Style 2: Reasonably balanced wines with less acidity and herbaceous flavour and with more colour, fruitiness, body and structure, Style 3: Full-bodied, fruity and complex wines without herbaceousness and with ripe tannins and good colour (comparable to style 4 in Fig. 16), and Style 4: Full-bodied wines which lack structure and with jammy, over-ripe flavours and harsh tannins (comparable to style 5 in Fig. 16). The Style 3 wines were preferred by the tasting panel. The different styles corresponded to grape composition parameters and ratios determined in a simultaneous study at the time (Hunter et al. 2004) and in which five wine styles were evident (Fig. 16). If ratios of 0B:Titratable acidity are considered, these styles would correspond to ratios of < 2.0, < 3.0, < 4.0, < 5.5 (preferred) and > 5.5 (Fig. 16).
Spain
Similar to results found in South Africa, berry size was reduced during the last stages of the ripening periods in both Priorat and Tarragona (Figs 17 & 18). Growth conditions clearly impacted to a large extent on berry size; the berry sizes reached in the Tarragona region being larger, already at véraison. This also affected the soluble solid concentrations. Lower soluble solid concentrations were reached in grapes of the Tarragona region, despite the ostensible delay in initial soluble solid accumulation in the Priorat region. Given the differences in terroir (Priorat being dry, low humidity climate and vines grown on Schist soils and Tarragona being high humidity, maritime influenced and vines grown on loamy-clay soil), the reaction of the vines in the different regions and the impact on the grapes may have been predicted. In the berries from the Tarragona region, total and extractable anthocyanin contents stayed at the same level during the two harvests (47 & 52 days after véraison), whereas in berries from the Priorat region, anthocyanin contents continued to increase with further ripening (from 45 to 58 days after véraison) and were generally much higher than those found in the Tarragona region. Similar results were found for total phenol contents (data not shown). The higher soluble solid concentrations in grapes from the priorat region resulted in higher wine alcohol contents, whereas the wine colour intensity, and total phenol, tannin monomer as well as polymerised tannin contents increased (Table 1).
The wine sensorial quality data showed that wines from the second harvests in both regions were more concentrated with less astringency; these wines generally scored higher for all characteristics evaluated (Figs. 19 & 20). Four different styles of wines could be distinguished: Style 1: Wines with low concentration and flavour (Tarragona, Ripeness level 1), Style 2: Wines with reasonable colour and body, but unbalanced because of excessive astringency and a lack of flavour (Tarragona, Ripeness level 2), Style 3: Wines with good balance, colour, concentration and fruitiness (Priorat, Ripeness level 1), and Style 4: Wines with good body, but not well-balanced and with over-ripe characteristics, despite the good colour (Priorat, Ripeness level 2). Wines from the first harvest from the Priorat region were preferred by the tasting panel.
Conclusions
The extraction and concentration of anthocyanins, tannins and total phenolics generally increased with grape ripening. In over-ripe grapes, phenol extraction from the skins seemed limited, despite an increase of values in the wine. However, more extraction of flavan-3-ols from the seeds may occur in such grapes, explaining the values obtained in wine. Polymerisation of phenolics in seeds increased according to ripeness level. It can be concluded that the ripeness level of the skins affect the phenol content of the wine and the ripeness level of the seeds affect the nature of phenols in the wine. The results obtained on phenolic patterns during fermentation were matched with wine quality. Corresponding wine styles could easily be differentiated between the different regions. Distinguished wine styles generally ranged from: low-bodied, herbaceous and astringent, unbalanced wines lacking flavour and colour; light style wines, yet fruity and balanced; full-bodied, balanced wines high in fruitiness and tannin structure and typical of Syrah; to high alcohol wines typically showing an unbalanced structure with jammy, over-ripe flavours - in such wines, colour is poorly related to organoleptic quality.
Terroir selection and matching scion-rootstock combination, the quality of graft material, and the choice and execution of long term (soil preparation, spacing, establishment, vine training and trellising, row direction) and seasonal practices (canopy management, fertilisation, water management) are critical in the realisation of the full potential of the terroir-cultivar combination and the impact of the vine on grape composition and chemical and physical balance at the time of harvest. The level of judiciousness in selection and execution will be displayed in the quality of the grapes and wine, the synchronisation of maturation of both canopy and grapes, as well as the variety of wine styles that may be obtained (this also applies to different cultivar clones during initial evaluation for inherent properties). A uniformly growing vineyard that allows the obtainment of a range of wine styles would also increase the possibilities for introducing unique characteristics into a wine by way of blending (from grapes within a vineyard or from cross-blending between vineyards). Wine style refers to a wine with different characteristics that should never be confused with inferior quality. It represents a complex combination of biochemical and physical berry parameters, leading to a unique wine suiting the requirements of a competitive and changing market.
Acknowledgements
The authors gratefully acknowledge the technical contributions of personnel in the Viticulture Department, ARC Infruitec-Nietvoorbij, the many informative discussions with members of the SA Wine Industry, and financial support by the SA Wine Industry through Winetech.
Contacts
Kobus Hunter can be reached at ARC Infruitec-Nietvoorbij, Private Bag X5026, 7599 Stellenbosch, South Africa and the Department of Viticulture and Oenology, University of Stellenbosch, Stellenbosch, South Africa.
Neels Volschenk can be reached at ARC Infruitec-Nietvoorbij, Private Bag X5026, 7599 Stellenbosch, South Africa.
Montse Nadal can be reached at CeRTA, Dept de Bioquímica i Biotecnologia, Facultat d'Enologia de Tarragona, Universitat Rovira i Virgili, Campus Sescelades, Marcel·lí Domingo, s/n, 43007 Tarragona.
Literature
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