Evaluation of parameters to determine optimum ripeness in Cabernet Sauvignon grapes in relation to wine quality

Botes, Matthys Petrus (2009-03)

Thesis (MscAgric (Viticulture and Oenology))--University of Stellenbosch, 2009.

Thesis

South Africa is the eighth largest wine producing country in the world and face stiff competition on the world market. Cabernet Sauvignon is the most planted red cultivar in the world as well as in South Africa and can be seen as the wine by which countries are judged. The aim of this study was to investigate suitable, practical maturity parameters or combinations thereof to determine the optimal time to harvest Cabernet Sauvignon grapes under South African conditions. The following parameters were investigated during this study: seed lignification, maturity indexes, anthocyanin concentration per berry, sensory criteria (grape skins tasting and wine) and phenolic content. Berry development in four Cabernet Sauvignon vineyards in different South African winegrowing areas were investigated over the 2003, 2004 and 2005 seasons. The first parameter to be investigated was seed lignification percentages. Seasonal differences at commercial harvest were observed with values of 2004 varying between 73% and 91% compared to 59% and 80% for the 2003 and 2005 seasons but commercial harvest was two weeks later during the 2004 season. During this study it was found that seeds never reached 100% lignification for Cabernet Sauvignon as was found in previous work to indicate grape maturity. The development of anthocyanins also peaked well before the maximum seed lignification was reached. It therefore appears that seed lignification is not suitable for the determination of grape maturity for Cabernet Sauvignon grapes under South African conditions. The second parameter to be investigated was maturity indexes (Balling / Titratable Acidity (TA), Balling × pH, Balling × pH2). The best wine values were used to determine the optimal maturity index values. Morgenster was the only vineyard to consistently give values that corresponded to previously reported data (index values). Anhöhe and Plaisir de Merle reported higher maturity values than that reported in literature and seasonal variation was observed. Maturity index values for the best wines varied between 88 and 101 (Balling × pH) for Anhöhe during 2003 and 2005 seasons, but increased too between 97 and 107 (Balling × pH) for 2004. The maturity index values were found to be vineyard and season dependant, with warmer areas reaching higher values. From this study it appears that maturity index values as a singular maturity parameter does not give a good indication of berry maturity in all seasons or vineyards. Thirdly, the berry anthocyanin concentration (mg / berry and mg / g berry) were investigated and comparable trends were found between the four vineyards. However vineyards in warmer, drier regions (Anhöhe) tended to have higher anthocyanin concentrations per gram berry. The more vigorous vineyard of Morgenster consistently exhibited a higher anthocyanin concentration per berry. This can be explained by the ratio of skin to pulp between small berries (Anhöhe, 0.95 g - 2004) and larger berries South Africa is the eighth largest wine producing country in the world and face stiff competition on the world market. Cabernet Sauvignon is the most planted red cultivar in the world as well as in South Africa and can be seen as the wine by which countries are judged. The aim of this study was to investigate suitable, practical maturity parameters or combinations thereof to determine the optimal time to harvest Cabernet Sauvignon grapes under South African conditions. The following parameters were investigated during this study: seed lignification, maturity indexes, anthocyanin concentration per berry, sensory criteria (grape skins tasting and wine) and phenolic content. Berry development in four Cabernet Sauvignon vineyards in different South African winegrowing areas were investigated over the 2003, 2004 and 2005 seasons. The first parameter to be investigated was seed lignification percentages. Seasonal differences at commercial harvest were observed with values of 2004 varying between 73% and 91% compared to 59% and 80% for the 2003 and 2005 seasons but commercial harvest was two weeks later during the 2004 season. During this study it was found that seeds never reached 100% lignification for Cabernet Sauvignon as was found in previous work to indicate grape maturity. The development of anthocyanins also peaked well before the maximum seed lignification was reached. It therefore appears that seed lignification is not suitable for the determination of grape maturity for Cabernet Sauvignon grapes under South African conditions. The second parameter to be investigated was maturity indexes (Balling / Titratable Acidity (TA), Balling × pH, Balling × pH2). The best wine values were used to determine the optimal maturity index values. Morgenster was the only vineyard to consistently give values that corresponded to previously reported data (index values). Anhöhe and Plaisir de Merle reported higher maturity values than that reported in literature and seasonal variation was observed. Maturity index values for the best wines varied between 88 and 101 (Balling × pH) for Anhöhe during 2003 and 2005 seasons, but increased too between 97 and 107 (Balling × pH) for 2004. The maturity index values were found to be vineyard and season dependant, with warmer areas reaching higher values. From this study it appears that maturity index values as a singular maturity parameter does not give a good indication of berry maturity in all seasons or vineyards. Thirdly, the berry anthocyanin concentration (mg / berry and mg / g berry) were investigated and comparable trends were found between the four vineyards. However vineyards in warmer, drier regions (Anhöhe) tended to have higher anthocyanin concentrations per gram berry. The more vigorous vineyard of Morgenster consistently exhibited a higher anthocyanin concentration per berry. This can be explained by the ratio of skin to pulp between small berries (Anhöhe, 0.95 g - 2004) and larger berries (Morgenster, 1.82 g – 2004). Wine colour density (A420+A520) followed the same trend as the anthocyanin concentrations of the homogenate. Grape skins (G) were used to make an artificial wine that was evaluated by an expert panel to determine the development of the grapes. Wines (W) made from sampled batches were also evaluated by an expert panel for: colour intensity, vegetative, red berry, black berry with spice, acidity, astringency and general quality. Vegetative aromas and acidity decreased and red and black berry with spice increased during ripening for both berries and wine. Colour intensity also increased, corresponding to an increase in perceived general quality score. Correlations between general quality of both the grape skins tasting and wines were investigated. Balling showed a strong correlation with general quality of the grape skins tasting (r = 0.76; p = 0.00) but not as strongly with subsequent wines (r = 0.57; p = 0.00). Anthocyanin concentration (mg / g berry) of the berries (r = 0.36; p = 0.00), perceived colour intensity of grapes (r = 0.69; p = 0.00) and wine (r = 0.84; p = 0.00) correlated with general wine quality. The tasting panel identified wines that were statically better than the rest for each season and vineyard. Maximum berry anthocyanin concentration coincided with wines rated as the best by the tasting panel. More than one wine was identified during the maximum anthocyanin peak that did not differ statistically from the best wine. It appears from this study that a window period exists at the maximum anthocyanin peak, where wines of comparable quality, but different style, can be produced. Principal component analysis (PCA) was used to determine the least number of suitable parameters that could distinguish between unripe and ripe grapes in order to establish a grape maturity model. These differences were successfully described by Balling, TA, pH, potassium (K+), tartaric and malic acid. Anthocyanin concentration could further distinguish between ripe and overripe grapes in the model. From these parameters the minimum and maximum values were used to construct a universal ripeness model containing data from all four vineyards. Variation between the four vineyards caused too much overlapping in the universal model data as the vineyards were situated in different climatic regions according to the Winkler temperature model. On a per vineyard basis this did not occur to the same extend. The best rated Cabernet Sauvignon wines correlated strongly with soluble solid content; colour and quality perceptions of grapes, but large seasonal differences resulted in larger grape compositional variances than that of the individual vineyards in the different climatic zones. This illustrated the difficulty of pinpointing a specific parameter to indicate optimal ripeness. From this study it is clear that a universal maturity model for Cabernet Sauvignon berries is not attainable at present, but individual vineyard models shows the most potential. A preliminary study into the differences of the phenolic composition was done using reverse phase high performance liquid chromatography (RP-HPLC) on the homogenate and wine. Malvidin-3-glucoside and total anthocyanins followed comparable trends to that found for the Iland method. Strong correlations (r > 0.9) were found between the malvidin- 3-glucoside and malvidin-3-glucoside-acetate and p-coumarate; this was also true for the total anthocyanins in both homogenate and wine. Wines identified by a tasting panel to be the best quality, corresponded with the maximum anthocyanin concentration (mg / L) peak in the homogenate. Dense canopies at the Morgenster vineyard over the three seasons lead to lower total anthocyanin and quercetin-3-glucuronide concentrations compared to the Anhöhe and Plaisir de Merle vineyards. The shading of bunches by the dense canopy most likely contributed to this. Catechin, epicatechin, proanthocyanidin and polymeric phenol concentrations decreased significantly from veraison until harvest. Seasonal differences were noted in the four vineyards. No correlations could be found between the general wine quality and the phenolic compounds, but a weak trend was observed for total anthocyanins in the homogenate. A trend was found with the total flavan-3-ol to anthocyanin ratio determined by RP-HPLC analysis of the grape homogenates (r = 0.40, p = 0.00). This ratio varied between 1 and 3 for the wines rated as being the best quality. Phenols by themselves do not give a clear indication of optimal harvest time. From this study it appears that no single parameter could consistently indicate optimal ripeness over the seasons or per vineyard, but the maximum berry colour (anthocyanin concentration) did give an indication of optimal harvesting time. It is clear that a combination of parameters could predict the optimal time more precisely as with the above mentioned model but more research is needed to this end.

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