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

[摘要] South Africa is the eighth largest wine producing country in the world and face stiffcompetition on the world market. Cabernet Sauvignon is the most planted red cultivar inthe world as well as in South Africa and can be seen as the wine by which countries arejudged.The aim of this study was to investigate suitable, practical maturity parameters orcombinations thereof to determine the optimal time to harvest Cabernet Sauvignon grapesunder South African conditions. The following parameters were investigated during thisstudy: seed lignification, maturity indexes, anthocyanin concentration per berry, sensorycriteria (grape skins tasting and wine) and phenolic content.Berry development in four Cabernet Sauvignon vineyards in different South Africanwinegrowing areas were investigated over the 2003, 2004 and 2005 seasons. The firstparameter to be investigated was seed lignification percentages. Seasonal differences atcommercial 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 wastwo weeks later during the 2004 season. During this study it was found that seeds neverreached 100% lignification for Cabernet Sauvignon as was found in previous work toindicate grape maturity. The development of anthocyanins also peaked well before themaximum seed lignification was reached. It therefore appears that seed lignification is notsuitable for the determination of grape maturity for Cabernet Sauvignon grapes underSouth African conditions.The second parameter to be investigated was maturity indexes (Balling / TitratableAcidity (TA), Balling × pH, Balling × pH2). The best wine values were used to determinethe optimal maturity index values. Morgenster was the only vineyard to consistently givevalues that corresponded to previously reported data (index values). Anhöhe and Plaisirde Merle reported higher maturity values than that reported in literature and seasonalvariation was observed. Maturity index values for the best wines varied between 88 and101 (Balling × pH) for Anhöhe during 2003 and 2005 seasons, but increased too between97 and 107 (Balling × pH) for 2004. The maturity index values were found to be vineyardand season dependant, with warmer areas reaching higher values. From this study itappears that maturity index values as a singular maturity parameter does not give a goodindication of berry maturity in all seasons or vineyards.Thirdly, the berry anthocyanin concentration (mg / berry and mg / g berry) wereinvestigated and comparable trends were found between the four vineyards. Howevervineyards in warmer, drier regions (Anhöhe) tended to have higher anthocyaninconcentrations per gram berry. The more vigorous vineyard of Morgenster consistentlyexhibited a higher anthocyanin concentration per berry. This can be explained by the ratioof 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 stiffcompetition on the world market. Cabernet Sauvignon is the most planted red cultivar inthe world as well as in South Africa and can be seen as the wine by which countries arejudged.The aim of this study was to investigate suitable, practical maturity parameters orcombinations thereof to determine the optimal time to harvest Cabernet Sauvignon grapesunder South African conditions. The following parameters were investigated during thisstudy: seed lignification, maturity indexes, anthocyanin concentration per berry, sensorycriteria (grape skins tasting and wine) and phenolic content.Berry development in four Cabernet Sauvignon vineyards in different South Africanwinegrowing areas were investigated over the 2003, 2004 and 2005 seasons. The firstparameter to be investigated was seed lignification percentages. Seasonal differences atcommercial 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 wastwo weeks later during the 2004 season. During this study it was found that seeds neverreached 100% lignification for Cabernet Sauvignon as was found in previous work toindicate grape maturity. The development of anthocyanins also peaked well before themaximum seed lignification was reached. It therefore appears that seed lignification is notsuitable for the determination of grape maturity for Cabernet Sauvignon grapes underSouth African conditions.The second parameter to be investigated was maturity indexes (Balling / TitratableAcidity (TA), Balling × pH, Balling × pH2). The best wine values were used to determinethe optimal maturity index values. Morgenster was the only vineyard to consistently givevalues that corresponded to previously reported data (index values). Anhöhe and Plaisirde Merle reported higher maturity values than that reported in literature and seasonalvariation was observed. Maturity index values for the best wines varied between 88 and101 (Balling × pH) for Anhöhe during 2003 and 2005 seasons, but increased too between97 and 107 (Balling × pH) for 2004. The maturity index values were found to be vineyardand season dependant, with warmer areas reaching higher values. From this study itappears that maturity index values as a singular maturity parameter does not give a goodindication of berry maturity in all seasons or vineyards.Thirdly, the berry anthocyanin concentration (mg / berry and mg / g berry) wereinvestigated and comparable trends were found between the four vineyards. Howevervineyards in warmer, drier regions (Anhöhe) tended to have higher anthocyaninconcentrations per gram berry. The more vigorous vineyard of Morgenster consistentlyexhibited a higher anthocyanin concentration per berry. This can be explained by the ratioof 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 asthe anthocyanin concentrations of the homogenate.Grape skins (G) were used to make an artificial wine that was evaluated by anexpert panel to determine the development of the grapes. Wines (W) made from sampledbatches were also evaluated by an expert panel for: colour intensity, vegetative, red berry,black berry with spice, acidity, astringency and general quality. Vegetative aromas andacidity decreased and red and black berry with spice increased during ripening for bothberries and wine. Colour intensity also increased, corresponding to an increase inperceived general quality score. Correlations between general quality of both the grapeskins tasting and wines were investigated. Balling showed a strong correlation with generalquality of the grape skins tasting (r = 0.76; p = 0.00) but not as strongly with subsequentwines (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 werestatically better than the rest for each season and vineyard. Maximum berry anthocyaninconcentration coincided with wines rated as the best by the tasting panel. More than onewine was identified during the maximum anthocyanin peak that did not differ statisticallyfrom the best wine. It appears from this study that a window period exists at the maximumanthocyanin peak, where wines of comparable quality, but different style, can beproduced.Principal component analysis (PCA) was used to determine the least number ofsuitable parameters that could distinguish between unripe and ripe grapes in order toestablish a grape maturity model. These differences were successfully described byBalling, TA, pH, potassium (K+), tartaric and malic acid. Anthocyanin concentration couldfurther distinguish between ripe and overripe grapes in the model. From these parametersthe minimum and maximum values were used to construct a universal ripeness modelcontaining data from all four vineyards. Variation between the four vineyards caused toomuch overlapping in the universal model data as the vineyards were situated in differentclimatic regions according to the Winkler temperature model. On a per vineyard basis thisdid not occur to the same extend. The best rated Cabernet Sauvignon wines correlatedstrongly with soluble solid content; colour and quality perceptions of grapes, but largeseasonal differences resulted in larger grape compositional variances than that of theindividual vineyards in the different climatic zones. This illustrated the difficulty ofpinpointing a specific parameter to indicate optimal ripeness. From this study it is clear thata universal maturity model for Cabernet Sauvignon berries is not attainable at present, butindividual vineyard models shows the most potential.A preliminary study into the differences of the phenolic composition was done usingreverse phase high performance liquid chromatography (RP-HPLC) on the homogenateand wine. Malvidin-3-glucoside and total anthocyanins followed comparable trends to thatfound 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 bethe best quality, corresponded with the maximum anthocyanin concentration (mg / L) peakin the homogenate. Dense canopies at the Morgenster vineyard over the three seasonslead to lower total anthocyanin and quercetin-3-glucuronide concentrations compared tothe Anhöhe and Plaisir de Merle vineyards. The shading of bunches by the dense canopymost likely contributed to this.Catechin, epicatechin, proanthocyanidin and polymeric phenol concentrationsdecreased significantly from veraison until harvest. Seasonal differences were noted in thefour vineyards. No correlations could be found between the general wine quality and thephenolic compounds, but a weak trend was observed for total anthocyanins in thehomogenate. A trend was found with the total flavan-3-ol to anthocyanin ratio determinedby RP-HPLC analysis of the grape homogenates (r = 0.40, p = 0.00). This ratio variedbetween 1 and 3 for the wines rated as being the best quality. Phenols by themselves donot give a clear indication of optimal harvest time.From this study it appears that no single parameter could consistently indicateoptimal 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 thata combination of parameters could predict the optimal time more precisely as with theabove mentioned model but more research is needed to this end.

[发布机构] Stellenbosch University