The influence of Cabernet Sauvignon grape maturity on the concentration and extraction of colour and phenolic compounds in wine

Abstract

Extended maturation of wine grapes is employed to achieve optimum berry flavour development and phenolic maturity for the desired wine style. While it has been suggested that fruit maturity may also influence the extraction efficiency of colour and mouthfeel compounds from grapes into wine during processing, this has not been thoroughly evaluated. One aim of this research was to determine the impact of grape harvest date on the colour metrics and phenolic compounds in wines made from grapes harvested beyond historic or traditional maturity levels. To investigate this, berry phenolic composition and concentration were measured over two seasons (2008 and 2009) throughout post-veraison maturity of Vitis vinifera L. cv. Cabernet Sauvignon grapes, along with the composition and concentration of colour and phenolics in the wines produced from these grapes. The data did not support the notion of increased extractability of phenolic compounds with grape maturity. However, the relative wine phenolic concentrations themselves might be more commercially relevant than extractability. Based on the 2008 grape and wine phenolic data, concentrations in wine appeared directly related to the grape concentrations. Unfortunately, the trends were not as clear in 2009. Grape malvidin-3-glucoside and polymeric tannin concentrations increased with ripening and the wine concentrations trended similarly. Grape caftaric acid, catechin, epicatechin, and B2 dimer concentrations declined with ripening, and this was reflected in their concentrations in the wine. Phenolic compounds were measured as they are known to provide colour, astringency and bitterness to wines. Descriptive analysis was performed in order to determine how grape ripeness affected the wines made from these grapes. Principal component analysis of the sensory data differentiated the wines by harvest week; however, the phenolic compounds measured did not fully explain the changes in wine sensory properties. Prediction models of sensory attributes describing colour and astringency were reasonable in 2008, but not 2009. This was likely due to the weaker chemical concentration trends in 2009. Additional metrics are likely needed to explain the complex nature of the wine. Harvesting grapes at higher maturities also results in increased alcohol concentrations in the resulting wines. This can result in wines which possess undesirable sensory aspects such as excessive alcohol, as well as stuck fermentations due to alcohol inhibition of yeast growth. In some cases, incoming must may be diluted with water to adjust the final alcohol content of the wine to approximately 14% (v/v). To test the impact of dilution, wines were made from Chardonnay and Zinfandel grapes harvested at high sugar levels. The pre-fermentation sugar concentrations were lowered with water or dealcoholized wine, and compared to wines made with no sugar adjustment. The concentration of both the phenolic and aroma compounds of these wines was assessed and correlated to sensory data. Using PCA, the Chardonnay control wines were separated from the treatment wines based on phenolic chemistry and descriptive analysis, but the aroma compound concentrations were not diluted by the water or dealcoholized wine addition. In Zinfandel, PCA of the phenolic compound concentrations did not separate the control and water added treatment; however, the aromas were more similar between the control and dealcoholized wine treatment. Sensorially, the Zinfandel control wines could be separated from the treatments, which also differed from one another.