Toward a model of grape proanthocyanidin extraction during vinification

Abstract

PAs are compartmentalised within the grape berry, and differ in their composition and degree of extractability. Within each compartment, the CWM limits PA extraction firstly by its degree of permeability and secondly its ability to complex with PA molecules. Grape CWM shows a high affinity for PA of higher MM. In addition, grape flesh cell walls bind PA more strongly than skin CWM, which renders flesh-derived PA inextractable in dilute alcohol solution. Analysis of winePA has shown that skin-derived PAs are preferentially extracted over those derived from the seed in riper grapes, and PAs of higher MM are absent. The reason for the lack of high MM PA may be two-fold. Firstly, skin PAs of higher MM are poorly extracted in dilute alcohol, and require a strong solvent to break their association with the CWM. Secondly, suspended CWM modulates the MM distribution of extracted PA during fermentation, removing that of higher MM. Using ‘Cabernet sauvignon’ for a study of grape berry ripening, skin-derived PA was found to become more extractable in dilute alcohol solution as ripening progressed, and the MM of the extracted PA increased. This may reflect changes in both cell wall permeability and affinity for PA. A conceptual model for PA extraction is presented which proposes the inclusion of rate constants for total PA extraction (k_E), adsorption to suspended CWM (k_A) and desorption of CWM-bound PA (k_D).