Chemical Changes in Grape Stem and Their Relationship to Stem Color throughout Berry Ripening in Vitis vinifera L. cv Shiraz.

Little attention has been paid to the color change or chemical compositional changes that occur in grape stems and how this correlates with the berry ripening process. Recently we have found that the change in grape peduncle color of Shiraz (Vitis vinifera) from green at veraison to predominantly brown at harvest occurs in parallel with berry ripening and as such may represent a new way of assisting in the prediction of grape maturity and harvest date. We have now investigated further the link between certain key chemical compositional changes that occur in the grape stem (peduncle and rachis) from veraison to harvest and how these attributes correlate with the observed color change in the vineyard. We report that peduncle moisture content has an excellent linear correlation with the color hue value and is negatively correlated in a strong fashion with the chlorophyll and carotenoid pigment ratio (Ca+b/Cx+c) within the peduncles. Significant differences in the moisture content, total chlorophylls (including chlorophyll a and b levels), total carotenoids, total phenolics, and the antioxidant capacity (DPPH) levels between the peduncles and rachises were found as they evolve from veraison to harvest. Finally, we have demonstrated for the first time that peduncle moisture content codevelops with the prototypical berry ripeness parameters (Brix, pH, TA), which provides for the development of a new approach for viticulturists and winemakers to evaluate grape ripeness through peduncle moisture levels and therefore assist in harvest decision making.