Soft, Sweet, and Colorful: Stratified Sampling Reveals Sequence of Events at the Onset of Grape Ripening

Abstract

Asynchronous development of grape berries leads to high variation among berry samples collected during veraison. We applied a stratified sampling method that groups berries by firmness to the touch and visible skin color to study the sequence of physical and chemical changes occurring at the onset of and during ripening. Method robustness and reproducibility were tested by measuring berry weight, diameter, deformation, and total soluble solids (TSS) in samples collected from 10 wine, table, and juice grape cultivars varying in skin color. Additionally, Syrah berries sampled by five different individuals were compared in one year, and Syrah and Merlot berry samples and cluster images were evaluated over three years to account for environmental variation. Merlot berries collected in one year were classified into seven developmental stages to measure changes in berry weight, elastic modulus, TSS, titratable acidity (TA), pH, malate, tartrate, and anthocyanins. Stratified sampling reliably differentiated among developmental stages when applied by different individuals and in different cultivars and years. Once the berries softened, they took 11 to 14 days to complete their color change. Softening occurred mostly before sugar accumulation and renewed berry expansion began. Sugar accumulation was concurrent with malate degradation, but its onset preceded anthocyanin accumulation. The increase in TSS was closely associated with decreases in elastic modulus, TA, and malate and increases in berry weight, skin mass per area ratio, and pH. Ripening-related changes in TA and pH were strongly driven by changes in malate but not tartrate. The stratified sampling method allows novel insights into the timing of physical and chemical changes that occur in grape berries during the period commonly referred to as veraison.