Physiochemistry of blackberries (Rubus L. subgenus Rubus Watson) affected by red drupelet reversion

Red drupelet reversion (RDR) is a physiological disorder causing individual or groups of drupelets on blackberries that are black at harvest to turn red during postharvest cool storage. The objectives of this study were to examine and quantify the physiochemical changes occurring in flesh affected by RDR. Drupelets were classified as ‘fully black’, ‘partially red’, or ‘fully red’. The total anthocyanin concentration in black, partially, and fully red drupelets was 1841 mg kg⁻¹, 1064 mg kg⁻¹ and 769 mg kg^-1 fresh weight respectively. Anthocyanins containing acylated or disaccharide sugar moieties were more stable than anthocyanins with non-acylated monosaccharide sugar moieties. The pH of partially red (3.05) and fully red drupelets (3.01) was lower than black drupelets (3.32). Firmness of partially red (1.90 N) and fully red drupelets (1.77 N) was lower than that of fully black drupelets (2.39 N). Examination by light and electron microscopy showed cell disruption, separation, and loss of integrity in the upper mesocarp of affected drupelets. Electrolyte leakage over 24 h was significantly higher from partially red (84.8%) and fully red (90.0%) than fully black drupelets (64.9%). The data are consistent with RDR in blackberries arising from mechanical damage that causes cell decompartmentalisation and subsequent anthocyanin degradation.