Antioxidant Capacity and Phenolic Antioxidants of Midwestern Black Raspberries Grown for Direct Markets Are Influenced by Production Site

Black raspberries ( Rubus occidentalis L.) are rich in phytonutrients that have demonstrated chemoprotective properties against certain degenerative diseases. To estimate variability in phytonutritional quality among sources of black raspberry, 19 samples representing four common midwestern cultivars obtained from eight production sites were assayed for their antioxidant capacity [2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing ability of plasma (FRAP) methods], total phenolic content (TP), total monomeric anthocyanin levels (TMA), and levels of cyanidin 3-rutinoside. The antioxidant potential among samples averaged 2.92 ± 0.29 and 4.62 ± 0.88 mmol TE·100 g −1 fresh weight by the DPPH and FRAP methods, respectively; TP, TMA, and cyanidin 3-rutinoside means averaged 449 ± 62, 336 ± 109, and 244 ± 84 mg·100 g −1 fresh weight, respectively. Levels of FRAP, TP, TMA, and cyanidin 3-rutinoside were strongly correlated ( r = +0.85 to +0.96). Mean antioxidant capacities and phenolic constituent levels were similar among ‘Bristol’, ‘Jewel’, and ‘MacBlack’ samples; values for a single sample of ‘Haut’ were lower but comparable to levels found in individual samples of the other three cultivars. Black raspberry production site differences were statistically significant for FRAP, TMA, cyanidin 3-rutinoside, and titratable acidity (TA) levels. Inverse relationships ( r = –0.65 to –0.74) among black raspberry samples for FRAP, TMA, or cyanidin 3-rutinoside levels versus levels of TA suggested that site differences may be partially attributable to fruit ripeness at harvest. Relationships among these parameters versus regional differences in soil temperatures were also significant but weak. Regardless of its environmental or physiological drivers, point-source variation in fruit phytonutrient contents may be a relevant concern in health-related studies or clinical applications. Moreover, it may impact the nutritional benefits to the consumer and affect the quality advantages associated with direct-marketed black raspberries.