- Blackberry (Rubus sp.) fruit are a good source of phenolic compounds and anthocyanins, and are consumed in fresh and processed forms. Though organic products become more popular, limited information is available about how different organic production methods affect the post-harvest quality of blackberry fruit. The objective of this project was to investigate the effects of different organic production systems and harvest times on the physicochemical and nutraceutical properties of four cultivars of blackberry fruit during refrigerated and frozen storage. Trailing blackberry 'Obsidian' and semi-erect blackberry 'Triple Crown' were studied under refrigerated storage while two trailing blackberry 'Marion' and 'Black Diamond' were studied under frozen storage.
'Obsidian' and 'Triple Crown' were grown organically and treated with three different organic fertilizers: processed poultry litter, soy meal, and a blend of fish emulsion and hydrolysate. Each fertilizer was applied at the same rate of 56 kg nitrogen/ha in 2012 and 2013. Samples were hand-picked three times per season at 1 wk intervals, packed immediately into clamshell containers, and stored at 4.0 ± 0.2 ºC and 90 ± 5% relative humidity for up to 12 d. Physicochemical properties, including decay, leakage, pH, titratable acidity (TA), weight loss, firmness, and moisture content, and antioxidant content and capacities, including total phenolic content (TPC), total monomeric anthocyanins (TMA), radical scavenging activity (RSA), oxygen radical absorbance capacity (ORAC), and ferric reducing ability of plasma (FRAP), were measured prior to and during refrigerated storage. Harvest date and storage time showed more effect on the physicochemical properties than that of fertilizer type. During storage, late-harvest fruit of both cultivars had the least decay in 2012 while early-harvest fruit exhibited the least decay in 2013. Fruit leakage in both cultivars increased during storage, reaching 54.3% and 62.5% in 'Obsidian' and 62.3% and 73.0% in 'Triple Crown' in 2012 and 2013, respectively. Fruit pH increased while titratable acidity decreased during storage in both cultivars. Firmness of 'Obsidian' fruit was significantly higher in 2012 than in 2013. Overall, fruit firmness decreased during storage. 'Obsidian' fruit had a 2.52% weight loss while that of 'Triple Crown' fruit had a 3.15% weight loss after 10 d of storage. The type of fertilizer only affected fruit weight loss in 'Obsidian'. 'Obsidian' also had as much as 37% higher ORAC values than 'Triple Crown' at harvest. Late-harvest fruit from plants fertilized with fish emulsion showed 29% higher ORAC values than fruit harvested from plants in the other fertilizer treatments. Generally, 'Obsidian' blackberry showed greater variability in antioxidant properties than 'Triple Crown'.
'Marion' and 'Black Diamond' were also grown organically but, in this case with three different weed management strategies: non-weeding, hand weeding, and weed mat. Fruit were machine-harvested three times at 1 wk intervals in 2012, sorted by hand to exclude molded and damaged samples, frozen in a forced-air freezer at -25 ºC, and stored at the freezer for up to 9 mo. Physicochemical properties, including pH, TA, and total soluble solids (TSS), and antioxidant content, including TPC and TMA were measured prior to and during frozen storage. Antioxidant capacities, including RSA, ORAC, and FRAP were also measured prior to frozen storage. Although weed management had no significant effect on the physicochemical properties of the fruit, it had numerous effects on TPC, TMA, RSA, ORAC and FRAP. Late-harvest 'Marion' fruit had the highest ORAC and FRAP values compared to 'Black Diamond' fruit and 'Marion' fruit from the earlier two harvests. Fruit from the hand-weeded treatments had up to 30% higher antioxidant content and capacity during the first second harvests than fruit from the non-weeded and weed mat treatments.
This study provided important information about the effects of organic production systems on post-harvest quality of blackberry fruit during refrigerated and frozen storage. Such information will be helpful for providing guidelines to the organic berry industry (growers, packers and processors) for making decisions on the selection of organic fertilizers and weed management practices, the timing of fruit harvest, and the maximum amount of time in which the fruit should be refrigerated or frozen without a significant loss in quality.