Effects of climatic conditions during harvest and handling on the postharvest expression of red drupelet reversion in blackberries

Red drupelet reversion (RDR) causes individual drupelets on blackberries to revert from black at harvest to a red colour postharvest, reducing the quality and marketability of fruit. The objective of this trial was to assess the effects of time of harvest and associated climatic variables, as well as handling of fruit during harvest, on postharvest RDR expression and fruit quality. Fruit were harvested on ten occasions over two days by one of two methods: either hand-harvested into shallow buckets and transferred to industry standard 125 g clamshell punnets (standard practice) or harvested carefully without handling by cutting the pedicel and placing each fruit into individual cotton wool-lined trays. The number of partially red (PR) and fully red (FR) drupelets per fruit was counted, firmness was measured by compression, and skin firmness measured by a penetrometer. Air and fruit skin temperature, relative humidity, vapour pressure deficit, and soil water tension were all influenced by the time of day. 85% of fruit that was handled during harvest had at least one drupelet develop RDR, whilst only 6% of fruit not handled during harvest had any RDR. In handled fruit, warmer skin temperature at harvest was associated with increased RDR incidence and severity (P = 0.001). The skin firmness of fully black (FB) drupelets, measured by a penetrometer, also decreased significantly by an average of 0.56 N when harvested during warmer temperatures compared to fruit that was not handled. The data indicate that mechanical injury incurred during harvest is a major cause of RDR in fresh blackberries, and that harvest times associated with warmer temperatures result in significantly higher rates of RDR and reduced postharvest quality.