Hydrothermal microwave-assisted treatments for blackberries as postharvest control against Botrytis cinerea

Abstract. Blackberry (Rubus adenotrichus Schltdl.) is a fruit rich in antioxidant compounds, such as phenolics and anthocyanins. However, they are highly perishable even at low temperatures. The mold Botrytis cinerea attacks berries, causing a gray fuzzy appearance in the fruits, even at 5ºC. The objective of this study was to develop and evaluate hydrothermal microwave-assisted treatments in blackberries, in order to control the presence of B. cinerea during the storage of the berries at 7ºC. The treatments consisted in batches of 100 g of blackberries immersed in 200 g of water and submitted to 270, 380 or 470 W power in a domestic microwave oven, until reach 48ºC and hold it for 6 min. These conditions were reported previously as enough to inactivate B. cinerea in strawberry puree. After heating, berries were removed immediately and immersed in an ice water bath at 4°C. Then, berries were let dry placed in plastic trays with slotted cover (to avoid condensation from respiration) and were stored at 7°C for 14 days. A set of berries was left as a control (without microwave treatment). Physicochemical properties (pH, titratable acidity total soluble solids, firmness, color) and content of anthocyanins, phenolic, and flavonoids compounds were analyzed. Anthocyanin content was between 8.3 and 9.3 mg of cyanidin 3-glucoside/100 g fresh fruit at day 1, and increased during the storage. However, higher contents were observed in the control (untreated) berries, with contents around 23 mg of cyanidin 3-glucoside/100 g fresh fruit, while the treated berries had 13.35 to 14.37 mg of cyanidin 3-glucoside/100 g fresh fruit (p<0.05). The total phenolic compounds ranged from 33.76 to 44.35 mg gallic acid/100 g of fresh berry, without differences among treatments (p>0.05). Flavonoids also increased with storage time, the highest values were observed in the control samples with values of 60.94 mg quercetin/g fresh fruit. The better treatment for blackberries was the performed at 270 W. Under these conditions, mold presence was prevented, and blackberries did not show significant changes in the physicochemical properties and preserved their biocompounds content.