Influence of hormetic heat treatment on quality and phytochemical compounds of broccoli florets during storage

The effect of moist hot air treatment applied to broccoli florets was studied in order to maintain quality and phytochemical compounds during postharvest storage at 4 °C. Exposure to hormetic heat doses of 41 °C for 180 min (low temperature, LT) and 47 °C for 12 min (high temperature, HT) delayed yellowing for 21 d compared with non-heated florets that yellowed after 14 d. Chlorophyll content was also higher in florets treated with both the LT and HT heat treatments. The respiration rate of heat-treated broccoli was significantly higher immediately after heat treatments, being 10-times greater in LT-treated and 15-times greater in HT-treated florets on day 0 when compared with the control florets. However, after 7 d of storage differences were not significant, even though respiration rates were lower in treated broccoli after 21 d of storage compared with non-heated florets. Off-odors were also detected in HT-treated broccoli. Titers of indole-type glucosinolates were significantly enhanced by both heat treatments, while the glucoraphanin content of florets only increased with the HT treatment. A similar pattern was observed with gene expression, where overexpression of tryptophan N-hydroxylase (CYP79B3) was greater than the expression of dihomomethionine N-hydroxylase (CYP79F1) in heat-treated broccoli florets. Titers of hydroxy-cinnamic acids of florets were increased by both heat treatments. The total antioxidant capacity was significantly enhanced by the HT treatment. Similarly, overexpression of coumarate ligase (CoL), chalcone synthase (CHS) and phenylalanine N-hydroxylase (CYP79A2) was triggered by the HT treatment. The results indicate hormetic heat treatments can enhance the content of phytochemicals in broccoli florets during storage. However, the application of heat at 41 °C (LT) was superior to the HT treatment in maintaining quality, although the enhancement of phytochemicals was less.