Influence of hormetic heat treatment on quality and phytochemical compounds of broccoli florets during storage
Citation
Duarte-Sierra, A., Forney, C.F., Michaud, D., Angers, P., Arul, J. (2017). Influence of hormetic heat treatment on quality and phytochemical compounds of broccoli florets during storage. Postharvest Biology and Technology, [online] 128 44-53. http://dx.doi.org/10.1016/j.postharvbio.2017.01.017
Plain language summary
Broccoli is considered to be a nutritious vegetable that is reputed to have many health benefits including benefits to digestion, the cardiovascular system and the immune system, as well as having anti-inflammatory and possible cancer-preventing properties. Phytochemicals in broccoli that may be responsible for these health promoting properties include glucosinolates and hydroxy-cinnamic acids. However, fresh broccoli is highly perishable and new technologies to preserve the quality of fresh broccoli while maintaining or enhancing health-promoting phytochemicals are needed. Mild heat treatments applied in the form of moist hot air have been shown to slow yellowing and senescence of broccoli and extend its market-life. Understanding how these heat treatments impact the healthful compounds in broccoli and how they are maintained during cold storage was determined in this study. Heat treatments of 41 °C for 180 min and 47 °C for 12 min delayed yellowing for 21 days by slowing the loss of the green pigment chlorophyll. However, the 47 °C - 12 min treatment induced off-odor production in treated broccoli. Concentrations of some glucosinolates were significantly enhanced by both heat treatments. The concentrations of hydroxy-cinnamic acids were also increased as well as total antioxidant capacity of florets. Associated with these increased concentration was an enhancement of related gene expression. While heat treatments enhanced the content of health-promoting phytochemicals in broccoli florets, the 41 °C – 180 min treatment was considered superior to the 47 °C - 12 min treatment in that it did not induce the production of off-odors, although the enhancement of phytochemicals was less.
Abstract
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.