Methodology for the determination of hormetic heat treatment of broccoli florets using hot humidified air: Temperature-time relationships

Broccoli (Brassica oleracea) is one of the most consumed produce among Brassica crops because of its content in bioactive compounds such as glucosinolates and flavonoids. Preservation of this vegetable is a challenging task because of its rapid senescence, manifested as floret yellowing. In order to delay this undesirable aspect, several postharvest treatments have been explored including heat treatment, to extend its marketable life. Although heat treatments using arbitrary combinations of temperature and time have been found effective in slowing down the yellowing of broccoli florets, there is no clear methodology for the selection of temperature-time variables for heat application. The objective of this work was to establish a temperature-time relationship using membrane electrolyte leakage response as an indicator of heat severity. Broccoli florets were treated with hot humidified air at temperatures from 32 to 52 °C for periods ranging from 5 to 1440 min. Electrolyte leakage was determined by measuring the conductivity of cell eluate from broccoli stems in 0.4 M mannitol solution. The percentage of electrolyte leakage increased with exposure time at each temperature test following zero order kinetics. The electrolyte leakage rate increased with temperature, but the Arrhenius plot showed a clear broken linear pattern with a break with a transition or critical temperature zone of 42-45 °C. Although equivalent times for heating at different temperatures can be estimated from the kinetics of electrolyte leakage, the selection of treatment temperature needed to be below 42 °C, where the florets stored at 10 °C/95% RH for 10 days, showed changes in color with the progress of senescence without causing excessive anaerobic conditions and/or tissue damage. Heat treatment of florets at temperatures in the critical zone led to excessive accumulation of ethanol as a result of anaerobic respiration, while treatments with temperatures above the critical zone (>45 °C) led to severe anaerobic conditions as well as tissue damage, despite enhanced color retention of broccoli florets treated with temperatures above 42 °C. Heat treatment at 41 °C for 180 min as hormetic heat dose for broccoli florets is suggested. The results of this work suggest that the selection of treatment temperature is of primary consideration for heat treatment of fresh produce.