Influence of Cold or Frozen Storage on Temporal Changes in Sulforaphane and Objective Taste Values of Broccoli (Brassica oleracea var. italica) Florets

Abstract

Sulforaphane, an organosulfur compound obtained from cruciferous vegetables, is known to suppress gastric cancer (Zhang et al., 1992), and food supplements containing high concentrations are currently being marketed (e.g., Super Sprout, vegetable cotyledons). Sulforaphane is produced via catalysis by myrosinase of the precursor glucoraphanin (Hirai et al., 2007). However, it has been reported that the enzyme and its substrate do not coexist in the same cell (Andreasson et al., 2001), and so some studies have attempted to bring them into contact using physical methods to injure the vegetable tissues. Van Eylen et al. (2009) reported that high-pressure treatment of broccoli (Brassica oleracea var. italica) heads at 300 MPa for 35 minutes caused glucoraphanin to be converted into sulforaphane, while Matusheski et al. (2004) found that mild heating of fresh broccoli sprouts or florets to 60°C prior to homogenization accelerated this conversion process. Pérez et al. (2014) proposed an optimized process consisting of blanching at 57°C for 13 minutes. Furthermore, Ezaki and Onozaki (1982) reported that grating caused isothiocyanates, including sulforaphane, to be produced in radish (Raphanus sativus L.). Makino et al. (2018) reported that sulforaphane concentration was increased between 1.6 and 2.3 times in broccoli florets kept in a hypoxic atmosphere at 20°C for 2 days compared to florets in a normoxic atmosphere, demonstrating for the first time a method that increased sulforaphane without the need for physical injury to the plant tissue. However, the concentration began to decrease after 2 days even under hypoxic conditions. Also, it appears that hypoxia may cause an off-odor containing ethanol due to fermentation (Thompson et al., 2002). In the previous study (Makino et al., 2018), in-package ethanol concentration was significantly increased after 2 days. In the present study, the aim was to maintain sulforaphane concentration by cold storage or by freezing, either of which could be expected to depress the reactions to reduce causing sulforaphane loss and off-odors such as ethanol after 2 days storage. Also, changes in the taste of broccoli florets over time at different storage temperatures were investigated by objective measurement. This experiment was conducted to confirm the influence of hypoxia, storage period, or temperature on the taste of broccoli florets.