Broccoli, Kale, and Radish Sprouts: Key Phytochemical Constituents and DPPH Free Radical Scavenging Activity

Abstract

Our research group previously found that broccoli sprouts possess neuroprotective effects during pregnancy. The active compound has been identified as sulforaphane (SFA), obtained from glucosinolate and glucoraphanin, which are also present in other crucifers, including kale. Sulforaphene (SFE), obtained from glucoraphenin in radish, also has numerous biological benefits, some of which supersede those of sulforaphane. It is likely that other components, such as phenolics, contribute to the biological activity of cruciferous vegetables. Notwithstanding their beneficial phytochemicals, crucifers are known to contain erucic acid, an antinutritional fatty acid. The aim of this research was to phytochemically examine broccoli, kale, and radish sprouts to determine good sources of SFA and SFE to inform future studies of the neuroprotective activity of cruciferous sprouts on the fetal brain, as well as product development. Three broccoli: Johnny's Sprouting Broccoli (JSB), Gypsy F1 (GYP), and Mumm's Sprouting Broccoli (MUM), one kale: Johnny's Toscano Kale (JTK), and three radish cultivars: Black Spanish Round (BSR), Miyashige (MIY), and Nero Tunda (NT), were analyzed. We first quantified the glucosinolate, isothiocyanate, phenolics, and DPPH free radical scavenging activity (AOC) of one-day-old dark- and light-grown sprouts by HPLC. Radish cultivars generally had the highest glucosinolate and isothiocyanate contents, and kale had higher glucoraphanin and significantly higher sulforaphane content than the broccoli cultivars. Lighting conditions did not significantly affect the phytochemistry of the one-day-old sprouts. Based on phytochemistry and economic factors, JSB, JTK, and BSR were chosen for further sprouting for three, five, and seven days and subsequently analyzed. The three-day-old JTK and radish cultivars were identified to be the best sources of SFA and SFE, respectively, both yielding the highest levels of the respective compound while retaining high levels of phenolics and AOC and markedly lower erucic acid levels compared to one-day-old sprouts.

Keywords: antioxidant activity; broccoli; cruciferous; erucic acid; glucosinolates; kale; radish; sulforaphane (SFA); sulforaphene (SFE); total phenolics.

Figure 1

Structures of sulforaphane, sulforaphene, and erucic acid.

Figure 2

Chromatograms of selected one-day-old broccoli, kale, and broccoli cultivars (227 nm) showing glucosinolates and phenolic compounds. Broccoli: Johnny’s Sprouting Broccoli (JSB), Gypsy F1 (GYP); Kale: Johnny’s Toscano Kale (JTK); Radish: Black Spanish Round (BSR), and Miyashige (MIY). Compounds: 1: Glucoiberin; 2: glucoraphanin; 3: glucoraphenin; 4: 4-hydroxyglucobrassicin; 5: glucoiberverin; 6: glucoerucin; 7: glucoraphasatin; 8: glucobrassicin; 9–11: Unidentified phenolics.

Figure 3

Total glucosinolates (Tot GSL), glucoraphenin (GRE), glucoraphanin (GRA), sulforaphene (SFE), and sulforaphane (SFA) levels in dark-grown radish, kale, and broccoli cultivars. (For the ITC of JSB, light-grown sprouts are used.) Broccoli: Johnny’s Sprouting Broccoli (JSB), Gypsy F1 (GYP) Mumm’s Sprouting Broccoli (MUM); Kale: Johnny’s Toscano Kale (JTK); Radish: Black Spanish Round (BSR), and Miyashige (MIY), Nero Tunda (NT). GRE and SRE are in radishes only; GRA and SFA are in kale and broccoli (n = 3; ±SD). For a given analyte type, values followed by the same letter and font type are not significantly different (p ≤ 0.05). Upper-case letters are used for GRA only.

Figure 4

HPLC Chromatogram of a mixture of sulforaphene (SFE: t_R16.08 s) and sulforaphane (SFA: t_R16.62 s) at 205 nm.

Figure 5

Total phenolics (µmol Rutin equivalents/g DW) and antioxidant capacity (DPPH radical scavenging activity: µmol Trolox equivalents/g DW) of one-day-old sprout extracts grown under dark and light conditions. Radish: Black Spanish Round (BSR), Miyashige (MIY), and Nero Tunda (NT); kale: Johnny’s Toscano Kale (JTK); broccoli: Johnny’s Sprouting Broccoli (JSB), Gypsy F1 (GYP), and Mumm’s sprouting broccoli (MUM). (n = 3; ±SD). For a given analyte and lighting condition, values followed by the same letter are not significantly different (p ≤ 0.05). For all the sprout varieties, there was no significant difference observed in the analyte levels for different lighting conditions.

Figure 6

Content of major isothiocyanate in three-, five-, and seven-day radish, kale, and broccoli cultivars: sulforaphene is found in Black Spanish Round (BSR) radishes, and sulforaphane is found in Johnny’s Toscano Kale (JTK) and Johnny’s Sprouting Broccoli (JSB); (n = 3; ±SD). Different lowercase and upper-case letters above bars, respectively, indicate significant differences for samples of the same sampling day and samples of the same variety at p < 0.05, according to Tukey’s Multiple Comparison Test.

Figure 7

Total phenolics (uM rutin equivalents/g DW) (a) and DPPH radical scavenging activity (µmol TE/g DW) (b) of three-, five-, and seven-day-old sprout extracts. Black Spanish Round radish (BSR); Johnny’s Toscano Kale (JTK); Johnny’s Sprouting Broccoli (JSB). Different letters above bars indicate significant differences at p < 0.05 according to Tukey’s Multiple Comparison Test.

Figure 8

Principal component analysis showing: (a) correlation of phytochemical parameters with PC1 and PC2, and (b) relationship of three cruciferous cultivars at three harvest dates. PCA was performed on the correlation matrix of average values of phytochemical parameters and antioxidant capacity (DPPH).