What are the best cooking methods to retain sulfur-containing compounds in cruciferous vegetables?

Observational studies suggest higher intake of cruciferous vegetables (e.g., broccoli, cauliflower, kale) is associated with lower chronic disease risk (1,2) . Glucosinolates (GSL) and cysteine sulfoxides such as S- methyl cysteine sulfoxide (SMCSO) are sulfur-containing compounds found in high amounts in these vegetables (3) . Currently, no data exists on SMCSO levels in Australian-grown cruciferous vegetables and limited data exists for glucosinolates (GSL). The levels of SMCSO retained in cruciferous vegetables after various domestic cooking methods is unknown, and measurement of SMCSO and GSL levels in cooked Australian-grown cruciferous vegetables is limited. This study sought to (1) quantify SMCSO and GSL in Australian-grown cruciferous vegetables and (2) identify the most preferable cooking methods to retain levels in these vegetables. Using liquid chromatography mass spectrometry, we quantified SMCSO and ten GSL in seven cruciferous vegetables before and after steaming. We further quantified levels in broccoli before and after microwaving, stir-frying, and boiling. Each cooking method; steaming (3 minutes), microwaving (2 minutes), boiling (3 minutes), stir-frying (4 minutes); was chosen so vegetables remained firm and not overcooked to mimic healthy cooking recommendations (4) . Student t-tests were used to compare the differences in raw and steamed levels for all vegetables, and analysis of variance with Tukey post-hoc assessed the differences in raw and cooked broccoli (i.e., steamed, microwaved, boiled, stir-fried). Overall, SMCSO contributed greater dry weight (0.6–1.9%) than total GSL combined (0.3–1.2%). SMCSO levels from lowest to highest were Chinese cabbage < white cabbage < cauliflower < kale < red cabbage < broccoli < Brussels sprouts (6–19 mg/g dry weight [DW]) and GSL levels were cauliflower < Chinese cabbage < red cabbage < kale < broccoli < white cabbage < Brussels sprouts (3–12 mg/g DW). SMCSO increased after steaming (1–24%) in all vegetables except white cabbage (−31%), kale (−18%), and Chinese cabbage (−5%), but only reached statistical significance in Brussels sprouts (+16%, p < 0.05). Most vegetables increased total GSL (ranging 1–34%) after steaming, except kale (−38%) and Chinese cabbage (−8%). Stir-frying and boiling broccoli led to significant losses in SMCSO (−34% and −50%, respectively) and in the two dominant GSL in broccoli; glucoraphanin (−47% and −52%, respectively) and glucobrassicin (−46% and –51%, respectively) (all p < 0.05). We have quantified SMCSO and GSL levels in a selection of Australian-grown cruciferous vegetables (broccoli, kale, Brussels sprouts, cauliflower, red, white, and Chinese cabbages) before and after cooking. SMCSO and GSL levels were relatively stable after light steaming. Additionally, light steaming or microwaving were the most preferable methods to retain SMCSO and GSL levels in broccoli. Boiling or stir-frying broccoli were the least favourable. These results have important implications when estimating intake of these beneficial sulfur-containing compounds.