Cooking method significantly effects glucosinolate content and sulforaphane production in broccoli florets

It is known that glucosinolate levels in Brassica vegetables can be affected during cooking but little is documented about the effect of cooking on isothiocyanate production. In this study, three cooking methods were evaluated for their effects on the contents in broccoli florets of the glucosinolates, glucoraphanin (GR), glucobrassicin (GB), neoglucobrassicin and progoitrin, as well as on sulforaphane (SF) and sulforaphane nitrile (SFN) production in broccoli florets. Two broccoli cultivars, ‘Marathon’ and ‘Booster™’, were analysed raw and after they were steamed, microwaved (with water) or boiled for 2 or 5 min. Residual cooking water from all treatments was collected and analysed for GR and GB to determine the extent of leaching of intact glucosinolates. Irrespective of time, steaming resulted in significantly greater retention of GR, GB and SF, while boiling and microwave cooking resulted in significant losses of GR, GB and SF in both varieties. Glucosinolate content in the residual cooking water was highest after boiling and microwaving. Loss of SF production was primarily due to both leaching of GR into cooking water and thermal inhibition of ESP and myrosinase once internal floret temperatures exceeded 70 °C. Cooking method can significantly alter content of potentially beneficial compounds in broccoli florets and optimal SF ingestion may be obtained by eating raw or lightly steamed broccoli florets.