Enhancement of sulforaphane content in cabbage outer leaves using hybrid drying technique and stepwise change of drying temperature

• Rates of formation and degradation of sulforaphane are dependent on temperature.
• Sulforaphane first increased before reaching the maximum and then decreased by the end of drying.
• LPSSD followed by vacuum drying could be used to enhance sulforaphane content in dried cabbages.

Sulforaphane is a hydrolysis product of glucosinolates, which are plant secondary metabolites found in various kinds of Brassica vegetables including white cabbages (Brassica oleracea L. var. capitata). Sulforaphane is of interest as it is claimed to possess chemoprotective effect against an array of cancers. The formation of sulforaphane is via enzymatic reactions and the rates of formation and degradation are strongly dependent on the temperature. Careful regulation of the material temperature during drying to maximize the formation and minimize the degradation of sulforaphane should therefore be possible. In this study, the effect of stepwise change of medium temperature during hot air drying, vacuum drying and hybrid drying, i.e., low-pressure superheated steam drying (LPSSD) followed by vacuum drying, on the evolution of sulforaphane in white cabbage outer leaves was investigated; the results were compared with those in the cases of constant drying medium temperature. Similar evolution patterns of sulforaphane were noted in all cases; sulforaphane first increased before reaching the maximum and then decreased towards the end of drying. Nevertheless, the cabbages undergone LPSSD at 60 °C for 10 min and then vacuum drying at 45 °C until reaching the final moisture content exhibited the highest amount of sulforaphane.