Bioactive compositions of extracts from cabbage outer leaves as affected by drying pretreatment prior to microwave-assisted extraction

•Effect of drying on MAE was assessed in terms of bioactive composition of extracts.•GLS, sulforaphane, TPC and TAA of extracts were assessed.•GLS, TPC and TAA of extracts from the dried cabbages were lower.•Sulforaphane contents of extracts from the dried cabbages were higher, however.

In order to add value to cabbage outer leaves, which are generally considered as a waste product, extraction and use of their health-beneficial bioactive compounds should be made. Among many extraction alternatives microwave-assisted extraction (MAE) is one of the most promising and has indeed been applied to extract bioactive compounds from cabbages. However, study is lacking on the effect of drying prior to MAE, which is sometimes required since extraction cannot always be performed on fresh materials. This study was divided into 2 parts, with the first part investigating the effects of selected drying methods, i.e., hot air drying (HD) and low-pressure superheated steam drying (LPSSD), on selected bioactive compounds, namely, glucosinolates (GLS), sulforaphane, phenolics, as well as the total antioxidant activity (TAA) of the dried cabbages. In the second part the effects of the drying methods on subsequent MAE were assessed. The results suggested that HD but not LPSSD led to a significantly decreased GLS content of the dried cabbages. Sulforaphane and total phenolics contents as well as TAA of the dried cabbages decreased upon both HD and LPSSD. The GLS and total phenolics contents as well as TAA of the extracts from the dried cabbages prepared by HD and LPSSD were lower than those from the fresh sample. The results on the sulforaphane content of the extracts were nevertheless opposite.