Enhancement of microwave-assisted extraction of bioactive compounds from cabbage outer leaves via the application of ultrasonic pretreatment

Abstract

Although microwave-assisted extraction (MAE) has proved to be a rapid alternative to extract bioactive compounds from plant materials, it might be possible to enhance MAE further through structural modification of the plant matrix. In this study, the effect of plant structure modification via ultrasonic pretreatment prior to MAE was investigated. The evolutions of selected bioactive compounds, namely, glucosinolates, sulforaphane, vitamin C and phenolics, and antioxidant activity of the extract from cabbage outer leaves during sonication, which is indeed equivalent to ultrasonic-assisted extraction (UAE), were first monitored to identify a suitable time for the pretreatment prior to subsequent MAE. For comparison MAE and Soxhlet extraction were conducted and their results compared with those belonging to UAE and UAE+MAE. Microstructural changes of cabbages, as observed via confocal laser scanning microscopy (CLSM) and quantified via the use of the fractal dimension, undergone different extraction methods were observed and used to explain the extraction results. Energy consumption of different extraction methods was also evaluated. UAE+MAE led to higher contents of extractable bioactive compounds due to the effects of acoustic cavitation and subsequent internal heating within the plant cells by microwave irradiation, which resulted in more structural damage and hence better release of the compounds. The contents of the extractable bioactive compounds from UAE, MAE and UAE+MAE were significantly lower than those from Soxhlet extraction in almost all cases; Soxhlet extraction time was much longer, however. MAE exhibited the highest energy efficiency compared with UAE, UAE+MAE and Soxhlet extraction.