Enhancement of microwave-assisted extraction via intermittent radiation: Extraction of carotenoids from carrot peels

Abstract Despite being regarded as waste, carrot residues contain large amounts of β-carotene, around 58 ± 6 mg/100 g (d.b.), and other carotenoids. It is therefore worthwhile to search for an appropriate means to extract these carotenoids from, and hence increase the value of, the residues. Microwave-assisted extraction (MAE) has noted to be a promising alternative for such a purpose; however, extraction yields achievable through MAE are in many cases lower than those achievable through conventional solvent extraction. Extending the MAE time further would nevertheless result in thermal degradation of carotenoids. In this study, the use of intermittent microwave radiation at various values of the intermittency ratio, which refers to the fraction of the microwave radiation time to the total processing time in one cycle (α = 1/2, 1/3 and 1/4), to allow extended MAE without causing excessive thermal degradation of β-carotene and carotenoids from carrot peels was investigated; the antioxidant activity of the extracts was also assessed. The effects of various microwave powers and solvent-to-sample ratios were also investigated. The use of α = 1/4 along with appropriate microwave powers and solvent-to-sample ratios (180 W/75 mL:2 g and 300 W/150 mL:2 g) resulted in larger amounts of extractable β-carotene and total carotenoids as compared with the use of the corresponding continuous MAE. Prolonging the off period and subsequently turning on the microwave radiation resulted in higher antioxidant activity of the extracts in almost all cases, especially when assessed by the ABTS assay.