Effect of drying conditions on bioactive compounds and antioxidant activity of broccoli (Brassica oleracea L.)

The effect of air-drying parameters on antioxidant activity and changes in antioxidant compounds of broccoli (Brassica oleracea L.) were studied. Broccoli samples were dried to 70 g kg?1 moisture content using temperatures ranging from 50 to 100 ?C and air flow-rates from 1.20 to 2.25m s?1, resulting in drying times from 25 to 90 min. Temperature, owing to its positive effect on the oxidation kinetics, was negatively correlated with ascorbic acid and free and total polyphenol contents but not with kaempferol, even though the sample dried at the highest temperature showed the lowest kaempferol content. The air flow-rate was positively correlated with the total (TPP) and free (FPP) polyphenol contents because it led to a reduction in the drying time. No correlation was found between air flow-rate and kaempferol content. Air flow-rate and temperature positively affected the antioxidant activity by reducing the drying time. High-temperature, short-time processes maximised the antioxidant activity of broccoli owing to the negative effect of drying time on antioxidant activity. The antioxidant activity of broccoli was positively and significantly correlated with the FPP content but not with TPP and kaempferol. The evaluation of hydroxymethylfurfural content as an indicator of the occurrence of a Maillard reaction in dried broccoli did not support the hypothesis of a contribution of MRPs to the antioxidant activity of dried broccoli. `a