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

Abstract

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 2.25 m 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. INTRODUCTIONThe health-promoting capacity of plant foods depends strictly on their processing history. 1 The conditions of storage, processing and preparation have been proved to have significant effects on the content of antioxidants 2,3 but the impact of processing on the antioxidant activity of vegetables is still a neglected area and little information is available. 2 Broccoli is a vegetable that shows high antioxidant activity owing to the presence of bioactive compounds, 4 -8 among which polyphenols seem to be have the greatest effect. 8,9 The effect of thermal treatment (cooking) on the polyphenol content of broccoli 10,11 and the effect of freezing on polyphenol content and antioxidant activity 12 have been studied, but the effect of processing on the antioxidant activity of these vegetables is still mostly unknown. One study has been carried out on the quality of dehydrated broccoli, 13 but the effect of drying on the antioxidant activity of broccoli was not considered.Several studies investigated the antioxidant activity changes after drying of tomatoes 14,15 and onion powders 16 and in both cases there was an increase in antioxidant activity due to hot air-drying. In the case of tomato, it was hypothesised that the

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