Effects of Different Cooking Methods on Nutritional and Physicochemical Characteristics of Selected Vegetables
CRISTIANA MIGLIO,† EMMA CHIAVARO,‡ ATTILIO VISCONTI,§VINCENZO FOGLIANO,§ AND NICOLETTA PELLEGRINI*,†
Department of Public Health, University of Parma, via Volturno 39, 43100 Parma, Italy, Department of Industrial Engineering, University of Parma, viale GP Usberti 181/A, 43100 Parma, Italy, and Department of Food Science, University of Napoli “Federico II”, Parco Gussone, 80055 Portici, Napoli, Italy

J. Agric. Food Chem. 2008, 56, 139–147

The objective of the present study was to evaluate the effect of three common cooking practices (i.e., boiling, steaming, and frying) on phytochemical contents (i.e., polyphenols, carotenoids, glucosinolates, and ascorbic acid), total antioxidant capacities (TAC), as measured by three different analytical assays [Trolox equivalent antioxidant capacity (TEAC), total radical-trapping antioxidant parameter (TRAP), ferric reducing antioxidant power (FRAP)] and physicochemical parameters of
three vegetables (carrots, courgettes, and broccoli). Water-cooking treatments better preserved the antioxidant compounds, particularly carotenoids, in all vegetables analyzed and ascorbic acid in carrots and courgettes. Steamed vegetables maintained a better texture quality than boiled ones, whereas boiled vegetables showed limited discoloration. Fried vegetables showed the lowest degree of softening, even though antioxidant compounds were less retained. An overall increase of TEAC, FRAP, and TRAP values was observed in all cooked vegetables, probably because of matrix softening and increased extractability of compounds, which could be partially converted into more antioxidant chemical species. Our findings defy the notion that processed vegetables offer lower nutritional quality and also suggest that for each vegetable a cooking method would be preferred to preserve the nutritional and physicochemical qualities.
 

..........In conclusion, the present study clearly indicates that physicochemical and nutritional qualities of vegetables are deeply modified by domestic cooking and that modifications of the evaluated parameters are also strongly dependent upon the vegetable species. However, the cooking conditions used here, chosen to reproduce the common Italian cooking practices, were less severe than thermal conditions usually applied in previous studies. These conditions would have promoted the
release of antioxidant compounds from the vegetable matrix and determined the formation of new antioxidant compounds. Moreover, it is also likely that matrix softening and increased extractability upon cooking were accompanied by the conversion of polyphenol into very active chemical species, which were not yet identified and concurred synergistically to determine the high antioxidant capacity. The overall increase of TAC values observed in the present study is in partial disagreement with the concept that processed vegetables have lower nutritional quality than the raw ones. Moreover, our results suggest that for each vegetable a preferential cooking method could be selected to preserve or improve its nutritional and physicochemical qualities. This selection may help consumers on the choice of cooking practices to improve the nutritional quality of foods, as well as their acceptability.