The Effects of Processing on Hydrophilic Antioxidant Capacity of Black Beans

Dry, uncooked black beans have been found to contain a significantly high amount of antioxidants compared to many other foods. There is little information available on the influence of thermal processing on antioxidant capacity of foods. The antioxidant capacity, reported in μmol Trolox Equivalents per gram of uncooked, blanched, or retorted black beans was evaluated using the oxygen radical absorbance capacity (ORAC) assay. A boiling water blanch of black beans for 30, 60 or 90 sec resulted in a 33% reduction in antioxidant capacity compared to uncooked beans. There was no significant difference among antioxidant capacity of beans blanched for the varying times. There was a significant effect of thermal processing black beans at 110° C for 10 and 30 min. Compared to uncooked beans (229.18 μmol TE/g), there was a decrease of 91% in antioxidant capacity for cooked beans (19.13 μmol TE/g). The 30 min cook time gave a significantly lower antioxidant capacity than the 10 min (p<0.01). When antioxidant capacity of black beans was calculated on a dry weight basis, it was shown that the difference in antioxidant capacity was due to an increased moisture uptake during the longer cook time. Increased water absorption decreased the amount of antioxidants per gram. Containers to be retorted were each filled with 100g of black beans and 150, 186 or 233 ml of water. Black beans filled with less water had significantly less reduction in antioxidant capacity than the other two fill ratio samples. This was determined to be due to loss of materials from beans that resulted in lower antioxidant capacities. Two commercial samples of black beans, one processed in a retort without agitation and one processed in an agitating retort at a higher temperature but for a shorter time were found to contain similar antioxidant capacity. Samples were cooked to similar moisture contents.

Black beans that are thermally processed contain reduced antioxidant capacity per 100g due to water absorption during blanching. There is continued moisture increase during retorting but there is also up to 80% antioxidant capacity lost from the solid material in black beans during the two types of thermal processing.