Effects of different cooking conditions on the anthocyanin content of a black rice (Oryza sativa L. ‘Violet Nori’)

The study is focused on the effect of different cooking conditions on the antioxidant content, particularly anthocyanins, of Oryza sativa L. ‘Violet Nori, a new black rice cultivar. 10 different cooking tests were performed. The selected cooking conditions allowed to evaluate the effect of boiling, roasting, oven cooking, risotto cooking, and oriental cooking. The total anthocyanins amounts were evaluated by both a spectrophotometric pH differential method and High-Performance Liquid Chromatography (HPLC), together with the Total Phenolic Content (TPC) and the Radical Scavenging Activity (RSA). The obtained results showed that boiling with a low water amount (100 g rice/650 mL water) and oriental cooking, which allows a reduction of boiling times thanks to the lid of the pot, allow effectively saving at least part of their anthocyanins content. Similar results were obtained by boiling if rice was introduced in cold or boiling water, but on the contrary a ratio of 100 g rice/1000 mL water greatly enhanced the loss of antioxidant compounds. Risotto and oven cooking allowed obtaining results that were roughly intermediate between the ‘best’ and the ‘worst’ results. A high correlation existed between the amounts of total anthocyanins and both TPCs and RSAs. The ‘best’ results show that although cooking necessarily decreases the rice content of valuable antioxidants, a careful choice of the operative conditions allows effectively preserving amounts of total anthocyanins higher than 100 mg/100 g rice portion which are close to or even higher than in other well-known sources of dietetic anthocyanins.