Influence of Cooking Methods on Free and Bound Phenolic Acids in Korean Black Rice

Abstract

The composition and thermal stability of free and bound phenolic acids were investigated in Korean black rice cultivars, Sintoheugmi and Sinnongheugchal. Protocatechuic acid, 4-hydroxybenzoic acid, vanillic acid, p-coumaric acid and ferulic acid were identified and quantified. Protocatechuic acid was predominant (81–93% of the total amount) in free form, whereas ferulic acid was predominant (about 60%) in bound form. The contents of free and bound phenolic acids were three times higher in raw Sinnongheugchal compared with raw Sintoheugmi. The thermal stability of the phenolic acids was assessed by boiling, steaming, pan-frying and roasting. All cooking methods caused significant increases in the total amount of free phenolic acids. In particular, free protocatechuic acid content increased 4–11 times in both cultivars. The free protocatechuic acid content correlated negatively with the total anthocyanins contents indicating that protocatechuic acid was generated from the decomposition of anthocyanins during the thermal processing. Practical Applications Black rice is thermally processed by varying cooking methods. All cooking methods changed the profile of the free phenolic acids. Particularly, the free protocatechuic acid content increased up to 11-fold in cooked black rice compared with raw rice. This indicates that the cooking temperature and cooking time affect differently the contents as well as the composition of phenolic acids. These results will be useful in predicting the influence of thermal processing on phenolic acids in black rice.