Elucidating the effect of polyphenol-protein interactions on rheological properties of purple waxy rice

Abstract

Processing purple rice causes interactions between polyphenols and protein. The effect of covalent and non-covalent interactions on the viscosity of non-pigmented (Neches) and purple (HB-1) waxy rice cultivars was studied by using dithiothreitol (DTT), urea, sodium chloride (NaCl), and acidic (pH 2) and alkaline (pH 12) environments. Urea did not affect pasting properties of both cultivars. The pasting viscosity of whole grain Neches and HB-1 and milled Neches increased by NaCl, but not that of milled HB-1. DTT reduced the pasting viscosity of Neches, whereas that of HB-1 decreased moderately. Whole grain Neches and HB-1 and milled Neches exhibited greater pasting viscosity at pH 2, whereas milled HB-1 was unaffected. Pasting viscosity of Neches was slightly affected at pH 12, whereas that of whole grain HB-1 increased and milled HB-1 decreased. Protein fractions of both cultivars shared similar molecular weight and secondary structures, but polyphenols affected the tertiary structure. The polyphenol-protein cross-links played the predominant role in the viscosity development of purple rice and may enhance the stability of the protein network at pH 2 and in the presence of DTT, but were disrupted at pH 12. Polyphenol-protein interactions may improve starch pasting properties without the need for chemical modifications.