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

Authors: JANNASCH, ANNEGRET - University Of Arkansas; WANG, YA-JANE - University Of Arkansas; THALLAPURANAM, SURESH - University Of Arkansas; ALRAAWI, ZEINA - University Of Arkansas; McClung, Anna

Submitted to: Journal of Cereal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2024
Publication Date: 3/1/2024
Citation: Jannasch, A., Wang, Y., Thallapuranam, S., Alraawi, Z., McClung, A.M. 2024. Elucidating the effect of polyphenol-protein interactions on rheological properties of purple waxy rice. Journal of Cereal Science. https://doi.org/10.1016/j.jcs.2024.103877.
DOI: https://doi.org/10.1016/j.jcs.2024.103877

Interpretive Summary: There is global interest in developing "clean label" food products which contain natural, simple ingredients free from artificial chemicals. Starch is an ingredient widely used in many processed foods but in its natural form it has limited commercial application due to its instability during mixing and due to extremes in temperature and pH. Chemical modifications of starch are commonly used to improve its stability during processing and storage however these are not considered as clean-label ingredients. This study was conducted to determine if there are other natural or minimal processing methods that can be used to enhance functionalities to produce clean-label starch ingredients. Although rice is generally consumed milled or with the brown bran layer intact, there are rice varieties with red or purple pigmented brans which are rich in natural phenolic compounds that are associated with health-promoting benefits such as anti-inflammatory, anticancer and antidiabetic properties. These phenolic compounds have been shown to modify flour pasting and viscoelastic properties through their interaction with cereal proteins, starch, and non-starch polysaccharides. Covalent and non-covalent interactions between these pigmented compounds and grain proteins may affect emulsifying, foaming and gelling properties, solubility, and thermal stability of rice starch used in food products. A study was conducted using chemical agents that disrupt specific grain chemistry interactions to better understand the role of polyphenols on starch rheological properties. These agents included using reducing agents that disrupt native protein structures, pH which changes the solubility of proteins and starch, urea which affects hydrophobic chemical interactions, and salts which can disrupt protein-protein bonds. A brown bran waxy rice (cv. Neches) and a purple bran waxy rice (cv. HB-1) were used to produce both whole grain flour and milled (bran removed) flour. The gelatinization and paste viscosity properties of these flours were evaluated directly and with different combinations of chemical agents. The results of this study found that covalent polyphenol-protein interactions were predominantly involved in the viscosity of the starch of the pigmented waxy rice (HB-1). This may be due to polyphenol-protein cross-linking which stabilized the protein-network, and which may provide starch with enhanced pasting viscosity and stability in neutral and acidic environments, similar to that of chemically cross-linked starch. This study demonstrates that pigmented rice has the potential to alter starch pasting properties and may serve as a clean-label ingredient in starch-based products.

Technical Abstract: Covalent and non-covalent interactions between polyphenols and protein are formed during hydrothermal processing of pigmented rice. Chemical agents including dithiothreitol (DTT), urea, and NaCl provided a tool to study the role of disulfide bonds, hydrogen bonds, and hydrophobic and electrostatic interactions in the viscosity development of whole grain and milled rice of non-pigmented (Neches) and pigmented (HB-1) waxy rice cultivars. Acidic (pH 2) and alkaline (pH 12) environments were used to investigate the presence of covalent polyphenol-protein cross-links. Urea did not affect pasting properties of Neches and HB-1. NaCl increased pasting viscosity of whole grain and milled Neches and whole grain HB-1, but did not affect pasting viscosity of milled HB-1. The pasting viscosity of Neches was significantly decreased by DTT, whereas that of HB-1 was only slightly reduced. Whole grain and milled Neches and whole grain HB-1 exhibited greater pasting viscosity at pH 2, whereas milled HB-1 was unaffected. At pH 12, the pasting viscosity of Neches was only slightly affected, whereas that of whole grain HB-1 increased and milled HB-1 decreased. The protein fractions of Neches and HB-1 shared similar molecular weight profiles and secondary structures, but the presence of medium to high polyphenol concentrations affected the tertiary and quaternary structure and mediated the conformational changes of rice proteins by DTT. Covalent polyphenol-protein interactions played the predominant role in the viscosity development of pigmented waxy rice. These cross-links altered protein structure to strengthen the protein network at pH 2 and in the presence of DTT, but were disrupted at pH 12. Polyphenol-protein interactions may be utilized to develop clean-label starch ingredients with improved pasting properties without the need for chemical modification.