Investigating Phenolic-Mediated Protein Matrix Development for Potential Control of Cereal Starch Digestion

Leigh Christine R Schmidt, Purdue University

Abstract

Shifts in the human diet to more refined foods and ingredients have contributed to the rise in metabolic disease rates associated with long-term consumption of foods causing swift rises in blood glucose response. Foods which result in a more moderate blood glucose curve are considered healthier by increasing satiety and reducing oxidative stress. Sorghum products contain naturally slowly digested starch. The matrix of sorghum porridges contains kafirin protein bodies which cross link around gelatinizing starch molecules, while similar nascent matrices in other cereals aggregate and collapse. The 3-deoxyanthocyanidin pigments unique to sorghum may be accountable for the difference in matrix stability. The density of the starch entrapped in the matrices is thought to partially inhibit α-amylase access to the starch, reducing overall starch digestion and thereby mitigating glucose response. The purpose of this work was to increase our understanding of how phenolic compounds in sorghum interact with endosperm proteins to create a stable matrix, and to explore if the knowledge might be translated to other starchy cereal products. In the first study, phenolic extracts from flours (sorghum, corn masa, white rice) were characterized for phenolic content, antioxidant activity, phenolic components, and their ability to interact with a model protein system (ovalbumin) in order to examine protein polymerization. While neither phenolic content nor antioxidant activity were found to predict polymerization, sorghum extracts demonstrated more diverse molecular weight (MW) products than masa or rice. In the second study, specific phenolic compounds in sorghums (p-coumaric, sinapic, and gallic acids; (+)-catechin; and apigeninidin, a 3-deoxyanthocyanidin found in sorghums) were interacted in the model protein system at different concentrations to observe extent and type of protein polymerization, and promising compounds subjected to fluorescence quenching spectroscopy to examine the nature of the interactions. Apigeninidin produced a wide range of MW products linked by disulfide bonds, while gallic acid stimulated oxidation-driven polymerization with disulfide and other non-amide peptide linkages. Catechin, gallic acid, and apigeninidin quenching of ovalbumin native tryptophan fluorescence intensified as concentration (0-50 μM) and temperature (25-95°C) increased. At higher concentrations (25-50 μM), apigeninidin had the greatest effect on protein structure, indicated as a red shift (+6 ±1 nm) in peak emission wavelength due to increased hydrogen bond formation. The number of binding sites per ovalbumin tended to rise with temperature for apigeninidin and catechin, as did the binding affinity, but temperature had less effect on gallic acid binding. With the polymerization and binding data, apigeninidin was determined as a prospective molecule for increasing protein matrix stability in foods without causing excessive oxidation-driven aggregation. The final study explored the effects of apigeninidin addition to a yellow corn flour and naturally present anthocyanin (blue corn) on starch digestion and microstructure of porridges by utilizing an in vitro α-amylase assay and confocal microscopy. Notably, apigeninidin was not found to inhibit α-amylase at the levels used and would not have contributed to slower digestion. Porridges with apigeninidin evinced a reduction of initial starch digestion rate through 20 min digestion, and increased protein matrix formation and stability compared to an untreated control. Enhanced matrix remained more stable through 60 min αamylase digestion. Blue corn protein microstructure was less extensive and less stable than apigeninidin-treated samples but improved compared to the untreated yellow corn.

Degree

Ph.D.

Advisors

Jones, Purdue University.

Subject Area

Physiology|Agronomy|Atomic physics|Food Science|Physics

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