Controlling nonenzymatic browning reactions by selected dietary polyphenols in chemical and food models

Nonenzymatic browning reactions, which readily proceed with the starting reactants of sugar and protein during thermal processing and storage of food, are of unignorable significance to the formation of color, aroma and flavor compounds. In addition to its role in the sensory properties of food, the process also contributes to loss of essential nutrients, generation of beneficial compounds such as antioxidants, as well as production of toxicants, including 5-hydroxymethylfurfural (5-HMF), reactive carbonyl species (glyoxal and methylglyoxal) and advanced glycation end products (AGEs). These exogenous dietary toxicants can enter body circulation after digestion. The absorption, metabolism, and exposure to these toxicants have been found to cause oxidative cell damage, genetic mutation, pathological development of chronic diseases and tumor genesis. Dietary polyphenols from daily food intake have been reported to mitigate the toxicants formation primarily through free radical scavenging and dicarbonyl trapping activities in chemical models under simulated physiological conditions. However, there’s lack of studies on the phenolics’ antioxidant and antiglycation activity under thermal conditions or in real food matrix. The present study aims to evaluate the potential of dietary polyphenols in controlling the thermal proceeding of nonenzymatic browning reactions, specifically on the formation of colorants, antioxidants and toxicants. Six polyphenols commonly present in diet were selected, including phloretin, naringenin, quercetin, epicatechin, chlorogenic acid and rosmarinic acid, which belong to different structural categories. In sugar caramelization model, polyphenols were found to increase the browning intensity and antioxidant capacity of caramel. The chemical reactions in the system of sugar and polyphenol, which include formation of polyphenol-sugar adducts, were suggested to be partially responsible for the formation of brown pigments and heat-induced antioxidants. In addition, rosmarinic acid was demonstrated to significantly inhibit the formation of 5-HMF. When added into the glucose-casein glycation model, polyphenols were capable of inhibiting the formation of both total fluorescent AGEs and nonfluorescent carboxymethyllysine (CML).The thermal stability and transformation of polyphenols was likely an important factor affecting their antioxidant activity and inhibitory efficacy of dicarbonyls formation. We further discovered that treatment with epicatechin would not only lower AGEs formation but AGEs-induced cytotoxicity and oxidative stress to human retinal pigment epithelial (ARPE-19) cells. Finally, the practicality of polyphenols as functional food ingredients was explored in a cookie model. Results indicated that the enhancement on the antioxidant capacity was not as obvious as expected because the phenolics’ antioxidant activity was seriously lowered by the baking process due to thermal degradation and transformation. Meantime, the tested polyphenols, especially quercetin, showed inhibition against formation of both reactive dicarbonyls and total fluorescent AGEs. Last but not the least, polyphenol fortification could also induce colorimetric changes and alterations in selected cookie quality attributes. To conclude, the research contributed to the knowledge of the antioxidant and antiglycation activity of dietary polyphenols during high temperature thermal treatment. The findings support dietary polyphenols as functional food additives in purpose of increasing the level of health-beneficial antioxidants whereas lowering the load of harmful compounds and their in vitrotoxicity.published_or_final_versionBiological SciencesDoctoralDoctor of Philosoph