In modern food industry a major challenge is to maintain the quality and extend the shelf-life of processed foods. Multiple types of chemical reactions have been found to cause quality loss of processed foods. Oxidation and browning reactions have been identified as the two well-known types. The present study focuses on lipid oxidation and the Maillard reaction (known as nonenzymatic browning). Lipid oxidation in foods is mainly related to the unsaturated lipids, and the Maillard reactions is a ubiquitous reaction between reducing sugars and amino groups as found in amino acids, peptides and proteins. Lipid oxidation as well as the Maillard reactions can affect the quality of foods. An efficient way to retard or inhibit both types of reactions can be achieved by utilizing natural antioxidants, which are normally presumed to be safe when the source is plant material, “Generally Recognized as Safe” (GRAS-status).
In this PhD project, green tea rich in polyphenol compounds catechins was selected as the natural source of antioxidants. The aim of the project was to investigate green tea as a source of natural antioxidants for use in processed foods. First of all, the antioxidant effects of green tea catechins and green tea extract were investigated in terms of scavenging efficiency of the food-derived free radicals. The relevant experimental methods were based on electrochemical detection of oxygen consumption, electron spin resonance (ESR) spectroscopy, Rancimat test and liposome oxidation test. In addition, antioxidant interactions for individual green tea catechins or green tea extract in combination with endogenous antioxidant, α-tocopherol, were studied. The results showed that green tea catechins possess excellent antioxidant activity against the oxidative damage of lipids induced by free radicals. Moreover, the combinations of green tea catechins and green tea extract with α-tocopherol were found to exert synergistic antioxidant effects in the heterogeneous systems such as o/w emulsions.
Secondly, the ability of green tea catechins to reduce the protein radicals generated from the hypervalent heme pigment ferrylmyoglobin (MbFe(IV)=O) was analyzed using a kinetic method, stopped-flow absorption spectroscopy. The results showed that tea catechins such as epicatechin (EC) and epigallocatechin gallate (EGCG), particularly EGCG, have very potent activity to deactivate the MbFe(IV)=O. However, combinations of tea catechins were not observed to have a synergistic effect to reduce the MbFe(IV)=O, but merely an additive effect or even antagonism was observed.
Thirdly, green tea catechins including EC and EGCG were found to efficiently inhibit the formation of intermediary radicals formed during the Maillard reactions by ESR. A mechanistic study of influencing Maillard reaction products by green tea catechins was mainly explored by liquid chromatography-mass spectrometry (LC-MS). The results revealed that the quinone form of EC can undergo either a Michael type addition or a “Strecker like” reaction to react with amino acids such as lysine, in effect inhibiting the Maillard reactions, indicating that the inhibitory effect is not only correlated with the abilities of catechins to scavenge radicals and quench sugar fragmentation products.
Taken together, the results indicate that green tea can provide extremely strong antioxidative protection against oxidative damage of foods and at the same time, against the Maillard reactions, and thereby can be used as a very good source of natural antioxidants for processed foods.