Enzymatic synthesis and physicochemical characterisation of phloridzin oxidation products (POP), a new water-soluble yellow dye deriving from apple

Abstract

Abstract Phloridzin is a phenolic compound specifically found in apple and particularly concentrated in the pomace. Its oxidation by mushroom polyphenol oxidase followed a very specific pathway: phloridzin was first almost totally converted to a colourless intermediate (POPi), which was then converted to a yellow pigment (POPj) without any significant side reactions. The pigment could be an alternative to tartrazin for use as a food dye. Maximum yields were 84% for both compounds, depending on the incubation time. POPj had a bright yellow colour at pH 3 to 5, turning to orange for higher pHs. Half saturation at pH 3 was obtained for a concentration close to 30 mg/L. Kinetics of degradation of POPj in aqueous solution were determined as a function of pH and temperature. Treatments for several hours at high temperature (above 80 °C) resulted in a partial degradation, which was accelerated when the pH increased. However, POPj was only weakly degraded by prolonged storage at ambient temperature. Esterification of POPi and POPj in ethanol/HCl yielded the corresponding ethyl esters with 60 and 98% yields, respectively. POPi and its ethyl ester showed free radical scavenging activities (DPPH method) comparable to those of well-known antioxidants such as ascorbic acid, trolox or (−)-epicatechin. Industrial relevance In the field of food dyes, industries are more and more interested in alternatives to synthetic dye. The purpose of this study is to show the benefits and also the limits of a new bright yellow and highly hydrosoluble dye resulting from the enzymatic oxidation of phloridzin by polyphenoloxidase. Phloridzin is concentrated in apple pomace which is the major by-product of apple juice and cider production. Therefore, the use of apple pomace for the production of a new yellow dye might be possible. The pigment could be an alternative to tartrazin for use as a food dye.