Potential of quercetin in combination with antioxidants of different polarity incorporated in oil-in-water nanoemulsions to control enzymatic browning of apples

Quercetin in combination with other antioxidants from natural sources and different polarity, including quercetin-3-rhamnoside, ascorbic acid, and α-tocopherol, were incorporated into oil-in-water (O/W) nanoemulsions to evaluate their effects on the activity of polyphenol oxidase (PPO) and browning of Granny Smith apple fruits. Additionally, molecular modeling methods were used to determine the ability and possible mechanisms of action of quercetin and quercetin-3-rhamnoside to inhibit PPO and to reduce the o-quinones derived from (–)-epicatechin, chlorogenic acid, catechol and methylcatechol (PPO substrates). Nanoemulsions were prepared using a low-energy emulsification method. In all nanoemulsions, Dynamic Light Scattering (DLS) analysis showed a size distribution of droplets between 100 and 500 nm diameter, whereas microscopical observation by freeze-fracture transmission electron microscopy (FF-TEM) corroborated the presence of the droplets and their size. Also, occasional arrangements consisting of stacked layers, with vesicle-like structures between them, were seen. The nanoemulsion 1, that contained the four antioxidants was the one that showed the best effect on PPO, inhibiting it by 41%, and inhibiting the browning of the apple flesh between 81 and 74% in a period of 6 h. Calculations with molecular modeling methods suggest that quercetin may be a more effective reducing agent than ascorbic acid and quercetin-3-rhamnoside, and could also act as a non-competitive inhibitor of PPO. While quercetin-3-rhamnoside represents a selective and competitive inhibitor of PPO. The most important function of α-tocopherol in the nanoemulsions could be the regeneration of quercetin to prolong its antioxidant activity.