Polyphenol oxidase activity and colour changes of ‘Starking’ Apple cubes coated with alginate and dehydrated with air

The objective was to study the effect of alginate coating on polyphenol oxidase (PPO) activity and colour of ‘Starking’ apple cubes during dehydration with hot air. Apple cubes were dehydrated at 20°C, 35°C or 40°C, with a parallel air-flow. Analysis of PPO activity, colour (L*, a*, b*) and dry matter were performed along the dehydration process at each temperature. All samples presented a peak in relative PPO activity in the beginning of the drying. Exponential models fitted well the experimental data after the peak. Cubes without coating presented lower PPO activity, suggesting lower browning than coated samples throughout the dehydration process, for all temperatures. Better results for coated samples were obtained with a perpendicular airflow drying at 40°C, after dipping the whole apple in water at 60°C for 10 minutes. In order to prevent coated samples from browning, drying by perpendicular airflow preceded by a thermal treatment of the whole apple is required.