Polyphenol oxidase inactivation and process adaptation for the valorization of banana peels into dietary fiber ingredients

Bananas (Musa sp.) are among the most widely produced crops worldwide. Banana processing generates substantial quantities of peels as a by-product, rich in dietary fiber and polyphenols, yet often discarded as waste. Key obstacles to their use are rapid deterioration and enzymatic browning caused by polyphenol oxidase (PPO), limiting their utilization in the food industry. The aim of this thesis was to characterize banana peel PPO and establish suitable conditions for its inactivation, followed by the evaluation of thermal PPO inactivation and hot-air drying as stabilization methods and their effects on the yield, purity, and functional properties of dietary fiber fractions. In the first part, extraction conditions were adjusted to increase PPO activity, followed by characterization of 'Prata' banana peel PPO, showing highest activity at 30–40 °C and pH 6.0–6.5, with highest affinity towards dopamine (KM = 0.94 mM). Complete inactivation of 'Prata' and 'Cavendish' PPO was achieved at 90 °C after 5 and 15 min, whereas plasma processed air reduced activity to only 46 %. A biphasic model indicated at least two isoenzymes, with the highest thermal stability attributed to membrane-bound PPO. The second part investigated thermal PPO inactivation in banana peels. Blanching at 90 °C for 15 min achieved complete inactivation, improving soluble dietary fiber (SDF) content, water-holding, and oil-binding capacity, without affecting antioxidant capacity or phenolic content. Blanching increased total dietary fiber (TDF) content of the dietary fiber concentrate (DFC) by 11% and galacturonic acid content of the SDF fraction by 52%. Hot-air drying preserved insoluble dietary fiber (IDF) functionality and enhanced its yield by 9%, while SDF purity increased by 17%. Ripening degree had a pronounced effect on fiber fractionation. Riper bananas yielded DFC with higher TDF content and 197% higher SDF purity. Overall, the results demonstrate that thermal stabilization can effectively overcome PPO-related limitations in banana peel utilization while maintaining or improving the yield and functional properties of dietary fiber fractions. Together with the pronounced influence of ripening degree, these findings provide a basis for the targeted processing of banana peels into functional dietary fiber ingredients and their valorization as a by-product.