Cold atmospheric plasma inactivation of polyphenol oxidase: Focus on the protective and boosting effect of mono- and disaccharides

Polyphenol oxidase (PPO) is among the most detrimental enzymes in processed plant foods, being responsible for enzymatic browning. To propose a "mild" alternative to traditional enzymatic inactivation methods, this study investigated the effect of cold atmospheric plasma (CAP) on PPO inactivation and highlighted the role of different sugars on both inactivation and structural modification of this enzyme. Different model systems were prepared in phosphate buffer using a purified PPO either alone or added with glucose, fructose, sucrose, and trehalose at different concentrations. CAP treatments (6 KV; 23 KHz; duty cycle 10%) were applied at times ranging from 5 to 30 min. Different spectroscopic analyses were conducted before and after treatments to evaluate the PPO activity and changes in tertiary and secondary structures. CAP induced a significant reduction (p < 0.05) in PPO activity across all systems, ranging from 70% to 94% after 30 min of treatment. Among sugars, fructose enhanced (p < 0.05) the PPO inactivation (+23% on average with respect to the phosphate buffer system), possibly by promoting the loss of secondary structures containing the copper-binding site of the catalytic pocket. The effect of other sugars on PPO inactivation was strictly dependent on their type and concentration; specifically, disaccharides at the highest concentrations and treatment times showed a protective effect on the structure and functionality of the protein. Thus, the results of this study highlight that sugars can modulate the effectiveness of CAP, offering promising perspectives for optimizing this food processing technology. PRACTICAL APPLICATION: Cold atmospheric plasma (CAP) is a promising nonthermal technology for food preservation. In particular, surface dielectric barrier discharge (SDBD) CAP could be applied as an alternative to chemical and thermal treatments to inactivate polyphenol oxidase (PPO), an enzyme responsible for browning reactions and quality loss in most processed fruit and vegetable products. However, as shown by this study, PPO inactivation induced by CAP is affected by sugars. Specifically, fructose can positively influence the inactivation of this enzyme. Therefore, CAP potentially could find main applications for the PPO stabilization of high fructose-content plants (e.g., pears, apples, bananas, grapes, peppers, and squashes).