Trypsin is a serine protease, which has been proved to be a novel superoxide scavenger. The burst of superoxide induced by polychlorinated biphenyls can be impeded by trypsin in both wild type and sod knockout mutants of Escherichia coli. The experimental results demonstrated that the activities of superoxide scavenging of trypsin were significantly accelerated by Cu ions. Also, with the addition of Cu ions, a new β-sheet (β7) transited from a random coil in the Cu(II)-trypsin (TP) system, which was favorable for the formation of more contacts with other sheets of trypsin. Residue–residue network analysis and the porcupine plots proved that the Cu ion in trypsin strengthened some native interactions among residues, which ultimately resulted in much greater stability of the Cu(II)-TP system. Moreover, compact and stable trypsin structures with Cu ions might be responsible for significantly provoking the activity of superoxide scavenging.

Flesh browning is fundamentally important to the quality of minimally processed fruit. The development of browning is associated with reactive oxygen species accumulation in the fruit tissue. The novel superoxide scavenging activity of trypsin has been shown in our previous work. In the current study, the effects of mixtures of trypsin and ascorbic acid (AsA) on the processing quality of apple homogenate were evaluated. In the presence of trypsin and AsA, the browning of apple homogenate was significantly impeded, and changes in the titration of acid and in the concentration of soluble solids were also delayed. The mixture of trypsin and AsA could significantly reduce cell membrane damage and the activity of peroxidase. Above all, trypsin could protect the cells of the fruit due to its superoxide scavenging activity. The utilization of trypsin will provide a new strategy for controlling quality in the processing of apples or other fruits and vegetables. Practical applications Pure, natural fruit juice with fruit flavor, especially pure, cloudy juice with a variety of natural nutrients, is increasingly widely accepted. Eliminate or inhibit the free radical accumulation is one of the most efficient methods to control the browning of apple tissue. Trypsin had a significant effect on the superoxide anion scavenging activity and showed a good protective effect on the cells. This article investigates the antioxidant and cell protective activity of trypsin–ascorbic acid (AsA) mixture during the processing of apple homogenate. The application of trypsin provides a new strategy for the quality control of fruit juice. Food additives with trypsin have a broad market and development potential.