Cold plasma effects on enzyme activity in a model food system

Polyphenoloxidase (PPO) and peroxidase (POD) are enzymes that need to be inactivated or inhibited in order to avoid undesirable browning reactions and the loss of sensorial or nutritional quality of fruits and vegetables. As a promising gentle alternative to traditional methods, such as pasteurisation or the use of antibrowning compounds, the present study investigates cold plasma's ability to inactivate PPO and POD in a model food system, as well as possible inactivation mechanisms. The study shows that cold plasma is capable of reducing the activity of both PPO and POD in the model food system. The activity of PPO was reduced by about 90% after a treatment time of 180 s. POD was more stable and was reduced by about 85% after 240 s. Circular dichroism and tryptophan fluorescence measurements indicate that the reason for their loss of activity is based on a plasma-induced modification of their secondary structure. A decrease in the alpha-helix content was accompanied by an increase of the percentage of beta-sheet regions. Reduced and red-shifted tryptophan fluorescence intensities supported these changes. Industrial relevance: The quality of freshly cut fruits and vegetables greatly depends on the activity of naturally occurring enzymes such as PPO and POD, which catalyse browning reactions at cut surfaces. The presented study shows that cold plasma, as a promising non-thermal pasteurisation technology, is capable of reducing the activity of these enzymes in a model food system. In addition, it describes the impact of different treatment parameters and gives insights into inactivation mechanisms. The results contribute to the understanding of cold plasma effects on enzyme activity and could be a basis for a possible industrial implementation. (C) 2013 Elsevier Ltd. All rights reserved.