Cold Plasma Treatment Effects on Nutritional Quality and Shelf Life Extension of Fresh-Cut Vegetables

Abstract

Minimal processing (peeling, slicing, shredding) breaks tissue, causing the intracellular contents to trigger a wounding response which results in increased respiration and ethylene production, increased enzymatic browning rates, and fast microbial growth, making a product of high nutritional value only last for a few days. It has been proposed that cold plasma may be a promising non-thermal solution to this issue as it is able to inactivate surface microorganisms and oxidative enzymes without raising the temperature of tissues. However, the reactive oxygen and nitrogen species it creates are the very ones that oxidate ascorbic acid, polyphenols and pigments, which has an inherent quality trade-off to the technology, instead of a simple benefit. This review reviews that trade-off quantitatively. Evidence is compiled on the inactivation of polyphenol oxidase (PPO), peroxidase and pectin methylesterase, behaviour of vitamin C, phenolic compounds, chlorophyll and texture, and microbial and shelf-life results obtained in lettuce, rocket, chicory, melon, carrot and apple systems. In the studies reviewed, enzyme inactivation ranged from 42 to 77%, losses of immediate ascorbic acid were generally less than 4–7%, and microbial reductions of 1.7–5 log CFU/g resulted in a shelf life of 2.5–3 days in fresh-cut melon and more than 4 days in ready-to-eat rocket. The pattern is biphasic, that is, a dose response curve with high doses causing a decrease in quality and low doses causing the retention of quality. Hence, the process parameters (gas composition, voltage, exposure mode and time) determine whether cold plasma is a stressor or a preservative, and the paper finds the most important design criteria is to optimize the process parameters according to a multi-parameter quality index rather than the maximum microbial inactivation. This work is concerned with the potential of cold plasma to extend the shelf life and improve nutritional quality of fresh-cut vegetables in the context of polyphenol oxidase (PPO) inactivation, ascorbic acid production, and plasma-activated water application.