Preliminary Study on the Reduced Pressure Cold Plasma Processing of Fresh Cut Salads: Rheological Assessment of Modifications in the Plant Tissue Structure

Abstract

Cold plasma is a novel nonthermal food processing technology that uses energetic, reactive gases to inactivate contaminating microbes and to achieve an enzymatic depletion on foods. The primary modes of action are due to UV light and reactive chemical products of the cold plasma ionization process. A wide array of cold plasma systems that operate at atmospheric pressures or in low-pressure treatment chambers are under development. Optimization and scale up to commercial treatment levels require a more complete understanding of plasma interaction modes with food. In particular, little attention to date has been paid to structural and textural modification induced by plasma in food matrices. Results that are presented in this paper refer to the study of low pressure cold plasma treatment of fresh vegetable, precisely fresh cut salad leaves (Valerianella locusta Laterr). Throughout dynamic rheology tests, it has been evaluated the impact of the non-thermal plasma treatment on the structural modifications occurring in the salad tissue at a mesoscopic scale-length where cell wall materials (CWM) account for the perceived texture of the salad. The rheological behaviour of CWM water dispersions in steady-state regime proved that plasma treatment is able to induce an immediate modification in flow behaviour, which persists during the shelf-life period. CWM dispersions behave as weak gels. Specifically, cold plasma treatment of salad leaves generates CWM gels with higher coordination of the gel polymers network, as quantified by means of the Bohlin’s theory on week gels. Moreover, it has been proved that the relaxation time distribution spectrum may provide useful information on understanding the underlying stress relaxation mechanisms in gels. An in-depth investigation of physical events could be of help in view of the optimization of the cold plasma technology application in plant foods preservation.