Time domain nuclear magnetic resonance to monitor mass transfer mechanisms in apple tissue promoted by osmotic dehydration combined with pulsed electric fields

Pulsed electric field (PEF) technology is gaining momentum as a pre-treatment to enhance mass transfer of vegetable tissues obtained by further processing. In this study PEF pre-treatment increased osmotic dehydration (OD) effectiveness, in terms of water loss and solid gain in apples, as a function of electric field strength and number of pulses. Mass transfer was particularly high when average electric fields of 250 and 400V cm-1 were applied. Time domain nuclear magnetic resonance (TD-NMR), with the use of a contrast agent, clarified structural changes that drive mass transfer. Treatments at 100V cm-1 redistributed water between vacuole, cytoplasm and extracellular space, while at 250 and 400V cm-1 the membrane breakages caused the loss of cellular compartmentalization. Two non-destructive and fast acquirable parameters, the longest measured relaxation time (T2) and water self diffusion coefficient (Dw), allowed the separate and accurate observation of PEF treatment and osmotic dehydration effects. Industrial relevance: The developed non-destructive method, here described, allows the measure of the effects of PEF treatment on apple tissue which can be exploited to have reliable control of the process within minutes. Since mass transfer parameters depend on subcellular water redistribution, the present work provides a tool to boost the development and optimization of agri-food processes on fresh vegetable tissues.