Osmotic dehydration processing of kiwifruit pericarp tissue studied by means of LF-NMR relaxometry

Osmotic dehydration is a partial dewatering process by immersion of cellular tissue in a hypertonic solution, which is accompanied by solutes counter-diffusion into the tissue. Low field-nuclear magnetic resonance (LF-NMR) seems to be a promising technique to follow the distribution of water/solutes through the cellular tissue during treatment. The objective of the present study was to evaluate the cellular compartment modifications of kiwifruit (Actinidia deliciosa) outer pericarp tissue caused by osmotic treatment in a 61.5% sucrose solution through the quantification of transversal relaxation time (T2) and water self diffusion coefficient (Dw) obtained by LF-NMR means. Proton T2 of the samples was measured using CPMG sequence. Three T2 were obtained of around 30, 200 and 1000 ms, which could be ascribed to the protons located in the cell walls, in the cytoplasm/ extracellular space, and in vacuoles, respectively and could be directly compared to those described in the literature for several fruits and vegetables. Vacuoles T2 represented around 66% of total kiwifruit protons. The leakage of water leading to vacuoles shrinkage seemed to cause a concentration of solutes, retained by the tonoplast, making the vacuoles T2 decrease at each treatment time. Dw were measured by means of the pulsed field spin-echo (PFSE) sequence. As expected, the kiwifruit Dw values measured in the raw kiwifruit were lower than free water one, as the structures and solutes of raw kiwifruit reduce water mobility, and decrease even more during osmotic treatment, due to the water loss and sugar gain. The coefficient measured by means of PFSE represented an average value of the whole kiwifruit tissue protons. In order to obtain Dw values specific for each cellular compartment, a two compartment fitting was also used. The Dw correspondent to vacuoles was much higher than the average one Dw.