Modification of Transverse NMR Relaxation Times and Water Diffusion Coefficients of Kiwifruit Pericarp Tissue Subjected to Osmotic Dehydration

The objective of the present study was to evaluate cellular compartment modifications of kiwifruit (Actinidia deliciosa) outer pericarp tissue caused by osmotic treatment in a 61.5 % sucrose solution, through the quantification of transverse relaxation time (T2) and water self-diffusion coefficient (Dw) obtained by low field nuclear magnetic resonance means. Raw material ripening stage was taken into account as an osmotic dehydration (OD) process variable by analyzing two different kiwifruit groups, low (LB) and high (HB) °Brix. Three T2 values were obtained of about 20, 310, and 1,250 ms, which could be ascribed to the proton populations, located in the cell walls, in the cytoplasm/extracellular space, and in the vacuoles, respectively. According to T2 intensity values, vacuole protons represented between 47 and 60 % of the total kiwifruit protons, for LB and HB kiwifruits, respectively. The leakage of water leading to vacuole shrinkage seemed to cause concentration of solutes, retained by the tonoplast, making the vacuole T2 value decrease along the OD. As expected, the Dw values of raw kiwifruits were lower than the value of the free pure water. The water mobility (and hence Dw), depending on the kiwifruit distinctive cellular structures and solutes, decreased even more during OD due to water loss and sugar gain phenomena. Dw represents an average value of the diffusion coefficient of the whole kiwifruit tissue protons. In order to obtain Dw values specific for each cellular compartment, a multiple component model fitting was also used. According to these results, the vacuole water self-diffusion coefficient (Dw,v) was much higher than Dw.