NMR studies of changes in subcellular water compartmentation in parenchyma apple tissue during drying and freezing

Abstract

The potential of Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) for non‐invasively monitoring the subcellular and intercellular redistribution of water in cellular tissue during drying and freezing processes is assessed and it is concluded that despite exciting advances in NMR micro‐imaging and NMR microscopy, nonspatially resolved NMR relaxation and diffusion techniques still provide the best probes of subcellular water compartmentation in tissue. The power of the NMR relaxation technique is illustrated by using the changes in the distribution of NMR water proton transverse relaxation times to monitor the subcellular compartmentation of water and ice during the drying and freezing of parenchyma apple tissue. The NMR drying data are analysed with a numerical model of the cell and show that mild air‐drying in a fluidized bed results in loss of water from the vacuolar compartment, but not from the cytoplasm or cell wall regions. The loss of vacuolar water is associated with overall shrinkage of the cell and only a slight increase in air space. During freezing the vacuolar compartment is found to be the first to freeze, with the cytoplasmic and cell wall compartments only freezing at much lower temperatures. Freeze‐drying apple tissue gives much lower water contents than fluidized bed drying, but the NMR data confirms that it destroys membrane integrity and causes cell wall collapse.