The effect of osmotic adjustment on the mechanical properties of potato parenchyma

Abstract

Changes in the mechanical properties of potato tubers can be primarily attributed to physiological changes which affect structural components (cell wall, middle lamella) and to turgor pressure changes within the cells (which are affected by water loss and membrane integrity). In order to separate the effect of physiological changes from those associated with water loss, the cell turgor pressure of potato parenchyma was adjusted by hyper- and hypotonic solutions of mannitol. Tubers of the cv. Russet Burbank were stored for long (10 months) and short (1 month) periods. Slices and cylinders of parenchyma were obtained from the centres of the tubers along three axes; slices were also taken from the outer part of the tuber along two of the axes. Slices were subjected to small strain oscillatory shear at 0.02, 0.2 and 2 Hz and the storage and loss moduli (shear stiffnesses) obtained. Cylinders were compressed to 10.5% strain at 2 and 20 cm min −1 and initial and end moduli (longitudinal stiffnesses), and energy absorption terms obtained. Variability in measured mechanical properties was less when cells were fully turgid. Anisotropicity in shear stiffness was evident, although anisotropicity in longitudinal stiffness was dependent on turgor pressure. Shear stiffness was greater in outer parts of the tuber when slices were adjusted to common turgor pressures. Longitudinal stiffness was affected much more by osmotic adjustment than was shear stiffness. Although quantitatively different, hypotonic solutions caused the load-deformation curves of long term storage tubers to resemble those of short-term storage tubers. Similarly, hypertonic treatment brought about a load-deformation response in new tubers that made them appear as though they had been stored for ten months. The results indicate the importance of water content when analysing the mechanical properties of potato parenchyma.