Gas in Scattering Media Absorption Spectroscopy (GASMAS) Detected Persistent Vacuum in Apple Tissue After Vacuum Impregnation

The microstructure and the capillary pressure of the pore space are important variables for better understanding of the complex phenomena occurring during vacuum impregnation (VI) of plant tissues. In this study, we used GASMAS (Gas in Scattering Media Absorption Spectroscopy) of oxygen to, non-destructively, measure the dynamics of the internal pressure in apple pieces after restoration of the atmospheric pressure. Apple pieces were impregnated with isotonic sucrose solution (18% w/v) at different reduced pressures (15, 30, 45 kPa (abs.)). After restoration of the atmospheric pressure, the pressure of the remaining pore space gas could remain as low as 50 kPa (abs) and rise slowly toward ambient over a time scale of hours. Both the residual vacuum and the timescale of pressure equilibration with ambient varied with applied vacuum level and apple variety. It is proposed that at least a part of the pore space of apples may be hydrophobic, giving rise to a negative Laplace pressure, and thus the convective flow of impregnating solution is arrested at a mechanical equilibrium where internal pressure is lower than external pressure. Further pressure equilibration can then only be achieved either by gas diffusion in gas phase, or by gradual wetting of the pores.