A microscopic method was developed to detect pressure thresholds for gas outflow and solution impregnation during vacuum impregnation of plant materials. Raw materials with different porosities (apple and spinach) were impregnated with an isotonic sucrose solution at a minimum pressure of 150 mbar. An automatic vacuum controller system (AVCS) was used to control the pressure during vacuum impregnation. Micrographs of tissues subjected to vacuum impregnation were recorded as the pressure in the treatment chamber was varied. Image analysis allowed the evaluation of the pressure at which gas was released from the pores (seen as bubbles) during the application of vacuum, and the pressure at which tissue impregnation took place during the restoration of atmospheric pressure. Spinach tissues showed gas release at a much lower pressure than apple, and impregnation commenced at a much higher pressure. These differences in pressure threshold could be caused by the narrow pores in spinach and possibly by changes in leaf volume. Industrial relevance: Vacuum impregnation is considered a promising technology to facilitate the impregnation of vegetable tissues with different solutions containing, e.g., firming, antioxidant or antimicrobial agents. The mass transfer taking place during vacuum impregnation depends on the characteristics of the tissue pores. Effective vacuum impregnation requires the efficient removal of air from the tissue during vacuum treatment to obtain complete filling of the tissue during the subsequent impregnation step. The findings of this study may allow food manufacturers to optimize vacuum impregnation parameters depending on the porosity characteristics of the raw material. (C) 2013 Elsevier Ltd. All rights reserved.