Vacuum-assisted impregnation of pectin methylesterase (PME) solution has been recognized as an efficient pretreatment to improve the firmness of heat-treated fruit. In order to improve the control of solute infusion into fruit pieces, the effect of the osmolarity and viscosity of vacuum impregnation solution on solute penetration and distribution was studied in 1.5 cm apple cubes, using model PME-based solutions containing sodium chloride and/or sodium alginate. While vacuum impregnation of either a viscous hypotonic or a non-viscous hypertonic, solution infused solutes homogeneously into fruit pieces, the penetration of viscous hypertonic solutions was much lower, and PME or chloride infusion was limited primarily to the superficial zone of apple cubes. Similar penetration was observed with a highly concentrated sucrose solution. These findings may result from a synergistic combination of negative osmotic and friction effects during liquid flow into fruit pores. (C) 2007 Elsevier Ltd. All rights reserved.