Water addition to the solid substrate preceding autoclaving increased substrate porosity and phytase production in solid state fermentation. In comparison with dry sterilization, the phytase activity increased 6-, 8.5-, and 10-fold when the autoclaving time was 20, 40, and 60 min, respectively. Autoclaving increased the void space of sterilized lentils, and the increase was 16% higher when water was supplemented to the lentils before sterilization. Image analysis of SEM pictures of the solid substrate showed that water supplementation presterilization portended greater micro-fissure surface area, which also increased with increasing the sterilization time. SEM pictures of the fermentation product showed that fungal growth into the center of the solid substrate was ubiquitous when water was supplemented before sterilization but was absent when water was supplemented post sterilization. Similarly, spore formation on the substrate surface for the presterilization water supplementation samples far exceeded spore formation for samples that received supplementation poststerilization. This evidence suggests that improved mass transfer into the solid substrate resulting from additional pore volume and the formation of micro-fissures on the substrate surface is responsible for the observed gains in phytase productivity in solid state fermentation. (c) 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012