BACKGROUND: Soybean hulls result from the processing of the bean for producing oil and protein products. This by-product generated massively in America has virtually no commercial value, so substantial effort is being applied to its exploitation for generating value-added goods. This work evaluates soybean hulls as inducer of the production of pectinolytic enzymes, through optimization studies regarding polygalacturonase production by Aspergillus sojae in submerged cultures. RESULTS: A 2-fold improvement in polygalacturonase yield was found by varying the initial pH of the culture in a very narrow acid pH range (2.40-2.80). The optimized fermentation process was successfully transferred to stirred-tank bioreactors in terms of volumetric productivity, and final polygalacturonase yields were 42 U mL(-1) and 1.39 U g(-1) soybean hulls, which are among the highest reported with this by-product. Morphological characterization of A. sojae during cultivation showed that the fungus mainly developed in dispersed mycelia at initial pH of 2.40-2.80 while, conversely, fungal pellets predominated in cultures performed at initial pH of 5.40. CONCLUSION: High enzyme titers are possibly connected to the formation of dispersed mycelia, as well as to acid-induced expression of the respective gene/s. It is foreseen that this data will be helpful regarding the production of fungal pectinases or other acid-induced enzymes. (C) 2017 Society of Chemical Industry