The effect of polymorphism on the purity of L-glutamic acid (L-Glu) is investigated by combining experiments and molecular simulations using L-aspartic acid (L-Asp) as an impurity. The results show that the content of L-Asp in beta form is higher than that in a form, and the purity L-Glu decreases with the increase of the amount of beta form in the final product. Molecular simulation is employed to account for the different contents of L-Asp in different polymorphs of L-Glu. It reveals that L-Asp is mainly incorporated into the (011) surface of a form and the (020) surface of beta form. Because the area for alpha (011) is much smaller than that for 13 (020), beta crystal can accommodate more L-Asp than does a crystal. (C) 2012 Elsevier B.V. All rights reserved.