The objective of this study was to examine the mechanical behavior of poly(lactic acid)-based packaging, which was subjected to an external load while undergoing dimensional and material property changes due to the diffusion of liquid through its thickness. We defined the characteristics of the material response by taking into account the changes in the properties due to liquid sorption. The material properties at any given location and time were dependent on the liquid content. In this article, we present the evolution of the mechanical behavior using a numerical stress model that accounts for the effects of the time, liquid content, temperature, and swelling-induced strain. We assumed that the deformation depended on the liquid concentration, but the liquid concentration could be obtained without the knowledge of the stress or strain. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40600.