Amaranth (Amaranth spp) is considered a pseudocereal with valuable nutritional characteristics. Amaranth grain is commonly used in cereal industry as expanded product obtained from the puffing operation. In this work, the puffing process of amaranth seeds with different water content was studied. Moreover, the seeds and puffed kernels were analyzed by Differential Scanning Calorimetry (DSC), image analysis, dielectric spectroscopy and Cryo-SEM. The results of the study allowed identifying different levels of water retention in amaranth seeds. Moreover, the expansion process was modeled determining the critical points, obtaining the state variables and the final product conditions. It was also performed a structural study of the amaranth seed and the puffed product. The dielectric properties were also measured for determining the optimal time for the puffing operation. (C) 2011 Elsevier Ltd. All rights reserved.