We have used differential scanning calorimetry (DSC) to determine the unfolding properties of commercial products of human serum albumin (HSA) prepared from pooled human blood, transgenic yeast, and transgenic rice. The initial melting temperatures (T-m1) for the unfolding transitions of the HSA products varied from 62 degrees C to 75 degrees C. We characterized the samples for purity, fatty acid content, and molecular weight. The effects of adding fatty acids, heat pasteurization, and a low pH defatting technique on the transition temperatures were measured. Defatted HSA has a structure with the lowest stability (T-m of approximate to 62 degrees C). When fatty acids are bound to HSA, the structure is stabilized (T-m of approximate to 64-72 degrees C), and prolonged heating (pasteurization at 60 degrees C) results in a heat-stabilized structural form containing fatty acids (T-m of approximate to 75-80 degrees C). This process was shown to be reversible by a low pH defatting step. This study shows that the fatty acid composition and bioprocessing history of the HSA commercial products results in the large differences in the thermal stability. (c) 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:62-69, 2015