The secondary structure of proteins in legumes, cereals, milk products and chicken meat was studied by diffuse reflectance infrared spectroscopy in the region of the amide I band. Major secondary structure components ( β-sheets, random coil, α-helix, turns), together with the low- and high-frequency side contributions, were resolved and related to the in vitro digestibility behaviour of the different foods. A strong inverse correlation between the relative spectral weights of the β-sheet structures and in vitro protein digestibility values was measured. Structural modifications in legume proteins induced by autoclaving were monitored by the changes in the amide I spectra. The results indicate that the β-sheet structures of raw legume proteins and the intermolecular β-sheet aggregates, arising upon heating, are primary factors in adversely affecting the digestibility.