As the mechanism underlying the sense of smell is unclear, different models have been used to rationalize structure–odor relationships. To gain insight into odorant molecules from bread baking, binding energies and vibration spectra in the gas phase and in the protein environment [7-transmembrane helices (7TMHs) of rhodopsin] were calculated using density functional theory [B3LYP/6-311++G(d,p)] and ONIOM [B3LYP/6-311++G(d,p):PM3] methods. It was found that acetaldehyde (“acid” category) binds strongly in the large cavity inside the receptor, whereas 2-ethyl-3-methylpyrazine (“roasted”) binds weakly. Lys296, Tyr268, Thr118 and Ala117 were identified as key residues in the binding site. More emphasis was placed on how vibrational frequencies are shifted and intensities modified in the receptor protein environment. Principal component analysis (PCA) suggested that the frequency shifts of C–C stretching, CH3 umbrella, C = O stretching and CH3 stretching modes have a significant effect on odor quality. In fact, the frequency shifts of the C–C stretching and C = O stretching modes, as well as CH3 umbrella and CH3 symmetric stretching modes, exhibit different behaviors in the PCA loadings plot. A large frequency shift in the CH3 symmetric stretching mode is associated with the sweet-roasted odor category and separates this from the acid odor category. A large frequency shift of the C–C stretching mode describes the roasted and oily-popcorn odor categories, and separates these from the buttery and acid odor categories.