CH₃OH and CO gas-to-ice ratios (N_gas/N_ice) for the SVS 4 sources in the Serpens molecular core. The shaded azure areas represent the ranges estimated in this paper for both molecules. The curves represented by the solid black lines indicate the theoretical ratio estimated from the astrochemical model by Cazaux et al. (2016). The dashed light blue line in the left panel marks the ratio estimated by Öberg et al. (2009).