Panel a: observed abundance ratios, R, between G135.27 MM1 and G31.41. Panel b: abundance ratios, R, of models (A_V,init = 3 mag) with low and normal metallicity. In both panels, the ratios correspond to η = X/X_CH3OH. Molecules with R lower than one-third (dashed red line) or higher than three (dashed green line) are highlighted in red or green, respectively, while those with R within a factor of two of unity (1/2 < R < 2; dashed blue lines) are highlighted in blue. The other molecules are shown in black. The unfilled bar with arrow in panel a represents an observational upper limit for G135.27 MM1. In panel b, the solid bars correspond to the best-fit model of G135.27 MM1 (t_wu = 1 × 10⁶ yr), while the hatched bars represent the ratios for the other two timescales.