Upper and lower panels: IRAM-WideX spectra (in F_ν scale) extracted at the L1157-B1b blueshifted position (see Fig. 1 and text). The spectral resolution is 1.95 MHz (see Sect. 3). A zoom-in in intensity is shown so that the weakest lines can be seen. The transitions producing the emission lines are labelled in blue, with the corresponding frequencies marked by vertical blue lines. The detected CH₃OH and CH₃COH lines (analysed here) are reported in Table 1.