  Aim: In the past, observations of protostellar shocks have been able to set
constraints on the formation route of formamide (NH2CHO), exploiting its
observed spatial distribution and comparison with astrochemical model
predictions. In this work, we follow the same strategy to study the case of
acetaldehyde (CH3CHO). Method: To this end, we used the data obtained with the
IRAM-NOEMA interferometer in the framework of the Large Program SOLIS to image
the B0 and B1 shocks along the L1157 blueshifted outflow in methanol (CH3OH)
and acetaldehyde line emission. Results: We imaged six CH3OH and eight CH3CHO
lines which cover upper level energies up to 30 K. Both species trace the B0
molecular cavity as well as the northern B1 portion, i.e. the regions where the
youngest shocks (1000 yr) occurred. The CH$_3$OH and CH$_3$CHO emission peaks
towards the B1b clump, where we measured the following column densities and
relative abundances: 1.3 x 10^16 cm-2 and 6.5 x 10-6 (methanol), and 7 x 10^13
cm-2 and 3.5 x 10-8 (acetaldehyde). We carried out a non-LTE LVG analysis of
the observed CH3OH line: the average kinetic temperature and density of the
emitting gas are Tkin = 90 K and nH2 = 4 x 10^5 cm-3, respectively. The CH3OH
and CH3CHO abundance ratio towards B1b is 190, varying by less than a factor 3
throughout the whole B0-B1 structure. Conclusions: The comparison of
astrochemical model predictions with the observed methanol and acetaldehyde
spatial distribution does not allow to distinguish whether acetaldehyde is
formed on the grain mantles or rather on the gas-phase, as its gas-phase
formation, dominated by the reaction of ethyl radical (CH3CH2) with atomic
oxygen, is very fast. Observations of acetaldehyde in younger shocks, e.g. 10^2
yr old, or/and of the ethyl radical, whose frequencies are not presently
available, are necessary to settle the issue.
