Capsules consist of droplets enclosed by a membrane with shear resistant properties especially when fabricated by interfacial cross-linking. In many applications, the protection and release of the internal medium need to be strictly controlled. It is possible to tune the membrane mechanical properties by changing the physico-chemical conditions of the fabrication process, but a good control of the production requires their characterization, which is a scientific challenge, since the objects are a few tens of microns in size at most. One advantageous approach is to resort to microfluidic techniques. We study the transient response of capsules having a cross-linked human serum albumin (HSA) membrane, as they flow through a sudden expansion. We determine the characteristic time scales of the capsule relaxation and compare them to the ones predicted by a full numerical model of the relaxation of a capsule flowing in a rectangular channel, for which the membrane is assumed to be purely elastic. We show that the membrane is viscoelastic and that the relaxation is solely a function of the ratio of the relaxation time to the convective time.