Constant flux performance in time is achieved with polyethersulfone (PES) polymeric microsieves when filtering protein solutions, skimmed milk and white beer in combination with backpulsing. Such microsieves are fabricated by phase separation micromolding (PSμM) and possess pores around 2 μm. The filtration of bovine serum albumin (BSA) solutions at neutral pH results in constant flux when backpulsing. The constant flux performance is related to the ability of polymeric microsieves to flex during permeate pressure pulsing. Their flexibility allows pressure pulse transmission to the feed and, therefore, almost no flow reversal occurs. The membrane motion affects the hydrodynamics in the feed channel and disturbs the polarization layer and the cake deposited. Reference experiments with stiff SixNy-based microsieves, nuclepore and macroporous microfiltration membranes show different behavior: the permeate pressure pulse hardly translates into the feed channel. Backpulsing for these membranes is less effect as anti-fouling strategy. Backpulsing of polymeric microsieves also allows stable flux operation for other complex feeds like skimmed milk and white Belgian beer.