Hypotheses concerning the fouling of membranes during beer clarification via crossflow microfiltration are reviewed. Beer has been classified into three groups of components, each having a different kind of fouling mechanisms - but also having interactions with other modes of fouling. The membrane fouling also strongly depends on the characteristics of the membrane. An optimal pore diameter and membrane morphology can be identified. The various hypotheses have been formulated in terms of mathematical models, which are tested using experimental data of dead-end filtration of beer. This comparison shows that our hypotheses are quite likely to be valid, and form a good basis for further model-based exploration of the optimization of the beer clarification process. Due to the similarity of beer with biotechnological broths, the presented fouling hypotheses extend beyond the original application of beer microfiltration. (C) 2012 Elsevier B. V. All rights reserved.