This investigation highlights the application of particle size distribution measurement in the brewing process steps of mash separation, wort boiling, and wort clarification. Large-scale trials carried out in seven industrial operations applying different process techniques resulted in the appearance of fine (5 mu m), medium (20-25 mu m), and coarse particles (>200 mu m). During mash separation, the medium and coarse particles were removed while fine particles became dominant and were responsible for the remaining lauter turbidity. Further, prior to start of wort boiling medium particles representing hot trub are formed while the relative portion of fine particles is reduced. This formation is substantially completed when wort boiling temperature is reached, a fact not conforming to current expectations. The coarse particles occurring during wort boiling were identified as originating from hop pellets, while the addition of soluble hop extracts did not cause this signal. Finally, wort clarification by whirlpooling, which is a hydrodynamic separation step, could be described by reduction of coarse and partly of medium particles, while fine particles dominate the resulting wort. The particulate changes observed seem to correlate with chemical analytes conventionally used to characterize these process steps. (C) 2007 American Institute of Chemical Engineers.