The improvement of the cleaning efficiency in cleaning in place (CIP) systems is playing a key role in enhanced food production. To achieve this goal it is essential to understand the mechanisms controlling complex cleaning processes. In this work, cleaning tests are conducted that combine complex geometries with complex food soils using a novel test method based on the local optical detection of phosphorescence (LPD). It combines (i) a cohesive food soil, (ii) complex pipe geometries and (iii) continuous observation of the cleaning progress to investigate the mechanisms of cleaning in immersed CIP systems. The experimental work is supported by CFD simulations. The obtained results show that for the specifications tested the cleaning process can be described as diffusion-controlled. This assumption is supported by CFD simulations that correspond well with the measured local cleaning times. Furthermore, the knowledge of the mechanisms that control cleaning is essential for any optimization of these processes. Based on the presented results it is now possible to visualize the effect of complex pipe geometries or inappropriate hygienic design.