In this paper, the first experimental results related to fouling taking place at heat exchangers during heat treatment of milk is presented. Instead of milk, simulated milk ultrafiltrate was used and the experiments were conducted in a simplified model geometry consisting of one heating plate (dimensions 32 mm by 40 mm) made of stainless steel in a flat flow channel (height 2.0 mm) in a closed flow loop. The three-dimensional deposit structure was measured in real time with optical coherence tomography (OCT) with ca. 4 μm resolution. Fouling was observed in a flow rate range 300 − 1500 ml/min (Reynolds number 250 − 1250). The largest amount of deposited layer was 70 g/m 2 in 7.5 hours at the flow rate of 1500 ml/min. The thickness of the deposited layer was ca. 300 μm. In some cases, we observed detachment of deposit flocks, which was seen both in the OCT images and in temporary decrease of the plate temperature (indicating improved heat transfer). The system seemed to be here in a dynamic equilibrium; reflocculation and floc detachment followed each other the plate temperature fluctuating in a range of a few degrees.