For a 2-stage homogenizer designed with springs outside the milk flow, 2 problems are common. First, it is possible to see a wear ring (Figure 5A) form on the ball over time where the ball makes contact with the seat. This happens as consequence of a pulsing singlepiston pump forcing milk against the ball, resulting in repeated opening the narrow gap between the ball and the seat and bringing the ball to its original position against the seat when pressure is not applied (Wenrich, 1946; Trout, 1950; Walstra et al., 2005). In this type of homogenizer, it appears that the balls do not rotate, so the same area on the ball is always opening and closing against the seat. A localized erosion of the ball lets milk flow through this point with a lower pressure decrease across the seat and will cause poor homogenizer performance. Second, damage to the seat occurs on the edge that contacts the ball, leading to the accumulation of material between the ball and seat (Figure 5B).