Understanding how proteins stabilize amorphous calcium orthophosphate (ACP) phases is of great importance in biology and for pharmaceutical or food applications. Until now, most of the former investigations about ACP−protein stability and equilibrium were performed under conditions where ACP colloidal nanoclusters are surrounded by low to moderate concentrations of peptides or proteins (15−30 g L−1). As a result, thequestion of ACP−protein interactions in highly concentrated protein systems has clearly been overlooked, whereas it corresponds to actual industrial conditions such as drying or membrane filtration in the dairy industry for instance. In this study, the structure of an ACP phase is monitored in association with one model phosphorylated protein (casein) using solid-state nuclear magnetic resonance (ssNMR) under two conditions of high protein concentration (300 and 400 g L−1). At both concentrations and at 25 °C, it is found that the caseins maintain the mineral phase in an amorphous form with no detectable influence on its structure or size. Interestingly, and in both cases, a significant amount of the nonphosphorylated side chains interacts with ACP through hydrogen bonds. The number of these interacting side chains is found to be higher at the highest casein concentration. At 45 °C, which is a destabilizing temperature of ACP under protein-free conditions, the amorphous structure of the mineral phase is partially transformed at a casein concentration of 300 g L−1, while it remains almost intact at a casein concentration of 400 g L−1. Therefore, these results clearly indicate that increasing the concentration of proteins favors ACP−protein interactions and stabilizes the ACP clusters more efficiently.