Calcium is one of the several elements that can be found in milk distributed between the micellar and the serum milk phase. Calcium is important from a nutritional point of view, but its contribution to the functional and structural properties of dairy products has only recently been acknowledgement. The presence of calcium in a dynamic equilibrium between the serum and the micellar milk phase make the distribution susceptible to certain physicochemical conditions and to technological treatments of milk resulting in fluctuations in pH and temperature and also sensitive to addition of calcium salts. The perturbation of calcium equilibria by these factors will affect the final properties of acid, calcium and rennet milk gels.

By decreasing the pH from 6.0 to 5.2 (acid gels), the calcium equilibrium was significantly
affected by temperature (4, 20, 30, 40 oC), and different combinations of temperature and pH may result in different final structure properties in dairy products such as cheese. A significant amount of calcium remained in the micelles between pH 4.8 and 4.6, this can contribute to the final strength of acid milk gels, such as in yogurt or in cream cheeses. After the gelation point, a sudden solubilization of micellar calcium was observed at 50 oC and 60 oC, which revealed an interesting role of calcium during acidification at elevated temperatures. After enrichment of milk with calcium D-lactobionate, the added calcium was distributed between the micellar and serum milk phase at pH 6.6-6.0, but at pH 5.7-5.4 the added calcium remained mainly in the
serum milk phase. The importance of pH for the distribution of the added calcium between the micellar and serum milk phase may affect bioavailability from enriched products and the distribution should be taken into consideration when designing new enriched dairy products.

Calcium gels can be produced by addition of a calcium salt and heat treatment at temperatures higher than 70 oC for several minutes. The combination of heat treatment and calcium addition to milk with pH values between 6.6 and 5.6, will produce calcium milk gels with unique and dense gel structure and with little seperation of whey due to participation of calcium to the final gel structure. On the other hand, the combination of heat treatment and calcium addition to milk with pH values lower than 5.6 will still produce gel structures which are dominated by the decrease of pH and not by the addition of calcium.

For rennet gels, different gels were produced with different amount of micellar calcium, as colloidal calcium phosphate (CCP), while keeping the pH and the ionic environment of the milk constant. A lower amount of micellar calcium was correlated with less time for the rennet gels to be formed. In addition the low amount of micellar calcium caused a more compact gel structure with many protein aggregates.

The results of this study highlighted the importance of calcium for the formation of acid, calcium and rennet gels. The content and the interactions of calcium with proteins during milk processing are crucial for final texture and structure of different milk gels. The results of this study should be useful for improvements of existing dairy products and for design of novel dairy products with unique structure and texture and mineral bioavailability