Influence of the protein content on structural characteristics of stirred fermented milk

Milk proteins are the major constituents modified during the manufacture of fermented dairy products, such as yogurt and fresh cheese. The aim of the study was to observe the influence of protein concentration on the structural characteristics of stirred fermented milk. The experiments were conducted in order to establish the connections existing between rheological properties and the microstructure of stirred fermented milk. Increasing the protein concentration of skim milk by means of nanofiltration from 3.5 to 7.0% led to firmer gels and to a higher viscosity and serum-holding capacity of the stirred fermented products; even the degree of solvation was increased. These structural properties and electron micrographs indicate that the higher the protein concentration, the finer the gel network. In contrast, the addition of rennet at the beginning of the fermentation induced a coarser network. Therefore, the gels made by acidification and rennet action are firmer and the stirred products hold less serum; however, the apparent viscosity is higher compared to stirred products made by acidification.