In this talk, we present results on the direct interactions between casein micelles in skim milk during yogurt formation by addition of the acidifier glucono-delta-lactone. From diffuse light, dynamic cross-correlation and heterodyne near field scattering, we obtain valuable information on the micelle-micelle interactions as the acidity of the medium is changed. The micelles form factor is described with Mie theory for spherical particles and the static structure factor by solving the Ornstein-Zernike equation with the soft mean spherical approximation considering an effective potential among micelles which includes different contributions, i.e., Van der Waals attraction and electrostatic and steric repulsions. Our results indicate that aggregation of casein micelles during acidification of skim milk is primarily due to the collapse of the steric layer over the micellar surface. Milk acidification reduces the electrostatic and steric repulsions, thus allowing the Van der Waals attraction to overcome the repulsive forces. As the total potential becomes attractive, the distance between casein micelles is gradually reduced until they form a gel at a characteristic acidity of the system.