Formation of yogurt microstructure and three-dimensional visualization as determined by confocal scanning laser microscopy

Confocal scanning laser microscopy was used to observe coagulum formation resulting from milk acidification. Reconstituted nonfat dry milk was steamed for 15 min, inoculated with yogurt culture or mixed with glucono-delta-lactone, and incubated at 40 degrees C on a warm microscope stage. The pH was monitored on the slide simultaneously with observation of gel microstructure development. Coagulum formation was observed as a continuous process; gelation of the milk was initiated at pH 5.35, as determined by casein micelle aggregation and the cessation of bacterial cell movement. As the pH continued to drop, nonreflective, casein-free spaces around encapsulated microorganisms increased, which was attributed to contraction of the casein network. Direct-acidified milk at pH 5.0 had a three-dimensional microstructure with more uniform sizes and distribution of nonreflective spaces than did cultured yogurt. At pH 4.4, the structural differences between the various acidified milk gels were less apparent. The pH gradient within the milk gel was observed using a fluorescent molecular probe that was sensitive to pH. Areas of relatively high pH corresponded to the light-reflecting areas of the gel, confirming their identity as casein.