Microscopic observation of the effect of some reagents on the physical state of casein

Summary

Observations were made by anoptral phase-contrast microscopy of the effect of various reagents on the state of dispersion of casein micelles in skim-milk and in lactic acid and rennet coagula. Among the hydrogen-bond-breaking reagents, urea at higher (4M) concentration dispersed the casein micelles in skim-milk and in acid and rennet coagula. Sodium salicylate was effective at low (M) concentration. Guanidine hydrochloride at low concentration caused aggregation of the micelles in skim-milk but at high concentration caused dispersion, while at both concentrations it dispersed acid and rennet coagula. Lithium iodide and potassium thiocyanate flocculated the casein in skim-milk, but dispersed it in both types of coagula. Flocculation in skim-milk by these reagents was followed by slow coalescence of the casein.

At substantially lower molar concentrations, sodium dodecyl sulphate, an anionic detergent, which attacks either the hydrophobic bonds or the salt linkages in proteins, dispersed micelles and both types of coagula. Sodium thioglycollate, a disulphide-bond-breaking reagent, had no visible effect on the casein.

In rennet coagulum in which the calcium bridges have been destroyed by the sequestering action of EDTA or in lactic acid coagulum, urea at low concentrations caused a quick and almost complete coalescence of casein. Fibres were formed in the rennet coagulum when the coalesced material, which presumably consists of randomly coiled chains of unfolded casein macromolecules, was subjected to stretching or flowing. Similar chemical and physical conditions would appear to prevail when fibre formation takes place in cheddaring cheese curd.