Effect of formaldehyde on heat stability of milk

Formaldehyde, at concentrations of 5-15 mmol/L, strongly increased heat stability of milk, especially at pH 6.8-6.9. Also in the absence of urea (another stabilizing agent naturally present in milk), formaldehyde stabilized milk against heat. Formaldehyde inhibited the heat-induced dissociation of casein from the micelles, notably kappa-casein. Formation of cross-links is the most probable explanation. Preheating of formaldehyde with each of several amino acids before addition to the milk showed that formaldehyde reacted preferentially with the epsilon-amino group of lysine. These results and the literature suggest that formaldehyde cross-links the epsilon-amino group of lysine with asparagine, glutamine or arginine residues. Turbidity showed that the heat-induced growth of casein particles was retarded substantially by formaldehyde and that cross-links were formed only locally, not throughout the micelles. The stabilizing action of formaldehyde was thought to be due to immobilization of kappa-casein onto the micelle, such that the micelles retained their colloidal stability because of steric repulsion by the immobilized kappa-casein; without formaldehyde, kappa-casein dissociated easily from the micelle at pH > 6.7 upon heating, leaving unstable micelles.