The age-thickening of sweetened condensed milk: III. The effect of ions

Summary

Age-thickening of sweetened condensed separated milk can be greatly reduced by removing part of the diffusible salts by dialysis of the milk before processing. This treatment is still effective after high-temperature preheating, which indicates that denaturation of the whey proteins does not influence age-thickening independently of the milk salts.

When added at the level of 20 m-equiv./kg of condensed milk to the milk before condensing, those polyvalent anions (phosphate, oxalate, citrate, ethylenediaminetetra- acetate) which have the property of forming undissociated or insoluble calcium salts greatly increase the rate of age-thickening. Fluoride, which forms an insoluble calcium salt but is monovalent, has the opposite effect.

It is suggested that both natural and added polyvalent anions destabilize the condensed milk by forming linkages between the calcium atoms of the calcium caseinate micelles, producing the larger, more asymmetric units which were postulated in Part I to account for the viscosity changes during age-thickening. Fluoride would have the opposite effect by combining with the calcium, and so preventing such linkage formation by the polyvalent anions.

When added to the milk at the same level as the anions, divalent cations, especially calcium and zinc, reduce the rate of age-thickening. It is suggested that this is due to removal of milk-serum anions, particularly phosphate and citrate, as insoluble or undissociated salts.

The effect of both anions and cations added after condensing is almost the opposite of that produced by the same ion before condensing, with the exception that phosphate increases age-thickening in both cases. There is as yet no clear explanation for this observation.