Layering of proteins in stored UHT milk samples

Two UHT milk samples were stored undisturbed at ambient conditions for 18 months. After storage, layers were removed and analysed for protein content and composition. The upper layers were enriched in alpha(S)+beta-casein and whey proteins whereas the bottom layers were enriched in aS thorn O-casein indicating the gravitational settling of kappa-casein-depleted casein micelles. One milk sample showed higher levels of gelling at the bottom of the pack and also had a greater concentration of kappa-casein-depleted casein micelles in the lower layers when compared with the second milk. These results are consistent with the age gelation mechanism for UHT milk where the gravitational settling of kappa-casein-depleted casein micelles forms a concentrated layer at the bottom of the pack that subsequently aggregate to forming a gel layer.(C) 2022 Elsevier Ltd. All rights reserved.