A number of changes occur when caramel is heated. The pH decreases, serum (whey) proteins are denatured, casein micelles and/or whey proteins may aggregate, and the Maillard reaction occurs. Caseins are very heat stable; they are considered “disordered” proteins because they have little secondary or tertiary structure and as a result, they are not typically denatured. While casein is not subject to heat denaturation in the same way as globular whey proteins, it may aggregate under certain conditions, especially if it is in micellar form. At neutral pH, k-casein stabilizes casein micelles against aggregation by way of steric and electrostatic repulsion; highly charged macropeptide tails physically prevent the approach and interaction of hydrophobic regions of the native casein that would lead to aggregation and gelation. Whey proteins are considerably more heat labile than casein; they are easily denatured at temperatures >60 C and are fully denatured at 90 C. Beta lactoglobulin (belg) is the most abundant whey protein, followed by alpha lactalbumin (a-lac): the properties of b-lg tend to dominate reactions occurring upon heat treatment. In high sugar or high solids systems, denaturation of b-lg and a-lac is reduced.