The next step in the process is "preheating" during which the standardised milk is heated to
temperatures between 75 and 120C and held for a specified time from a few seconds up to
several minutes (cf. pasteurisation: 72C for 15 s). Preheating causes a controlled
denaturation of the whey proteins in the milk and it destroys bacteria, inactivates enzymes,
generates natural antioxidants and imparts heat stability. The exact heating/holding regime
depends on the type of product and its intended end-use. High preheats in WMP are
associated with improved keeping quality but reduced solubility. Preheating may be either
indirect (via heat exchangers), or direct (via steam injection or infusion into the product), or a
mixture of the two. Indirect heaters generally use waste heat from other parts of the process
as an energy saving measure.