Abstract: Plasticization and glass transition of amorphous components in food powders often result in stickiness and caking. The glass transition temperature (Tg) of milk powders was measured by differential scanning calorimetry (DSC) and a viscometer method was used to determine sticky-point temperatures. Water sorption isotherms were established for varying solids compositions. Lactose contents were analyzed by high-performance anion exchange chromatography with pulsed amperometric detection (HPAE-PAD) and proteins were identified using SDS-PAGE gel electrophoresis. Solids composition and water affected both the Tg and stickiness behavior. Stickiness was governed by carbohydrates and water plasticization. At low protein contents, precrystallization of lactose decreased the sticky point temperature, but increasing protein content in all milk powders decreased stickiness at all water activities. The results showed that glass transition can be used to describe time-dependent stickiness and crystallization phenomena, and it can be used as a parameter to control and reduce stickiness of dairy solids with various compositions.
Practical Application: Glass transition of component sugars in milk powders with various water contents was responsible for a solid-liquid transformation which resulted in their viscous flow at particle surfaces and stickiness of the powders. Stickiness leads to wall deposition in dehydration and caking of powders in storage when the amorphous carbohydrate-rich components gain liquid characteristics. High protein contents in milk powders decreased stickiness, but precrystallization of lactose prior to spray drying increased stickiness at low protein content. Milk powders in storage gained higher water contents with increasing protein contents, but stickiness was reduced and lactose crystallization was delayed which improved storage stability.