Investigating phase stability of ultra-high-temperature reconstituted skim milk

The phase stability, also referred to as physical stability, of UHT milk is limited by sedimentation and age gelation, with complex causes. This study investigated underlying relationships between the physical instability of UHT reconstituted skim milk (RSM) and a series of characteristics. Physicochemical data and Raman spectra were collected from RSM before and after UHT processing and storage. The total solid depletion rate in the sample liquids with a value > 5% was used to indicate phase instability. Raman spectra revealed that lower levels of solids depletion in UHT milk were associated with reduced exposure of aromatic and aliphatic amino acids, fewer hydrophobic interactions, fewer changes in disulfide bonds, and possibly limited Maillard reactions. Physicochemical properties such as particle size and pH impacted the extent of solids depletion. The results suggested that UHT treatment induced physicochemical changes in the samples (e.g., protein structural evolution and enhanced protein-protein interactions). Plasmin hydrolysis as evidenced by gel electrophoresis analyses resulted in changes in UHT-RSM, and the subsequent physicochemical alterations promoted protein aggregation, leading to sedimentation and formation of gel-like material. Aggregation and sedimentation could also occur in the absence of plasmin activity, driven by protein structural evolution. The obtained knowledge may benefit both ingredient and consumer food processors by enabling the prediction of ingredient functionality and quality defects in UHT-RSM.