Physical Properties and Formation Mechanism of Xylooligosaccharides-Milk Gel

The aim of this study was to investigate the changes in the physical properties and gelation mechanism of xylooligosaccharides (XOS)-milk system during heat treatment. Firstly, the preparation conditions of XOS-milk gels were optimized using sensory evaluation. At the optimal XOS concentration, the changes in pH, color, gel strength, water-holding capacity, and rheological parameters of the XOS-milk system were studied during heat treatment. The gelation mechanism was analyzed by using solubility, particle size, and polyacrylamide gel electrophoresis analysis. The results showed that the XOS-milk gel was evaluated to be the highest sensory score at an XOS addition level of 5% (w/v) for 6 h of heat treatment. The pH of the XOS-milk system decreased with the increase of heating time (pH5.40 at 8 h), and the gel strength increased (144.4 g at 8 h). The system exhibited solid-like properties (G'>G'') after heat treatment for 2 h. The water-holding capacity initially increased (53.1% at 6 h) and then decreased (49.1% at 8 h) with prolonged heat treatment. Particle size and polyacrylamide gel electrophoresis analysis showed that milk proteins aggregated after heat treatment. The formation of gel network structure was observed using the scanning electron microscope (SEM). The solubility analysis showed that ionic bonds were the main driving force for protein aggregation and gel network formation, which contributed from 46.8% to 66.3% in samples of different heating time, while hydrophobic interactions and hydrogen bonds displayed the less contribution. As the heat treatment being prolonged, the total contribution of non-covalent bonds such as ionic bonds gradually decreased, which may be related to the formation of Maillard-related covalent bonds. Therefore, XOS promoted the formation of the heat-induced milk gel and its properties might have the potential to be used for the development of new milk products.