Use of milk-derived protein powders in high-protein bars has grown significantly in recent years. Such products undergo deteriorative, hardening reactions during storage. This study explored the physical characteristics of whey protein powders, particle packing and ageing in bar matrices. The stability of a whey protein isolate (WPI) was compared with that of three WPI hydrolysates. The onset of solidity (ψ) was dependent on powder type, volume fraction (ϕ) and a particle interaction energy (U). Bars containing hydrolysates did not harden to the same extent as those containing intact WPI. For WPI, ψ occurred at ϕ of 0.73, compared with approximately 0.55 for two of the hydrolysates. Bars containing the most extensively hydrolysed proteins did not exhibit an equivalent liquid-solid transition. Hardening was lower in systems in which ψ occurred at low ϕ. Rheological characterisation of the liquid-solid boundary provides a means to better understand structural development in concentrated food systems.
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