Equilibrium and non-equilibrium structures in complex food systems

The physics ruling the macroscopic behaviour of foods is discussed in a few selected examples in which the equilibrium/non-equilibrium nature can be directly related to the typical length scale of the food structure and the specific interaction strength, the latter describing the decrease in specific free energy (e.g., per mole of species or per single event) when going from a non-equilibrium to the equilibrium configuration. Liquid-crystalline phases in foods, such as those based on self-assembled lipids and water, are discussed as an example of equilibrium system, in which low correlation length scales (∼nm) enable very-short times to re-organize molecules into equilibrium configurations. High internal phase emulsions and dry foams where length scales are very large (∼101–104 μm), are discussed as a typical example of non-equilibrium system, which however, can be stable for long times owing to the high free energy barrier and the large correlation lengths. Finally double emulsion systems are considered as an example where both interaction strength and correlation lengths have intermediate values. This allows tuning morphologies and making equilibrium behaviour to prevail over non-equilibrium by suitably tailoring the energetic interactions. In particular the case of osmotic pressure in the water phase of water/oil/water double emulsions is discussed as a viable route to control interaction strength in complex food systems.