Estimation of equilibrium properties in formulation or processing of liquid foods

Water is the main component in most foods and equilibrium properties of water are considered as reference properties in food processing. Water activity is an example. This work is based on the UNIFAC method [AIChE Journal 21 (1975) 1086], which estimates equilibrium properties from the excess Gibbs energy of all compounds within the solution. It relies on the idea of group contribution in which each molecule is considered as a collection of basic building blocks, the functional groups. The advantage of the group contribution approach is that, from a relatively small number of groups, the properties of many different molecules can be obtained. This gives vapour–liquid properties, liquid–liquid or solid–liquid properties, osmotic properties, and by using dissociation constants values, this affords pH values. The UNIFAC model was modified. The relationship of Larsen et al. [Ind. Eng. Chem. Res. 26 (1987) 2274] accounts for short-range interactions; a Pitzer term based on the Debye-Hückel theory accounts for long-range interactions. Finally, hydration of ions or non-dissociated molecules is considered via a hydration number. This number characterises a specific ion, not a given salt, which facilitates the treatment of mixtures containing many dissociated or partly dissociated compounds. The model was used to estimate equilibrium properties (aw, activity coefficient of salts and sugars) needed for modelling osmotic dehydration of foods, in production of flavouring agents by fermentation processes (pH, activity coefficient) and in food (e.g. cowʹs milk) formulation and control.