(224d) Molecular Modeling and Simulation Studies of the Structural and Energetic Evolution during Dehydration of Food Systems

From a molecular point of view, foodstuffs are mostly comprised of biopolymeric molecules that form highly porous structures in which solvents, solutes, additives, and other natural or artificial ingredients are accommodated. Because of the complexity of the food structures and compositions and because of the sensitivity of the pore structures and properties of food to physical and chemical changes arising externally or internally, foodstuffs are one of the most complex types of soft materials and are inclined to exhibit significant structure-property relationships in the physicochemical, functional, processing, and nutritional aspects of food science and engineering. In this respect, water (moisture) is a major factor in determining the properties of foods in various stages as it is not only a major component of food materials but also acts as space filler for stabilizing food structures, solvent for dissolving additive molecules, medium for heat and mass transfer, and participant in the chemical and/or biological reactions in food systems. .

One of the important ways for preparing and preserving foods is dehydration (water removal), which is a process that requires energy to be supplied to enable water molecules to break loose from their interactions and escape from the porous structures of the foods. The process of food dehydration and the preservation of dehydrated foods can thus be directly related to the interactions of water molecules with food macromolecules, solute ions, and additive molecules, because these interactions dictate the energy requirement for water removal during food dehydration as well as the structures of foods before, during, and after dehydration. Moreover, the magnitudes of these interactions depend on the densities and intermolecular distances of mutually interacting molecules, rendering the water molecules in foods to exist in different energetic states and making single system-wide thermodynamic parameters (e.g, water activity) inadequate in representing the diverse states of water molecules in food porous structures and in elucidating the microscopic and macroscopic structural evolution of food materials caused by dehydration. Capable of tackling these challenges is the molecular dynamics (MD) method which employs molecular-based interaction potentials to construct a model system that provides a satisfactory physicochemical representation of the real system and allows various structural and dynamic properties to be studied at a molecular level. Recently, we have employed MD modeling and simulations to construct an amylose-based food material and study the dehydration process in a continuous dynamic manner by periodically examining the energetics of all water molecules at the water-vapor interface and removing the one with the highest energy from the model food system. It was found that the strong interactions of water molecules with the food polymer stabilize the porous structure of the food material and makes the dehydration energy requirement to be greater than that involved in pure water vaporization. They also cause the water interaction energetics to be highly nonuniform in all three spatial dimensions. As dehydration proceeds, the surface tension of the descending water interface coupled with strong water-polymer interactions exert downward stresses on the food polymer to induce polymer conformational compaction and food structural reduction.

We further added two functional food ingredients to the model food systems in our MD simulation studies. The first one is potassium sorbate, which is a commonly used food preservative against the growth of bacteria, fungi, molds, and yeast in foods. The second one is fructose-1,6-diphosphate (FDP), which is a bioprotective agent and a promptly available high energy substrate and can promote growth and improve health. The stability of the food porous structures was found to be enhanced by the presence of the food additive molecules which function as additional fillers in the food pore space and interact strongly with the polymer chains and water molecules of the food material. During dehydration, the descending water level tends to accumulate the additive molecules, in particular sorbate with a nonpolar hydrocarbon moiety, at the vapor-liquid interface, which helps to form dense clusters of food polymer chains at the interface, reduces the level of structural reduction, but increases the energy requirement for water removal. The additive molecules in the interior regions of the food structures have strong tendencies to stay close to food polymer and hence exhibit nonuniform density distributions as well. Extensively dehydrated food porous structures were found to have substantially improved stability due to the water molecules remaining in the pore structure that effectively “bind” the food polymer and additive molecules together. Which makes it very difficult to recover the original pore structures of the food systems that existed before the commencement of dehydration. The findings of these MD studies suggest that the original pore structures could be reduced less during dehydration and could be recovered more effectively by rehydration, if (i) the food material has a higher polymer density, (ii) solute or additive molecules have elongated shapes to act as cross-linkers, and (iii) the effects of water surface tension can be minimized or eliminated using special dehydration methods such as freeze drying. The MD modeling and simulation approach developed in this work could form a basis for systematic evaluation of food porous structures and their structural and energetic evolution and mechanisms in molecular detail and for providing scientific explanations to the macroscopic trends and empirically observed phenomena during food dehydration.