ReviewMulti-scale modelling of polymers: Perspectives for food materials
Introduction
Carbohydrate polymers and proteins are two of the main components of food. They are often used as viscosity enhancers and structure builders in solutions and gels (Dickinson, 1991, Hill et al., 1998). In the melt and glassy state they serve as matrix material for the encapsulation, stabilization and the release of active food ingredients like flavours, bacteria and nutrients (Ubbink and Schoonman, 2003, Risch and Reineccius, 1995). Smell, taste, texture as well as digestion are then the result of a complex interplay of many different physical and chemical processes in such a multi-component system.
These underlying processes result from phenomena, which are governed by a wide range of time and length scales. They range from specific interactions which depend on (quantum mechanical) processes on the Å or nm scale up to the rearrangement of the global polymeric conformation which is mainly determined by entropic and topological constraints, on a typically μm scale.
The complexity of the problems inhibits the ability to globally relate the macroscopic properties of food materials with the atomistic and molecular constituents. The number of situations that can be treated by analytical theory and which are accessible to direct experimental measurements is very limited. As we will show in the following for a large number of problems molecular dynamics (MD) simulations within a multi-scale approach can serve as a powerful method to complement and extend our knowledge of structure–property relations in complex food materials.
Simulations can extend theoretical concepts beyond idealized cases and can prove their applicability in specific more complicated experimental situations. Simulations can also be used to interpret experimental measurements and thus are able to establish a link between observed (macroscopic) quantities to basic molecular structures and dynamics. In addition since one can apply a multitude of different analysis tools to the same ‘sample’ one is able to relate a wealth of different properties with each other. Such methods have been very successfully developed and applied in polymer science, where the major difference is that the polymer molecules are mostly of synthetic rather than biological origin and, more important, the number of different components is typically rather small (Attig et al., 2004, Holm and Kremer, 2004, Holm and Kremer, 2005, Baschnagel et al., 2000).
In this article we want to give a perspective for which types of questions in the field of food materials science and technology state of the art multi-scale modelling techniques promise to be successful and which limitations we see. Since computer simulations have hardly been used for food materials, we will use examples from polymer physics to illustrate their applicability.
Due to space limitations we will focus on carbohydrate polymers, in mind that there is a large number of problems which can be tackled with similar techniques in the field of protein solutions, aggregation and gelation as well as in the phase behaviour of amphiphilic systems containing fats and lipids (Cooke et al., 2005, Srinivas et al., 2004, Loison et al., 2004).
Section snippets
Questions
In this section, we list a number of problems and questions where MD simulations can improve our understanding of food materials. But we are aware that this list is far from being complete.
Understanding the barrier properties of glassy carbohydrates requires knowledge about their glass transition as well as the molecular packing. The dependence of the glass transition temperature on carbohydrate type, chain length and presence of plasticizers like water and small sugars can be investigated by
Multi-scale modelling—method
Describing a system from first principles in the frame work of quantum mechanical density functional theory is only possible for small molecules and very short time scales (femtoseconds). Other more accurate methods are even more restricted. More practical is a description of the system on the level of atoms. Integrating out the electronic degrees of freedom yields an atomistic force field. A large number of such force fields for carbohydrates has been described in the literature (Damm et al.,
Structure of amorphous polymer matrix
Due to the intrinsic disorder of amorphous material the information obtained from scattering methods (X-ray and neutron scattering) as well as NMR spectroscopy is rather limited. Positronium annealing life-time spectroscopy (PALS) gives information on the average free path length of a diffusing positronium, but not of the local structure. For the interpretation one needs assumptions on the electronic density around the holes, determined by their shape. With MD simulations one can directly
Outlook
The work described here is so far mostly based on synthetic polymers. This is due to the traditional starting point of modern materials science. With more basic scientific methods entering food technology, it becomes also more and more apparent, that there are huge similarities in some important areas, such as morphology development, processing, material stability, etc. There are of course many aspects, which are different, especially those related to biological function or the fact that food
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