Crystal morphology control structure, metastable

In contrast to natural structures the morphological features of structures in fabricated foods are in principle within our control. The source of the many structures of foods, even those made from a single raw material (e.g., wheat flour), lies in the ingredient mix and the fact that thermodynamic equilibrium is practically never required or achieved during processing. These metastable structures can be attained because they are favored kinetically, that is, the approach to equilibrium is slow. At any point during the development of a particular structure a process of shape stabilization sets in, usually by vitrification, partial crystallization, phase separation and/or formation of a network (Figure 12.5). 

The goal of this article is to describe ways in which crystal structure, morphology, and crystallization kinetics can be utilized to reproducibly maintain metastable states and control solid-state outcomes. Experimental methods that can be employed to investigate the factors that regulate crystallization from solution will be presented. 

The understanding of the structure and properties of semi-crystaUine polymers involves many different experimental techniques, scientific disciplines and theoretical approaches. The totality of the problem, and the interrelation between its different facets, are shown schematically in Fig. 1.1. (2) Essentially, all properties are controlled by the molecular morphology, that in turn is determined by the crystallization mechanisms. Information about mechanisms is obtained from studies of crystallization kinetics. In order to interpret kinetics, the equilibrium requirements need to be established. It has long been recognized that the crystaUine state that is actually observed in polymers, more often than not represents one that is not at equilibrium and can be considered to be metastable. However, knowledge of the equilibrium requirements is vital to understanding all aspects of the crystallization process and the final state that eventually evolves. Based on the overview... 

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