A commonly used parameter that represents the state of food
during dehydration is water activity, aw, which by its definition is
a macroscopic empirical thermodynamic property. During food
dehydration, as water content decreases, water activity also decreases
and exhibits very non-linear relationships (Marinos-Kouris
and Maroulis, 2006; Bhandari and Adhikari, 2008). Empirically,
strongly-bound water refers to the last small amount of water
molecules that remain trapped until very low water vapor pressuresor equivalently very low aw values are realized. Based on
the results and discussion presented above and in Figs. 3–5, strong
water–macromolecule interactions underlie the strongly-bound
water in foods and hence are a cause of the lowering of the water
activity aw in foods. From this point of view, the decreased pore
sizes resulting from higher food macromolecular densities can
have a twofold effect that depresses aw: increased capillary effect
(e.g. Barbosa-Canoras et al., 2007, pp. 116) and enhanced water–
macromolecule interactions as shown in this work. Since the
former cannot completely explain the significant aw changesobserved during water adsorption and desorption and between different
food systems, the latter must play a very important role in
determining aw and be included in our pursuit of an adequate
understanding of food science and engineering. This aspect also
points out the possibility of non-uniform ‘‘microscopic or local’’
water activity within the same food system during dehydration
due to distributions of pore sizes and water–macromolecule
interactions. It is also worth noting here that the capillary effect occurs
due to a combination of surface tension (caused by cohesion
within the liquid or more fundamentally attractive liquid–liquid
intermolecular interactions) and adhesion between the liquid and
surrounding pore surface (caused by attractive liquid–pore intermolecular
interactions). The potential models employed in this
work include both proper water–water interaction potential for
the former and water–macromolecule interaction potential for
the latter. Thus, the potential models employed in the MD simulations
of this work have an adequate capability to generate and
investigate the capillary effect in molecular detail in model food
systems where food macromolecules are closely aligned together
to form walled pores.