In most foods of high or intermediate water content,Dis not very
much smaller thanD 0 , rarely by more than a factor 10. For example, for
diffusion of salt in waterD& 10 ^9 m^2 ?s^1 , whereas in meatDis about
half that value, and in hard cheeses about 0.2 timesD.Dof sucrose in most
fruits is about 10^10 m^2 ?s^1. For drier foods, the differences become larger.
All factors mentioned above give a stronger effect for a higherjvalue.
Figure 5.16a shows some results onDfor water in foods of variable water
content, and it is seen that the effects are large; some studies on drying
indicate an even stronger reduction ofD*at very low water content. It
should also be considered that the activity coefficient of water may be
materially decreased at very low water content; hence a smaller activity
gradient, hence slower diffusion. Constriction, i.e., Factor 3 mentioned
above, implies that the diffusion of larger molecules will be hindered more
than that of smaller ones. This is illustrated by results shown in Figure
5.16b. See also Section 8.4.1.
FIGURE 5.16 Examples of hindered diffusion. (a) The effective diffusion
coefficientðDÞof water in various materials as a function of their mass fraction
of waterðowÞ. (Adapted from Bruin and Luyben; see Bibliography.) (b) The effective
diffusion coefficientðDÞof some molecules and a virus in polymer gels of various
concentrations (%). (Very approximate results, obtained from Muhr and Blanshard;
see Bibliography.)