Low moisture foods

The preference for low protein, low starch damage flour in biscuits is obvious when the role of protein and damaged starch as water binders is considered. The aim in making biscuits is to produce a low moisture food. Incorporating components that bind water makes that aim more difficult. Table 2 considers the relative properties of biscuit and bread flours. 

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

Roos (1995) noted that the decrease in viscosity above Tg is responsible for various changes, such as stickiness and collapse of dried foods, agglomeration, and crystallization of food components (e.g., lactose in dried milk). In addition, the crispness of various low moisture foods is lost above Tg. Determination of Tg values as a function of solids or water content and water activity can be used to establish state... 

H. Levine and L. Slade, Interpreting the behavior of low-moisture foods, in Water and Food Quality (T. M. Hardman, eds.), Elsevier, London, 1989, pp. 71-135. 

The many structures of low moisture foods viewed by scanning electron microscopy (a) bread (b) chocolate biscuit (c) whey protein powder (d) milk chocolate. 

Seymour, S.K. and Hamann, D.D. Crispness and crunchiness of selected low moisture foods, /. Texture Stud., 19, 79,1988. 

Biomaterial films can be used to prevent water sorption by low moisture foods or pharmaceutical solids and, thus, to maintain product quality, and to improve shelf life and storage stability. For many biomaterials water acts as a plasticizer and therefore affects stability and quality (Roos and Karel, 1990, 1991). Hence, it is important to control water sorption properties of low moisture foods or pharmaceutical products. Edible films can be used as barriers or retarders of water sorption of low moisture products. 

Effects of Sugar Crystallization on Nonenzymatic Browning Kinetics in Low-Moisture Food Systems... 

We may conclude that in most aqueous solutions, simple bimolecular reactions will proceed according to Eq. (4.12). For bimolecular reactions in a system containing very little solvent, however, the reaction rate will be mostly diffusion controlled, implying that the rate is inversely proportional to the viscosity. In low-moisture foods, the situation may often be intermediate. It appears logical to combine Equations (4.12) and (4.16). Since we must essentially add the times needed for encountering and for the reaction itself, the resultant rate constant would then follow from... 

Chemical reaction kinetics proceeds on the (often implicit) assumption that the reaction mixture is ideally mixed, and does not consider the time needed for reacting species to encounter each other by diffusion. The encounter rate follows from the theory of Smoluchowski. It turns out that most reactions in fairly dilute solutions follow chemical kinetics, but that reactions in low-moisture foods may be diffusion controlled. In the Bodenstein approximation, the Smoluchowski theory is combined with a limitation caused by an activation free energy. Unfortunately, the theory contains several uncertainties and unwarranted presumptions. 

In many intermediate-moisture foods, most chemical reactions will not be diffusion limited. In low-moisture foods (e.g., uw <0.2), however, the most important cause of chemical stability will generally be small diffusivity. This would apply to all examples in Figure 8.10, except for oxidation of carotene. A relation like that in Figure 8.10d is generally observed for oxidation reactions, where water is not a reactant, and even at awx 0 perceptible diffusion of 02 may occur see further point 6, below. 

This equation well describes the relation between water content of potato starch and Tg as given in Figure 16.3a the relation is less perfect for several other binary mixtures. Many low-moisture foods are mixtures of polymers, water, and several other small-molecule components. Prediction of Tg from composition then is generally uncertain in practice a (narrow) transition range rather than a sharp transition point is often observed. 

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