Drying curves generalized

Over time, diagrams were developed relating water activity with enzyme activity, dormancy of stored seed, loss of dry product crispness by moisture absorption, pigments and vitamins degradation, nonenzymatic browning, and fat oxidation. Response curves generally are not linear, and readers working with food or feed formulations are referred to the technical literature about their products. 

The general features of wet and dry curves obtained from capillary flow porometry are shown in Figs. 5.4 and 5.5 for a porous poly(e-caprolactone) film sample. 

Equation 10.15 is a new form of the generalized drying curve with the reduced time (t/s) and the index of intensity of heating (n). The semi-theoretical origin of Eq. 10.15 allows more accurate predictions to be made of drying kinetics with a minimum of experimental data, and also enables the preheating drying period to... 

Sousa, L.H.C.D., Monteiro, A.S., Perri, V.R., Motta Lima, O.C., Pereira, N.C., Mendes, E.S., 2004. Generalization of the drying curves in convective and conductive/convective textile fabric drying. In 14th International Drying Symposium, Sao Paulo, Brazil, pp. 710-717. 

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

Each food or food ingredient shows a characteristic equiHbrium relative humidity at a given moisture content and temperature. Thus as a food is dried and its moisture content is reduced from its fresh value where water activity is generally 1.0, to lower and lower values, the equiHbrium water activity of the food decreases as a complex function of residual moisture. The shape of the equiHbrium relative humidity—moisture content curve is set by the chemistry of the food. Foods high ia fmctose, for example, biad water and thus show lower water activities at high moisture contents. Dried pmnes and raisias are examples. Drying can be terminated at any desired moisture content and hence any water activity. 

When a solid is dried experimentally, data relating moisture content to time are usually obtained. These data are then plotted as moisture content (dry basis) W versus time 0, as shown in Fig. 12-41 7, This curve represents the general case when a wet sohd loses moisture first by evaporation from a saturated surface on the sohd, followed in turn by a period of evaporation from a saturated surface of gradually decreasing area, ancf finally, when the latter evaporates in the interior of the solid. 

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