Drying Rate.The drying rate of grain in a grain-drying system depends on the drying rate of the individual grain kernels. In general, small kernels lose moisture more readily than large kernels, and naked seeds dry faster than covered seeds. This is illustrated in Fig. 1.19, in which the drying curves of maize, rice, and wheat at 49°C are drawn. Maize contains the largest kernels and dries the slowest; rice and wheat seeds are of comparable size but rice kernels are covered and therefore dry slower than wheat kernels.
Grain kernels dry at a falling rate. This is shown in Fig. 1.20, in which the drying and the drying-rate curves are drawn for a typical grain species. In Table 1.18 data are tabulated for the relative drying rates of several grains. As the kernels increase in size, and decrease in moisture content, the drying rate decreases.
Typical equations for the drying rate of the major grains follow.
Figure 1.19. Comparison of drying rates of dent maize, medium rice, and soft wheat at 49◦C.
Figure 1.20. Grain drying during falling-rate period.