Heat transfer effects on drying rate of food dehydration

Abstract

This work studies the consequences of neglecting heat transfer effects in the Fick's law solution of food dehydration problems. A semi‐empirical approach is presented which correlates temperature and moisture changes of sugar beet root during the first falling rate period of drying; while the coupled heat and mass transfer equations are not solved rigorously, the approach gives a fairly accurate representation of the experimental results. An improved analysis through Fick's law is possible when the effect of changing sample temperature is considered.

Drying kinetics of grape seeds

“…Temperature dependence of the effective diffusivity has been shown to follow an Arrhenius relationship (Jason, 1958;Saravacos and Charm, 1962;Vaccarezza et al, 1974;Suarez et al, 1980):…”

“…External resistances to mass transfer become important for a Biot number for mass transfer, Bi m = kÃL/Dq s less than 10 (Vaccarezza et al, 1974). In addition, experimental studies of the effect of air velocity on the moisture content of the material during drying can be used to determine whether external resistances are important to mass transfer (Vaccarezza et al, 1974). When convective hot air is equal to or greater than 1 m/s, external resistances become negligible (Jason, 1958).…”

“…Thus, the temperatures used in this study, 40, 50, and 60°C, were in the range of the low end of typical drying temperature. External resistances to mass transfer become important for a Biot number for mass transfer, Bi m = kÃL/Dq s less than 10 (Vaccarezza et al, 1974). In addition, experimental studies of the effect of air velocity on the moisture content of the material during drying can be used to determine whether external resistances are important to mass transfer (Vaccarezza et al, 1974).…”

Diffusional Analysis during Air Drying of a Starch Food System

“…Air velocities of 0.5, 1.0 and 1.5m.s-' were used which, although restricted by equipment performance, ensured that internal and not external resistance to mass transfer was the limiting factor. This was theoretically verified by calculation of a mass transfer Biot number greater than 10 [lo] and experimentally corned as no measurable difference in drymg-rates was found in Qylng tests carried out at 0.5 and 1.5m.s". Samples of cylindrical conf&p'ation were considered, allowing both one dimensional and two dimensional diffusional analysis; sample radii of 7.5mm, 9.25mm and 13.5mm were tested, being restricted due to practical considerations of sample preparation.…”

“…As shown in Figure 12, no s i m c a n t temperature Werence was observed at the four points. Similar temperature observations were recorded by thermmuples (9,10,11 and 12) positioned in a single vertical plane below the circumferential surface. These results may indcate that the major extent of heat transfer occurs in the radial direction, i.e.…”