ENTHALPY-ENTROPY COMPENSATION IN FOOD VAPOR ADSORPTION

Enthalpy-entropy compensation was analysed for sorption isotherms of p otatoes, macadamia nuts, apricots, figs, currants, prunes and raisins. Plots of (Delta H-dif)(T) vs (Delta S-dif)(T) for potatoes and macada mia nuts presented two isokinetic temperatures: T-B1 = 272.0 +/- 57.7 K (-1 degrees C) for potatoes, T-B1 = 265.0 +/- 18.8 K (-8 degrees C) for macadamia nuts and T-B2 = 382.5 +/- 7.3 K (109.5 degrees C) for bo th products. The first isokinetic temperature (T-B1) appeared only at the upper portion of the temperature range tested (50, 60 and 70 degre es C for potatoes and 50 and 60 degrees C for macadamia nuts). The two isokinetic temperatures observed for potatoes and macadamia nuts sugg ested that during the initial stages at low a(W) T-B1 is controlled by changes in the entropy of water whereas the second isokinetic tempera ture (T-B2) is considered to be enthalpy-controlled. Dried fruits pres ented only one isokinetic curve T-B = 315.7 +/- 3.5 K (42.7 degrees C) , for raisins, currants and Jigs (75.2-82.3% db. sugars) and T-B = 317 .7 +/- 4.6 K (44.7 degrees C) for prunes and apricots (51.5-54.5% db. sugars), indicating an enthalpy-controlled adsorption process for the whole range of moisture contents covered. Copyright (C) 1996 Elsevier Science Limited