DRYING KINETICS AND QUALITY OF WALNUTS

The drying operation is an essential step in the preservation of the q uality of the unshelled walnut: drying which is too slow can produce c oloured kernels, drying which is too fast can produce split shells; bo th phenomena correspond to valueless products on the market. In the pr esent study, carried out on Franquette walnuts during three harvesting periods (1990, 1991, 1992), four constitutive parts of the whole waln ut were studied to analyse the drying: the shell, the membrane, the in tersticial air layer and the kernel. The evolution of the moisture con tent during ripening was recorded showing the moisture content range f or the whole nut and for each constituent. Sorption curves were determ ined. The moisture change of each constituent during low temperature c onvective drying was also determined. This result is a key factor beca use it allows the end point of drying of the unshelled walnut to be co ntrolled in relation to the required trading moisture content of the k ernel. The drying experiments were performed in a laboratory scale dry er where temperature, relative humidity and air velocity were regulate d and monitored. The product temperature and mass losses were recorded . In preliminary studies, a representative shape of the drying correla tion was selected. This empirical model is analogous to Newton's cooli ng law. Then, using the experimental data, the shape (k) and bending ( n) factors were obtained from the relevant parameters of the process ( dry bulb temperature, humidity of the drying air and initial moisture content of the nut). Air velocity in the tested range, does not appear to be a relevant parameter of the process and, in fact, no constant r ate drying phase is observed for harvested walnuts.