THERMOPHYSICAL PROPERTIES OF FRESH AND DRIED WHITE ONION SLICES

The variation of the specific heat, effective thermal conductivity and density of onion (Allium cepa L., cv. Southport White Globe) slices w ith moisture content at near ambient temperature was determined by the method of mixtures, the line heat-source probe and the liquid displac ement method respectively. The specific heat, the effective thermal co nductivity and the calculated effective thermal diffusivity were found to vary linearly with moisture content. The effective thermal conduct ivity of the onion slices did not change significantly in the measurem ent temperature range 30.7-33.0-degrees-C and the porosity range 0.09- 0.27. None of the additive models tested accurately predicted the spec ific heat and the effective thermal conductivity of onion slices at di fferent moisture contents. The true density of the onion slices increa sed linearly from 1066-1416 kg/m3 as the slices were dried. The appare nt density and internal porosity as a function of moisture content wer e correlated by third-order polynomials.