Propriedades termofísicas de polpa de cajá (Spondia lutea L.) a baixas temperaturas.

Abstract

This work consists in characterizing the pulp of yellom monbin fruit and in determining some of its thermophysicaJ properties, such as specific bulk and heat, and thermal diffusibility, and conduction, for those temperatures which are below freezing point. The characterization of the yellom monbin pulp was accomplished by determining some aspects, namely, pH, total soluble solids, acidity and moisture. The specific bulk was obtained through the ratio between bulk and volume when freezing the samples of pulp in a known volume bin at 0,-18, -35, -70, -100 and -180°C temperatures. The specific heat below freezing point was measured by using Moline's method (MOLINE el al., 1961) at temperatures o f - 3 5 , -70, -100 and -180°C, and also the mixture method at -18, -35, -70, -100, -180 and -196°C. The thermal diffusibility was determined at the temperatures o f 0, -18, -35, -70 and -100°C, using Dicherson's methodology (DICHERSON, 1965). The thermal conduction was measured by the yield between specific bulk, specific heat and thermal diffusibility. Some models from the literature, based on the water content for specific heat measurements, were used. The resulting values were compared with the experimental ones. From the results regarding the study on the pulp of yellom monbin, the following conclusions were drawn: (a) the average values of pH, soluble solids, acidity and moisture were approximately 4,16 pH, 8, 18° Brix, 1,23% and 90,06% (wet base), respectively; (b) the specific bulk decreased from 1025 to 986 kg/m3 , together with a decrease in temperature from 0°C to -180°C, describing an exponential curve, (c) the specific heat obtained through Molines's methodology (MOLINE el al., 1961) ranged from 2,78 kj/kg°C to 3,81kj/kg°C for a temperature range of-35 to -180°C. These values are higher than those reported by the literature, and also than the ones obtained from the mixture method; (d) the specific heat in the yellom monbin pulp, obtained thorough the mixture method, ranged from 1,96 kj/kg°C to 0,72 kj/kg°C, achieving a temperature range from -18 to -196°C, respectively, thus describing a linear equation; (e) the thermal diffusibility and conduction increased as temperature decreased where thermal diffusibility values of 6,41xl0"7m2/s to 8,83"7m2/s were found for a temperature range of 0 -100°C. The thermal conduction ranged from 1,43 w/m°C, for temperatures ranging from -18 to -100°C.