Secagem osmo-convectiva da acerola.

Abstract

This work deals with the study of osmotic-convective drying of West Indian cherry (Mafpighia emarginata DC} by dehydration in sucrose solutions, followed by convective drying in air under several operating conditions. The effects of immersion time, agitation, concentration and temperature of the osmotic solution and temperature of the drying air on the osmotic dehydration kinetics and convective drying were studied by means of three factorial designs. The operational conditions were: 40, 55 and 70 °Brix; the operating temperature at the osmotic stage varied 28, 30, 34, 35 and 40 °C while in the convective step it was between 60, 65 and 70 °C; the immersion time in the sucrose solution varied from short times (30, 60 and 90 min), intermediate times (4 and 7 h) and longer times (24, 48 and 72 h). Relative to the external transfer aspects, the effect of agitation in the osmotic stage was analyzed, varying no agitation to up to 780 rpm and in the differential cell 20 rpm, whereas in the convective stage the air velocity in the dryer was fixed at 1.3 m/s. The influence of several osmotic treatment conditions in the equilibrium isotherms, in the drying kinetics and in the quality of the product was also analyzed. For a better comprehension of the mechanisms involved in the osmotic dehydration, studies were made on the permeability of the West Indian cherry skin in a diffusion cell. It was found that the time for osmotic dehydration and convective drying diminishes when the concentration of sucrose and the immersion time increases. Simple Fickian models were used to analyze the water diffusion in the osmotic and convective stages, as well as the mass transfer through the skin in the diffusion cell experiments. The water diffusion coefficient in the whole fruit varied between 0.40x10"® and 8.0x10"5 cnvVmin, in the osmotic stage and O.SxIO"4 and 1.8x1(T*cm3/min in the convective. As expected, the diffusion coefficient on the skin was less than on the entire fruit, although in the same order of magnitude, varying between 0.14x10"s and 1,0x1 or5 cm2/min confirming that it is on the external waxy film where the greatest resistance to the mass transfer is found. The sensorial analyses of the processed West Indian cherry juice presented a good acceptability, in taste, aroma, color and appearance. The West Indian submitted to osmotic convective drying presented less loss of vitamin C during the convective drying when compared to the "in natura" fruit. These results showed that the osmotic pretreatment is efficient in order to diminish the convecting drying time, preserving the quality of the West Indian cherries.