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Pumpkin pieces were dehydrated by conductive hydro drying using the Refractance Window (RW TM ) technology. The water bath temperature was fixed at 90 ± 2°C and the drying was carried out for 5h. This study evaluated the convective effects of heat transfer by natural convection (NC) and forced convection (FC) due to the air in the top of the sample and the conductive effects on a Mylar sheet (MY) and a Mylar sheet covered with aluminum foil (MY/AL). Temperatures were measured at three sample points (bottom, middle and top) during drying. The highest and lowest average temperatures were observed in NC-MY and FC-MY, respectively. The highest and lowest drying rates were obtained in FC-MY/AL and NC-MY, respectively. The ANOVA indicated that the convection and convection-film interaction were statistically significant. Heat transfer due to the different mechanisms in the involved systems (water, drying film, product and air) was examined.

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Guava is widely consumed because of its agro-industrial use, and its antioxidant properties attributed to vitamin C and carotenoids content. However, it has a short shelf life. Guava has been dried by atomization, fluidized bed, lyophilization (FD) and convective drying (CD). CD requires long operation times and the product characteristics are not desirable. In contrast, FD produces high quality products, but requires long processing times, high energy consumption and high operation costs. As an alternative, the Refractance Window® (RW) drying is relatively simple and cheap technique. The objective of this study was to compare the effect of CD, FD and RW techniques, on the moisture content, water activity, color, porosity, volume change, vitamin C and carotenoids content in guava samples. The samples dried by RW required less time to reduce the moisture content and exhibited smaller changes in color than CD or FD. There were greater losses of carotenoids and vitamin C when drying by CD whereas RW had similar losses than FD. Lyophilized products exhibited minor change in volume and greater porosity. RW results in better retention of properties, compared with other drying techniques. Based on this, RW is a promising technique for the development of guava snacks.