Study of Thermodynamic, Structural, and Quality Properties of Yacon (Smallanthus sonchifolius) During Drying

The thermodynamic properties and the changes in volume were studied during hot air drying of yacon. The results indicated high rates of moisture and water activity losses during the first 150 min of drying. Shrinkage of the product was evident during drying, with a reduction of about 89 % as observed by three-dimensional laser scanning and by the structural changes observed during scanning electron microscopy, indicating severe tissue shrinkage and collapse during air drying. The experimental sorption data of air-dried yacon were applied to various isotherm equations (GAB; Oswin; Chung–Pfost; Henderson, Smith, and Halsey). The Chung–Pfost model was best for characterizing the sorption behavior at 25, 30, 35, and 40 °C. The sorption isotherms of the dehydrated yacon were of type III, characteristic of high-sugar-content products. The values obtained for the differential enthalpy and entropy were higher at low moisture contents and decreased with increase in moisture content. The value obtained for the free energy of −70.7 J mol −1 suggests the process was spontaneous. A color analysis was also carried out on the yacon samples dehydrated by hot air and by freeze drying, with and without prior blanching at 100 °C for 4 min. The parameter L * was significantly higher, and the values for a * and b * significantly lower for the freeze dried yacon as compared to the forced air dehydrated samples, resulting in lighter, greenish, and bluish samples.