The desorption isotherms of apples of the Granny Smith variety, treated with 0.5% citric acid aqueous solution, were determined using the gravimetric static method. Experiments at three temperature levels: 20^°C, 40^°C and 60^°C and ten water activity levels from 0.112 to 0.856 were carried out. Along with five well known models - modified Chung-Pfost, modified Halsey, modified Oswin, modified Henderson and GAB (Guggenheim-Anderson-de Boer), a new fraction-linear model has been applied to fit the data and to calculate the isosteric heat of desorption determination.

In this research, the possibilities of applying the extrusion process to produce functional food from quinoa enriched with goji berries were studied. The physical (expansion ratio, bulk density, hardness, and specific mechanical energy) and functional (water solubility index, water absorption index) characteristics were determined and optimized by the response surface methodology. Extrudates were produced using a laboratory single-screw extruder. The full factorial experimental design was used (N = 2²) with three complementary center points to show the interactions of the amount of goji berry (1, 3, and 5%) and the feed moisture content (13, 16, and 19%) of the mixture on the physical and functional characteristics. Increasing moisture content from 13 to 19% resulted in extrudates with a lower expansion ratio, water absorption index, water solubility index, specific mechanical energy, and higher density and hardness. Increasing the amount of goji berries from 1 to 5% led to a decrease in expansion ratio, water absorption index, and hardness, and an increase in density, water solubility index, and specific mechanical energy. Optimal extrusion conditions for production of extrudates from quinoa enriched with goji berry were 16.3% feed moisture content and 1.32% goji berry’s amount.