Water Adsorption Properties of Two Types of Powdered Chrysanthemums Based on Structural Characterization and Thermodynamic Analysis

To evaluate the correlation between the water absorption characteristics of dried chrysanthemums and their structure, water activity (A_w), and storage temperature, two types of powdered chrysanthemums ("Xiaohuangju" and "Gongjuwang") were analyzed through SEM, FTIR, and XPS. The results indicated that the surface of the powdered chrysanthemums was rough and contained a large number of indentations and cavities. In comparison to "Xiaohuangju", "Gongjuwang" powder had smaller particle sizes and more hydrophilic groups on the surface, such as -OH, O-C-O, etc. To examine the water adsorption behavior of two varieties of powdered chrysanthemums, the static gravimetric method was employed at 20, 30, and 40 ℃ and a range of A_w values from 0.112 to 0.976. Model fitting and thermodynamic analysis were conducted to gain insight into the water adsorption mechanism. An increase in water activity (A_w) resulted in a rise in the equilibrium water content (X_e) of powdered chrysanthemums, conversely, the higher the temperature, the lower the X_e values at the equivalent A_w levels. The fitting results demonstrated that the Peleg model was the most suitable model for depicting the water adsorption isotherms. Results from the thermodynamic analysis suggested that the net isosteric heat of adsorption (q_st) decreased when the equilibrium water content (X_e) increased, and the q_st values of "Xiaohuangju" and "Gongjuwang" powder remained steady in the presence of X_e values greater than 0.14 g/g and 0.24 g/g, respectively. The monolayer water content (X_m) of "Xiaohuangju" at 20, 30, and 40 ℃ were 0.0690, 0.0525, and 0.0505 g/g (d. b), respectively, while those of "Gongjuwang" were 0.0645, 0.0591 and 0.0584 g/g (d. b). When the temperature and water content increased, the effective pore size (r_p) of "Xiaohuangju" powder increased, exhibiting mesoporous characteristics when X_e>0.09 g/g (d. b). Conversely, the internal pores of “Gongjuwang” powder shifted from micropore to mesopore only when X_e>0.11 g/g (d. b). The results of the study can be employed as a guide for improving the drying process of chrysanthemums and determining the most suitable storage conditions.