Modeling of moisture sorption isotherm and evaluation of net isosteric heat for spray-dried fortified coconut (Cocos nucifera L.) powder

In order to understand interactions a_w vs equilibrium moisture content (EMC) in fortified coconut powder, moisture sorption isotherms were constructed under different storage conditions in order to predict the changes in their physical, chemical and microbiological properties that occur during storage and processing, which are unique to each food.

For which the moisture sorption isotherms were determined at three different temperatures (15, 25 and 35 °C), in a range of water activity from 0.1 to 0.90. Nine models, namely, the GAB, BET, Oswin, Smith, Halsey, Henderson, Chung and Pfost, Peleg and Caurie equations, were fitted to the sorption data. Various statistical tests were adopted as criteria to evaluate the fit performance of the models.

Of the models tested, the Peleg model gave the best fit to experimental data (R² = 0.997; R_MSE = 0.276), across the full range of water activities and at different temperatures. Humidity of the monolayer (m_o) was found between 2.54 and 2.34%, a fundamental parameter to define the storage and control conditions, given that it is considered the value at which the product is more stable. The net sorption isosteric heat (Q_st) increased to maximum and then diminished with increased moisture content (X_w); maximum values were obtained in the X_w interval between 0.48 and 2.87% (db), being between 35.72 and 99.26 kJ/mol, where the maximum value indicates coverage of the strongest bond sites and higher adsorbate-adsorbent interaction.

These results provide reliable experimental data on water absorption isotherms of the CP + FAC important to determine optimal processing, storing and packaging conditions.