Moisture sorption isotherms of sunflower seeds: Thermodynamic analysis

Abstract

The study of the thermodynamic properties provides the knowledge of the sorbent affinity by the water and the spontaneity of the sorption process, important to design drying equipment and to predict hygroscopic behavior during storage. This work aimed to determine and evaluate isotherms of hygroscopic equilibrium and thermodynamic properties for different equilibrium conditions (water contents and temperatures) in sunflower seeds, using the indirect static method. Sunflower seeds with initial water content of 0.164 (decimal, dry basis - d.b.) were used. Fifteen mathematical models were tested for hygroscopic equilibrium modeling, and the Smith model presented the best fit. It was verified that the thermodynamic properties were influenced by the water content, the integral isosteric desorption heat increased with the decrease of the equilibrium water content, ranging from 1.787 to 2.670 kJ kg-1 to 0.198 to 0.0518 (decimal, d.b.), respectively, the desorption process of the sunflower seeds was controlled by the enthalpy and the Gibbs free energy, which varied from 3.6363 to 200.1730 kJ kg-1, was positive for the temperatures studied, with increase to during the desorption process, proving to be a non-spontaneous process.

Thermodynamic properties of moisture desorption isotherms of ryegrass (Lolium multiflorum L.) seeds

“…The isokinetic temperature was higher than the harmonic average temperature, showing that the process is controlled by enthalpy. These results are in line with what has been observed by several researchers who have applied the isokinetic theory of sorption of different agricultural products (Oliveira et al, 2010;Goneli et al 2010;Hassini et al, 2015;Zeymer et al, 2018a, Campos et al, 2019.…”

“…It was noted that Gibbs free energy increased with decreasing moisture content, being positive for all Ciência e Agrotecnologia, 44:e027619, 2020 temperatures studied, tending to stabilize at higher levels of equilibrium moisture content (Figure 3). This trend was also observed by Oliveira et al (2010) and by Campos et al (2019), studying Zea mays L. and Helianthus annuus L. seeds, respectively.…”

“…With the linearity between the differential enthalpy relation and the differential sorption entropy, the isokinetic theory, or enthalpy-entropy compensation theory, can be considered valid for the moisture desorption phenomenon. Other studies have also reported this same behavior (Oliveira et al, 2010;Goneli et al 2010;Hassini et al, 2015;Zeymer et al, 2018a;Campos et al, 2019).…”

“…For the same temperature, with increasing equilibrium moisture content, there is an increase in water activity. Previous research with different agricultural products also reported this behavior (Oliveira et al, 2011;Costa et al, 2015;Zeymer et al, 2018b;Campos et al, 2019). Water activity reports the amount of water molecules available for different reactions in the product, as well as the development of microorganisms; therefore, by increasing the equilibrium moisture content, a greater amount of moisture is available for these reactions, resulting in an increase in water activity (a w ).…”

“…Recently, thermodynamic parameters such as enthalpy, entropy, Gibbs free energy and isosteric heat, among others, have been investigated for different types of grains and seeds, such as Opuntia ficus indica seeds (Hassini et al, 2015), Salvia hispanica L. seeds (Velázquez-Gutiérrez et al, 2015), Piper nigrum L. seeds (Yogendrarajah et al, 2015), Tamarindus indica L. seeds (Alpizar-Reys et al, 2017), Beta vulgaris L. seeds (Oliveira et al, 2017), Phaseolus vulgaris L. grains (Miano;Sabadoti;Augusto, 2018), Lactuca sativa L. seeds (Zeymer et al, 2018a), Oryza sativa L. grains (Zeymer et al, 2018b), Helianthus annuus L. seeds (Campos et al, 2019), Triticum vulgare L. grains (Mattioda;Jorge;Jorge, 2019). However, studies involving the thermodynamic properties of moisture sorption of ryegrass seeds have not yet been performed.…”