Moisture sorption isotherm and thermodynamic analysis of quinoa grains
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ORIGINAL
Moisture sorption isotherm and thermodynamic analysis of quinoa grains Sultan Arslan-Tontul 1 Received: 6 July 2020 / Accepted: 29 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In literature, most studies have only focused on sorption characteristics of quinoa grains. This study is one of the limited studies that deal with the sorption properties of quinoa with a thermodynamic approach. In the designed study, the equilibrium moisture content of whole quinoa grains determined by the static gravimetric method and ten different sorption models were applied for the description of the relationship between detected water activity and equilibrium moisture content. The thermodynamical properties of sorption have been determined by calculation of isosteric heat, entropy, Gibb’s free energy and spreading pressure. The best fitted sorption models are Brunauer–Emmett–Teller, Henderson, Peleg and Smith according to the statistical significance parameters. The monolayer moisture content of quinoa grains varies between 3.72–5.25%. The adsorption isotherm of grains are classified in Type-II by giving a sigmoidal shape. According to the thermodynamical analysis, the increase in moisture content leads to a decrease in isosteric heat of sorption from 50.4 kJ/mol to 43.7 kJ/mol. Additionally, the reduction in sorption heat is more gradual after 8% moisture content. Like isosteric heat of sorption, sorption entropy decreases from 15.9 J/molK to 1.5 J/ molK with increasing water activity. Spreading pressure which means the surface excess-free energy, has ranged between 0.01 and 0.15 J/m2 and increase with water activity. Increasing moisture contents and temperatures have also decreased Gibb’s free energy level. Keywords Quinoa . Sorption equations . Entropy . Spreading pressure
1 Introduction Quinoa (Chenopodium quinoa Willd.) is a native plant, harvested in the Andean region (Peru, Bolivia and Argentina). It has been widely used for food and feed for more than 7.000 years. Quinoa is classified as a pseudo-cereal due to its starch-like endosperm. In recent years, there has been an increasing interest in quinoa grains because of its unusual rich nutritional and balanced composition. Recent researches have suggested that protein, fat and fiber contents of quinoa are higher than that of the present in wheat and other grains such as barley, corn and rice. Moreover, its amino acid composition is considered as ideal according to FAO (Food and Agriculture Organisation) who has declared 2013 as the “International Year of Quinoa” [1–5]. On the other hand, the * Sultan Arslan-Tontul [email protected] 1
Agricultural Faculty, Department of Food Engineering, Selçuk University, 42130 Konya, Turkey
gluten-free composition makes quinoa a vital additive for food formulation improved for celiac diseased patients. However, the highly hygroscopic outer layer of quinoa grains lead to influence the grain from the relative humidity of the air, and the storage stability decreases quickly by gaining
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