Desorption isotherms and isosteric heat of niger grains (Guizotia abyssinica (L. f.) Cass.)

Abstract

Niger seed oil has excellent quality and quantity (30-50%) and is rich in polyunsaturated fatty acids, mainly linoleic acid (about 70%). Thus, control of environmental temperature and relative humidity, and of seed moisture content is essential to maintain seed oil quality during storage. Thus, the objective of this study was to determine the desorption isotherms of niger seeds under different temperature and water activity conditions, to test the experimental data with different mathematical models, and discover which best represents the phenomenon, as well as calculate the isosteric heat value for the product. Hygroscopicity was determined by the static gravimetric method, at temperatures of 30, 35 and 40 ºC, and water activity between 0.07 and 0.79 (decimal). For the experimental data, mathematical models commonly used to test hygroscopicity in agricultural products were evaluated. From the results obtained, it was concluded that: water activity increases as moisture content rises; based on the statistical parameters, the models of Chung and Pfost, modified Henderson, Oswin, Sabbah, Sigma Copace, Halsey, Smith and Copace appropriatrely represented the desorption phenomenon of niger seeds; total isosteric desorption heat increased with reduction of moisture content, with values varying from 2539.62 to 3081.48 kJ kg -1 , within the range of equilibrium moisture content of 2.4 to 12.2 (% dry basis).Resumo: O niger é diferenciado pelo óleo dos grãos apresentar ótima qualidade e quantidade (30 a 50%) e por ser rico em ácidos graxos poli-insaturados, principalmente, o ácido linoleico, cerca de 70%. Com isso, o controle da temperatura e da umidade relativa do ambiente e do teor de água dos grãos é necessário para a manutenção da qualidade dos grãos de espécies oleaginosas durante o armazenamento. Assim, objetivou-se com o presente trabalho determinar as isotermas de dessorção dos grãos de niger para diferentes condições de temperatura e atividades de água, ajustar diferentes modelos matemáticos aos dados experimentais, selecionando aquele que melhor representa o fenômeno, bem como calcular o calor isostérico para o produto. A higroscopicidade foi determinada pelo método estático gravimétrico, nas temperaturas de 30, 35 e 40 ºC e atividades de água entre 0,07 e 0,79 (decimal). Aos dados experimentais, foram ajustados os modelos matemáticos frequentemente utilizados para representação da higroscopicidade de produtos agrícolas. A partir dos resultados obtidos, concluiu-se que: a atividade de água aumenta conforme é elevado o teor de água; com base nos parâmetros estatísticos, os modelos de Chung e Pfost, Henderson modificado, Oswin, Sabbah, Sigma Copace, Halsey modificado, Smith e Copace foram satisfatórios para representação do fenômeno de dessorção dos grãos de niger; o calor isostérico integral de dessorção aumenta de acordo com a redução do teor de água, e seus valores apresentaram variação de 2539,62 a 3081,48 kJ kg -1 , na faixa de teor de água de equilíbrio de 2,4 a 12,2 (% base seca). Palavras-chave:Equilíbrio h...

Adsorption isotherms and thermodynamic properties of Carthamus tinctorius L. seeds

“…It can be observed that the harmonic mean had values lower than the range of isokinetic temperature, showing that the process is controlled by enthalpy. These results are consistent with those found by several researchers who have successfully applied the isokinetic theory to the sorption of the most diverse products (Hassini et al, 2015;Goneli et al, 2016;Oliveira et al, 2017;Resende et al, 2017;Silva et al, 2018;Campos et al, 2019).…”

“…Several authors studying the hygroscopicity of seeds have also observed better fit to the experimental data with the Chung-Pfost model, such as Oliveira et al (2013) for cotton (Gossypium hirsutum L.) seeds, Corrêa et al (2016) for beet (Beta vulgaris L.) seeds, Ullmann et al (2016) for sweet sorghum (Sorghum bicolor (L.) Moench) grains, Siqueira et al (2018) for niger (Guizotia abyssinica Cass.) seeds, Fonseca et al (2020) for sorghum (Sorghum bicolor (L.) Moench) grains and Isquierdo et al (2020b) for passion fruit (Passiflora alata Curtis) seeds.…”

“…In addition, together with the weather forecast, it is possible to predict in a long period of time what may occur with the stored seeds, supporting the decision making of the professional in charge (Corrêa et al, 2016). The sorption isotherms and the other thermodynamic properties have been studied by several researchers (Goneli et al, 2016;Resende et al, 2017;Siqueira et al, 2018;Fonseca et al, 2020).…”