Comparison between desorption isotherm curves of ryegrass (Lolium multiflorum L.) and flax (Linum usitatissimum L.) seeds

Comparação entre curvas isotérmicas de dessorção de sementes de azevém (Lolium multiflorum L.) e linhaça (Linum usitatissimum L.)

ABSTRACT

It is necessary to determine the sorption isotherms of seeds to develop adequate systems of storage and drying. The chemical composition of a product affects the sorption process; products with a high oil content adsorb a lower amount of moisture from the environment than products with a high carbohydrate content. Given the importance of the hygroscopicity of different agricultural products, this work aimed to determine, model and evaluate the difference between desorption isotherms of ryegrass and flax seeds grown at different temperature and relative humidity conditions. Ryegrass and flax seeds, which contained initial moisture contents of 10.4 and 8.7% (db), respectively, were used. The equilibrium moisture content of the seeds was determined using a static-gravimetric method at different temperatures (10, 20, 30, 40, and 50 ± 1 °C) and relative humidity values (between 11 and 96 ± 2%), in three replicates. Seven mathematical models were adjusted to the equilibrium moisture content experimental data of the seeds. The Chung Pfost model best fit the experimental data of ryegrass seeds, whereas the Smith model was determined to be the best fit for flax seeds. The equilibrium moisture content of the seeds was found to decrease as the temperature increased when the value of water activity was constant. The desorption isotherms of ryegrass seeds (Type II) and flax seeds (Type III) are different, according to Brunauer’s classification, which is caused by the composition (starch and oil content) of each product.

Index terms:
Equilibrium moisture content; chemical composition; mathematical modeling; Chung Pfost; Smith.