Adsorption isotherms and thermodynamic properties of Carthamus tinctorius L. seeds

ABSTRACT Safflower is a crop of high economic value with high oil concentration in its seeds and great industrial versatility, besides various benefits to human health. As with other agricultural crops, it is common to store safflower to make it available in different periods of the year and, due to its hygroscopic characteristics, studies evaluating the effect of temperature and air relative humidity on its moisture content become relevant. Thus, the objective of the present study was to determine the water adsorption isotherms of safflower seeds and analyze their thermodynamic properties. Moisture contents of 6.5, 6.9, 7.3, 7.7, 8.3 and 9.1% (dry basis) were obtained by adsorption under controlled conditions of temperature (30 °C) and relative air humidity (90%). The adsorption isotherms were obtained by the indirect static method at different temperatures (10, 20, 30 and 40 °C). As temperature increased, for the same moisture content, there was an increase in water activity and, for constant water activity, the values of equilibrium moisture content decreased with increasing temperature. Chung-Pfost model showed the best fit to describe the phenomenon of hygroscopicity of safflower seeds. The thermodynamic properties were influenced by the moisture content of the seeds, reducing the energy necessary for water absorption in the product with the increase in adsorption, and the enthalpy-entropy theory was controlled by enthalpy.摘要：红花是一种经济价值颇高的油料作物，其种子含油量优异，工业应用场景广泛，同时对人体健康具有多重益处。与其他农作物类似，为保障全年稳定供应，红花通常需要进行贮藏；而由于其吸湿性特性，探究温度与空气相对湿度对其含水率的影响具备重要研究价值。本研究旨在测定红花种子的水分吸附等温线（water adsorption isotherms），并分析其热力学性质。实验通过在温度30℃、空气相对湿度90%的可控环境中开展吸附处理，获得了干基含水率（dry basis）分别为6.5%、6.9%、7.3%、7.7%、8.3%及9.1%的样品。采用间接静态法，在10、20、30和40℃的不同温度条件下，获取了各温度下的水分吸附等温线。研究结果表明：在相同含水率条件下，水分活度（water activity）随温度升高而升高；而在水分活度恒定的情况下，平衡含水率（equilibrium moisture content）随温度升高而降低。Chung-Pfost模型（Chung-Pfost model）对红花种子的吸湿性现象具有最优的拟合效果。热力学性质受种子含水率影响，随着吸附过程的推进，物料吸附水分所需的能量逐渐降低，且焓-熵理论（enthalpy-entropy theory）由焓主导。