Errors in Static Desiccator Method of Water Sorption Isotherms Estimation

摘要

Sorption isotherms find numerous applications in food science and technology. They can be measured by different methods, but static desiccator method is one of the most frequently used. It is simple and many samples can be placed in one desiccator. Hence, isotherms of many products can be obtained in one experiment. In this work 20 products, mainly herbs and spices, were placed in one desiccator and 13 desiccators were used to obtain water sorption isotherms. Water activity of each sample was measured in Hygroscope DT2. The effect of the individual operations such as opening the desiccator, transferring samples to the balance and measuring water activity on the results was assayed. It was shown that the most important is the disturbance of equilibrium caused by opening the desiccator, taking the sample and closing it again. The disturbances cause adsorption of water from the surrounding air by samples with low water activities, and desorption of water from the samples having high water activity. Analysis showed that the process of desorption is slow and affects results insignificantly. On the other hand adsorption is fast and leads to the increase of water activity of samples. The increase depends on water activity of the sample and number of openings of the desiccator. With 20 openings the increase in water activity can be two to threefold in the range 0 ≤ a_w ≤ 0.1. Some recommendations for standardization of methods using static desiccator method are suggested.