Comparison of the new vapor sorption analyzer to the traditional saturated salt slurry method and the dynamic vapor sorption instrument
Water is a key component of food materials. One of the most useful aspects of the water activity (aw) concept is the moisture sorption isotherm, which plots the moisture content of a material as a function of aw at the same temperature. Moisture sorption isotherms are useful for a variety of processing and product stability applications. The most common methods of generating equilibrium moisture sorption isotherms are using saturated salt slurries (SSS) or humidity generating instruments. The saturated salt slurry method has been a standard method for many years, but there are several drawbacks to this method, including long equilibration times, extensive labor required, and mold growth at high aw values. Many of the disadvantages of the SSS method have been overcome by humidity generating instruments represented by the Dynamic Vapor Sorption (DVS) instrument from Surface Measurement Systems Ltd. There have been several studies done to show that the humidity generating instruments are reliable in terms of their accuracy compared to the SSS method. The Vapor Sorption Analyzer (VSA) is a new type of isotherm generating instrument. The VSA collects aw and weight change data similar to the DVS, but has the added benefit of being able to generate non-equilibrium isotherms as well. Currently, there are a few methods able to produce non-equilibrium isotherms, but no instrument has the capability of producing both types of isotherms. Within this one instrument, the uniqueness of each type of isotherm can be used in order to obtain the most information about the moisture characteristics of a material. Since the VSA still overcomes most of the drawbacks of the saturated salt slurry method and also has the capability of generating both types of isotherms, it would be beneficial to make use of this new instrument. However, no research has been published that investigates the accuracy of the VSA compared to the SSS method or other humidity generating instruments. This research investigated the validity of the new VSA by comparing the equilibrium isotherms generated to the SSS and DVS isotherms. Working isotherms of five different types of food materials (microcrystalline cellulose, soy protein isolate, pregelatinized corn starch, crystalline sucrose, and corn flakes) were obtained in duplicate at 25°C at 11 aw values, ranging between 0.064 and 0.936. Results were analyzed using a mixed model ANOVA and pairwise tests were performed to determine if there was a statistical difference in the mean moisture content between the different methods at each aw. The Scheffe method was used to correct for the p-value to account for the multiple test comparisons. p-values below 0.05 were considered to be significant. The SSS, VSA, and DVS methods all produce similar equilibrium sorption isotherms for the five materials tested. Very few differences were found to be significant and can all be attributed to the heterogeneous nature of the food material or the expected variation around that particular aw value for the material. Thus, the VSA has proved to be an accurate instrument in terms of generating equilibrium sorption isotherms for food materials.
water activity, moisture content, isotherm
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign