Comparison Between Water Vapor Sorption Isotherms Obtained Using The New Dynamic Dewpoint Isotherm Method and those Obtained Using The Standard Saturated Salt Slurry Method

Abstract

The Dynamic Dewpoint isotherm method directly determines sample a_w using chilled-mirror technology, while changes in sample weight are tracked gravimetrically. The objective of this research was to compare the Dynamic Dewpoint isotherms to saturated salt slurry isotherms for five materials: dent corn starch, isolated soy protein, microcrystalline cellulose, crystalline sucrose, and corn flakes. The Dynamic Dewpoint isotherms were obtained using the AquaSorp Isotherm Generator between 0.10 and 0.95 a_w at 25°C. Comparison working isotherms were obtained using 12 saturated salt slurries, between 0.064 and 0.973 a_w in desiccators and proximity equilibration cells at 25°C. The Dynamic Dewpoint isotherms exhibited similar sorption behavior to the saturated salt slurry isotherms, except for corn flakes, where there was a marked decrease in moisture content compared to the saturated salt slurry isotherms between 0.40 and 0.70 a_w. This difference was attributed to the slow diffusion of water into the very dense laminated corn flake matrix, highlighting the rate of sorption (time) dependency of the Dynamic Dewpoint isotherm method. The Dynamic Dewpoint isotherm method offers the opportunity for real-time investigation of water sorption-related material properties, such as the glass to rubbery transition, recrystallization, hydrate formation, and deliquescence.

Keywords:

• Isotherm
• Water activity
• Saturated salt slurries
• Dynamic Dewpoint isotherm
• AquaSorp Isotherm Generator

ACKNOWLEDGMENTS

This project was supported in part by the USDA National Institute of Food and Agriculture, Hatch project number 385. In addition, the authors gratefully acknowledge the financial support of the 2008 Marcel Loncin Research Prize from the Institute of Food Technologists and the 2008 Midwest Advanced Food Manufacturing Alliance (University of Nebraska-Lincoln) grant. The helpful scientific discussions with Brady Carter of Decagon Devices (Pullman, WA, USA) and Dr. Anthony J. Fontana, Jr. of Silliker, Inc. (Homewood, IL, USA) are also gratefully acknowledged.

Notes

¹The user selected flow rate can range from 10 to 1000 ml/min, where a lower flow rate allows more time for moisture penetration.

²Subsequent to the data collected herein, to minimize this overlap the manufacturer now suggests conditioning the sample chamber between sample runs by running an activated charcoal sample. In addition, the manufacturer added a 30 min stabilization step before the sorption run begins.

³For clarity purposes, only one microcrystalline cellulose DDI was plotted. However, the other two microcrystalline cellulose DDIs behaved similarly.