Determine water content in low-moisture, high-sugar fruit powder by different analytical techniques.

Water content is an essential stability, cost and regulatory parameter. However, the accuracy and reproducibility of the techniques used to determine water content depend on several factors, including the composition of the sample.

Determining the water content of low-moisture, high-sugar-content materials is especially difficult. For these types of samples, the widely used Karl Fischer Titration (KFT) analytical technique is recommended by scientists. However, despite its usefulness, KFT has some limitations, including issues involving the specificity of solvent and titrant samples. There also are method-associated inconveniences.

So, scientists at the University of Illinois compared the KFT-determined water content of blueberry powder that had been dried using refractance window drying (RWD) to three more convenient methods: vacuum-oven drying (VOD), halogen drying (HD) and dynamic vapor sorption (DVS). RWD is reported to retain beneficial nutritional compounds. The conditions for KFT involved the extraction of water using a 7-to-3 anhydrous methanol and formamide solution for 30 minutes at 25 C and performing titration with Hydranal-Composite 5.

The environment for performing VOD was 70 C, using762 mm of mercury for 6 hours. For HD, the conditions were standard mode, 80 C and 1 mg mass loss in a 50-second termination criterion. For DVS, the conditions included exposure to 0% relative humidity for 24 hours at 25 C. The water content of RWD blueberry powder for KFT was 2.9993 [+ or -] 0.0189%. For VOD, it was 3.4635 [+ or -] 0.3932%. For HD, it was 2.89 [+ or -] 0.07%, and for DVS, it was 2.7041 [+ or -] 0.3748%.

The VOD water content was significantly different from the others. However, the standard deviation of the DVS method was much greater than those obtained for the KFT and HD methods. The HD technique is suitable for measuring the water content of low-moisture, high-sugar-containing fruit powder. For accurate results, the HD technique must be calibrated by a direct method, such as the KFT method used in this research. So, it appears that the HD method is most useful for routine sample analysis.

Further information. Shelly Schmidt, Department of Food Science and Human Nutrition, 367 Bevier Hall, University of Illinois, 905 S.Goodwin Ave., Urbana, IL 61801; phone: 217-333-6369; fax: 217-265-0925; email: sjs@uiuc.edu.