'Hidden parameters' of infrared drying for determining low water contents in instant powders.

Drying techniques are very frequently used and in many cases official methods for moisture determination. These methods, however, do not yield the water content as a result but a mass loss which is caused not only by the evaporation of water but by all substances volatile under the drying conditions, be they original components of the product or be they produced by decomposition reactions during the drying process. This mass loss varies therefore with the parameters applied like time, temperature, form of energy transfer, atmospheric pressure or surrounding humidity. To shorten determination times of many hours in common air ovens with convective heating, techniques with more efficient heating principles have been developed. One of these is infrared drying. With such methods, however, the danger of product decomposition and, consequently, of wrong results rises, particularly when the water content is low. It could be shown, however, that analyses are possible, even for beverage instant powders with very low water contents. Moreover, parameter sets could be found to match the infrared results exactly with the true water content determined by Karl Fischer titration. Another essential finding was that not only the parameters for the drying programme itself like time, temperature and end-point criterion are important, but also, and this to a surprisingly great extent, the number of consecutive measurements and the duration of the intervals between analyses. This effect again depends extremely on the type of apparatus.