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

Drying techniques are very frequently used and in many cases official metho ds for moisture determination. These methods, however, do not yield the wat er content as a result but a mass loss which is caused not only by the evap oration of water but by all substances volatile under the drying conditions , be they original components of the product or be they produced by decompo sition reactions during the drying process. This mass loss varies therefore with the parameters applied like time, temperature, form of energy transfe r, atmospheric pressure or surrounding humidity. To shorten determination t imes 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 decompo sition and, consequently, of wrong results rises, particularly when the wat er content is low. It could be shown, however, that analyses are possible, even for beverage instant powders with very low water contents. Moreover, p arameter 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 l ike 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 depe nds extremely on the type of apparatus. (C) 1999 Elsevier Science B.V. All rights reserved.