Water as a source of errors in reference materials.

In the field of reference materials, long-term stability is of highest importance. Particularly for biological matrices, any kind of deterioration must be avoided during shelf-life. In the production precautions are taken to prevent these, but possible alterations caused by intensive drying have been underestimated until today. This presentation discusses several mechanisms of deterioration related to water activity and water content. Comparison of the results for water and moisture for six reference materials and their water sorption isotherms serve as a base to discuss the importance of accurate dry mass corrections for certified values. It is shown that the method chosen for dry mass correction influences certified contents significantly and a method description must be given. Furthermore, it is confirmed that for biological matrices optimal ranges of water activity and water content exist, where hygroscopicity is low without significant danger of product deterioration. Additionally, handling is facilitated in the optimised ranges.

Investigations on the water content of candidate reference material CRM 679 cabbage powder.

The determination of the residual water content by thermal methods and especially by Karl Fischer titration in a candidate reference material of the European Commission (candidate CRM 679 cabbage powder) is described and discussed.

'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.

[Comparison of methods to characterise thermally altered frying fats and oils]. / Methodenvergleich zur Charakterisierung thermisch veränderter Fritierfette und -öle.

During the process of deep fat frying the fat or oil undergoes several chemical and physical changes. To guarantee an effective quality control for used frying fats simple and rapid methods for the measurement of heat abuse are needed. Therefore several frying oils were heated with and without foodstuff and the change of polar parts, acid number, colour acid number, specific absorption and dielectric properties with prolonged heating time were determined. It could be shown that under usual frying conditions acid number and colour acid number, which are often used in praxis, as well as the specific absorption are not unrestrictedly useful to characterise heated frying fats. It turned out, however, that the determination of the dielectric properties with a foodoil-sensor is a useful tool to investigate heat abuse of frying fats and oils in routine analysis.

Determination of total polar parts with new methods for the quality survey of frying fats and oils.

During the frying process the fat undergoes several chemical and physical changes caused by heat, water and atmospheric oxygen. With prolonged heating time the accumulation of deterioration products leads to organoleptic failures and a decrease of the nutritive value. For the quality control of frying fats or oils the determination of total polar parts by preparative column chromatography is an approved standard method. Many attempts were undertaken to replace this time and chemical consuming method. To substitute for the determination of polar components by column chromatography the measurement of dielectric properties with a food oil sensor, nuclear magnetic resonance spectroscopy (NMR) and near infrared spectroscopy (NIR) proved to be suitable.