The effects of drying on biological matrices and the consequences for reference materials

In the field of reference materials (RMs), long-term stability is of highest importance. Particularly for biological matrices, any kind of deterioration must be avoided during shelf-life. Extensive precautions during production are taken to prevent microbiological and enzymatical activities, but possible alterations caused by intensive drying have been underestimated until today. This paper discusses several mechanisms of deterioration related to water activities and water contents. The four Reference Material matrices chosen as examples are freeze-dried whole grain wheat flour, spray-dried skimmed milk powder, freeze-dried tuna muscle powder and freeze-dried plankton powder. Contents of water and mass loss were measured by Karl Fischer titration (KFT) and with three different drying methods. Based on these data and results and sorption isotherms (25°C, range of water activities 0.05–0.80) possible influences on the certified values by hygroscopicity and the differences between the mass loss and water content are discussed. It was shown that the method chosen for dry mass correction influences certified contents significantly and a method description must be given. Furthermore, it was 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.