This can occur with samples that are very low in fat. Moisture left in low fat samples after the extraction is the issue. As AOCS Procedure AM 5-04 indicates, when the Filter Bags are removed from the Analyzer, they should be placed in an oven at 100 to 105°C for 15-30 minutes. The bags are then placed in a desiccant pouch an allowed to cool to room temperature (10 to 15 minutes) and reweighed.

It is possible that the extraction time was not long enough to gain a complete fat extraction from the low fat sample. In the vast majority of samples, a 60 minute extraction is sufficient. However, a longer extraction period may be necessary if the results show minus values.

Negative values may also occur is the sample is holding onto the solvent in spite of the post extraction drying step. You can determine if this is the case by drying the samples either for an additional 3 hours at 100°C or for an hour at 110 - 120°C. Both of these scenarios help to release solvent binding. It should be noted that with the higher temperature drying, there is a concern that more than just the solvent can be blown off so the lower temperature drying may be preferable.

It is important to make certain that the small desiccant packets should be blue. If the contents are any other color, the desiccant is no longer absorbing moisture. Remove the packets from the zip lock bag and place them in an oven at 100 to 105°C for at least an hour and they will be renewed (blue color will return.) At ANKOM, we perform this renewal process at least once a month. In addition, to insure the integrity of the zip lock bag, regular replacement should be considered.