Why do I get negative crude fat values?

Negative results can occur when: 

1. Very low fat content samples are not dried properly after solvent extraction. Refer to AOCS AM 5-04 for proper technique. After solvent extraction, filter bags must be dried in an oven at 100 - 105°C for 15-30 minutes. The bags should then be placed in a desiccant pouch, allowed to cool to room temperature (10 to 15 minutes), and weighed. You can determine if drying was adequate by continuing to dry the samples either for an additional three hours at 100°C or for one hour at 110-120°C. Both conditions will help to release bound solvent. Be aware that drying at 110-120°C may drive off volatile compounds which will skew fat results.
2. Make sure that the desiccant is reactivated regularly. A blue color is an indication that the desiccant is dry and active. If the contents are any other color, the desiccant is no longer absorbing moisture. To dry desiccant, remove the desiccant packets from the zip lock bag and place them in an oven at 100 - 105°C for at least one hour. To ensure the integrity of the zip lock bag, regular replacement should be considered.
3. Insufficient extraction time could result in incomplete fat extraction from low fat samples. Most often, a 60-minute extraction is sufficient. However, a longer extraction time may be necessary if negative fat values are obtained.