Effect of Tween 80 on incubation period of Lactobacillus casei assay of serum folate.

Contamination of folic acid assay medium with oleate led to a lag phase when Lactobacillus casei cultures were grown on folic acid but not when whole serum provided the folate source. This led to high assay values when using 20-hour incubation periods but satisfactory results were obtained after 48 hours' incubation. The addition of Tween 80 to the medium eliminated the lag phase and the assay results were consistent after 13, 24, and 48 hours' incubation.