1967 Volume 35 Issue 6 Pages 451-456
In the preceding report of this series, a cup-plate method with Streptococcus faecalis as test organism was found to be available for the microbiological assay of pure folic acid solution from the basis of its high precision and the presence of a linear relation between the sizes of growth zones and the logarithms of the vitamin concentration. In order to confirm the availability of this method for the determination of folic acid in biological materials, several animal and vegetable samples were used and parallel assay was made with the turbidimetric method. In the case of the assay of free folic acid of some materials, the response lines from the extracts were not parallel to that from pure folic acid solutions. Also, the recoveries of the added folic acid to these extracts were unsatisfactory. This phenomenon is presumably due to the presence in the samples of growth factors or some factors affecting the assay results. The valid and reliable assays of folic acid in natural products by the plate method are not possible unless an improvement in extracting procedure of the vitamin from them will be detected.
