Abstract
A microbiological assay method for the quantitative determination of niacin in natural materials by a cup-plate technique using Lactobacillus arabinosus was presented. The assay conditions for this method were proposed as follows. Assay medium : Iwai's uniform medium for microbioassay of the B-vitamins with 1% purified agar, 1.5mm plate thickness, pH 6.0 to 7.0. Niacin solution : pH 6.0 to 6.5. Inoculum size of test organism : 0.025 OD in final. Incubation : 37℃, 20 to 24 hours. Under these conditions a reproducible and sharply defined growth zone in the assay plate was steadily obtained and there was a linearity of the doseresponse to pure niacin solution over a wide range of 0.125 to 16 μg per ml. In the statistical evaluation of the results the probable errors amounted to only 5 to 8%. The results of niacin content in the several samples from animal and vegetable sources estimated by the plate method and by turbidimetric ones were nearly equal and the recoveries of the added vitamin were satisfactory enough except for the case of a few sample. Also, the dose-response line for all samples used hardly had any drift from that for pure niacin solution. These results suggest that the cup-plate method is applicable to the detection of niacin in the biological materials, and is suitable for the routine assay of large numbers of samples.
