A differential assay of folic acid and total folate in foods containing enriched cereal-grain products to calculate μg dietary folate equivalents (μg DFE)

A method was developed to differentiate folic acid from total folate in enriched cereal products. By elimination of the protease and conjugase digestion steps from the trienzyme digestion used to measure total folate, folic acid can be directly quantified by the microbiological assay using Lactobacillus casei (ssp. rhamnosus) ATCC 7469. Determination of total folate by the trienzyme digestion together with assay of folic acid by the procedure reported here allows calculation of food folate and μg dietary folate equivalents (μg DFE). Recovery studies conducted at five concentration levels in a non-fortified wheat flour gave a mean recovery of 95.5 (RSD% 9.7) for folic acid and 98.5 (RSD% 5.0) for total folate. Recoveries from a variety of enriched cereal foods ranged from 88% to 99% for the folic acid extraction and 90% to 96% for the trienzyme digestion. Analysis of SRM 1846 Infant Formula ( n = 11 ) gave means±s.d. of 123.5±6.9 μg/100 g of folic acid and 136.6±6.7 μg/100 g of total folate (reference concentration value=129±28 μg/100 g). Intermediate precision determined as intra-laboratory reproducibility(r) gave RSDr % values of 4.32 for the folic acid assay and 3.15 for the total folate assay. Microgram DFE values were determined for several enriched cereals by calculating food folate as the difference between total folate and folic acid and the formula μg DFE=μg food folate+(1.7×μg folic acid). The differential assay of folic acid and total folate is a simple and accurate procedure to determine μg DFE in enriched cereal-grains or foods containing enriched flour.