Low bioavailability of folacin has been previously reported for a variety of foods of plant origin. This study was conducted to examine the possible role of various types of dietary fiber on the bioavailability of folic acid monoglutamate. Cellulose, pectin, lignin, sodium alginate and wheat bran were selected for their differing physical and chemical properties. In vitro binding studies by equilibrium dialysis showed no evidence of physical or chemical binding of folic acid under physiological conditions. In vivo effects were evaluated by a chick bioassay with graded levels of folic acid in semipurified diets containing the fiber materials at 3% (weight/weight). Total liver and plasma folacin concentration and chick growth were used as response indicators. Dose-response curves indicated that pectin, lignin and alginate significantly reduced chick growth at all levels of dietary folacin. Plasma and liver folacin dose-response curves were not significantly different for any of the fiber materials, which indicated that the growth impairment was not due to a fiber effect on folic acid absorption. These results suggest that added dietary fiber has little or no effect on the bioavailability of folic acid monoglutamate.

The biological activity of 10-formylfolic acid (10-formyl-FA) and 5-methyl-5,6-dihydrofolic acid (5methyl-DHF) was examined to determine the potential contribution of these oxidized folates to the folacin activity of foods. Both compounds exhibited high folacin activity for Lactobacillus casei in microbiological assays under standard conditions in the presence of ascorbate. Chick bioassays with plasma folacin as the indicator of biological activity revealed approximately 100% activity for 10formyl-FA and 80% activity for the natural l isomer of 5-methyl-DHF, respectively, relative to folic acid. Tritiated forms of these compounds were synthesized and compared with tritiated folic acid for their incorporation into the liver folacin pools of rats after oral dosing. Fractionation of conjugase-treated liver extracts by HPLC indicated that 10-formyl-FA and 5-methyl-DHF were absorbed and metabolized in a manner that was qualitatively and quantitatively similar to that of tritiated folic acid. These results suggest that the formation of 10-formyl-FA and 5-methyl-DHF in foods by oxidation of reduced folates would not yield important losses of folacin activity.

The effects of thermal processing on folacin bioavailability in processed and unprocessed lactose-casein liquid model food systems containing either folic acid or 5-methyltetrahydrofolic acid (5-CH3-THF) were examined by using a chick bioassay. Microbiological and high-performance liquid chromatographic (HPLC) analyses indicated that folic acid was very stable during thermal processing at 120 °C for 20 min while 5-CH3-THF was approximately 75% degraded. Both derivatives were found to be biologically available after processing, which indicated that no complexes were formed during processing that inhibited their utilization. For the model systems there was general agreement between chick bioassay, microbiological, and HPLC analyses. Thermal processing effects on naturally occurring folacin in beef liver and cabbage were also examined. Folacin from cooked beef liver appeared to be fully available. Raw cabbage folacin, which had undergone enzymatic deconjugation during diet preparation and storage, was found to be completely biologically available. Approximately 60% of the folacin in cooked cabbage, which corresponded to the polyglutamate fraction, was not biologically available.

The results of reverse phase high performance liquid chromatographic (HPLC), competitive binding radiometric, and LactobacilZus cusei methods were compared for the determination of total folacin in raw cabbage, a fortified cereal, and a fortified infant formula. A cation exchange procedure was developed which permitted the analysis of cabbage folacin by reverse phase HPLC after hydrolysis of folacin polyglutamates with endogenous conjugase. Although general agreement was observed between assays for the cereal product, marked variation was found for cabbage and the infant formula. These results suggest that the accuracy of the radiometric and L. casei assays may be influenced by the distribution of folacin derivatives present and other sample components. Reverse phase HPLC provides a direct analysis which is not subject to such variability.

An organic acid profile provides valuable information regarding authenticity of apple juice. The presence of D-malic acid is a clear indication of adulteration because this isomer does not occur naturally. Fumaric and citric acid levels above trace amounts are also inconsistent with pure apple juice; therefore, measurement of these organic acids may also be used as an authenticity check. Citric acid, total malic acid, and fumaric acid were determined in a single scan by liquid chromatography (LC) for 30 known pure apple juice samples. The L-isomer of malic acid was measured by an enzyme-specific method, and the D-isomer was calculated as the difference between total malic acid and L-malic acid.