Determination of folates in foods by high-performance liquid chromatography with fluorescence detection after precolumn conversion to 5-methyltetrahydrofolates

A liquid chromatographic–fluorimetric determination of folates in foodstuffs including their extraction, without or with deconjugation, chemical conversion to 5-CH3-H4PteGlun and purification of the extract by affinity chromatography is reported. The conversion enables the analysis of total folates and also of the contents of the different mono- and polyglutamate forms of the folates. The method has a satisfactory day-to-day repeatability (never more than 10%) and a very low detection limit (0.02 pmol per injection). Depending on the folate studied, the recovery rates varied from 78% (10-CHO-PteGlu) to 98% (5-CHO-H4PteGlu). Furthermore it has been possible to show that the deconjugation of the folates by rat plasma conjugase was incomplete in foodstuffs whereas chicken pancreas conjugase effectively converted the different folate polyglutamates into folate diglutamates. It could not be demonstrated that prior hydrolysis with a protease and amylase was useful for the analysis of the different foodstuffs studied (yeast, spinach, beef liver, beef fillet and peas) when deconjugation was performed with the chicken pancreas conjugase.