Improved photostability of folic acid by the radical-scavenging effect of tannic acid

Abstract Folic acid (FA) is sensitive to ultraviolet (UV) light and decomposes into photoproducts with higher fluorescence quantum yield: 6-formylpterin (Fop) and p-aminobenzoylglutamic acid (PABA-Glu). Fop is subsequently oxidized into 6-carboxypterin (Cap). The increased fluorescence intensity of FA solution with increasing irradiation time proved the degradation. Addition of tannic acid (TA) at the TA/FA molar ratio of 0.025/1 could protect 99% FA compared to that no FA could be detected without addition of TA after 120 min of UV irradiation. The retention of FA without TA at pH 5–6 was below 0.4% and above 93% at pH 3–4, but the retention of FA with TA was over 97% for pH 3–6. Moreover, the retention of FA with TA in gummy was over 81% which was significantly higher than that of the one without TA. No covalent bonds were formed between TA and FA. Ethylenediaminetetraacetic acid could not protect FA against photodegradation. Ascorbic acid (Vc) could protect FA to the same extent as TA, but with more addition. It agreed with the results that the DPPH radical-scavenging effect of Vc is lower than TA. These results demonstrated that TA could improve the photostability of FA by the radical-scavenging effect.