SummaryRiboflavin (RF), also known as vitamin B2, belongs to the class of water-soluble vitamins and is widely present in a variety of food products. It is sensitive to light and high temperature, and therefore, needs a consideration of these factors for its stability in food products and pharmaceutical preparations. A number of other factors have also been identified that affect the stability of RF. These factors include radiation source, its intensity and wavelength, pH, presence of oxygen, buffer concentration and ionic strength, solvent polarity and viscosity, and use of stabilizers and complexing agents. A detailed review of the literature in this field has been made and all those factors that affect the photo, thermal and chemical degradation of RF have been discussed. RF undergoes degradation through several mechanisms and an understanding of the mode of photo- and thermal degradation of RF may help in the stabilization of the vitamin. A general scheme for the photodegradation of RF is presented.

In the present investigation the photolysis of riboflavin (RF) in the presence of citrate species at pH 4.0-7.0 has been studied. A specific multicomponent spectrophotometric method has been used to assay RF in the presence of photoproducts during the reactions. The overall first-order rate constants (k obs ) for the photolysis of RF range from 0.42 to 1.08×10(-2) min(-1) in the region. The values of k obs have been found to decrease with an increase in citrate concentration indicating an inhibitory effect of these species on the rate of reaction. The second-order rate constants for the interaction of RF with total citrate species causing inhibition range from 1.79 to 5.65×10(-3) M(-1) min(-1) at pH 4.0-7.0. The log k-pH profiles for the reactions at 0.2-1.0 M citrate concentration show a gradual decrease in k obs and the value at 1.0 M is more than half compared to that of k 0, i.e., in the absence of buffer, at pH 5.0. Divalent citrate ions cause a decrease in RF fluorescence due to the quenching of the excited singlet state resulting in a decrease in the rate of reaction and consequently leading to the stabilization of RF solutions. The greater quenching of fluorescence at pH 4.0 compared to that of 7.0 is in accordance with the greater concentration of divalent citrate ions (99.6%) at that pH. The trivalent citrate ions exert a greater inhibitory effect on the rate of RF photolysis compared to that of the divalent citrate ions probably as a result of excited triplet state quenching. The values of second-order rate constants for the interaction of divalent and trivalent citrate ions are 0.44×10(-2) and 1.06×10(-3) M(-1) min(-1), respectively, indicating that the trivalent ions exert a greater stabilizing effect, compared to the divalent ions, on RF solutions.

a b s t r a c tA study of the photodegradation of 5 × 10 −5 M riboflavin (RF) in 0.2-1.0 M phosphate buffer in the presence and absence of 2.50 × 10 −4 M caffeine at pH 6.0-8.0 has been carried out. RF in phosphate buffer is photodegraded simultaneously by normal photolysis (photoreduction) and photoaddition reactions giving rise to lumichrome (LC) and cyclodehydroriboflavin (CDRF) as the main final products, respectively. RF and its photoproducts, formylmethylflavin (FMF), lumiflavin (LF), LC and CDRF in degraded solution have been determined by a specific multicomponent spectrophotometric method with an accuracy of ±5%. The apparent first-order rate constants for the photodegradation of RF and for the formation of LC and CDRF are 5.47-15.05 × 10 −3 min −1 , 1.06-8.30 × 10 −3 min −1 and 4.31-8.05 × 10 −3 min −1 , respectively. An increase in phosphate concentration leads to an increase in the rate of formation of CDRF and alters the photodegradation of RF in favor of the photoaddition reaction. This photoaddition reaction is further enhanced in the presence of caffeine which results in a further decrease of the fluorescence of RF in phosphate buffer. Caffeine may facilitate the photoaddition reaction by suppression of the photoreduction pathway of RF.

The physical mixtures of AA and CT showed no interaction between the two compounds by FTIR spectrometry indicating that the reduction in rates of degradation is due to the antioxidant activity of CT which protected AA to some extent from the atmospheric oxygen.

The polymerization of 1–3 M 2-hydroxyethyl methacrylate (HEMA) initiated by riboflavin/triethanolamine system has been studied in the pH range 6.0–9.0. An approximate measure of the kinetics of the reaction during the initial stages (~5% HEMA conversion) has been made to avoid the effect of any variations in the volume of the medium. The concentration of HEMA in polymerized solutions has been determined by a UV spectrophotometric method at 208 nm with a precision of ±3%. The initial rate of polymerization of HEMA follows apparent first-order kinetics and the rates increase with pH. This may be due to the presence of a labile proton on the hydroxyl group of HEMA. The second-order rate constants for the interaction of triethanolamine and HEMA lie in the range of 2.36 to 8.67 × 10−2 M−1 s−1 at pH 6.0–9.0 suggesting an increased activity with pH. An increase in the viscosity of HEMA solutions from 1 M to 3 M leads to a decrease in the rate of polymerization probably as a result of the decrease in the reactivity of the flavin triplet state. The effect of pH and viscosity of the medium on the rate of reaction has been evaluated.