Kinetics of Rebaudioside A Degradation in Buffer Solutions as Affected by UV Light Exposure

Abstract

Rebaudioside A is a natural non-caloric high-potency sweetener extracted from the leaves of Stevia rebaudiana. With rebaudioside A use increasing in foods, understanding factors affecting its stability is necessary. The literature contains contradictory data about the photostability of rebaudioside A. In addition, kinetic data are lacking regarding the effect of light on rebaudioside A stability. The objective of this project was to determine the degradation rates of rebaudioside A in buffer solutions as a function of ultraviolet (UV) light intensity. Six solutions containing rebaudioside A were prepared: 0.1 M sodium phosphate (pH 3 and 7), 0.1 M sodium citrate (pH 3 and 7), and water adjusted to pH 3 and 7. Eleven 3.7-mL glass vials containing 2 mL of each solution were stored at 32.5°C in darkness, under low intensity UV radiation (365 nm, 27 μW/cm2), and under high intensity UV radiation (365 nm, 190 μW/cm2). Samples were removed at regular time intervals for up to 205 days. Rebaudioside A concentrations were determined using high performance liquid chromatography. Pseudo-first-order rate constants with 95% confidence intervals were calculated for the degradation of rebaudioside A. Rebaudioside A stability was adversely affected by light exposure. Under dark conditions, rebaudioside A in water, citrate buffer solutions, and phosphate buffer solutions was relatively stable. The degradation rate constants increased significantly (p<0.05) with increasing light intensity in all solutions. In both light-protected and light-exposed groups, rebaudioside A in water and citrate buffer solutions was more stable at pH 7 than pH 3, but in phosphate buffer solutions rebaudioside A was more stable at pH 3 than pH 7. The degradation rate constants of rebaudioside A in phosphate buffer solutions at pH 3 and pH 7 were both significantly (p<0.05) higher than the values in water and citrate buffer solutions, suggesting the susceptibility of rebaudioside A degradation in phosphate buffer solutions when protected from light. In darkness, this rapid degradation of rebaudioside A occurring in phosphate buffer at pH 7 was hypothesized to be the result of the dibasic phosphate anion catalyzing its hydrolysis. However, exposure to UV light resulted in rebaudioside A degradation occurring approximately 10 times faster in citrate buffer than phosphate buffer at both pH levels. The highest degradation rate constants occurred in pH 3 and 7 citrate buffers, which were not significantly different (p>0.05). The sensitivity of rebaudioside A to UV light was thus greater in citrate buffers than in water or phosphate buffers. The accelerated degradation of rebaudioside A in citrate buffer was hypothesized to result from free radicals generated by Fe (III)-citrate complexes during UV light exposure. Manufacturers and distributors of beverages containing rebaudioside A must recognize the detrimental effects of light exposure on the stability of rebaudioside A. Appropriate product formulations, packaging, and storage are needed to optimize the shelf life and quality of the rebaudioside A products.