Stability of sodium ascorbyl phosphate in the water-glycerol system

https://doi.org/10.1016/j.jpba.2020.113103Get rights and content

Highlights

  • An analytical method for determination of sodium ascorbyl phosphate and ascorbic acid content simultaneously by HPLC was established.

  • The degradation mechanism of sodium ascorbyl phosphate in the water-glycerol system was revealed and a thermal degradation prediction model was established.

  • The stability of sodium ascorbyl phosphate in the water-glycerol system during storage was investigated under different conditions comprehensively.

Abstract

Sodium ascorbyl phosphate is a hydrophilic derivative of ascorbic acid with better stability compared to the parent compound. However, sodium ascorbyl phosphate is not as stable in solution as it is in the solid state, and it has been found to degrade, with accompanying discoloration, under the influence of different conditions. Here, the degradation mechanism of sodium ascorbyl phosphate in the water-glycerol system was revealed and the thermal degradation kinetics was shown to follow second-order kinetics. A thermal degradation prediction model was established and successfully fitted to the experimental data. In addition, the stability of sodium ascorbyl phosphate in the water-glycerol system during storage was investigated under different conditions, including changes in concentration, temperature, pH, light and oxygen, and metal ions. Sodium ascorbyl phosphate content was quantitatively measured via HPLC, and the color and pH values of the sample were qualitatively measured using a spectrophotometer and a pH meter, respectively. It was found that temperature and pH are the most important factors affecting the stability of sodium ascorbyl phosphate.

Introduction

Ascorbic acid (AA) is one of the most important water-soluble micronutrients [1], and is widely used in pharmaceutical agents, cosmetic ingredients, and dietary supplements as a preservative, color fixing agent, or antioxidant [[2], [3], [4], [5], [6], [7]]. However, it can be easily oxidized due to the existence of the unstable enediol in its structure, causing it to lose its biological activity and thereby limiting its application [[8], [9], [10]]. Sodium ascorbyl phosphate (SAP), with a phosphate group in the second position of the cyclic ring, is a synthesized derivative of ascorbic acid [11]. The presence of the phosphate group is conducive to protecting the enediol structure from oxidation. Thus, SAP is more stable than ascorbic acid [12] and is widely used as a food additive, preservative, cosmetic additive [13], and pharmaceutical raw material [14] due to its high water solubility. Besides, SAP is a valuable fine chemical with broad development prospects, to promote collagen formation, inhibit the production of skin melanin [15] and plays an important role in anti-inflammation [16,17].

One disadvantage of SAP is that, while it has good stability in its solid form, it tends to degrade in solution. Segall and Mayano [11] focused on a typical cosmetic product formulation, finding that SAP degraded under different conditions and had a higher retention rate in the absence of light. Smaoui et al. [18] reported a rate of loss of 5 % SAP in o/w emulsion with an initial concentration of 2.0 % after two months. Spiclin et al. [17,19] found that the rate of loss of the SAP in o/w and w/o microemulsions stored in the dark at 1.00 % (m/m) initial concentration after two months was 5 %. However, few studies have been carried out on the degradation mechanism of SAP to date, and most investigations into the factors affecting degradation reported in the literature are not comprehensive. For cosmetic products containing SAP, its degradation leads to the loss of biological activity, accompanied by undesirable changes in color and acidity. Hence, understanding the degradation behavior of SAP in solutions is of key industrial importance.

In the present study, the water-glycerol system was used to study the stability of SAP, with the main objectives of this work being to investigate the degradation mechanism of SAP and to reveal the causes of discoloration and changes in activity. In addition, the thermal degradation kinetics of SAP was studied, and a predictive model for thermal degradation was established. Finally, the SAP content, color change, and pH values of the system were compared over a wide range of conditions by varying the concentration, temperature, pH, light and oxygen, and metal ions. The results obtained from this study are aimed at providing technical guidance and a theoretical basis for overcoming SAP degradation in solutions in industrial applications.

Section snippets

Chemicals and reagents

Sodium ascorbyl phosphate (≥99 %) was purchased from DSM Nutritional Products (UK) Ltd. l-ascorbic acid, potassium phosphate monobasic, magnesium chloride hexahydrate and calcium chloride dehydrate were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Glycerol, acetonitrile, citric acid and phosphoric acid were purchased from Shanghai Macklin Biochemical Co., Ltd. Hydrochloric acid was purchased from Tianjin Rionlon Pharmaceutical Chemistry co., Ltd. All chemicals were of analytical

SAP degradation and its mechanism

Increasing the storage time and temperature leads to different degrees of discoloration of the SAP solution, which indicates that certain compounds in the solution have changed. The color of the initial control sample and the control samples stored at different temperatures for 5 days is shown in Fig. 1.

High performance liquid chromatography was used to reveal the SAP degradation mechanism. The spectra peak of SAP and AA were identified at 2.38 min and 2.62 min by the UV spectrum respectively.

Concluding remarks

In this work, the stability of SAP in the water-glycerol system was studied, and the mechanism of SAP degradation was probed by using HPLC and Raman spectroscopy techniques. The results showed that SAP is hydrolyzed and degraded into AA when dissolved in water, which is subsequently easily oxidized to form the light yellow-colored DHA—which can further degrade to brown-colored products such as 2,3-diketogulonic acid and other osones. The thermal degradation of the SAP dissolved in the

Declaration of Competing Interest

The authors have declared no conflict of interest.

CRediT authorship contribution statement

Xiaohui Dong: Conceptualization, Methodology, Software, Writing - original draft. Ting Zhang: Conceptualization, Validation. Hongyuan Wei: Resources, Supervision. Leping Dang: Investigation, Writing - review & editing.

Acknowledgements

We thank the financial support by the National Natural Science Foundation of China (No. 21676196).

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