Individual hydrophilic vitamins are considered unstable molecules. In addition to external factors, their degradation rate is also influenced by their mutual interactions, which are still relatively unexplored. Due to the complexity of the factors that may affect their stability and the lack of appropriate analytical methods, stability studies in the literature provide inconsistent results that are mostly qualitative. However, with the growing use of vitamin products, there is also an increasing need for reliable stability studies, as knowledge on vitamins stability and their interactions is necessary to ensure the quality of such products.
Therefore, the central goal of this master's thesis was to quantitatively evaluate the stability of all individual hydrophilic vitamins in their commonly used forms in vitamin products as well as their mixtures in solutions under different stress conditions. For this purpose, we initially validated the previously developed stability-indicating analytical method following the ICH guidelines and found that it meets the set criteria in terms of specificity, linearity, accuracy, precision, detection limit, quantitation limit and stability of control samples.
Stress testing under extreme conditions (light, elevated temperature, oxidation, acidic and alkaline hydrolysis) gained insight into the instability reactions of individual vitamins and identified vitamin C and 5-methyltetrahydrofolic acid (the active form of folates) as the least stable vitamins. We also found different susceptibility to degradation of the tested vitamins in solutions of individual vitamins and their mixtures. The most notable was the increased oxidative stability of vitamins B1, B2, B5, B6, B9 and B12 and the decreased photostability of vitamins B3, B5, B6, B7, B9 and B12 in the mixtures. The stress testing was upgraded with an in-depth evaluation of the stability of individual vitamins and selected mixtures at different pH values, storage temperatures and oxidant (hydrogen peroxide) concentrations. First order kinetics best described their degradation and was used for the quantitative stability evaluation. We defined the conditions with decisive effect on the stability of the studied vitamins and investigated the interactions between the vitamins in the mixtures. For most vitamins, we confirmed the expected accelerated degradation at higher temperature, while the pH of the medium had a different effect on the stability of the individual vitamins. When evaluating the oxidative stability of vitamins in the mixtures, we found complex interactions in which we recognized the role of vitamin C as key.
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