Modified Kinetic Modeling of Stevioside and Rebaudioside A Degradation for Shelf-Life Prediction of Stekurmin MD Polyherbal Syrup

The chemical stability of active pharmaceutical ingredients (APIs) is a critical determinant of the shelf life and efficacy of herbal formulations, particularly in complex polyherbal systems. Therefore, this study aims to evaluate the shelf life of Stekurmin MD, a polyherbal syrup containing extracts of Stevia rebaudiana, Curcuma zanthorrhiza, and honey, using an accelerated shelf-life test (ASLT) approach combined with modified kinetic modeling. Stevioside and rebaudioside A, the major antidiabetic steviol glycosides, were selected as stability markers and quantified by validated RP-HPLC during 35 days of storage at accelerated temperatures (25, 35, 45, and 55 °C). The conventional kinetic evaluation results showed apparent first-order behavior, but concentration–time profiles exhibited non-linear trends characterized by an initial increase before degradation. This behavior suggested degradation–reformation equilibria and multi-component matrix interactions within the polyherbal system, deviating from classical Arrhenius assumptions. The apparent activation energy (Eₐ) values were calculated as 5.60 kJ/mol for rebaudioside A and 7.16 kJ/mol for stevioside, markedly lower than those reported for single-compound systems. Therefore, a modified kinetic approach using polynomial regression was applied to better describe the observed non-linear degradation patterns and improve shelf-life prediction. Based on this model, the estimated shelf life at room temperature was 28.10 days for rebaudioside A and 27.58 days for stevioside. These results show that non-conventional kinetic modeling provides a more reliable framework for predicting the shelf life of complex polyherbal formulations influenced by multi-component interactions.