Research Progress on the Stability of Anthocyanins and Their Interactions with Proteins: a Perspective Based on the pH-Dependent Multistate System of Anthocyanins

Anthocyanins are water-soluble natural colorants found in plants, known for their multiple health benefits. However, their structural instability, which leads to color fading and degradation in response to pH changes, presents significant challenges for their application as natural food colorants in complex food matrices with varying pH conditions. To understand the mechanisms behind anthocyanin instability and to develop effective stabilization strategies, it is essential to examine their pH-dependent multistate behavior. Under highly acidic conditions (e.g., pH≤1), anthocyanins exist solely as the flavylium cation (AH+). As the pH increases, AH+ undergoes reversible reactions, resulting in a mixture of AH+, quinoidal base, hemiketal, cis- and trans-chalcones. Thus, the fading of anthocyanins is significantly influenced by pH, arising from reversible reactions within their multistate system (i.e., transformation from colored into colorless forms) and potentially irreversible degradation. Furthermore, the interactions between proteins and anthocyanins have been extensively studied to develop strategies to stabilize anthocyanin. In this context, it is essential to analyze the role of proteins in regulating anthocyanin color and in stabilizing their color or content, from the perspective of the pH-dependent multistate system of anthocyanins. This review introduces the pH-dependent multistate system of anthocyanins, explores the mechanisms of fading, and concludes with an examination of their interactions with proteins. The objective is to provide theoretical insights and innovative strategies for stabilizing anthocyanins and developing anthocyanin-based natural food colorants.