Title: Citric Acid-Stabilized Amorphous CaCO₃ as a High-Surface-Area Carrier for Functional Food Ingredients

Amorphous CaCO₃ (ACC) has an intrinsically large specific surface area, rendering it a promising platform for developing various functional materials. However, its practical use is severely limited by an extremely short lifespan, which is typically only a few seconds, before its spontaneous recrystallization into vaterite or calcite. In this study, we investigate the use of citric acid to suppress the rapid crystallization of ACC in a mixed ethanol/ethylene glycol medium and to obtain reproducible high-surface-area ACC under controlled conditions. Experiments involving systematic variations in citric acid concentration (0.00-1.33% w/v) revealed that crystallization was completely inhibited at concentrations above 0.39% w/v, as confirmed by X-ray diffraction. Brunauer-Emmett-Teller analysis showed that ACC samples synthesized with 0.44-1.11% w/v citric acid achieved exceptionally high surface areas (600-700 m²/g). The observed suppression of crystallization is discussed in relation to previously reported interactions between citrate species and calcium carbonate phases, rather than being directly demonstrated at the molecular level. As a proof of concept, curcumin was loaded onto the prepared ACC samples, and enhanced dissolution behavior was observed, which plateaued above a specific surface area threshold. This finding highlights the importance of the surface-area-to-cargo ratio in future formulation strategies. The resulting stable high-surface-area ACC is a promising platform for the design of functional materials.