Sustainable synthesis of calcium propionate from postbiotic propionic acid produced by Schleiferilactobacillus harbinensis–Pichia kudriavzevii co-culture using alternative calcium sources: Physicochemical characterization and feed additive potential

Background and Aim: Calcium propionate is a multifunctional feed additive widely used to improve energy metabolism, prevent metabolic disorders, and enhance feed preservation in livestock production. However, commercial calcium propionate is predominantly synthesized from petrochemical-derived propionic acid, raising concerns regarding sustainability and resource dependence. This study aimed to synthesize calcium propionate using bio-derived propionic acid produced by a co-culture of Schleiferilactobacillus harbinensis and Pichia kudriavzevii, compare four alternative calcium sources, and identify the most suitable material for producing high-quality calcium propionate through comprehensive physicochemical characterization. Materials and Methods: Bio-derived propionic acid obtained from an optimized microbial co-culture fermentation was reacted with eggshell flour, shellfish meal, bone meal, and indigenous limestone flour under optimized synthesis conditions (200-mesh particle size, 15% propionic acid, solid-to-liquid ratio of 0.14:1, 80°C, and 2.5 h). The synthesized products were characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) to evaluate morphology, elemental composition, functional groups, and crystalline structure. Results: The calcium source markedly influenced the physicochemical characteristics of the synthesized calcium propionate. Eggshell-derived calcium propionate exhibited the most porous and heterogeneous microstructure, indicating a greater apparent surface area than products synthesized from the other calcium sources. EDS analysis confirmed that calcium, carbon, and oxygen were the predominant elements in all products, while the eggshell-derived product showed the most homogeneous elemental composition with fewer detectable impurities. FTIR spectra confirmed successful calcium propionate formation in all treatments through characteristic carboxylate absorption bands, whereas the eggshell-derived product exhibited sharper and better-defined absorption peaks, indicating greater chemical uniformity. XRD analysis further demonstrated a more ordered crystalline structure and fewer secondary mineral phases in the eggshell-derived product than in the shellfish meal-, bone meal-, and limestone-derived products. Collectively, SEM–EDS, FTIR, and XRD consistently identified eggshell flour as the most suitable calcium precursor for producing high-quality calcium propionate. Conclusion: Eggshell flour is a promising sustainable calcium source for producing high-quality calcium propionate from bio-derived propionic acid. Integrating postbiotic propionic acid production with eggshell waste valorization provides an environmentally friendly approach for manufacturing value-added feed additives while supporting circular bioeconomy principles and sustainable livestock production.