Antimicrobial Properties of Thermally Processed Oyster Shell Powder for Use as Calcium Supplement

Abstract

Oyster shells, though rich in calcium, are mostly discarded and contribute to environmental issues. Developing calcium-based materials with antimicrobial functionality offers a promising solution. However, their low bioavailability limits their direct use, requiring processing to enhance their applicability. Therefore, this study aims to evaluate the physicochemical properties and antimicrobial activity of thermally processed pulverized oyster shells (TPOS) and citric acid-treated TPOS (TPOSc) compared with those of fibrous calcium carbonate (FCC) and coral-derived calcium product (CCP), which are used as reference materials. The solubility values were 0.7 mg/g for FCC, 0.5 mg/g for TPOS, 0.4 mg/g for TPOSc, and 0.05 mg/g for CCP. The average particle sizes were 476 (FCC), 1000 (TPOS and TPOSc), and 1981 nm (CCP). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses revealed calcium ion release and structural changes in TPOS and TPOSc. Antibacterial testing further confirmed that these samples exhibited significant antimicrobial activity. Furthermore, to assess their practical applicability, TPOS and TPOSc samples with antimicrobial properties were incorporated into rice cakes. All samples retained antimicrobial activity at 0.3 wt%, while higher concentrations led to deterioration in their textural properties. These findings support the potential of thermally processed oyster shell powders for food applications that require microbial control with minimal impact on product quality.

Keywords: antimicrobial; bioavailability; calcium; oyster shells; thermal processing.

Figure 1

Solubility of samples and calcium compounds: Different letters above the bars indicate significant differences between groups according to Duncan’s test (p < 0.05).

Figure 2

pH changes in calcium-based sample suspensions in DI water over time.

Figure 3

pH changes in calcium-based sample suspensions in 50% acetic acid solution (initial pH 1.4) over time.

Figure 4

SEM-EDS analysis of TPOS before and after dissolving in DI water.

Figure 5

SEM-EDS analysis of TPOSc before and after dissolving in DI water.

Figure 6

SEM-EDS analysis of FCC before and after dissolving in DI water.

Figure 7

SEM-EDS analysis of CCP before and after dissolving in DI water.

Figure 8

XRD analysis of polymorphic calcium carbonate: CCNPs, calcium carbonate nanoparticles.

Figure 9

XRD patterns of calcium-based samples before and after dissolving in DI water: (A,B) TPOS, (C,D) TPOSc, (E,F) FCC, (G,H) CCP. Each pair of subfigures shows the XRD spectra of the respective sample before (left) and after (right) dispersion in distilled water, highlighting potential phase changes or crystallinity differences.

Figure 10

Antimicrobial effects of calcium-based samples against Escherichia coli: (A) TPOS, (B) TPOSc, (C) FCC, (D) control and CCP.

Figure 11

Effects of calcium-based samples addition on the (A) pH and (B) yellowness (b*) values of rice cakes at different concentrations (0.1, 0.3, and 0.5 wt%).

Figure 12

Changes in natural microbial levels in rice cakes supplemented with calcium-based samples over three days at concentrations of (A) 0.1 wt%, (B) 0.3 wt%, and (C) 0.5 wt%.

Figure 13

Hardness and chewiness of rice cakes supplemented with calcium compounds at (A,B) 0.1 wt%, (C,D) 0.3 wt%, and (E,F) 0.5 wt% concentrations. Different letters indicate significant differences among groups according to Duncan’s test (p < 0.05).