Bactericidal Activity of Calcium Oxide (CaO, Heated Scallop-Shell Powder) against Listeria monocytogenes Biofilms on Egg Shell and Stainless Steel Surfaces

Introduction: Scallop shells are waste products of scallop harvesting in districts of Korea and Japan. When scallop shells are heated to >700°C, calcium carbonate (CaCO₃), the main component of scallop shells, is converted to calcium oxide (CaO), which has potent bactericidal activity. The persistence of L. monocytogenes biofilms on food surfaces and surfaces that contact food is the main attribute facilitating its environmental spread and subsequent contamination of ready to-eat meat products.

Purpose: This study examined the bactericidal activity of 0.05–0.50% calcium oxide (CaO, heated scallop-shell powder) against planktonic cells and biofilms of Listeria monocytogenes (ATCC 19113) on egg shell and stainless steel surfaces, which represent the potential major surfaces found in egg processing plants.

Methods: 0.05–0.50% CaO was used to inactivate planktonic cells and biofilms of L. monocytogenes ATCC 19113. Hunter colors of “L” (lightness), “a” (red/green), and “b” (yellow/blue) were used for the quality of egg. Non-linear Weibull model was fitted to determine 3-log reduction of CaO (%) for the planktonic cells and biofilms of L. monocytogeneson the surfaces.

Results: On both surfaces, the bactericidal activity of CaO against planktonic cells and biofilms increased over log reductions of magnitude with increasing concentrations of CaO (p < 0.05). On egg shell surfaces, exposure to 0.05–0.50% CaO for 1 min reduced planktonic cell numbers in cell suspensions by 0.47–3.86 log (CFU/mL) and biofilm cell numbers by 0.14–2.32 log (CFU/cm²). On stainless steel surfaces, exposure to 0.05–0.50% CaO reduced planktonic cell numbers in cell suspensions by 0.03–4.43 log (CFU/mL) and biofilm cell numbers by 0.03–2.86 log (CFU/cm²). The color of eggs shells, which are represented by Hunter color parameters “L” (lightness), “a” (red/green), and “b” (yellow/blue), was not changed by 0.05–0.50% CaO treatments. The concentration required to reduce bacterial cell numbers by 3 log (CR = 3) (99.9% reduction), based on fits using the non-linear Weibull model, were significantly different for planktonic cells in cell suspension (0.31% CaO), biofilms on egg shell surfaces (0.57% CaO) and biofilms on stainless steel surfaces (0.46% CaO) (p < 0.05). Biofilms of L. monocytogenes were more resistant to CaO treatment on egg shell surfaces than on stainless steel surfaces due to their surface properties.

Significance: CaO represents a promising substitute for chemical disinfectants that are currently used on eggs and in egg processing plants.

Keywords: Listeria monocytogenes Biofilms; Calcium oxide; Egg shell surface; Stainless steel surface; Weibull model