Antilisterial Effect of a Natural Formulation Based on Citrus Extract in Ready-To-Eat Foods

Abstract

Controlling Listeria in food is a major challenge, especially because it can persist for years in food processing plants. The best option to control this pathogen is the implementation of effective cleaning and disinfection procedures that guarantee the safety and quality of the final products. In addition, consumer trends are changing, being more aware of the importance of food safety and demanding natural foods, minimally processed and free of chemical additives. For this reason, the current consumption model is focusing on the development of preservatives of natural origin, from plants or microorganisms. In sum, this study aimed to evaluate the antimicrobial effectiveness of a citrus extract formulation rich in flavonoids against several L. monocytogenes and L. innocua strains, using in vitro test (agar diffusion test, minimum bactericidal concentration (MBC), and time-kill curves) and challenge test in food trials (carne mechada, salami, fresh salmon, lettuce, brine, and mozzarella cheese). The results presented in this work show that citrus extract, at doses of 5 and 10%, had a relevant antimicrobial activity in vitro against the target strains tested. Besides this, citrus extract applied on the surface of food had a significant antilisterial activity, mainly in carne mechada and mozzarella cheese, with reductions of up to eight logarithmic units with respect to the control. These results suggest that citrus extract can be considered a promising tool to improve the hygienic quality of ready-to-eat foods.

Keywords: Listeria; flavonoids; food safety; natural preservatives.

Figure 1

Microbial viability evolution of (A) L. monocytogenes CECT 4032; (B) L. monocytogenes CECT 5366, (C) L. monocytogenes DMC 1-23, (D) L. monocytogenes DMC 3-17, (E) L. innocua CECT 4030, (F) L. innocua DMC 4, (G) L. innocua DMC 5-1, (H) L. innocua DMC 6-2, and (I) L. innocua DMC 7-3 in the presence of citrus extract. Concentrations tested for each strain: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5%, and ○ 10% of citrus extract. Values are means with SD in bars. * p < 0.05; ** p < 0.001 respect to control.

Figure 2

(A) Evolution of L. monocytogenes on the surface of carne mechada fillets during 25 days of conservation at 4 °C against the different concentrations tested: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5% y, and ○ 10% of citrus extract. Values are means with SD in bars. * p <0.05; ** p <0.001 with respect to the control. (B) Evolution of L. monocytogenes on the surface of salami during 15 days of conservation at 4 °C against the different concentrations tested: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5% y, and ○ 10% of citrus extract. Values are means with SD in bars. * p < 0.05; ** p < 0.01 with respect to the control.

Figure 3

(A) Evolution of L. monocytogenes on the surface of salmon during 7 days of conservation at 4 °C: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5% y, and ○ 10% of citrus extract. Values are means with SD in bars. * p < 0.05 with respect to the control. (B) Evolution of L. monocytogenes on the surface of lettuce during 7 days of conservation at 4 °C: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5% y, and ○ 10% of citrus extract; X 150 mg/L of sodium hypochlorite. Values are means with SD in bars. * p < 0.01; ** p < 0.001 with respect to the control.

Figure 4

(A) Evolution of L. monocytogenes in (A) mozzarella cheese and (B) brine during 25 days of conservation at 4 °C: ● 0 ppm (negative control), □ 0.5%, ♦ 1%, Δ 5%, and ○ 10% of citrus extract. Values are means with SD in bars. * p < 0.05; ** p < 0.001 with respect to the control.