Antimicrobial activity of lactic acid and copper on growth of Salmonella and Escherichia coli O157:H7 in laboratory medium and carrot juice
Introduction
With greater availability of information about the health impacts and better reporting standards, consumers are now, not only paying closer attention to the risk of food-borne pathogens, but also to the presence of artificial chemical preservatives included to control food-borne pathogens. As a result, consumers are calling for greater use of natural ingredients to ensure the safety of consumable products. In response to this demand, the food industry is looking to use more natural food preservatives that have strong antimicrobial activity to ensure safe wholesome food products. Two of these natural additives are lactic acid and copper. Traditionally, lactic acid, a weak-organic acid, has been widely used to control growth of pathogenic bacteria in foods for several decades. The antimicrobial activity occurs through the diffusion of lactic molecules into microbial cells until equilibrium is reached, in accordance with the pH gradient, causing membrane disruption, inhibition of essential metabolic reactions, stress on intracellular pH homeostasis and accumulation of toxic anions and ultimate death of microbial cells (Brul & Coote, 1999).
It is common knowledge that microorganisms require low concentrations of copper ions as essential micronutrients and as vital cofactors for processing of metalloproteins and certain enzymes (Gadd, 1993, Nies, 1999). Higher concentrations of copper (250 ppm) can cause inhibition of growth or even death of microorganisms. The toxic effect of copper on microorganisms occurs by the displacement of essential ions, thereby obstructing functional groups of proteins, inactivating enzymes, producing hydroperoxide free radicals, and altering membrane integrity (Nies, 1999). Copper compounds have been widely used as algicide, fungicide, molluscide and acaricide agents in agriculture (Borkow & Gabbay, 2005).
Copper plated surfaces have been shown to have a significant antibacterial activity against a wide range of microorganisms, including Salmonella enteric and Campylobacter jejuni (Faúndez, Troncoso, Navarrete, & Figueroa, 2004). Copper (3.72%) added to stainless steel was found to impede the adhesion of microorganisms during an initial 48 h period, but this antimicrobial effect disappeared after longer incubation periods (Kielemoes & Verstraete, 2001). Copper was added to phosphate-based glasses to combat oral microorganisms and to inhibit the adherence of several microorganisms (Mulligan, Wilson, & Knowles, 2003). Therefore, there is a potential for copper to be used as a preservative to inhibit pathogens in food products. Currently, limited information is available on the effect of copper on the growth inhibition of food-borne pathogens in food products. Previous work, using pigs, suggests that the combination of lactic acid and copper may be an effective combination against undesired microbes. In a study by Beal, Niven, Campbell, and Brooks (2003), lactic acid (150 mM) and copper sulfate (50 ppm) were added to liquid pig feed and found to cause a 10-fold decrease in the Dvalue of Salmonella typhimurium DT104:30. Given the growing attention being paid to natural antimicrobial compounds, which may be more readily accepted by consumers, it is sensible to explore the possibility of combining compounds to obtain a better and more cost efficient reduction of pathogenic microbes. Therefore, the objective of this study was to investigate the effect of lactic acid alone, or in combination with copper sulfate, on the growth inhibition of Salmonella spp. and Escherichia coli O157:H7 in laboratory medium and carrot juice.
Section snippets
Bacterial strains
Thirty eight strains of Salmonella spp. strains (Table 1) and six strains of E. coli O157:H7 (Table 2) were used in this study. Salmonella spp. strains were obtained from the Food Microbiology Laboratory in North Carolina A&T State University. E. coli strains were supplied by Dr. S.S. Summer, Department of Food Science and Technology at Virginia Tech. These strains were maintained on tryptic soy agar (TSA, Difco Laboratories, Becton Dickinson, Sparks, MD) slants at 4 °C. Strains were then
Preliminary study
A preliminary study was conducted to determine the lowest concentration of copper and lactic acid alone that caused slight growth inhibition of Salmonella spp. and E. coli O157:H7. Table 3 shows the effect of four different concentrations (0.0 ppm, 50 ppm, 100 ppm, and 200 ppm) of copper sulfate on the growth of Salmonella spp. and E. coli O157:H7. Results showed that slight growth inhibition was obtained with 50 ppm. A significant growth inhibition was observed with 100 ppm and 200 ppm.
A similar
Acknowledgments
The research work was funded through the Cooperative State Research, Education and Extension Service of the United States Department of Agriculture, Project No. NC.X-173-5-02-170-1, in the Agricultural Research Program, North Carolina Agricultural and Technical State University. We would like to express our gratitude to Dr. C.S. Turner, Associate Dean for Research for her support and Dr. K. Gruber for technical editing.
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