Inhibition Of Listeria And Salmonella In Refrigerated Meats - Brief Article

Nutrition Research Newsletter, August, 2001 by Evelyne Mbandi, Leora A. Shelef

Foodborne illness caused by Listeria monocytogenes is a major concern in the food industry and to the general public because of its high mortality rate. Salmonellosis caused by Salmonella enterica is another concern because it is the most frequently identified foodborne infection in the United States.

Because of the seriousness of these organisms, different techniques must be investigated as to which is the most effective in controlling foodborne illness outbreaks. Organic acids such as lactic and acetic acid are known for their antimicrobial effects in foods. The salts of these two acids are safe and are approved as direct ingredients for use in foods. Even though the antimicrobial activities of sodium lactate (SL) and sodium acetate (SA) are well documented, there is limited information on the effect of their combination or of the combination of SL with sodium diacetate (SDA) on survival and growth of Listeria monocytogenes and Salmonellae in meats.

Effects of SL (1.8 and 2.5%), SDA (0.1 and 0.2%) or SA (0.2%) and their combinations on the behavior of L. monocytogenes and Salmonella enterica were investigated in sterile beef. The organisms or a combination of the organisms were added to cool samples of the beef. Samples were then stored at 5 or 10 [degrees] C. Cells were counted in the meat samples immediately after inoculation and at five-day intervals until levels of approximately 8 log CFU/ g were reached or for 25 days at 10 [degrees] C and 30 days at 5 [degrees] C.

Although L. monocytogenes grew at a faster rate than Salmonella at 10 [degrees] C, concentrations of approximately 8 log CFU/g of both pathogens were reached by day 20 in the untreated samples. SL (1.8%) decreased the growth rate of both L. monocytogenes and Salmonella. However, SDA (0.2%) was more effective than SL in decreasing the growth rate of L. monocytogenes and it caused a more than 1 log CFU/g decline in initial numbers of Salmonella Enteritidis during storage for 25 days at 10 [degrees] C. Salmonella was more sensitive than L. monocytogenes to each of the salts. Combinations of 2.5% SL and 0.2% SDA were bacterio-static to L. monocytogenes and bactericidal to Salmonella after 20 days at 10 [degrees] C. Although SA was consistently and significantly less inhibitory than SDA, its mixtures with SL also demonstrated synergistic activity against both pathogens.

The results showed that combinations of 2.5% SL and 0.2% SDA can be expected to greatly enhance the safety of refrigerated and temperature-abused ready-to-eat meats. The synergistic effect of the salts was 3.7 times (2.5% SL and 0.1% SDA) and 4.5 times (2.5% SL and 0.2% SDA) larger than the sum of reductions obtained with the single salts. It was thought that the acids altered the pH of the meat and therefore changed the environment for the survival of pathogens. However, the inhibitory effects of the salts used in this study are not dependent on pH reduction, since the meat pH remained constant throughout the storage periods. Further, since Salmonella Enteritidis is associated with poultry and eggs, the use of the antimicrobials combination can be extended to ready to eat (RTE) foods containing such products. Testing the effect of the inhibitory system on these two pathogens in RTE products is under way.

Evelyne Mbandi and Leora A. Shelef. Enhanced Inhibition of Listeria monocytogenes and Salmonella Enteritidis in Meat by Combinations of Sodium Lactate and Diacetate, Journal of Food Protection 64(5): 640-644 (May 2001) [Correspondence to: Leora A. Shelef, Phone: 313-577-2998; Fax: 313-577-8616; E-mail: lshelef@sun.science.wayne.edu].