Called competitive inhibition, this biocontrol approach uses non-pathogenic microorganisms to prevent the growth of pathogens in targeted substances. For example, LAB cultures may be applied to MPR fruits and vegetables (e.g., already peeled, and possibly sliced, grated, or shredded) to inhibit the growth of pathogens that may be present, including Salmonella, Shigella, Escherichia coli O157:H7, Staphylococcus aureus, and others.
"Whether these pathogens grow and cause disease depends on the type of product, conditions of storage (time, temperature, atmosphere, etc.), and competing microflora," wrote Fred Breidt, Ph.D., and Henry P. Fleming, Ph.D., the article's authors. "Altering the normal microbial ecology of these products through cutting, processing, modified-atmosphere packaging, and refrigerated storage may have the unintended effect of allowing the growth of pathogenic bacteria [without biocontrol]."
Naturally found in fermented foods such as pickles and sauerkraut, lactic acid bacteria can prevent the growth of pathogens and spoilage organisms in minimally processed produce and other non-fermented foods. In fermented foods, these cultures are being studied for ways to manipulate the fermentation process to enhance flavor and shelf life.
LAB, such as the Lactobacillus species found in yogurt, are generally much more resistant to acid than other bacteria, thus can survive in environments that are lethal to most pathogens. Moreover, they can produce a variety of metabolites [substances produced by or taking part in metabolism], including lactic and acetic acids which increase acidity in foods, inhibiting the growth of other microorganisms.
"In the event of prolonged storage or temperature abuse of MPR foods, the biocontrol culture (LAB) should grow, and prevent the growth of pathogenic microorganisms by competitive inhibition," Breidt and Fleming wrote.
Inhibitory metabolites produced by lactic acid bacteria include organic acids, hydrogen peroxide, enzymes, and bacteriocins. They are produced during the LAB metabolism of sugar (sucrose, fructose, or glucose) naturally found in or added to foods. The type of metabolite produced depends on the type of LAB in foods and their metabolic response to them.
"Biocontrol cultures will likely be product specific, as growth of bacteria in plant materials may be affected by the availability of nutrients and naturally present inhibitors," Breidt and Fleming wrote.
Washing procedures, including the addition of chlorine or other compounds to the wash water, generally have not been successful in reducing microorganisms on minimally processed fruits and vegetables, the authors noted.
"The ineffectiveness of washing or sanitizers to remove bacteria from produce is likely due to microorganisms located in protected regions near the surface of the plant material," Breidt and Fleming wrote.
Studies with endive leaves and salad products have shown that lactic acid bacteria can prevent the growth of pathogens like Listeria monocytogenes, Salmonella typhimurium, and S. aureus. Therefore, products for salad bars and fresh-cut, pre-packaged salads sold at retail may be good applications for these cultures. Competitive inhibition with LAB also shows great potential for enhancing the safety of fresh fruits and vegetables.
However, Breidt and Fleming noted that the use of protective cultures should only supplement good manufacturing practices, not substitute for the proper handling and packaging of produce.
A current application of competitive inhibition is the "Wisconsin process" for ensuring the safety of bacon. This process uses LAB cultures with reduced levels of nitrite to prevent the growth of harmful microorganisms.
Biocontrol has also been studied for use in poultry to reduce the presence of Campylobacter jejuni, in cattle to eliminate E. coli O157:H7 prior to slaughtering, and in refrigerated meat products to guard against spoilage.
For a copy of the article or Sept. 1997 issue of Food Technology, contact Angela Dansby at (312) 782-8424 X134.
Fred Breidt, Ph.D. is senior researcher in the Depts. of Biological and Agricultural Engineering and Food Science at North Carolina State University. Henry P. Fleming, Ph.D., is location coordinator and research leader of the U.S. Dept. of Agriculture's Agricultural Research Service and professor in the Dept. of Food Science at North Carolina State University.
Founded in 1939, the Institute of Food Technologists is a non-profit scientific society of 28,000 members working in food science, technology and related professions in industry, academia, and government. As the society for food technology, IFT brings sound science to the public discussion of food issues.