Different antimicrobials are added in the manufacturing of dairy products such as flavored yogurts and processed cheese. Potassium Metabisulfite (PM) and Potassium Nitrite (PN) have been reported to have antimicrobial effect on pathogenic microorganisms such as Clostridium botulinum and Listeria monocytogenes. Yogurt is known for its health benefits, due to the presence of cultured bacteria. PM and PN are not commonly used in the dairy industry hence it would be interesting to study their influence in yogurt culture bacteria. The objectives of this study were: 1. to elucidate the influence of PM and PN at various concentrations, on the growth of yogurt culture and 2. to determine the possible acquisition of resistance after prior exposure to low doses of these antimicrobials. For the first objective different concentrations of PM and PN (100, 1,000, 10,000, 100,000 and 1,000,000ppm) were separately added to broth previously inoculated with Lactobacillus bulgaricus LB-12 and Streptococcus thermophilus ST-M5. Control samples did not receive any antimicrobial. Growth was determined by plating at 0, 24, 48 and 72 hours of incubation. For the second objective, treatments consisted of separately exposing cultures to 100 and 1,000ppm of both antimicrobials and after 24, 48 and 72 hours, transferring them into 10,000 and 100,000ppm, of both antimicrobials. Growth was measured at 0, 24, 48 and 72 hours of incubation. Data were analyzed using Proc Mixed and Repeated Measures model of the Statistical Analysis System SASŪ. Differences of Least Square Means where used to determine significant differences. Neither PM nor PN had an antimicrobial effect on yogurt culture Lactobacillus bulgaricus LB-12 or Streptococcus thermophilus ST-M5. Both antimicrobials at 1,000,000ppm significantly increased counts of Streptococcus thermophilus ST-M5 6 log CFU/mL compared to control. Prior exposure of Streptococcus thermophilus ST-M5 to both
antimicrobials at 100 and 1,000ppm for 72 hours showed resistance to 10,000 and 100,000ppm of both PM and PN with a significant increase of 6 log CFU/mL compared to control. Commercial applications of this study would be to incorporate PM and PN in yogurt manufacture for inhibition of spoilage and pathogenic bacteria to ensure good preservation of the product and improved shelf life.