Investigating the Effect of Different Concentrations of Citric Acid on Listeria monocytogenes and Escherichia coli O157 : H7 in Fermented Soy Protein Isolate Systems With Bovine Milk Yogurt as a Control

ABSTRACT This study examined the antimicrobial effects of citric acid (0% and 1%) on Escherichia coli O157:H7 and Listeria monocytogenes in soy protein isolate (SPI)–based yogurt systems, with bovine milk yogurt serving as a control. Fermentation (0, 15, and 24 h) and storage (48 h) phases were monitored to evaluate changes in pH, total titratable acidity (TTA), and microbial counts. All samples showed a gradual decline in pH, indicating successful lactic acid bacteria (LAB) fermentation, with the greatest reduction occurring within the first 15 h. The addition of 1% citric acid moderated lactic acid production, likely due to altered metabolic activity and buffering effects. LAB effectively inhibited both pathogens, with citric acid enhancing this antimicrobial action. While L. monocytogenes showed the highest reduction in dairy samples, 1% citric acid in SPI exhibited a synergistic effect with LAB, leading to greater pathogen control than in 0% citric acid samples. Overall, citric acid improved microbial safety and fermentation stability in plant‐based yogurt systems, though further work is needed to assess its impact on sensory and textural quality.