Enhanced antimicrobial effect of thyme oil nanoemulsion combined with Lactobacillus acidophilus La-5 against Listeria monocytogenes in laboratory-prepared soft cheese
Abstract
The development of natural and sustainable preservation strategies is important for improving food safety, extending shelf life, and reducing reliance on synthetic additives in dairy products. This study evaluates the antimicrobial effectiveness of thyme oil nanoemulsion and Lactobacillus acidophilus La-5, individually and in combination, against Listeria monocytogenes in soft cheese during refrigerated storage (4°C) for 21 days. The thyme oil nanoemulsion exhibited physicochemical characteristics, including a mean droplet size of 85.3 ± 3.2 nm, a polydispersity index of 0.21 ± 0.02, and a zeta potential of –32.4 ± 1.8 mV, indicating colloidal stability. Four treatment groups were evaluated: control, thyme oil nanoemulsion (0.5% v/w), L. acidophilus La-5 (10⁷ CFU/g), and a combination of both treatments. The combined treatment achieved the greatest reduction in L. monocytogenes counts, decreasing from 6.2 to 1.8 log CFU/g after 21 days, whereas individual treatments produced a reduction of approximately 2.4–2.5 log CFU/g. Viability of L. acidophilus remained above 6.7 log CFU/g throughout storage, demonstrating compatibility between probiotic culture and nanoemulsion system. All pH values remained within acceptable range, and sensory attributes maintained scores of >7.0 on a 9-point hedonic scale, indicating good acceptability. The enhanced antimicrobial performance of the combined treatment is potentially associated with complementary mechanisms involving disruption of bacterial cell membranes by thyme oil constituents and antimicrobial metabolite production by L. acidophilus. These findings support the use of thyme oil nanoemulsion and L. acidophilus La-5 as a natural biopreservation strategy for improving microbial safety, maintaining product quality, reducing dependence on synthetic preservatives, and contributing to sustainable dairy production systems.