Control of Listeria monocytogenes With Undissociated Organic Acids: Staged Evaluation in Broth, Milk, and a Proof-of-Concept Surface Application on Queso Fresco

Queso fresco (QF) is a high-moisture, near-neutral pH fresh cheese that supports the growth of Listeria monocytogenes during refrigerated storage. This study evaluated whether delivering undissociated organic acid (UDA) can suppress L. monocytogenes across staged systems: broth, milk, and QF surface. Four dairy L. monocytogenes strains and Scott A were grown in non-pH-adjusted brain heart infusion (BHI) broth containing acetic (AA; total acid: 10-25 mM, pH: 5.24-6.81), lactic (LA; total acid: 20-40 mM, pH: 4.27-6.35), or propionic acid (PA; total acid: 2-25 mM, pH: 5.10-6.89). Growth (OD595nm) was modeled (Logistic 4P model, JMP Pro18) to estimate lag phase duration (LPD), and the pH of treatments was used to calculate the UDA. LPD increased with total acid concentration and correlated significantly with UDA for AA and PA, whereas LA effects were pH-dependent (p < 0.05). UDA targets (0, 2, 5, mM) from the broth screen were then applied in milk, with AA and PA slightly inhibiting growth of L. monocytogenes at low UDA (2 mM - pH: AA 5.93; PA 6.39), and controlling growth at high UDA concentrations (5 mM - pH: AA 5.39; PA 5.61). In contrast, LA inhibited growth only at low pH (4.62 ± 0.06). To assess the effect of UDA on QF, cubes of cheese (20 g; 3.81 × 3.81 cm) were surface-treated (20 min) with AA or PA to achieve low (<20 mM), moderate (40-50 mM), or high (>100 mM) UDA, inoculated with a five-strain L. monocytogenes cocktail (3 log CFU/g), vacuum-packaged, and stored at 7 °C for 28 days. In untreated, low and moderate groups, L. monocytogenes increased by >1 log CFU/g (Harlequin Listeria agar; 37 °C, 48 h), whereas high UDA treatments were bacteriostatic throughout storage. At sufficient UDA levels, AA and PA suppressed L. monocytogenes growth on near-neutral-pH QF, while LA efficacy depended on pH. These findings support targeted, surface-directed UDA interventions to suppress L. monocytogenes on QF without lowering bulk product pH.