Control of Listeria monocytogenes in whole milk using antimicrobials applied individually and in combination.

Dairy product recalls and dairy-related illnesses are often the result of contamination with Listeria monocytogenes, which can occur throughout the dairy production and supply chains. The use of antimicrobial compounds is one practical approach for controlling pathogen survival and growth in foods. The goal of this study was to use fluid milk as a model system to identify listeristatic or listericidal treatments that show promise for application in fluid milk and for further evaluation in other dairy products (e.g., cheese). Caprylic acid (CA), ε-polylysine (EPL), hydrogen peroxide, lauric arginate (LAE), and sodium caprylate (SC) were added individually or in combination to whole milk inoculated with L. monocytogenes at ˜4 log10 cfu/mL. Samples were stored at 7°C for 21 d, and L. monocytogenes counts were determined weekly. Inhibitory concentrations of LAE (800 mg/L) and EPL (100-400 mg/L), as well as SC and CA (3,200 mg/L each), were identified. The addition of EPL at 800 mg/L reduced L. monocytogenes counts by >3 log10 cfu/mL from initial inoculation levels after 21 d. Addition of hydrogen peroxide to milk reduced counts by >3 log10 cfu/mL from initial inoculation within 24 h (400 and 800 mg/L) or by d 7 (200 mg/L). Although the combinatory treatments of EPL + CA, EPL + LAE, and LAE + SC were characterized as indifferent, EPL + SC worked synergistically to reduce L. monocytogenes populations in milk over 21 d. Overall, these data identify potential antimicrobial treatments to control L. monocytogenes in milk and serve as a foundation for the continued development of antimicrobial controls for L. monocytogenes in dairy products.

Efficacy of Antimicrobials Applied Individually and in Combination for Controlling Listeria monocytogenes as Surface Contaminants on Queso Fresco.

Outbreaks of listeriosis are continually attributed to the consumption of Hispanic-style soft cheeses contaminated with Listeria monocytogenes postpasteurization. Once contaminated, L. monocytogenes can grow rapidly in cheeses like Queso Fresco (QF) even when stored at refrigeration temperatures. Several antimicrobials, including acidified calcium sulfate with lactic acid (ACSL), ε-polylysine (EPL), hydrogen peroxide (HP), lauric arginate ethyl ester (LAE), and sodium caprylate (SC), have demonstrated antilisterial activity in food. The objectives of this study were to determine the efficacy of these antimicrobials used individually and in combination to control L. monocytogenes as surface contaminants on QF and to identify additive and synergistic interactions. Cheeses were surface inoculated at ∼4 log CFU/g, dipped in antimicrobial solutions, vacuum packaged, and then stored at 7°C for 35 days. L. monocytogenes counts were determined 24 h after application of the antimicrobials and then weekly throughout storage. Dip treatments in a 5% (v/v) HP solution reduced L. monocytogenes counts to <0.5 log CFU/g within 24 h with no increase in counts through day 35. Dip treatments in LAE at 2 and 5% alone and in combination with EPL at 10% produced initial reductions in pathogen counts (1.5 to 1.8 CFU/g) but did not inhibit pathogen growth compared with the sterile water control. Dip applications of ACSL at 25% also produced an initial ∼1.5-log reduction in L. monocytogenes counts followed by regrowth. Application of SC at 10% alone and in combination with either EPL or LAE inhibited growth to <1 log CFU/g through 21 days of storage. The combination of ACSL+SC worked synergistically to inhibit the growth of L. monocytogenes on QF to <1 log CFU/g through 35 days. These data indicate that HP alone and treatments containing EPL, LAE, or ACSL in combination with SC are promising postlethality treatments and process controls for L. monocytogenes on QF through a 21-day shelf life.