Listeria monocytogenes is responsible for an infectious disease called listeriosis, which occurred after the consumption of foods. Among various foods, meat products such as RTE meats, frankfurters, deli meat, and pate have been ranked first as the types of food vehicles involved in listeriosis outbreaks due to the extended storage and frequent consumption with no additional heat. The overall goal of this study was to investigate the antilisterial effects of different organic acid salts, hop acid extracts, and their combinations in liquid media and processed meats. To achieve the overall goal, four separated studies were conducted. In study I, nine different organic acid salts were investigated for Listeria inhibition, physicochemical changes and organoleptic characteristics in full- and low-sodium frankfurters. Potassium acetate and potassium diacetate (PAPD) out of the nine organic acid mixtures was the most effective in inhibiting Listeria in full- and low-sodium frankfurters during storage at 4, 7 or 10oC. The sensory characteristics of all formulations were similar except a low score was seen for flavor and overall acceptability in low-sodium frankfurters containing PAPD. In study II, eight different hop acid extracts were investigated for Listeria inhibition with/without PAPD in trypticase soy broth containing yeast extract (TSBYE). Five hop acid extracts (α-acid, β-acid, acid-tetra, K-tetra, and K-hexa) out of the eight including acid-iso, K-iso, and K-rho significantly inhibited Listeria in liquid media at 25 and 50 ppm at 37oC. The combinations of these five hop acids at 25 or 50 ppm with 0.5% PAPD led to better inhibition of Listeria than any single hop acid or 0.5% PAPD alone. After 30 min exposure at 85oC, all of the five hop acids were heat stable with the best inhibitory activity seen for the α-acid, regardless of heating time. At 7oC in liquid media, the mixture of 5 ppm hop acid/0.5% PAPD was listeristatic, whereas none of the single hop acids showed any Listeria inhibition except β-acid. In study III, the antilisterial activity of hop α- and β-acids at 5 ppm with/without 0.5% PAPD were investigated in deli-style turkey meats during storage at 4 and 7oC. Both α- and β-acids at 5 ppm did not inhibit L. monocytogenes during storage at 4 and 7oC, while the hop/PAPD combinations were listeristatic, regardless of the hop acid type. Similar results of no inhibition were observed in skim milk and 2% milk containing α- or β-acid at 5 ppm. In study IV, antilisterial activities of hop α- and β-acids at various concentrations were investigated in turkey slurry at 7 and 37oC. Hop α- and β-acids exhibited antilisterial activity at the concentration > 750 ppm at 37oC and > 500 ppm at 7oC, respectively. These results indicate that formulation of single hop acid required > 500 ppm at 7oC to inhibit Listeria in meat products, which is 100 or more times greater than the 5 ppm hop acid in liquid media.
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