Antimicrobial Interventions to Reduce Listeria spp. Contamination on Shrimp
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The effects of selected antimicrobials, applied singularly or in combination, and frozen or refrigerated storage conditions on the survival of Listeria spp. on inoculated shrimp was evaluated in this study. A combination of 0.5% CPC (Cetylpyridinium Chloride) with a water wash at room temperature and freezing of the shrimp at -22.3 degrees C was the only treatment that had a significant antimicrobial effect on the Listeria spp. Antimicrobial effects and the mode of action of PEF (Pulsed Electric Field) and CPC on Listeria cells were evaluated in detailed studies. PEF in 0.1% sodium chloride had a bacterostatic effect toward Listeria spp. during refrigerated storage, but no immediate or bacteriostatic effect was caused by freezing the samples. A concentration of 1% sodium chloride reduced the Listeria spp. population after freezing by 1.1 log; however, the pungent chlorine odor that was generated during treatment might cause discomfort for employees in shrimp processing facilities. Also, chlorine might cause corrosion of metal surfaces of processing equipment. There was no difference in the antimicrobial effects on the survival of Listeria spp. by PEF between the exposure times of 1 or 2 min, as well as in the sodium chloride concentrations of 0.1 and 0.5%. PEF treatment in the presence of 0.1% sodium chloride is recommended. A solution of 0.5% CPC effectively inhibited all of the strains of Listeria spp. in the cell suspensions. A treatment of 0.5% CPC combined with PEF treatment in a sodium chloride concentration of 0.1% caused a delayed effect on the Listeria spp. after 2 d of refrigerated storage. After 2 d of frozen storage, the formation of ice crystals was decreased in the number of Listeria spp. when contaminated samples were treated with water. The results indicated chemicals (e.g. CPC and NaCl) might protect Listeria spp. from the formation of ice crystals. TEM (Transmission Electron Microscopy) micrographs revealed that cell membranes were damaged by PEF treatment and that cells were ruptured by CPC treatment. A maximum reduction of 2.76 log10 CFU/g of Listeria spp. on shrimp was achieved by a combination of PEFCPC.