Destruction of Listeria monocytogenesin Sturgeon (Acipenser transmontanus) Caviar by a Combination of Nisin with Chemical Antimicrobials or Moderate Heat

The objective of this study was to investigate the effect of nisin in combination with heat or antimicrobial chemical treatments (such as lactic acid, chlorous acid, and sodium hypochlorite) on the inhibition of Listeria monocytogenesand total mesophiles in sturgeon (Acipenser transmontanus) caviar. The effects of nisin (250, 500, 750, and 1,000 IU/ml), lactic acid (1, 2, and 3%), chlorous acid (134 and 268 ppm), sodium hypochlorite (150 and 300 ppm), and heat at 60°C for 3 min were evaluated for a five-strain mixture of L. monocytogenesand total mesophiles in sturgeon caviar containing 3.5% salt. Selected combinations of these antimicrobial treatments were also tested. Injured and viable L. monocytogenescells were recovered using an overlay method. Treating caviar with ≥500 IU/ml nisin initially reduced L. monocytogenesby 2 to 2.5 log units. Chlorous acid (268 ppm) reduced L. monocytogenesfrom 7.7 log units to undetectable (<0.48 log units) after 4 days of storage at 4°C. However, there were no synergistic effects observed for combinations of nisin (500 or 750 IU/ml) plus either lactic acid or chlorous acid. Lactic acid caused a slight reduction (~1 log unit) in the microbial load during a 6-day period at 4°C. Sodium hypochlorite was ineffective at the levels tested. Mild heating (60°C for 3 min) with nisin synergistically reduced viable counts of L. monocytogenesand total mesophiles. No L. monocytogenescells (<0.48 log units) were recovered from caviar treated with heat and nisin (750 IU/ml) after a storage period of 28 days at 4°C.