Validation of the Use of Composite Sampling for the Detection of Listeria monocytogenes in Frozen Vegetables

Introduction: Multiple methods have been validated to detect Listeria monocytogenes in small samples (25 g) of frozen vegetables. Little work has been done to determine if these methods are effective at detecting low levels of Listeria monocytogenes in 125 g and 375 g composited frozen vegetable samples.  In this study, the U.S. Food and Drug Administration’s Bacteriological Analytical Manual (Online Edition), enzyme-linked fluorescent immunoassay (ELFA) and polymerase chain reaction (PCR) methods were used to evaluate the efficacy of detecting low and medium levels of Listeria monocytogenes in composited frozen samples of broccoli, tomato, potato and onion.

Purpose: The purpose of this study was to evaluate the efficacy of compositing frozen vegetables using multiple detection methods for Listeria monocytogenes at low and medium inoculation levels.

Methods: A four-strain Listeria monocytogenes cocktail was used to spike 25 g, 125 g and 375 g samples of frozen vegetables at low and medium levels.  Samples were enriched for each test method with volumes of pre-enrichment modified to accommodate the 125 g and 375 g samples sizes.  Data were analyzed using the chi squared test to determine statistical significance.

 Results: At the medium inoculation level, Listeria monocytogenes was detected by all methods in all 25 g, 125 g and 375 g samples.  When considering all vegetables and methods using the low inoculation level, the ability to detect Listeria monocytogenes in the 375 g samples was significantly lower than the 25 g samples.

 Significance: This study provides data for evaluating the use of sample compositing of frozen vegetables using multiple methods of detection for Listeria monocytogenes.