Purpose: The purpose was to select culture(s) with anti-listeria activity, to validate their efficiency and neutral taste when sprayed on vegetables and to apply them in an industrial environment.
Methods: Around 1,500 lactic acid bacteria were tested in laboratory conditions for their anti-Listeria effect. Further tests (bacteriocin and acid production, taxonomic identification via DNA sequencing, safety verification, etc.) were performed on the most promising candidates. Then the selected bacteria were sprayed onto green salad and challenge tests against cocktails of Listeria monocytogenes were performed. Metagenomics study was also performed.
Results: The Lactobacillus curvatus allowed inhibiting the growth of L. monocytogenes during the shelf life (13 days) of the green salad stored at cooling temperature. In the control batch, the pathogenic bacteria growth potential was 1.4 log unit. During this test, no taste deviation was reported. Metagenomics results have shown a good survival and a strong dominance of the protective strain. In addition of the strong ability to compete for space (“jameson effect”) this unique Lactobacillus curvatus is able to produce bacteriocin. For industrial application, a device was designed to spray a solution of bacteria, using water as carrier. The goal was to guarantee an optimal distribution of bacteria on the leaves while adding a low quantity of water (around 0.8%) not having any effect on the humidity or water activity of the product.
Significance: The study has brought a laboratory concept to an industrial application, already used in Europe by several manufacturers to prevent the growth of L. monocytogenes in their products.