Effect of sodium cinnamate, coumarate, caffeate and ferulate mixtures on the viability, morphometry and ultrastructure of lactic-acid bacteria and Listeria monocytogenes

The aim of this study was to investigate the effect of mixtures of sodium cinnamate, coumarate, caffeate and ferulate against L. monocytogenes and four lactic-acid bacteria strains. The mixtures with bactericidal activity against L. monocytogenes were: coumarate 20 mM – ferulate 20 mM, coumarate 20 mM – caffeate 20 mM, coumarate 20 mM – cinnamate 20 mM, and a ternary mixture of caffeate 3.2 mM (0.06%) – coumarate 13.2 mM (0.25%) – cinnamate 3.2 mM (0.05%). When testing the effect of these mixtures on the LAB, the results showed that L. casei Shirota survived with the four mixtures tested, S. thermophilus survived with three mixtures, while L. plantarum and L. delbrueckii subsp. bulgaricus only survived with the ternary mixture. Ultrastructure studies revealed that the ternary mixture caused degradation of the cytoplasmic content and rupture of the cell membrane of L. monocytogenes. On the contrary, this mixture allowed in some degree the survival of food important lactic-acid bacteria preserving the integrity of the cell wall and membranes. The mixtures of cinnamic and hydroxycinnamic salts showed antilisterial activity at neutral pH; which may be an advantage in the application of these salts as preservatives in low-acid foods.