Effect of a(w), controlled by the addition of solutes or by water content, on the growth of Listeria innocua in broth and in a gelatine model

The effect of a(w) on the growth of Listeria innocua was investigated in broth and on the surface of a gelatine food model. In broth, a(w) was controlled from 0.91 to 0.99 by the addition of solutes such as NaCl, KCl, glucose, sucrose and glycerol. In the gelatine food model, a(w) was controlled by removal of water. In the a(w) range, 0.92-0.99, the generation times observed in broth in the presence of NaCl, KCl, sucrose and glucose were similar but were longer than those in glycerol. For lag times, the inhibition of L. innocua growth followed the order: NaCl=KCl=sucrose>glucose>glycerol. When comparing growth at a(w) 0.95 for the three media--broth+NaCl, gelatine gel (a(w) controlled by removal of water) and gelatine gel with NaCl (gel+NaCl, a(w) controlled by NaCl)--the shortest generation time was observed in broth+ NaCl, followed by gel+NaCl and, finally, on gel with a larger gap between the last two. The generation time on gel was five times greater than the generation time in broth+NaCl and 2.5 times greater on gel+NaCl. It was concluded that not only the structure of the media (solid or liquid) had an effect on Listeria inhibition but also and mainly the way the a(w) was adjusted. Removal of water was more stressful to Listeria than the addition of NaCl.