Characterization and growth of Bacillus spp. in heat-treated cream with and without nisin

Summary

Aims: To study the germination and growth of both inoculated and naturally occurring Bacillus strains in heat-treated cream with and without nisin. Methods and Results: In heat-treated cream (90 degreesC for 15 min) stored at 8 degreesC, growth was dominated by naturally occurring Bacillus strains such as Bacillus pumilus and B. licheniformis. Only six of the 52 isolated strains were B. cereus/thuringiensis. All of the B. cereus strains, but none of the other strains, produced enterotoxin when tested with the TECRA and reverse passive latex agglutination kits. Bacterial growth during storage of the cream at 8 or 10 degreesC was completely inhibited by low concentrations of nisin. Conclusions: The high number of Bacillus strains surviving the heat treatment represent a risk for heat-treated food that contains cream. The safety of the cream, for instance in 'ready-to-eat' products, can be improved by the addition of low concentrations of nisin. Significance and Impact of the Study: Spores of several Bacillus species may survive heat treatment of cream, but low concentration of nisin with inhibit germination and growth.