Effect of copy number of the spoVA2mob operon, sourdough and reutericyclin on ropy bread spoilage caused by Bacillus spp

Abstract

Bacillus spp. cause ropy bread spoilage of bread, which is characterized by a rotten fruity odor, followed by discoloration and degradation of the crumb. Bacillus spp. are wheat grain endophytes and form heat resistant endospores, therefore, process hygiene and heating during baking do not prevent ropy spoilage. This study used 8 strains of Bacillus subtilis and Bacillus amyloliquefaciens to determine whether the presence and the copy number of spoVA^2mob operon influences survival after baking; in addition, the spoilage phenotype was correlated with the presence of amylolytic enzymes in genomes of Bacillus spp.. The presence and copy number of the spoVA^2mob operon had only a minor effect on survival of Bacillus endospores. Strains of B. amyloliquefaciens caused ropy spoilage faster than strains of B. subtilis, this difference correlated to the number and type of extracellular amylases encoded in the genomes of the strains of B. amyloliquefaciens and B. subtilis. The inhibitory effect of sourdough on ropy spoilage was determined by addition of 3-24% sourdough fermented with L. reuteri TMW1.656. Addition of 12% and 24% sourdough, corresponding to a bread pH of 5.93 ± 0.041 and 5.53 ± 0.040, respectively, delayed ropy spoilage for 2 and more than 5 d, respectively. The comparison of addition of 12% sourdough fermented with the reutericyclin producing L. reuteri TMW1.656 and the isogenic reutericyclin-negative strain L. reuteri TMW1.656ΔgtfAΔrtcN demonstrated that reutericyclin produced in sourdough inhibits growth of Bacillus in bread. In conclusion, sourdough inhibits germination of Bacillus spores in bread and the effect of sourdough is enhanced by reutericyclin.

Keywords: Bacillus spp.; Extracellular amylase; Reutericyclin; Ropy spoilage; Sourdough.