Aerobic spore-forming bacteria associated with ropy bread: Identification, characterization and spoilage potential assessment.

Aerobic spore-forming (ASF) bacteria have been reported to cause ropiness in bread. Sticky and stringy degradation, discoloration, and an odor reminiscent of rotting fruit are typical characteristics of ropy bread spoilage. In addition to economic losses, ropy bread spoilage may lead to health risks, as virulent strains of ASF bacteria are not uncommon. However, the lack of systematic approaches to quantify physicochemical spoilage characteristics makes it extremely difficult to assess rope formation in bread. To address this problem, the aim of this study was to identify, characterize and objectively assess the spoilage potential of ASF bacteria associated with ropy bread. Hence, a set of 82 ASF bacteria, including isolates from raw materials and bakery environments as well as strains from international culture collections, were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and their species identity confirmed by 16S rRNA and gyrA or panC gene sequencing. A standardized approach supported by objective colorimetric measurements was developed to assess the rope-inducing potential (RIP) of a strain by inoculating autoclaved bread slices with bacterial spores. In addition, the presence of potential virulence factors such as swarming motility or hemolysis was investigated. This study adds B. velezensis, B. inaquosorum and B. spizizenii to the species potentially implicated of causing ropy bread spoilage. Most importantly, this study introduces a standardized classification protocol for assessing the RIP of a bacterial strain. Colorimetric measurements are used to objectively quantify the degree of breadcrumb discoloration. Furthermore, our results indicate that strains capable of inducing rope spoilage in bread often exhibit swarming motility and virulence factors such as hemolysis, raising important food quality considerations.

Ropiness in Bread-A Re-Emerging Spoilage Phenomenon.

As bread is a very important staple food, its spoilage threatens global food security. Ropy bread spoilage manifests in sticky and stringy degradation of the crumb, slime formation, discoloration, and an odor reminiscent of rotting fruit. Increasing consumer demand for preservative-free products and global warming may increase the occurrence of ropy spoilage. Bacillus amyloliquefaciens, B. subtilis, B. licheniformis, the B. cereus group, B. pumilus, B. sonorensis, Cytobacillus firmus, Niallia circulans, Paenibacillus polymyxa, and Priestia megaterium were reported to cause ropiness in bread. Process hygiene does not prevent ropy spoilage, as contamination of flour with these Bacillus species is unavoidable due to their occurrence as a part of the endophytic commensal microbiota of wheat and the formation of heat-stable endospores that are not inactivated during processing, baking, or storage. To date, the underlying mechanisms behind ropy bread spoilage remain unclear, high-throughput screening tools to identify rope-forming bacteria are missing, and only a limited number of strategies to reduce rope spoilage were described. This review provides a current overview on (i) routes of entry of Bacillus endospores into bread, (ii) bacterial species implicated in rope spoilage, (iii) factors influencing rope development, and (iv) methods used to assess bacterial rope-forming potential. Finally, we pinpoint key gaps in knowledge and related challenges, as well as future research questions.