Bacillus species responsible for rope in bread

Abstract

To assess rope inducing potential in bread, eleven flour samples from different sources were subjected to the methods of Watkins (1906), Russ et al., (1961), the mesophilic spore count and the baking test. The methods were compared.

No correlation was found between the mesophilic spore count and rope development in bread or the methods of Watkins (1906) and Russ et al., (1961). The results of this study supported previously established trends of either low or no correlation between microbial counts and rope production in bread.

As part of an attempt to improve the correlation, colonies were randomly selected from the mesophilic spore count plates and their ability to induce rope on bread slices tested. In an effort to produce maximum symptom expression spore suspensions of all isolates were either not treated or heated before inoculation onto sterile slices of bread and observed daily for development of rope. The results show that spores of heated rope isolates caused rope symptoms to develop more rapidly than non treated spores.

The 34 rope and 34 non rope producing isolates were characterised by a series of biochemical tests. Of the isolates examined, 47.1% were identified as B.subtilis, 30.8% as B.licheniformis, 17.6% as B.cereus and 4.41% as B.megaterium and there was little difference in this ratio between rope and non rope producers.

All isolates were tested for rate of acetoin production and it was found that rope producers produced acetoin at a faster rate than non rope producers. It was possible to distinguish the rope producing isolates from those which did not produce rope as 97% of the rope isolates were positive for acetoin within 28 h while only 47% of the non rope producers were positive for acetoin after 28 hours. After 48 h 97% of the rope producers were positive for acetoin, 55% of these showed high levels of acetoin. Growth rates of all isolates was examined but the results failed to show a difference between isolates which produced rope and those which did not.