When a red substance builds up in a DO-regulated E. coli fermentation process, it probably means that a pH indicator—like phenol red—is present, which changes color in response to pH changes. Phenol red turns red in an acidic environment, which can be created during fermentation. To show pH changes and the fermentation of particular sugars like lactose, other indicators, such as neutral red, can also be used in fermentation media.

Please provide the composition and volume of your medium , initial and final pH and OD reading at the end, as well as type of reaction vessel together with rpm if an impeller or agitator is being used.

Dear Lukas,

depending on the medium and nutrient availability, your E. coli strain might produce red pigments or heme under the chosen conditions to cope with the lower temperature and the oxygen availablity:

https://pubmed.ncbi.nlm.nih.gov/4865682/

https://pubmed.ncbi.nlm.nih.gov/4567136/

https://pubmed.ncbi.nlm.nih.gov/34826004/

Best

Michael