Designing a fermentation medium: carbon and nitrogen sources

Every fermentation medium answers two questions: what will the cells eat for energy and carbon, and where will they get their nitrogen? The answers drive growth rate, by-product formation and, ultimately, titre.

Carbon sources

Carbon usually makes up the largest share of a medium by mass and cost. The common choices behave quite differently:

Nitrogen sources

Nitrogen comes in two forms:

• Inorganic, mainly Ammonium sulphate and ammonium hydroxide (which doubles as the pH-control base). Defined, cheap and consistent, but it gives cells only nitrogen.
• Complex organic, such as Yeast extract powder and Soya peptone. These supply amino acids, peptides, vitamins and trace nutrients. They speed up growth and often lift product yields, at the cost of lot-to-lot variation.

Many processes combine both: a complex source in the batch medium to get the culture going, then a defined feed of carbon and ammonia during the fed-batch phase.

Balance the C:N ratio and the phosphate

A high carbon-to-nitrogen ratio favours storage products and secondary metabolites. A lower ratio favours biomass and protein. Phosphate salts such as Potassium phosphate, dibasic provide phosphorus and buffering capacity, but at high concentration they precipitate with magnesium and trace metals, so add those from separate stocks.

Defined vs complex media

Defined media make scale-up and regulatory filing easier because every component is known. Complex media are faster to develop and usually give higher early titres. A practical route is to start complex, find which components matter, then replace them one by one with defined equivalents.

Browse Carbon Sources, Organic Nitrogen Sources and the wider Fermentation Nutrients range.

Products in this guide

Dextrose (D-glucose), anhydrousCAS 50-99-7 GlycerolCAS 56-81-5 SucroseCAS 57-50-1 MaltoseCAS 6363-53-7 Lactose monohydrateCAS 10039-26-6 Yeast extract powderCAS 8013-01-2 Soya peptonePeptones & Hydrolysates Ammonium sulphateCAS 7783-20-2 Potassium phosphate, dibasicCAS 7758-11-4

Related collections

Carbon Sources 132 Organic Nitrogen Sources 19 Fermentation Nutrients 519 Microbial Fermentation 702