The three numbers
- Total nitrogen (TN) is measured by the Kjeldahl method; multiplied by 6.25 it approximates crude protein. Peptones typically run 10 to 14%, yeast extracts around 10 to 12%.
- Amino nitrogen (AN) is the nitrogen in free amino groups, representing the fraction already hydrolysed to amino acids and short peptides.
- AN/TN, the ratio of the two, indicates the degree of hydrolysis. Enzymatic peptones are often 20 to 40%; acid-hydrolysed casamino acids can exceed 60%.
How to use them
- For fast growth (seed cultures, high-density fermentation) choose a higher AN/TN: more small molecules that cells use directly.
- For secondary metabolites (antibiotics, enzymes) choose a lower AN/TN: nitrogen is released more slowly and early overgrowth is avoided.
- When changing supplier, compare TN and AN/TN first, then ash and sodium chloride. Close numbers mean a low-risk switch.
Other common items
- Ash: inorganic salt content, affecting osmolarity and buffering.
- Sodium chloride: from neutralisation steps; high values affect salt-sensitive organisms.
- pH (2% solution) and solubility: whether the solution is clear after preparation.
- Moisture: hygroscopicity, affecting weighing accuracy and storage.
- Microbial limits: total aerobic count, E. coli, Salmonella and others, confirming the raw material itself is not contaminated.
Values vary slightly from batch to batch; the specification sheet gives ranges, and variation within range normally does not affect a process.