What is peptone?
Peptone is what you get when protein is broken down with enzymes or acid: a mixture of peptides of various lengths plus free amino acids. Microorganisms cannot take up intact protein, but they absorb these small fragments readily, which is why peptone is the principal nitrogen source in culture media and appears in almost every formulation.
The name is literal. Greek pepsis means digestion, so peptone is "digested protein".
How is peptone made?
The raw material is a protein: casein from milk, soybean, meat, yeast, wheat or pea are the common ones. The protein is suspended in water and broken down in one of two ways.
- Enzymatic hydrolysis: pancreatic enzymes, papain or similar cut the protein under mild conditions. The result has a wide spread of peptide lengths and keeps all the amino acids intact. Most peptones are made this way.
- Acid hydrolysis: hydrochloric acid at high temperature cuts the protein all the way down to free amino acids. Casamino Acids is the classic product, but tryptophan is destroyed in the process.
The hydrolysate is filtered, concentrated and spray-dried into a pale yellow powder. Complete solubility, light colour and the absence of growth inhibitors are checked on every batch before release.
Types of peptone
The same word covers products that behave very differently, depending on the raw material and the degree of hydrolysis. By raw material:
| Class | Typical names | Character |
|---|---|---|
| Casein (milk) | Tryptone, Casein Peptone, Casamino Acids | Complete amino acid profile, high tryptophan, main nitrogen source of LB and TSB |
| Soybean | Soy Peptone, Soytone, Papaic Digest of Soybean Meal | Plant-based, contains carbohydrate, second nitrogen source in TSB |
| Yeast | Yeast Peptone, Yeast Extract | Rich in B vitamins and nucleotides, the standard for lactic acid bacteria and probiotics |
| Meat | Meat Peptone, Proteose Peptone, Beef Extract | Traditional formulations such as Nutrient Broth |
| Other plants | Wheat Peptone, Pea Peptone, Potato Protein | Animal-free and non-GMO, used in vaccine and cell culture work |
By degree of hydrolysis, the number to look at is the amino nitrogen to total nitrogen ratio (AN/TN). A high ratio means more small molecules that cells use immediately, so growth starts fast. A low ratio means nitrogen is released more slowly, which helps avoid nitrogen limitation later in a fermentation. Every specification sheet lists it.
Tryptone versus peptone
Tryptone is one kind of peptone: casein hydrolysed specifically with pancreatic (tryptic) enzymes. It is high in tryptophan and is the main ingredient of LB medium for E. coli. So every tryptone is a peptone, but not every peptone is a tryptone.
How to choose a peptone
- E. coli and general bacteria: tryptone or casein peptone, which the standard formulations are built on.
- Lactic acid bacteria, bifidobacteria, probiotics: yeast peptone or yeast extract combined with soy peptone, for vitamins and growth factors.
- Bacillus, actinomycetes and other large-scale fermentations: soy or yeast sources, for low cost and steady nitrogen supply.
- Halal, vegetarian or animal-origin-free claims: soy, yeast, wheat or pea peptones; avoid meat and casein.
- Cell culture and vaccines: ultrafiltered plant peptones with a narrow molecular weight distribution and no animal components.
Peptones of the same class differ between suppliers. When switching, run a small trial first and compare growth curves and product yield.
How to store peptone
Sealed, dry, at room temperature and away from light. The powder is hygroscopic, so close the container tightly after opening. Caking usually means moisture uptake rather than degradation, but discard any powder that gives a turbid solution. Typical shelf life is three to five years; the specification sheet is the reference.
Frequently asked questions
Is peptone edible? Food-grade peptone is used in fermented products that are consumed directly, such as probiotic powders and starter cultures for fermented drinks. Look for a food-grade declaration and production under a food safety management system (ISO 22000).
Is peptone the same as protein hydrolysate? Protein hydrolysate is the broader term; peptone is the class used for microbial culture. The cell culture field tends to say hydrolysate, the microbiology field says peptone.
What does AN/TN mean on a specification sheet? Amino nitrogen as a share of total nitrogen, which indicates how far the protein was hydrolysed. See the glossary for details and the COA article for how it is measured.