Methionine is one of the standard amino acids that make up proteins, and it also occurs on its own, unattached, in foods, feeds, fermentation samples, and cell culture media. A lab reports methionine as part of a wider amino acid profile, not in isolation. Methionine is one of the sulfur-containing amino acids in the standard set, a chemistry point that distinguishes it from most other panel analytes. This page covers what it is, where a free or total result applies, and which panel reports it. Handling claims that would need a verified source are left out rather than stated without one.
Chemistry in plain terms
Standard references classify methionine as an essential amino acid for humans, meaning it must come from food rather than being made by the body in sufficient amounts. Its side chain contains sulfur, the feature behind its grouping with the sulfur-containing amino acids. PubChem lists L-methionine as a standard proteinogenic amino acid, built into protein chains as well as present free in solution. Both forms, bound and free, are real, and they answer different questions.
Standard biochemistry texts also note methionine's role at the start of protein chains, where newly made proteins begin with methionine. That role concerns protein synthesis in living cells. It is stated here as identity context only, not as a claim about any sample.
Where it occurs
Methionine occurs as a building block in proteins across foods, feeds, and biological materials. Free methionine also occurs in cell culture and spent media, fermentation liquids, protein hydrolysates, animal feeds and pet foods, and food and supplement products, where a free profile describes the sample as it is.
This page gives no typical level for methionine in any material. Levels vary by recipe, ingredients, organism, process, and sampling point; no verified figure is available. Measure your sample rather than borrowing a number.
Free vs total for methionine
A free methionine result measures methionine already dissolved and unattached in the sample as received, with no hydrolysis step. That is usually the relevant number for a question about the sample as used or as eaten: what is in the medium now, or what free profile a product presents.
A total result is measured after acid hydrolysis, the step that breaks protein chains apart and releases bound amino acids. It therefore includes free methionine plus methionine bound inside protein. This page gives no free-to-total ratio for any material, because that ratio is a property of the specific sample.
Methionine has no special hydrolysis conversion of the kind that affects asparagine and glutamine, whose amide groups are lost during acid hydrolysis and which are therefore reported together with aspartic acid and glutamic acid respectively. The general total-panel caution still applies: a hydrolysed measurement describes the sample after the method has acted on it, not a photograph of its native state.
Method notes: how methionine is measured here
At OpenChemWorks, methionine is measured as part of a panel using AQC pre-column derivatization, the Waters AccQ-Tag approach, with HPLC-UV detection. The amino acids are chemically tagged, separated on the HPLC column, and quantified against calibrated standards. AABA, short for alpha-aminobutyric acid, is added at a known amount as the internal standard. AABA is the standard, added by the lab; methionine is the analyte, measured from your sample. Different compounds, different roles.
This page makes no oxidation or stability handling claim for methionine: no verified source for such notes is available, so they stay out. No resolution, recovery, or detection-limit figure is quoted, because no verified figure from this laboratory's validated method is available.
Which panel includes methionine
Methionine is included in AA-1, the free amino acid panel: 19 amino acids plus ammonia, measured by AQC HPLC-UV. Cysteine is excluded from AA-1 and needs a separate AA-3 workup. Methionine is also part of a total result under AA-T, where the sample is hydrolysed first and methionine is reported on that total basis.
Prices are only these: AA-1 costs $150 per sample for liquids, $180 per sample for powders ($150 plus $30 powder preparation), and $150 flat for first-time customers, with $125 per sample at 25 or more samples per month. AA-T, total amino acid analysis, costs $230 per sample. The pilot is $250 for 5 samples, credited in full against a first contract signed within 90 days.
OpenChemWorks, in Davis, California, is validating now and reserving pilot slots. The lab is not GLP, GMP, or ISO/IEC 17025 accredited, and no turnaround is promised here. Email info@openchemworks.com with your sample description and whether your question is about the sample as it is, or its protein as a whole.
Frequently asked questions
Is methionine a sulfur-containing amino acid?
Yes, in standard classifications methionine is grouped with the sulfur-containing amino acids because its side chain contains sulfur. That is a chemistry classification, not a level in any sample.
Is AABA the same as methionine?
No. AABA, alpha-aminobutyric acid, is the internal standard added by the lab for quantification. Methionine is a different compound, and it is the analyte measured from your sample.
Do I need free or total methionine?
Free answers a question about the sample as received. Total answers a question about protein composition after hydrolysis, and includes the free fraction.
Is methionine reported on its own?
At OpenChemWorks it is reported as part of the AA-1 free panel or the AA-T total panel, alongside the other panel analytes.
References
- PubChem — L-Methionine (CID 6137) — Identity reference for the amino acid named on this page.
- Waters — Derivatization of Amino Acids Using Waters AccQ-Tag Chemistry — Method background for AQC derivatization and HPLC-UV amino acid analysis.
- British Journal of Nutrition (2023) — Asparagine and glutamine lose an amide residue during acid hydrolysis and are measured with aspartate and glutamate respectively.
- OpenChemWorks — Pricing — Published service and pricing copy (2026).
OpenChemWorks Laboratory · Reviewed by the laboratory director, PhD chemist · Published October 29, 2026