Red yeast is the clearest example in the botanical trade of a material that has to be bought on two numbers at once: the compound you want and the compound you do not. Monacolin K is the reason the material is bought; citrinin is the reason it is scrutinised. A supplier who quotes one and is silent on the other has not finished the specification. This note sets out the chemistry behind both and the documents that make a lot of red yeast extract auditable.
The wanted compound: monacolin K, in two forms
Monacolin K is lovastatin, CAS 75330-75-5, C24H36O5, 404.5 g/mol (PubChem CID 53232). It occurs in the material in two interconvertible forms: the closed lactone and the open hydroxy acid. The difference is one water: hydrolysis of the lactone ring adds 18.02 g/mol, giving C24H38O6 at 422.5 g/mol. The two forms have different chromatographic behaviour and different polarity, and they interconvert with pH and with storage time.
That single fact explains most disputes between two laboratories testing the same lot. If one method converts everything to the acid form and the other quantifies the lactone as received, the two results differ by the ratio of the two forms — not by any error. The RFQ must state which form is being reported.
| Compound | CAS | Formula | MW (g/mol) | Role |
|---|---|---|---|---|
| Monacolin K (lactone) | 75330-75-5 | C24H36O5 | 404.5 | Target marker |
| Monacolin K (hydroxy acid) | — | C24H38O6 | 422.5 | Open-ring form, +18.02 |
| Citrinin | 518-75-2 | C13H14O5 | 250.25 | Unwanted co-metabolite |
| Ergosterol | 57-87-4 | C28H44O | 396.65 | Fungal biomass marker |
| GABA | 56-12-2 | C4H9NO2 | 103.12 | Fermentation co-product |
The unwanted compound: citrinin
Citrinin (CAS 518-75-2, C13H14O5, 250.25 g/mol, CID 54680783) is a polyketide mycotoxin produced by the same Monascus strains under some fermentation conditions. It is a small, highly fluorescent molecule, which is why the standard determination is HPLC with fluorescence detection — more sensitive than UV by a wide margin and appropriate for a compound whose limit is set far below any UV quantitation range. The relevant point for a buyer is procedural: the strain and the fermentation conditions determine whether citrinin appears at all, so the control is upstream, not at the finishing stage.
What the fermentation actually produces
Monascus species produce a family of monacolins, not a single compound, plus the pigment series that gives the material its colour and ergosterol at 396.65 g/mol as a fungal membrane sterol. Ergosterol is useful to a buyer for an unexpected reason: it is a marker of fungal biomass, so it moves with how much mycelium is in the material rather than with how much monacolin it contains. A lot with high ergosterol and modest monacolin is a lot of fungus with modest potency.
Gamma-aminobutyric acid at 103.12 g/mol is another co-product of the same fermentation and is sometimes reported; it is not a potency marker and should not be used as one.
Documents to request with the first sample
| Document | What it must state |
|---|---|
| Certificate of analysis | Monacolin K value with lactone or acid form named, and citrinin result |
| Chromatogram | Resolved monacolin K peak, and the citrinin trace at the reporting limit |
| Strain statement | Monascus species and strain identifier |
| Fermentation record | Substrate, duration, and whether solid-state or submerged |
| Stability data | Lactone-to-acid drift over the claimed shelf life |
Stability: the drift that surprises buyers
Because the lactone and the acid interconvert, a monacolin K figure is a snapshot in time, not a constant. Heat and moisture during storage push the equilibrium toward the open acid form, and a specification written only for the lactone will show an apparent decline that is really a conversion. The fix is cheap: specify total monacolins expressed as monacolin K, or specify both forms separately and sum them. Either approach removes an argument that otherwise recurs on every stability pull.
Frequently asked questions
Why did two laboratories report different monacolin K on one lot?
Almost always the lactone-versus-acid question. The two forms differ by 18.02 g/mol and by retention behaviour.
Is citrinin removed by processing?
It is controlled by strain selection and fermentation conditions, not by downstream finishing. Ask for the strain and the process record.
Is a high ergosterol figure a positive?
Not by itself. It tracks fungal biomass at 396.65 g/mol, not monacolin content.
Identity data in this article is taken from PubChem (US National Library of Medicine) records for CIDs 53232, 54680783, 444679, 119 and 5460093. Batch-specific commercial values — assay, moisture, particle size, packaging, shelf life and minimum order quantity — are issued per lot on the certificate of analysis and are not published here.