How protein percentage is actually measured, and why two isolates at 90% are not equal

Protein content is rarely measured directly. It is inferred from nitrogen, and the arithmetic between the two is where two suppliers quoting 90% can be selling meaningfully different material.

The Kjeldahl convention

The standard method measures total nitrogen and multiplies by a conversion factor to estimate protein. The general factor is 6.25, which assumes protein is 16% nitrogen. That assumption holds loosely across many proteins and poorly for others — which is why specific factors exist, such as 5.7 for many plant proteins and 6.38 for dairy.

The consequence is direct: the same material assayed with 6.25 instead of 5.7 reports roughly 10% higher protein. Neither number is dishonest. Only one of them is comparable to the quote sitting next to it.

The moisture basis

"As is" and "dry basis" are different figures for the same powder. A protein at 90% on a dry basis carrying 6% moisture is about 84.6% as it ships. Specifications that state a protein figure without the basis are ambiguous by omission, and it is always worth resolving before an order rather than after.

Nitrogen that is not protein

Because the method measures nitrogen, any nitrogen-bearing adulterant inflates the result. This is the mechanism behind the melamine adulteration scandals, and it is the reason serious buyers of protein do not rely on Kjeldahl alone.

Practical defences:

• Amino acid profile by HPLC — the total of the individual amino acids should be consistent with the declared protein. A gap between the two is the flag.
• Non-protein nitrogen testing where the value or the supply chain justifies it.
• Combustion (Dumas) as a cross-check, faster than Kjeldahl and useful for routine verification.

What to put on the specification

• Protein percentage, the conversion factor and the moisture basis — all three, together.
• Amino acid profile, at least on a type-test basis per production campaign.
• Moisture, ash and fat, which account for the rest of the material.
• Solubility and pH in the conditions your process actually uses.
• Microbiological limits and, for any dairy-derived protein, an allergen statement.

Comparing quotes honestly

Send the same three requirements to everyone you are asking: factor, basis, and an amino acid profile. Suppliers who answer all three promptly are quoting the same thing as each other, and you can compare on price. Those who cannot are not really in the comparison yet.