Reference
Amino Acid Score (Chemical Score)
**Amino Acid Score (Chemical Score)** is a measure of protein quality that expresses the content of a protein's most limiting essential amino acid as a percentage of that amino acid in a reference requirement pattern. It is the uncorrected foundation on which the more familiar PDCAAS metric is built.
How it is calculated
The score compares the amino acid profile of a food protein against a standard reference pattern derived from human amino acid requirements. For each essential amino acid, the amount supplied is divided by the amount the reference pattern calls for. The lowest of these ratios — the one belonging to the limiting amino acid — becomes the chemical score, because that amino acid restricts how much of the protein the body can use for synthesis.
A score of 100% means the protein meets or exceeds the reference pattern for every essential amino acid. A lower score signals a shortfall in at least one. In new pea genotypes, for example, the sum of methionine plus cysteine is the limiting amino acid, producing a chemical score of 46% — the sulfur amino acids constrain the rating even though lysine is abundant.
Relationship to PDCAAS and DIAAS
The amino acid score on its own ignores how well a protein is actually digested. PDCAAS addresses this: the most limiting amino acid score is corrected for the protein’s true digestibility, then truncated at 1.00. DIAAS, proposed by the FAO in 2013 to replace PDCAAS, uses ileal digestibility and does not truncate, so values can exceed 100%. Both metrics inherit the limiting-amino-acid logic of the chemical score; they refine it rather than discard it.
The practical stakes are large. By one FAO estimate, a protein with a zero amino acid score yields a Net Protein Utilization of roughly 25%, meaning four times the minimal quantity would be needed to meet requirements. The chemical score is therefore a first, structural read on whether a protein can stand alone.
Relevance to potato protein
The amino acid score of potato protein is 65%. That figure reflects the uncorrected pattern; once digestibility is accounted for, potato protein performs considerably better, with reported PDCAAS values among the highest of any vegetable source. This gap between a moderate chemical score and a strong corrected score illustrates exactly why the digestibility-adjusted metrics were developed — the raw pattern alone understates a highly digestible protein’s usable quality.
Related terms