Page 37 - FoodFocusThailand No.244 August 2026

STRONG
STRONG QC & QA QC & QA

PROTEIN QUALITY ASSESSMENT

FROM A NUTRITIONAL PERSPECTIVE:

FROM AMINO ACID SCORE TO PDCAAS AND DIAAS

Protein is an essential macronutrient required for growth, tissue repair, enzyme and hormone synthesis,
immune function, and muscle maintenance. However, protein quantity alone does not determine nutritional
value, as proteins from different foods vary in amino acid composition and digestibility. For example, 20 g
of protein from milk, eggs, soybeans, rice, or wheat may provide different nutritional benefits; if a protein
has a low essential amino acid content or poor digestibility, its utilization for bodily protein synthesis will
be limited. Therefore, protein quality has been developed as a concept to evaluate how effectively dietary
proteins meet human essential amino acid requirements.

Protein Quality and the Amino Acid Score truncated at 1.00, making it impossible to distinguish among
Protein quality reflects the ability of a dietary protein to proteins of superior quality. In addition, PDCAAS is based on
supply essential amino acids in adequate proportions while fecal protein digestibility, which may not accurately reflect amino
considering digestibility and bioavailability. Essential amino acid absorption because of microbial activity in the large intestine.
acids cannot be synthesized by the human body, or cannot To overcome these limitations, the FAO recommended the
be synthesized in sufficient amounts. The essential amino Digestible Indispensable Amino Acid Score (DIAAS) in 2013.
acid present in the lowest proportion relative to human DIAAS is based on the ileal digestibility of each indispensable
requirements is called the limiting amino acid, which restricts amino acid, providing a more accurate estimate of the amino
protein utilization. For example, cereals are usually low acids available for metabolism. Unlike PDCAAS, DIAAS values
in lysine, whereas legumes contain low levels of sulfur- are not truncated at 100, allowing high-quality proteins to be
containing amino acids. Therefore, consuming a variety of differentiated. DIAAS is calculated by comparing the amount
foods or combining different protein sources can improve of each digestible indispensable amino acid with age-specific
protein quality. amino acid reference patterns, with the lowest ratio representing
The Amino Acid Score (AAS), also known as the Chemical the final score. According to FAO criteria, proteins with DIAAS
Score, compares the content of each essential amino acid in values ≥100 are classified as excellent-quality proteins, those
a test protein with the human reference amino acid pattern. with values of 75–99 as good-quality proteins, and those with
The lowest ratio identifies both the amino acid score and values <75 as low-quality proteins for protein quality claims.
the limiting amino acid. Although AAS is simple to calculate
from amino acid composition, it does not account for protein Toward more Accurate Protein Quantification
digestibility or amino acid absorption. Consequently, it may Before evaluating protein quality, the protein content of a food
overestimate protein quality, particularly in foods containing must be determined. Generally, protein is estimated from total
dietary fiber or antinutritional factors such as phytate, tannins, nitrogen using a nitrogen-to-protein conversion factor; therefore,
and trypsin inhibitors. the resulting value represents crude protein rather than true
protein. The Kjeldahl method is the reference method for protein
PDCAAS and DIAAS: Nutritional Indices for determination and measures total nitrogen through digestion,
Protein Quality Assessment distillation, and titration. Although it is accurate and internationally
PDCAAS (Protein Digestibility-Corrected Amino Acid Score), accepted, it is time-consuming, requires hazardous chemicals,
introduced by the FAO/WHO in 1991, is calculated by and cannot distinguish between protein nitrogen and non-
multiplying the AAS by the true digestibility of the protein. protein nitrogen. The Dumas (combustion) method uses high-
High-quality proteins such as egg, casein, whey, and soy temperature combustion of the sample, making it faster, requiring
protein isolate typically achieve PDCAAS values close to or fewer chemicals, and making it suitable for high-throughput
equal to 1.00. PDCAAS is simple and suitable for nutritional analysis. However, like the Kjeldahl method, it also measures
evaluation. However, it has important limitations. Scores are total nitrogen.

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