What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Protein Quality Assessment: Methods and Applications

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Protein Quality Assessment

Protein quality is a measure of how well a dietary protein can satisfy the metabolic demands for nitrogen and essential amino acids in humans or animals. It is not a single property but a composite of two fundamental parameters: the amino acid composition of the protein and the efficiency with which those amino acids are absorbed and made available for protein synthesis. A protein that contains all nine indispensable amino acids in the correct proportions but resists digestion will be of poor quality, just as a highly digestible protein lacking one essential amino acid will fail to support growth.

What is Protein Quality?

Proteins are polymers of amino acids linked by peptide bonds. Of the 20 standard amino acids used in protein synthesis, nine are indispensable (essential) in humans: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The human body cannot synthesize these amino acids at rates sufficient to meet physiological demands, so they must be supplied from the diet. Additionally, cysteine and tyrosine are considered conditionally indispensable because they can be synthesized from methionine and phenylalanine, respectively, but only when those precursors are available in adequate amounts.

Protein quality assessment asks a practical question: given a specific protein source, how much of its amino acid content will actually be absorbed and used to meet human requirements? This matters because the body does not store amino acids in the way it stores fat or glycogen. Amino acids not immediately used for protein synthesis are deaminated, and the nitrogen is excreted as urea. Therefore, a protein that provides amino acids in excess of requirements, or in imbalanced proportions, will have a lower net value than one that closely matches the body's needs.

Why Assess Protein Quality?

Protein quality assessment has direct applications in food labeling, clinical nutrition, agricultural feed formulation, and regulatory policy. Regulatory bodies such as the Food and Agriculture Organization (FAO) and the U.S. Food and Drug Administration (FDA) use protein quality scores to determine recommended dietary allowances and to regulate nutrition claims on packaged foods. For example, a food product labeled as a "good source of protein" must meet a minimum protein quality threshold, not merely contain a certain gram amount of protein.

In clinical settings, protein quality determines the design of enteral formulas for patients with malabsorption, the management of renal disease where protein restriction is necessary, and the formulation of infant formulas, which must closely mimic the amino acid profile of human milk. In agriculture, protein quality of feed ingredients such as soybean meal, fish meal, and corn gluten determines growth rates in livestock and poultry, directly affecting production economics. Without a reliable method to quantify protein quality, none of these applications would be possible.

Amino Acid Composition and Scoring Patterns

The first step in assessing protein quality is determining the amino acid composition of the protein source. This is typically done by acid hydrolysis of the protein followed by chromatographic analysis, most commonly using high-performance liquid chromatography (HPLC) or ion-exchange chromatography with post-column derivatization. The protein is hydrolyzed in 6 N hydrochloric acid at 110°C for 24 hours, which cleaves all peptide bonds. However, this method destroys tryptophan and partially degrades methionine and cysteine. Therefore, separate alkaline hydrolysis is required for tryptophan, and performic acid oxidation is used before acid hydrolysis to convert methionine and cysteine to stable derivatives (methionine sulfone and cysteic acid) that survive the process.

Essential Amino Acid Profiles

The result of amino acid analysis is a profile expressed in milligrams of each amino acid per gram of protein (mg/g protein). This profile is then compared against a reference pattern, which represents the amino acid requirements of the target population. The essential amino acid profile of a protein is the primary determinant of its intrinsic quality. For example, wheat gluten is rich in glutamine and proline but contains very little lysine, making it a poor source of this indispensable amino acid. Conversely, milk proteins (casein and whey) contain all nine indispensable amino acids in proportions close to human requirements, which is why dairy proteins are considered high quality.

The limiting amino acid is the indispensable amino acid present in the lowest amount relative to the reference pattern. This concept is rooted in the "Liebig's barrel" principle: protein synthesis proceeds only as fast as the least available amino acid permits. If lysine is the limiting amino acid in wheat, then all other amino acids in wheat are in relative excess and will be catabolized rather than used for protein synthesis.

Reference Protein Patterns

Reference protein patterns are established by expert committees based on amino acid requirement studies. The FAO/WHO established the first reference pattern in 1957, and it has been revised multiple times since. The current reference pattern for older children, adolescents, and adults was established in 2007 and is based on stable isotope studies of amino acid oxidation. This pattern specifies requirements in mg per kg body weight per day, which are then converted to a pattern per gram of protein using the average protein requirement of 0.66 g protein per kg per day.

For infants, a different reference pattern is used, based on the amino acid composition of human milk. The infant pattern has higher requirements for most indispensable amino acids, particularly tryptophan and cysteine, reflecting the demands of rapid growth. The choice of reference pattern is critical because it determines which amino acid is limiting and therefore the final quality score.

Protein Digestibility: Key Factor in Quality

Amino acid composition alone is insufficient to predict protein quality. The protein must be digested into absorbable amino acids and di- or tri-peptides. Digestibility is defined as the proportion of ingested nitrogen that is absorbed from the gastrointestinal tract. It is a measure of the bioavailability of the amino acids contained in the protein.

True vs. Fecal Digestibility

Digestibility can be measured at different points along the gastrointestinal tract, giving rise to distinct measures. Fecal digestibility is the classical measure, calculated as:

Fecal digestibility = (Nitrogen intake − Fecal nitrogen) / Nitrogen intake

This measure is simple but has a significant flaw: it does not account for nitrogen that is absorbed in the small intestine but then secreted back into the large intestine as part of microbial protein or endogenous secretions. The colon contains a large population of bacteria that metabolize undigested protein and also incorporate nitrogen into their own biomass. This means that fecal nitrogen includes both unabsorbed dietary nitrogen and nitrogen of microbial origin, leading to an overestimation of true absorption.

True digestibility corrects for endogenous nitrogen losses—the nitrogen from digestive enzymes, sloughed intestinal cells, and mucus that is not absorbed. It is calculated as:

True digestibility = (Nitrogen intake − (Fecal nitrogen − Endogenous fecal nitrogen)) / Nitrogen intake

Endogenous fecal nitrogen is measured by feeding a protein-free diet and measuring fecal nitrogen, which then represents only endogenous losses. True digestibility is always higher than apparent digestibility because the correction subtracts endogenous losses from the fecal nitrogen.

Ileal digestibility is a more accurate measure because it samples digesta at the terminal ileum, before the large intestine and its microbial fermentation. This requires ileal cannulation in animal models or the use of ileal digesta from slaughtered animals. Ileal digestibility avoids the confounding effects of colonic bacteria and is considered the gold standard for protein quality assessment in monogastric animals.

Factors Affecting Digestibility

Several factors influence protein digestibility. Antinutritional factors such as trypsin inhibitors in raw soybeans, lectins in legumes, and tannins in certain cereals can reduce digestibility by binding to digestive enzymes or to the protein itself. Heat processing generally improves digestibility by denaturing proteins and inactivating these inhibitors, but excessive heat can cause Maillard reactions that bind lysine and other amino acids in indigestible complexes.

Protein structure also matters. Proteins with extensive disulfide bonds, such as those in hair, feathers, and nails (keratins), are highly resistant to proteolysis. Plant proteins are often less digestible than animal proteins because plant cell walls (cellulose, hemicellulose, lignin) physically entrap proteins and limit access by digestive enzymes. The digestibility of a typical mixed diet is around 90-95%, but individual proteins range from less than 50% (some legume proteins) to nearly 100% (egg white protein).

PDCAAS: Protein Digestibility-Corrected Amino Acid Score

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was adopted by the FAO and WHO in 1989 as the preferred method for assessing protein quality in human nutrition. It replaced the older Protein Efficiency Ratio (PER) method, which had been the standard in the United States since 1919. PDCAAS is a chemical score that combines the amino acid profile of the protein with its true fecal digestibility.

Calculation of PDCAAS

The PDCAAS calculation proceeds in four steps:

Determine the amino acid profile of the protein by acid hydrolysis and chromatographic analysis, expressing each indispensable amino acid in mg per gram of protein.
Compare to the reference pattern: For each indispensable amino acid, calculate the ratio of the amino acid in the test protein to the amino acid in the reference pattern. The lowest ratio is the amino acid score (also called the chemical score).
Determine the true fecal digestibility of the protein, typically using rat assays. The rat is the standard model because its digestive physiology is similar to that of humans for most protein sources.
Multiply the amino acid score by the true fecal digestibility to obtain the PDCAAS.

The formula is:

PDCAAS = Amino acid score × True fecal digestibility

For example, if a protein has an amino acid score of 0.80 (meaning its limiting amino acid is present at 80% of the reference pattern) and a true fecal digestibility of 0.90, the PDCAAS would be 0.72. PDCAAS values are capped at 1.00; any value above 1.00 is truncated to 1.00 because consuming more than the requirement does not confer additional benefit.

Limitations of PDCAAS

PDCAAS has several well-documented limitations. First, it uses fecal digestibility rather than ileal digestibility. As discussed, fecal digestibility overestimates amino acid absorption because it fails to account for colonic fermentation and microbial nitrogen. This is particularly problematic for proteins that are poorly digested in the small intestine but fermented in the colon, such as some plant proteins.

Second, PDCAAS truncates scores at 1.00. This means that high-quality proteins like whey, casein, egg, and soy all receive a score of 1.00, even though they differ in their actual metabolic value. This truncation makes it impossible to distinguish between proteins that exceed the reference pattern and those that merely meet it.

Third, PDCAAS is based on the total protein content, not on individual amino acid digestibility. The digestibility of different amino acids within a single protein can vary considerably. Lysine, for example, is particularly susceptible to Maillard reactions during heat processing, which reduces its digestibility more than that of other amino acids. PDCAAS assumes uniform digestibility across all amino acids, which is not correct.

Fourth, PDCAAS was developed using the rat as the animal model. Rat fecal digestibility values are generally lower than human values for the same proteins, which can underestimate protein quality for human nutrition. The FAO now recommends using human or pig ileal digestibility data when available.

DIAAS: Digestible Indispensable Amino Acid Score

In 2013, the FAO convened an expert consultation that recommended replacing PDCAAS with the Digestible Indispensable Amino Acid Score (DIAAS). DIAAS addresses several of the limitations of PDCAAS and represents the current state of the art in protein quality assessment.

DIAAS vs. PDCAAS

The fundamental difference between DIAAS and PDCAAS is the use of ileal digestibility values for individual amino acids rather than a single fecal digestibility value for total protein. DIAAS is calculated as:

DIAAS = (mg of digestible indispensable amino acid in 1 g of protein / mg of the same indispensable amino acid in 1 g of reference protein) × 100

The "digestible" amount is determined by multiplying the content of each indispensable amino acid by its individual ileal digestibility coefficient. This means that the digestibility of lysine, for example, is measured separately from that of tryptophan, reflecting the fact that these amino acids can have very different bioavailabilities in the same protein.

DIAAS values are not truncated at 100. A protein with a DIAAS above 100 is considered an excellent protein source, and the actual value is reported. This allows differentiation among high-quality proteins. For example, whey protein has a DIAAS of approximately 109, while soy protein has a DIAAS of approximately 90-95. Under PDCAAS, both would have been capped at 1.00.

Determining DIAAS Values

DIAAS values are determined using ileal digestibility data from growing pigs or humans. The pig is the preferred model because its digestive physiology closely resembles that of humans, and ileal cannulation allows repeated sampling from the same animal. The procedure involves feeding a test diet containing the protein of interest, collecting ileal digesta, and measuring the flow of each amino acid at the terminal ileum.

The ileal digestibility of each amino acid is calculated as:

Ileal digestibility = (Amino acid intake − Ileal amino acid flow) / Amino acid intake

The ileal amino acid flow must be corrected for endogenous losses, which are measured by feeding a protein-free diet or using enzyme-hydrolyzed casein and ultrafiltration methods. The resulting true ileal digestibility values are then used to calculate DIAAS.

DIAAS values are categorized into three quality levels: excellent (DIAAS ≥ 100), good (DIAAS between 75 and 99), and no claim (DIAAS < 75). These categories are intended to guide food labeling and nutrition claims. The FAO recommends that DIAAS be adopted as the standard method for protein quality assessment, but implementation has been gradual because it requires more sophisticated data than PDCAAS.

In Vitro Protein Quality Assessment Methods

Animal-based methods for protein quality assessment are expensive, time-consuming, and raise ethical concerns. In vitro methods use enzymes and laboratory techniques to simulate digestion and estimate protein quality without the need for live animals. These methods are particularly useful for screening large numbers of samples or for routine quality control in the food industry.

Enzymatic Assays

The most common in vitro method is the multi-enzyme assay, which simulates the digestive process using a mixture of proteolytic enzymes. A typical protocol uses trypsin, chymotrypsin, and peptidase, or a combination of pepsin and pancreatin, to digest the test protein under controlled conditions of pH, temperature, and time.

One widely used approach is the pH-stat method. The protein is suspended in water, and the pH is adjusted to 8.0. A solution of trypsin, chymotrypsin, and peptidase is added, and the pH is maintained at 8.0 by continuous titration with sodium hydroxide. The amount of NaOH added over a fixed time (usually 10 minutes) is proportional to the number of peptide bonds cleaved, which reflects the digestibility of the protein. The slope of the pH-stat curve is compared to that of a reference protein (usually casein) to calculate a relative digestibility value.

Another approach is the two-step enzymatic method using pepsin at low pH followed by pancreatin at neutral pH. The protein is first incubated with pepsin in 0.1 N HCl at 37°C for 2 hours, simulating gastric digestion. The pH is then adjusted to 7.0, and pancreatin is added for an additional 2 hours, simulating intestinal digestion. The undigested protein is precipitated with trichloroacetic acid, and the soluble nitrogen in the supernatant is measured and expressed as a percentage of total nitrogen. This method correlates well with in vivo digestibility for many proteins but tends to underestimate digestibility for proteins that are poorly soluble.

Cell-Based Assays

Cell-based assays use cultured intestinal epithelial cells (such as Caco-2 cells) to measure the absorption of digested protein. Caco-2 cells, derived from a human colorectal adenocarcinoma, spontaneously differentiate into enterocyte-like cells with brush border membranes and tight junctions when cultured on permeable supports. They express many of the peptidases and transporters found in the small intestine, making them a useful model for studying amino acid absorption.

In a typical cell-based assay, the protein is first digested enzymatically as described above. The resulting digest is then applied to the apical surface of the Caco-2 monolayer, and the appearance of amino acids and peptides in the basolateral compartment is measured over time using HPLC or mass spectrometry. This approach provides information not only on digestibility but also on the bioavailability of specific amino acids.

Cell-based assays are more physiologically relevant than simple enzymatic assays because they account for brush border peptidase activity and active transport. However, they are more complex, require specialized equipment, and are not yet standardized for routine protein quality assessment. They are primarily used in research settings to investigate the mechanisms of amino acid absorption and to compare the bioavailability of different protein sources. For high-throughput quantification of amino acids in such experiments, methods such as Automated Protein Quantification or Protein Quantification Mass Spectrometry are often employed.

Biological Methods: Protein Efficiency Ratio and Nitrogen Balance

Biological methods measure the actual metabolic response of an animal to a test protein. These methods were the first to be developed and remain important for validating other approaches, even though they are no longer the primary methods for regulatory purposes.

Protein Efficiency Ratio (PER)

The Protein Efficiency Ratio (PER) is the oldest standardized method for protein quality assessment. It is defined as the weight gain of a growing animal divided by the protein consumed:

PER = Weight gain (g) / Protein intake (g)

The standard PER assay uses weanling rats (21-23 days old) fed a diet containing 10% protein for 28 days. The rats are housed individually, and food intake is measured daily. The PER is calculated at the end of the study and is corrected against a casein standard, which is assigned a PER of 2.5.

The PER method has several serious limitations. It does not account for the maintenance protein requirement; it measures only growth, not the full range of protein functions. It is also influenced by the protein content of the diet—a protein that is marginal in quality may show a higher PER at lower dietary levels because more of the protein is used for growth rather than maintenance. The PER method also assumes that all weight gain is protein, which is not true, and it does not correct for differences in body composition.

Despite these limitations, PER was the official method for protein quality assessment in the United States until 1993, when the FDA adopted PDCAAS. It is still used in Canada and in some regulatory contexts, particularly for infant formula labeling.

Nitrogen Balance Studies

Nitrogen balance is the difference between nitrogen intake and nitrogen excretion (urinary, fecal, and miscellaneous losses such as sweat and hair). A positive nitrogen balance indicates net protein accretion, while a negative balance indicates net protein loss. Nitrogen balance studies measure the minimum amount of a given protein needed to maintain nitrogen equilibrium, which is the state where intake equals excretion.

The nitrogen balance method is the gold standard for determining protein requirements in humans. In a typical study, subjects are fed a diet containing a fixed amount of the test protein for a period of 7-14 days to allow adaptation. Nitrogen intake is calculated from the protein content of the diet, and nitrogen excretion is measured by collecting urine and feces over the final 3-5 days of each period. The protein intake is then varied across periods to establish the relationship between intake and balance.

The slope of the nitrogen balance response curve reflects the quality of the protein. A high-quality protein will show a steep slope, meaning that small increases in intake produce large increases in nitrogen balance. A low-quality protein will show a shallow slope because much of the additional nitrogen is excreted rather than retained.

Nitrogen balance studies are expensive, require strict metabolic control, and are not practical for routine protein quality assessment. However, they provide the most direct measure of protein utilization in humans and are used to validate other methods. They are also used to establish amino acid requirement values, which form the basis of the reference patterns used in PDCAAS and DIAAS calculations.

Common Pitfalls and Practical Considerations

Students and practitioners alike make predictable errors when working with protein quality assessment. Understanding these pitfalls is essential for correctly interpreting protein quality data.

Misinterpreting Scores

The most common error is confusing PDCAAS and DIAAS. PDCAAS uses a single fecal digestibility value for the entire protein and truncates scores at 1.00. DIAAS uses individual ileal digestibility values for each amino acid and does not truncate scores. A protein with a PDCAAS of 1.00 and a DIAAS of 85 is not contradictory; it simply reflects the different methodologies. The PDCAAS is capped, while the DIAAS provides a more granular view.

Another frequent error is assuming that a higher score always means a better protein for all purposes. A protein with a high score may still be limiting in a specific amino acid for a particular population. For example, the reference pattern for infants is different from that for adults, so a protein that scores well for adults may score poorly for infants.

Students also often confuse "protein quality" with "protein content." A food can be high in protein but low in quality, or low in protein but high in quality. Gelatin, for example, is nearly pure protein but is of very low quality because it lacks tryptophan and is low in several other indispensable amino acids.

Choosing the Right Method

The choice of protein quality assessment method depends on the purpose of the assessment. For regulatory labeling in most countries, PDCAAS is still the accepted method, although the FAO recommends DIAAS. For research purposes, DIAAS is preferred because it provides more detailed information. For routine quality control in food manufacturing, in vitro enzymatic assays are the most practical because they are rapid, inexpensive, and do not require animal facilities.

When using in vitro methods, it is important to remember that they provide an estimate, not a direct measurement, of digestibility. The correlation between in vitro and in vivo results varies with the protein source, and in vitro methods tend to be less accurate for proteins that are poorly soluble or that contain antinutritional factors. It is always advisable to validate in vitro results against in vivo data when possible.

Another practical consideration is the choice of reference pattern. The 2007 FAO reference pattern for older children, adolescents, and adults is appropriate for most applications, but the infant reference pattern should be used when assessing proteins for infant formulas. Using the wrong reference pattern can lead to incorrect conclusions about which amino acid is limiting.

Finally, be aware that protein quality values are not fixed properties of a food. They vary with the growing conditions of the plant or animal, the processing methods used, and the presence of other dietary components. A soybean protein isolate may have a different PDCAAS than a soybean protein concentrate, and both may differ from the value for whole soybeans. Always report the specific product and processing conditions when citing protein quality values.

Frequently Asked Questions

What is the difference between PDCAAS and DIAAS?

PDCAAS (Protein Digestibility-Corrected Amino Acid Score) uses a single fecal digestibility value for the entire protein and truncates scores at 1.00. DIAAS (Digestible Indispensable Amino Acid Score) uses individual ileal digestibility values for each indispensable amino acid and does not truncate scores. DIAAS is considered more accurate because it accounts for differences in digestibility among amino acids and avoids the confounding effects of colonic fermentation.

How is protein quality measured?

Protein quality is measured by combining two parameters: the amino acid composition of the protein relative to a reference pattern, and the digestibility of the protein. The most common methods are PDCAAS and DIAAS, which both involve amino acid analysis, comparison to a reference pattern, and digestibility measurement. In vitro methods using enzymes or cultured cells can also estimate protein quality without animal studies.

What does PDCAAS stand for?

PDCAAS stands for Protein Digestibility-Corrected Amino Acid Score. It is a method for evaluating protein quality that was adopted by the FAO and WHO in 1989. It is calculated by multiplying the amino acid score (the lowest ratio of an indispensable amino acid in the test protein to the reference pattern) by the true fecal digestibility of the protein.

Why is protein quality important?

Protein quality is important because it determines how effectively a dietary protein can meet the body's needs for indispensable amino acids and nitrogen. Low-quality proteins may be deficient in one or more essential amino acids, limiting protein synthesis even when total protein intake is adequate. Protein quality assessment is used in food labeling, clinical nutrition, and agricultural feed formulation.

What is the best method for protein quality assessment?

DIAAS is currently considered the best method because it uses ileal digestibility values for individual amino acids and does not truncate scores, allowing differentiation among high-quality proteins. However, PDCAAS is still the most widely used method for regulatory purposes because it is simpler and more data are available. The choice of method depends on the purpose of the assessment.

What are the limitations of PDCAAS?

PDCAAS has several limitations: it uses fecal rather than ileal digestibility, which overestimates amino acid absorption; it truncates scores at 1.00, making it impossible to distinguish among high-quality proteins; it assumes uniform digestibility across all amino acids; and it uses rat rather than human digestibility data. These limitations can lead to inaccurate assessments for certain proteins.

How is protein digestibility measured?

Protein digestibility is measured by feeding a known amount of protein and measuring the nitrogen or amino acids that appear in feces (fecal digestibility) or at the terminal ileum (ileal digestibility). True digestibility corrects for endogenous losses by measuring nitrogen excretion on a protein-free diet. In vitro methods use enzymes to simulate digestion and estimate digestibility without animal studies.

Key Takeaways

Protein quality is determined by both amino acid composition and digestibility; a protein must provide all indispensable amino acids in adequate amounts and be efficiently absorbed.
The nine indispensable amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine; the limiting amino acid determines the quality score.
PDCAAS is the traditional regulatory method, calculated as amino acid score × true fecal digestibility, with scores capped at 1.00.
DIAAS is the newer FAO-recommended method that uses individual ileal digestibility values for each amino acid and does not truncate scores, allowing differentiation among high-quality proteins.
In vitro methods using enzymes or Caco-2 cell monolayers provide rapid, animal-free estimates of protein quality but require validation against in vivo data.
Biological methods like PER and nitrogen balance studies measure actual metabolic responses but are expensive and not practical for routine assessment.
Protein quality values are product-specific and vary with processing conditions, so always report the specific product and method used when citing values.