Last summer, researchers at the Ohio State University conducted a study to compare the digestibility of proteins in chicken breast and a meat analogue. They found that the chicken reacts with water more efficiently than the meat analog, increasing the penetration of digestive enzymes into the protein mass. It also had a higher proportion of small, hydrophilic peptides compared to the meat analog. As a result, the chicken released more than 500 peptides during digestion, compared to about 110 peptides in the meat analog.
The concentration of essential amino acids from digested chicken was more than 400 mmol/m compared to about 300 mmol/m in the meat analog digest. Both samples contained several essential amino acids, including valine, leucine/isoleucine, threonine, and phenylalanine; however, the meat analog had fewer non-essential amino acids compared to chicken (https://doi.org/10.1021/acs.jafc.2c01711).
Later in the year, a different research group conducted similar digestive experiments, comparing pork and beef to meat analogs (https://doi.org/10.1016/j.foodchem.2022.132917). In March, yet another group ran experiments on commercial products made from lab-grown meat and reached the same conclusion (https://doi.org/10.3390/foods12051061).
"Larger quantities of potential bioactive peptides were released from the meat than from the plant-based meat analogs during digestion," one research team wrote.
By 2025, the projected growth of the meat analogs market is 8.6% with a total market share of $21 billion. Although sales reports indicate a recent dip in market share for meat alternatives, the segment has grown so significantly in the past five years, analysts report it can weather these market adjustments (https://tinyurl.com/yuwema5f). In fact, this September, during the meeting of the United States’ Dietary Guidelines Advisory Committee, Heather Eicher-Miller reported that in the US, protein consumption across all age groups is increasing (https://tinyurl.com/mrx87axu).
Consumers value plant-based proteins as a more sustainable means of feeding a growing global population; or they may avoid eating animals for moral reasons. Whatever their driver, many are unaware that plant-based proteins do not pack the same nutritional punch as animal proteins. While the adoption of plant-based proteins into the human diet has grown, our understanding of protein quality lags.
WHAT THE BODY NEEDS
Proteins, carbohydrates, and fats are the macronutrients necessary for a balanced diet. Proteins are crucial for the growth, maintenance, and physiological function of the human body. Amino acids contribute to the synthesis and function of muscles, organs, enzymes, and hormones. These molecules also play a role in cellular structure, growth and repair, transport of nutrients, and a robust immune response.
The human body synthesizes some amino acids de novo, while others must be acquired through dietary intake. Nutritional experts have identified nine essential, or indispensable, amino acids that are necessary to meet the body’s metabolic demands: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (Table 1).
Animal proteins, including red meat, fish and seafood, poultry and eggs, and dairy, contain all nine essential amino acids. Plants vary in the composition and amount of these essential molecules. Most plant proteins, such as from beans, lentils, whole grains, and nuts, are incomplete. Soy and quinoa do contain all the essential amino acids, but not in the recommended amino acid ratios. Anyone eating a strictly plant-based diet should be intentional about pairing complementary plant proteins to provide the most comprehensive and balanced amino acid ratio for optimal nutrition.
However, understanding reported protein quality metrics is not as straightforward for consumers as other nutrients. Recommendations for vitamin C or calcium, for example, correspond to a single nutrient, while metrics for protein represent nine indispensable amino acids, plus dietary nitrogen.
"For amino acids from a protein to be utilized by the human body, the protein first needs to be hydrolysed into free amino acids and small (di- and tri-) peptides, which can subsequently be absorbed in the bloodstream," writes Shiksha Adhikari of Wageningen University & Research (https://doi.org/10.3390/nu14050947).
Hence, protein quality is determined by the digestible dietary essential amino acids that are bioavailable for the support of human growth and development.
MEASURING PROTEIN QUALITY
Over the years, many different groups have developed and implemented methods to determine protein quality according to their digestibility. The digestibility of a protein is typically defined as the proportion of ingested protein that is hydrolysed into amino acids, di- and tripeptides, which are available for absorption.
The Food and Agriculture Organization/World Health Organization (FAO/WHO), Expert Consultation on Protein Quality Assessment recommends the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), which compares the amino acid profile of a specific protein against a standard. In 1993, the US Food and Drug Administration adopted this metric for determining protein quality. Two decades later, the PDCAAS was replaced with the Digestible Indispensable Amino Acid Score (DIAAS), believed to be a more accurate measure of quality since it accounts for the amino acid digestibility at the end of the small intestine. (https://doi.org/10.3390/nu12123704).
This score provides a way to evaluate the dietary indispensable amino acids and non-essential nitrogen in food. The score for animal proteins, like eggs, milk, whey, and casein, is very near 1.00, while plant proteins vary in the concentration of indispensable amino acids. For example, soy has a DIAAS score near 1.00, while potato, pea, quinoa, and canola have a DIAAS score around 0.75.
The Canada government, on the other hand, bases protein quality on a change in body weight, known as the protein efficiency ratio (PER). The method’s advantage is how easy it is to conduct. Rodents are weighed and then fed a test protein or a reference protein. After a period, they are reweighed, and weight gain is expressed relevant to the amount of protein consumed.
Though there may be several options for measuring protein quality, each has one or more flaws that lead food scientists to lament that there should be a better method.
AN IMPROVED METHOD FOR SOYBEAN
Soy protein is a common ingredient in plant-based diets and a handful of studies indicate positive health outcomes associated with eating it. Qi Sun, an associate professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health, described how soy consumption lowers heart disease risk and lengthens lifespan (https://tinyurl.com/3n76dnk6).
"It’s almost a no-brainer that people should choose tofu and other plant-based proteins instead of animal source proteins," Sun said in the article. Adding, eating more plant-based foods can "not only improve human health, but also improve the health of the globe."
To improve the nutritional value of soy-based products, it is important to assess protein quality accurately and cost-effectively. According to Keshun Liu, a research chemist with the United States Department of Agriculture, trypsin inhibitor activity correlates to protein quality. A trypsin inhibitor is an antinutrient that prevents the trypsin enzyme from breaking down proteins during digestion. Lui found it was possible to understand protein availability by measuring the inhibitor’s activity.
AOCS approved a method to measure trypsin inhibitor activity in 1975 (AOCS method Ba 12-75). According to Liu, this method requires three reagents that include the substrate, trypsin enzyme, and the inhibitor. During the first step of this method, the trypsin and inhibitor are combined followed by the substrate. Because the inhibitor is a protein, the trypsin hydrolyzes the inhibitor and at the same time undergoes autolysis. The two biological reactions depend on the pH of the medium, duration, and other factors, and may result in variable and inaccurate results.
"We find that this order is not as good as if you mix inhibitor and substrate first and then the trypsin," said Liu. "The enzyme-last order produces consistent results, as it avoids the limited hydrolysis of the inhibitor by the enzyme and trypsin autolysis before the colorimetric reaction starts."
In 2019, Liu tweaked AOCS Ba 12-75 by changing the sequence of reagent addition and reduced the total assay volume by half. Twelve international labs completed a collaborative study to validate the proposed method to analyze 10 samples with varying levels of trypsin inhibitor activity. The new method (Ba 12a-2020) gave results that were repeatable and reproducible. It also reduced the overall cost and environmental footprint of the analysis. The AOCS Uniform Method Committee has approved Ba 12a-2020 as an official method, and in May 2023, Ba 12-75 was moved to a surplus method.
Liu’s lab at the USDA also developed a new method to measure the protein solubility index in soy and other plant protein products (https://doi.org/10.1002/aocs.12643). The index strongly correlates to the trypsin inhibitor activity and therefore provides an indirect but faster and cheaper assessment of protein quality. Liu aims to conduct a large-scale international collaborative study to validate the method and, if successful, present the results to AOCS Uniform Method Committee for consideration and approval as an AOCS official method.
One challenge that persists in current methods to measure trypsin inhibitor activity is differentiating between two types of inhibitors. The Kunitz trypsin inhibitor and the Bowman-Birk inhibitor have different physicochemical properties and implications for health. Measuring activity and inhibitor type simultaneously is particularly important for soybean and animal feed industries.
To address this need, Liu’s lab is developing a method to measure not only proteinase inhibition activity but also the concentration of each type of inhibitor in soybeans. With this method on hand, researchers can focus on the inhibitors of most concern for nutritional applications and breed healthier soybeans.
EFFECTS ON PROTEIN QUALITY
As valuable as it is to have a better means of measuring soybean quality, that is still just one ingredient. We generally consume food as meals, combining different items that complement each other in terms of digestible amounts of essential amino acids (FIG. 3). Hence, researchers have determined that the DIAAS value of a meal can be predicted using the values of each individual item (https://doi.org/10.1093/jn/nxaa398).
According to these calculations, when essential amino acids are lacking in a plant-based food it is possible for another food to compensate. However, there are also cases where combinations of foods do not fully compensate for incomplete essential amino acid ratios.
Add to that, processing steps involving heat and alkaline treatment can result in lost or reduced digestibility. Processinginduced changes in protein digestibility can arise from effects on protein structure, anti-nutritional factors, accessibility for the digestive enzymes, but also through effects on the non-protein constituents. Sometimes processing leads to a negative effect, but sometimes the effect is positive.
For example, proteins in some meat analogs go through thermal–mechanical treatment with an extruder or a high-pressure shear cell to form a dense mesh structure or to align fibrils and create texture. The treatment unfolds proteins and damages heat-labile antinutritional factors, which benefits protein digestion. However, as the recent experiments between meat and meat analogs show, the protein aggregates that are formed reduce the contact area for digestive enzymes and hinder digestion.
A plant-based diet requires consumer consciousness concerning protein quality. Simply eating more plant proteins will not ensure proper nutrition. They must hit the right balance. Overconsuming results in an inefficient use of protein because excess amino acids are oxidized.
As more plant-based protein products enter the marketplace, manufacturers could assist consumers by ensuring their products deliver the essential nutrients needed. Meat analogs offer a promising substitute to animal products in terms of sustainability and nutrition. Future formulation improvements will continue to better mimic animal protein taste and texture. Manufacturers will have to consider protein quality too.
About the Author
is editor-in-chief of INFORM magazine. Stacy Kish is a science writer for INFORM and other media outlets. She can be contacted at earthspin.science@