Study Finds Soybean Meal Processing Dramatically Reduces Protein Quality in Pig Diets

University of Illinois research shows excessive heat treatment to soymeal cuts bioavailable lysine by 44%, highlighting the importance of ingredient quality and processing in swine nutrition. This potentially may have similar implications for poultry?

Urbana, Illinois | 23 June 2026 – New research from the University of Illinois Urbana-Champaign has demonstrated that the nutritional value of soybean meal—the world’s most widely used protein source in swine diets—depends not only on its chemical composition but also on how it is processed.

The study, conducted by L. Torrez-Mendoza and Professor Hans H. Stein from the University’s Department of Animal Sciences, found that excessive heat exposure during soybean meal processing can substantially reduce the availability of essential amino acids, potentially compromising pig growth performance despite conventional laboratory analyses indicating adequate crude protein levels.

The findings underscore the growing importance of evaluating ingredient quality based on nutrient digestibility and amino acid availability rather than crude protein content alone.

Heat Processing Alters Nutritional Value

Soybean meal undergoes controlled heat treatment during oil extraction to deactivate naturally occurring anti-nutritional factors such as trypsin inhibitors. However, excessive heating can trigger chemical reactions that damage amino acids, reducing their biological availability to animals.

To investigate this effect, the researchers conducted a controlled feeding trial involving 160 weaned pigs, comparing diets formulated with soybean meal processed under three different thermal conditions:

• normally processed soybean meal;

• moderately overheated soybean meal; and

• severely overheated soybean meal.

While all diets contained soybean meal as their principal protein source, the degree of thermal processing resulted in marked differences in amino acid quality.

Reactive Lysine Declined by 44%

One of the study’s most significant findings involved reactive lysine, the bioavailable fraction of lysine that animals can absorb and utilise for protein synthesis. Lysine is generally considered the first limiting amino acid in swine diets and plays a critical role in:

• skeletal muscle development;

• lean tissue accretion;

• immune function;

• enzyme production; and

• overall growth performance.

The researchers reported that severe overheating reduced reactive lysine concentrations by 44%, falling from 2.84% in normally processed soybean meal to just 1.59% after excessive heat treatment. Although total lysine concentrations may appear relatively unchanged using conventional chemical analyses, a substantial proportion becomes chemically modified during overheating, making it unavailable for digestion and metabolism by pigs.

Why Heat Damages Amino Acids

According to the researchers, excessive thermal processing promotes the Maillard reaction, a well-known chemical interaction between amino acids—particularly lysine—and sugars.

Once lysine becomes bound in these complexes, it can no longer be available for effective protein synthesis despite remaining detectable through routine compositional analysis / NIR Tests.

As a result, feed ingredients may appear nutritionally adequate on paper while delivering significantly lower levels of metabolically available amino acids.

These findings reinforce the growing importance of measuring reactive lysine rather than relying solely on total lysine concentrations when evaluating protein quality of a feed / feed ingredient.

Growth Performance Depends on Amino Acid Availability

Modern pig genetics require highly digestible amino acids to support rapid lean tissue deposition and efficient feed conversion. Reduced lysine availability may lead to:

• slower daily weight gain;

• poorer feed conversion efficiency;

• reduced carcass lean percentage;

• increased feed costs per kilogram of gain; and

• lower overall production profitability.

Because lysine serves as the reference amino acid for balancing ideal protein formulations, reductions in its availability can also alter the utilisation efficiency of other essential amino acids within the diet.

Implications for Feed Manufacturers

These findings carry important implications for feed ingredient suppliers, oilseed processors and compound animal feed manufacturers.

While heat treatment remains essential for destroying anti-nutritional compounds in soybean meal, excessive processing can inadvertently reduce protein quality and increase nutritional variability between production batches.

The researchers suggest that quality-control programmes should increasingly incorporate indicators of amino acid bioavailability—including reactive lysine and standardised ileal digestibility—in addition to traditional crude protein measurements.

Such approaches could enable nutritionists to formulate diets with greater precision while reducing unnecessary amino acid supplementation.

Economic Importance for the Global Swine Industry

Soybean meal remains the dominant protein ingredient in pig production worldwide, accounting for the largest share of supplemental protein used in commercial swine diets.

With feed costs representing approximately 60–70% of total pig production costs, even modest reductions in amino acid availability can translate into significant economic losses through reduced growth performance and lower feed efficiency.

As precision nutrition becomes increasingly central to modern livestock production, accurate assessment of ingredient quality is expected to play an even greater role in maximising productivity and sustainability.

Shift Toward Functional Feed Evaluation

The University of Illinois study highlights a broader evolution in animal nutrition—from evaluating feed ingredients based solely on their chemical composition to assessing their functional nutritional value.

The researchers conclude that the origin and processing conditions of soybean meal can substantially influence the proportion of amino acids that pigs are actually able to absorb and utilize, reinforcing the need for more sophisticated quality assessment tools throughout the feed supply chain.

For swine producers and feed manufacturers alike, the message is clear: not all soybean meal delivers the same nutritional value, and processing conditions can have a profound impact on animal performance even when conventional nutrient analyses appear satisfactory.