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Soybean is the most valuable oilseed meal on the animal feed market due to its protein content and favorable amino acid profile. Soybean also responds well to heat treatment used to destroy anti-nutritional factors (ANF) necessary for maintaining the health of monogastric animals.

Although heat treatment is needed to destroy ANF, it can also diminish other nutritious components of the soybean meal protein. Therefore, to develop the highest quality soybean meal, ideal processing conditions should have controlled high heat with a short residence time. Meal manufacturers can then perform tests to evaluate the quality of the soybean meal for animal feed.

The value of soybeans for animal meal

Protein is an essential ingredient of animal feeds. It is necessary for animal growth, body maintenance, reproduction, and the output of products such as milk and eggs. Among edible oilseeds, soybeans have the highest crude protein content that is also highly digestible.

Amino acids boost immunity and help improve the performance and productivity of animals. Soybean meal has a strong profile of essential amino acids released from the protein. It has the highest lysine digestibility of any of the commonly available protein sources. Since monogastric animals can’t synthesize lysine, this makes soybean meal a valuable protein source in diets. Soybeans also rank high in methionine, cysteine, and threonine digestibility.

The ANF in soybeans, such as trypsin inhibitors and lectins, can negatively impact animal health and performance by decreasing feed digestibility and increasing gut inflammation. Though harmful, the ANF can be destroyed easily with sufficient heat treatment. However, it is crucial to avoid overheating the oilseeds during this process.

Excessive heat can result in the breakdown of protein, loss of digestibility, destruction of amino acids, and render a portion of lysine unavailable for poultry and swine.

Since finding this balance can be challenging, meal manufacturers and their customers in the animal feed industry need reliable methods to monitor their soybean meal protein quality. The most common methods for monitoring quality are the protein solubility (PS) test and the protein dispersibility index (PDI).

Protein Solubility vs. Protein Dispersibility Index

PS is a long-established method of evaluation that uses a potassium hydroxide solution to determine if soybean meal is overprocessed, and high numbers can indicate whether the meal has been under processed. The solubility of the protein in the potassium hydroxide solution is inversely related to the degree of heat treatment.

The top of the PS test value range is raw soybean with a PS of 100%. Meal heated to a dark brown color has a PS as low as 30 to 40%. The range of acceptability varies across processors but generally falls within 78%-84%. In some cases, the acceptable protein solubility can increase to 89% for laying hens and older broilers less sensitive to ANF. By contrast, values lower than 78% and especially lower than 74% reflect decreased lysine availability for animals.

A second testing method, PDI, has been used in the feed industry for over 25 years but has only recently gained traction to distinguish the quality of soybean meals. PDI measures the amount of soybean meal dispersed in water after blending a meal sample and water in a high-speed blender.

Some studies indicate that soybean meal with a 45% or lower PDI is acceptable as adequately processed. However, practical experience has shifted recommendations for soybean meals with PDI values between 15 and 30%. Further studies are needed to determine the optimal range and maximum level for PDI in commercially processed soybean meals.

Soybean processors often combine PDI with the urease activity (UA) test, which involves mixing soybean meal with urea and water. This method determines the residual UA of soybean products and helps detect under processed soybean meal. Combining these two tests provides a better monitor of the full scope of soybean meal quality.

PS and PDI are the most commonly used tests among others available today. PS is better established and has more defined criteria for quality, while PDI is an easier and less costly method of evaluating soybean meal quality.

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Processing for optimal soybean meal protein

The differences observed in meal quality come down to processing conditions such as moisture, drying time, toasting, and drying temperature. Therefore, manufacturers should ensure that the heat used during processing is the correct temperature, consistent, and timed for ideal exposure to provide the highest quality protein.

One method used to increase soybean meal protein quality is to use a high-shear extruder, like the Anderson Dox™, ahead of an expeller press. The Dox works ahead of the press to mechanically shear, cook, and dry soybeans without expensive steam-heated cooking vessels. The Dox deactivates harmful enzymes, flash-dries excess moisture, and adequately cooks the proteins with a quick, 30 second residence time.

In addition to promoting high-quality soybean meal protein, the Dox can add value to your plant operations. It can:

  • Add flexibility to soybean oil processing. Read more here.
  • Advance greener oil milling. Read more here.
  • Meet consumer demand for high-oleic soybean oil. Read more here.

To learn more about the Dox high-shear extruder, request a same-day quote or speak with an Anderson expert now.


1. Fraanje, W. & Garnett, T. (2020). Soy: food, feed, and land use change. (Foodsource: Building Blocks). Food Climate Research Network, University of Oxford.
2. Eys, J.E. Van. Manual of Quality Analyses For Soybean Products in The Feed Industry, U.S. Soybean Export Council.
Căpriţă, Rodica & Caprita, Adrian & Iuliana, Cretescu. (2010). Protein Solubility as Quality Index for Processed Soybean. J. Anim. Sci. Biotechnol. 43.
3. Amino Acids. Amino Acids – Extension | Poultry Science. (n.d.).