Soybean meal fed to pigs undergoes heat treatment to destroy trypsin inhibitors and other antinutritional factors that impair the digestion of protein and thus reduce performance. However, heat treatment can damage nutrients as well. In particular, the Maillard reaction reduces amino acid digestibility by combining amino acids with sugars to produce biologically unavailable compounds.

An experiment was conducted to determine the digestibility of amino acids in pigs fed soybean meal that had been heat treated in varying ways and for varying times. Conventional soybean meal was divided into four batches. One batch was not heated; one was autoclaved at 125°C for 15 minutes; one was autoclaved at 125°C for 30 minutes; and the last one was oven dried at 125°C for 30 minutes. Ten growing barrows were fed a total of five different diets. The experimental diets contained 40% each of the four different soybean meals being tested. An N-free diet was also formulated and fed to measure the basal endogenous loss of protein and amino acids.

The Maillard reaction describes a process in which the NH₂ group of an amino acid combines with a reducing sugar in the presence of heat to produce Amadori compounds and melanoidins.  These compounds are not bioavailable; therefore, they reduce amino acid digestibility and growth performance.  In addition, advanced Maillard reaction products can themselves react with amino acids and make them unavailable. Lysine is particularly susceptible to the Maillard reaction because it has an exposed NH₂ in the ε position on its side chain, which readily reacts with the carbonyl group of reducing sugars.

Among the compounds produced by the Maillard reaction are melanoidins, which impart a brown color to the substance. The oven-dried soybean meal in Figure 1 is similar in color to the unheated soybean meal, but the autoclaved samples are darker brown. This color change is due to production of melanoidins and indicates possible heat damage.  Because the Mailard reaction destroys amino acids, heat damage often results in a reduction in the concentration of lysine and other amino acids, as shown in Table 1. 

The standardized ileal digestibility (SID) of crude protein and amino acids in the four experimental diets is shown in Table 2. The SID of crude protein was not reduced by autoclaving soybean meal for 15 minutes or by oven-drying, but was reduced by autoclaving for 30 minutes. 

Autoclaving the samples for 15 minutes decreased the digestibility of lysine and aspartic acid, but the digestibility of the other amino acids remained the same as in the unheated samples. Autoclaving for 30 minutes reduced the digestibility of all amino acids relative to their digestibility in unheated soybean meal; in addition, the digestibility of lysine and aspartic acid was reduced relative to the samples that were autoclaved for 15 minutes. The digestibility of all amino acids did not differ between the samples that were oven-dried and the unheated samples.

As shown in Tables 1 and 2, autoclaving reduced the concentration and the digestibility of lysine in soybean meal. However, the concentration of crude protein is not changed by heat damage. It is, therefore, possible to estimate the degree of heat damage in a feed ingredient by calculating the lysine to crude protein ratio (Table 3). If the ratio of lysine to crude protein in a sample is reduced compared with that in unheated soybean meal, it is an indication that the sample has been heat damaged.

Heat damage may also be estimated by determining the furosine concentration in the samples, because the concentration of furosine increases in samples that are heat damaged.  

• Autoclaving soybean meal for 15 minutes at 125°C reduced the digestibility of lysine and aspartic acid, but did not alter the digestibility of crude protein or any other amino acids.
• Autoclaving soybean meal for 30 minutes at 125°C reduced the digestibility of all amino acids.
• Oven-drying soybean meal does not damage amino acids. Moisture is required for the Maillard reaction to take place.
• Heat damage in a sample of soybean meal may be estimated by comparing the lysine to crude protein ratio or the concentration of furosine in the sample to that of unheated soybean meal.

Figure 1. Soybean meal subjected to different thermal treatments. Starting in upper left to lower right: no heat, autoclaved 15 minutes, autoclaved 30 minutes, and oven dried 30 minutes.

Table 1. Chemical composition of soybean meal after heat treatment

Table 2. Standardized ileal digestibility by growing pigs of CP and AA in soybean meal subjected to different thermal treatments

^a–c Within a row, means without a common superscript differ (P < 0.05).

Table 3. Lysine:crude protein ratio and furosine content in soybean meal after heat treatment

This report is based on: González-Vega, J. C., B. G. Kim, J. K. Htoo, A. Lemme, and H. H. Stein. 2011. Amino acid digestibility in heated soybean meal fed to growing pigs. J. Anim. Sci. 89:3617-3625.