Ideal extruder temperature to produce best full fat soybean meal

Although there are different types of plant proteins used as low cost and sustainable protein sources, soybean meals are the most widely used source of high-quality plant protein for livestock, aquaculture, and especially for poultry feed. It is also considered as an effective feed ingredient that are less expensive and more sustainable than fish meal. Therefore, the understanding of its processing allows us to take advantages.

Danny Patino
Commercial Nutritionist
Trouw Nutrition, Central & South America
Michael Joseph
Assistant Professor and Extension Specialist
North Carolina State University

PROCESSING OF DIFFERENT TYPES OF SOYBEAN MEAL (SBM)
To obtain the SBM, it may be processed in a variety of different ways to increase the protein concentrations and to deactivate anti-nutritional factors. During different stages of processing, this feed ingredient is exposed to varying degrees of heating. Based on the processing, we can obtain different types of SBM such as full fat SBM (FFSBM), extruded expelled SBM, and solvent extracted SBM. Both full fat and extruded expelled SBM are obtained primarily from the dry extrusion process. The extruded expelled SBM undergoes an additional step of mechanical expelling of oil from the FFSBM obtained after extrusion. The solvent extracted SBM is obtained after the solvent extraction of the oil from the soybeans. This SBM is the most popular and is widely used across the feed industry. However, there is a need for the use of extruded FFSBM due to their extra benefits. In addition, there is a lack of clarity amongst many extruder operators on the optimum processing conditions for obtaining the best quality of this potential feed ingredient.

BENEFITS OF FULL FAT SOYBEAN MEAL
According to Ramos (2021), FFSBM is an excellent source of energy with high stability on fat. The nutritional quality and amino acid digestibility of full fat soybean meal can be better than the conventional solvent extracted SBM, if properly processed. Additionally, use of FFSBM is useful for feed mills that have not invested in the costly equipment like pumps and tanks that are needed for liquid (oil) addition. Further, the use of full fat soybeans helps lower the energy requirements (by not using pumps to add in the oil externally into the formulation) of producing feeds and thus contributes to lowering the carbon footprint of feed production. However, it is important to understand that proper heat treatment is needed to lower the heat labile anti-nutritional factors like trypsin inhibitor, to extract the maximum benefits of FFSBM. Overzealous operation of extruder at higher temperatures can lead to overheating can damage protein and reduce the nutritional value. Therefore, it is important to understand that the quality of SBM could be affected by the process used, the different amount of nutrients in the beans, and the antinutritional factors.

FINDING THE OPTIMAL EXTRUDER PROCESSING TEMPERATURE
This study was conducted to find the optimal processing temperature during extruder operation by determining the quality of FFSBM exposed to different processing temperatures. Common lab tests for raw soybeans and extruded FFSBM included crude protein (CP) content analysis and other tests for measuring FFSBM quality. These included official methods (Palic et al., 2008) – urease index (UI) and protein dispersibility index (PDI) and unofficial methods like KOH protein solubility (PS) and trypsin inhibitor (TI) activity. The measurements are an indicator of over processing and under processing of FFSBM when subjected to extrusion process. A single screw dry extruder was used to produce 6 experimental FFSBM using different die temperatures of 135°C, 145°C, 155°C, 160°C, 165°C, and 170°C in a commercial feed mill. The operating conditions were selected to be between the limitations of the equipment and to achieve both under and overcooking of the SBM. Data were analyzed using one-way ANOVA and means were separated using Tukey’s test.

Figure 1. Residual Urease Index (UI) at different extruder die temperatures
Figure 2. Trypsin Inhibitors (TI) levels at different extruder die temperatures

RESULTS
The UI more than a 0.15 increase in pH units suggest under processing (US Soy in International Buyers’ Guide), the content of UI of the raw beans were 2.09. However, after the extrusion process UI decreased less than 0.15 for all the FFSBM produced (Figure 1). The TI levels at 170°C was found to be the least at 0.60 mg/g and was statistically different from the other FFSBMs. TI values higher than 3.5 mg/g compromised growth performance due to low digestibility and values lower than 1 mg/g could indicate overheating (Woyengo et al. 2017). Therefore, FFSBM produced with temperatures of 135, 145, and 155°C could compromise the growth performance in the animals and FFSBM obtained by 170°C suggests overprocessing (Figure 2).

Figure 3. Protein Dispersibility Index (PDI) at different extruder die temperatures
Figure 4. KOH Protein Solubility (PS) at different extruder die temperatures
Figure 5. Lysine, Methionine, and Lysine: Crude Protein Ratio at different extruder die temperatures

The PDI values decreased with increase in temperature and all values were less than 45% indicating adequate heat treatment (Figure 3). However, the quality standards by US Soy in International Buyers’ Guide recommends a PDI between 20 and 35% to be the best for animal performance. So, temperatures of 155°C and above were in the right PDI range. The PS between 73-88% is considered of acceptable quality and lower values suggest over processing. FFSBM, thus processed at temperatures of 165°C and 170°C are over processed. For the FFSBM produced at temperatures lower than 165°C, there was no statistical difference (Figure 4). Both PS and PDI values suggested overprocessing at temperatures of 165°C and 170°C. The CP increased from 37.59% in raw beans to 45.23% at 170°C with consistent increase as the processing temperature was raised. As the temperature rises more water is evaporated. Therefore, this evaporation decreases the moisture content and leads to an increase in crude protein levels. There were no statistical differences between the different extruder operating temperatures for the Lysine and Methionine content (Figure 5). However, we also calculate the ratio between lysine and crude protein (Lys: CP) as this is an indicator of heat damage. When an ingredient is heat-damaged to an extreme level (final stage of Maillard reaction), the concentration of lysine is reduced, but the concentration of crude protein is not (González-Vega et al., 2011). For SBM, a Lys: CP ratio of 6 and above is generally accepted as an indicator of good protein quality. Therefore, as the green line shows the only value lower than 6 is the SBM obtained by 170°C, which suggests that the quality of protein will be affected in temperatures higher than 170°C.

TAKE HOME MESSAGE
Overall, this study (Table 1) showed that between temperature range of 155 to 170°C, FFSBM has an adequate PDI. However, the KOH Protein solubility analysis suggests that 165°C and 170°C overprocesses the SBM. In addition, TI values higher than 3.5 mg/g can compromise the growth performance in animals, therefore values under this value are preferred. Finally, the protein quality of the SBM should be higher than that of 6 when considering Lys:CP ratio. Therefore, based on the above facts, this study showed that 160°C was the ideal operating temperature to get the best extruded FFSBM. All the test parameters are essential to determine the thermal effect of processing on FFSBM quality. These test parameters would be useful to soybean processors and animal nutritionists for better monitoring of extruded SBM quality. Additionally, right operating parameters are important to minimize the carbon footprint by lowering the energy costs and wastage of low quality products and thus support sustainability.

About Danny Patino
Danny Patiño is from Ecuador. He obtained his bachelor’s degree in Food Science and Technology from Zamorano University, Honduras and his master´s degree in Poultry Science with a minor in animal nutrition from North Carolina State University, USA. During studies and career, he continued to gain more interest and knowledge in the area of animal nutrition and feed processing. He did various studies around the use of plant proteins in animal feeds during his master. He has recently joined the team from Trouw Nutrition as their commercial nutritionist to support Central & South America.


About Michael Joseph

Michael Joseph, Ph.D. is an Assistant Professor and Extension Specialist in the Prestage Department of Poultry Science at North Carolina State University, Raleigh NC USA. He is a food and feed extrusion specialist. His experience ranges from infant foods to fish feeds and pet foods. His research focuses on finding value-added solutions for by-products and co-products from the agriculture and meat processing industries to enhance sustainability.