A closer look at the energy values of processed soybean

Table 1. Energy content (AMEn, kcal / kg) of roasted soybeans according to institutions and used prediction equation (90% DM)

On the other hand, the presence of oligosaccharides and fibrous components could affect the digestibility and energy content of beans, depending on the amount of beans included in the feed, as well as the age of the bird (Dilger et al., 2004; Choct et al., 2010; Ravindran et al., 2014b). Thus, under certain conditions, the oligosaccharides (fermentation limited to volatile fatty acids and possible “prebiotic” effect) and the fiber fraction (if they are included in very low fiber feed) could even improve the use of other nutrients present in the feed.

However, excess heat negatively affects amino acid (AA) digestibility, as well as the overall nutritional value of FFSB due to the higher incidence of Maillard reactions (Fontaine et al., 2007; González-Vega et al., 2011). Due to its complexity and the cost of directly measuring the content and type of trypsin inhibitors (TI), Maillard reactions and in vivo digestibility of the protein fraction, indirect techniques have been developed for its assessment, including urease activity (UA), dispersion index of protein (PDI), the solubility of the protein in KOH (KOH) and the thermal processing index (PCI).

Trypsin inhibitors (TI) are the main antinutritional factor (ANF) of raw soybeans (Liener, 1981; García-Rebollar et al, 2016). TIs present in beans are thermolabile in nature and are largely inactivated after proper heat treatment. The processing improves not only the nutritional value of the bean in relation to the availability of AA but also in relation to its energy content.

The work has studied the content and quality of the lipid and protein fractions of the processed soybean, estimating the energy value of samples from different sources:

• Africa (n = 5),
• Argentina (n = 4),
• Central Europe (n = 2),
• Southern Europe (n = 2),
• Eastern Europe (n = 4)
• USA (n = 10)

These fractions are subjected to various types of thermal processing.

However, the objective of the work is not to compare the influence of the processing or the origin on the nutritional value of the bean but to give an overview on the quality of soybeans commercialized in areas of our economic environment.

The relative importance of the energy content of soybean in relation to the protein value has grown in relevance in recent years. This gradual change is due to the greater availability of crystalline AA, without notable changes in terms of energy availability in the various production areas. There are prediction equations that allow estimating the AMEn content of the FFSB based on its chemical composition.

In this study, it was estimated that the crude protein (CP) digestibility coefficient was 88% for all samples. This estimate would not be correct if the type of processing (roasted vs. extruded vs. cooked) or the origin of the original bean is known. Likewise, based on laboratory data, it has been estimated that the ether extract (EE) value obtained by previous acid hydrolysis is on average 1.5 % higher than the content determined without acid hydrolysis (case of the prediction equation proposed by the WPSA, 1989).

Here we have used 2 equations, widely used by the feed manufacturing sector, to predict the AMEn content of FFSB in poultry:

• European Tables (WPSA, 1989) that are based on the CP content, ether extract without acid hydrolysis (EE) and N-free extract (NFE) of the evaluated sample but that does not take into account the digestibility protein or the content of sugars or ANF thereof (García-Rebollar et al., 2016; Ibáñez et al., 2020)
• CVB tables (2019 ) for broilers, which, in addition to the proximal composition, take into account the content and digestibility of CP and EE (eg with previous acid hydrolysis), as well as the sugar content.

The NFE term of this equation, which represents the nitrogen-free extract, is calculated by the difference between the dry matter (DM) and the sum of CP, EE, crude fiber (FB) and ash. Therefore, the equation proposed by the WPSA (1989) gives the same energy value to the sugar fraction as to the cellulose or lignin fraction.

Using these predictive equations, it is possible to determine the energy of the FFSB samples based on the chemical composition of the samples. It should be noted that the AMEn values obtained using one of these 2 equations for the analyzed samples are very dispersed, varying between 3,325 and 3,489 kcal / kg with the WPSA equation (1989) and between 2,995 and 3,244 kcal / kg with the CVB equation (2019) (Table 2).

Following these same criteria, we have assessed the energy content of the FFSB based on the average chemical composition indicated by various International Institutions specialized in animal nutrition. Taking these criteria into account, the energy content varied between 3,178 and 3,496 kcal / kg according to the WPSA (1989) and 3,006 and 3,149 kcal / kg according to the CVB (2019) (Table 2).

Table 2. Energy value of roasted soybeans (AME, kcal / kg) according to the prediction equation used (90% DM)

The above data clearly indicate that the use of regression equations, although recommended, often entails over- or under-titrations of the actual energy content of the analyzed FFSB samples. An additional problem related to the use of these equations is the lack of knowledge about the laboratory methods used in the analysis of the various components of the sample under study (eg, whether or not acid hydrolysis was used for oil; Kjeldahl or Leco-type combustion for protein and polarimetry or enzymatic method for starch) which affects the precision and confidence interval of the estimated energy value (Mateos et al., 2019).

• Given the high existing variability, feed manufacturers must be aware of the economic importance an adequate assessment of the quality of the beans used in formulation entails.
• Even though the soybean is an ingredient with less variability in terms of its nutritional composition than the rest of the commercialized protein or lipid sources, it requires an improvement in quality control.
• The origin of the bean, the pre- and post-harvest conditions of the original bean, and the type and quality of processing are key factors to consider.
• The reduction of the variability of the original bean, together with the improvement of the processing conditions, will allow optimizing the economic results and the productivity of the birds under current conditions.

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