The quality of soybean meal (SBM) for animal feeding depends of the balance betweeen heat inactivaction of antinutritional factors, and the Maillard reactions rendering insoluble proteins due to excessive heat treatment. To estimate SBM quality, the feed industry uses three methods: measuring urea activity, KOH protein solubility, and protein dispersion index (PDI). This article analyses the advantages and disadvantages of each test.
Soybeans contain variable amounts of antinutritional factors (ANF), including trypsin inhibitors (TI). They have negative effects on the activity of the enzymes trypsin and chymotrypsin, interfering with amino acid (AA) digestibility.
The antitrypsin factors are thermolabile, so their content and activity are reduced after heat processing. Therefore, the adequate processing of soybean increases AA digestibility and significantly improves the nutritional value of the soybean meal (SBM).
To know the real quality of SBM protein, it is not enough to analyze TI levels. It is also important to determine the amount of Lys that remains active at the end of the heating process. SBM with TI values lower than 1-2 mg/g could indicate overheated products if the incidence of Maillard reactions is high.
The existing techniques to determine and estimate both TI and Maillard reactions are complicated and expensive. Therefore, it is fundamental to use correctly and carefully the methods available in the laboratory
There are several methods to control the protein quality in SBM. Among them, the most used ones in the animal feed industry are:
Soybeans contain variable amounts of urease, a thermolabile enzyme whose destruction by heat treatment has, roughly speaking, a similar dynamic to the inactivation of TI (Balloun, 1980; Waldroup et al., 1985). The urease test measures the increase in pH due to ammonia production from urea in the presence of urease. The pH increases in unprocessed or poorly processed meals. This method indirectly measures the degree of heat processing applied to the SBM and, therefore, the degree of TI destruction.
Characteristics of the method
High urease values indicate raw or poorly processed SBM, while values close to zero can indicate well processed or overheated SBM.
The solubility of the globulins, which are the major storage proteins in beans, is high but it decreases with heat processing. Before the processing, the bean protein is
very soluble, and the KOH value is above 90-95%.
The KOH protein solubility test allows us to measure soybean protein solubility in a 0.2% KOH solution, indirectly determining the degree of protein denaturation due to heat processing. The method is effective to determine the overheating of the bean or meal, however it is less effective in detecting poorly processed beans (Anderson- Haferman et al., 1992).
It should be noted that the handling of the samples during analysis – grinding size and time, stirring, temperature, pH of the solution, time, etc. – may influence the results (Whittle and Araba, 1992). In any case, the KOH index is less affected by the storage time than other methods such as PDI (Protein Dispersion Index) (Serrano et al., 2013).
Protein Dispersion Index (PDI)
PDI measures the solubility in water of the nitrogen present in soybean samples.
The method allows to determine the optimal level of processing of a bean or SBM. According to Hsu and Satter (1995), Batal et al. (2000) and Dudley-Cash (2001), It is more accurate than the urease test and, sometimes, than the KOH protein solubility test.
As indicated for the KOH protein solubility method, the handling and sample processing can affect the laboratory results. In particular, the duration of storage tends to reduce PDI values much more than its effects on KOH and TI (Sueiro et al., 2009a, b; Serrano et al., 2013) (Table 1). For this reason, PDI values of a SBM individual sample are possibly lower when it is analysed in the country of destination than it would be if analysed in the country of origin.
Table 1. Influence of duration of storage of soybean meal on protein quality indicators (UA = Urease activity (mg N/g); KOH = Protein solubility in KOH (%); PDI = Protein Dispersion Index (%); ATI = Activity of trypsin inhibitor (mg/g DM)) .
Despite the different existing criteria (Lee et al., 2007; from Coca-Sinova et al., 2008), KOH protein solubility, and to a lesser extent PDI, better estimate the quality of SBM processing than urease activity. It is important to consider that SBM from USA shows average KOH and PDI values 4-5 points higher than those for SBM from Brazil or Argentina (García-Rebollar et al., 2016).
Based on the present knowledge, it would probably be more convenient or more adequate to use SBM with urease values close or equal to zero and KOH protein solubility values above 85-88%.These KOH values are higher than the current recommendations put forward by most authors. Table 2 shows the acceptable values of soybean protein quality (KOH protein solubility, PDI, urease activity and ATI), according to van Eys (2012).
Table 2. Values of soybean meal quality indicators most commonly accepted by the animal feed industry (van Eys, 2012)
About antinutritional factors
We need to consider….
For TI, the values are presented as activity units (ATI) and not as TI units (TIU).
The trypsin inhibitors cause pancreatic hypertrophy in poultry, since the birds try to compensate for the enzyme loss. This is not observed in pigs.
Stachyose and raffinose
The oligosaccharides (stachyose and raffinose), simple carbohydrates that cannot be digested by the animal but can be fermented in the large intestine, are additional relevant ANF found in SBM.
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