Guide for assessing the protein quality in soya feed products

Practical Recommendations

Processing intensity is key to quality

Common procedures for the heat treatment of soya beans are toasting, steaming and extrusion. The purpose of these procedures is to deactivate anti-nutritive components such as trypsin inhibitors. However, applying high temperatures inevitably leads to nutrient damage so the goal is to balance processing intensity. For toasted soya beans, the intensity is a function of processing time and temperature.

Crude protein content is a standard feed parameter, but it does not provide information on the digestibility. Processing indicators are measurable components of soya feed products which make the quality of soya bean processing (heat treatment) quantifiable. Table 1 summarises processing indicators which best predict the digestibility of the feed. Nutrient availability can be high if the values for trypsin-inhibitor activity (TIA) and protein dispersibility index (PDI) are within the target range (see numbers in Table 1). On the other hand, a poor feed conversion ratio becomes more likely if, for example, the TIA value in soya cake is higher than 4 mg/g (see Figure 1).

Table 1: Overview of processing indicators in soya bean-based feed products.  Values are based on dry matter (88%). Sources: various, see in further information.

Monitoring and testing of processing indicators

The monitoring of processing indicators provides key information for ensuring the quality of soya feed products over the whole year. All common kinds of soya feed products (toasted soya beans, expeller, or solvent-meal) can be tested on urease activity, TIA and PDI. Most laboratories with a specialisation on feed analysis can measure urease activity and PDI. Analysis of TIA or amino acids are less commonly offered by laboratories in Central Europe (see also in “Further information”).

Even under routine operating conditions it is fundamental to collect a correct, representative sample so that any subsequent analytical work and interpretation makes sense. The monitoring of the processing indicators of soya products can be compared with the guidelines applied in other crop quality management and assurance schemes.

The following list contains guidance points which are special for soya bean processing indicators:

• When soya bean processing equipment is operating for the first time, frequent testing is necessary until a constant product quality can be reached.
• More frequent testing is recommended if the processing equipment is not continuously in use or if the processing settings are frequently changed.
• Experienced operators of soya bean processing equipment can detect changes in processing performance by the taste of the soya product. However, tasting provides only a rough indication and cannot replace lab tests.
• An adequate sample weight is usually 0.5 kilogram.
• Occasional testing of unprocessed soya beans is advisable since the quality can differ significantly between each batch.
• It is essential that soya beans are standardised for particle size, purity, and moisture content prior to processing. Ideal conditions can also differ depending on the processing equipment used.