After solvent extraction of the lipids in soybeans, the meal must be heated to eliminate remaining solvent (usually hexane), but also to deactivate antinutritional factors such as trypsin inhibitors and lectin. Inadequate heating fails to completely destroy the antinutritional factors, which may have a detrimental impact on animal performance. Excessive heating reduces the availability of lysine (via the Maillard reaction) and possibly, to a lesser extent, other amino acids. Laboratory tests are thus needed to determine whether samples of soybean meal have received adequate, but not excessive, heat treatment following oil extraction. Of tests commonly used, the evaluation of urease activity (UA) is the easiest to perform, and is especially useful in detecting underprocessed soybean meal. It is less reliable for detecting overprocessed meal. The protein solubility (PS) test is the most commonly used assay to detect overprocessed soybean meal, although very high values are indicative of underprocessed meal. The protein dispersibility index (PDI) is also used primarily to detect overprocessed or adequately processed soybean meal.
A. Urease Test: Overview
The enzyme urease is present in soybeans, but is of very limited interest in monogastric nutrition. However, much like trypsin inhibitors and lectins, its activity is reduced by heating. As a determination of urease activity is far easier to conduct than are assays for trypsin inhibitor, urease activity is frequently used as a “marker” to indirectly reflect the presence of antinutritional factors in soy products. Historically, urease activities in excess of a 0.15 increase in pH units suggested underprocessing, while activities of less than 0.05 units indicated overprocessing. However, during the past several decades, a change in soybean processing has led to the production of meals with much lower than 0.05 change in pH with no apparent detrimental effect on animal performance. In addition, the former maximum acceptable level of 0.15 pH units is no longer considered as absolute. Older birds, especially laying hens, can easily tolerate meals with a urease value of 0.25 or possibly even higher. Turkey poults may be most sensitive, as turkey starter diets frequently contain in excess of 40% soybean meal. The relative insensitivity of the urease assay as a means of quantifying overprocessing of soybean meal is due to the fact that there is no negative scale in this assay. The test is unable to distinguish between meals which may be barely acceptable versus those which are grossly overprocessed.
B. Protein Solubility (PS): Overview
The solubility of soybean protein in potassium hydroxide solution is inversely related to degree of heat treatment. The PS of raw soybean flour approaches 100%, while meals which have been heated to a dark brown color may have PS as low as 30 or 40%. Although definitive values are hard to establish, it is reasonable to accept that protein solubilities in the range of 78-84% reflect optimum soybean processing. Meals from 84-89% may be fully acceptable for laying hens and older broilers, which are less sensitive to antinutritional factors. In contrast, values lower than 78, and especially lower than 74% PS, reflect an incremental decrease in lysine availability for all animals.
C. Protein Dispersibility Index: Overview
An alternative means of evaluating the adequacy of soybean meal processing is by the protein dispersibility index (PDI). Protein dispersibility index has been used in the feed industry for almost a quarter of a century, but has only recently gained attention as a method to distinguish soybean meal quality for feed use. Protein dispersibility index measures the amount of soybean meal protein dispersed in water after blending a sample with water in a high speed blender. Work by Batal et al. (2000) suggests that PDI is a more consistent and sensitive indicator of adequately heat processed soybean meal than urease index or protein solubility in KOH. Soybean meal with a PDI of 45% of lower is adequately heat processed. This value is somewhat higher than the range of 15 to30% recommended by the National Soybean Processors Association. Further work is needed to determine the optimum range and maximum level for PDI in commercially-processed soybean meal. Combing the PDI test with the urease test could be useful to soybean processors and poultry nutritionists for better monitoring of soybean meal quality. For example, a soybean meal containing low urease (0.3 or below) and high PDI (40 to 45%) may indicate that the sample is definitely high quality because it has been adequately heat processed, but not overprocessed.