An Update on Soybean Meal Quality Considerations Robert A. Swick, Ph.D. American Soybean Association 541 Orchard Road, #11-03 Liat Tower Singapore 238881 April 2001 Introduction The cultivation of soybeans originated in China and for centuries this legume has been used in foods after being processed by various methods. The first attempts to feed soy to animals were unsuccessful with poor growth observed as compared to feeds containing other sources of protein. In 1917, Osborne and Mendel found that good growth could be demonstrated if soybeans were first heated before incorporation into the diet of animals. Later it was found that heating resulted in denaturation of protease inhibitors that interfered with digestion. Soybean meal now enjoys the largest worldwide market share of all protein meals used in poultry and swine feed. The world production of solvent extracted soybean meal was estimated to be 114 million metric tonnes out of a total soybean production of 165 million metric tonnes in 2000. Soybean meal represents more than half of the total production of all protein meals. Use of vegetable protein in animal feed is becoming increasingly important as fish stocks decline and concerns increase over the possibility of disease transmission from animal protein meals. In a typical broiler or pig diet, soybean meal supplies around 50% of the protein and amino acids and about 25% of the metabolizable energy (ME). Although properly toasted soybean meal is considered to have a high level of digestible essential amino acids and other nutrients, opportunities do exist to further improve the utility of soybean meal. For example, soybean meal is high in lysine, but this lysine is sensitive to denaturation during the toasting operation. Furthermore, the ME content of soybean meal is low compared to grains. Being able to measure and determine which opportunities may offer the greatest rewards for feed producers will be a major step toward enhancing the value of soybean meal and increase its inclusion rate. Advances in analytical techniques, processing and genetics will all contribute to the future increased use of soy in poultry feed. Protein and Amino Acids Soybean meal is a rich source of protein that is high in lysine. Methionine and cystine however are limiting. The proximate and amino acid analyses of a large number of soybean meal samples obtained from various international locations are given in Table 1. The soybean meal from the U.S. was dehulled as noted by the high protein level and low level of crude fiber. The other meals were mostly represented by non-dehulled samples. Meals from Brazil and India were higher in protein as compared to meals from Argentina and China with comparable average fiber levels. This observation may be related to warmer climate, agronomic conditions or seed genetics. Lysine content as a percent of crude protein was not constant between the locations suggesting differences relative levels of protein fractions in beans from various locations. The ratio of lysine to crude protein varied from 6.09% to 6.32% with the highest levels found in meals from temperate locations using improved genetic lines. The ratio of other important amino acids to protein such as methionine, cystine, threonine and tryptophan did not to vary as much as lysine.
The crude protein values reported in these data were obtained solely from determinations by the Kjeldahl method and have been adjusted to 12% moisture. Recently, the Dumas combustion method for crude protein determination has gained favor in many countries as a quick, accurate, safe and more environmentally acceptable method for crude protein determination. This method was approved by A.O.C.S. in 1998. The procedure uses an instrument where a sample of soybean meal is subjected to electric furnace temperatures of 600 C in a sealed reactor in the presence of oxygen. Nitrogen content of the combustion gas is then measured using a thermal conductivity detector. Each determination requires only about 2 minutes. The release of nitrogen is usually more complete than the Kjeldahl method and there is less chance for human error. Table 2 shows the results of two commercial laboratories testing the same samples of soybean meal. The results show that the combustion method typically gives slightly higher results than the Kjeldahl and that differences exist between laboratories conducting the Kjeldahl procedure. While the differences between Kjeldahl and Dumas may seem small numerically, the case for argument becomes large when multiplied by 50,000 mt in a typical commercial soybean shipment. As more laboratories using the Kjeldahl technique have abandoned the use of the hazardous mercury catalyst, the problem of low reported protein values has increased. More skill is required in the laboratory to digest aromatic and indole containing amino acids in soybean meal without the mercury catalyst. Importance of Metabolizable Energy Soybean meal contains 10% more gross energy than corn but only has about 72% of the ME of corn. Carbohydrate components in soybean meal are hulls, sugars and non-starch polysaccharides all of which are poorly digested in monogastric animals. The dilution effect of these undigested carbohydrates reduces nutrient density and makes high energy feeds more difficult and costly to formulate. Soybean hulls can be removed prior to oil extraction resulting in a meal that is higher in both protein and energy. The oligosaccharide and polysaccharide sugars are more difficult to remove requiring an additional extraction step with water and alcohol. The resulting product, called soy protein concentrate, is not costeffective for poultry feed. The use of supplemental alpha-galactosidase enzyme has been investigated as a potential means to increase ME content by releasing energy from these indigestible sugars but has thus far shown inconsistent benefits. Most nutritionists adjust the amino acid content of soybean meal when new shipments are received based on the crude protein content. Fewer however, adjust the energy content (ME) of soybean meal in their least cost computer program data bases. Unfortunately, the ME content of soybean meal is variable as shown in Table 3. Being able to quickly predict the ME content of soybean meal would greatly improve the cost performance of poultry feed. As energy is valuable, adjustments should be made, based on at the very least, a simple dilution calculation taking into consideration moisture, fiber, and ash content. Based on this, dehulled meal with 4% less fiber and ash would contain around 100 kcal/kg more poultry ME than non-dehulled meal. A further improvement would be the use of prediction equations. Several have been reported in the literature based on ME experiments with live birds (Janssen, 1989). The equation reported for solvent extracted soybean meal is ME(n) = 37.5 x CP + 46.39 x EE + 14.9 x NFE, where CP is crude protein, EE is ether extract (fat) and NFE is nitrogen free extract (100 - % CP - %EE - %ash - % moisture). This equation returns a value of 2285
kcal/kg ME for dehulled meal containing 48% C.P., 0.5% fat, 3.5% crude fiber 5% ash and 12% moisture. For non-dehulled meal with 44% crude protein, 0.5% fat, 7.0% crude fiber and 12% moisture, the equation returns a value of 2143 kcal/kg ME. Published results taken from the literature (Table 3) suggest higher values and a wider spread of 206 kcal/kg between dehulled and non-dehulled meal. Another equation published by Janssen (1989) for heat treated soybean pellets, ME(n) = 38.79 x CP + 87.24 x EE + 18.22 x NFE appears to give more reasonable results with soybean meal. Using the above proximate values, this equation returns values of 2490 kcal/kg ME for dehulled soybean meal and 2340 kcal/kg ME for nondehulled soybean meal. As these values and spread are more in line with literature values and thus this latter equation is hereby recommended for routine feed mill use. The latter prediction equation provides a quick way to differentiate energy content of meals given proximate analysis values. The importance of knowing the ME content of soybean meal can be easily demonstrated. The value of the ME in the meal can be compared to feed grade fat being used in the feed industry in a given location. A 200 kcal/kg ME difference between high quality dehulled soybean as compared to an average non-dehulled soybean meal, for example, is worth U.S. $93,750 per year in a feed mill producing 60,000 MT of feed per year. This figure assumes the feed on average contains 25% soybean meal and feed grade fat is priced at U.S. $250 per mt and contains 8,000 kcal/kg ME. Quality Assessment and Its Importance Soybean meal must be processed with a precise amount of toasting after solvent oil extraction. Heat labile antinutritional factors are reduced while at the same time preventing over-toasting that would cause a loss of digestible protein. The protease inhibitors including soy trypsin inhibitor are a major concern. Under toasted soybean meal may contain a high level of these proteins that bind and inactivate digestive enzymes produced in the pancreas of animals. Digestion is reduced as the pancreas attempts to produce more enzyme to make up for the excreted loss. As these lost enzymes are rich in sulfur amino acids a deficiency may occur. Slight to moderate over-toasting can result in poor bioavailability of lysine due to the Amadori and/or Maillard reactions. This is undetected in the routine amino acid assay as sugars and aldehydes that bind to lysine during over-toasting are released during the acid hydrolysis step in the assay. Urease and KOH Protein Solubility The presence of active trypsin inhibitor is indirectly determined by measuring the activity of urease enzyme present in soy. Both of these proteins are denatured and deactivated during heating. The laboratory method for urease involves mixing soybean meal with urea and water. Ammonia, which is alkaline, is released by the action of urease on urea. In the American Oil Chemists Society (AOCS) method, the end point is determined by measuring the increase in ph of the sample media. In the EEC method, the endpoint reflects the amount of acid required to maintain a constant static ph. Results of these two methods differ slightly from one another. Although this test is routinely done and often used in contract specifications, the results do not correlate well with animal performance. The urease test is only good for detecting under-processed meal. Meal with no urease activity may still have quite acceptable nutritive value. Average urease values of 72 samples of high quality dehulled soybean meal were recently measured using the A.O.C.S. method by an Australian feed
company laboratory and are reported in Table 4. An average value of 0.24 with a range of 0.05 to 0.37 ph unit rise was determined. All samples showed urease activity, with over half testing above 0.30 ph unit rise. The upper limit for urease after which a performance decline might be expected in young broilers was determined by Waldroup et al., (1985). Based on the results of several studies using meal that was heat treated for various lengths of time, an upper limit of 0.35 ph units rise was determined safe for poultry and swine. Figure 1 shows the results of one of the experiments where serious declines in performance were only observed in meal with urease above 1.75 ph unit rise. Over-processing can be estimated by measuring protein solubility in a solution of 0.2% potassium hydroxide (KOH-PS). There has been a great deal of interest in this procedure as results are correlated to growth rate in chickens and pigs (Parsons et al., 1991; Lee and Garlich, 1992; Araba and Dale, 1990). The results suggest a decline in performance at less than 72% KOH-PS. Lee and Garlich (1992) examined soybean meal processed at a commercial plant for various lengths of time with up to 50% additional residence time in the desolventizer-toaster. Effect on broiler performance and amino acid availability was then determined. KOH-PS and urease activity in the samples ranged from 81% to 92% soluble and 0 to 0.05 ph rise. Weight gain varied 10% between the six samples tested. In this work, the best performance and lysine digestibility was observed in meal with the highest KOH-PS and highest urease activity and lowest performance observed with the lowest KOH-PS and zero urease activity. Protein Dispersibility Index The protein dispersibility index (PDI) has been used in the feed industry for over 25 years but is only very recently gaining attention as a method for distinguishing quality of soybean meal for feed use. The PDI measures the amount of soybean meal protein dispersed in water after blending a sample with water in a high speed blender. Some recent investigations examined the ability of PDI to predict growth of chicks fed soybean meal samples heated by autoclaving for various lengths of time. The PDI results were then compared to protein solubility in 0.2% KOH and urease level as measure by rise in ph (Engram et al., 1999). The results appear in Figure 2. The results suggest that this test may be useful to further distinguish quality of SBM samples that are considered of high quality based on urease and KOH measurements. The work shows several samples with KOH solubility above 90% and high urease levels give gave varying growth rates in chicks. The PDI method was able to predict the growth supporting potential of these meals. In further work by Batal et al, 2000, it is clear that the PDI procedure demonstrates more consistent responses to heating of soy flakes than does urease or KOH protein solubility. The inconsistent and non-linear nature of the urease index to heating of soybean flakes contributes to the inconsistency among research studies for results on the maximum acceptable levels of urease activity in soybean meal. Soybean meal with a PDI between 45 and 50% and urease of 0.3 ph unit change or below may indicate that the sample is definitely high quality because it has been adequately heat processed but not overprocessed (Batal et al., 2000). Conclusion Soybean meal is traded on the basis of weight, moisture, protein, fat and urease level. Animal growth however, is more directly related to available energy and amino acid content of soybean meal. Feed mill purchasers often simply calculate the price per percent protein to
determine the value of a soybean meal. Although this may be somewhat useful, it is not completely valid when comparing meals of different fiber and ash content or meals from different origins or crushing plants. A major opportunity for increasing the utility of soybean meal is the recognition by feed millers of the value of available nutrient content, especially lysine and metabolizable energy. Quality specifications such as KOH-PS, PDI, and digestible lysine will be useful to estimate available lysine as these parameters are correlated to bird performance. Prediction of the ME content of soybean meal using equations based on proximate analysis results will result in increase performance and profit. Widespread adoption of these procedures will increase the utility of soybean meal and will, in turn, result in greater profit for feed producers. References ADAS. 1990. UK Tables of Nutritive Value and Chemical Composition of Feedingstuffs. Editor: Rowett Research Services, Aberdeen, Scotland. Coon, C. N., K. L. Leske, O. Akavanichan and T. K. Cheng. 1990. Effect of oligosaccharidefree soybean meal and true metabolizable energy and fiber digestion in adult roosters. Poultry Sci. 69:787-793. Batal, A.B., M.W. Douglas, A.E. Engram and C.M. Parsons. 2000. Protein dispersibility index as an indicator of adequately processed soybean meal. Poultry Science, 79:1592-1596. Dale, N. and H. L. Fuller. 1987. Energy values of alternative feed ingredients (Project 139). Special report to Southeastern Poultry and Egg Association. Athens Georgia, U.S.A. Engram, Douglas, Shirley and C. M. Parsons. 1999. Unpublished. In Methods for determining quality of soybean meal important. Ed: W. A. Dudley-Cash in: Feedstuffs, Jan 4, 1999 pp 10-11. INRA. 1989. L'alimentation des Animaux Monogastriques 2e edition. Editions INRA, Paris, France. Janssen, W.M.M.A. 1989. European Table of Energy Values for Poultry Feedstuffs. 3 rd ed. Beekbergen, Netherlands: Spelderholt Center for Poultry Research and Information Services. Lee, H. and J. D. Garlich. 1992. Effect of overcooked soybean meal on chicken performance and amino acid availability. Poultry Sci. 71:499-508. Leske, K. L., O. Akavanichan, T. K. Cheng and C. N. Coon. 1991. Effect of ethanol extract on nitrogen-corrected true metabolizable energy for soybean meal with broilers and roosters. Poultry Sci. 70:892-895. Mateo, C. D. 1998. Comparative performance of broilers fed U.S. dehulled, Argentine dehulled and Indian non-dehulled soybean meals. University of the Philippines, Los Baños. Unpublished, Personal Communication.
Muztar, A. J., H. J. A. Likuski and S. L. Slinger. 1981. True metabolizable energy of a number of feedingstuffs and complete diets as determined in two laboratories. Poultry Sci. 60:373-377. NRC. 1994. Nutrient Requirements for Poultry, 9th Edition. National Research Council. National Academy Press, Washington D.C. Parsons, C. M., K. Hashimoto, K. J. Wedekind and D. H. Baker. 1991. Soybean protein solubility in potassium hydroxide: An in-vitro test of in-vivo protein quality. J. Anim. Sci. 69:2918-2924. Parsons, C. M., K. Hashimoto, K. J. Wedekind, Y. Han and D. H. Baker. 1992. Effect of over-processing on availability of amino acids and energy in soybean meal. Poultry Sci., 71:133-140. RPAN. 1993. Rhodimet Nutrition Guide, 2nd Edition. Feed ingredients formulation in digestible amino acids. Rhone Poulenc, Antony Cedex, France. Sibbald, I. R. 1976. The true metabolizable energy values of several feedingstuffs with roosters, laying hens, turkeys and broiler hens. Poultry Sci. 55:1459-1463. Sibbald, I. R. 1977. The true metabolizable energy values of some feedingstuffs. Poultry Sci. 56:380-382. Waldroup, P. W., B. E. Ransey, H. M. Hellwig and N. K. Smith. 1985. Optimum processing for soybean meal used in broiler diets. Poultry Sci. 64:2314-2320. Wolynetz, M. S. and I. R. Sibbald. 1984. Relationship Between Apparent and true metabolizable energy and the effects of a nitrogen correction. Poultry Sci. 63:1386-1399.
Table 1. Analyzed Nutrient Levels of Soybean Meal: Global Results From Four Laboratories Country Location of Origin U.S. Brazil Argentina India China No. samples tested 937 120 78 69 46 Crude protein, % 48.1 46.1 43.5 46.8 45.0 Crude fiber, % 2.88 5.21 5.54 5.89 5.20 Ash, % 6.21 5.73 5.89 7.11 4.16 Fat, % 1.43 1.54 1.70 1.24 1.45 Urease, ph.05.04.04.06.04 Protein sol. 85.6 83.6 79.9 76.3 83.4 in.2% KOH Lysine, % 3.04 2.84 2.70 2.86 2.74 Lysine: Protein 6.32 6.18 6.21 6.09 6.09 Methionine, %.68.63.63.65.64 Met + Cys, % 1.36 1.30 1.27 1.31 1.30 Threonine, % 1.87 1.78 1.72 1.79 1.76 Trytophan, %.68.61.58.62.60 Values adjusted to 88% dry matter basis Averages of samples tested by United Soybean Board (Woodson Tenent Laboratories and Iowa State University Grain Quality Laboratory), Novus International (University of Missouri, Dept of Agric. Chem,), Degussa (Feed Analysis Laboratories, New Jersey, USA and Frankfurt, Germany) and Archer Daniels Midland (Decatur, Illinois).
Table 2. Comparison of Crude Protein Determination of Soybean Meal by Kjeldahl and Dumas (combustion) Methods Sample Laboratory 1 Laboratory 2 Laboratory 2 Description Kjeldahl Kjeldahl Dumas 1 46.2 45.5 46.1 2 46.0 45.9 46.6 3 45.8 45.5 46.6 4 46.0 45.7 47.1 5 46.1 46.4 46.7 Average 46.0 45.8 46.6 Table 3. Metabolizable Energy (kcal/kg) of Soybean Meal for Poultry Author Dehulled Non-dehulled Muztar et al., 1981 * 2571 2389 Sibbald, 1976 * 2676 - Wolnetz ad Sibbald, 1984-2330 Sibbald, 1977 * 2671 2292 Dale and Fuller, 1987 2449 - Coon et al., 1990-2458 Leske et al., 1991-2255 - 2240 Parsons et al., 1992 2518 - RPAN, 1993 2405 2160 INRA, 1990 2550 2420 ADAS, 1990 2503 2373 NRC, 1994 2385 2204 Ave 2518 2312 Range 2385-2676 2160-2458 Standard deviation 107 98 All adjusted to 88% dry matter basis * Corrected for N using Sibbald, 1984 ratio of.94 for SBM TME(n):TME
Figure 1. Effect of Urease Activity on 7 to 21 day Old Broiler Performance (from: Waldroup et al., 1985) 450 2.50 400 2.00 Wt Gain, grams 350 300 250 FCR Wt Gain, grams 1.50 FCR 1.00 200 0.50 150 0.00 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 Urease level, ph rise Waldroup et al, 1985 Table 4. Typical Urease Values of High Quality Soybean Meal Average, rise in ph units 0.24 Number of samples tested 72 Minimum value 0.05 Maximum value 0.37 Standard Deviation 0.07 Coefficient of Variation, % 27.3
Figure 2. Effect of Protein Solubility in 0.2% KOH, Urease Activity, Protein Dispersibility Index in Autoclaved Extracted Soy Flakes on Chick Growth (from: Engram, Douglas, Shirley and Parsons, 1999) 220 210 Gain, g 200 190 180 170 160 150 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 Urease activity, ph 220 Gain, g 210 200 190 180 170 160 150 0 10 20 30 40 50 60 70 80 90 100 Protein Solubility in 0.2% KOH, % 220 Gain, g 210 200 190 180 170 160 150 0 10 20 30 40 50 60 70 80 90 100 Protein Dispersibility Index, %