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Effects Of Processing On Nutrient Content of Soybean meal

K. C. Rhee
Food Protein Research and Development Center
Texas A&M University
USA

Introduction
Formulation of balanced diets is fundamental to economical animal production, and this process depends on the knowledge of nutrient requirements of the animal and the nutritional attributes of nutrient sources. The task of correctly assessing the protein nutritive value is rather challenging. The specific protein requirements must be established for each animal species of interest, with due considerations of the breeds, sex, age, weight, intended use, and other particulars of the animal. Also, amino acid contents of the protein source and their availability to animals require careful assessment.

Considerations In Proteins For Feeds
The fact that proteins differ greatly in nutritive value was first demonstrated grossly by comparing the performance of animals fed diets containing approximately the same amount of protein. It has been known that the less-than-desirable performance of an animal on a low-quality protein diet can be compensated for, to a large extent, by increasing the amount of the same low-quality protein. It has also been established that the nutritional quality of proteins is usually related to the amount and availability of amino acids in them, since the supplementation of diets containing low-quality proteins with appropriate amino acids improves their nutritive values.

Dietary requirements for protein are actually requirements for the amino acids contained in the dietary protein. These amino acids are used by animals to fulfill a series of functions. For example, amino acids, as proteins, are primary constituents of structural and protective tissues, such as skin, hair, feathers, bone matrix and ligaments as well as of the soft tissues, including organs and muscles. Also, the digested/absorbed amino acids and small peptides serve a variety of metabolic functions and as precursors of many important non-protein body constituents. An adequate intake of dietary amino acids is required because body proteins are in a dynamic state, with synthesis and degradation occurring continuously. If dietary protein (amino acids) is inadequate, there is a reduction or cessation of growth or productivity and a withdrawal of protein from less vital body tissues to maintain the functions of more vital tissues.

There are 18-21 amino acids that are physiologically essential for proper body functions and maintenance. Nutritionally, these amino acids are divided into essential and nonessential amino acids. Essential amino acids are those that the animal cannot synthesize at all or rapidly enough to meet metabolic requirements. They include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Nonessential amino acids are those that can be synthesized by the body in enough quantities from other amino acids, and include alanine, aspartic acid, cysteine, glutamic acid, glycine, proline, serine, and tyrosine. The essential amino acids must be supplied by the diet. If the diet does not provide adequate quantities of nonessential amino acids, the animal must synthesize them from essential amino acids. Thus, stating dietary requirements for both protein and amino acids is an appropriate way to ensure that all amino acids (both essential and nonessential) needed physiologically are provided for from diets.

Protein and amino acid requirements vary considerably according to the animal species, breeds, body size, growth rate, sex, reproductive state, etc. Genetic differences in amino acid requirements may occur because of differences in efficiency of digestion, nutrient absorption and metabolism of absorbed nutrients (NRC, 1975). Such factors, as dietary energy, bulk density and ambient temperature, that affect feed consumption also will affect quantitative intake of protein and amino acids which will consequently influence the dietary concentration of these nutrients needed to provide adequate nutrition (NRC, 1987).

Each amino acid can be metabolized independently of others, but relationships exist between certain amino acids, some with beneficial effects and some with antagonism. Shortfalls for cystine requirement can be met by methionine because two moles of methionine can be used to synthesize one mole of cystine, but the requirement for methionine can only be met by methionine. The need for tyrosine can be satisfied by phenylalanine because tyrosine is the initial degradation product of phenylalanine. Serine can be converted to glycine on a mole-to-mole basis, and this reaction is reversible. The essential amino acids are related to one another to support the need for production and maintenance. Requirement for any one essential amino acid represents the combined need for maintenance and production. Each essential amino acid is unique in its catabolism, and an inadequacy of any one of them (the first limiting) usually necessitates some catabolism of the others. The animal¡ãOs respons to amino acid deficiency can vary with the essential amino acid, the extent of its inadequacy and the existing relationships among the remaining essential amino acids. For example, animals performed better on diets with less-than-adequate amount of methionine/cystine, leucine, lysine, or arginine than on diets with inadequate quantities of all essential amino acids. On the other hand, animals lost additional weights when the amount of one of the following essential amino acids was reduced in diets: phenylalanine, tyrosine, tryptophan, isoleucine, valine or threonine.

Antagonisms exist between certain amino acids: leucine-isoleucine-valine, arginine-lysine, and threonine-tryptophan (D¡ãOMello and Lewis, 1970), with the most practical importance on leucine isoleucine antagonism. Certain feed combinations (i.e., corn plus corn gluten meal) can lead to practical diets with exceptionally high leucine levels while isoleucine is marginally adequate. Amino acid toxicity requires a particularly high level of one amino acid relative to all other amino acids. Such an occurrence is unlikely under practical circumstances because most of feedstuffs do not have such a large difference in amino acid contents. However, formulation error may lead to amino acid toxicity; excessive methionine is toxic (Ueda et al., 1981; Edmonds and Baker, 1987).

In general, the recommended requirements of proteins and amino acids are intended to support maximum growth and production. However, achieving maximum growth and production may not always ensure maximum economic returns, particularly when prices of protein sources are high. If a slightly decreased performance can be tolerated, dietary concentrations of amino acids may accordingly be reduced somewhat to maximize economic returns.

Prediction Of Protein Nutritive Values
It is well known that the nutritive value of a protein ingredient depends on the composition and availability of amino acids that vary greatly among ingredients. Many factors can influence the composition and availability of amino acids in grains and protein supplements. It is therefore desirable to know the amino acid composition of the actual ingredient to be used in the diet and their availability to animals for accurate and economical feed formulation. However, in general, it is not feasible to analyze all samples of feed ingredients for amino acid availability prior to their use in a particular feed formulation. Therefore, a series of indirect methods have been proposed and used to predict the nutritive value of protein ingredients that can be used in formulating the diets.

Kjeldahl nitrogen determination has been used as a quick and preliminary method to estimate the protein nutritive value of various ingredients. Although the method often overestimates the true protein content and does not give any information on the composition and availability of amino acids, it does provide gross crude protein content of the test material. Under many practical situations, the gross crude protein content data may provide sufficient information to formulate acceptable diets. Needless to say, this information becomes far more powerful and useful when used along with amino acid composition data.

Biological value (BV), defined as the percentage of ingested nitrogen retained in the body, has long been the choice for estimating the nutritive value of proteins. When combined with digestibility (D) data, BV reflects fairly well the nutritive value of the protein. These values are obtained by measuring the fecal and urinary nitrogen of an animal fed a test protein diet and then correcting for the amounts excreted when a nitrogen-free diet was fed. )

D = { [ I - ( F - F 0 ) ] / I } x 100

BV = { [ I - ( F - F 0 ) - ( U - U0 ) ] / [ I - ( F - F0 ) ] } x 100

where, I is the nitrogen intake of test protein; F, fecal nitrogen; F , fecal nitrogen on a nitrogen-free 0 diet; U, urinary nitrogen; and U , urinary nitrogen on nitrogen-free diet.

The overall nutritive value of a protein should then be obtained by BV x D, which is identical with Net Protein Utilization described below.

Determination of Net Protein Utilization (NPU) also provides an estimate of nitrogen retention by measuring the difference between the body nitrogen contents of animals fed no protein and those fed a test protein. This value, divided by the amount of protein consumed, is NPU, which is defined as the percentage of the dietary protein retained.

NPU = { [ I - ( F - F 0 ) - ( U - U ) ] / I } x 100

In chemical score, sometimes called amino acid score, the content of each essential amino acid in ) is expressed first as a ratio of total essential amino acids (E ) in the diet. These ratios the protein (A x x ) and the total are then expressed as percentages of the ratios between each amino acid in egg (A e essential amino acids of egg (E ). The chemical score (A E /A E ) is the lowest of all these percentages. e x e e x ) in Chemical score can also be determined by another method. The content of each amino acid (A x ). a test protein is expressed as a percentage of the same amino acid in the same amount of egg (A e The amino acid showing the lowest percentage is called the limiting amino acid and this percentage /A ). The results obtained by these two methods are almost identical and have is the chemical score (A x e a rather high correlation ? = 0.86) with Biological Values when egg protein was used as the standard.

Qualitative differences in protein quality can be demonstrated by many methods. The Protein Efficiency Ratio (PER), defined as the weight gain per gram of protein eaten, has been the most widely used method because of its relatively simple procedure. In practice, corrected PER is calculated on the basis of an assumed PER of the standardized casein of 2.50 to normalize inter-laboratory variations.

Processing Effects On Soybean Meal Quality
Soybeans are rich in protein with well-balanced amino acid profile. However, soybeans contain several antinutritional factors that adversely affect protein nutrition unless they are properly controlled. Traditionally, soybeans are processed into defatted meals before they are used as ingredients to formulate diets, particularly for swine and poultry. During the past several years, however, increasing amounts of full-fat soybean meals have also been used as animal feeds. Several steps involved in manufacturing these products can have either positive or negative effect on the quality of the meal protein depending on the conditions used in the processing.

The single most important parameter that affects the soybean meal protein quality is the heat applied at different stages of processing. Proper processing conditions (moisture content, heating time and temperature) inactivate antinutritional factors (i.e., trypsin inhibitors, hemagglutinins, lectins, etc.) in soybean meal, which results in much improved growth when fed to monogastric animals (Araba, 1990a). The heat treatment must be controlled carefully because overheating can result in deterioration of protein quality by destroying heat-sensitive amino acids (methionine, lysine and cystine) and by causing Maillard reaction. Urease assays have generally been used by the feed industry in monitoring soybean meal quality because it is easier to determine urease activity than trypsin inhibitors and urease is more heat resistant than trypsin inhibitors. The U.S. feed industry has long used a maximum urease rise of 0.3 pH units as the standard for processing soybean meal for all types of poultry feeds. Damage to the soy protein from overheating is more serious when dietary lysine concentrations are marginal, and the heat damage can be monitored by measuring the solubility of the protein in potassium hydroxide solution either by the Kjeldahl or by the dye-binding method (Araba and Dale, 1990b; Kratzer et al., 1990).

In the classical soybean processing of conditioning, flaking and then extraction followed by desolventizing and toasting, a number of heat treatment steps are involved. Under the current commercial practice, the conditioning step has little effect on protein quality. The conditioning temperature (approximately 71!C, 11% moisture) is not high enough to denature the protein or inactivate urease and trypsin inhibitors. Flaking itself has no effect on protein quality either; however, more heat is required in the desolventizer/toaster to inactivate urease and trypsin inhibitors in thicker flakes, which will also denature more proteins. It is therefore very important to maintain uniform flake thickness of 0.25-0.3 mm to produce meals with desirable quality. After oil extraction, the meal has about 35% hexane hold-up, which is removed by directly injecting live steam. The desolventized meal with approximately 16-24% moisture is then toasted or cooked at 105-110!C for 15-30 minutes to bring the urease activity down to 0.2 pH units. A considerable amount of heat is required to bring the moisture level down to a safe cool storage level of about 12%. The protein solubility of the resulting product normally ranges between 80-85%, with reduced available lysine content of as much as 40% due to protein denaturation caused by excessive heat treatment.

Many soy processors currently use expanders to improve the efficiency of solvent extraction. Soybeans are dried to 10% moisture, cracked, dehulled, conditioned to about 11% moisture at 55-82!C, flaked to 0.3-0.5 mm thickness, expander processed with steam to exit temperatures of about 105-120!C to form collets, and cooled to 60!C for extraction. Due to the improved solvent percolation and drainage, resulting from the high porosity of the collets, the extracted collets have only about 20% hexane hold-up, 43% less than the flake. These collets can be desolventized and toasted using a considerably smaller amount of steam than flake, leaving less moisture in the meal from collets, about 13.5% (Watkins, 1998). This level of moisture is high enough to effectively inactivate the urease to 0.3 pH units and easy to dry the product with cooling alone to the desired storage moisture level with minimum protein denaturation (protein solubility of about 90-92%) and consequently with minimum loss of protein nutritive value. A complete study on the availability of amino acids will be necessary to fully understand the implications. However, poultry and swine feeding trials at Texas A&M University and others indicate that the product (SoyMAX?) provides higher rates of amino acid digestibility and higher energy values that produce better feed conversion rates and higher body weight gains (Wright, 1998). When combined with other factors, such as increased plant capacity and slightly more oil yield, the expander process becomes an attractive option. It almost doubles the plant capacity, saves energy, saves equipment cost, and produces finished meals with high protein solubility and improved nutritive value over the classical flake method.

Dry extruders are used to prepare full-fat soybean meals. While this process is very effective in inactivating various antinutritional factors, the excessively high operating temperature (somewhere around 150!C) is detrimental to protein nutrition even at the relatively low moisture level used in the process. However, the recently developed double-expander process has been reported to produce full-fat soybean meals (Super Soy) with high lysine availability and low trypsin inhibitor activity.

Other processes used to produce full-fat soybean meals include cooking/autoclaving, micronizing, and roasting. Cooking is a relatively simple and straightforward method. The raw beans are soaked and then boiled at least 30-120 minutes, dried and fed to animals as whole, ground or rolled. Autoclaving is another cooking procedure under steam pressure. These processes are however inefficient and not flexible.

Micronizing is a process of cooking soybeans with the heat generated by vibrating molecules under the influence of infrared rays. Some European countries have used this method primarily to produce human food due to its high investment and operational costs.

Roasting is a process of dry heating soybeans at 110-170!C, depending on the type of equipment used and the desired nutritive value of the full-fat soybean meal. The various types of roasting range from salt bed or heated ceramic tile roasting to common grain dryers and conventional rotary drum type roasters to fluidized bed and hot air roasters. Various types of rotary drum type roasters are popular for full-fat soybean meal processing because of the low investment cost, portability and simplicity of operation. This type roasters are direct fired, and the quality, uniformity, degree of cooking, and color of products can vary greatly. The fluidized bed system utilizes superheated and pressurized air to roast the beans under controlled temperature and residence time. This method is highly efficient, versatile, dependable, uniform, clean, simple and cost effective in roasting full-fat soybean meals.

References
Araba, M. and N. M. Dale, 1990a. Evaluation of protein solubility as an indicator of under processing soybean meal. Poult. Sci. 69: 1749.
Araba, M. and N. M. Dale, 1990b. Evaluation of protein solubility as an indicator of over processing soybean meal. Poult. Sci. 69: 76.
D¡ãOMello, J. P. F. and D. Lewis, 1970. Amino acid interactions in chick nutrition. 3. Independence i amino acid requirements. Br. Poult. Sci. 11:367.
Edmonds, M. S. and D. H. Baker, 1987. Comparative effects of individual amino acid excesses when added to a corn-soybean meat diet: Effects on growth and dietary choice in the chick. J. Anim. Sci. 65: 699.
Krazter, F. H., S. Bersch, P. Vohra and R. A. Ernst, 1990. Chemical and biological evaluation of soybean flakes autoclaved for different durations. Anim. Feed Sci. Technol. 31: 247.
NRC, 1981. Nutrient Requirements of: Goats, 1985. Sheep, Sixth Revised Edition, 1988. Swine, Ninth Revised Edition, 1989. Dairy Cattle, Sixth Revised Edition, 1993, Fish, 1994. Poultry, Ninth Revised Edition, and 1996. Beef Cattle, Seventh Revised Edition, National Research Council, National Academy Press, Washington, D.C.
NRC, 1975. The Effect of Genetic Variance on Nutritional Requirements of Animals. National Research Council, National Academy Press, Washington, D.C.
NRC, 1987. Predicting Feed Intake of Food-producing Animals. National Research Council, National Academy Press, Washington, D.C.
Ueda, H., S. Yabuta, H. O. Yokota, and I. Tasaki, 1981. Involvement of feed intake and feed utilization in the growth retardation of chicks given the excessive amounts of leucine, lysine, phenylalanine or methionine. Nutr. Rep. Int. 24: 135.
Watkins, L. R, 1998. Private communication.
Wright, J, 1998. News Release.

 

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