Soybean Amino Acids in Health, Genetics, and Evaluation

Abstract

Soybean is an important source of protein and amino acids for humans and livestock because of its well-balanced amino acid profile. This chapter outlines the strengths and weaknesses of soybean as a complete amino acid source as well as the relative importance of individual amino acids. Special attention is paid to the sulfur-containing amino acids, methionine and cysteine. Breeding and genetic engineering efforts are summarized to highlight previous accomplishments in amino acid improvement and potential avenues for future research. Agronomic properties and processing methods that affect amino acid levels in soybean food and feed are also explained. A brief introduction into current amino acid evaluation techniques is provided. By understanding the complexities of amino acids in soybean, protein quality for humans and livestock can be maximized.

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Provisional chapter
Soybean Amino Acids in Health, Genetics, and
Evaluation
William Monte Singer, Bo Zhang,
M.A. Rouf Mian and Haibo Huang
Additional information is available at the end of the chapter
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
William MonteSinger, BoZhang, M.A. RoufMian
and HaiboHuang
Additional information is available at the end of the chapter
Abstract
Soybean is an important source of protein and amino acids for humans and livestock
because of its well-balanced amino acid prole. This chapter outlines the strengths and
weaknesses of soybean as a complete amino acid source as well as the relative importance
of individual amino acids. Special aention is paid to the sulfur-containing amino acids,
methionine and cysteine. Breeding and genetic engineering eorts are summarized to
highlight previous accomplishments in amino acid improvement and potential avenues
for future research. Agronomic properties and processing methods that aect amino
acid levels in soybean food and feed are also explained. A brief introduction into cur-
rent amino acid evaluation techniques is provided. By understanding the complexities
of amino acids in soybean, protein quality for humans and livestock can be maximized.
Keywords: essential and nonessential amino acids, soybean meal, methionine, cysteine,
sulfur-containing amino acids
1. Introduction
Soybean is one of the world’s most economically and nutritionally important crops. In 2018,
soybean were 61% of international oilseed production with 397.9 tons harvested worldwide
[1]. The United States and Brazil were the largest producers at 4545 and 4299 million bushels,
respectively, with China being the largest importer of U.S. whole soybeans valued over $3 bil-
lion U.S. [2, 3]. Soybean products, namely meal and oil, are popular in a myriad of industries
for their versatility and utility. Soybean oil provides the most versatility with uses in fuel,
solvents, candles, cosmetics, construction, and foam. However, soybean meal is the driving
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.

factor for 70% of the plant’s value with 97% of all U.S. soybean meal being used for animal
feed [4]. As such, an enormous portion of soybean’s importance lies with its nutritional capa-
bilities for livestock and humans.
Nutritionally speaking, soybeans are a highly valued protein source. Proteins are a crucial
macromolecule needed in the diets of human and livestock. However, the true signicance of
soybean protein is due to its well-balanced amino acid prole that aligns with dietary needs
of humans and animals [5]. Amino acids are the functional subunits of proteins that, when
linked together in dierent orders, generate the variety of proteins critical to life. Amino acids
are also important intermediates for many biosynthesis pathways [6]. Deciencies in single
or multiple amino acids can negatively impact an individual’s growth and development [7,
8]. Intriguingly, an excess of certain amino acids has also been shown to worsen feed intake,
nitrogen eciency, and growth rate in livestock [9–11]. The importance of amino acid levels
on human health has also been well documented [12, 13].
Amino acids are characterized by having amine (−NH2) and carboxyl (−COOH) functional
groups as well as a “R-group” that is unique to each amino acid [14]. Amino acids are abun-
dant in both proteinogenic (protein-incorporated) and non-proteinogenic forms [15]. The 20
common, proteinogenic amino acids are generally the focus of research in soybeans as they
are the dening nutritive feature. Of those twenty, nine amino acids are essential for humans
to consume. Livestock usually require these same amino acids from feed and might require
others because of their biological systems [16]. Soybeans contain some level of all nine essential
amino acids which creates a suitable nutritional foundation for livestock feed and human food.
2. Essential amino acids
Essential amino acids are ones that living organisms are unable to biosynthesize themselves
and must obtain from their food source [7, 12, 16]. Therefore, in this term, “essential” refers to
the amino acid requirements in dietary ingredients. The nine standard essential amino acids
for humans present in soybean are: histidine, isoleucine, leucine, lysine, methionine, phenyl-
alanine, threonine, tryptophan, and valine [17]. Arginine is regularly considered an essential
amino acid for sh, poultry and sometimes swine due to absent or decient urea cycles [9,
16]. Poultry and reptiles also require dietary glycine because of diering waste excretion
pathways [16]. While crude protein content is normally recognized as the driving nutritional
factor for soybean meal, these essential amino acids provide true utility.
It has long been recommended that protein quality is based upon essential amino acid content.
However, for many reasons, animal feed and human food markets have only recently begun
assessing accordingly [5, 18, 19]. Equipment required for accurate amino acid measurement
and the diversity of markets for amino acids makes it dicult for supply chain evaluators like
elevator operators to appraise amino acid content on site. To some degree, the well-balanced
soybean amino acid prole also devalues the need to measure individual amino acid levels.
Since all essential amino acids are present, less aention is paid to decient amino acids such
as methionine and tryptophan [17, 20].
Soybean for Human Consumption and Animal Feed2

Deciencies in soybean’s essential amino acid prole has led to a large section of the livestock
industry focusing on feed mixing and supplementation. Rationing with other feed sources
such as cereal grain and synthetic amino acid augmentation can eectively resolve the issue.
Although, this comes with economic and environmental problems. Supplementing amino
acids adds costs to farmers. For example, the average cost for amino acids supplementation
for dairy farmers is 20 cents per head per day [21]. Maximizing crude protein for a growth
limiting factor also negatively impacts livestock nitrogen-use-eciency and environmental
nitrogen outputs [8, 22]. Synthetic amino acid production can produce hazardous environ-
mental waste and synthetic methionine, the most limiting soybean amino acid for poultry,
has also been banned for organic poultry production [20, 23]. Movement towards sustainable
agriculture will pressure the feed industry to alter how soybean meal is enhanced for essential
amino acid livestock maximization. Furthermore, the increasing popularity of meat-less diets
in humans will create new markets for soybean’s well-balanced amino acid prole.
3. Nonessential amino acids
Nonessential amino acids should not be misconstrued as unimportant amino acids. Of the 20
proteinogenic amino acids, those considered nonessential are still necessary for living organ-
isms. Healthy organisms are able to biosynthesize them and are not required from food and
feed consumption. The 20 standard nonessential amino acids for humans found in soybean
are: arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine,
proline, serine, and tyrosine [24–26]. As previously mentioned, the necessity of amino acids
such as arginine and glycine can dier amongst species. Some nonessential amino acids are
also aected by the presence and amounts of essential amino acids.
Cysteine not provided from food consumption is directly biosynthesized from methionine
via trans-sulfuration [16, 27–30]. Consequently, if cysteine is not provided in the diet, then
enough methionine must be provided to compensate for both amino acid needs. For that
reason, feed research for poultry occasionally measures methionine and cysteine jointly [10,
31, 32]. Tyrosine is also directly formed from phenylalanine via hydroxylation [16, 29, 33].
Other amino acids like arginine, glycine, and proline can be required from the diet when
animals are young, old, sick, or otherwise decient in body protein regulation. As human
and livestock diets become more sustainably plant-based, it will become more important to
evaluate nonessential amino acids, specically the ones immediately aected by essential
amino acids.
4. Proteinogenic sulfur-containing amino acids
The two proteinogenic sulfur-containing amino acids, methionine and cysteine, are critical to
evaluating soybean meal as food and feed. While present in soybean, methionine and cysteine
levels are both inadequate for consumer needs [8, 20, 24]. Similar to research determinations,
the nutritional requirements for methionine and cysteine intake are often grouped together
Soybean Amino Acids in Health, Genetics, and Evaluation 3

as overall sulfur consumption from protein. Adult humans are recommended to intake 910–
1120 mg methionine and cysteine (based on body weight) per day [13]. For livestock, sulfur
amino acids recommendations can vary based on species, age, end-use, and diet formulation.
The importance of dietary sulfur amino acids for livestock is greatly emphasized in literature,
especially with soybean meal as base feed [8, 20, 34, 35].
Methionine and cysteine are vital to biological functions because of the sulfur contained in
their R-groups and versatility in macromolecule synthesis. Methionine is well-known for being
the typical initiating amino acid for protein synthesis and has hydrophobic properties when
incorporated into proteins [36]. These hydrophobic properties usually result in methionine
incorporation within the core of proteins. However, certain proteins have surface-exposed
methionine susceptible to oxidation that is associated with age-related disease [30, 37, 38].
S-adenosylmethionine, a methionine metabolism intermediate, is widely-used with functions
in methylation as well as amine, methylene, and sulfur atom donation [30, 39, 40]. Methionine
is especially important for poultry production as birds have exceedingly high sulfur amino
acid requirements, and low methionine levels can negatively aect growth rate, carcass yield,
fat content, and disease immunity [41–43]. Cysteine’s ability to form disulde bonds makes
it incredibly important to tertiary protein structure and can occur with and without enzyme
interactions [30, 44]. Cysteine is involved with keratin and feather production in poultry and
deciencies have been correlated to poor breast muscle development [45, 46]. Swine also need
higher amounts of cysteine as they age to compensate for body maintenance [29, 47].
5. Breeding eorts
Soybean has an extensive cultivated history dating back thousands of years to its country
of origin, China. Glycine max, the contemporary species of soybean, was domesticated from
the wild species Glycine soja and has been continually been improved through selective and
molecular breeding [48–50]. Once harvesting traits such as seed shaer and lodging were
improved in the late 1930s to make soybean a competitive row crop, other cultivar improve-
ments became a valuable research goal [49]. Current breeding programs tend to focus on traits
such as yield, disease resistance, abiotic stress tolerance, and seed composition. Seed compo-
sition improvements include protein and oil content, fay acid levels, anti-nutritional factors,
isoavones, and amino acids proles. Before 1972, there had been zero reported research
for improvement of soybean amino acid proles, rather with emphasis on overall protein
content [51]. Modern breeders are also inclined to concentrate eorts on protein content and
consider amino acid levels an afterthought [52]. TN04-5321 is the only released germplasm
in the United States that maintains yield and protein content while improving amino acid
balance by increasing methionine and cysteine to levels recommended livestock needs [53].
The major soybean storage proteins are 11S (glycinin) and 7S (conglycinin) and provide the
bulk of amino acids with limited non-proteinogenic amino acids in seed [54–56]. By increasing
11S and 7S quantity, more protein can ultimately be present in food and feed. Overemphasis on
crude protein content can have negative ramications on overall protein quality, specically
Soybean for Human Consumption and Animal Feed4

decient amino acids. While an increase in protein content would theoretically entail an
increase in amino acids including methionine and cysteine [57], the opposite eect has been
more notable [58, 59]. Molecular breeding techniques have recently improved the under-
standing of amino acid genomic regulation. Multiple quantitative trait loci (QTL) studies have
been performed to identify genomic regions that control amino acids levels in soybean seed
[60–64]. A myriad of QTL’s were found to create amino acid phenotypic variation. Individual
amino acids had reoccurring or proximal QTL’s discovered such as Sa 518 [60], ss107913002
[61], and BARC-048619 [62] for glycine and threonine. QTL’s for methionine and cysteine
were also discovered which could lead to valuable improvements for soybean livestock feed
[60–64]. Other genomic studies such as genome-wide association studies (GWAS) and genetic
diversity analyses would further improve genetic understanding.
6. Genetic engineering
Genetic engineering experiments such as genetically modied organisms (GMOs) and gene
editing are also promising avenues for improving amino acid proles of soybeans. Compared
to conventionally bred varieties, transgenic soybeans face additional adversity from registra-
tion requirements and public opinion. Transgenic eorts generally have one of three targets:
magnifying biosynthesis genes, adjusting biosynthesis regulation, and modifying storage
proteins. The earliest example would be a Brazil nut gene transfer in 1992. This successfully
increased protein content and methionine biosynthesis, however a major food allergen was
also transferred making commercialization impossible [65]. Expressing zein proteins from
corn has also been shown to increase sulfur-containing amino acids levels in soybean [66, 67].
Altering biosynthesis feedback regulation amplied both non-proteinogenic and proteogenic
lysine by circumventing normal enzymatic pathways [68]. Tryptophan in soybean also exhib-
ited increased non-proteinogenic levels when a feedback-insensitive enzyme was transferred
[25]. While soybean is not decient in lysine or tryptophan, corn is decient in both. By increas-
ing lysine and tryptophan concentrations, soybean becomes an even more useful feed additive
to corn rations. Even with limited research on modifying overall amino acids proles in soy-
beans, modifying 11S and 7S storage proteins ratios [69] or silencing their expression entirely
[70] has displayed increased amino acids levels. Similarly, a study using irradiated mutant soy-
beans lacking storage proteins as breeding parents demonstrated increased non-proteinogenic
amino acids contents [54]. In addition, further research should be conducted to determine the
bioavailability and digestibility of increased non-proteinogenic amino acids in soybean.
7. Agronomic relations
Amino acids concentrations in soybean are not only aected by their genetic potential.
Agronomic properties greatly impact the nal levels of amino acids. Agronomy encompasses
all aspects of crop production including environmental eects, climatic variables, and abiotic
factors. Perhaps the most considered agronomic factor is soil nutrient availability. Insucient
Soybean Amino Acids in Health, Genetics, and Evaluation 5

soil nutrient levels of nitrogen, potassium, phosphorous, sulfur, calcium, and magnesium
create poor amino acid proles in soybean plants [71]. Increased phosphorous rates have
been shown to increase the percentage of methionine and tryptophan in seed but had no
eect on protein content percentage [72]. Applications of sulfur, phosphorous, and nitrogen
(individual and combined) produced a variety of dierent methionine and cysteine seed con-
centrations [73]. Sulfur deciencies were also shown to inhibit the production of 11S proteins
while almost eliminating methionine and cysteine in 7S proteins [74]. It is becoming more
popular to also apply biological substances such as amino acids to plants through foliar and
seed application. Amino acid uptake by soybean and wheat have been proven, and improved
soybean growth rates and antioxidant eects have also occurred [75, 76]. Further research
should be conducted to determine if biofortication solutions are possible through amino
acid application.
Amino acid variation has also been shown to occur across environments [24, 59]. Specic
correlations have emerged in response to temperature, solar radiation, and rainfall. One
study shows that increased temperature leads to increased concentrations of all proteino-
genic amino acids [77], while another concludes that only methionine and cysteine increase
alongside temperature [78]. Increased solar radiation and greater available water appeared to
have a negative relationship with amino acid content [77]. These favorable conditions would
increase yield which has been shown to have a negative correlation with overall protein con-
tent [57]. The multitude of agronomic factors that aect amino acid proles in soybean make
it exceedingly important to compensate for variables when researching.
8. Processing impacts
The diversity in food, feed, and industrial used for soybean require the whole seed or seed
components to be processed. Processing can aect the nutritional value of soybean protein
and presence of amino acids in food and feed. Processing procedures can either separate seed
components for dierent purposes or convert the entire seed into a product (usually human
food). Some human soy foods such as edamame and soybean sprouts need lile to no process-
ing. Others including soymilk, tofu, nao, and soy sauce involve more processing. Soymilk
and tofu processing are interconnected. Soymilk is a water-extract of whole or crushed soy-
beans that is coagulated and pressed into tofu [48]. While not all seed proteins convert into
protein in tofu, 11S/7S storage protein ratios have been shown to be both positively and nega-
tively correlated with tofu hardness [79, 80]. Nao is a soy food created by fermenting whole
soybeans with Bacillus subtilis. Fermentation time aects nal amino acid concentrations, and
proper fermentation length could potentially increase nutritional values [81]. Soy sauce is
produced by traditional and commercial methods, but both are based around whole seed or
meal fermentation with Aspergillus sp. However, commercial methods have a lower amino
acid to nitrogen ratio [48].
Soybean meal processing also impacts the level of amino acids in livestock feed. The rst step
in soybean meal processing is essentially separating protein from oil. A variety of methods
Soybean for Human Consumption and Animal Feed6

exist including solvent extraction, screw pressing, and extruding [48, 82, 83]. All three pro-
cesses have three nal products: oil, meal (usually toasted to lessen anti-nutritional factors),
and hulls. Over processing of solvent extracted soymeal has been shown to decrease lysine,
cysteine, and arginine levels [84, 85]. Protein solubility and dispersibility measurements may
be a useful indicator of over processing [86, 87]. Soybean hulls are sometimes added to live-
stock feed for additional ber, however an increase in hull/meal ratios decrease the digest-
ibility of amino acids [88]. While soybean is renowned for its protein and amino acid content,
actual nutritional values can be decreased through certain processing methods.
9. Evaluation methods
All previously mentioned aspects of soybean production in regard to amino acid levels and
human and animal nutrition depend on a single common denominator: amino acid quanti-
cation. Amino acids must be reliably, eectively, and accurately identied, measured and
evaluated. A Google Scholar search of “amino acid analysis” will display over 1 million results.
Several reviews have been published regarding the development of amino acid analysis [89–91].
In general, contemporary analysis of amino acids from any source will be performed by chroma-
tography or near-infrared reectance spectroscopy. Chromatography is the common method
with specic techniques including ion exchange chromatography (IEC), high-performance
liquid chromatography (HPLC), and gas chromatography (GC). HPLC is the more validated
method for soybean amino acid analysis. It is more ecient than IEC, and it does not require the
transformation into volatiles like GC [91–94]. Near-infrared reectance spectroscopy (NIRS) is a
more recent addition to amino acid analysis, and it has the potential to drastically improve the
eciency in soybean feed evaluation [19]. The inability to actually measure amino acid levels is
main hindrance for NIRS amino acid analysis. NIRS methods must be developed from a calibra-
tion set of raw data (often from HPLC) [95–97]. Nonetheless, eciency improvements should
persuade researchers to continually explore future NIRS amino acid analysis applications.
10. Conclusion
Soybean is a valuable source of protein and amino acids for humans and livestock. Soybean’s
well-balanced amino acid prole provides all essential amino acids as well as most nones-
sential. However, there is much room for nutritional improvement. Proteinogenic sulfur-
containing amino acids, methionine and cysteine, are decient in soybean and are especially
needed in livestock rations. Increased levels of these amino acids would augment soybean
meal value and lessen the need for synthetic amino acid supplements. Breeding eorts have
made lile progress in adjusting amino acid proles thus far, however signicant develop-
ments in understanding genomic control regions promise future success. Genetic engineering
eorts have shown promising amino acid improvements, but regulations and public opinion
made commercialization dicult. New gene-editing technology could be the key to unlock-
ing true nutritional improvement.
Soybean Amino Acids in Health, Genetics, and Evaluation 7

Agronomic properties and processing methods both impact the nal quantities of amino acids
available to humans and livestock. Understanding these impacts are essential to improve the
nutritional quality of soybeans. Amino acid evaluation through HPLC provides reliable and
ecient quantication, yet even quicker measurements are possible through NIRS. As the
world’s population continues to grow, soybeans will be essential to both human and livestock
for amino acid requirements. Wholesome approaches that understand the complexities of
amino acids in soybean will be required to maximize overall success and feed the world with
balance soy proteins.
Acknowledgements
The authors would like to acknowledge Virginia Polytechnic Institute and State University’s
Open Access Subvention Fund for funding the publishing fee.
Conict of interest
The authors declare there is no conict of interest.
Author details
William Monte Singer1*, Bo Zhang1, M.A. Rouf Mian2 and Haibo Huang3
*Address all correspondence to: wilmsing@vt.edu
1 School of Plant and Environmental Sciences, Virginia Polytechnic and State Institution,
Blacksburg, Virginia, USA
2 United States Department of Agriculture, Agricultural Research Service, Raleigh,
North Carolina
3 Department of Food Science and Technology, Virginia Polytechnic Institute and State
University, Blacksburg, Virginia, USA
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