Soybean Hulls - a brief review of literature
Soybean seed coats, or hulls, represent about 8 to 10% of the weight of soybean grain (Sessa and
Wolf, 2001; R. Glahn, personal communication). Mullin and Xu (2001) reported the following major
constituents of hulls, on a dry weight basis:
cellulose 14 to 25 g/100g
hemicellulose 14 to 20
pectin 10 to 12
protein 9 to 12
uronic acid 7 to 11
ash 4 to 5
lignin 3 to 4
Thus soybean hulls, though high in fiber, are a significant source of protein, comparable to corn grain in the
amount of crude protein.
Hulls as feed
The primary use for soybean hulls is feed. Hulls are routinely removed during crushing of soybeans
but are returned to the processing stream to be added to the meal fraction. Hulls are withheld from the meal
only if their inclusion would cause the product to exceed the limit of allowable fiber. Excess hulls may be
sold as feedstuffs or discarded as waste. Removal of hulls costs processors 5 to 10 cents per bushel.
Soybean hulls can be used as the primary dietary ingredient for cattle. Their small particle size and
high specific gravity, however, may reduce digestibility through rapid passage from the rumen (Löest et al.,
2001). Addition of alfalfa forage to soybean hulls improved digestibility but did not alleviate the problem
of rapid passage (Trater et al., 2001). Methionine has been identified as the first-limiting amino acid for
steers fed hull-based diets (Greenwood and Titgemeyer, 2000). Soybean hulls, fed in combination with
whole, raw soybeans, were as valuable as whole cottonseeds as a ration for lactating dairy cows (Abel-
Caines et al., 1997).
Soybean hulls can be an effective source of fiber in diets for dogs (Cole et al., 1999). Sows can be
successfully fed diets containing up to 15% soybean hulls (Kornegay, 1981).
Hulls as nutraceuticals
In developed countries today there is widespread interest in the health benefits of foods. Dietary
fiber has the potential to reduce serum cholesterol levels in humans. Soybean hulls are low in soluble fiber,
however, and have had only modest and inconsistent effects on serum cholesterol levels (Shinnick et al.,
Soybean Bowman-Birk trypsin inhibitor is an anti-cancer agent that can be obtained from seedcoats
of soybean cultivar Williams (Sessa and Wolf, 2001).
Iron deficiency is a serious problem worldwide, particularly for women and children. Soybean grain
has been identified as a good source of available iron (Murray-Kolb et al., 2003). Soybean hulls have less
phytate than the rest of the seed. The presence of phytate reduces availability of iron in foods. Hulls of the
soybean cultivar „Tokyo‟ are believed to have more iron in available form, and hulls of this cultivar are
high in digestible iron (R. Glahn, personal communication).
Although fiber is believed to reduce the availability of zinc and calcium, rat feeding trials showed
no effect of inclusion of soybean hulls on the availability of either mineral (Weingartner et al., 1979).
Hulls as industrial raw material
Soybean hulls of some cultivars have a high peroxidase activity, 100-fold greater than that of
cultivars containing low activity. The difference in activity is conditioned by a single gene pair, designated
Ep/ep (Gizjen et al., 1993). Hulls from cultivars with high activity were used in the 1990s as a commercial
source of peroxidase for industrial products.
Protein extracts from soybean hulls contained a chitinase, expressed late in seed development. This
compound is believed to serve a defensive function, perhaps in protecting the seed from fungal attack
(Gizjen et al., 2001).
Enayati and Parulekar (1995) studied the enzymatic degradation of soy hulls and reported they were
highly biodegradable, a desirable property in some industrial applications.
Genetics of hull characteristics
There has been very little effort to study genetic variability for hull characteristics. Differences in
peroxidase activity (Gizjen et al., 1993) were noted earlier. In the 1920s and 1930s USDA plant explorers
collecting soybean germplasm in Eastern Asia were often told that cultivars with “thin hulls” were
desirable for certain food uses. Potential varietal differences for such hull characteristics have not been
Alvarez et al. (1997) reported significant differences among Brazilian soybean cultivars for lignin
content of the seed coat, ranging from 4.2 to 6.2%. They reported a high correlation between lignin content
and resistance to mechanical damage. In their study, the cultivars were replicated in the field, but were
produced in a single location for a single year. Thus, the possibility of cultivar × environment interaction
for lignin content could not be examined.
Cole et al. (1999), in their study of hulls as a dietary fiber source for dogs, acquired nine samples of
soybean hulls from around the U.S. They noted wide variation among samples for total dietary fiber (64 to
81%), the ratio of insoluble:soluble fiber (5:1 to 15:1), and crude protein content (9 to 19%). It is not
known how much of this variation may be genetic.
Mullin and Wu (2001) analyzed seeds and hulls of six soybean cultivars and observed differences in
composition. They did not replicate the cultivars, however, so it is not possible to interpret their data as
demonstrating genetic differences.
Additional knowledge of soybean hulls, particularly genetic variation, would be very helpful in
efforts to improve the quality of U.S. soybeans for feed, food, and industrial purposes.
Potential research questions
What percentage of the fiber in the grain occurs in the hull?
What types of fibers are present?
Is there useful genetic variation for the physical and chemical characteristics of the hull? Examples
of characteristics include hull percentage of total grain weight, protein content of the hull, solubility of
fiber, availability of iron or other minerals, Bowman-Birk inhibitor.
Could something (e.g., protein, sugar, vitamins) be genetically added to the hull to increase its
Do hulls have any potential as nutraceuticals or as industrial raw material?
Abel-Caines, S. F., R. J. Grant, and S. G. soybean hull-based diets. J. Anim. Sci. 78:1997-2004.
Haddad.1997. Whole cottonseeds or a combination of Kornegay, E. T. 1981. Soybean hull digestibility by
soybeans and soybean hulls in the diets of lactating dairy sows and feeding value for growing-finishing swine. J. Anim.
cows. J. Dairy Sci. 80:1353-1357. Sci. 53:138-145.
Alvarez, P. J. C., F. C. Krzyzanowski, J. M. G. Löest, C. A., E. C. Titgemeyer, J. S. Drouillard, D. A.
Mandarino, and J. B. França-Neto. 1997. Relationship Blasi, and D. J. Bindel. 2001. Soybean hulls as a primary
between soybean seed coat lignin content and resistance to ingredient in forage-free diets for limit-fed growing cattle. J.
mechanical damage. Seed Sci. Tech. 25:209-214. Anim. Sci. 79:766-774.
Cole, J. T., G. C. Fahey, Jr., N. R. Merchen, A. R. Mullin, W. John, and Weili Xu. 2001. Study of
Patil, S. M. Murray, H. S. Hussein, and J. L. Brent Jr. 1999. soybean seed coat components and their relationship to water
Soybean hulls as a dietary fiber source for dogs. J. Anim. Sci. absorption. J. Agric. Food Chem. 49:5331-5335.
77:917-924. Murray-Kolb, Laura E., Ross Welch, Elizabeth C.
Enayati, Nader, and Satish J. Parulekar. 1995. Theil, and John L. Beard. 2003. Women with low iron stores
Enzymatic saccharification of soybean hull-based materials. absorb iron from soybean. Am. J. Clin. Nutr. 77:180-184.
Biotechnol. Prog. 11:708-711.
Gizjen, Mark, Robert van Huystee, and Richard I. Sessa, D. J., and W. J. Wolf. 2001. Bowman-Birk
Buzzell. 1993. Soybean seed coat peroxidase. A comparison inhibitors in soybean seed coats. Industrial Crops and
of high-activity and low-activity genotypes. Plant Physiol. Products 14:73-83.
103:1061-1066. Shinnick, F. L., R. Mathews, and S. Ink.1991. Serum
Gizjen, Mark, Kuflom Kuflu, Dinah Qutob, and cholesterol reduction by oats and other fiber sources. Cereal
Jacqueline T. Chernys. 2001. A class I chitinase from Foods World 36:815-821.
soybean seed coat. J. Exp. Bot. 52:2283-2289. Trater, A. M., E. C. Titgemeyer, C. A. Löest, and B.
Greenwood, R. H., and E. C. Titgemeyer. 2000. D. Lambert. 2001. Effects of supplemental alfalfa hay on the
Limiting amino acids for growing Holstein steers limit-fed digestion of soybean hull-based diets by cattle. J. Anim. Sci.
Weingartner, K. E., J. W. Erdman, Jr., H. M. Parker,
and R. M. Forbes.1979. Effect of soybean hull upon the
bioavailability of zinc and calcium from soy flour-based
diets. Nutr. Rep. Int. 19:223-231.