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., 1991).

            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 studied.

            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 value?

            Do hulls have any potential as nutraceuticals or as industrial raw material?

References

            Abel-Caines, S. F., R. J. Grant, and S. G. Haddad.1997. Whole cottonseeds or a combination of soybeans and soybean hulls in the diets of lactating dairy cows. J. Dairy Sci. 80:1353-1357.

            Alvarez, P. J. C., F. C. Krzyzanowski, J. M. G. Mandarino, and J. B. França-Neto. 1997. Relationship between soybean seed coat lignin content and resistance to mechanical damage. Seed Sci. Tech. 25:209-214.

            Cole, J. T.,  G. C. Fahey, Jr., N. R. Merchen, A. R. Patil, S. M. Murray, H. S. Hussein, and J. L. Brent Jr. 1999. Soybean hulls as a dietary fiber source for dogs. J. Anim. Sci. 77:917-924.

            Enayati, Nader, and Satish J. Parulekar. 1995. Enzymatic saccharification of soybean hull-based materials. Biotechnol. Prog. 11:708-711.

            Gizjen, Mark, Robert van Huystee, and Richard I. Buzzell. 1993. Soybean seed coat peroxidase. A comparison of high-activity and low-activity genotypes. Plant Physiol. 103:1061-1066.

            Gizjen, Mark, Kuflom Kuflu, Dinah Qutob, and Jacqueline T. Chernys. 2001. A class I chitinase from soybean seed coat. J. Exp. Bot. 52:2283-2289.

            Greenwood, R. H., and E. C. Titgemeyer. 2000. Limiting amino acids for growing Holstein steers limit-fed soybean hull-based diets. J. Anim. Sci. 78:1997-2004.

            Kornegay, E. T. 1981. Soybean hull digestibility by sows and feeding value for growing-finishing swine. J. Anim. Sci. 53:138-145.

            Löest, C. A., E. C. Titgemeyer, J. S. Drouillard, D. A. Blasi, and D. J. Bindel. 2001. Soybean hulls as a primary ingredient in forage-free diets for limit-fed growing cattle. J. Anim. Sci. 79:766-774.

            Mullin, W. John, and Weili Xu. 2001. Study of soybean seed coat components and their relationship to water absorption. J. Agric. Food Chem. 49:5331-5335.

            Murray-Kolb, Laura E., Ross Welch, Elizabeth C. Theil, and John L. Beard. 2003. Women with low iron stores absorb iron from soybean. Am. J. Clin. Nutr. 77:180-184.

            Sessa, D. J., and W. J. Wolf. 2001. Bowman-Birk inhibitors in soybean seed coats. Industrial Crops and Products 14:73-83.

            Shinnick, F. L., R. Mathews, and S. Ink.1991. Serum cholesterol reduction by oats and other fiber sources. Cereal Foods World 36:815-821.

            Trater, A. M., E. C. Titgemeyer, C. A. Löest, and B. D. Lambert. 2001. Effects of supplemental alfalfa hay on the digestion of soybean hull-based diets by cattle. J. Anim. Sci. 79:1346-1351.

            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.