S OLUBLE S OYBEAN P OLYSACCHARIDES (S OYA F IBER )

4.3.2 S OLUBLE S OYBEAN P OLYSACCHARIDES (S OYA F IBER )

4.3.2.1 Source

Water-soluble soybean polysaccharide (SSPS) or soya fiber is the cell wall material of the cotyledons in soybeans. Soya fiber is also referred to as soybean hemicellulose. Commercial SSPS or soya fiber is extracted from okara, a by-product produced during production of soybean protein isolates and tofu. 82

4.3.2.2 Method of Production

Soybean fiber or SSPS is produced on an industrial scale in Japan by Fuji Oil Co. Ltd. It is extracted from the by-product okara in the production of soy protein isolates

or tofu. The industrial production of soya fiber is similar to the procedure described below: Okara in dilute acid (pH 5, adjusted by hydrochloric acid) is autoclaved at

120°C for 1.5 h. 82,83 After cooling to room temperature, the suspension is centrifuged at 10,000 g for 30 min to remove the residue. The remaining SSPS in the residue is washed with distilled water and centrifuged again. The supernatants are combined and then concentrated and dried. This process gives a yield of about 45% based on the starting okara material.

4.3.2.3 Chemistry and Structural Features

Soybean fiber contains 64 to 80% soluble fiber, 4.8 to 14% proteins, and 6 to 8.6% ash. Highly purified SSPS contains 83.6% carbohydrates, 4.7% proteins, and 5.3% ash. SSPS is composed of galactose (42 to 46%), arabinose (18 to 27%), rhamnose (4.8 to 6.5%), xylose (2.3 to 5.1%), and small amounts of fucose and glucose (1 to 3%), in addition to the presence of galacturonic acid (18 to 24%) 82,83

Soy fiber or SSPS is a highly branched pectic polysaccharide, and the backbone consists of two types of structures: a galacturonan (GN) and a rhamnogalacturonan (RG). 83 The RG structure is comprised of a diglycosyl repeating unit: (1 →4)-α-D-

galacturonic acid-(1 →2)-α-L-rhamnopyranose units with β-D-galactan side chains. The side chains are substituted with L-fucosyl and L-arabinosyl residues, which are linked to the C-4 of the rhamnosyl residues. The degree of polymerization of the side chains is estimated to be 43 to 47, which is longer than those of fruit pectins. SSPS is highly polydispersed in molecular weight. Three to four fractions of molec- ular weight ranging from 4700 to 542,000 Da, based on pullulan standards, were identified by gel permeation chromatography. 83,84

A structure model of SSPS was

proposed by Nakamura and coworkers 83 (Figure 4.3).

4.3.2.4 Functional Properties and Applications

SSPS is a low-viscosity gum and its flow behavior is similar to that of gum arabic (Figure 4.4). It is soluble in cold water and exhibits Newtonian flow behavior at a

Functional Food Carbohydrates

2 Rha 14 GalA 1 4 GalA 1 2 Rha 1 4 GalA 1 (Main back bone)

Gal 6 1 Ara 5 Gal 4 1 Gal 4 (Side chain) 4

Gal 4 1 Gal 4 Side chain q

FIGURE 4.3 Structural features of soluble soybean polysaccharides. (Adapted from Maeda, H., in Handbook of Hydrocolloids, Phillips, G.O. and Williams, P.A., Eds., CRC Press, Boca Raton, FL, 2000, p. 309.)

10 5 Guar gum

SOYAFIBE-S-En100

10 4 HM Pectin

SOYAFIBE-S-DN or 10 3 SOYAFIBE-S-DA100

SOYAFIBE-S-LA200 10 2 sity (mPa/S) co

1 Gum arabic V is 10 10

Concentration (%)

FIGURE 4.4 Comparison of viscosity–concentration relations of soluble soybean polysac- charides (SSPSs) and some commercial gums. (Reproduced from Maeda, H., in Handbook of Hydrocolloids , Phillips, G.O. and Williams, P.A., Eds., CRC Press, Boca Raton, FL, 2000, p. 309.)

concentration as high as 10% (w/w). However, when the concentration is increased to 20% (w/w), it exhibits shear-thinning flow behavior. Unlike pectins, the viscosity

of SSPS solution is not sensitive to salts, including NaCl, CaCl 2 , and KCl. However, the viscosity increases with the increase of sugar concentration (e.g., sucrose). The viscosity of SSPS solution is also pH and temperature sensitive: the viscosity tends to decrease with the decrease of pH and increase of temperature, and the process is reversible. 82

SSPS could be used in the food industry as a stabilizing and thickening agent. For example, SSPS exhibits superior properties regarding volume, color, and crumb grain when added to a Japanese sponge cake at 4%. 82 Addition of 10% SSPS has

Seed Polysaccharide Gums

less effect than wheat bran on color and surface smoothness of Chinese steamed bread, but has a stronger detrimental effect on volume and texture. 82 Recent appli- cations of SSPS include stabilizing milk proteins under acidic conditions. 85 It requires lower SSPS concentrations than pectin to stabilize and disperse protein

particles, and the fine milk protein structures (about 0.6 μm) and prevent agglom- eration. The mechanism for stabilization of milk proteins was attributed to the steric

effect of the thick layer of the neutral sugar side chains; this is different from pectin, which stabilizes milk proteins mainly by electrostatic repulsion. 85 A more detailed application of SSPS in food systems is described by Maeda. 82

4.3.2.5 Physiological Properties and Health Benefits

SSPS is classified as a food ingredient and food additive with no limitation of application in Japan. 82 Recently, the European Commission Health and Consumer

Protection Directorate-General evaluated the safety of soybean hemicellulose (soy fiber) as a food additive and concluded that the use of soybean hemicellulose in

food products at levels ranging from 0.5 to 30 g/kg is acceptable. 86 In a study for supporting a GRAS (generally recognized as safe) notice submission to the U.S. Food and Drug Administration (FDA), SSPS was fed to approximately 6-week-old Sprague Dawley rats at three dosages for 3 months. The animal toxicity test revealed no observed adverse effect at intake levels of 2.43 g/kg of body weight for males

and 2.91 g/kg of body weight for females. 87 Tests on animal models also revealed that SSPS appeared to have favorable effects on the intestinal functions, as it is partially metabolized into organic acid by enteric bacteria and it effectively shortens

the gastrointestinal transit time in rats. 87 Ingestion of SSPS also could improve postgastrectomy osteopenia and partially prevent calcium malabsorption in rats. A patented method describes the use of SSPS as a cholesterol oxide-adsorbing agent. 88 All these data demonstrate that SSPS is a safe and effective dietary fiber in addition to its functional properties.