Effects of Extrusion on Dietary Fiber and Isoflavone Contents of Wheat
Extrudates Enriched with Wet Okara
V. E. A. Rinaldi,1 P. K. W. Ng,1,2 and M. R. Bennink1
Cereal Chem. 77(2):237–240
Okara is the residue left after soymilk or tofu production. In North
analyzed. The radial expansion ratio decreased as fiber content increased.
America, okara is used either as animal feed, fertilizer, or landfill. The
On the other hand, both bulk density and breaking strength increased as
purpose of this study was to use wet okara to produce and enrich
fiber content increased. Combining okara with soft wheat flour resulted
extruded cereal products and to study the effects of extrusion on the
in increased protein, dietary fiber, and isoflavone contents compared with
dietary fiber and isoflavone contents. Wet okara was combined with soft
soft wheat flour alone. Extrusion of the formulations resulted in decreased
wheat flour to produce two different formulations (33.3 and 40% okara)
insoluble fiber (?25.5%) and increased soluble fiber (?150%) contents of
and extruded using four combinations of two screw configurations and
extrudates. Extrusion decreased the total detectable isoflavones (?20%)
two temperature profiles. Various physicochemical properties, dietary fiber
and altered the distribution of the six detected isoflavones.
by enzymatic-gravimetric method, and isoflavone content by HPLC were
Soybeans have grown in popularity in recent years due to their
okara and flour were weighed in the appropriate proportions and
many attributes and versatility. The most notable attributes of soy-
mixed for 3 min with a mixer (A-200, Hobart Mfg. Co., Troy, OH).
beans are their oil and protein contents, however they are also a
The materials were mixed in order to develop a flow-like consis-
good source of dietary fiber and isoflavones.
tency that was imperative for delivering a consistent amount of
Throughout their existence, soybeans have been used to make
material per unit time into the barrel of the extruder. Extrusion was
many food products. Soy foods are typically divided into two
performed using a corotating and intermeshing twin-screw extruder
categories: nonfermented (soy flour, tofu, soymilk, and okara) and
(model MP19TC-25, A PV Baker, Grand Rapids, MI), with a 19-mm
fermented (tempeh, miso, and soy sauce) (Golbitz 1995). Two
barrel diameter and 25:1 length to diameter ratio. The materials
popular products in the United States are tofu and soymilk. The
were fed into the barrel of the extruder with a twin-screw volu-
production of each of these results in a by-product called okara
metric feeder (K2M, K-Tron Corp., Pittman, NJ). The die used for
(soy pulp). In the United States, okara is typically discarded but it
this study had a single circular orifice with a 3-mm diameter opening.
may be used as animal feed or fertilizer. In Japan, however, it is
Two screw configurations (Table II), low shear (LS) and high
used as a food source (e.g., okara tempeh). In any country, dis-
shear (HS), and two temperature profiles, low temperature (LT;
carding okara as waste is potentially an environmental problem
40, 60, 125, 135, and 145°C from feed port to die exit) and high
because okara is highly susceptible to putrefaction. Okara also has
temperature (HT; 40, 60, 130, 155, and 170°C), were combined to
a high moisture content (?80%), making it difficult to handle and
form four extrusion conditions (LS-LT, LS-HT, HS-LT, and HS-HT)
too expensive to dry by conventional means.
for this study. Each formulation was extruded in two replicates at
Okara contains >20% protein and >50% dietary fiber (Watanabe
each of the four possible extrusion conditions. The replicates were
and Kishi 1984). Wang and Murphy (1996) reported that okara con-
run on different days. The screw speed was held constant at 100 rpm
tains ?10% isoflavones, the much sought after phytochemicals
and the feed rates for samples A and B were 1.95 and 3.71 kg/hr,
present in raw soybeans. In light of these attributes, okara is a suit-
respectively. The extrusion conditions were chosen after a series
able candidate for nutritional enrichment for cereal-based products.
of preliminary studies intended to produce extrudates similar to
Finding convenient ways to incorporate okara into food could
commercial extruded high-fiber breakfast cereal products. The
eliminate a possible source of pollution and add economic value to
extrusion process used for this study was unique in that no water
this currently valueless product. Extrusion could provide a conven-
was injected into the barrel of the extruder in contrast to traditional
ient and relatively low cost way to incorporate the okara into food
extrusion processes. The only source of water for the extrusion
products. The extruder could serve as a cooker, drier, and sterilizer
process was the existing water in the samples, derived mainly from
for okara-enriched products.
the wet okara.
The purpose of this study was to use wet okara to produce ex-
Samples (2 kg) were collected and dried in an air oven (45°C)
truded enriched cereal products and to determine the effects on
overnight. After drying, 1 kg of extrudate was ground using a
dietary fiber and isoflavone contents of okara-enriched extruded
cyclone mill (Udy Corp., Fort Collins, CO) with a 1.00-mm mesh
cereal products due to extrusion.
screen. The remaining 1 kg was left unground. Both ground and un-
ground samples were stored frozen (–10°C) until analyzed.
MATERIALS AND METHODS
Measurement of Physicochemical Properties
Wet okara, derived from the production of soymilk, was obtained
Proximate analyses were performed using AACC Approved
from American Soy Products Inc. (Saline, MI). Soft wheat flour
Methods (AACC 1995), with the exception of the total fat analysis.
was obtained from the King Milling Co. (Lowell, MI). Both the
Total fat analysis was based on the procedure described by Foster
okara and the flour were stored frozen (–10°C) until needed.
and Gonzales (1992) using an extraction unit (Soxtec HT 1043,
Wet okara (thawed) and soft wheat flour were combined to form
Tecator, Hoganas, Sweden) with modifications. The samples were
two workable formulations designated A and B (Table I). The wet
not weighed into the extraction thimbles and mixed with sand. They
were weighed onto 11.0-cm diameter Whatman No. 1 filter papers
1 Department of Food Science and Human Nutrition, Michigan State University,
and placed into the extraction thimbles.
East Lansing, MI 48824-1224.
Radial expansion ratio (Harper 1981), bulk density (Park et al
2 Corresponding author. E-mail: firstname.lastname@example.org
1993), and breaking strength were analyzed for all unground extru-
date samples. Breaking strength was determined with a texture
Publication no. C-2000-0212-06R.
© 2000 American Association of Cereal Chemists, Inc.
analyzer (TA-XT2, Stable Micro Systems, Surry, England) with a
Vol. 77, No. 2, 2000 237
1.27-cm diameter stainless steel sphere as the crushing implement.
were performed and Fisher’s least significant difference (LSD)
Extrudate pieces from a sample were placed into a cylinder with a
was used for multiple means comparison.
1.5-cm diameter opening and a 10-cm depth. Sample weight was
recorded, and the sample was then crushed a distance of 1.0 cm at
RESULTS AND DISCUSSION
a rate of 1 mm/sec.
Dietary fiber was determined using the enzymatic-gravimetric
The proximate analysis data (Table IV) for samples of the two
AOAC method 991.43 (1990). The amounts of total dietary fiber
formulations reflect the combination of okara and flour in different
(TDF) and insoluble dietary fiber (IDF) were directly determined
ratios. The increased protein content of each of the samples compared
by analysis, while the soluble dietary fiber (SDF) was indirectly
with flour alone demonstrates that okara can be used to enrich the
determined by the difference (TDF – IDF = SDF).
protein content of soft wheat flour.
Isoflavone analysis was based on the procedure described by
Wang and Murphy (1994), with the exception of the use of meth-
anol as the mobile phase in place of acetonitrile. Sample sizes
were adjusted such that the samples A and B contained 1.000 g of
okara on a dry basis. Therefore, all isoflavone results are based on
Radial Expansion Ratio, Bulk Density, and Breaking Strength
for Extruded Samples
1.000 g of dry okara.
The isoflavone extracts were analyzed by HPLC with an AGP-1
gradient pump and EDM-2 eluent degas module (Dionex Corp.,
Sunnyvale, CA), a tunable absorbance detector (486b, Waters
Corp., Milford, MA), and a Microsorb MV column (5 µm, 100 Å,
18) (Rainin Instrument Co., Woburn, MA). Samples were
injected onto a 20-µL loop with an autoinjector (AS 3000, Spec-
trasystem, Freemont, CA). The isoflavones were detected at 260
nm. A linear gradient (Table III) of two solvents were used.
All statistical analyses were performed with a statistical analysis
system (SAS Institute, Cary, NC) using the general linear model
a Extrusion conditions used (screw configuration-temperature profile): LS-LT
procedure. One-way and two-way analyses of variance (ANOVA)
= low shear-low temperature; LS-HT = low shear-high temperature; HS-LT
= high shear-low temperature; HS-HT = high shear-high temperature.
b Values followed by the same letter in the same column are not significantly
different (P < 0.05).
Wet Okara and Soft Wheat Flour Used for Samples A and B
Soft Wheat Flour (%)
Insoluble, Soluble, and Total Dietary Fiber Contents (%, db) for
Nonextruded and Extrudeda Samples A and B
a A = 11.0% okara (db), 33.0% water, and 56.0% flour (db); B = 14.0% okara
(db), 37.5% water, and 48.5% flour (db).
Screw Elementsa Used for Low Shear (LS) and High Shear (HS)
8 D twin lead screws (TLS), 7 × 30° forward kneading elements
(FKE), 8 D TLS, 3 × 60° FKE, 3 × 30° reverse KE (RKE), 2 D
single LS (SLS), 4 × 60° FKE, 3 × 30° RKE, 2 D SLS
8 D TLS, 7 × 30° FKE, 8 D TLS, 4 × 60° FKE, 4 × 30° RKE, 2 D
TLS, 6 × 60° FKE, 4 × 30° RKE, 1 D SLS, 7 × 90° KE, 2 D SLS
a One kneading element = 0.25 D (1 D = 19 mm).
Linear Gradient Program Used for HPLC Analysis of Isoflavones
Solvent A (%)a
Solvent B (%)b
Extrusion conditions used (screw configuration-temperature profile): LS-LT
= low shear-low temperature; LS-HT = low shear-high temperature; HS-LT
= high shear-low temperature; HS-HT = high shear-high temperature.
b Values followed by the same letter in the same column for Samples A or B
are not significantly different (P < 0.05).
c Least significant difference.
a 89.9% water, 10% methanol, 0.1% acetic acid.
Individual and Total Detected Isoflavone Contents (µg/g of dried okara)
b 99.9% methanol, 0.1% acetic acid.
of Nonextruded Samples
Proximate Analysis Data (%, db) for Nonextruded Samples
238 CEREAL CHEMISTRY
Radial Expansion Ratio, Bulk Density, and Breaking Strength
and B decreased for all extrusion conditions when compared with
Radial expansion ratio, bulk density, and breaking strength data
their respective nonextruded samples. These results were in agree-
are listed in Table V. Increasing the content of certain ingredients
ment with Wang et al (1993), Qian and Ding (1996), and Ralet et
has affected the radial expansion of extruded products (Moore et
al (1990). Extrudates from sample A had significantly (P < 0.05)
al 1990, Lue et al 1991, Jin et al 1995). Radial expansion ratio de-
decreased IDF contents at all conditions. However, of sample B
creased in the present study as fiber content was increased (sample
extrudates, only those extruded at LS-LT and HS-HT conditions
A vs. sample B). Jin et al (1995) suggested that increasing fiber
had significantly decreased (P < 0.05) IDF contents. A significant
content caused thickening of the cell walls and decreased air cell
(P < 0.001) effect on IDF contents was observed when the temper-
size in the microstructure of the extrudate, resulting in decreased
ature profile was changed from low to high for sample B. On the
radial expansion. A decreasing trend was observed in the radial ex-
other hand, changing the screw configuration from low to high
pansion ratio as the extrusion conditions became increasingly more
resulted in a significant (P < 0.01) effect for sample A. It appears
severe for both samples A and B. This may be due to the breaking
that the extrusion conditions required to affect the IDF contents
down of components into smaller particles, which may interfere with
bubble expansion, reducing the extensibility of the cell walls and
The SDF contents for the extruded samples A and B increased sig-
causing premature rupture of steam cells in the extrudate micro-
nificantly (P < 0.05) for all extrusion conditions when compared with
structure resulting in decreased radial expansion (Guy 1985).
their respective nonextruded samples, with the exception of sample
The bulk density values in the present study increased as radial
A extruded at LS-LT. These observations were in general agreement
expansion decreased (Table V). This is in agreement with the
with Bjorck et al (1984), Caprez et al (1986), Siljestrom et al
findings of other researchers (Hsieh et al 1991, Lue et al 1991, Jin
(1986), Ralet et al (1990), Wang et al (1993), and Qian and Ding
et al 1995). The increase in bulk density is due to an increase in
(1996). No one parameter had a significant effect on the SDF content
the density of the microstructure of the extrudate as the radial
for extruded A samples. However, both temperature and screw con-
figuration had independent but significant effects (P < 0.01) on the
The present study revealed that the breaking strength increased
SDF contents of sample B. In general, extrusion resulted in decreased
as radial expansion ratios decreased (Table V). The increase in break-
IDF and increased SDF contents for both samples A and B.
ing strength is presumed to be due to the thickening of the cell walls
and the decrease in air cell size. This may provide increased strength
Isoflavone Contents in Nonextruded and Extruded Samples
to the extrudate microstructure and increased resistance to fracture.
There were no detectable isoflavones in flour, based on spectro-
scopy at 260 nm. Six isoflavone compounds were identified in okara
Dietary Fiber Contents in Nonextruded and Extruded Samples
and in the nonextruded products and extruded samples A and B. In-
The total dietary fiber (TDF), insoluble dietary fiber (IDF), and
creasing soy and soy isoflavone consumption by non-Asians appears
soluble dietary fiber (SDF) contents for the nonextruded samples
to be highly desirable given the potential for soy and soy isofla-
are listed in Table VI. As a result of the combination of okara and
vones to mitigate onset of chronic disease (Messina et al 1998). The
flour, the TDF contents for the nonextruded samples A and B
six isoflavones identified were (in order of elution) daidzin, genistin,
increased 147 and 162%, respectively, compared with the TDF con-
malonyl daidzin, malonyl genistin, acetyl genistin, and genistein. The
tent of flour alone, demonstrating that okara can be used to
levels of detected isoflavones for nonextruded samples A and B
increase the TDF content of soft wheat flour.
are listed in Table VII.
Sample A extruded at LS-LT conditions was the only extruded
The glucoside, malonyl, and acetyl forms of daidzin and genistin
sample to significantly differ (lower) (P < 0.05) from the nonex-
combined represent 90–98% of the total detectable isoflavones in
truded sample A in TDF content (Table VI). Sample B extruded at
okara and the nonextruded samples A and B. Wang and Murphy
HS-LT conditions was the only extruded sample to significantly
(1994) and Anderson and Wolf (1995) also found ?90% of the total
differ (higher) (P < 0.05) from the nonextruded sample B in TDF
detectable isoflavones to be a combination of the glucoside, malonyl,
content. The increased TDF content for this sample B may be due
and acetyl isoflavones in soy products. Malonyl daidzin and malonyl
to the formation of enzyme-resistant starch. Englyst et al (1995)
genistin were the most abundant of the isoflavone compounds in
suggested the increase in TDF contents of starchy foods, such as
okara and the nonextruded samples A and B. Soybeans typically
those containing white flour, after heat processing is due to the for-
contain 50% more malonyl glycosides than the simple glucosidic
mation of enzyme-resistant starch during the cooling of the product.
forms of the isoflavones. On the other hand, soy milk contains more
The IDF contents of the extruded products of both samples A
glucosidic than malonyl forms of the isoflavones. The ratio of gluco-
Individual and Total Detected Isoflavone Contents (µg/g of dried okara) for Nonextruded and Extrudeda Samples A and B
a Extrusion conditions used (screw configuration/temperature profile) as in Table V.
b Least significant difference.
c Values followed by the same letter in the same row are not significantly different (P < 0.05).
Vol. 77, No. 1, 2000 239
sides to malonyl isoflavones in okara suggests that extraction of
soy protein to produce soy milk preferentially extracts the glucosidic
forms of daidzein and genistein, leaving more of the malonyl forms
Partial support of this research from the Crop and Food Bioprocessing
in the okara. There was little difference in total isoflavone contents
Center of Michigan State University and the Michigan Agricultural
Experiment Station is gratefully acknowledged.
among the samples. Nonextruded samples of A and B had decreased
levels of daidzin, genistin, malonyl daidzin, and malonyl genistin,
and increased acetyl genistin and genistein levels when compared with
the okara sample. A possible explanation for the redistribution of iso-
American Association of Cereal Chemists. 1995. Approved Methods of
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[Received May 5, 1999. Accepted December 9, 1999.]
240 CEREAL CHEMISTRY