Ultra high pressure homogenized soy flour for tofu making

Food Hydrocolloids 32 (2013) 278e285
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Food Hydrocolloids
journal homepage: www.elsevier.com/locate/foodhyd
Ultra high pressure homogenized soy flour for tofu making
Hsiao-Hui Liu a, John-Tung Chien b, Meng-I Kuo b, *
a
b
Ph.D. Program in Nutrition and Food Sciences, Fu-Jen Catholic University, 510 Jhong-Jheng Road, New Taipei City 24205, Taiwan
Department of Food Science, Fu-Jen Catholic University, 510 Jhong-Jheng Road, New Taipei City 24205, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2012
Accepted 3 January 2013
Using whole soybean for tofu making can effectively reduce the okara waste, increase the utilization of
raw material, and lower the production cost. However, the fiber in whole soybean would alter the protein
concentration in soymilk and might weaken the structure of tofu. In the current study, UHPH was used
for reducing the particle size of soy flour, non-thermally denaturing the soy proteins, and making a high
quality tofu. Two soy flour suspension concentrations (15% and 20%) were treated by UHPH with four
combinations of pressures and cycles. Glucono-d-lactone (GDL) was used as coagulant for tofu making.
Soy flour suspensions heated at 95 C for 10 min were used as the control. The result showed that the
UHPH treated soy flour suspension had a smaller and more uniform particle size than in the control.
Under appropriate pressure and cycles the hardness, gumminess and chewiness of tofu made from soy
flour treated with UHPH was similar to the control tofu. The tofu made from 20% soy flour suspension
with 150 MPa UHPH for 3 cycles had the lowest expressible water and syneresis. Control tofu showed
a particles inlaid, incomplete honeycomb-like network, whereas UHPH tofu had a regular, continuous
honeycomb-like structure. The above results indicated that UHPH reduced the particle size of soy flour,
denatured the soy proteins, and could produce a favorable tofu with high level of functional components
such as fiber.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Tofu
High pressure homogenization
Soy flour
Texture
Microstructure
1. Introduction
Global prices of raw materials for food production increased
rapidly over the past decade. It will be a challenge for food manufacturers to use crops more efficiently during food processing to
lower the production cost while keep making high quality product.
Tofu is part of traditional cuisine in East Asia and vegetarian food
worldwide. During typical tofu making process, soybeans are
soaked, ground with water, and filtered to produce soymilk. The
soymilk is then heated up to 90 C for more than 10 min. The hot
soymilk is cooled down to 60e70 C for adding coagulants and is
reheated to 80 C for gelation. Thermal treatment is typically used
for making tofu to dissociate, denature, and aggregate the soy
proteins, inhibit the microbial growth, reduce the beany flavor,
inactivate undesirable biological compounds such as trypsin inhibitors and lipoxygenase (Kumar, Rani, Tindwani, & Jain, 2003; Liu,
1997). However, temperature adjustment during tofu making is
energy consuming.
On the other hand, only 53% of soy materials (in dry basis)
become the final tofu product and the rest remain in okara. In
average, okara contains 28.52% of protein, 9.84% of oil, 55.48% of
* Corresponding author. Tel.: þ886 2 29052019; fax: þ886 2 22093271.
E-mail addresses: 062998@mail.fju.edu.tw, mengikuo@gmail.com (M.-I. Kuo).
0268-005X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.foodhyd.2013.01.005
dietary fiber, 2.56% of carbohydrates and 3.61% of ash (Liu, 1997;
Redondo-Cuenca, Villanueva-Suárez, & Mateos-Aparicio, 2008).
Besides the macronutrients, dietary fiber in the okara is a beneficial
food ingredient which possesses a positive effect on human health
(Gallaher, Locket, & Gallaher, 1992; Rodríguez, Jiménez, FernándezBolaños, Guillén, & Heredia, 2006). However, short shelf life, beany
flavor, fibrous texture, unsavory eating quality, and easily browning
during drying process limited the utilization of okara. Therefore,
using the whole soybean for tofu making might be a solution for
limiting the okara waste problem. It can also increase the soybean
usage percentage and reduce the production cost. But the fiber in
whole soybean can alter the protein concentration in soymilk and
weaken the structure of tofu.
Ultra high pressure homogenization (UHPH) is a continuous,
non-thermal processing technique. This process requires the liquid
or material with colloidal character to pass thorough a highpressure valve in the range of 100e350 MPa. Two-stage homogenization is commonly used for higher efficiency. The primary stage
is designed to reduce the size of colloidals and the secondary stage
is to disrupt the clusters formation. Concerning factors that
involved in the UHPH process include high shear force, pressure,
cavitation, friction, turbulence, and high velocity in combination
with heat generation. UHPH has been introduced in food processing for emulsification, dispersion, particle size reduction, enzyme
H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
279
Fig. 1. Light microscope images of 15% soy flour suspension with thermal treatment and ultra high pressure homogenization (UHPH). (a) thermal treatment, observed using visible
light; (b) thermal treatment, observed using polarized light; (c) 150 MPa UHPH for 3 cycles, observed using visible light; (d) 150 MPa UHPH for 3 cycles, observed using polarized
light. The scale bar represents 300 mm. LP: Large non-soluble particles.
inactivation, and mixing especially in juice, dairy product and
soymilk (Cruz et al., 2007, 2009; Datta, Hayes, Deeth, & Kelly, 2005;
Hayes, Fox, & Kelly, 2005; Hayes & Kelly, 2003; Suárez-Jacobo et al.,
2011; Thiebaud, Dumay, Picart, Guiraud, & Cheftel, 2003; Zamora,
Ferragut, Jaramillo, Guamis, & Trujillo, 2007). However, none of
them studied the effect of UHPH on whole soybean for tofu making.
The objective of this study was to compare the quality of tofu made
by different concentrations of soy flour suspension using both
traditional thermal treatment and UHPH.
2. Materials and methods
2.1. Materials
Soybean was obtained from Neco Seeds (Non-GMO soybean,
Neco Seeds Farms Inc., Garden City, USA). Hydrochloric acid solutions were used as the pH-adjusting solution. All chemicals were
analytical grade and obtained from Panreac Química S.A.U. (Barcelona, Spain) and Sigma Chemical Co., (St. Louis, MO, USA). Soybean was ground into flour by cyclone mill (UDY Corporation, Fort
Collins, Colorado, USA) and passed through a 100-mesh sieve. The
soy flour was stored in desiccators at room temperature for further
treatment in 2 weeks.
2.2. Sample preparation
Two soy flour suspensions concentrations (15% and 20%) were
prepared and placed at room temperature for 20 min before
treatment. The soy flour suspensions were either thermally treated
by heating at 95 C for 10 min or non-thermally treated by UHPH in
a valve-mode homogenizer (APV 2000; SPX Co., Charlotte, NC,
USA). Each UHPH sample was homogenized for two or three cycles
at two different pressures (100 and 150 MPa). The treated soy flour
suspensions were stored at 4 C in the refrigerator for tofu making
and further analysis.
2.3. Tofu making
Freshly prepared 10% GDL solution was added to the treated soy
flour suspension. The suspension was then poured into a 10 cm
8 cm 3 cm (length width height) container. The final
concentration of GDL in the suspension was 0.5%. After mixing for
5 min, the container was covered and transferred to an 80 C water
bath. Tofu curd was formed after 30 min of incubation and was
cooled down immediately to the room temperature. The pH of
mixture was recorded during heating. The tofu was stored at 4 C in
the refrigerator for 1 day before further investigation.
2.4. Appearance and microstructure of soy flour suspension and
tofu
The microstructure of soy flour suspension was examined by the
optical microscope (Elipse E600, Nikon Co., Tokyo, Japan) under
visible light and polarized light. A drop of suspension was placed on
slide and covered by micro cover glass before examination.
A digital camera (EOS 1000D, Canon Inc., Tokyo, Japan) equipped
with a Canon EF-S 18e55 mm f/3.5e5.6 lens was used to observe
the appearance of tofu. Tofu was cut into a cylinder of 2 cm
diameter and 2 cm height.
The microstructure of tofu was closely examined by the scanning
electron microscope (SEM, S-3000N, Hitachi Science Systems,
Tokyo, Japan). The sample preparation for SEM followed the method
of Liu and Kuo (2011) with some modifications. Tofu was cut into
several 2 mm 2 mm 10 mm cubes. These cubes were rapidly
frozen in the liquid nitrogen (196 C) and dehydrated in a freeze
dryer. Dried samples were then placed on an aluminum stub and
immobilized with double-sided adhesive carbon-tape. The natural
broken side of the sample was face up on the tape and coated with
gold using the gold sputter (Desk-2, Denton Vacuum, Moorestown,
NJ, USA) for 90 s at 25 A current. The microstructure of tofu sample
was examined with SEM at the accelerating voltage of 15 kV.
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H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
Texture Analyzer (Stable Micro Systems Ltd., Haslemere, Surrey,
UK). The tofu sample was cut into cylinders with 20 mm diameter
and 20 mm height. The samples were compressed twice to 30% of
their original height by a cylinder probe at a constant cross-head
speed of 2.0 mm/s. The following texture parameters of the samples were measured: hardness, springness, cohesiveness, adhesiveness, gumminess and chewiness.
2.6. Determination of expressible water, entrapped water and
syneresis of tofu
The expressible water of tofu was measured according to the
method of Lee (2008) with modification. The tofu sample (15 g) was
prepared in a 30 mL centrifuge tube. After storing at 4 C in the
refrigerator overnight, the sample was centrifuged at 2500 g for
75 min at 4 C. The expressed fluid was decanted into a weighing
pan and weighted. The expressible water of tofu was calculated as
the percentage of expressed fluid in the original tofu sample by
weight. The entrapped water of tofu was obtained by subtracting
the expressible water content from its moisture content (%).
The method for measuring syneresis was modified from the
method proposed by Amstrong, Hill, Schrooyen, and Mitchell
(1994). Tofu sample was cut into a cylinder with 15 mm diameter
and 5 mm height. Six cylinders were placed on a plastic grid,
weighted and then sealed into a container to prevent the moisture
evaporation. After storing at 4 C for 24 h, the effluent water was
weighted. The syneresis of tofu was expressed as the percentage of
effluent water in the original tofu sample by weight.
3. Results and discussion
3.1. The appearance of soy flour suspension and tofu
Fig. 2. The pH of soy flour suspension with thermal treatment and ultra high pressure
homogenization (UHPH) containing 0.5% (w/w) GDL during heating at 80 C for 30 min
(a) 15% soy flour suspension; (b) 20% soy flour suspension.
2.5. Determination of tofu texture
The texture characteristic of tofu was analyzed according to the
texture profile analysis (TPA) (Bourne, 1978) using a TA-XT2i
Thermal treatment is a traditional process during tofu making,
and was chosen as the control in this study. Fig. 1 shows optical
light microscope images of soy flour suspension with thermal
treatment and UHPH. The soy flour suspension with thermal
treatment contained many large non-soluble dark particles (LP)
(Fig. 1a). The brown color of soy flour suspension with thermal
treatment was darker than that with UHPH (Fig. 1c). Lipids in
soymilk were in the form of oil globules surrounded with protein
(Guo et al., 2002). Floury, Desrumaux, and Legrand (2002) observed
the reduction of droplet sizes of emulsions by UHPH. We thought
that the regions of brown color in soy flour suspension were
Fig. 3. The appearance of tofu made from different concentrations (15% and 20%) of soy flour suspension with thermal treatment and ultra high pressure homogenization (UHPH).
(a) 15%, thermal treatment; (b) 15%, 100 MPa UHPH for 2 cycles; (c) 15%, 100 MPa UHPH for 3 cycles; (d) 15%, 150 MPa UHPH for 2 cycles; (e) 15%, 150 MPa UHPH for 3 cycles; (f) 20%,
thermal treatment; (g) 20%, 100 MPa UHPH for 2 cycles; (h) 20%, 100 MPa UHPH for 3 cycles; (i) 20%, 150 MPa UHPH for 2 cycles; (j) 20%, 150 MPa UHPH for 3 cycles.
H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
emulsions and their sizes were reduced by UHPH since the light
could easily penetrate these small emulsions during microscope
observation. The diameter of LP in suspension decreased from
300 mm to less than 30 mm after UHPH (Fig. 1c). The crystalline
structure of LP that has been observed under polarized light
(Fig. 1b) also showed the decrease in size after UHPH (Fig. 1d).
These results indicate that UHPH had the ability to reduce the sizes
of LP and emulsion droplets in the soy flour suspension system.
Cruz et al. (2007) and Roesch and Corredig (2003) who studied the
effect of UHPH on emulsions prepared with soy protein concentrate
or soymilk had similar observations.
Fig. 2 shows the pH of soy flour suspension with thermal and
UHPH treatments containing 0.5% GDL during heating at 80 C for
30 min. The initial pH of untreated soy flour suspension with different concentrations was between 6.14 and 6.3 and decreased
slightly during heating. The initial pH of soy flour suspension with
thermal and UHPH treatments containing GDL was lower than
untreated suspension. The pH of soy flour suspension with thermal
treatment containing GDL decreased gradually during heating.
Similar changes in pH of soy flour suspension with UHPH treatments containing GDL were observed. The final pH of treated soy
flour suspensions with different concentrations was between 4.6
and 4.8. These results indicated that the concentration of soy flour
and UHPH treatment did not affect the pH change of soy flour
suspension containing GDL during heating.
The appearance of tofu made by different concentrations of soy
flour suspension with thermal and UHPH treatments is shown in
Fig. 3. The protein content ranged from 3.5% in the homemade
silken (soft) tofu to 11.8% in the extra firm tofu in a wet basis (Lee,
2008; Yuan & Chang, 2007). In this study, the whole soybean had
35.91% of protein. Therefore, the protein content of 15% and 20% soy
flour suspensions were equivalent to 4.34% and 5.57%, respectively.
Thus, the protein concentration in soy flour suspensions was sufficient for gelation in making tofu.
The tofu made from 15% soy flour suspension with 100 MPa
UHPH was softer and less stable (Fig. 3b and c) than that made with
thermal treatment (Fig. 3a). This may be the reason that we saw
some liquid exuded and broken tofu fragments dispersed on the
table when it was cut. This resulted in the samples with less height.
As the pressure of UHPH was increased to 150 MPa, the appearance
of UHPH tofu was more intact and complete (Fig. 3d and e). The tofu
made from 20% soy flour suspension with thermal treatment had
a coarse surface with small particles (Fig. 3f). UHPH tofu made from
higher soy flour content show less or no liquid exuded on the table
after cutting (Fig. 3gej). It’s clear that tofu with higher solid content
can absorb or hold more water and has less water exuded. The tofu
made from 150 MPa UHPH had smoother and straighter appearance
especially for these treated with 3 cycles (Fig. 3e and j). The above
281
results suggest that UPHP could non-thermally denature the proteins in the soy flour suspension. Therefore, UPHP treated soy flour
suspension can be used to manufacture tofu, given that the pressure
and cycle for UHPH are selected properly.
3.2. The texture of tofu
Texture analysis is an important method to understand the tofu
quality. The texture properties of tofu made from different concentrations of soy flour suspension with thermal and UHPH treatments are shown in Table 1. The hardness is an indicator of tofu for
its resistance to the destructive force during processing and
application. Comparing tofus made from 15% soy flour suspension,
the hardness of tofu made by 100 MPa UHPH was the lowest, and
the hardness of tofu made by thermal treatment was the highest.
Increasing the UHPH pressure to 150 MPa significantly increased
the hardness of tofu. The hardness of tofu made by 150 MPa UHPH
for 3 cycles was closed to the tofu made by thermal treatment.
The hardness of tofu made from 20% soy flour suspension was
higher than tofu made from 15% soy flour suspension except the
control tofu prepared with thermal treatment (Table 1). Soy protein
was the main component in soymilk to develop the gel structure
(Kohyama, Sano, & Doit, 1995). The increase of the solid content in
suspension implies the increase of soy protein concentration. This
may cause the increasing hardness of tofu. We found that hardness
of UHPH tofu increased with increasing the pressure and cycle. It is
still interesting to observe that tofu made by 150 MPa UHPH for 3
cycles is harder than tofu made by thermal treatment. The effects of
soy flour percentage and processing method on the tofu hardness
should be considered and discussed.
A lower hardness of tofu made from 20% soy flour suspension
with thermal treatment was observed (Table 1). In the present
study, whole soybean was used for tofu making. Besides the soy
protein, there were many large and non-soluble particles (Fig. 1a) in
the soy flour suspension participated in the tofu gelation. These
particles might cause some unfavorable effect on gel network formation. They could block the protein chain connection during
gelation. When UHPH was applied, the size of these particles was
reduced (Fig. 1c) and the unfavorable effect was decreased,
resulting in the increase of the hardness of tofu. Cruz et al. (2009)
also mentioned that the large fat droplets in soy-yogurt gel disrupt the network homogeneity and cause the gel with lower
firmness, and UHPH could improve the firmness of gel.
Other texture properties such as springiness, cohesiveness and
chewiness of tofu with different treatments showed similar tendency (Table 1). Springiness means that a product physically
springs back after being deformed during the first compression.
High springiness products possess a higher elasticity and a greater
Table 1
The texture characteristics of tofu made from different concentrations (15 and 20%) of soy flour suspension with thermal treatment and ultra high pressure homogenization
(UHPH).a
Concentration
15%
20%
a
Treatments
Hardness
Adhesiveness
Springiness
Cohesiveness
Chewiness
(G)
(g mm)
(mm)
e
(g mm)
27.95A
46.97C
50.66C
52.84CD
36.93B
0.953AB
0.850D
0.834D
0.844D
0.947AB
0.491B
0.308E
0.311E
0.298E
0.424C
112.09B
23.91F
23.36F
29.21F
82.05C
59.2DE
54.75C
55.98CD
64.88E
46.78C
0.827D
0.901C
0.901C
0.926BC
0.974A
0.322E
0.369D
0.368D
0.396CD
0.584A
26.93F
39.35EF
51.43DE
63.88D
197.56A
Thermal (control)
100 MPa-UHPH for
100 MPa-UHPH for
150 MPa-UHPH for
150 MPa-UHPH for
2
3
2
3
cycles
cycles
cycles
cycles
238.50B
90.93G
86.02G
115.83F
204.50C
Thermal (control)
100 MPa-UHPH for
100 MPa-UHPH for
150 MPa-UHPH for
150 MPa-UHPH for
2
3
2
3
cycles
cycles
cycles
cycles
101.02GF
118.06F
154.76E
174.23D
346.17A
Values are means of ten replicates. Means with different superscript letters within a column are significant different (P < 0.05).
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H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
Table 2
The expressible water, entrapped water and syneresis of tofu made from different
concentrations (15% and 20%) of soy flour suspension with thermal treatment and
ultra high pressure homogenization (UHPH).a
Concentration
Treatments
15%
Thermal (control)
100 MPa-UHPH
for 2 cycles
100 MPa-UHPH
for 3 cycles
150 MPa-UHPH
for 2 cycles
150 MPa-UHPH
for 3 cycles
Thermal (control)
100 MPa-UHPH
for 2 cycles
100 MPa-UHPH
for 3 cycles
150 MPa-UHPH
for 2 cycles
150 MPa-UHPH
for 3 cycles
20%
Expressible
water
Entrapped
water
Syneresis
(%)
(%)
(%)
28.13B
37.22A
58.67CD
50.77FG
9.98A
7.55B
35.06A
53.37EF
7.91B
38.18A
49.81G
5.50C
26.16CB
62.13BC
3.10E
27.10B
23.30CD
56.42DE
61.07BC
3.49E
5.11CD
23.02CD
61.51BC
4.05DE
21.1D
64.28B
3.09E
8.68E
75.86A
1.14F
for 3 cycles had the highest cohesiveness. Increase the UHPH cycle
at the pressure of 150 MPa increased the cohesiveness of tofu. We
thought that UHPH reduced the particle size of components in the
soy flour suspension leading to an increase in their surface area and
consequently increased the association between proteineprotein,
proteineoil and proteinefiber inside the tofu.
Chewiness represents how easy the tofu is to swallow (Obatolu,
2008). The values of chewiness of tofu made by different treatments distributed in a wide range in this study. Even though the
chewiness of tofu made from 20% soy flour suspension with
150 MPa UHPH for 3 cycles was up to twice higher than other tofus
prepared in this study, its chewiness was still less than the firm tofu
reported by other study (Shen, 2008).
Adhesiveness is the energy required to break up the attractive
forces between the surface of the food and the surface of other
materials. Tofu made from 15% soy flour suspension with UHPH had
higher adhesiveness than thermal treated tofu, and the tofu made
by 150 MPa UHPH for 2 cycles had the highest adhesiveness.
However, increase the cycle of UHPH at the pressure of 150 MPa
decreased the adhesiveness of tofu. The adhesiveness of tofu
increased significantly with the concentration of soy flour suspension except the tofu made by 100 MPa UHPH.
a
Values are means of three replicates. Means with different superscript letters
within a column are significant different (P < 0.05).
3.3. The syneresis and water distribution in tofu
chewiness, requiring consumer to spend more energy to eat them.
The springiness of tofus made from 15% soy flour suspension with
150 MPa UHPH for 3 cycles was found to be closer to the tofu made
from 15% soy flour suspension with thermal treatment (Table 1).
Cohesiveness measures how well a product withstands a second
deformation relative to the first deformation. It can be interpreted
as how tight the binding inside the gel to resist the deformation.
The tofu made from 20% soy flour suspensions with 150 MPa UHPH
The water distribution within tofu directly influences its texture
and stability. In a gel system, water might either bind to a functional
group, or hold in the pores of gel network. The syneresis, expressible water and the entrapped water of tofu made from different
concentrations of soy flour suspension with thermal and UHPH
treatments are shown in Table 2. Moisture that can be separated
from tofu by centrifugation is classified into expressible water, and
the water remained is named entrapped water. The expressible
water of UHPH tofu decreased and the entrapped water increased
Fig. 4. Scanning electron micrographs of tofu made from different concentrations (15% and 20%) of soy flour suspension with thermal treatment and ultra high pressure homogenization (UHPH) at 300 magnification. (a) 15%, thermal treatment; (b) 20%, thermal treatment; (c) 15%, 150 MPa UHPH for 3 cycles; (d) 20%, 150 MPa UHPH for 3 cycles. The
length of scale bar represents 100 mm.
H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
283
Fig. 5. Scanning electron micrographs of tofu made from different concentrations of soy flour suspension with ultra high pressure homogenization (UHPH) at 1500magnification.
(a) 15%, 100 MPa UHPH for 2 cycles; (b) 15%, 100 MPa UHPH for 3 cycles; (c) 15%, 150 MPa UHPH for 2 cycles; (d) 15%, 150 MPa UHPH for 3 cycles; (e) 20%, 100 MPa UHPH for 2 cycles;
(f) 20%, 100 MPa UHPH for 3 cycles; (g) 20%, 150 MPa UHPH for 2 cycles; (h) 20%, 150 MPa UHPH for 3 cycles. The length of scale bar represents 20 mm.
with the increase of the concentration of soy flour suspension. This
can be explained that the increase in soy flour content increased the
concentration of protein, and enlarged their association in tofu, and
consequently more developed structure leading to an increase in
the water binding properties (Cruz et al., 2009; Kovalenko & Briggs,
2002). The expressible and entrapped water of thermal treated tofu
did not change with the increase of soy flour suspension concentration. The tofu made from 20% soy flour suspension with 150 MPa
UHPH for 3 cycles had the lowest expressible water and the highest
entrapped water.
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H.-H. Liu et al. / Food Hydrocolloids 32 (2013) 278e285
The syneresis is defined as the water expelled or passive diffused from tofu during storage. The syneresis of tofu made from
20% soy flour suspension was less than tofu made from 15% soy
flour suspension under the same process condition. Increase the
UHPH cycle at the pressure of 150 MPa decreased the syneresis of
tofu. The tofu made from 20% soy flour suspension with 150 MPa
UHPH for 3 cycles had the lowest syneresis. From Table 1, the tofu
made from 20% soy flour suspension with thermal treatment had
weaker gel structure. Because the solid content of this tofu was
higher, its hydration ability should be better. However, water in this
tofu was easily to be separated by centrifugation. Thus, in contrast
with the tofu made from 15% soy flour suspension, a lower syneresis but similar amount of expressible water was observed in tofu
made from 20% soy flour suspension with thermal treatment.
The texture and syneresis of homemade soft tofu made from
soymilk had been tested under the same condition in our laboratory (Lee, 2008). The hardness of homemade soft tofu was lower
and the syneresis was higher than that of tofu made from soy flour
suspension with UHPH at the pressure of 150 MPa. We thought that
the solid components in addition to the soy protein in soy flour
suspension also had contribution to the tofu hardness, and reduced
the tofu syneresis as well.
3.4. The microstructure of tofu
Figs. 4 and 5 show the scanning electron micrographs of tofu
made by different concentrations of soy flour suspension with
thermal and UHPH treatments. A honeycomb-like three dimensional protein network was observed in tofu (Fig. 4). The thermal
treated tofu had several particles inlaid in the network (Fig. 4a and
b). Large amount of particles was found in the protein structure of
tofu made from 20% soy flour suspension with thermal treatment
(Fig. 4b). This might decrease the continuation of the protein network, resulting in an unstable structure with lower hardness,
springiness and cohesiveness (Table 1). The tofu made by 150 MPa
UHPH showed a regular honeycomb-like structure without large
particles (Fig. 4c and d). The above results provide the evidence that
UHPH reduced the size of particles in the soy flour suspension
which will be a great benefit to the formation of tofu structure.
The tofu made from 15% soy flour concentration with 100 MPa
UHPH for 2 cycles showed an incomplete structure as compared
with the tofu made by other treatments. A structure of randombound protein clusters was observed (Fig. 5a). The tofu made
from 20% soy flour concentration showed a denser network (Fig. 5e
and h). The tofu made by 150 MPa UHPH for 3 cycles had almost
continuous protein network (Fig. 5d and h).
Floury, Desrumaux, and Legrand (2002) and Keerati-U-Rai and
Corredig (2009) found that UHPH caused not only the reduction
in droplet sizes of emulsions but also the denaturation of soy
proteins due to strong mechanical forces. In this study, the proteins
in soy flour suspension were denatured by thermal treatment or
UHPH. However, the extent of protein denaturation in soy flour
suspensions treated thermally or non-thermally by UHPH might be
different and was affected by concentration of soy flour, homogenization pressure and cycle. Cruz et al. (2009) reported that
thermal-treated and UHPH-treated soymilks exhibited different
coagulation mechanism. These might be the reasons for the different texture and microstructure of tofu made from soy flour
suspensions with different treatments in this study.
It is important to mention that the fiber in soy flour play a role in
the stability of tofu. Roesch and Corredig (2003) suggested that soy
fiber might interact with emulsions and occupy the continuous
phase. This might limit the movement of water and emulsions in
tofu, increasing their stability to syneresis or creaming. However,
we observed that the size of soy fiber influenced the texture and
microstructure of tofu. It seems that UHPH was able to destroy the
cell structure and degrade the soy fiber (Floury, Desrumaux, Axelos,
& Legrand, 2002), but thermal treatment did not change it (Roesch
& Corredig, 2003). Thus, when whole soybean was used as material
for tofu making, UHPH had a favorable effect on the formation of
continuous and stable gel network with more functional components and less syneresis.
4. Conclusions
The results of this study showed that UHPH denatured the soy
protein, reduced the particles size of whole soy flour, and produced
tofu with texture characteristics similar to thermal treated regular
tofu. The texture and microstructure of UHPH tofu were affected by
the homogenized pressure and cycle and the solid content of soy
flour suspension. The tofu made from 20% soy flour suspension
with 150 MPa UHPH for 3 cycles showed the best quality. The
microstructure of UHPH tofu showed a regular honeycomb-like
structure and had a better water holding capacity during storage.
UHPH could be a favorable technique for making tofu with high
dietary fiber.
Acknowledgments
We are grateful for the financial support from the National
Science Council in Taiwan (NSC), grant number (100-2628-E-030001-MY2). The Department of Chemistry in Fu Jen Catholic University provided the SEM facility. We also thank Dr. P. T. Huang for
supporting the gold sputter. Special thanks go to Dr. S. F. Guo for the
critical comment on the manuscript.
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