the viscoelastic properties of soybean curd (tofu) as

8/3/2019 The Viscoelastic Properties of Soybean Curd (Tofu) As

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Original article

The viscoelastic properties of soybean curd (tofu) as

affected by soymilk concentration and type of coagulant

Yongqiang Cheng,1 Naoto Shimizu2 & Toshinori Kimura2*

1 China Agricultural University, Now Doctoral Program in Agricultural Sciences, University of Tsukuba, Tennodai 1-1-1,

Tsukuba-shi 305-8572, Ibaraki-ken, Japan

2 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba-shi 305-8572,

Ibaraki-ken, Japan

(Received 15 March 2003; Accepted in revised form 17 August 2004)

Summary The viscoelastic properties of different types of tofu were investigated. Soymilk

concentrations were 5, 6, 7, 8 and 9%. Coagulants used were 30 mm CaSO4 or 30 mm

glucono-delta-lactone (GDL). As the concentration of soymilk was increased viscosity and

handling difficulties increased. A high concentration of soymilk in tofu gave a high break

stress and produced hard tofu. The four-element Burgers model fitted the creep behaviourand both viscous and elastic parameters could be acquired from model analysis, reflecting

changes in elasticity and viscosity of tofu. The constant viscous parameter in the model

increased with increasing soymilk concentration. The viscous parameters of viscoelastic

materials like tofu gel, obtained from small deformation tests, seemed to correlate, to some

extent, with the break stress obtained from large deformation tests. For hard tofu

production increasing the soymilk concentration within a certain range and the partial

replacement of calcium sulphate coagulant by GDL could be effective options.

Keywords Burgers model, creep behaviour, hard tofu, true strain, viscosity.

Introduction

The healthy effects of soy products have aroused

the attention of many researchers and consumers

(Hawrylewicz et al., 1995; Hasler, 1998). The

protein intake in China is still inadequate, even

today. Because of the high nutrient value of soy

protein, soy products are considered as excellent

protein resources. As a soy product, tofu is a

traditional product in some Asian countries like

China and Japan. Promoting the production of

tofu in China could be a good way of solving the

problem of protein intake there. Generally speak-

ing, the Chinese seem to prefer hard tofu.

Currently, there are still some problems in theindustrial production of hard tofu in China

(Cheng et al., 2000). To improve tofu production,

texture evaluation of tofu products is necessary.

Break stress behaviour alone cannot reflect both

the elastic and viscous properties of food materi-

als. For hard tofu manufacture, a study of the

effects of processing on the texture of tofu is

necessary. In a creep test, a load is kept constant

and the strain increases with time, causing creep

behaviour. Analysis of the creep behaviour by

models can give both elastic and viscous param-

eters. Knowledge of changes in elasticity and

viscosity, as affected by different processing con-

ditions, could help to improve hard tofu produc-

tion in China. The creep properties of agar gels

(Isozaki et al., 1976), 20% soybean gel and egg

white gel (Kuwahata & Nakahama, 1975), whey

protein gels (Katsuta et al., 1990) and porcineserum-myosin gel (Ni & Hayakawa, 2001) have

been investigated and fitted to different models.

Kuwahata & Nakahama (1975) investigated the

creep behaviour of a 20% soybean gel and

compared it with an egg white gel and a 1.5%

agar gel. They reported that the creep behaviour*Correspondent: Fax: +81 29 855 2203;

e-mail: [email protected]

International Journal of Food Science and Technology 2005, 40, 385390 385

doi:10.1111/j.1365-2621.2004.00935.x

2005 Institute of Food Science and Technology Trust Fund


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of the three types of materials could be fitted by a

four-element model. However, up to now, few

studies have been reported describing the creep

properties of tofu with different soymilk concen-

trations and coagulants. In this study, the creep

properties of different types of tofu were investi-

gated and analysed by viscoelastic models. Based

on the results of creep tests, ways to improve hard

tofu production in China are also described.

Materials and methods

Materials

Soy protein isolate (SPI) of 90% dry base (84.6%

wet base) protein content (FujiproE, Fuji Oil Co.,

Osaka, Japan) was used to prepare soymilk.

Analysis-grade glucono-delta-lactone (GDL) and

food-grade CaSO42H2O were purchased from

Wako Pure Chemicals Industries Co. (Osaka,

Japan). They were used without further purifica-

tion.

Tofu production in the laboratory

The tofu production method used in this laborat-

ory was based on the industrial kinugoshi (silken)

tofu production method used in Japan (Watanabe,

1997) and was also reported in our previous paper

(Cheng et al., 2002). SPI was dissolved in distilled

water to prepare 5, 6, 7, 8, and 9% (w/v) protein

soymilk. The soymilk was heated to 100 C and

kept at this temperature for 3 min, then was

cooled to below 10 C. GDL solution or CaSO4suspension was added as coagulant and the tofu

made in the laboratory was referred to as either

GDL-tofu or Ca-tofu. Their final concentrations

were adjusted to 30 mm. The soymilk mixture was

poured into a steel tank, containing glass moulds

(30 mm in height, 36 mm in diameter) to shape the

columnar tofu samples. The soymilk was then

heated to 75 C in a water bath and kept for more

than 40 min to let the soymilk coagulate. Samples

were stored at room temperature for 60 minbefore the viscoelastic tests.

Measurement of the viscosity of soymilk

Viscosity is a very important index of soymilk

concentration. The viscosity of soymilks with

different concentration was measured in order to

determine the influence of concentration on vis-

cosity. A LV2000 viscometer (Cannon Instrument

Company, State College, PA, USA) was used, and

the rotation speed was 30 r.p.m. for all the tests.

The experiments were repeated more than three

times and the coefficients of variation for all

determinations were


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fitted to different models. Mohsenin (1986) sum-

marized the rheological evaluation of plant and

animal materials in detail and described different

models for the analysis of stress relaxation andcreep behaviour. The Maxwell model (Fig. 1a),

with a spring model and a dashpot model in series

and the Vigot model (Fig. 1b), with a spring model

and a dashpot model in parallel, were used as the

primary units. A least square non-linear algo-

rithm in Sigmaplot (Jandel Corporation, San

Rafael, CA, USA) was used to fit experimental

data to the models. The best fit was determined on

the basis of the standard error and coefficient of

variation of the parameters, the value of the

regression coefficient and by comparison of the

residual value between the plots of the experimen-

tal and fitted data. The appropriate type of model

was determined on the basis of the dependence

criterion for the non-linear algorithm in Sigmaplot.

After the model was determined, the parameters of

the model were calculated by iteration from

different initial values using Sigmaplot software.

Results and discussion

Change in viscosity with soymilk concentration

The viscosity of soymilk increased with increasing

soymilk concentration (Fig. 2). As protein was themajor component of the solid content of the SPI

used, a higher soymilk concentration means a

higher protein content. Circle et al. (1964) repor-

ted that soymilk viscosity increased exponentially

with soymilk concentration. Our study showed a

similar trend, particularly at the lower concentra-

tions. In industrial practice, too high a viscosity

may cause difficulties in transportation in pipes in

some cases (e.g. when a centrifugal pump is used).

Thus, the viscosity of soymilk should be controlled

to an appropriate level.

Stress-true strain behaviour of tofu

In our previous study, we reported the stress-strain

behaviour of different tofus (Cheng et al., 2002).

However, considering the area change during

compression, it would be better to determine the

stress-true strain relationship at the same time.

The stress-true strain behaviours of GDL-tofu and

Ca-tofu are shown in Fig. 3. For the same soymilk

concentration, all the GDL-tofus showed a higher

break stress than Ca-tofu, as also previously

reported by Saio (1979). True strain has been

used by many researchers (Lee et al., 1983; Van

Kleef, 1986; Kim et al., 2001). Compared with

normal engineering strain (Cheng et al., 2002),

true strain gave a larger value in all the cases,

which meant that a larger break strain was

obtained when using true strain, although it did

not change the break stress of the different tofus.

During the compression tests, the area of the tofu

sample changed under the compression and thus

the use of true strain would give a better indication

of the different factors influencing tofu texture.

Generally speaking, increasing the soymilk con-

centration gave a harder tofu (Fig. 3). This result

corresponded with the findings of Saio (1979). Thetotal amount of protein increased with increasing

soymilk content, the density of the network

becoming higher resulting in a higher break stress.

The break stress and strain of 9% GDL-tofu and

Ca-tofu decreased a little compared with the 8%

tofus. The contents of GDL and CaSO4 were

En

E1 1

n

E

E

(a) (b) (c)

Figure 1 Characteristics of the different models. (a) Max-

well model; (b) Vigot model; (c) Four-element Burgers

model. E, elasticity of Hookes body; g, viscosity of

dash-pot body.

Figure 2 Effect of soymilk concentration on its viscosity.

Viscoelastic properties of tofu Y. Cheng et al. 387

2005 Institute of Food Science and Technology Trust Fund International Journal of Food Science and Technology 2005, 40, 385390


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constant at 30 mm for all cases and the protein

amount increased. The decrease of break stress

and strain for 9% GDL-tofu and Ca-tofu may be

because of the decreasing coagulant/protein ratio;

i.e. it might be that 30 mm GDL or CaSO4 may be

insufficient coagulant for 9% protein. For hard

tofu manufacture, the results show that use of

GDL is better for the production of tofu with

harder texture. Saio (1979) reported that the

network of GDL-tofu consisted of flocculent

aggregates and that of Ca-tofu showed a spongy

structure with a tight framework. The coagulation

mechanisms of Ca-tofu and GDL-tofu are a little

different although hydrophobic interactions play

an important role in both (Kohyama et al., 1995).

The existence of gluconic acid in GDL-tofu andcalcium ions in Ca-tofu may be important factors

that differentiate between the textural properties

of Ca-tofu and GDL-tofu. Because of the acidic

taste of GDL-tofu, in the practical production of

hard tofu, only partial replacement of CaSO4 by

GDL should be considered. These results also

indicated that the concentration of soymilk and

type of coagulant had a great influence on the

texture of tofu gel.

Creep behaviour of tofu

For texture evaluation, both large and small

deformations are used, large deformations in

compression tests and small deformations in stress

relaxation, creep and dynamic viscoelastic tests. A

stress relaxation test on tofu was studied in our

previous research (Cheng et al., 2002). The dy-

namic viscoelasticity of tofu and soybean gel has

also been reported previously (Van Kleef, 1986;

Kohyama & Nishinari, 1992). Compliance (the

reciprocal of elasticity) was used as an index of

creep in this study. A typical creep curve is shown

in Fig. 4. The compliance increased with time,

initially quickly then more slowly. After unloadingthe sample, a constant deformation remained,

which showed that tofu was a viscoelastic mater-

ial. At the start point, the sudden increase reflected

the existence of constant elasticity. From the

characteristics of the creep curve, a four-element

model could be considered. It was found that a

four-element Burgers model (Mohsenin, 1986)

could fit the curve. The model is shown in Fig. 1c.

The relationship between compliance and the

viscoelastic parameters could be expressed as

follows (Kawabata, 1989):

Jt 1En

1E11 e tsret1 t

gn2

where, J(t) 1/E(t) is compliance (Pa)1); t is

time (s); En is the constant elasticity parameter

(Pa); E1 is the elastic parameter of the Vigot model

(Pa); gn is the constant viscosity parameter; and

sret1 is retardation time (s), which is defined as

(a)

(b)

Figure 3 Stress-true strain curves of different tofu. (a)

CaSO4-coagulated tofu; (b) GDL-coagulated tofu. Soymilk

concentration: d, 5%; , 6%; , 7%; s, 8%; , 9%.

Figure 4 Typical creep curve of tofu.

Viscoelastic properties of tofu Y. Cheng et al.388

International Journal of Food Science and Technology 2005, 40, 385390 2005 Institute of Food Science and Technology Trust Fund


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sret1 g1/E1 (g1 is the viscous parameter of the

Vigot model).

The creep behaviour of Ca-tofu produced from6 to 9% soymilk is shown in Fig. 5. Tofu from

higher soymilk concentration gave lower compli-

ance, i.e. a smaller deformation. For the same

measurement conditions, a smaller deformation

indicates a firmer material structure. Hence creep

behaviour indicated that a higher soymilk concen-

tration resulted in a stronger tofu structure, and

this is consistent with the stress-true strain results.

A higher soymilk concentration means a higher

protein content. It has been suggested that the

network of tofu gel may be formed via hydrogen

bonding, hydrophobic associations, ionic inter-

actions and electrostatic cross-links and also

through some sulphydryl-disulphide linkages of

unfolded polypeptides (Catsimpoolas & Meyer,

1971; Utsumi & Kinsella, 1985). Soy protein plays

an important role in the gel formation. The total

amount of protein increased with increasing

soymilk content. For a high protein content, the

density of the network becomes higher and this

results in higher break stress.

Iterative calculations using Sigmaplot software,

based on a least square algorithm, gave the creep

parameters of the four-element Burgers model

(Tables 1 and 2). The regression coefficients (R2

)in all cases were above 0.99. The constant viscosity

parameter, gn, increased with increasing concen-

tration of soymilk, whereas the constant elasticity

parameter, En, varied irregularly with concentra-

tion. For both Ca-tofu and GDL-tofu, the retar-

dation time (sret1) deceased with increasing

soymilk concentration. The changes of both gnand sret1 reflected the formation of a firmer

structure with increase in soymilk concentration,

i.e. higher viscosity and shorter retardation time.

GDL-tofu had a higher gn than Ca-tofu for the

same soymilk concentration, which is consistent

with the stress-true strain results shown in Fig. 3.

It would seem that the viscous parameters of creep

behaviour correlate with changes in break stress,

similar to our previous findings from stress relax-

ation analysis of tofu (Cheng et al., 2002). Shim-

oyamada et al. (1999) showed that the gel strength

of the freeze-gel formed from soymilk was related

to the viscosity of the soymilk before freezing.

Although both large deformation properties

(stress-strain test) and small deformation proper-

ties (stress relaxation, creep, dynamic viscoelastic

tests) reflect tofu texture, they are not always

consistent (Kuwahata & Nakahama, 1975). For

analytical simplicity and a better understanding of

the models involved, the creep test is a very

effective small-deformation measurement. Forfood gels, eating characteristics as well as func-

tional properties such as handling and cutting are

strongly dependent on their larger-deformation

and fracture characteristics. To understand the

overall properties of tofu, both large and small

deformation tests are necessary.

Time (s)

Comp

liance(Pa1)

6%

7%

8%

9%

Figure 5 Creep properties of CaSO4-coagulated tofu from

different soymilk concentrations:, 6%; , 7%; , 8%; ,

9%.

Table 1 Parameters of the four-element Burgers model

(Fig. 1c, eqn 2) for the creep behaviour of CaSO4-

coagulated tofu

Soymilk concentration 6% 7% 8% 9%

En(103 Pa) 5.7 4.4 7.7 4.5

E1(10

3

Pa) 5.6 7.6 7.4 8.5g1 (10

6 Pa s) 0.38 0.26 0.10 0.16

gn(106 Pa s) 1.9 2.2 4.5 4.3

sret1(s) 68 34 13 19

Table 2 Parameters of the four-element Burgers model for

the creep behaviour of GDL-coagulated tofu

Soymilk concentration 5 % 6% 7% 8% 9%

En (103 Pa) 5.0 8.3 8.7 5.2 6.1

E1 (103 Pa) 6.4 9.4 14.8 17.7 17.2

g1 (106 Pa s) 0.79 0.70 1.0 0.87 0.72

gn(106 Pa s) 1.5 2.6 3.8 6.9 8.9

sret1(s) 124 75 68 50 42

Viscoelastic properties of tofu Y. Cheng et al. 389

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Conclusions

The results of this study on the stress-strain and

creep properties of Ca-tofu and GDL-tofu with

different concentrations of soymilk suggest the

following conclusions:

1 True strain gave larger values thanengineering

strain andwas a morerealistic concept, considering

the area changes that occur during compression.

2 Higher concentration of soymilk resulted in

tofu with a higher break stress. For the same

soymilk concentration, GDL-tofu had a higher

break stress than Ca-tofu.

3 The creep behaviour of tofu could be represen-

ted by a four-element Burgers model. The

parameters obtained from the model reflect

both the elastic and viscous changes in tofu.

For both Ca-tofu and GDL-tofu, the constant

viscous parameter gn showed a consistent

increase with increasing soymilk concentration.

The viscous parameters obtained from the

small deformation test might have a more

consequent relationship with the break stress

obtained from the large deformation test.

For producing the hard tofu preferred by

Chinese consumers, increasing the soymilk con-

centration within a certain range and the partial

replacement of calcium sulphate coagulant by

GDL coagulant could be effective options.

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