Journal of Agricultural Chemistry and Environment, 2014, 3, 7-13
Published Online April 2014 in SciRes.
How to cite this paper: Lien, C.-C., Shen, Y.-C. and Ting, C.-H. (2014) Ohmic Heating for Tofu MakingA Pilot Study. Journal
of Agricultural Chemistry and Environment, 3, 7-13.
Ohmic Heating for Tofu Making—A Pilot
Cheng-Chang Lien1, Yu-Chieh Shen1, Ching-Hua Ting2*
1Department of Biomechatronic Engineering, National Chiayi University, Chiayi, Chinese Taip ei
2Department of Mechanical and Energy Engineering, National Chiayi University, Chiayi, Chinese Taipei
Email: *
Received January 2014
The aim of this study is to explore the relationship between temperature and electrical conductiv-
ity of soya milk under ohmic heating in tofu making. The soya milk of 10 Brix was heated to a
steady temperature of 90˚C. The applied voltage was increased and the temperature rising rate
was investiga ted for adequate heating profiles in tofu making. Experimental results showed that
the electrical conductivity of soya milk is proportional to the heating time. The temperature rising
rate was increased from 1.46˚C to 3.82˚C/min as a result of increased voltage. Hence ohmic heating
could be an efficient, convenient heating measure in tofu making.
Soya Milk; Ohmic Heating; Electrical Conductivity; Temperature Rising Rate
1. Introduction
Tofu is a salt- or acid-coagulated water-based gel, with soya lipids and proteins as well as other constituents
trap ped in gel net works [1]. I t is made by coagulating soya milkfollowed by either pressure or heat treatment
after the addition of a coagulant. The taste of tofu is significantly affected by its final texture [2], i.e., by the
firmness of the tofu gel’s structure. The textural property is determined by the concentration of soya milk, the
type and c oncentr atio n of co agulant, the gelating press ure and temperature, and the gelation time [3].
The quality of a tofu can be exami ne d usi ng destructive testing. A textural analyzer is suitable for characte-
rizing the mechanical property; while microscope analysis explores the chemical composition and the micr o-
struc ture [4,5]. T he above two methods are of a destructive nature and require laboratory practice. They are not
suitable for o n-line app licatio n as a quali ty indicator . Ultr a sound which can give a big amount of information in
a short time has been shown to be effective in detecting the gelation process and the final product in an efficient
way [3,5]. While ultraso und is a means feasible for on-line quality detection, its setup is ex pensive.
The formation of a tofu gel req uires soya milk mixed with a co agulant to be heated to a reaction temperature.
In gelation, protein clusters are formed as a mechanism of ion exchanging between soya milk and the added
coa gul ant. The concentrati on of the exchangin g ions, anions orca tions, is a result of the ongoing gelation process.
The ion concentration can be easily characterized as electrical conductivity, an easy, economic, and efficient
*Corresponding author.
Author, Author
meas ure me nt [6].
The objective of this study is to investigate the feasibility of using ohmic heating in tofu making with ele c-
trical conductivity as a quality indicator. Adequate temperature rising rates of soya milk is examined b y apply-
ing different voltages onto the soya milk. The temperature profile can be used in tofu making with heat treat-
ment and the electrical co nduc ti vi ty response can be for feedback control of gelation.
2. Ohmic Heating for Tofu Making
2.1. Mechanism of Tofu Formation
Addition of a coagulant into heated soya milk triggers the formation of tofu gels. The formation is a two -step
mechanism [1]: soya proteins are denatured and then aggregated by the coagulant. Soya beans contain 35% -
40% of stor a ge and whey pro te ins by wei g ht. The 7S pro t ein, c o ngl yci ni n, a nd 11 S pr o te in, gl yci ni n, ar e t he t wo
major proteins that affect the property of a tofu produce [7]. Figure 1 depicts the mechanism of soya milk
transforming to t ofu gels. When soya milk is heated to 65˚C and beyond, the oil body is released. This is the first
step of transformation. The 7S and 11S proteins will escape from the subunits at 75˚C and 80˚C, respectively.
Subunits are surrounded by protein particles wit h a size of 100nm [8-10]. The transformation completes.
2.2. Principle of Ohmic Heating
The performance of ohmic heating in food processing depends on the electrical conductivity of the foodstuff.
DC (d ir ec t c urr e nt) o hmi c he a ti ng has p ower co nd uc tion by the moving ions i n t he fo od st u ff fl ui d. T he visc o sit y
of the fluid decreases in response to a hotter fluid. T his spee ds up the ionic motion and hence the elec trical con-
ductivity. The food fluid consists of conductive and resistive components which in turn, convert the electric
current into heat. Figure 2 shows a model of soya milk under ohmic heating. The traditional heating method
conve ys heat fro m the heat ge nera tor to the hea rt of the me dium t hrough he at con vectio n or co nducti on. A te m-
perature gradient forms in traditional heating and leading therefore to a non-uniform temperature d istrib ution, i.e.
some medium may be overheated. Ohmic heating distributes electrical energy uniformly in the medium by
means of the free ions and hence the medium is uniformly heated. The electrical conductivity of the processed
medium follows [6]:
with σ in Sm1 the electrical conductivity of the medium, I in A the applied current, V in V the applie d voltage, L
in m the distance between two electrodes, and A in m2 the contacting area between the medium and the elec-
Figure 1. Process of prot ein separati on in heated soya milk [9].
Author, Author
Figure 2. Soya milk under ohmic heating [11].
2.3. Circuit model of Ohmic Heating
Ohmic heating for fluid can be represented with an equivalent circuit model as shown in Figure 3. The r esistor
stands for the intrinsic electrical conductivity of the soya milk. It converts electrical energy from the voltage
source (V) to heat. The capacitor describes the capacitive effect built by the t wo electro de plates with the so ya
milk as the electrolyte in between. The circuit has the following first-order dynamics:
V dv
R dt
= +
2.4. Factors Effecting Ohmic Heating
An ohmic heating system heats a medium by applying an electrical field into the medium. A voltage (V) applied
on the two electrode plates forms the electric field (E) with the correla tion E = V/l and the ene rgy d ensi t y stor ed
in the medium between the two electrodes is u = 1/2ϵE2 with ϵ the permititivit y constant of the so ya milk. T he
electrical power dissipated by the medium is:
PIR= ⋅
The steady state of Equation (2) indicates that a larger vol t a ge applicatio n will result in a larger current. A
larger current heats the medium in an efficient and powerful way. While the temperature of the soya milk in-
creases, the electrical conductivity of the so ya mil k incre ases in a p ositive trend [ 6]:
is the electrical conductivity at current tempe rature T and
is the electrical conductivity at the
reference temperature Tref. Ions in an electrolyte are easily to escape from the substance molecules. Different
substances will have different pro p er ties and hence the
is substance-specific [12-14].
3. Materials and Methods
3.1. Materials
Washed soya beans (800 g) were soaked for 8 - 12 hrs at a constant temperature (30˚C) in reverse osmosis (RO)
water. One ti me b y weight of RO water was added to the equivalent dry weight of the soya beans. The mixtures
were ground and centrifugally filtered for 2 min to remove t he r esi d ue . The soya milk produce was adjusted to a
concentration of 10˚Brix. The procedure of preparing soya milk is summarized as Figure 4.
3.2. Equipment
Figure 5 illustrates the set up f or oh mic heatin g exper iments. Soya milk is placed in a steel co ntainer (L * W: 11
* 7 cm2) with ceramic coating. The two titanium electrodes are powered with a DC power supply (PSW-720, In-
stek, Taiwan) which is remotely controlled by a computer using the LabVIEW software (V8.3, National Instru-
ment, USA). The soya milk is continuously stirred for uniform heat distribution. The temperature of the soya
Author, Author
Figure 3. The equivalent circuit model of ohmic heating.
Figure 4. Procedure of soya milk preparation.
Figure 5. The ohmic heating setup.
milk is detected using a TM-925 temperature transducer (Prosperous, Taiwan) and the electrical conductivity is
picked on line using by an electrical conductivity meter (Con 10, Eutch, Singapo re) .
4. Results and Discussion
4.1. Effect of Voltage Application on Temperature Rising Rate
The soya milk was heated to 90˚C by ohmic heating with different voltage but the same current applications
within 2 min. Figure 6 shows temperature responses to different applied voltages. A larger voltage results in a
faster temperature rise.
Author, Author
Figure 6. Temperature profiles of soya milks heated by different
DC voltages.
Table 1 summarizes the temperature rising rates of different heating voltages. Clearly, increasing the vol-
tage increases the temperature rising rate.
4.2. Current Respon se of Ohmic Heating
The temperature of the soya milk rises as a result of heating time in controlled current application. Figure 7
shows the current response at a temperature rising rate of 3.82˚C/min. The current was large at the beginning as
dominated by the first-order dynamics of Equation (2).
4.3. Electrical Conductivity in Ohmic Heating
Figure 8 shows measured and calculated electrical conductivities. There is a big discrepancy in between. This
may be caused by the error of current measurement. By referri ng to Figure 7, there is large fluct uat ion in t he cur-
rent measurement.
4.4. Electrical Conductivity of Different Applied Voltages
Fig ure 9 sho ws calculated electrical conductivities of thre e voltage levels. T he estimate is positively propo r-
tional to the applied voltage level. When the temperature is between 22 and 60˚C, the electrical conductivity
of the soya milk has a positive temperature effect. Afterwards, it saturates.
This first-order dynamics effect is caused by the electro-osmotic effect [6]. Only in the beginning, a large
amount of ions and salt in the soya solid dissolves in the milk.
5. Conclusion
In this paper, we have presented the potential app lication of co mbing ohmic heati ng and electrical co nductivity
measurement in tofu making by heat treatment. In theory, the soya milk can be uniformly cooked by ohmic
Table 1. Temperature rising rates o f different applied vo ltages.
Applied voltage(V)
140 150 160
Sam ples 3 3 3
Mea n 1.46
Std 0.24 0.7 0.61
1Mean ± Std. with different superscripts in th e same row are significantly different (P < 0.05), using the Scheffe test.
0500 10001500 2000 2500 3000
Author, Author
Figure 7. Heating current for a temperature rising rate of
Figure 8. M eas ured and calculated el ectrical conductivities.
Figure 9. Calculated electrical conductivities at different volta-
ge levels.
0200 400 600 8001000 1200 1400
Temperat ure
0200 400600 80010001200 1400
Electrical conductivity
20 3040 5060 7080 90100
Author, Author
heating. The electrical conductivity responses to different voltage levels show clear cuts around 60˚C, at this
temperature the oil body of the soya milk is released. At temperatures around 75˚C and 80˚C, the electrical con-
ductivity response reaches a minimum saddle and starts to increase. These two temperatures correspond to the
release of the 7S and 11S proteins, major proteins in tofu produce. Hence the electrical conductivity measur e-
ment could be a fast, convenient, and non-destructive tes ting method for on-line indicatio n of tofu quality.
Thi s work was fina ncial ly sup po rted by the N atio nal Scie nc e Co uncil of T ai wan und er t he G rant N o. 102-2221-
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