Washed soya beans (800 g) were soaked for 8 - 12 hrs at a constant temperature (30˚C) in reverse osmosis (RO) water. One time by 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 the residue. The soya milk produce was adjusted to a concentration of 10˚ Brix. The procedure of preparing soya milk is summarized as Figure 1.

The performance of ohmic heating in food processing depends on the electrical conductivity of the foodstuff. An electrical current flowing through the foodstuff has power conduction by the moving ions in the foodstuff fluid. The viscosity of the fluid decreases in response to a hotter fluid. This speeds up the ionic motion and hence the electrical conductivity. The food fluid consists of conductive and resistive components which in turn, convert the electric current into heat [9] [10].

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 storage and whey proteins by weight. The 7S protein, conglycinin, and 11S protein, glycinin, are the two major proteins that construct the property of a tofu produce [11].

Figure 2 depicts the mechanism of soya milk transforming to tofu 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 with a size of 100 nm [11]-[13]. The transformation completes.

The process of tofu coagulation by ohmic heating is graphically illustrated in Figure 3. The ions become a good conductor to the impinging current in ohmic heating and the milk is therefore heated up by the current. Ions dissolve in the milk uniformly. Hence the milk will have a homogeneous temperature distribution in the whole volume.

Figure 4 presents ohmic heating in soya milk as a circuitry model. The resistance R characterizes the intrinsic electrical conductivity of the soya milk and the capacitance C describes the capacitive effect built by the two electrode plates with the soya milk as the electrolyte in between. The circuit has the following first-order dynamics:

The capacitive effect is transient. In theory, it becomes a barrier to DC (direct current) power and energy storage to alternate current (AC).

While the temperature of the soya milk increases, the electrical conductivity of the soya milk increases in a positive trend as a result of more ionized subunits [1] [11]-[12]:

where is the electrical conductivity at current temperature T and is the electrical conductivity at the reference temperature T_ref. Ions in an electrolyte are easily to escape from the substance molecules at a hot atmosphere. Different substances will have different properties and hence the is substance-specific [10] [14].

The setup for ohmic heating experiments and ultrasonic measurement is shown in Figure 5. Readers are referred to [3] for details of ultrasonic measurements. Soya milk is placed in a steel container (L × W: 11 × 7 cm²) with ceramic coating. The two titanium electrodes are powered with an AC/DC power supply (APS-1102, Instek, Taiwan) which is remotely controlled by a computer using the LabVIEW software (V8.3, National Instrument, USA). The temperature of the soya milk is detected using a TM-925 temperature transducer (Prosperous, Taiwan).

A 1 MHz ultrasonic transducer (WesternNDE, Canada) is mounted onto the sample chamber. A computercontrolled ultrasonic instrument (WT-UT-001A, Western NDE, Canada) excites the transducer to emit a chirp of ultrasound waves into the chamber, amplifies the received ultrasound waves at a gain of 60 dB, and acquires responsive waveforms at a sampling frequency of 100 MHz. Ultrasonic responses and inferred data are displayed on-line for visual inspection and recorded for subsequent off-line analysis. The transducer was calibrated

with RO water as the reference medium.

In theory, the velocity of an ultrasound wave travelling in a medium is determined by:

where E is the elastic modulus and ρ the density of the milk. The modulus of elasticity increasing as a function of time in coagulating tofu gels and should, according to the above equation, increase the velocity.

Figure 6 shows ultrasonic measurements of ultrasound waves travel in coagulating tofu gels under traditional heat treatment [3]. Its velocity profile is contradictory to the above statement. This is because, in traditional heat treatment, the formation of tofu gels (clusters and air bubbles) alters the molecular structure and intermolecular interactions [4] and hence the elasticity changes in a dramatic degree because of the presence of airbubbles.

Figure 7 show ultrasonic velocity measurements under ohmic heating, which have totally contradictory profiles to Figure 6. There is no doubt that the soya tofu gels are in a liquid state at the beginning of coagulation. But in the course of ohmic heating more protein clusters aggregate probably in a homogeneous way and hence the elastic modulus of tofu gels strengthens. The homogeneous coagulation comes from that the electrical current of ohmic heating can penetrate every protein in the soya milk and hence all proteins could have the same intense of ionisation. This homogeneity of protein ionisation is unlikely to occur in traditional heat treatment that heats the soya milk through non-uniform heat convection.

Figure 8 shows electrical conductivities of 4 tofu gels with different concentrations of coagulant. The EC measurement has first-order dynamics. This behaviour is identical to that of ultrasonic velocity and attenuation. The denser the coagulant is the higher the electrical conductivity. The EC is low in the beginning of ohmic heating and nearly saturates in steady state.