Food Sci. Technol. Res., 19 (6), 1071–1075, 2013

Filtration Behavior during Soymilk Separation Process

Shiori idogawa1,2*, Kensuke ito

1 and Tomoyuki Fujii2

1 Taishi Foods Inc., 68 Aza-Okinaka, Kawamorita, Sannohe-machi, Sannohe-gun, Aomori 039-0141, Japan2 Innovative Research Center for Agricultural Sciences, Graduate School of Agricultural Science, Tohoku University, 1-1 Amamiya-machi, Tsutsumidori, Aoba-ku, Sendai, Miyagi 981-8555, Japan

Received August 11, 2012; Accepted August 7, 2013

In this study, constant-pressure filtration was carried out to examine the blocking mechanisms during soymilk separation. In the filtration experiments, okara was deposited on filter paper of the membrane module in view of the separation process in soymilk. We found that a standard blocking model could be applied to explain the filtration behavior of soymilk by the okara layer at temperatures between 70 and 90℃. The initial filtration rate monotonically increased depending on decreasing viscosity, but the block-ing coefficient decreased as the soymilk temperature increased below 85℃, but increased at 90℃. In or-der to clarify the temperature dependence of the dispersion states of soymilk particles, we determined the particle size distribution and ζ-potential of soymilk particles. The temperature dependence of the block-ing coefficient was related to the average size of particles in soymilk. Therefore, the behavior of blocking in the okara layer would be considered the main issue in the soymilk separation process.

Keywords: soymilk, okara, filtration, blocking mechanism, soymilk particle

*To whom correspondence should be addressed.E-mail: s-itono@taishi-food.co.jp

IntroductionSoymilk is mainly obtained from the heated slurry pro-

duced by grinding soybeans in water. The slurry is ladled into a coarsely woven cloth sack. The sack is then pressed either with a traditional lever or with a more modern hydrau-lic press or centrifuge (Shurtleff and Aoyagi, 1975). In recent years, membrane separation has been the most commonly adopted soymilk separation process, compared to centrifugal and other processes, because it offers energy, dewatering, and detergency advantages (Sasaki, 1981). However, this process is complicated by the presence of proteins, lipids, and poly-saccharides that range in size from nanometers to millimeters (Ono et al., 1991; Morita, 1965). The protein particle aggre-gates formed by the rearrangement of soy protein subunits during heating were found to be 40 − 200 nm in diameter, and these particles interacted with soymilk lipids (Ono et al., 1996; Ren et al., 2009). In contrast, the tofu manufacturing process requires handling of the slurry at a high temperature (Toda et al., 2007), because the yields of protein and lipid in soymilk are affected by the temperature in the squeezer. The hot slurry is divided into the soymilk, which passes through a

screen, and okara is deposited on a screen in the squeezer. In such a case, separation in a squeezer is determined not by the screen mesh size (usually approx. 100 μm) but by the cake layer, like okara in soymilk membrane separation, that forms on the screen. Many factors, such as the characteristics of the multicomponent system, temperature and state of the cake layer, would change separation efficiency and create a major obstacle for the theoretical analysis of constituent complexity (Nabetani, 1998).

To date, there have been many investigations that have attempted to understand filtration mechanisms in the field of microfiltration (Grace, 1956; Granger et al., 1985; Hurt and Barbano, 2010; Tung et al., 2010; Vishwanathan et al., 2011). The filtration behavior in a membrane could be ana-lyzed using a blocking model based on mathematical expres-sions (Hermans and Bredée, 1936). However, the slurry is a highly concentrated system and high-temperature treatment is generally involved in the separation processes of tofu manufacturing. Few studies have applied blocking mecha-nisms to concentrated or heated slurry despite the industrial importance of these variables. There are many examples in the food industry of research into membrane separation, such as refining of dairy whey (Seki et al., 2012) or oligo-saccharide (Matsubara et al.,1996), fruit juice (Ohta, 1996),

S. idogawa et al.

Okara (30 g) was placed into a mold to form its layer on nonwoven rayon fiber filter paper (Advantec Toyo Kaisha, Ltd., Tokyo, Japan) and set into the filter unit. An okara layer was formed at a thickness of 1.7 ± 0.4 mm, as determined using a caliper. Increase in filtrate weight was measured us-ing an electronic balance. We first checked whether the filter paper would have filtration resistance. When the preliminary filtration test was conducted without the okara layer, we con-firmed that the permeation of 4 L of soymilk did not show filtration resistance (data not shown).

Pore blocking model For constant-pressure filtration (Hermans and Bredée, 1936), the following equation is valid:

dvd t k dv

dt n

2

2

= d n (1)

where n denotes a constant, t the filtration time, and v the filtrate weight. The k-value for a particular mode of filtration depends on the system, the filter medium, and the specific fil-tration conditions. In constant-pressure filtration, n suggested three possible mechanisms to measure the blocking of filter media; complete blocking: n = 2, standard blocking: n = 1.5, and intermediate blocking: n = 1.

Evaluation of physicochemical properties of soymilk par-ticle The particle size distribution of soymilk was evaluated from the autocorrelation function determined using a dy-namic light scattering (DLS) technique. DLS measurements were carried out at a scattering angle (θ ) of 165° using an Otsuka ELSZ-2 analyzer (Otsuka Electronics Co. Ltd., Osa-ka, Japan). We analyzed the autocorrelation functions with the CONTIN program and obtained multimodal particle size distribution curves. Soymilk was analyzed as a non-dilutive solution at the described temperatures.

The ζ-potential was evaluated using an aqueous electro-phoresis technique. Aqueous electrophoresis measurements were also carried out using soymilk diluted 1:100 in water in an ELSZ-2 analyzer (Otsuka Electronics Co. Ltd., Osaka, Japan).

Results and DiscussionProperty of filtration behavior of soymilk separation

Figure 2a shows the courses of filtrate weight (v) for five temperature conditions. The initial permeation rate of the fil-trates increased with increasing temperature, but the final fil-trate weights decreased at temperatures over 85℃. Figure 2b shows the double logarithm plot of the values of d2t/dv 2 and dt/dv calculated from the data in Fig. 2a on the basis of equa-tion (1). As shown in Fig. 2b, these plots could be described with a single straight line (coefficient of determination R2 = 0.861 to 0.903) at each temperature. The linear slope n was approximately 1.5 (standard deviation 0.07), corresponding to the standard blocking model. However, the inclination

processing of soy sauce (Furukawa, 1997) and improvement of the flavor of soymilk (Harada, 2004). However, there is no example of research on high temperature filtration.

Filtering highly concentrated slurry resulted in the forma-tion of a cake layer on the filter medium, making analysis difficult. This study was conducted on the assumption that the cake layer comprised okara in soymilk separation. We constructed constant-pressure filtration experiments un-der concentrated, multi-component, heated conditions and analyzed the results using a blocking model to assess the temperature dependence of each parameter. We also evalu-ated the influence of both high temperature and particle size distribution on the separation process from the viewpoint of a dispersion system.

Materials and MethodsMaterials Soymilk and okara were provided by Taishi

Foods Inc. (Aomori, Japan). Soymilk was adjusted with wa-ter to a total solid content of 11.5% (w/w) and contained 5% proteins, 3.2% lipids, 2.6% carbohydrates, and 0.7% ash.

Filtration experimental apparatus A schematic diagram of the experimental apparatus used in this study is shown in Fig. 1.

The soymilk (4 L) was pre-heated at a given temperature. In each experiment, filtration pressure was supplied by com-pressed nitrogen and adjusted to 0.15 MPa using a pressure regulator. The filter unit (Advantec Toyo Kaisha, Ltd., Tokyo, Japan) was maintained in a thermostatically controlled water bath. The diameter of the membrane module was 150 mm.

In the squeezer for soymilk separation, the deposited okara layer is continuously discharged using a screw. There-fore, the okara layer is maintained at a fixed thickness on the screen. Before filtration experiments, okara was deposited on filter paper in view of the separation process in soymilk.

Fig. 1. Schematic diagram of the experimental apparatus for constant-pressure filtration.

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(Fig. 4b). The temperature dependence of the blocking coef-ficient Ks suggests that the filtration behavior with high tem-perature was different from that with low temperature.

of the slope at 90℃ was a little small. Each plot was repro-duced in five separation experiments. Soymilk was an emul-sion comprised of protein and lipid particles, which were ≤ 1% the size of the okara particles. This might result in a filtration scenario where colloid particles pass through the okara layer and accumulate in its interspace. As mentioned above, it was judged that application of a filtration model could be performed.

Analysis of filtration behavior using a standard blocking model It is assumed that the okara layer has uniform capil-larity, in which the number and length remain constant but the diameter decreases as okara residue accumulates during the filtration process:

π(r02 − r2)LN = hv (2)

N: number of capillaries [-]r: radius of capillary at any time [mm]r0: radius of capillary at t = 0 (and v = 0) [mm]h: volume of okara residue accumulation per unit weight

of filtrate [mm3/g]L: average capillary length [mm]

According to Poiseuille’s law, filtration rate (dv/dt) is related with the radius of the capillary and can be rewritten as

dtdv

dtdv

dtdv

dtdv

rr

LNrh v

K v

1

1 s

0 0

4

0 02

2

0

2

�

=

=

=

-

-

d d

d d

d ]

n n

n n

n g (3)

where Ks is expressed as

K LNrh

s02�= (4)

The integral of equation (3) is

vt

dtdv K t1

s

0

= +d n

(5)

Therefore, if the experimental results can be linearly ex-pressed in a t/v vs. t plot, the filtration behavior can be ex-plained using a standard blocking model. Then the intercept (dv/dt)0

−1 indicates okara medium resistance at the initial fil-tration, and the slope Ks indicates the blocking coefficient for level of membrane blocking. Figure 3 shows the t/v vs. t plot for soymilk separation in the range of 70 to 90℃. It demon-strated that the standard blocking model could be applicable to the filtration behavior at each temperature. The initial filtration rate (dv/dt)0 monotonically increased with soymilk temperature (Fig. 4a), and was reproduced in three separation experiments. Ks decreased as soymilk temperature increased below 85℃, but increased at 90℃ in all three experiments

Fig. 3. t/v vs. t plots of soymilk filtration at each temperature.

Fig. 2. Weight vs. time (v vs. t ) plots for each temperature (a). The double-logarithmic plots of equation (1) were applicable to a linear model (b).

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size distribution and ζ-potential of soymilk particles. Figure 5 shows the particle size distribution of soymilk particles. The results shown in Fig. 5 indicate that the average size of soy-milk particles decreased as the temperature increased below 85℃. On the other hand, the particle size increased at 90℃. From this result, it was suggested that particulate capture in the pore of okara layer would be effective at 90℃ because particle diameter would be large at this temperature. Figure 6 shows the result of the ζ-potential of soymilk particles. There was an increase in the magnitude of the electrical charge of soymilk particles depending on temperature, which suggested that the particles were stabilized as the temperature increased. This electrostatic effect might result from the denaturated protein particles and/or the thermal change of the state of in-terfacial layer surrounding the lipid particles.

Analysis of temperature dependence of the blocking pa-rameter The blocking model could be applied to the soy-milk separation process. The initial filtration rate (dv/dt)0 was inversely proportional to dispersion media viscosity (Grace, 1956; Sugimoto, 1992). In this study, the dispersion medium was an aqueous solution, and its viscosity exhibited a mono-tonic decrease as temperature increased. The initial filtration rate (dv/dt)0 was inversely correlated with viscosity (data not shown). The blocking coefficient Ks became convex down-ward as the soymilk temperature increased. The behavior of the blocking coefficient depended on the volume of okara residue accumulation per unit volume of filtrate h, because L, N, and r in equation (4) were thought to remain constant dur-ing the separation process. A larger h indicated that more and/or larger particles would be caught in the pores of a separa-tion-active layer, like the okara layer in this case. Therefore, in order to clarify the temperature dependence of the disper-sion states of soymilk particles, we determined the particle

Fig. 5. Particle size distribution of soymilk particle at each temperature.

Fig. 6. The electrical charge (ζ-potential) of soymilk particle at each temperature.

Fig. 4. The initial filtration rate (dv/dt)0 (a) and the blocking coef-ficient KS (b) of three experiments at each temperature.

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Matsubara, Y., Iwasaki, K., Nakajima, M., Nabetani, H. and Nakao, S. (1996). Recovery of oligosaccharides from steamed soybean waste water in tofu processing by reverse osmosis and nanofiltra-tion membranes. Biosci. Biotech. Biochem., 60, 421-428.

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Ono, T., Takeda, M. and Shuntang, G. (1996). Interaction of protein particles with lipids in soybean milk. Biosci. Biotech. Biochem., 60, 1165-1169.

Ren, C., Tang, L., Zhang, M. and Guo, S. (2009). Structural charac-terization of heat-induced protein particles in soy milk. J. Agric. Food Chem., 57, 1921-1926.

Sasaki, F. (1981). Dewatering of sludge by FKC-Screw-Press. Ja-pan Tappi J., 34, 593-603 (in Japanese).

Seki, N., Kinoshita, K., Saitoh, H., Ochi, H., Iwatsuki, K., Okawa, T., Ohnishi, M., Tamura, Y. and Ito, A. (2012). Analysis of flux change on nanofiltration of dairy whey. Kagaku Kogaku Ronbun-shu, 38, 90-101 (in Japanese).

Shurtleff, W. and Aoyagi, A. (1975). The book of tofu; food for mankind Vol. I. Soyfoods Center, pp. 87.

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Toda, K., Chiba, K. and Ono, T. (2007). Effect of components ex-tracted from okara on the physicochemical properties of soymilk and tofu texture. J. Food Sci., 72, C108-113.

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Vishwanathan, K.H., Govindaraju, K., Singh, V. and Subramanian, R. (2011). Production of okara and soy protein concentrates using membrane technology. J. Food Sci., 76, E158-164.

Generally speaking, the behavior of the blocking coef-ficient corresponds with the average sizes of the particles in a filtration medium. From the ζ-potential of 90℃ having been smaller than that of 85℃, it was assumed that the behavior of Ks was affected not by an electrostatic effect but a particle size effect. In defatted soymilk, most protein particles range from 40 to 200 nm in diameter (Ono et al., 1996). Therefore, the > 400-nm-diameter particles shown in Fig. 5 would be lipid particles. However, measurement of the diameter of a particle in such a high concentration system was the first in soymilk, and the characteristics of these large particles are still unclear. The soymilk particles might interact with each other or dis-sociate with increased temperature. It is not clear how or why soymilk particle sizes change depending on temperature. Our next investigation will examine the effect of each component and the cross-interaction of proteins and lipids.

In conclusion, a standard blocking model was applicable to the filtration behavior of soymilk at temperatures between 70 and 90℃. Our results indicated that the filtration behavior of the soymilk separation process was dependent on temper-ature. Furthermore, it was found that control of colloid par-ticle size in soymilk emulsions would be an important factor in the soymilk separation process.

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Hurt, E. and Barbano, D.M. (2010). Processing factors that influ-ence casein and serum protein separation by microfiltration. J. Dairy Sci., 93, 4928-4941.

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