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Effects of moisture content on mechanical properties, transparency, and thermal stability of yuba film
Food Chemistry 243 (2018) 202–207
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Effects of moisture content on mechanical properties, transparency, and thermal stability of yuba film Siran Zhang, Nayeon Kim, Wallace Yokoyama, Yookyung Kim
MARK
⁎
Department of Scientific Research, Zhaoqing University, Guangdong, China Western Regional Research Center, Agricultural Research Service, Albany, CA, USA Department of Human Ecology, Graduate School, Korea University, Seoul, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Yuba film Sorption characteristics Transparency Thermal stability Mechanical properties
Yuba is the skin formed at the surface during the heating of soymilk. The 3rd, 7th, and 11th films were evaluated for properties at different RH. At 39% RH, the 11th film had the lowest moisture, while the 3rd film had the highest moisture. However, at 75% RH, reverse moisture results were obtained. The tensile strengths of the 3rd and 11th films were highest at 15% moisture, whereas the tensile strength of the 7th film was highest at 25% moisture. Elongation of the 3rd (127%) and 11th (85%) films were highest at 25% moisture. The light transmittance of the films was low and opaque at 5% moisture. The films were transparent at 23%–28% moisture, but became opaque as the moisture increased. The films at 39% RH (ΔH, 113–203 J/g) had higher thermal stability than those at 87% RH (ΔH, 315–493 J/g). Moisture content markedly changed the yuba film properties.
1. Introduction Yuba or tofu skin is a soy protein-lipid network film that forms on the surface of heated soymilk. Successive films can be lifted from the heated milk and dried for storage (Su & Chang, 2002). As a wrapping for meats and vegetables, dry yuba rehydrates in water and becomes a soft and elastic milky wet film ready for use (Alan, 2008). However, like other soy protein-based films, yuba is fragile in the dry state and has poor moisture barrier properties (Shurtleff & Aoyagi, 2013). Fresh or completely wet yuba is less fragile, but its mechanical properties such as tensile strength are inadequate to wrap and pack food (Chen & Ono, 2010). Many studies have been carried out to optimize yuba processing to increase yield and formation rate (Wang, Swain, Kwolek, & Fehr, 1983), to characterize and improve its appearance in foods (Chen, Yamaguchi, & Ono, 2009), and to improve its mechanical and storage properties (Yuan, Hu, & Yu, 2012). However, the utilization of yuba in non-conventional foods is limited. A potential use could be edible packaging film. Yuba is typically used and studied in the completely wet or dry state. The properties of yuba when between the completely wet and dry states, half-dried yuba (Shurtleff & Aoyagi, 2013), is being studied to develop new applications. The properties of the half-dried phase differ from those of the wet and dry states, and yuba seems to have suitable characteristics for use as edible packaging films. Water is not only the hydrating agent but is also an effective plasticizer that improves the flexibility and mechanical properties of the film (Gontard,
⁎
Guilbert, & Cuq, 1993). The objective of this study was to evaluate the mechanical properties, transparency, moisture sorption properties, and thermal stability of yuba films obtained at different points of successive skimming and stored at different relative humidities. 2. Materials and methods 2.1. Yuba processing Yuba films were prepared by using a modified method of Chen et al. (2009). Soymilk was prepared by soaking soybeans with a water ratio of 1:7.5 (concentration of soybean: 8.6%). The boiled soymilk was then transferred to a stainless steel yuba forming pan (15 × 18 × 6 cm) placed in a water bath. The soymilk level was adjusted to 2.3 cm and the temperature of the soymilk was maintained at 85 °C. Successive yuba films were collected at 15 min intervals and labeled sequentially. Eleven sheets of yuba were formed, and the third (Y3), seventh (Y7), and eleventh (Y11) films were chosen for analysis. Yuba was dried (Convection Oven, Cho Sun Science Co., Seoul, Korea) at 60 °C for 120 min, wrapped in plastic film, and stored at −18 °C until use. 2.2. Moisture sorption characteristics 2.2.1. Moisture sorption curve The dry yuba films (20 × 25 mm) were weighed, and the initial
Corresponding author at: Department of Human Ecology, Graduate School, Korea University, Seoul, South Korea. E-mail address: [email protected] (Y. Kim).
http://dx.doi.org/10.1016/j.foodchem.2017.09.127 Received 2 June 2017; Received in revised form 25 September 2017; Accepted 26 September 2017 Available online 28 September 2017 0308-8146/ © 2017 Elsevier Ltd. All rights reserved.
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Tokyo, Japan). The yuba samples of 3.4 mg equilibrated at 39% RH or 87% RH at 25 °C were placed in an aluminum DSC pan. The pan was hermetically sealed and equilibrated at 4 °C for 12 h. The blank was an empty aluminum DSC sealed pan. The thermal transition of yuba was measured by heating from 20 to 275 °C at a rate of 5 °C/min. The onset temperature (To), peak temperature (Tp), glass transition temperature (Tg), and denaturation enthalpy (ΔH) were recorded.
moisture content (M0) was measured using a halogen moisture analyzer (HB43-S, Mettler Toledo, Greifensee, Switzerland). The yuba specimens were then placed in separate desiccators containing saturated salt solutions at 25 °C, and controlled by a hygrometer (Testo 608-H2, Lenzkirch, Germany). The specific relative humidity (RH) of the saturated salt solutions at 25 °C were 39.5% (CaCl2), 57% (NaBr), 75% (NaCl), and 86% (KCl). Deionized water was used to provide 100% RH. The weights of the yuba specimens were measured at intervals of 2 or 4 h until two the consecutive weightings were the same; at this point, it was assumed that an equilibrium condition had been reached. The moisture adsorption curves of yuba were fitted to Peleg (1988).
t M (t ) = M0 + k1 + k2 t
2.6. Statistical analysis Statistical analysis was performed using the software program SPSS 19.0. All data for the analysis of variance (ANOVA) and significance was set at p < 0.05. Duncan’s multiple range test was applied to determine the significant differences among the mean values. All experiments were performed in triplicate, except for sorption curve equations and DSC.
(1)
where M(t) is the moisture after time t, M0 is the initial moisture, k1 is the Peleg rate constant (h%−1), and k2 is the Peleg capacity constant (%−1).
3. Results and discussion 2.2.2. Sorption isotherm curve The equilibrium moisture content was calculated from the increase in mass of the dry sample after equilibration at a given relative humidity. The Guggenheim–Anderson–de Boer (GAB) model (Bizot, 1983) was used to fit the soy protein film sorption isotherm data, and the monolayer values for moisture were calculated from the equations.
3.1. Moisture sorption characteristics
2.5. Differential scanning calorimetry
3.1.1. Moisture adsorption kinetics of yuba films As shown in Fig. 1A, all yuba films absorbed moisture rapidly in the first few hours and reached a plateau in less than 10 h at 37% RH and 75% RH but did not plateau until after 20 h at 99% RH; the plateau indicates equilibration has been reached at the specific relative humidity (Cho & Rhee, 2002). Although moisture content was much higher at the higher RH at the start of the rehydration, a longer time was required for the yuba film to reach moisture equilibration when the storage relative humidity was higher. At RH of 99% and 75%, the yuba films that were collected at a later processing time showed higher moisture at equilibrium and reached equilibrium later than the films collected earlier. This may be explained by the higher hydrophobic components (aggregated protein and lipid) in the earlier films and the higher hydrophilic components (carbohydrates) in the later yuba sheets (Wu & Bates, 1972). At was 39% RH, all films reached equilibrium in 10–20 h. Y11 was the last while Y3 was the first to reach equilibrium at 39% RH. Y11 also showed the lowest moisture content at equilibrium, while Y3 showed the highest moisture content and Y7 showed an average moisture content. Data measured from the sorption curve of the yuba films were fitted to Eq. (1), and the sorption curve equations and their constants (k1 and k2) were calculated as shown in Table 1. The k1 values indicate the initial moisture adsorption rate of the yuba film; the higher k1 value indicates the lower adsorption rate. The k2 values indicate the moisture content of the film had absorbed when the film reached equilibrium; the higher k2 value indicates less absorbed moisture (Cho & Rhee, 2002). The coefficients of determination in all cases were very high (r2 > 0.99), indicating excellent fit of the equations to the experimental data. At 75% RH, the moisture isotherm equation k1 values of the successively collected yuba films increased (5.39, 5.79, and 8.03) from Y3 to Y11, but the k2 values decreased from 10.62, 8.91, and 6.47 for the Y3 to Y11 samples, respectively. This means that, as the soymilk was heated, the yuba collected earlier absorbed moisture more rapidly in the first few hours, but the maximum moisture content at a given RH (75% or 99%) was lower than the films collected later. However, when stored at 39% RH, both the k1 and k2 values of all films increased with the order of collection (k1 and k2 values increased from 11.29 to 39.89 and from 18.18 to 47.19, respectively). For the yuba films that were collected later, more time was needed to reach equilibrium, even though they gained the least amount of moisture. The results are consistent with the moisture sorption curves.
The thermal transition properties of yuba at two different relative humidity were determined using a Seiko SII-EXSTAR 6000-DSC-6200 differential scanning calorimeter (DSC 6200, SII Nano Technology Inc.,
3.1.2. Moisture sorption isotherms of yuba films The moisture adsorption data of the yuba films fitted to the GAB models (Table 2) and isotherms are shown in Fig. 1B. The moisture
W=
m 0 Cka w (1−k a w)(1−k a w + Ck a w)
(2)
where W is the moisture content of the material in a dry state, aw is the water activity, m0 is the moisture content (d.b) at a monolayer, and C and k are the constants depending on temperature. 2.3. Mechanical properties of yuba The yuba films were conditioned at a given RH to reach a constant moisture content. A 3 × 6 cm sheet of film was folded three times into 1 × 6 cm. A Compac-100II rheometer (Sun Scientific Co. Ltd., Tokyo, Japan) was used to measure the tensile strength and elongation of the film. The original distance of the strain was set as 4 cm and the moving speed of the table was set as 5 mm/s. Tensile strength and elongation were calculated using the software provided by the manufacturer (Rheology data system 3.0, Tokyo, Japan). 2.4. Transparency The yuba films (8 × 10 mm) were weighed (w0) before being soaked in deionized water for 5 min. The remaining water on the surface was gently removed with a kitchen towel. The wet yuba was placed onto the outer side of a spectrophotometer cell (wc) and the cell was inserted into the spectrophotometer (Ultrospec 2100, GE Healthcare Biosciences Corp., New Jersey, USA). The transmittance was read at 500 nm, the cell was weighed with the film (wt) at 5 min intervals for 75 min, and an empty spectrophotometer cell was used as the blank. The moisture content M (t) of the film at t min can be expressed as follows:
M (t ) =
(wt−wc )−w0 wt−wc
where M(t) is the moisture content at time t, w1 is the weight of the film at 5 min intervals for 75 min, wc is the weight of the spectrophotometer cell, and w0 is the initial weight of the film.
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(A)
(B) 0.5
RH 99%
0.4
0.3
0.2
RH 75%
0.1
Moisture content (g H 2O/g solids)
Moisture content (g H
2 O/g
solids)
0.5
0.4
0.3
0.2
0.1
0.0
RH 37%
0.0 0
10
20
30
40
0.0
50
0.2
0.4
0.6
0.8
1.0
Water activity
Time (h)
Fig. 1. Moisture sorption curves of yuba at various relative humidities (A) and sorption isotherms (B) for yuba at 25 °C.
sheets were collected. The mo were 0.045, 0.051, and 0.076 g H2O/g solids of monolayer moisture contents for the Y3, Y7, and Y11, respectively.
Table 1 Constant value (k1 and k2) and coefficient of determinations (r2) for sorption curve equations of yuba at selected relative humidity. Sample
Y3 Y7 Y11
39% RH
75% RH
99% RH
k1a
k2a
r2
k1
k2
r2
k1
k2
r2
11.29 13.48 39.89
18.18 20.88 47.19
0.9996 0.9974 0.9996
5.39 5.79 8.03
10.62 8.91 6.47
0.9995 0.9999 0.9984
6.44 8.26 8.54
2.98 2.61 1.99
0.9996 0.9995 0.9994
3.2. Mechanical properties of yuba films Fig. 2A shows the effects of moisture content on the tensile strength of yuba films. In general, the tensile strength of yuba films ranged from 1.06 to 1.71 MPa, when the moisture content was about 15%. In the cases of Y7 and Y11, as the moisture content increased to above 25%, the tensile strength decreased rapidly. The decreased tensile strength values for yuba films can be explained by the plasticizing effect of water (as the film moisture content increased) on the hydrophilic components such as proteins and carbohydrates (Ashley, 1985; Gennadios, Park, & Weller, 1993). However, Y7 had slightly increased tensile strength values at around 25%. Compared to Y11, the Y3 and Y7 films that were collected earlier had higher tensile strength values. When the moisture content of the films was 22%, the tensile strengths were 1.25 MPa, 1.08 MPa, and 0.84 MPa for the Y3, Y7, and Y11, respectively. The elongation changes of yuba films were observed as shown in Fig. 2B. The yuba films collected earlier showed greater elongation. In general, the elongation values of most yuba films were low (ranging from 5.60% to 6.90%) at low moisture contents (about 15%) followed by a rapidly increasing slope (except Y11), as the moisture content increased up to 25% and reached their peaks (127% of Y3 and 86% of Y7). As the moisture increased, the elongation values decreased sharply. However, for Y11, elongation slowly increased from 6.90% to 53.5% as the moisture content increased from 15% to 30%. The effects of moisture content of yuba on the mechanical properties of the films are due to the plasticizer (lipid, water etc.) to solid ratio, which affects the intermolecular and interfacial interactions of the film network leading to different tensile strengths and elongation values (Gennadios et al., 1993).
Y3: 3rd yuba, Y7: 7th yuba, Y11: 11th yuba. a Unit of k1 and k2 is g of solid/g H2O. Table 2 GAB model constants and coefficient of determinations (r2) for different yuba. Sample
m0a
Cb
kb
r2
Y3 Y7 Y11
0.045 0.051 0.076
1.998 2.447 1.017
0.8817 0.8807 0.8699
0.9976 0.9994 0.9982
Y3: 3rd yuba, Y7: 7th yuba, Y11: 11th yuba. a Monolayer moisture content in g H2O/g solids. b GAB constant.
sorption of all yuba films show a common trend where the equilibrium moisture content increased at a slow rate until storage aw reached 0.75, and then increased sharply at higher aw. Yuba films collected later showed higher equilibrium moisture content. When aw was 0.87, the equilibrium moisture contents of Y3, Y7, and Y11 were 16.64%, 19.23%, and 23.71%, respectively. The relative moisture content differed at a relatively low aw (0∼0.56). The equilibrium moisture content of the successively collected yuba (Y3, Y7, and Y11) were 0.45%, 0.43%, and 0.39%, respectively, at aw of 0.39. The results suggest that a certain threshold of aw is needed before water binding takes place, probably by the higher content of hydrophilic carbohydrates in yuba films collected later. At relatively low aw, the yuba films that were collected earlier with a higher protein content may initiate earlier moisture absorption. The data in Table 2 show that the calculated GAB model constants and coefficient of determination were very high (r2 > 0.99). When the sorption data was fitted to the GAB model, increased monolayer values were observed in the order in which the yuba
3.3. Transparency of yuba films The moisture desorption curves of each film in a given environment are shown in Fig. 3A. As the film thickness decreased, the initial moisture content observed also decreased (r2 = 0.854, p < 0.001). Regardless of the initial moisture content, the moisture content decreased rapidly in the first 45 min and then decreased slowly up to 80 204
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(A)
(B) 160
1.8
140
1.6
120
Elongation (%)
Tensile strength (MPa)
2.0
1.4 1.2 1.0 0.8
100 80 60 40
0.6
20
0.4
0
0.2
14
16
18
20
22
24
26
28
30
32
14
16
18
Moisture content (%)
20
22
24
26
28
30
32
Moisture content (%)
Fig. 2. Effects of moisture content on tensile strength (A) and elongation (B) on yuba film.
protein network and decreased difference in refractive indices (Yang & Paulson, 2000). This decreased the scattering surfaces and allowed the light to pass through, which increased the optical transmittance. However, as the moisture content continued to increase and approach saturation, intermolecular agglomeration occurred causing turbidity and more light scattering (Feng & Winnik, 1997). However, among the films with different moisture contents, Y11 showed the lowest %T. This may be due to the greater thickness, higher carbohydrate ratio, and more Millard reaction products of Y11 (Ou, Wang, & Jin, 2005; Han, Ishitani, & Li, 2005).
min. The rates of moisture content loss from fast to slow were in the order of Y3, Y7, and Y11 due to the higher carbohydrate ratio of Y11 which holds water tightly (Chen et al., 2009). The optical transparency differences of the yuba films with the change of moisture content were observed (Fig. 3B). The light transmission (%T) of the yuba films at 500 nm can be correlated with the moisture contents as shown in Fig. 3B. At a very low moisture content (about 5%), the light transmittance of the yuba films was low and the films appeared to be almost opaque. As the moisture content increased, the optical transmittances of the yuba films increased rapidly, and reached their peak values (73.9%, 75.7%, and 64.4% of transmittance for Y3, Y7, and Y11, respectively) at a moisture content of 23%∼28%. The films at this stage were nearly transparent. However, the optical transmittances decreased rapidly again as the moisture content in the yuba films increased and the films became even more opaque (%T < 0.70%) at saturated moisture contents. The films with lower moisture content (5%) were dense, and the light was scattered due to differences in refractive indices of the surfaces of solid particulates and lipid droplets (Lim, Mine, & Tung, 1999). When the moisture content increased, the hydration of the protein network reduced the inter-chain interactions and enhanced the chain mobility, resulting in a loosened
3.4. Differential scanning calorimetry of yuba films The differential scanning calorimetry (DSC) curves of the yuba films conditioned at 39% RH and 87% RH are shown in Fig. 4. Yuba films conditioned at 39% RH showed endothermic peaks (peak temperature, Tp) at 179–182°C, and the films conditioned at 87% RH showed endothermic peaks at 175°C. The onset temperature (T0) of films conditioned at 39% RH was also higher than those conditioned at 87% RH. The peaks of the films in the 39% RH group tended to become sharper at higher successive collection, while the peaks of the films in the 87%
(A)
(B)
600
80
500
Light transmittance (%)
Moisture content (g/kg)
400
300
200
60
40
20 100
0 0 0
20
40
60
80
0
100
200
300
400
Moisture content (g/kg)
Time (min)
Fig. 3. Desorption curves of yuba film (A) and light transmittance of yuba film (B).
205
500
600
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DSC/mW
Relative humidity
DSC/mW
Relative humidity
Fig. 4. DSC curves of yuba films at relative humidity of 39% (A) and 87% (B).
indices. The moisture content of the yuba film must be carefully controlled to meet the required mechanical and optical properties of the film for their specific applications. Based on its thermal stability and mechanical properties, yuba with 25% moisture content, stored at 39% RH at the 7th film collection appears to have the greatest potential for use as edible films.
RH group seemed to decrease in the same order. The moisture sorption patterns described earlier (Fig. 1) show that at 39% RH, the films that were collected later absorbed less water than the films that were collected earlier (moisture content in yuba films: Y3 > Y7 > Y11). However, at 87% RH, the results were reversed (moisture content in yuba films: Y3 < Y7 < Y11). Correspondingly DSC of films with higher moisture content showed flatter peaks, while films with less moisture showed sharper peaks. This extremely differences may have been caused by the combined effects of the composition ratios of yuba films. For example, when at 39% RH, although Y11 absorbed less water, the carbohydrate ratio was higher, binding water more tightly and absorbing more energy during analysis (Wang, Qi, & Yang, 2011). Further evidence of the effects of water content can be provided by the denaturation enthalpy (ΔH) values. The ΔH values of each pair of films in the 39% RH and 87% RH groups approach the same value in the order of which the yuba film was collected. Peaks of the high water content group were much sharper than those of the low water content, with the ΔH values in the 87% RH group (ranging from 315 to 493 J/g) being much higher than those in the 39% RH group (ranging from 113 to 203 J/g). Generally, higher ΔH values suggested more hydrophobic/ hydrophilic interactions and lower thermal stability (Ma & Harwalkar, 1991). A similar phenomenon was reported by Kitabatake, Tahara, and Doi (1990), whereby the denaturation temperature of soy protein (7S and 11S) was significantly influenced by the water content, where the lower water content of the sample resulted in higher Tp of the samples. The glass transition temperature (Tg) of yuba films containing lower water content was slightly higher than those with higher water content. Tg of the 39% RH group showed a decrease from 155 to 150 °C, while the 87% RH group showed a decrease from 151 to 149 °C with the increased number of yuba sheets collected. Jiang, Tang, and Wen (2008) suggested that when the moisture content absorbed by the polar groups of films is sufficient to form a BET (Brunauer-Emmet-Teller) monolayer, the monolayer water presents significant plasticizing effect and reduces the glass transition temperature. However, Sobral, Monterrey-Q, and Habitante (2002) reported that when the plasticized state of protein was at a maximum, the Tg of the film with rich protein did not change with moisture nor with the plasticizer concentration.
Acknowledgment This research was supported by the National Research Foundation (NRF) of Korea (NRF-2015R1C1A2A01051996). Reference Alan, P. U. (2008). New food engineering research trends. In L. Z., S. Q. & L. Q. (Eds.), The development of the processing of yuba(protein-lipid film), (pp. 195–223). New York: Nova Science Publisher Inc. Ashley, R. (1985). Permeability and plastics packaging. Polymer permeability (pp. 269– 308). Springer. Bizot, H. (1983). Using the'GAB'model to construct sorption isotherms. In R. Jowitt, (Ed.). Physical properties of foods. London and New York: Applied Science Publishers. Chen, Y., & Ono, T. (2010). The mechanisms for yuba formation and its stable lipid. Journal of Agricultural and Food Chemistry, 58(10), 6485–6489. Chen, Y., Yamaguchi, S., & Ono, T. (2009). Mechanism of the chemical composition changes of yuba prepared by a laboratory processing method. Journal of Agricultural and Food Chemistry, 57(9), 3831–3836. Cho, S. Y., & Rhee, C. (2002). Sorption characteristics of soy protein films and their relation to mechanical properties. LWT – Food Science and Technology, 35(2), 151–157. Feng, J., & Winnik, M. A. (1997). Effect of water on polymer diffusion in latex films. Macromolecules, 30(15), 4324–4331. Gennadios, A., Park, H., & Weller, C. L. (1993). Relative humidity and temperature effects on tensile strength of edible protein and cellulose ether films. Transactions of the ASAE, 36, 1867–1872. Gontard, N., Guilbert, S., & Cuq, J. L. (1993). Water and glycerol as plasticizers affect mechanical and water vapor barrier properties of an edible wheat gluten film. Journal of Food Science, 58(1), 206–211. Han, Z., Ishitani, T., & Li, Z. (2005). Effects of different soymilk concentrations and depth on the formation of yuba. Nongye Gongcheng Xuebao(Transactions of the Chinese Society of Agricultural Engineering), 21(11), 179–181. Jiang, Y., Tang, C., & Wen, Q. (2008). DSC analysis of soy protein isolate films. Food and Fermentation Industries, 34(1), 28–30. Kitabatake, N., Tahara, M., & Doi, E. (1990). Thermal denaturation of soybean protein at low water contents (Food & Nutrition). Agricultural and Biological Chemistry, 54(9), 2205–2212. Lim, L. T., Mine, Y., & Tung, M. (1999). Barrier and tensile properties of transglutaminase cross-linked gelatin films as affected by relative humidity, temperature, and glycerol content. Journal of Food Science, 64(4), 616–622. Ma, C.-Y., & Harwalkar, V. (1991). Thermal analysis of food proteins. Advances in Food and Nutrition Research, 35, 317–366. Ou, J., Wang, X., & Jin, Q. (2005). Effects of soy ingredients on properties of dried bean milk cream in tight rolls. China Oils and Fats, 30, 37–40. Peleg, M. (1988). An empirical model for the description of moisture sorption curves. Journal of Food Science, 53(4), 1216–1217.
4. Conclusion This study showed that yuba has mechanical and optical properties are adjustable by processing conditions and has potential as an edible film. Moisture content has a large effect on mechanical and optical properties because water is an efficient plasticizer and affects refractive 206
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Wang, H., Swain, E., Kwolek, W., & Fehr, W. (1983). Effect of soybean varieties on the yield and quality of tofu. Cereal Chemistry, 60(3), 245–248. Wu, L., & Bates, R. (1972). Soy protein-lipid films. 1. Studies on the film formation phenomenon. Journal of Food Science, 37(1), 36–39. Yang, L., & Paulson, A. (2000). Mechanical and water vapour barrier properties of edible gellan films. Food Research International, 33(7), 563–570. Yuan, Y., Hu, Z., & Yu, K. (2012). Effects of selected processing parameters on the yield and physical characteristics of the yuba of packaging film. Science and Technology of Food Industry, 33, 245–248.
Shurtleff, W., & Aoyagi, A. (2013). History of whole dry soybeans, used as beans, or ground, mashed or flaked (240 BCE to 2013). Lafayette, California: Soyinfo Center. Sobral, P. J., Monterrey-Q, E., & Habitante, A. M. (2002). Glass transition study of Nile Tilapia myofibrillar protein films plasticized by glycerin and water. Journal of Thermal Analysis and Calorimetry, 67(2), 499–504. Su, G., & Chang, K. (2002). Trypsin inhibitor activity in vitro digestibility and sensory quality of meat-like yuba products as affected by processing. Journal of Food Science, 67(3), 1260–1266. Wang, S., Qi, J., & Yang, X. (2011). Effect of soybean polysaccharides on gelatinization and gel properties of rice starch. China Brewing, 7, 155–158.
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Effects of moisture content on mechanical properties, transparency, and thermal stability of yuba film Siran Zhang, Nayeon Kim, Wallace Yokoyama, Yookyung Kim
MARK
⁎
Department of Scientific Research, Zhaoqing University, Guangdong, China Western Regional Research Center, Agricultural Research Service, Albany, CA, USA Department of Human Ecology, Graduate School, Korea University, Seoul, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Yuba film Sorption characteristics Transparency Thermal stability Mechanical properties
Yuba is the skin formed at the surface during the heating of soymilk. The 3rd, 7th, and 11th films were evaluated for properties at different RH. At 39% RH, the 11th film had the lowest moisture, while the 3rd film had the highest moisture. However, at 75% RH, reverse moisture results were obtained. The tensile strengths of the 3rd and 11th films were highest at 15% moisture, whereas the tensile strength of the 7th film was highest at 25% moisture. Elongation of the 3rd (127%) and 11th (85%) films were highest at 25% moisture. The light transmittance of the films was low and opaque at 5% moisture. The films were transparent at 23%–28% moisture, but became opaque as the moisture increased. The films at 39% RH (ΔH, 113–203 J/g) had higher thermal stability than those at 87% RH (ΔH, 315–493 J/g). Moisture content markedly changed the yuba film properties.
1. Introduction Yuba or tofu skin is a soy protein-lipid network film that forms on the surface of heated soymilk. Successive films can be lifted from the heated milk and dried for storage (Su & Chang, 2002). As a wrapping for meats and vegetables, dry yuba rehydrates in water and becomes a soft and elastic milky wet film ready for use (Alan, 2008). However, like other soy protein-based films, yuba is fragile in the dry state and has poor moisture barrier properties (Shurtleff & Aoyagi, 2013). Fresh or completely wet yuba is less fragile, but its mechanical properties such as tensile strength are inadequate to wrap and pack food (Chen & Ono, 2010). Many studies have been carried out to optimize yuba processing to increase yield and formation rate (Wang, Swain, Kwolek, & Fehr, 1983), to characterize and improve its appearance in foods (Chen, Yamaguchi, & Ono, 2009), and to improve its mechanical and storage properties (Yuan, Hu, & Yu, 2012). However, the utilization of yuba in non-conventional foods is limited. A potential use could be edible packaging film. Yuba is typically used and studied in the completely wet or dry state. The properties of yuba when between the completely wet and dry states, half-dried yuba (Shurtleff & Aoyagi, 2013), is being studied to develop new applications. The properties of the half-dried phase differ from those of the wet and dry states, and yuba seems to have suitable characteristics for use as edible packaging films. Water is not only the hydrating agent but is also an effective plasticizer that improves the flexibility and mechanical properties of the film (Gontard,
⁎
Guilbert, & Cuq, 1993). The objective of this study was to evaluate the mechanical properties, transparency, moisture sorption properties, and thermal stability of yuba films obtained at different points of successive skimming and stored at different relative humidities. 2. Materials and methods 2.1. Yuba processing Yuba films were prepared by using a modified method of Chen et al. (2009). Soymilk was prepared by soaking soybeans with a water ratio of 1:7.5 (concentration of soybean: 8.6%). The boiled soymilk was then transferred to a stainless steel yuba forming pan (15 × 18 × 6 cm) placed in a water bath. The soymilk level was adjusted to 2.3 cm and the temperature of the soymilk was maintained at 85 °C. Successive yuba films were collected at 15 min intervals and labeled sequentially. Eleven sheets of yuba were formed, and the third (Y3), seventh (Y7), and eleventh (Y11) films were chosen for analysis. Yuba was dried (Convection Oven, Cho Sun Science Co., Seoul, Korea) at 60 °C for 120 min, wrapped in plastic film, and stored at −18 °C until use. 2.2. Moisture sorption characteristics 2.2.1. Moisture sorption curve The dry yuba films (20 × 25 mm) were weighed, and the initial
Corresponding author at: Department of Human Ecology, Graduate School, Korea University, Seoul, South Korea. E-mail address: [email protected] (Y. Kim).
http://dx.doi.org/10.1016/j.foodchem.2017.09.127 Received 2 June 2017; Received in revised form 25 September 2017; Accepted 26 September 2017 Available online 28 September 2017 0308-8146/ © 2017 Elsevier Ltd. All rights reserved.
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Tokyo, Japan). The yuba samples of 3.4 mg equilibrated at 39% RH or 87% RH at 25 °C were placed in an aluminum DSC pan. The pan was hermetically sealed and equilibrated at 4 °C for 12 h. The blank was an empty aluminum DSC sealed pan. The thermal transition of yuba was measured by heating from 20 to 275 °C at a rate of 5 °C/min. The onset temperature (To), peak temperature (Tp), glass transition temperature (Tg), and denaturation enthalpy (ΔH) were recorded.
moisture content (M0) was measured using a halogen moisture analyzer (HB43-S, Mettler Toledo, Greifensee, Switzerland). The yuba specimens were then placed in separate desiccators containing saturated salt solutions at 25 °C, and controlled by a hygrometer (Testo 608-H2, Lenzkirch, Germany). The specific relative humidity (RH) of the saturated salt solutions at 25 °C were 39.5% (CaCl2), 57% (NaBr), 75% (NaCl), and 86% (KCl). Deionized water was used to provide 100% RH. The weights of the yuba specimens were measured at intervals of 2 or 4 h until two the consecutive weightings were the same; at this point, it was assumed that an equilibrium condition had been reached. The moisture adsorption curves of yuba were fitted to Peleg (1988).
t M (t ) = M0 + k1 + k2 t
2.6. Statistical analysis Statistical analysis was performed using the software program SPSS 19.0. All data for the analysis of variance (ANOVA) and significance was set at p < 0.05. Duncan’s multiple range test was applied to determine the significant differences among the mean values. All experiments were performed in triplicate, except for sorption curve equations and DSC.
(1)
where M(t) is the moisture after time t, M0 is the initial moisture, k1 is the Peleg rate constant (h%−1), and k2 is the Peleg capacity constant (%−1).
3. Results and discussion 2.2.2. Sorption isotherm curve The equilibrium moisture content was calculated from the increase in mass of the dry sample after equilibration at a given relative humidity. The Guggenheim–Anderson–de Boer (GAB) model (Bizot, 1983) was used to fit the soy protein film sorption isotherm data, and the monolayer values for moisture were calculated from the equations.
3.1. Moisture sorption characteristics
2.5. Differential scanning calorimetry
3.1.1. Moisture adsorption kinetics of yuba films As shown in Fig. 1A, all yuba films absorbed moisture rapidly in the first few hours and reached a plateau in less than 10 h at 37% RH and 75% RH but did not plateau until after 20 h at 99% RH; the plateau indicates equilibration has been reached at the specific relative humidity (Cho & Rhee, 2002). Although moisture content was much higher at the higher RH at the start of the rehydration, a longer time was required for the yuba film to reach moisture equilibration when the storage relative humidity was higher. At RH of 99% and 75%, the yuba films that were collected at a later processing time showed higher moisture at equilibrium and reached equilibrium later than the films collected earlier. This may be explained by the higher hydrophobic components (aggregated protein and lipid) in the earlier films and the higher hydrophilic components (carbohydrates) in the later yuba sheets (Wu & Bates, 1972). At was 39% RH, all films reached equilibrium in 10–20 h. Y11 was the last while Y3 was the first to reach equilibrium at 39% RH. Y11 also showed the lowest moisture content at equilibrium, while Y3 showed the highest moisture content and Y7 showed an average moisture content. Data measured from the sorption curve of the yuba films were fitted to Eq. (1), and the sorption curve equations and their constants (k1 and k2) were calculated as shown in Table 1. The k1 values indicate the initial moisture adsorption rate of the yuba film; the higher k1 value indicates the lower adsorption rate. The k2 values indicate the moisture content of the film had absorbed when the film reached equilibrium; the higher k2 value indicates less absorbed moisture (Cho & Rhee, 2002). The coefficients of determination in all cases were very high (r2 > 0.99), indicating excellent fit of the equations to the experimental data. At 75% RH, the moisture isotherm equation k1 values of the successively collected yuba films increased (5.39, 5.79, and 8.03) from Y3 to Y11, but the k2 values decreased from 10.62, 8.91, and 6.47 for the Y3 to Y11 samples, respectively. This means that, as the soymilk was heated, the yuba collected earlier absorbed moisture more rapidly in the first few hours, but the maximum moisture content at a given RH (75% or 99%) was lower than the films collected later. However, when stored at 39% RH, both the k1 and k2 values of all films increased with the order of collection (k1 and k2 values increased from 11.29 to 39.89 and from 18.18 to 47.19, respectively). For the yuba films that were collected later, more time was needed to reach equilibrium, even though they gained the least amount of moisture. The results are consistent with the moisture sorption curves.
The thermal transition properties of yuba at two different relative humidity were determined using a Seiko SII-EXSTAR 6000-DSC-6200 differential scanning calorimeter (DSC 6200, SII Nano Technology Inc.,
3.1.2. Moisture sorption isotherms of yuba films The moisture adsorption data of the yuba films fitted to the GAB models (Table 2) and isotherms are shown in Fig. 1B. The moisture
W=
m 0 Cka w (1−k a w)(1−k a w + Ck a w)
(2)
where W is the moisture content of the material in a dry state, aw is the water activity, m0 is the moisture content (d.b) at a monolayer, and C and k are the constants depending on temperature. 2.3. Mechanical properties of yuba The yuba films were conditioned at a given RH to reach a constant moisture content. A 3 × 6 cm sheet of film was folded three times into 1 × 6 cm. A Compac-100II rheometer (Sun Scientific Co. Ltd., Tokyo, Japan) was used to measure the tensile strength and elongation of the film. The original distance of the strain was set as 4 cm and the moving speed of the table was set as 5 mm/s. Tensile strength and elongation were calculated using the software provided by the manufacturer (Rheology data system 3.0, Tokyo, Japan). 2.4. Transparency The yuba films (8 × 10 mm) were weighed (w0) before being soaked in deionized water for 5 min. The remaining water on the surface was gently removed with a kitchen towel. The wet yuba was placed onto the outer side of a spectrophotometer cell (wc) and the cell was inserted into the spectrophotometer (Ultrospec 2100, GE Healthcare Biosciences Corp., New Jersey, USA). The transmittance was read at 500 nm, the cell was weighed with the film (wt) at 5 min intervals for 75 min, and an empty spectrophotometer cell was used as the blank. The moisture content M (t) of the film at t min can be expressed as follows:
M (t ) =
(wt−wc )−w0 wt−wc
where M(t) is the moisture content at time t, w1 is the weight of the film at 5 min intervals for 75 min, wc is the weight of the spectrophotometer cell, and w0 is the initial weight of the film.
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(A)
(B) 0.5
RH 99%
0.4
0.3
0.2
RH 75%
0.1
Moisture content (g H 2O/g solids)
Moisture content (g H
2 O/g
solids)
0.5
0.4
0.3
0.2
0.1
0.0
RH 37%
0.0 0
10
20
30
40
0.0
50
0.2
0.4
0.6
0.8
1.0
Water activity
Time (h)
Fig. 1. Moisture sorption curves of yuba at various relative humidities (A) and sorption isotherms (B) for yuba at 25 °C.
sheets were collected. The mo were 0.045, 0.051, and 0.076 g H2O/g solids of monolayer moisture contents for the Y3, Y7, and Y11, respectively.
Table 1 Constant value (k1 and k2) and coefficient of determinations (r2) for sorption curve equations of yuba at selected relative humidity. Sample
Y3 Y7 Y11
39% RH
75% RH
99% RH
k1a
k2a
r2
k1
k2
r2
k1
k2
r2
11.29 13.48 39.89
18.18 20.88 47.19
0.9996 0.9974 0.9996
5.39 5.79 8.03
10.62 8.91 6.47
0.9995 0.9999 0.9984
6.44 8.26 8.54
2.98 2.61 1.99
0.9996 0.9995 0.9994
3.2. Mechanical properties of yuba films Fig. 2A shows the effects of moisture content on the tensile strength of yuba films. In general, the tensile strength of yuba films ranged from 1.06 to 1.71 MPa, when the moisture content was about 15%. In the cases of Y7 and Y11, as the moisture content increased to above 25%, the tensile strength decreased rapidly. The decreased tensile strength values for yuba films can be explained by the plasticizing effect of water (as the film moisture content increased) on the hydrophilic components such as proteins and carbohydrates (Ashley, 1985; Gennadios, Park, & Weller, 1993). However, Y7 had slightly increased tensile strength values at around 25%. Compared to Y11, the Y3 and Y7 films that were collected earlier had higher tensile strength values. When the moisture content of the films was 22%, the tensile strengths were 1.25 MPa, 1.08 MPa, and 0.84 MPa for the Y3, Y7, and Y11, respectively. The elongation changes of yuba films were observed as shown in Fig. 2B. The yuba films collected earlier showed greater elongation. In general, the elongation values of most yuba films were low (ranging from 5.60% to 6.90%) at low moisture contents (about 15%) followed by a rapidly increasing slope (except Y11), as the moisture content increased up to 25% and reached their peaks (127% of Y3 and 86% of Y7). As the moisture increased, the elongation values decreased sharply. However, for Y11, elongation slowly increased from 6.90% to 53.5% as the moisture content increased from 15% to 30%. The effects of moisture content of yuba on the mechanical properties of the films are due to the plasticizer (lipid, water etc.) to solid ratio, which affects the intermolecular and interfacial interactions of the film network leading to different tensile strengths and elongation values (Gennadios et al., 1993).
Y3: 3rd yuba, Y7: 7th yuba, Y11: 11th yuba. a Unit of k1 and k2 is g of solid/g H2O. Table 2 GAB model constants and coefficient of determinations (r2) for different yuba. Sample
m0a
Cb
kb
r2
Y3 Y7 Y11
0.045 0.051 0.076
1.998 2.447 1.017
0.8817 0.8807 0.8699
0.9976 0.9994 0.9982
Y3: 3rd yuba, Y7: 7th yuba, Y11: 11th yuba. a Monolayer moisture content in g H2O/g solids. b GAB constant.
sorption of all yuba films show a common trend where the equilibrium moisture content increased at a slow rate until storage aw reached 0.75, and then increased sharply at higher aw. Yuba films collected later showed higher equilibrium moisture content. When aw was 0.87, the equilibrium moisture contents of Y3, Y7, and Y11 were 16.64%, 19.23%, and 23.71%, respectively. The relative moisture content differed at a relatively low aw (0∼0.56). The equilibrium moisture content of the successively collected yuba (Y3, Y7, and Y11) were 0.45%, 0.43%, and 0.39%, respectively, at aw of 0.39. The results suggest that a certain threshold of aw is needed before water binding takes place, probably by the higher content of hydrophilic carbohydrates in yuba films collected later. At relatively low aw, the yuba films that were collected earlier with a higher protein content may initiate earlier moisture absorption. The data in Table 2 show that the calculated GAB model constants and coefficient of determination were very high (r2 > 0.99). When the sorption data was fitted to the GAB model, increased monolayer values were observed in the order in which the yuba
3.3. Transparency of yuba films The moisture desorption curves of each film in a given environment are shown in Fig. 3A. As the film thickness decreased, the initial moisture content observed also decreased (r2 = 0.854, p < 0.001). Regardless of the initial moisture content, the moisture content decreased rapidly in the first 45 min and then decreased slowly up to 80 204
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(A)
(B) 160
1.8
140
1.6
120
Elongation (%)
Tensile strength (MPa)
2.0
1.4 1.2 1.0 0.8
100 80 60 40
0.6
20
0.4
0
0.2
14
16
18
20
22
24
26
28
30
32
14
16
18
Moisture content (%)
20
22
24
26
28
30
32
Moisture content (%)
Fig. 2. Effects of moisture content on tensile strength (A) and elongation (B) on yuba film.
protein network and decreased difference in refractive indices (Yang & Paulson, 2000). This decreased the scattering surfaces and allowed the light to pass through, which increased the optical transmittance. However, as the moisture content continued to increase and approach saturation, intermolecular agglomeration occurred causing turbidity and more light scattering (Feng & Winnik, 1997). However, among the films with different moisture contents, Y11 showed the lowest %T. This may be due to the greater thickness, higher carbohydrate ratio, and more Millard reaction products of Y11 (Ou, Wang, & Jin, 2005; Han, Ishitani, & Li, 2005).
min. The rates of moisture content loss from fast to slow were in the order of Y3, Y7, and Y11 due to the higher carbohydrate ratio of Y11 which holds water tightly (Chen et al., 2009). The optical transparency differences of the yuba films with the change of moisture content were observed (Fig. 3B). The light transmission (%T) of the yuba films at 500 nm can be correlated with the moisture contents as shown in Fig. 3B. At a very low moisture content (about 5%), the light transmittance of the yuba films was low and the films appeared to be almost opaque. As the moisture content increased, the optical transmittances of the yuba films increased rapidly, and reached their peak values (73.9%, 75.7%, and 64.4% of transmittance for Y3, Y7, and Y11, respectively) at a moisture content of 23%∼28%. The films at this stage were nearly transparent. However, the optical transmittances decreased rapidly again as the moisture content in the yuba films increased and the films became even more opaque (%T < 0.70%) at saturated moisture contents. The films with lower moisture content (5%) were dense, and the light was scattered due to differences in refractive indices of the surfaces of solid particulates and lipid droplets (Lim, Mine, & Tung, 1999). When the moisture content increased, the hydration of the protein network reduced the inter-chain interactions and enhanced the chain mobility, resulting in a loosened
3.4. Differential scanning calorimetry of yuba films The differential scanning calorimetry (DSC) curves of the yuba films conditioned at 39% RH and 87% RH are shown in Fig. 4. Yuba films conditioned at 39% RH showed endothermic peaks (peak temperature, Tp) at 179–182°C, and the films conditioned at 87% RH showed endothermic peaks at 175°C. The onset temperature (T0) of films conditioned at 39% RH was also higher than those conditioned at 87% RH. The peaks of the films in the 39% RH group tended to become sharper at higher successive collection, while the peaks of the films in the 87%
(A)
(B)
600
80
500
Light transmittance (%)
Moisture content (g/kg)
400
300
200
60
40
20 100
0 0 0
20
40
60
80
0
100
200
300
400
Moisture content (g/kg)
Time (min)
Fig. 3. Desorption curves of yuba film (A) and light transmittance of yuba film (B).
205
500
600
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DSC/mW
Relative humidity
DSC/mW
Relative humidity
Fig. 4. DSC curves of yuba films at relative humidity of 39% (A) and 87% (B).
indices. The moisture content of the yuba film must be carefully controlled to meet the required mechanical and optical properties of the film for their specific applications. Based on its thermal stability and mechanical properties, yuba with 25% moisture content, stored at 39% RH at the 7th film collection appears to have the greatest potential for use as edible films.
RH group seemed to decrease in the same order. The moisture sorption patterns described earlier (Fig. 1) show that at 39% RH, the films that were collected later absorbed less water than the films that were collected earlier (moisture content in yuba films: Y3 > Y7 > Y11). However, at 87% RH, the results were reversed (moisture content in yuba films: Y3 < Y7 < Y11). Correspondingly DSC of films with higher moisture content showed flatter peaks, while films with less moisture showed sharper peaks. This extremely differences may have been caused by the combined effects of the composition ratios of yuba films. For example, when at 39% RH, although Y11 absorbed less water, the carbohydrate ratio was higher, binding water more tightly and absorbing more energy during analysis (Wang, Qi, & Yang, 2011). Further evidence of the effects of water content can be provided by the denaturation enthalpy (ΔH) values. The ΔH values of each pair of films in the 39% RH and 87% RH groups approach the same value in the order of which the yuba film was collected. Peaks of the high water content group were much sharper than those of the low water content, with the ΔH values in the 87% RH group (ranging from 315 to 493 J/g) being much higher than those in the 39% RH group (ranging from 113 to 203 J/g). Generally, higher ΔH values suggested more hydrophobic/ hydrophilic interactions and lower thermal stability (Ma & Harwalkar, 1991). A similar phenomenon was reported by Kitabatake, Tahara, and Doi (1990), whereby the denaturation temperature of soy protein (7S and 11S) was significantly influenced by the water content, where the lower water content of the sample resulted in higher Tp of the samples. The glass transition temperature (Tg) of yuba films containing lower water content was slightly higher than those with higher water content. Tg of the 39% RH group showed a decrease from 155 to 150 °C, while the 87% RH group showed a decrease from 151 to 149 °C with the increased number of yuba sheets collected. Jiang, Tang, and Wen (2008) suggested that when the moisture content absorbed by the polar groups of films is sufficient to form a BET (Brunauer-Emmet-Teller) monolayer, the monolayer water presents significant plasticizing effect and reduces the glass transition temperature. However, Sobral, Monterrey-Q, and Habitante (2002) reported that when the plasticized state of protein was at a maximum, the Tg of the film with rich protein did not change with moisture nor with the plasticizer concentration.
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4. Conclusion This study showed that yuba has mechanical and optical properties are adjustable by processing conditions and has potential as an edible film. Moisture content has a large effect on mechanical and optical properties because water is an efficient plasticizer and affects refractive 206
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