Effects of ultrasound and microwave pretreatments on the ultrafiltration desalination of salted duck egg white protein

food and bioproducts processing 9 6 ( 2 0 1 5 ) 306–313

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Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp

Effects of ultrasound and microwave pretreatments on the ultrafiltration desalination of salted duck egg white protein Bing Zhou a , Min Zhang a,∗ , Zhong-xiang Fang b , Yaping Liu c a

State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China School of Public Health, International Institute of Agri-Food Security, Curtin University, GPO Box U1987, Perth, WA 6845, Australia c Guangdong Galore Food Co. Ltd, Zhongshan 528447, China b

a r t i c l e

i n f o

a b s t r a c t

Article history:

The effects of application of ultrasound and microwave treatments before the ultrafiltra-

Received 25 January 2015

tion desalination of salted duck egg white protein were investigated in this work. The

Received in revised form 19 June

desalination rate, color, microstructure, foaming capacity, emulsifying index and gelling

2015

property were determined. The results showed that ultrasound and microwave pretreat-

Accepted 15 September 2015

ments increased about 10% and 3% desalination rate, respectively, compared to that without

Available online 25 September 2015

any pretreatment sample, beside the product quality was also improved. The effect of ultrasound pretreatment was better than that of microwave in terms of foaming capacity and

Keywords:

emulsifying index of the product.

Duck egg white protein

© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Ultrasound Microwave Ultrafiltration desalination Functional property Pretreatment

1.

Introduction

China is the biggest duck egg producer in the world, with an annual production of 4.2 million tons in 2008 (Agriculture, 2008). Salted duck egg is one of the most traditional and popular egg products in China. The most valuable product of salted duck egg is the egg yolk which is widely used in traditional Chinese foods such as the filling of the moon cakes that offers attractive orange color, unique flavor and desired texture (Kaewmanee et al., 2009). The salted duck egg white protein (SDEWP) is a by-product of the salted egg yolk processing which has, so far, not been fully utilized due to its high salinity (7–12% w/w) (Wang et al., 2009). Currently, SDEWP is used directly as a food ingredient in manufacturing of other foods or bioactive compounds. For example, SDEWP

was used in producing high protein noodles (Liu and Zhang, 1994) and Frankfurt sausage (Lin, 1996). Zhang (2001) extracted bioactive peptides from enzymatically hydrolyzed SDEWP. In addition, Zheng et al. (2004) optimized the processing parameters in producing high quality SDEWP, and Huang et al. (1996) compared four different drying methods (freeze drying, spray drying, roller drying and hot air drying) on the SDEWP powder quality. However, no report is available on desalination of the SDEWP and the desalinated SDEWP may have wider applications with the low salt content. Therefore, desalination of SDEWP might be beneficial to the duck egg industry by providing a new type of high protein resource, and reduce the environmental impact by reducing the by-product waste. In practice, some common and traditional protein desalination methods are ion-exchange column chromatography

∗ Corresponding author at: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu Province, China. Tel.: +8651085917089; fax: +86 5108 5807976. E-mail address: [email protected] (M. Zhang). http://dx.doi.org/10.1016/j.fbp.2015.09.004 0960-3085/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

food and bioproducts processing 9 6 ( 2 0 1 5 ) 306–313

(Greiter et al., 2002), electrodialysis (Xu and Huang, 2008), and ultrafiltration (Wang et al., 2013). The ion-exchange column chromatography is time consuming and inefficient, which requires frequent regeneration of the column that is a tough task and labor intensive (Bartlett, 1959). Comparatively, the electrodialysis is more efficient and could achieve a higher desalination rate, but the desalted solution should be concentrated to a suitable concentration before further processes, and therefore is a relatively cumbersome procedure (Dong et al., 2013). The more widely used and efficient method for protein desalination might be ultrafiltration (Aider et al., 2008). To further improve the efficiency, a number of new technologies have been reported for ultrafiltration desalination of proteins, such as ultrasound-assist-ultrafiltration desalination (UAUD) and microwave-assist-ultrafiltration desalination (MAUD) which includes the application of ultrasound and microwave pretreatments before the desalination process (Wang et al., 2013). The ultrasound can enhance the mass transfer through acoustic-induced cavitation and has been successively used in extraction of a variety of bioactives to increase the efficiency (Chemat and Khan, 2011). The assistance of microwave can highly localize temperature and pressure which cause selective migration of target compounds from the material to the surroundings at a more rapid rate (Spigno and Faveri., 2009), and therefore may also increase the processing efficiency. The purpose of this work was to apply UAUD and MAUD to increase the desalination rate of SDEWP and investigate their effects on the physicochemical properties of the desalted duck egg white proteins.

2.

Materials and methods

Salted duck eggs (7.8 ± 0.08% salt content) were purchased from Shendan Healthy Food Co., Hubei Province, China. The eggs were cleaned and separated into egg yolk and egg white protein using an egg separator and then the egg white protein was kept at 4 ◦ C and 95% relative humidity in a refrigerator until further use. The protein content in the egg white protein was determined as 8.8 ± 0.11%. Sodium dodecyl sulfate (SDS), and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemical reagents used in this study were of analytical grade.

2.1.

Methods

The SDEWP was divided into three batches in this study. The first batch was subjected to ultrasound treatment, the second one to microwave treatment, and the third one without any pretreatment was used as the control.

2.1.1.

Ultrasound treatment of SDEWP

Ultrasound treatment was carried out using two laboratory scale ultrasound treatment apparatus, namely an ultrasound probe (model JY98-IIDN, NingBo Scientz Biotechnology Co. Ltd., Ningbo, Zhejiang, China) and an ultrasound bath (model SB-800D, NingBo Scientz Biotechnology Co. Ltd., Ningbo, Zhejiang, China). About 200 mL of SDEWP was added with 800 mL distilled water, placed in the sample container and treated under 3 different ultrasound conditions: (1) in the ultrasound probe system of 20 kHz and 1000 W for 20 min (U1); (2) in the ultrasound bath system of 40 kHz and 200 W for 20 min (U2); (3) in the ultrasound probe system of 20 kHz and 1000 W for 10 min and ultrasound bath system of 40 kHz and 200 W for 10 min (U3). All the treatments were conducted at 20 ◦ C

307

Fig. 1 – Schematic diagram of vacuum ultra-filtration desalination equipment system for salted duck egg white.

temperature and the ultrasound was operated as pulse on and pulse off 5 s each. After ultrasound treatments, the samples were poured into a 1000 mL beaker and then kept at 4 ◦ C and 95% relative humidity for subsequent tests and processes.

2.1.2.

Microwave treatment of SDEWP

Microwave experiment was carried out using a commercial microwave oven (2450 MHz, Gospell Electric Technology Co., Ltd., Shenzhen, China) and was modified in our laboratory, whose power can be adjusted from 100 W to 800 W with a microwave power controller (Gospell Electric Technology Co., Ltd., Shenzhen, China). About 200 mL of SDEWP was added with 800 mL distilled water and placed in the microwave turntable plate for treatment of 3 min at the microwave power of 500 W. After this, the sample was poured into a 1000 mL beaker and then kept at 4 ◦ C and 95% relative humidity for subsequent tests and processes.

2.1.3.

Vacuum ultra-filtration

The SDEWP with and without ultrasound and microwave pretreatments were desalted using a laboratory scale vacuum ultra-filtration unit (FZ-8, Wuxi Ultra-filtration Equipment Company, Jiangsu Province, China) which was custom designed for our laboratory (Fig. 1). This system consists of the following four basic components: (1) seven ultrafiltration membrane tubes (PVDF, 1.2 cm outer diameter and 80 cm length) with a cut-off molecular weight of 10 kDa; (2) a gas–water separator system; (3) a vacuum system equipped with a water-cooler and a water-ring vacuum pump; and (4) a pulse circulating system equipped with a set of water pumps (flow rate of 10 L/min) and a sample container wrapped with a jacket through which heating/cooling water is circulated. About 1000 mL SDEWP solution, with or without ultrasound and microwave pretreatments, was transferred to the sample container of the vacuum ultra-filtration unit respectively. The operation parameters were set based on preliminary trials, which were: (a) pressure in the ultra-filtration tube = 5 kPa; (b) temperature in the sample container = 20 ◦ C; (c) time of ultra-filtration = 2 h. After ultra-filtration, the desalted duck egg white protein (DDEWP) solutions were taken out from the sample container and poured in a 500 mL beaker and then kept at 4 ◦ C and 95% relative humidity for the following drying process.

308

2.1.4.

food and bioproducts processing 9 6 ( 2 0 1 5 ) 306–313

Drying process

A centrifugal spray dryer (Wuxi Linzhou Drying Equipment Co., Ltd., Jiangsu Province, China) was used in the drying process. About 200 g samples were put into the centrifugal spray dryer and dried at the parameters which were set as inlet temperature 150 ◦ C, feed temperature 20 ◦ C, atomizer speed of 10,000 r/min, outlet temperature 80 ◦ C until the final moisture content of about 3.5% (wet basis).

2.1.5.

Sodium chloride content measurement

The salt content in SDEWP and DDEWP was measured according to the AOAC method (AOAC, 2000). 1 g of the sample was weighed into 250 mL Erlenmeyer flask; 25 mL of 0.1 M silver nitrate and 10 mL of concentrated nitric acid were then added. The mixture was boiled gently for 10 min. Then, 50 mL of distilled water and 5 mL of ferric indicator were added to the solution before being titrated by 0.1 M potassium thiocyanate standard solution until the solution became permanent light brown. The percentage of salt was then calculated by Eq. (1): Salt (%) = 5.8 ×

(V1 × N1) − (V2 × N2) W

(1)

where V1 is the volume of AgNO3 (mL); N1 is the concentration of AgNO3 (N); V2 is the volume of KSCN (mL); N2 is the concentration of KSCN (N); and W is the weight of sample (g).

2.1.6.

Desalination rate measurement

Desalination rate (%) =

S1 − S2 S1

(2)

where S1 is the sodium chloride content before vacuum ultra-filtration and S2 is the sodium chloride content after vacuum ultra-filtration.

2.1.7.

2

2

(L0 ∗ −L∗) + (a0 ∗ −a∗) + (b0 ∗ −b∗)

2

(3)

where L0 *, a0 *, b0 * are the color readings of the dried desalted duck egg white protein powders without any pretreatment (control) while L*, a*, b* are those for the treated samples.

Scanning electron microscopy analyses

The images of microstructure of the dried duck egg white powders by centrifugal spray drying were obtained using a scanning electron microscope (S-4800, Hitachi, Tokyo, Japan) at an accelerating voltage of 1.0 kV. Dried samples were coated using a gold-palladium alloy coater (Bal-Tec Co., Manchester, NH, USA), and the samples were observed at 600× magnification.

2.1.9.

Vf × 100 V0

(4)

FS =

V0 − (V30 − Vl ) × 100 V0

(5)

where V0 is the volume of the duck egg white solution before stirring (m3 ); Vf is the volume of the bubble (m3 ); Vl is the volume of the egg white solution after stirring (m3 ); and V30 is the volume of these bubbles after stirring for 30 min (m3 ).

2.1.10. Emulsification properties measurement The above prepared 10% (dry matter w/w) duck egg white protein solution was used for emulsification property evaluation. About 3 mL protein solution and 1 mL soy bean oil was mixed thoroughly in a 10 mL centrifuge tube and then centrifuged (2-16PK, Sigma Co. Ltd., Germany) at 8000 rpm, 15 ◦ C for 90 s. Then, 50 ␮L emulsion was taken out from the centrifuge tube and mixed with 5 mL of 0.1% sodium dodecyl sulfate solution. The absorbance of the mixture was measured using UV2600 UV-Vis spectrophotometer (Shimadzu Co. Ltd., Tokyo, Japan) at 500 nm. Emulsification activity index (EAI) and emulsification stability index (ESI) were used to evaluate the emulsification properties of the duck egg white protein. The EAI was the absorbance value of the emulsion and ESI was calculated by using Eq. (6) as given by Guo and Mu (2011): A0 × 10 A0 − A10

(6)

where A0 and A10 are the absorbance values of the egg white protein emulsion measured at 0 min and 10 min, respectively. Average values of absorbance were calculated from three measurements for each sample.

2.1.11. Gel property measurement



2.1.8.

FC =

ESI =

Color measurement

The color properties of the dried protein powders were determined using a Konica Minolta model CR-400 Chroma Meter (Osaka, Japan). L* (lightless), a* (redness), b* (yellowness) values were recorded. Eq. (3) expresses the total color difference (E) and is used to describe the color changes between the UAUD-DEWP samples and MAUD-DEWP samples. E∗ =

Staufen, Germany) for 3 min, and then immediately poured into a 200 mL graduated container and the volume of the foam and protein solution was recorded. Foaming capacity (FC) and foam stability (FS) were calculated using Eqs. (4) and (5) respectively as described by Chang and Chen (2000):

About 20 mL prepared 10% (w/w) duck egg white protein solution was transferred into a 25 mL beaker and heated in a water bath at 90 ◦ C for 30 min. The gel solution was cooled down to 4 ◦ C in a refrigerator for 10 h and then cut into cylinders (diameter 20 mm, height 20 mm). The gel hardness was measured by a texture analyzer (TA-XT2TM , Stable Microsystems Ltd., Leicestershire, UK) with Texture Exponent 32 software (Stable Micro System Ltd., Surrey, UK). A flat-bottom cylindrical probe (MSMP/36R, 36 mm in diameter) was used to develop the force–time curve. The parameters were set as follows; pre-test speed = 5 mm/s, test speed = 2 mm/s, post-test speed = 5 mm/s, strain = 50% and trigger force = 5 g (Handa et al., 1998). The tests were carried out in triplicate and the average firmness values were used to express the gel strength of the samples.

3.

Data analysis

Foaming capacity measurement

The duck egg white protein powder was added into distilled water and blend gently to prepare a 10% (dry matter w/w) protein solution at 20 ◦ C. About 20 mL of duck egg white protein solution was transferred into a 200 mL beaker and stirred at 1500 rpm (T18BS25 stirring implement, KIA Co. Ltd.,

All measurements were carried out in triplicate and the data were analyzed by one-way analysis of variance. The results are reported as mean and standard deviation. Difference between the mean values was considered significant at p ≤ 0.05 (95% confidence level). All statistical calculations were performed

food and bioproducts processing 9 6 ( 2 0 1 5 ) 306–313

using SPSS software, version 11.5.1 (SPSS Inc., Chicago, IL, USA).

4.

Results and discussion

4.1.

Desalination rate of duck egg white protein

Salt content is an important quality parameter of the desalted duck egg white protein powder which may affect its application range. Table 1 presents the salt content of UAUD-DEWP and MAUD-DEWP which shows that the application of ultrasound and microwave pretreatments significantly (p ≤ 0.05) affected the salt content of the resulting product. The desalination rate of the SDEWP without any pretreatment was below 85%, while that of other samples were all higher than 85%. The ultrasound treatment, especially at higher power significantly (p ≤ 0.05) increased the desalination rate. The results also indicated that the desalination rates of UAUDDEWP samples were higher than that of MAUD-DEWP ones (Table 1), suggesting ultrasound treatment is more efficient than that of microwave treatment in the desalination of the present samples. Among the ultrasound treated samples, the lowest desalination rate was U2AUD-DEWP sample, followed by the U1AUD-DEWP sample, and the highest desalination rate was the U3AUD-DEWP sample, suggesting the combined ultrasound frequency (40 kHz and 20 kHz) was better than the single ultrasound frequency treatment in desalination of SDEWP.

4.2.

Color

Color changes of duck egg white powder observed by analyzing L* (lightness), a* (red-green chromaticity), b* (yellow-blue chromaticity) coordinates, and calculating the E values. Table 2 shows the influence of UAUD and MAUD on the color attributes of duck egg white protein. As expected, the L* value of duck egg white protein powder without any pretreatment was the highest. The color parameters of U1AUD-DEWP samples were close to that of the MAUD-DEWP sample. The MAUD-DEWP powder had higher lightness but lower redness values than those of the UAUD-DEWP samples. This might come from the shorter microwave treatment time, which resulted in less color change when compared to the UAUD-DEWP powder. The color parameters (L*, a*, and b*) and the total color difference (E) of the UAUD-DEWP powders were different from each other. The E and b* values were the lowest in the U1AUD-DEWP powder, whereas those in the U2AUD-DEWP powder was the highest. However, a* and L* values in these samples showed an opposite trend to those of the E and b* values (Table 2). This might have been because of the application of ultrasound have led to an increase in the protein surface hydrophobicity and a decrease in the sulfhydryl group content (Arzeni et al., 2012b; Karki et al., 2009; Martini et al., 2010), which resulted in an increase in the exposed area of embedded riboflavin molecules inside macromolecular proteins (Zhou, 2002; Yu and Huang, 2001) and changed the color parameters. The stronger ultrasound frequency (40 kHz) treated DDEWP may have lost a higher amount of small molecular weight proteins during the downstream ultra-filtration process. These observations suggested that the color quality of the MAUD-DEWP powder might be the best among these four samples, because of its highest lightness but lowest yellow and redness values, implying more attractive color.

4.3.

309

Microstructure

The morphological and structural properties are important aspects of the overall quality of dehydrated products. The pretreatment and drying methods significantly affect the microstructure (Krokida and Maroulis, 2001). The surface morphology or the surface microstructural feature of the dried duck egg white particles was examined to better understand the effect of different pretreatments on the quality of dried samples. From the scanning electron micrographs (Fig. 2), it can be observed that the DDEWP particles without any pretreatment and the MADDEWP particles exert regular and uniform pore size and integrity, and the microstructural feature of UAUDDEWP particles was different from that of DDEWP and MADDEWP particles. And these pores are caused by the rapid evaporation of water. It can be also observed from Fig. 2 that the UAUDDEWP particles have larger pore size and more pores inside compared to those of DDEWP and MADDEWP particles. The differences in the microstructure of different pretreatment samples can be explained from the fact that the effect of ultrasound was stronger than that of microwave, and that might indicate that cavitation and mechanical oscillations of ultrasound can alter the protein structure. For the UAUD-DEWP samples (Fig. 2c–e), it can be seen that the integrity of the U1AUDDEWP particles was higher than that of U2AUDDEWP particles and U3AUDDEWP particles, in addition, the integrity of the U3AUDDEWP particles was between U1AUDDEWP particles and U2AUDDEWP particles which suggested that the higher the ultrasound frequency treatment, the less integrity the sample particles. This might because the application of ultrasound have led to reduce binding energy among protein conglomerates and macromolecular proteins due to increased charges and protein surface hydrophobicity and decreased sulfhydryl group content (Arzeni et al., 2012a). And this also affects the functional properties of different samples, such as foaming capacity and emulsifying properties.

4.4.

Foaming capacity and foam stability

Foaming capacity (FC) and foam stability (FS) are important functional properties of proteins, which are influenced by processing conditions, nature of sample and method of sample preparation (Fennema, 1996). The FC and FS values of the desalted duck egg white protein samples are presented in Fig. 3. The FC of DDEW was significantly lower (p ≤ 0.05) than those of UAUD-DEWP and MAUD-DEWP, and the FC values of UAUD-DEWP were higher than that of MAUD-DEWP, which indicated that both ultrasound and microwave pretreatments increased the duck egg protein foaming capacity and the ultrasound pretreatment had the highest effect. In addition, the FC values of the ultrasound treated samples were different from each other. The FC values of U2AUD-DEWP (40 kHz) were higher (p ≤ 0.05) than that of U1AUD-DEWP (20 kHz), and the value of U3AUD-DEWP (both 20 kHz and 40 kHz) was in the middle, which suggested that the higher the ultrasound frequency treatment, the better the foaming capacity of the protein. Sun Bingyu (2006) studied the effects of ultrasonic treatment on the frothiness of soy protein concentrate and found that the increased ultrasonic frequency has caused partial loose of the peptide chain of soybean protein and promoted the foam formation. Jambrak et al. (2008) also reported that the foam capacity of whey protein was improved after ultrasound treatments at both 20 kHz and 40 kHz frequencies.

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Table 1 – Mean salt content and desalination rate of duck egg white protein with different pretreatments. Sample

SDEWP

Salt content before desalination (g/100 g) Salt content after desalination (g/100 g) Desalination rate (%)

7.80 ± 0.08 1.40 ± 0.01e 82.10 ± 0.00a a

U1AUD-DEWP

U2AUD-DEWP

U3AUD-DEWP

MAUD-DEWP

7.80 ± 0.08 0.76 ± 0.03b 90.31 ± 0.05d

7.80 ± 0.08 0.87 ± 0.00c 88.85 ± 0.13c

7.80 ± 0.08 0.62 ± 0.02a 92.16 ± 0.21e

7.80 ± 0.08a 0.95 ± 0.01d 87.84 ± 0.12b

a

a

a

The values are mean of triplicate ± SD. Different superscripts in the same line are significantly different (p ≤ 0.05). SDEWP: salted duck egg white protein; U1AUD-DEWP: ultrasound (20 kHz, 1000 W, 20 min)-assist-ultrafiltration desalination-duck egg white protein; U2AUD-DEWP: ultrasound (40 kHz, 200 W, 20 min)-assist-ultrafiltration desalination-duck egg white protein; U3AUD-DEWP: ultrasound (20 kHz, 1000 W, 10 min and 40 kHz, 200 W, 10 min)-assist-ultrafiltration desalination-duck egg white protein.

Fig. 2 – Scanning electron micrographs of differently treated duck egg white proteins.

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Table 2 – Effect of UAUD and MAUD on the color parameters of duck egg white protein powders. Sample

L*

DDEWP U1AUD-DEWP U2AUD-DEWP U3AUD-DEWP MAD-DEWP

95.41 90.56 88.30 88.73 92.39

± ± ± ± ±

a* d

0.02 0.00b 0.08a 0.04a 0.07c

−3.26 0.11 0.40 0.74 0.08

± ± ± ± ±

E

b* a

0.00 0.01c 0.02d 0.01e 0.01b

8.15 6.02 8.94 7.74 6.42

± ± ± ± ±

d

0.00 0.03a 0.02e 0.01c 0.11b

– 7.45 ± 11.03 ± 10.77 ± 7.10 ±

0.02b 0.05d 0.03c 0.11a

The values are mean of triplicate ± SD. Different superscripts in the same column are significantly different (p ≤ 0.05).

1.6

40

1.4

35

150

100

50

1.2 1.0

25

0.8 20 0.6 15 0.4 DDEWP

0

DDEWP

U1AUDDEWP U2AUDDEWP U3AUDDEWP MAUDDEWP

Fig. 3 – Foaming capacities (FC) and foaming stabilities (FS) of differently treated duck egg white proteins. The increases of foaming capacity in the ultrasound treated DDEWP might be the mechanical homogenization effect of the ultrasound. The homogenization effect of ultrasound usually results in the dispersion of the protein particles more evenly and then improves the foaming property (Jambrak et al., 2008). In addition, the protein structure may become partially unfolded after ultrasound treatment which also increased the foaming capacity (Zhang et al., 2011). The FS values of MAUD-DEWP were lower than those of UAUD-DEWP. This may be due to the fact that the ultrasound has increased the charges and protein surface hydrophobicity whereas that of microwave might not. High surface hydrophobicity can increased the viscosity of the protein and the dispersion becomes better (Townsend and Nakai, 1983). For UAUD-DEWP, however, the results of FS value had an opposite trend to that of the FC values, i.e. the higher the ultrasound frequency was, the greater the damage to the foam stability of the proteins. The results are opposite to those reported in literature. For example, the foaming stabilities of wheat germ globulin were enhanced with the increase of ultrasound power (Jia et al., 2009).

4.5.

30

Emulsifying stability index (min)

EAI ESI

Emulsifying activity index

Foaming capacity/Foaming stability(%)

FC FS

Emulsifying properties and emulsion stability

Emulsifying activity index (EAI) and emulsion stability index (ESI) are also two important functional characteristics of proteins that affect the protein behaviors (Hettiarachchy and Kalapathy, 1998). The EAI and ESI values of duck egg white protein samples are presented in Fig. 4, which showed that the EAI and ESI values of DDEW samples were lower (p ≤ 0.05) than those of the UAUD-DEWP and MAUD-DEWP samples. Interestingly, the ESI values have the same trend to that of

U1AUDDEWP U2AUDDEWP U3AUDDEWP MAUDDEWP

Fig. 4 – Emulsifying activities index (EAI) and emulsifying stabilities index (ESI) of differently treated duck egg white proteins. the EAI values in these samples. The results suggested that the ultrasound and microwave treatments have similar effects on the protein emulsifying properties (Koc¸ et al., 2011). These results are in agreement with previous reports by Sun and Shi (2006) and Wang et al. (2006). Sun and Shi (2006) reported that the ultrasonic treatment could destroy the integrity of the soy protein structure and result in looser protein molecules that improved the emulsifying capacity and emulsion stability. Wang et al. (2006) also reported that microwave could improve the emulsifying properties of soy protein. Under the microwave electromagnetic fields, protein molecule polarity orientation varies with the changes of the electromagnetic field to produce intense rotation. This regular periodic rotation has promoted the soy protein molecules to a high spiral state (Hua et al., 1995) and the aggregation of protein molecules weaken or disappear, thus greatly improved the emulsifying properties of the soy protein (Wang et al., 2006).

4.6.

Gel properties

The gelling characteristic of egg white protein could affect the adhesive and hydration properties of ham, sausage, and salami products when it is used as an ingredient (Liu and Zhang, 1994; Lin, 1996). The results indicated that the hardness values of UAUD-DEWP gels were higher than that of MAUDDEWP (Fig. 5), suggesting the effect of ultrasound on the duck gel strength is greater than that of microwave. Che et al. (2008) observed that the gel strength of egg white protein powder was only slightly increased after microwave treatment. However, the gelling properties of a soy protein isolate were significantly enhanced after the ultrasound treatment, and the gel hardness became higher with the increase of the ultrasound power,

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Hardness (g)

1500

1000

500

0

DDEWP

U1AUDDEWP U2AUDDEWP U3AUDDEWP MAUDDEWP

Fig. 5 – Gel hardness of differently treated duck egg white proteins. that is because the protein gel phase angle would decrease with the effect of ultrasound, and the smaller gel phase angle means the better the protein gel (Tang et al., 2005).

5.

Conclusions

In this study, the desalination rate, foaming capacity and foam stability, emulsifying properties and emulsion stability, and gel strength of the salted duck egg white protein were determined after different pretreatments and ultrafiltration desalination. The application of ultrasound and microwave pretreatments significantly (p ≤ 0.05) improved the desalination rate and the impact of ultrasound was greater than that of microwave. The foaming capacity, emulsifying properties and emulsion stability, gel strength of the desalted duck egg white protein was better when the ultrasound intensity and power was increased in the present studied range.

Acknowledgement This work was financially supported by the Guangdong Province R&D Project in China (No. 2012B091000125).

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