Food Chemistry 185 (2015) 90–98

Contents lists available at ScienceDirect

Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Analytical Methods

Preparation and physicochemical properties of soluble dietary fiber from orange peel assisted by steam explosion and dilute acid soaking Lei Wang a,b, Honggao Xu a, Fang Yuan a, Rui Fan a, Yanxiang Gao a,⇑ a b

Beijing Key Laboratory of Functional Food from Plant Resources, College of Food Science & Nutritional Engineering, China Agricultural University, Beijing 100083, China Beijing Research Institute of Nutritional Resources, Beijing 100069, China

a r t i c l e

i n f o

Article history: Received 12 October 2014 Received in revised form 25 March 2015 Accepted 28 March 2015 Available online 3 April 2015 Keywords: Orange peel Soluble dietary fiber Steam explosion Sulfuric-acid soaking Physicochemical properties

a b s t r a c t The coupled pretreatment of orange peel with steam explosion (SE) and sulfuric-acid soaking (SAS) was investigated to enhance the yield and improve the functionality of soluble dietary fiber (SDF). When orange peel was pretreated by SE at 0.8 MPa for 7 min, combined with 0.8% SAS, the content of SDF was increased from 8.04% to 33.74% in comparison to the control and SDF prepared with SE–SAS showed the high water solubility, water-holding capacity, oil-holding capacity, swelling capacity, emulsifying activity, emulsion stability and foam stability. SDF from orange peel treated by SE–SAS exhibited significantly (p < 0.05) higher binding capacity for three toxic cations (Pb, As and Cu) and smaller molecular weight (Mw = 174 kDa). Furthermore, differential scanning calorimetry (DSC) measurement showed that SDF from orange peel treated by SE–SAS had a higher peak temperature (170.7 ± 0.4 °C) than that of the untreated sample (163.4 ± 0.3 °C). Scanning electron micrograph (SEM) images demonstrated that the surface of SDF from orange peel treated by SE–SAS was rough and collapsed. It can be concluded that SDF from orange peel treated by SE–SAS has the higher potential to be applied as a functional ingredient in food products. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Worldwide industrial citrus byproducts are estimated to be more than 15  106 tons, as the amount of residues accounted for 50% of the whole fruit mass. During the processing of citrus fruits, peels were the primary byproduct, and a potential burden to environment without further treatment (Ramful, Tarnus, Aruoma, Bourdon, & Bahorun, 2011). Byproducts from citrus juice extraction had a potential use as a source of dietary fiber (DF). In recent years, DF received a great deal of attention from researchers, the food industry and consumers due to the health benefits that were associated with the consumption of fiber-rich products. Beneficial effects included lowering blood lipid and glucose levels, reducing risks from cardiovascular and colorectal cancer diseases, increasing satiety of host, and enhancing gastrointestinal immunity (Gunness & Gidley, 2010). Among those health benefits, soluble dietary fiber (SDF) was thought to play a major role. In addition, DF had hydrocolloidal properties that contributed technological implications in food manufacturing and final food products (Kosmala et al., 2013). ⇑ Corresponding author at: Box 112, No. 17 Qinghua East Road, Haidian District, Beijing 100083, China. Tel.: +86 10 6273 7034; fax: +86 10 6273 7986. E-mail address: [email protected] (Y. Gao). http://dx.doi.org/10.1016/j.foodchem.2015.03.112 0308-8146/Ó 2015 Elsevier Ltd. All rights reserved.

Therefore, DF was not only desirable for its nutritional value but also important in food formulation with its functional and physicochemical properties (Fabek, Messerschmidt, Freeport, & Goff, 2014). The main advantage of DF from orange peel, compared with other alternative sources, such as cereals, was its higher proportion of soluble dietary fiber. This was very important for the development of fiber-containing products, considering that the requirement for DF intake should be balanced, i.e. the water-soluble fraction should represent between 30% and 50% of the total dietary fiber (Jing & Chi, 2013). Steam explosion (SE) is an innovative processing technology, which is used for the pretreatment of lignocellulosic materials. This technology is based on exposing the samples to high-temperature pressurized steam for a short period and forcing the steam into the tissues and cell of samples, followed by explosive decompression completed in millisecond (Yu, Zhang, Yu, Xua, & Song, 2012). During the explosion, most of the steam and hot liquid water in the samples quickly expands and breaks free of the structure. Compared with other pretreatment methods, the advantages of SE included significantly lower energy consumption, lower capital investment and less hazardous process chemicals (Alvira, Tomás-Pejó, Ballesteros, & Negro, 2010). In the previous publication, the results showed that SE could significantly improve the extraction yield and physicochemical properties of cellulose from

L. Wang et al. / Food Chemistry 185 (2015) 90–98

Lespedeza stalks (Lespedeza crytobotrya) (Wang, Jiang, Xu, & Sun, 2009). Chen and Chen (2011) had reported that SE extraction was an effective method for extracting and conversing flavonoids from sumac fruits. However, the application of SE in SDF extraction from orange peel has not yet been reported. Dilute sulfuric-acid soaking (SAS) pretreatment is an effective and inexpensive way of hydrolyzing hemicellulose, reducing cellulose crystallinity, and increasing surface area and pore volume of the substrate (Fang, Deng, & Zhang, 2011). A few available researches demonstrated that the dilute-acid soaking treatment influenced the structure and function of soy protein. The improvement of functional properties of acid-modified soy protein was due to its decreased molecular size and increase in surface hydrophobicity induced by deamidation (Zhang, Yang, Zhao, Hua, & Zhang, 2013; Zhang, Zhao, et al., 2013). Based on the aforementioned viewpoint, a combination of SE and SAS pretreatment can simultaneously influence the structural and functional properties of SDF from orange peel. The objective of this study was to optimize the parameters of SE–SAS in which the maximum SDF extraction yield was obtained. In addition, the physicochemical properties and microstructures of SDF were also evaluated.

91

millisecond. The samples were carefully recovered and sealed in plastic bags. 2.4. Description of experimental design In the acid soaking concentrations (0–1.0%) experiments, acid soaked and non-acid soaked orange peels were treated at 0.6 MPa for 9 min, and the SDF extraction yield of orange peel was examined. In the optimization of steam pressure, the acidsoaked orange peel was then treated at 0.4 MPa, 0.6 MPa, 0.8 MPa, 1.0 MPa and 1.2 MPa for 7 min, and the SDF extraction yield of SE-treated orange peel was examined. In the optimization of residence time of SE, the acid-soaked orange peel was treated at 0.8 MPa for different residence times (3 min, 5 min, 7 min, 9 min and 11 min), and the SDF extraction yield of orange peel was examined. All experiments were carried out using the individual amount (500 g) of dried orange peels. All experimental set points were carried out in triplicate. For each experiment, the materials recovered from receiver were carefully mixed together and constituted a unique batch (Zhang, Yang, et al., 2013; Zhang, Zhao, et al., 2013). 2.5. Preparation of SDF from orange peel

2. Materials and methods 2.1. Materials The oranges [Citrus sinensis (L.) Osbeck] were purchased from the commercial orchards located in China. The one batch of oranges was used for independent replicates. The orange fruits were separated into edible and inedible portions (peel) and the peels were dried in an air-oven at 50 °C for 24 h. The moisture content of the dried peel samples was 5.83 ± 0.73%. The dried samples were then finely ground to pass through a 10-mesh screen and stored in polyethylene bags until used. All other reagents and chemicals were of analytical grade. 2.2. Acid soaking treatment The ground orange peel was treated with dilute sulfuric acid solution (orange peel/solution ratio = 1:2) in a stainless steel container immersed in a temperature-controlled water bath maintained at 80 °C for 2 min. The container was equipped with a stirrer to ensure proper mixing of orange peel with the acid solution. The sulfuric acid concentrations were examined in a range between 0% and 1.0% (w/v). Soaking in deionized water was referred to as a non-acid treatment condition. Following the acid and non-acid treatment, the slurry was filtrated through the fourfold gauze to separate excess sulfuric acid (Zhang, Yang, et al., 2013; Zhang, Zhao, et al., 2013). After filtration, the moisture content of soaked orange peel meal was about 65%.

To determine SDF content in orange peel, SDF was extracted according to the AOAC method 985.29, an enzymatic–gravimetric procedure (AOAC, 2001) with slight modifications. The pretreatment orange peel, dispersed in 4 times volume of deionized water, adjusted pH to 6.0 with 0.1 mol/L NaOH, added 0.1% (w/w) heatstable a-amylase, hydrolyzed at 95 °C under constant stirring of 120 rpm for 30 min, and 120 °C in oil bath for 5 min to inactivate. After the temperature of the hydrolysate was decreased to 60 °C, 0.016% (w/w) neutral protease was added and went further hydrolysis for 30 min under constant stirring of 120 rpm. At the end, the enzymatic hydrolysis reaction was quenched at 95 °C for 5 min and the hydrolysate was centrifuged at 4200 rpm for 20 min, the supernatant and sediment were collected, respectively. The supernatant was condensed to one-tenth in vacuum rotary evaporation system. Afterwards, the concentrated supernatant was mixed with 95% (v/v) ethanol at 4 °C for 12 h and subjected to centrifugation at 4200 rpm for 15 min. The precipitated flocculate was dried in a laboratory oven (DHG-9140A, Yiheng, Shanghai, China) at 60 °C for 48 h. The dried flocculate was milled (Thomas Scientific, Swedesboro, NJ, USA) and passed through a 60mesh sieve and stored at 4 °C, the powder obtained was SDF. 2.6. Determination of soluble dietary fiber The contents of soluble dietary fiber (SDF) in untreated and treated samples were measured according to the AOAC (AACC Method 32-45, 2010).

2.3. Steam explosion treatment

2.7. Physicochemical properties of SDF

About 500 g of acid soaked and non-acid soaked orange peels were loaded into a 5 L reactor of the SE system. It adopts a structure in catapult explosion mode that is principally composed of a cylinder and piston. The force of the piston drive system, which is composed of a linear actuator and a solenoid valve, comes from compressed air (Yu et al., 2012). The reactor is equipped with a high-pressure autoclave with gas inlet. When the saturated steam was quickly allowed to enter the reactor and steam pressure was maintained for given time, the steam inlet was shut off and the piston device was triggered. The explosion was completed in about

2.7.1. Solubility, water-and oil-holding capacities and swelling capacity The water solubility (WS) was determined in triplicate according to the method described by Zhang, Liang, Pei, Gao, and Zhang (2009) with minor modification. Dry sample (1.0 g) was gently mixed with 50 mL of distilled water in a beaker. Subsequently, the mixture was stirred at 90 °C for 30 min in thermostat water bath followed by centrifugation at 4200 rpm for 10 min. The supernatant was collected, freeze-dried and weighted. The WS was calculated as follows:

92

WSð%Þ ¼

L. Wang et al. / Food Chemistry 185 (2015) 90–98

W1  100 W

where W1 and W are the weights of supernatant after drying and SDF sample, respectively. The water-holding capacity (WHC) was determined in triplicate according to a method described in the literature (Mateos-Aparicio, Mateos-Peinado, & Ruperez, 2010) with minor modification. Briefly, 20 mL distilled water was mixed with 0.5 g of SDF at room temperature (25 °C) for 24 h. After centrifugation at 4200 rpm for 10 min, the sediment was collected and weighted. The WHC was calculated as follows:

WHCðg=gÞ ¼

W1 W

where W1 and W are the weights of water adsorbed and SDF sample, respectively. The oil-holding capacity (OHC) was determined in triplicate according to the method described by Zhang et al. (2009) with slight modification. Briefly, 5 mL olive oil was mixed with 0.5 g of SDF at 4 °C for 1 h. Then it was centrifuged at 4200 rpm for 15 min, the sediment was collected and weighted. The OHC was calculated as follows:

OHCðg=gÞ ¼

W1 W

where W1 and W are the weights of oil adsorbed and SDF sample, respectively. The swelling capacity (SC) was measured in triplicate according to the method described by Zhang, Bai, and Zhang (2011) and Zhang, Huang, and Ou (2011) with minor modification. Accurately weighed dry SDF (0.2 g) was placed in a test tube, 5 mL of water was added and it was hydrated for 18 h at 4 °C. The volume fraction and the volume (mL) of the SDF were recorded. The SC was calculated as follows:

SCðmL=gÞ ¼

V W

where V is the final volume occupied by SDF sample; W is the weight of SDF sample. 2.7.2. Emulsifying activity, emulsion stability and least gelation concentration Emulsifying activity (EA) and emulsion stability (ES) were evaluated following Chao, Cheung, and Wong (1997) with minor modification. Briefly, 2 g SDF was dispersed in 100 mL deionized water to obtain 100 mL SDF suspension, which was homogenized using a Caframo HD-1 homogenizer at 2000 rpm for 2 min. Then, 100 mL of corn oil (Mazola, CPI International) was added to each sample and homogenized for 1 min. The emulsions were centrifuged in 15 mL graduated centrifuge tubes at 1200 rpm for 5 min, and then emulsion volume was measured. EA was expressed as the mL of the emulsified layer volume of the 100 mL entire layer in the centrifuge tube. The EA was calculated as follows:

EAðmL=100 mLÞ ¼

V1  100 V

where V1 and V are the volumes of the emulsified layer and the total content, respectively. ES was determined by heating the prepared emulsions to 80 °C for 30 min, cooling them to room temperature (25 °C) and centrifuging at 1200 rpm for 5 min. ES was expressed as mL of the remaining emulsified layer volume of 100 mL of the original emulsion volume. The ES was calculated as follows:

ESðmL=100 mLÞ ¼

V1  100 V

where V1 and V are the volumes of the emulsified layer after heating and the emulsified layer before heating, respectively. The least gelation concentration (LGC) was determined by the method of Coffman and Garcia (1977) with slight modification. Suspensions were prepared in distilled water with SDF concentrations of 2%, 4%, 6%, 8%, 10% and 12% (w/v). Aliquots of these suspensions (5 mL) were transferred into tubes and placed in a water bath for 1 h at100 °C, and then placed in an ice bath for 1 h. The least gelation concentration was detected when the sample did not slide along the tube when the tube was inverted. 2.7.3. Foam stability Foam stability of SDF was determined according to the methods used by Miquelim, Lannes, and Mezzeng (2010) with minor modification. The SDF samples were dissolved in distilled water, 1.0% of sucrose solution and 1.0% of NaCl solution, respectively at the concentration of 1.5%. All the SDF solutions were prepared in a graduated beaker of 250 mL and a volume of 20 mL was transferred to another graduated beaker of 50 mL. The beaker was covered with a paraffin film and stirred vigorously at 1000 rpm for 5 min by a magnetic stirrer. The foam stability was analyzed as a function of time observed for the volume of foam to form after stirring. 2.8. Adsorption capacity of SDF for toxic ions The maximum binding capacity (BCmax) and the minimum binding concentration (BCmin) of the SDF for toxic ions were measured according to the method described by Zhang, Bai, et al. (2011) and Zhang, Huang, et al. (2011) with slight modification. When determining BCmax, 1.0 g of SDF was suspended in 100 mL of a solution containing 10 mmol/L of the following solutions, Pb(NO3)2, CuSO4, and NaAsO2, in a 250 mL conic flask; and the pH was adjusted to 2.0 and 7.0 to simulate the environments in the stomach and small intestine, respectively. The slurries were shaken (120 rpm) for 3 h in a water-bath incubator maintained at 37 °C. At the end of adsorption, 2 mL volume of the sample was collected, and absolute ethanol (8 mL) was added to precipitate the SDF. The mixture was centrifuged at 4200 rpm for 20 min. The concentrations of Pb, As, and Cu in the supernatant were determined by inductively coupled plasma-atomic emission spectroscopy (Optima 2000 DV, Perkin-Elmer, Norwalk, CT, USA). When determining BCmin, 2.5 g of SDF and 500 lmol/L of Pb(NO3)2, CuSO4, and NaAsO2 were used; all other conditions were the same as for BCmax. 2.9. Molecular weight The molecular weight was measured according to the method described by Wu, Wang, and Xu (2007) with minor modification. Briefly, 60 mg SDF sample was dissolved in 10 mL sodium nitrate (0.1 mol/L) and centrifuged at 10,000 rpm for 10 min. The supernatant obtained was analyzed for molecular weight distribution using a Waters™ 650E Advanced Protein Purification System (Waters Corporation, Milford, MA, USA) assembly running 10 lL sample at 0.5 mL/min. TSK gel 2000 SWXL (7.8 mm i.d.  300 mm) column was regulated at 30 °C. A calibration curve was obtained with bovine carbonic anhydrase (29,000 Da), horse heart cytochrome C (12,500 Da), aprotinin (6500 Da), bacitracin (1450 Da), gly-gly-tyr-arg (451 Da) and gly-gly-gly (189 Da). With the help of elution time of calibration materials, the linear regression equation was obtained for the calculation of molecular weight. The results were processed with Millennium32 Version 3.05 Copyright 1998 (Waters Corporation Milford, MA, USA).

93

L. Wang et al. / Food Chemistry 185 (2015) 90–98

2.10. Thermal analysis

Extraction yield of SDF/%

The simultaneous thermal analysis (differential scanning calorimeter DSC) was carried out using a STA 60A-device (Shimadzu, Japan) according to Einhorn-Stoll, Kunzek, and Dongowski (2007) with minor modification, the following conditions were used: linear heating rate 5 °C/min from 30 to 200 °C, dynamic inert nitrogen atmosphere (75 mL/min), empty pan was used as a reference, SDF sample weight was approximately 10– 20 mg. All runs were performed at least in triplicate.

40

a c

30

d

d

0.8

1.0

c

b a 20

10

2.11. Scanning electron microscopy (SEM) 0

2.12. Statistical analysis All SDF preparations and analyses were conducted at least in triplicate. To verify the statistical significance of all results the values of means ± S.D. were calculated. Data were subjected to analysis of variance (ANOVA) using the software package SPSS 12.0 for Windows (SPSS Inc., Chicago, USA). Significant differences (p < 0.05) of means were determined by the Tukey test.

0

0.2

0.4

0.6

Sulfuric acid concentration/% 40

b

c b

Extraction yield of SDF/%

The SDF scanning electron images were gathered using a scanning electron microscope (JSM-6360LV, JEOL, Japan). SDF samples were prepared according to the method described by Peerajit, Chiewchan, and Devahastin (2012). In brief, the SDFs were placed on a specimen holder with double-sided scotch tape and sputtercoated with gold (10 min, 2 mbar). Subsequently, each sample was transferred to the scanning electron microscope at an accelerating voltage of 15.0 kV and 1000-fold magnifications.

b

30

d a

a

20

10

0 0

0.4

0.6

0.8

1.0

1.2

Steam pressure/MPa

3. Results and discussion 40

3.1. Effect of sulfuric acid concentration on the extraction yield of SDF

3.2. Effect of SE parameters on the extraction yield of SDF The effect of SE treatment parameters on the extraction yield of SDF from acid soaked orange peel was evaluated (Fig. 1b and c). The orange peel was first soaked in 0.8% sulfuric acid and then

c Extraction yield of SDF /%

The effect of sulfuric acid concentration on the extraction yield of SDF was shown in Fig. 1a. The extraction yield of SDF was generally proportional to sulfuric acid concentration. The extraction yield of SDF from orange peel by conventional water-extraction with SE pretreatment was about 20.32%. However, after SE–SAS pretreatment, the extraction yield of SDF was significantly (p < 0.05) increased to 23.52%, 27.49%, 29.03%, 33.82% and 34.85% at the presence of 0.2%, 0.4%, 0.6% and 0.8% sulfuric acid, respectively. There were no significant differences in extraction yield of SDF between 0.8% and 1.0%. Acid catalyzed hydrolysis of polysaccharide samples containing insoluble cellulose hemicelluloses in plant cell walls is commonly used for the conversion of these polymers to soluble sugars (Johansson et al., 2006). It was concluded that SAS could solubilize the hemicellulosic fraction of the orange peel and soften the lignin structure. Therefore, the acid soaked sample had a loose and porous structure (Wang et al., 2011). More significantly, the loose and porous structure could result in an increase in high pressure steam permeation into the cell tissues of orange peel, which could form more powerful explosive decompression. Nevertheless, high sulfuric acid concentration is environment-unfriendly and has negative influence on the production equipment (Zhang, Yang, et al., 2013; Zhang, Zhao, et al., 2013). Based on the consideration of the extraction yield of SDF and sulfuric acid consumption, 0.8% sulfuric acid was preferred for soaking.

c b

30

d e

a

20

10

0 3

5

7

9

11

Residence time/min Fig. 1. Extraction yield of SDF at various SE–SAS treatment conditions: (a) orange peel (500 g) treated by SAS at different sulfuric-acid concentrations with SE treatment (0.6 MPa and 9 min); (b) orange peel (500 g) treated by SE at different steam pressures for 7 min after sulfuric-acid soaking (0.8% concentration); (c) orange peel (500 g) treated by SE at 0.8 MPa for different residence times after sulfuric-acid soaking (0.8% concentration). Different letters on the top of a column indicate significant differences (p < 0.05); n = 3.

treated by SE. As shown in Fig. 1b, after SE–SAS pretreatment, the SDF extraction yield was increased from 22.63% (only SAS treatment) to 23.87%, 31.53%, 33.74%, 30.17% and 26.87% at 0.4 MPa, 0.6 MPa, 0.8 MPa, 1.0 MPa and 1.2 MPa, respectively, at 7 min. When steam pressure was less than 0.4 MPa, there was no obvious effect of SE pretreatment on extraction yield of SDF. Meanwhile, when steam pressure was higher than 0.8 MPa, it was the true that SE treatment caused orange peel discolor. Moreover, cellulose and lignin molecules were degraded and

94

L. Wang et al. / Food Chemistry 185 (2015) 90–98

converted into small molecules, which were not precipitated with alcohol, reducing the extraction yield of SDF. According to the aforementioned analysis, steam pressure at 0.8 MPa was selected to evaluate the effect of different residence times of SE on the extraction yield of SDF. As shown in Fig. 1c, when the residence time was 7 min, the extraction yield of SDF was improved to 33.74%. Compared with extraction yield of SDF from SAS treated orange peel (22.63%), it indicated that after SE–SAS pretreatment, the extraction yield of SDF was significantly (p < 0.05) improved. Talebnia, Karakashev, and Angelidaki (2010) suggested that the SE treatment efficiency was a product of several factors, including steam pressure, residence time, moisture content and particle size. According to the data in Fig. 1b and c, it showed that the extraction yield of SDF could reach maximum level in an appropriate range of steam pressure and residence time. 3.3. Physicochemical properties of SDF from orange peel 3.3.1. Solubility, water-and oil-holding capacities and swelling capacity The effect of SE–SAS pretreatment on WS, WHC, OHC and SC of SDF was shown in Table 1-1. In general, the polysaccharide constituents of dietary fibers were strongly hydrophilic. The WS of SDF was increased from 86.74 ± 2.01% to 91.37 ± 1.56%, likely due to the change in the three-dimensional structure of SDF and an increase in the amount of short-chain dietary fiber. In addition, SE–SAS treatment significantly (p < 0.05) increased the initial WHC of SDF (3.63 ± 0.21 g/g db) to 5.52 ± 0.73 g/g db. The WHC of SDF was similar to those of fruit-based sources such as plum pomace (Kosmala et al., 2013). However, it was higher than that of cereal-based sources, including rice bran (Abdul-Hamid & Luan, 2000) and oat bran (Zhang et al., 2009). The WHC of processed dietary fiber from several vegetable and fruit sources generally varied from 2.8 to 42.5 g/g (Elleuch et al., 2011). The SC of SDF from orange peel treated by SE–SAS was increased from 4.83 ± 0.52 mL/g db to 6.28 ± 0.73 mL/g db. The SC was dependent on the characteristics of individual components and the physical structure (porosity and crystallinity) of the fiber matrix (Raghavendar et al., 2006), the increase in SC might be attributed to a rise in the amount of short-chains and the surface area of dietary fiber induced by SE. These results were in agreement with previous reports that focused on SDF from extruded oat bran (Zhang et al., 2009) and micronized buckwheat hulls (Zhu, Du, Li, & Li, 2014). The ability of dietary fiber to retain oil is important in food applications, such as preventing fat loss upon cooking and also nutrition value for the ability to absorb or bind bile acids (Schneeman, 1999). As shown in Table 1-1, the OHC of SDF sample was significantly (p < 0.05) increased to 1.72-fold after SE–SAS pretreatment. Some studies indicated that water related properties, oil retention ability and swelling capacity of SDF in functional foods were highly associated with the rheological properties of the final products and the mouth-feel experienced by consumers (Elleuch et al., 2011). It was concluded that SDF from orange peel treated by SE–SAS might be a better dietary resource for related food products.

3.3.2. Emulsifying activity, emulsion stability and least gelation concentration Emulsifying activity (EA) was a molecule ability to act as an agent that facilitates solubilization or dispersion of two immiscible liquids and emulsion stability (ES) was the ability to maintain an emulsion and its resistance to rupture (Lan, Chen, Chen, & Tian, 2012). As shown in Table 1-1, EA of SDF from untreated orange peel was 56.40 ± 2.16 mL/100 mL and its ES was 45.50 ± 1.62 mL/100 mL, while those of SDF from orange peel treated by SE–SAS were 82.30 ± 1.96 mL/100 mL for EA and 65.80 ± 1.50 mL/100 mL for ES. Compared with SDF from untreated orange peel, SDF from orange peel treated by SE–SAS might be a good emulsifying agent. SDF had effective EA via interfacial absorption and the subsequent formulation of condensed films with high-tensile strength that resisted coalescence of droplets. Furthermore, it stabilized oil/water emulsions by forming a strong multimolecular film around each oil globule and thus retarded the coalescence by the presence of a hydrophilic barrier between the oil and water phases (Prajapati, Jain, & Moradiya, 2003). The EA of SDF powder was also very indicative of its ability to adsorb biliar acids and had potential health benefits to increase feces excretion, consequently limiting absorption of those acids in the small intestine and thus reducing the blood cholesterol levels (Lan et al., 2012). Therefore, it should have a good function in foods requiring emulsion formation and long shelf life. The LGC was used as an index of gelation capacity, which was an essential property in the preparation and acceptability of many kinds of foods. In this study, the LGCs of SDF from untreated and SE–SAS treated orange peel were determined and there was significant (p < 0.05) difference (Table 1-1). LGCs of SDF from untreated and SE–SAS treated orange peel were 10.20 ± 0.50% (w/v) and 7.60 ± 0.40% (w/v), respectively. In general, SDF with lower LGC value showed better gelation characteristics. Compared with SDF from untreated orange peel, the LGC value of SDF from orange peel treated by SE–SAS was significantly (p < 0.05) reduced, indicating that SDF from orange peel treated by SE–SAS had a better gelation capacity. 3.3.3. Foam stability In order to measure the functional properties of SDF for food products, time-stability experiments of SDF foams were performed. The volume of foams formed in the beaker was observed as a function of time for the various solution formulations, as shown in Fig. 2. For the water solution, the volumes observed straight after the foaming process were 4.83 mL and 4.05 mL at 0 h for SDF from SE–SAS treated and untreated orange peel, after 1 h, the volumes of the foams reduced to 3.86 mL and 3.29 mL, respectively. Identical trend was found for the solutions containing 1.0% of NaCl and 1.0% sucrose at 0 h and 1 h. Compared with the foam volume in water solution, reduced foam volume of SDF was observed in 1.0% of sucrose solution while increased foam volume in 1.0% of NaCl solution was found at both 0 h and 1 h. Therefore, although individual foam cases followed specific kinetics in the decrease of the foam volume, the long-term foam stability was clearly correlated with the air–water interfacial properties, mediated by the SDF complexes (Zhang, Bai, et al. (2011) and Zhang, Huang, et al. (2011)).

Table 1-1 Effect of SE–SASA on physicochemical properties of orange peel SDF.

⁄

Sample

WS (%)

WHC (g/g)

OHC (g/g)

SC (mL/g)

EA (mL/100 mL)

ES (mL/100 mL)

LGC (%)

Untreated SE–SAS

86.74 ± 2.01a 91.37 ± 1.56b

3.63 ± 0.21a 5.52 ± 0.73b

1.76 ± 0.32a 3.03 ± 0.31b

4.83 ± 0.52a 6.28 ± 0.73b

56.40 ± 2.16a 82.30 ± 1.96b

45.50 ± 1.62a 65.80 ± 1.50b

10.20 ± 0.50a 7.60 ± 0.40b

The values represent means of triplicate ± standard deviation. Values in the same column with different letters are significantly different (p < 0.05) (WS: water solubility; WHC: water-holding capacity; OHC: oil-holding capacity; SC: swelling capacity; EA: emulsifying activity; ES: emulsion stability; LGC: least gelation concentration). A SE–SAS treatment parameters: SAS concentration 0.8%, SE pressure 0.8 MPa and residence time 7 min.

95

L. Wang et al. / Food Chemistry 185 (2015) 90–98

6 SDF from orange peel treated by SE-SAS (0h) 5 Foam volume (mL)

SDF from untreated orange peel (0h) 4 SDF from orange peel treated by SE-SAS (1h) 3 SDF from untreated orange peel (1h) 2 1 0 Distilled water

1.0% Sucrose solution

1.0% NaCl solution

Fig. 2. Foam stability of SDF samples from orange peel treated by SE–SAS in different solutions at different times. SE–SAS treatment parameters: SAS concentration 0.8%, SE pressure 0.8 MPa and residence time 7 min.

Food foams had behavior and stability directly related to their microstructure, bubble size distribution and interfacial properties. A high interfacial tension inherent to air/liquid foam interface affected its stability, and thus it had a direct impact on storage, processing and product handing. Many studies focused on the stabilization of foams (Zhang, Bai, et al. (2011) and Zhang, Huang, et al. (2011)). In this research, the result exhibited that SDF from orange peel treated by SE–SAS could greatly improve foam stability under the same condition. 3.4. Binding capacity of three toxic ions to SDF The maximum binding capacity (BCmax) was the level at which all the binding sites (including the physical and chemical binding sites) of the dietary fibers were saturated by the toxic ions. This parameter could be used to represent the maximum amount of bound heavy metal ions. The minimum binding concentration (BCmin) was the equilibrium concentration at which the amount of ions bound by the dietary fibers equals the amount of ions released from the dietary fibers, provided that the binding sites of the dietary fibers were not saturated by the toxic ions (Zhang, Bai, et al. (2011) and Zhang, Huang, et al. (2011)); it could be used to evaluate the affinity of the dietary fibers for the toxic ions and represent the minimum concentration of heavy metal ions below which the ions could not be bound by the dietary fibers. Table 1-2 showed that the two type of SDFs had higher BCmax and lower BCmix for the toxic heave metal ions at pH 7.0 than pH 2.0, indicating that they had high affinity for the toxic cations under the condition similar to those in the gastrointestinal tract. Moreover, based on the BCmax and BCmix values, it could be concluded that SDF from orange peel treated by SE–SAS showed higher binding capacity for three toxic cations than those of untreated sample at pH 2.0 and 7.0. Our intake of dietary fiber remains for only a short time (1–2 h) in the stomach, which is not the main organ to absorb minerals. It

is unsuitable to judge the detoxifying capacity of dietary fibers merely by their detoxifying ability in the stomach. In contrast, the small intestine, which is the main organ to absorb minerals, and foods and toxic materials remain there much longer than in stomach. Therefore, the small intestine is a more important organ in the assessment of the detoxifying capacity of dietary fibers. At pH level similar to that in the small intestine (pH 7.0), BCmax values were 2–3 times higher than those at pH 2.0 and BCmix values were sharply decreased for two types of SDFs (Table 1-2), these values were so reduced that the heavy metals would not cause any damage to the body (Ou, Gao, & Li, 1999). Three possible mechanisms were proposed for binding capacity of dietary fibers with heavy metals: chemisorption, physical sorption and mechanical sorption (Hu, Huang, Chen, & Wang, 2010). Chemisorption was connected with the presence in the fiber matrix of phenolic group from lignin and carboxyl groups from uronic acids. When the pH value was increased, the carboxyl groups were dissociated to carboxyl anions (RCOO), which showed stronger interaction with the toxic cations than that of undissociated carboxyl groups, resulting in higher binding capacity of the dietary fibers with the toxic cations. Physical sorption resulted from van der Waals’ force, which was temperature dependent, while mechanical sorption was depended on the degree of porosity of SDF and its ability to trap the substances in its spatial structure. 3.5. Molecular weight distribution Molecular weights of SDFs were detected by DAWN EOS gel permeation chromatography-laser light scattering spectrometer, and the corresponding results were shown in Table 2. The molecular weight of SDF was varied largely in the sight of its intricate composition including glucan, pectin, gums, and mucilage. The pretreatment methods obviously influenced molecular size distributions of SDFs from orange peel. Compared to that of SDF from untreated orange peel (weight-average molecular weight (Mw) = 470 kDa),

Table 1-2 Effect of SE–SASA on the values of the maximum binding capacity (BCmax) and the minimum binding concentration (BCmin) of the orange peel SDF for Pb, As and Cu at pH 2.0 and 7.0. Sample

Untreated SE–SAS Untreated SE–SAS ⁄

pH

2.0 2.0 7.0 7.0

BCmax (lmol/g)

BCmin (lmol/L)

Pb

As

Cu

Pb

As

Cu

76.8 ± 3.2a 193.3 ± 5.1b 223.4 ± 3.9a 376.8 ± 6.3b

54.8 ± 1.8a 169.7 ± 4.6b 179.3 ± 3.5a 321.5 ± 5.2b

37.4 ± 1.2a 89.5 ± 2.4b 79.2 ± 3.8a 196.4 ± 5.5b

253.4 ± 7.5a 164.7 ± 4.9b 89.3 ± 2.6a 27.4 ± 1.3b

324.7 ± 9.7a 198.5 ± 8.8b 102.4 ± 6.1a 57.6 ± 4.5b

298.3 ± 9.3a 212.4 ± 7.8b 136.5 ± 6.7a 63.1 ± 3.9b

The values represent means of triplicate ± standard deviation. Values in the same column with different letters are significantly different (p < 0.05). A SE–SAS treatment parameters: SAS concentration 0.8%, SE pressure 0.8 MPa and residence time 7 min.

96

L. Wang et al. / Food Chemistry 185 (2015) 90–98

Table 2 Effect of SE–SASa on molecular weight of orange peel SDF. Samples

Weight-average molecular weight Mw (kDa)

Number-average molecular weight Mn (kDa)

Polydispersity Pd (Mw/Mn)

SDF from untreated orange peel SDF from SE– SAS treated orange peel

470

143

3.29

174

88

1.98

part of hemicellulose and lignin in comparison with the untreatment. It was inferred that the obvious decrease in particle size after SE–SAS treatment could increase the porosity and capillary attraction of SDF and consequently enhance the physical entrapment of oil, hence increase the OHC (Chau & Huang, 2003). Number-average molecular weight (Mn) and polydispersity (Pd) exhibited corresponding changes with Mw, which showed that SDF from SE–SAS treated orange peel had narrow polydispersity. 3.6. Thermal analysis

⁄

Values are given as means of independent experiments. a SE–SAS treatment parameters: SAS concentration 0.8%, SE pressure 0.8 MPa and residence time 7 min.

Fig. 3. DSC analytical curve of SDF from orange peel (solid line: treated by SE–SAS, dotted line: untreated). SE–SAS treatment parameters: SAS concentration 0.8%, SE pressure 0.8 MPa and residence time 7 min.

DSC was a useful technique to provide certain thermodynamic information, such as the temperature and enthalpy associated with transitions in materials during endothermic or exothermic processes. Two typical endotherms with peak temperature of SDF from untreated orange peel (163.4 ± 0.4 °C) and that from SE–SAS treated orange peel (170.7 ± 0.3 °C) were observed in the DSC thermograms in Fig. 3. A significant (p < 0.05) upshift of the peak temperature of the treated sample demonstrated that the short-chain SDF content was enhanced in comparison with the untreated sample because short-chain SDF has strong hydrogen bonds within its junction zones that require more energy to decompose the SDF crystalline structure (Li, Cui, & Kakuda, 2006). This conclusion strongly agreed with the previous results obtained from soybean residue SDF (Chen, Ye, Yin, & Zhang, 2014). Interestingly, exothermic and endothermic processes between SDFs from untreated and treated orange peels did not change until a temperature of 200 °C was reached, indicating that SDF was highly thermally stable. Similarly, Zhang et al. Also indicated that the peak temperature of SDF from extruded oat bran was higher than that of SDF from untreated one (Zhang, Bai, et al. (2011) and Zhang, Huang, et al. (2011)). 3.7. Scanning electron morphology (SEM)

the main molecular weight of SDF from SE–SAS treated orange peel (Mw = 174 kDa) was significantly (p < 0.05) smaller, which implied that SE–SAS treatment could severely degrade IDF, such as cellulose,

a

Based on the extraction yield of SDF, the surface morphology of SDF from orange peel treated by SE at 0.8 MPa for 7 min and 0.8% sulfuric acid soaking was examined. The SEM images of SDF from

b

c

Fig. 4. Scanning electron microcopy (SEM) images of SDF from orange peel ((a) untreated, (b) soaked in 0.8% sulfuric-acid, (c) treated by SE at 0.8 MPa for 7 min and soaked in 0.8% sulfuric-acid).

L. Wang et al. / Food Chemistry 185 (2015) 90–98

orange peel treated by SE–SAS were shown in Fig. 4, compared with SDF from untreated orange peel as control sample. The electron micrograph in Fig. 4a revealed that the surface of the SDF from untreated orange peel was intact. After 0.8% sulfuric acid soaking, the surface of SDF was partially disintegrated, suggesting that the sulfuric acid soaking could hydrolyze hemicelluloses, leading to collapsed and distorted cell wall structure (Fig. 4b). As shown in Fig. 4c, SE–SAS treatment made the cell wall disintegrate and the texture began to loosen, there was near complete rupture of the surface of SDF with large numbers of fragmented cell matter and there were many perforations appeared on the surface of SDF following SE–SAS pretreatment. Porosity, dilatability and the regiochemistry properties (adsorption or binding of some molecules) could be accounted for some physiological effects of dietary fiber (Ubando, Navarro, & Valdivia, 2005). Therefore, it was supposed that the spatial structure of SDF was changed by SE–SAS treatment, making the SDF show much higher binding capacity for the toxic ions. The SEM result implied that the kinetic energy of SE–SAS could break free of the cell structure of orange peel. Hence, SDF from orange peel treated by SE–SAS, which featured a larger surface area, allowed more space for the holding of water molecules by hydrogen bonding and/or dipole formation. 4. Conclusion SE–SAS pretreatment of orange peel was an efficient method for improving the SDF yield and physicochemical properties under optimal condition (0.8% H2SO4 concentration, 0.8 MPa steam pressure and residence time 7 min). The extraction yield of SDF could be significantly (p < 0.05) increased by the SE–SAS treatment. And the physicochemical properties of SDF from SE–SAS treated orange peel were dramatically improved and exhibited higher binding capacity for three toxic cations (Pb, As and Cu), in comparison with SDF from untreated orange peel. The main weight-average molecular weight (Mw) of SDF from SE–SAS treated orange peel (174 kDa) was significantly (p < 0.05) smaller than that of untreated SDF (470 kDa). These results of SEM and thermal analyses revealed a high concentration of short-chain SDF with a tough, irregular surface and high thermal stability (up to 200 °C). Therefore, SE–SAS technology would be a preferable choice for the production of active SDF from orange peel. The detailed mechanism that SE significantly enhanced the extraction yield of SDF from orange peel was unclear. However, during SE treatment, the shear stress through thermal explosion decomposition and the flow of treated slurry created tensile forces that broke the orange peel and resulted in further substantial breakdown of the lignocellulosic structure. Acknowledgements This work was supported by Key Science and Technology Projects of Hunan province of China (No. 2011FJ1047) and China Postdoctoral Science Foundation – China (2013BH029). References AACC Method 32–45. (2010). American association of cereal chemistry international. MN: St Paul. Abdul-Hamid, A., & Luan, Y. S. (2000). Functional properties of dietary fiber prepared from defatted rice bran. Food Chemistry, 68, 15–19. Alvira, P., Tomás-Pejó, E., Ballesteros, M., & Negro, M. J. (2010). Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresource Technology, 101, 4851–4861. AOAC. (2001). Official methods of analysis (17th ed.). MD: Association of Official Analytical Chemists. Chao, C., Cheung, K., & Wong, Y. (1997). Functional properties of protein concentrate from three Chinese indigenous legume seeds. Journal of Agricultural and Food Chemistry, 45, 2500–2503.

97

Chau, C. F., & Huang, Y. L. (2003). Comparison of the chemical composition and physicochemical properties of different fibers prepared from the peel of Citrus sinensis L. Cv. Liucheng. Journal of Agricultural and Food Chemistry, 51, 2615–2618. Chen, G. Z., & Chen, H. Z. (2011). Extraction and deglycosylation of flavonoids from sumac fruits using steam explosion. Food Chemistry, 126, 1934–1938. Chen, Y., Ye, R., Yin, L., & Zhang, N. (2014). Novel blasting extrusion processing improved the physicochemical properties of soluble dietary fiber from soybean residue and in vivo evaluation. Journal of Food Engineering, 120, 1–8. Coffman, A. W., & Garcia, V. V. (1977). Functional properties and amino acid contents of protein isolate from mung bean flour. Journal of Food Technology, 12, 473–484. Einhorn-Stoll, U., Kunzek, H., & Dongowski, G. (2007). Thermal analysis of chemically and mechanically modified pectins. Food Hydrocolloids, 21, 1101–1112. Elleuch, M., Bedigian, D., Roiseux, O., Besbes, S., Blecker, C., & Attia, H. (2011). Dietary fibre and fibre-rich by-products of food processing: Characterisation, technological functionality and commercial applications: A review. Food Chemistry, 124, 411–421. Fabek, H., Messerschmidt, S., Freeport, V., & Goff, H. D. (2014). The effect of in vitro digestive processes on the viscosity of dietary fibres and their influence on glucose diffusion. Food Hydrocolloids, 35, 718–726. Fang, H., Deng, J., & Zhang, T. (2011). Dilute acid pretreatment of black spruce using continuous steam explosion system. Applied Biochemistry and Biotechnology, 163, 547–557. Gunness, P., & Gidley, M. J. (2010). Mechanisms underlying the cholesterollowering properties of soluble dietary fibre polysaccharides. Food & Function, 1, 149–155. Hu, G. H., Huang, S. H., Chen, H., & Wang, F. (2010). Binding of four heavy metals to hemicelluloses from rice bran. Food Research International, 43, 203–206. Jing, Y., & Chi, Y. J. (2013). Effects of twin-screw extrusion on soluble dietary fibre and physicochemical properties of soybean residue. Food Chemistry, 138, 884–889. Johansson, L., Virkki, L., Anttila, H., Esselstrom, H., Tuomainen, P., & Sontag-Strohm, T. (2006). Hydrolysis of b-glucan. Food Chemistry, 97, 71–79. Kosmala, M., Milala, J., Kolodziejczyk, K., Markowski, J., Zbrzez´niak, M., & Renard, C. M. G. C. (2013). Dietary fiber and cell wall polysaccharides from plum (Prunus domestica L.) fruit, juice and pomace: Comparison of composition and functional properties for three plum varieties. Food Research International, 54, 1787–1794. Lan, G. H., Chen, H. X., Chen, S. H., & Tian, J. G. (2012). Chemical composition and physicochemical properties of dietary fiber from Polygonatum odoratum as affected by different processing methods. Food Research International, 49, 406–410. Li, W., Cui, S. W., & Kakuda, Y. (2006). Extraction, fractionation, structural and physical characterization of wheat beta-D-glucans. Carbohydrate Polymers, 63, 408–416. Mateos-Aparicio, I., Mateos-Peinado, C., & Ruperez, P. (2010). High hydrostatic pressure improves the functionality of dietary fiber in okara by-product from soybean. Innovative Food Science and Emerging Technologies, 11, 445–450. Miquelim, J. N., Lannes, S. C. S., & Mezzeng, R. (2010). PH influence on the stability of foams with protein-polysaccharide complexes at their interfaces. Food Hydrocolloids, 24, 398–405. Ou, S. Y., Gao, K. R., & Li, Y. (1999). An in vitro study of wheat bran binding capacity for Hg, Cd, and Pb. Journal of Agricultural and Food Chemistry, 47, 4714–4717. Peerajit, P., Chiewchan, N., & Devahastin, S. (2012). Effects of pretreatment methods on health-related functional properties of high dietary fibre powder from lime residues. Food Chemistry, 132, 1891–1898. Prajapati, V. D., Jain, G. K., & Moradiya, N. G. (2003). Pharmaceutical applications of various gums, mucilages and their modified forms. In C. J. Bailey (Ed.), Textbook of diabetes (pp. 1–73). Oxford: Blackwell Science. Raghavendar, S. N., Ramachandra Swamy, S. R., Rastogi, N. K., Raghavarao, K. S. M. S., Kumar, S., & Tharanathan, R. N. (2006). Grinding characteristics and hydration properties of coconut residue: A source of dietary fiber. Journal of Food Engineering, 72, 281–286. Ramful, D., Tarnus, E., Aruoma, O. I., Bourdon, E., & Bahorun, T. (2011). Polyphenol composition, vitamin C content and antioxidant capacity of Mauritian citrus fruit pulps. Food Research International, 44, 2088–2099. Schneeman, B. O. (1999). Fiber, inulin and oligofructose: Similarities and differences. Journal of Nutrition, 129, 1424S–1427S. Talebnia, F., Karakashev, D., & Angelidaki, I. (2010). Production of bioethanol from wheat straw: An overview on pretreatment, hydrolysis and fermentation. Bioresource Technology, 101, 4744–4753. Ubando, J., Navarro, A., & Valdivia, M. A. (2005). Mexican lime peel: Comparative study on contents of dietary fiber and associated antioxidant activity. Food Chemistry, 89, 57–61. Wang, K., Jiang, J. X., Xu, F., & Sun, R. C. (2009). Influence of steaming explosion time on the physic-chemical properties of cellulose from Lespedeza stalks (Lespedeza crytobotrya). Bioresource Technology, 100, 5288–5294. Wang, H., Srinivasan, R., Yu, F., Steele, P., Li, Q., & Mitchell, B. (2011). Effect of acid, alkali, and steam explosion pretreatments on characteristics of bio-oil produced from pinewood. Energy and Fuels, 25, 3758–3764. Wu, J. H., Wang, Z., & Xu, S. Y. (2007). Preparation and characterization of sericin powder extracted from silk industry wastewater. Food Chemistry, 103, 1255–1262.

98

L. Wang et al. / Food Chemistry 185 (2015) 90–98

Yu, Z. D., Zhang, B. L., Yu, F. Q., Xua, G. Z., & Song, A. D. (2012). A real explosion: The requirement of steam explosion pretreatment. Bioresource Technology, 121, 335–341. Zhang, M., Bai, X., & Zhang, Z. S. (2011). Extrusion process improves the functionality of soluble dietary fiber in oat bran. Journal of Cereal Science, 54, 98–103. Zhang, N., Huang, C. H., & Ou, S. (2011). In vitro binding capacities of three dietary fibers and their mixture for four toxic elements, cholesterol, and bile acid. Journal of Hazardous Materials, 186, 236–239. Zhang, M., Liang, Y., Pei, Y., Gao, W. W., & Zhang, Z. S. (2009). Effect of process on physicochemical properties of oat bran soluble dietary fiber. Journal of Food Science, 74, C628–C636.

Zhang, Y. P., Yang, R. J., Zhao, W., Hua, X., & Zhang, W. B. (2013). Application of high density steam flash-explosion in protein extraction of soybean meal. Journal of Food Engineering, 116, 430–435. Zhang, Y. P., Zhao, W., Yang, R. J., Ahmed, M. A., Hua, X., Zhang, W. B., et al. (2013). Preparation and functional properties of protein from heat-denatured soybean meal assisted by steam flash-explosion with dilute acid soaking. Journal of Food Engineering, 119, 56–64. Zhu, F. M., Du, B., Li, R. F., & Li, J. (2014). Effect of micronization technology on physicochemical and antioxidant properties of dietary fiber from buckwheat hulls. Biocatalysis and Agricultural Biotechnology, 3, 30–34.