论文部分内容阅读
Abstract ［Objectives］ This study was conducted to investigate the feasibility of using modified peanut dietary fiber as a functional food ingredient.
［Methods］ Using peanut shells as a test material, the process parameters of soluble dietary fiber (SDF) modified by extrusion and expansion were studied, and the functional and structural characteristics of SDF before and after modification were discussed.
［Results］ The optimum conditions were as follows: screw speed 200 rpm, temperature 130 ℃ and moisture content 20 %, and the SDF extraction yield was 22.3%. The modified SDF showed BCmax values of (378.5±5.3), (278.3±3.2) and (167.2±2.5) μmol/g and BCmin of (30.4±1.3), (63.4±3.7) and (71.3±4.2) μmol/L, for Pb, As and Cu, respectively, indicating that the adsorption to the three heavy metals was enhanced. The modified SDF had a porous network like honeycomb and swelled structure.
［Conclusions］ Therefore, it is feasible to modify SDF by extrusion and expansion.
Key words Peanut shell; Soluble dietary fiber; Modification; Heavy metal adsorption; Structure property
Received: March 5, 2020Accepted: May 7, 2020
Supported by The High-level Talents Program of Hebei Province (A20190-1128); Tangshan Science and Technology Planning Project (19150204E).
Xishang XING (1965-), male, P. R. China, senior agronomist, devoted to research about functional ingredients and food additives.
*Corresponding author. E-mail: wanglei3730217@163.com; 108532895@qq.com.
In recent years, the important role of dietary fiber in food nutrition and clinical medicine has received more and more attention from people, and it is called the "seventh largest nutritional element" of human beings［1-3］. Although dietary fiber has no nutritional value, its physicochemical properties make it have unique physiological functions and nutritional health care effects. According to its solubility, it can be roughly divided into soluble dietary fiber (SDF) and insoluble dietary fiber (IDF)［4-8］. Soluble dietary fiber is more capable of exerting its metabolic function in physiological functions than insoluble dietary fiber. It has special effects in promoting the growth of intestinal probiotic bacteria and preventing diabetes, obesity, coronary heart disease, arteriosclerosis, and hyperlipidemia［9-15］.
China is the worlds largest producer, consumer and exporter of peanuts, and its planting area and annual output are among the highest in the world. The peanut industry produces a lot of by-products every year, such as peanut meal, peanut residue, peanut shells, peanut stems and leaves, etc. , which are potential dietary fiber resources. If these resources are not fully utilized, it will cause serious waste of resources and pollution to the environment. Therefore, processing these by-products of the peanut industry into natural dietary fiber can not only increase the added value of the peanut industry, but also bring huge social and economic benefits［16］. The principle of extrusion-expansion technology is that the material is suddenly reduced from high temperature and high pressure to normal temperature and pressure in an instant. During the process, the moisture inside the raw material vaporizes, the gas expands suddenly, and the blasting occurs, producing a blasting effect. Extrusion pretreatment can make the material structure sponge-like, the volume increase, some structural structures such as fiber bundles destroyed, and the contents exposed, which is conducive to the dissolution of the target substance, improves the effect of the enzyme on the substrate, and helps to improve the effect of the enzyme hydrolysis［17-18］.
In this study, we performed a detailed comparative analysis on the heavy metal adsorption capacity and structural characteristics of modified peanut dietary fiber, and proposed that modified peanut dietary fiber as a functional food ingredient has broad development and application prospects in China.
Materials and Methods
Materials and Instruments
Peanut shells, provided by Tangshan Runze Cereals, Oils and Foods Co., Ltd.
Sodium nitrite, Pb (NO3)2, CuSO4 and NaAsO2, analytical grade, purchased from Beijing Chemical Reagent Company; electronic analytical balance (AR1140), Ohaus International Trading (Shanghai) Co., Ltd.; electronic balance (ARC120), Ohaus International Trading (Shanghai) Co., Ltd.; constant temperature water bath (HWS-26), Jiangsu Taicang Experimental Equipment Factory; water bath constant temperature oscillator (DSHZ-300), Jiangsu Taicang Experimental Equipment Factory; electric constant temperature drying oven (DHG-9140A), Shanghai Yiheng Technology Co., Ltd.; atomic fluorescence photometer (AFS-230E), Beijing Haiguang Company; plasma emission spectrometer (Optima-5300DV), American PE Company; scanning electron microscope (UV-1800), Shimadzu Corporation, Japan.
Experimental methods
Process parameter optimization
Peanut shells were subjected to starch removal with α-amylase tolerant to high temperature and protein removal with neutral protease. The material was extruded by a twin screw extruder under the conditions of screw speed (160, 180, 200, 220, 240 rpm), temperature (110, 120, 130, 140, 150 ℃), and material moisture (10%, 15%, 20%, 25%, 30%), and extracted with water. The supernatant was precipitated with ethanol and centrifuged, and the precipitate was dried to obtain SDF. Evaluation of heavy metal adsorption capacity
Determination of the maximum binding capacity (BCmax ): 250 ml Erlenmeyer flasks were added with 1.0 g of dietary fiber sample and 10 mmol/L heavy metal solutions (Pb(NO3)2, CuSO4, NaAsO2), respectively. In order to simulate the stomach and intestine environment in vitro, the pH value was adjusted to 2.0 and 7.0, respectively, followed by shaking at 120 r/min and 37 ℃ for 3 h. After adsorption, in order to precipitate SDF, 8 ml of absolute ethanol was added to a 2 ml of sample collection solution, followed by centrifugation at 4 000 r/min for 10 min. Each supernatant was measured for the concentration of residual heavy metal ions by atomic absorption.
Determination of the minmum binding capacity (BCmin ): 250 ml Erlenmeyer flasks were added with 2.5 g of dietary fiber sample and 500 μmol/L heavy metal solutions (Pb(NO3)2, CuSO4, NaAsO2), respectively. Other conditions were the same as those in the determination of BCmax .
Electron microscopic observation of soluble dietary fiber
After pulverizing the SDF obtained by extrusion-expansion modification, it was sieved through a 0.5 mm sieve and gold plated by ion sputtering. The prepared sample was analyzed and observed through a scanning electron microscope to obtain the corresponding scanning electron microscope graph.
Data processing
Experimental design and data processing adopted Design-Expert 7.0.
Results and Analysis
Process conditions of peanut shell SDF modified by extrusion-expansion technology
Extrusion-expansion technology integrates various unit operations such as transportation, mixing, heating and pressurization. The materials are subjected to high temperature and high shear action in an extruder barrel, and a part of the macromolecular polymer can be directly or indirectly converted into soluble fiber in a short time. As shown in Fig. 1, the extraction rate of SDF showed a trend of increasing first and then decreasing with the increase of screw speed and extrusion temperature; and with the increase of the moisture content in the material, the trend was first to increase and then to be gentle. With the conditions of the screw speed of 200 rpm, the temperature at 130 ℃ and the material moisture content of 20%, the SDF extraction rate was the highest (22.3%).
Analysis of heavy metal adsorption capacity of soluble dietary fiber
Lead, arsenic and copper are chemical substances that endanger human health. Due to environmental pollution, these heavy metal elements can be detected in many foods. They are not easy to be discharged, and there is a potential crisis of enrichment in organisms. They can lead to poisoning and cancer after the amounts increasing to certain extents. The binding of dietary fiber to heavy metal ions mainly depends on chemical adsorption, while physical adsorption also exists. Chemical adsorption mainly relies on the binding of carboxyl group from uronic acid and phenolic acid of lignin in the fiber to heavy metal ions. Therefore, it is greatly affected by pH. As the pH value increases, the dissociation of these groups increases, and they can be ionically bonded to positively charged heavy metal cations. On the contrary, the dissociation of carboxyl groups is reduced, which may reduce the adsorption effect, that is, the acidic environment is not conducive to the absorption of heavy metal ions by dietary fibers. Physical adsorption is the result of van der Waals force, and is affected by temperature, and the reaction rate is generally very fast［19］. Table 1 shows the effects of extrusion-expansion treatment on the maximum binding capacity and minimum binding concentration of peanut shell SDF to heavy metals. It can be seen from Table 1 that in the adsorption process of the same heavy metal ion, the adsorption effect was better at pH 7.0 than at pH 2.0, indicating that the small intestine environment is more suitable for the absorption of heavy metal ions by dietary fiber. Thompson et al. ［20］ found that most dietary fibers have the strongest binding capacity for heavy metals at pH 6.8, and will desorb heavy metals at pH 0.65.
In this study, under the in-vitro simulated intestinal environment (pH 7.0), Control-SDF had a strong adsorption effect on Pb, As and Cu, with BCmax values of (239.5 ± 2.5), (198.5 ± 2.9) and (100.4 ± 3.1) μmol/g and BCmin of (90.5 ± 1.6), (126.4 ± 4.1) and (155.3 ± 4.2) μmol/L for the three heavy metals, respectively. Among the heavy metals, the adsorption of Pb was the strongest, followed by As, and the adsorption of Cu was weaker. After extrusion modification on SDF, the adsorption of heavy metal ions was enhanced, and the modified SDF showed BCmax values of (378.5 ± 5.3), (278.3 ± 3.2) and (167.2 ± 2.5) μmol/g and BCmin values of (30.4 ± 1.3), (63.4 ± 3.7) and (71.3 ± 4.2) μmol/L for the three heavy metals, respectively. Under the gastric environment (pH 2.0), SDF had a weaker adsorption effect on Cu, but the adsorption effects on Pb and As were significantly higher than that on Cu. Under the condition of pH 2.0, extrusion and expansion enhanced the adsorption of heavy metals. After peanut shell SDF is extruded and expanded, the sample particles may be highly fragmented, the specific surface area increases, and the tissue is loosened, so the SDF fully contacts heavy metal ions and acquires enhanced adsorption of heavy metal ions［21］.
Electron microscope observation of soluble dietary fiber
Scanning electron microscope (SEM) is a new type of electron optical instrument that has developed rapidly in the past three decades. It is characterized by its three-dimensional and realistic images, which can show the shape and size of particles. It has strong adaptability to samples and is suitable for intuitive research on dietary fiber particles, and can take photos of representative particle morphology.
Fig. 2 shows the scanning electron microscopy results of two dietary fibers with the scanning multiple of 3 000 times. It can be seen from the figure that the particles of the two dietary fibers were tiny, and had a slightly wrinkled surface and a clear loose flaky structure, but the peanut shell SDF particles after extrusion and expansion modification were finer and the surface had an obvious honeycomb structure, evenly distributed. The reason might be that after the peanut shell SDF was modified, the macromolecular material was degraded, and the molecular chain was cut, which led to relatively low molecular weight and reduced degree of polymerization, so the particles became smaller, and the microstructure and molecular size changed. The microstructure of SDF is closely related to its functional characteristics, so it is speculated that the modified SDF may have good adsorption capacity of cholesterol, sodium cholate and nitrite［22］. Conclusions
The optimal process conditions for the peanut shell SDF modified by extrusion and expansion were as follows: screw speed 200 rpm , extrusion temperature 130 ℃ and material moisture 20%, and the SDF extraction rate reached 22.3%.
The results showed that the adsorption of the same heavy metal ion was better at pH 7.0 than at pH 2.0. After modification by extrusion and expansion, under both the conditions of pH 7.0 and pH 2.0, the adsorption capacity of peanut shell SDF to heavy metal ions was improved.
Observed by scanning electron microscope, the particles of Control-SDF and E-SDF were tiny, and had a slightly wrinkled surface and clear loose flaky structure with pores for adsorbing other substances, but the peanut shell SDF modified by extrusion and expansion had denser particles, smaller pores, and an obvious honeycomb surface structure with a uniform distribution.
Xishuang XING et al. Study on Functional and Structure Properties of Soluble Dietary Fiber Modified by Extrusion-expansion Technology from Peanut Shells
References
［1］ YANG F, DUAN YF. Preparation, properties and application of the dietary fiber from Pyracantha fortuneana ［J］. Food Science and Technology, 2007(5): 79-81. (in Chinese)
［2］ ZHANG JF. Study on Preparation and properties of corn enzyme dietary fiber by double-enzyme method［J］. Food Research and Development, 2007, 28(4): 97-101. (in Chinese)
［3］ WANG L, YUAN F, XIANG J, et al. Functional properties and rheological behavior of soluble dietary fiber from ponkan residue［J］. Journal of Chinese Institute of Food Science and Technology, 15(3): 24-31. (in Chinese)
［4］ ZHANG AX, LU C, MA M. Dietary fiber and human health［J］. Food and Nutrition in China, 2005(3): 53-54. (in Chinese)
［5］ CHAWLA R, PATIL GR. Soluble dietary fiber［J］. Comprehensive Reviews in Food Science and Food Safery, 2010, 9(2): 178-196. (in Chinese)
［6］ SHI XM, LEI J, LIANG AH, et al. Comparison of antioxidant properties among three dietary fibers［J］. Food Science and Technology, 2013, 38(1): 71-75. (in Chinese)
［7］ LI Y, XIONG MZ, YIN CL, et al. Ultra-high pressure modification of sweet potato residue insoluble dietary fiber［J］. Transactions of the Chinese Society of Agricultural Engineering, 2012, 28(19): 270-278. (in Chinese)
［8］ BENITEZ V, MOLLA ESPERANZA, MATIN-CABREJAS MA, et al. Effect of sterilization on dietary fiber and physicochemical properties of onion by-products［J］. Food Chemistry, 2011, 27(2): 501-507. ［9］ WANG L, YUAN F, GAO YX, et al. Optimization of high-pressure homogenization extraction of soluble dietary fiber from ponkan residue using response surface methodology and its antioxidant activities［J］. Journal of Chinese Institute of Food Science and TEchnology, 2015, 15(5): 83-89. (in Chinese)
［10］ SUN P, LIU KZ, ZHAO F. Study on extraction of Lycium barbarum polysaccharide and its residue treatment［J］. The Food Industry, 2013, 4(1): 48-50. (in Chinese)
［11］ CAO ZX, LIU F, XIONG L, et al. Study on the Extraction of Bran Dietary Fiber by Double Enzyme Method［J］. Journal of Chinese Institute of Food Science and Technology, 2010, 0(2): 138-141. (in Chinese)
［12］ YAN H, WANG ZJ, XIONG J, et al. Development of the dietary fiber functional food and studies on its toxicological and physiologic properties ［J］. Food and Chemical Toxicology, 2012, 50(9): 3367-3374.
［13］ WANG L, XU HG, YUAN F, et al. Preparation and physicochemical properties of soluble dietary fiber from orange peel assisted by steam explosion and dilute acid soaking ［J］. Food Chemistry, 2015(185): 90-98.
［14］ CHEN L, GUO XH, LI FH, et al. Research progress on the function and application of dietary fiber from edible fungi［J］. Food Science, 2012, 33(11): 303-307. (in Chinese)
［15］ ZHANG XQ, REN LL, HE XL, et al. Study on technology for extraction of dietary fiber from red jujube by enzymatic hydrolysis［J］. Journal of Anhui Agricultural Sciences, 2012, 40(1): 113-115. (in Chinese)
［16］ YU LN, YANG QL, YU SL, et al. Research development and application of the peanut dietary fiber［J］. Science and Technology of Food Industry, 2010(3): 376-380. (in Chinese)
［17］ HAN YB, LIU GL, SHI XY, et al. effects of extrusion on physiochemical property of germinated brown rice［J］. Journal of The Chinese Cereals and Oils Association, 2010, 25(12): 1-5. (in Chinese)
［18］ NING GZ, ZHANG B, WEI YM, et al. Study on process parameters of extrusion and expansion of oat flour［J］. Journal of the Chinese Cereals and Oils Associatio, 2010, 25(12): 28-31. (in Chinese)
［19］ ZHANG N, HUANG CH, OU S. In vitro binding capacities of three dietary fibers and their mixture for four toxic elements, cholesterol, and bile acid［J］. Journal of Hazardous Materials, 2011(186): 236-239.
［20］ CARA C, RUIZ E, BALLESTEROS M, et al. Production of fuel ethanol from steam-explosion pretreated olive tree pruning ［J］. Fuel, 2008(87): 692-700.
［21］ LAN GS, CHEN HX, CHEN SH, et al. Chemical composition and physicochemical properties of dietary fiber from Polygonatum odoratum as affected by different processing methods ［J］. Food Research International, 2012(49): 406-410.
［22］ AL-SHERAJI SH, ISMAIL A, MANAP MY, et al. Functional properties and characterization of dietary fiber from Mangifera pajang Kort. Fruit pulp［J］. Journal of Agricultural and Food Chemistry, 2011(59): 3980-3985.
Editor: Yingzhi GUANGProofreader: Xinxiu ZHU
［Methods］ Using peanut shells as a test material, the process parameters of soluble dietary fiber (SDF) modified by extrusion and expansion were studied, and the functional and structural characteristics of SDF before and after modification were discussed.
［Results］ The optimum conditions were as follows: screw speed 200 rpm, temperature 130 ℃ and moisture content 20 %, and the SDF extraction yield was 22.3%. The modified SDF showed BCmax values of (378.5±5.3), (278.3±3.2) and (167.2±2.5) μmol/g and BCmin of (30.4±1.3), (63.4±3.7) and (71.3±4.2) μmol/L, for Pb, As and Cu, respectively, indicating that the adsorption to the three heavy metals was enhanced. The modified SDF had a porous network like honeycomb and swelled structure.
［Conclusions］ Therefore, it is feasible to modify SDF by extrusion and expansion.
Key words Peanut shell; Soluble dietary fiber; Modification; Heavy metal adsorption; Structure property
Received: March 5, 2020Accepted: May 7, 2020
Supported by The High-level Talents Program of Hebei Province (A20190-1128); Tangshan Science and Technology Planning Project (19150204E).
Xishang XING (1965-), male, P. R. China, senior agronomist, devoted to research about functional ingredients and food additives.
*Corresponding author. E-mail: wanglei3730217@163.com; 108532895@qq.com.
In recent years, the important role of dietary fiber in food nutrition and clinical medicine has received more and more attention from people, and it is called the "seventh largest nutritional element" of human beings［1-3］. Although dietary fiber has no nutritional value, its physicochemical properties make it have unique physiological functions and nutritional health care effects. According to its solubility, it can be roughly divided into soluble dietary fiber (SDF) and insoluble dietary fiber (IDF)［4-8］. Soluble dietary fiber is more capable of exerting its metabolic function in physiological functions than insoluble dietary fiber. It has special effects in promoting the growth of intestinal probiotic bacteria and preventing diabetes, obesity, coronary heart disease, arteriosclerosis, and hyperlipidemia［9-15］.
China is the worlds largest producer, consumer and exporter of peanuts, and its planting area and annual output are among the highest in the world. The peanut industry produces a lot of by-products every year, such as peanut meal, peanut residue, peanut shells, peanut stems and leaves, etc. , which are potential dietary fiber resources. If these resources are not fully utilized, it will cause serious waste of resources and pollution to the environment. Therefore, processing these by-products of the peanut industry into natural dietary fiber can not only increase the added value of the peanut industry, but also bring huge social and economic benefits［16］. The principle of extrusion-expansion technology is that the material is suddenly reduced from high temperature and high pressure to normal temperature and pressure in an instant. During the process, the moisture inside the raw material vaporizes, the gas expands suddenly, and the blasting occurs, producing a blasting effect. Extrusion pretreatment can make the material structure sponge-like, the volume increase, some structural structures such as fiber bundles destroyed, and the contents exposed, which is conducive to the dissolution of the target substance, improves the effect of the enzyme on the substrate, and helps to improve the effect of the enzyme hydrolysis［17-18］.
In this study, we performed a detailed comparative analysis on the heavy metal adsorption capacity and structural characteristics of modified peanut dietary fiber, and proposed that modified peanut dietary fiber as a functional food ingredient has broad development and application prospects in China.
Materials and Methods
Materials and Instruments
Peanut shells, provided by Tangshan Runze Cereals, Oils and Foods Co., Ltd.
Sodium nitrite, Pb (NO3)2, CuSO4 and NaAsO2, analytical grade, purchased from Beijing Chemical Reagent Company; electronic analytical balance (AR1140), Ohaus International Trading (Shanghai) Co., Ltd.; electronic balance (ARC120), Ohaus International Trading (Shanghai) Co., Ltd.; constant temperature water bath (HWS-26), Jiangsu Taicang Experimental Equipment Factory; water bath constant temperature oscillator (DSHZ-300), Jiangsu Taicang Experimental Equipment Factory; electric constant temperature drying oven (DHG-9140A), Shanghai Yiheng Technology Co., Ltd.; atomic fluorescence photometer (AFS-230E), Beijing Haiguang Company; plasma emission spectrometer (Optima-5300DV), American PE Company; scanning electron microscope (UV-1800), Shimadzu Corporation, Japan.
Experimental methods
Process parameter optimization
Peanut shells were subjected to starch removal with α-amylase tolerant to high temperature and protein removal with neutral protease. The material was extruded by a twin screw extruder under the conditions of screw speed (160, 180, 200, 220, 240 rpm), temperature (110, 120, 130, 140, 150 ℃), and material moisture (10%, 15%, 20%, 25%, 30%), and extracted with water. The supernatant was precipitated with ethanol and centrifuged, and the precipitate was dried to obtain SDF. Evaluation of heavy metal adsorption capacity
Determination of the maximum binding capacity (BCmax ): 250 ml Erlenmeyer flasks were added with 1.0 g of dietary fiber sample and 10 mmol/L heavy metal solutions (Pb(NO3)2, CuSO4, NaAsO2), respectively. In order to simulate the stomach and intestine environment in vitro, the pH value was adjusted to 2.0 and 7.0, respectively, followed by shaking at 120 r/min and 37 ℃ for 3 h. After adsorption, in order to precipitate SDF, 8 ml of absolute ethanol was added to a 2 ml of sample collection solution, followed by centrifugation at 4 000 r/min for 10 min. Each supernatant was measured for the concentration of residual heavy metal ions by atomic absorption.
Determination of the minmum binding capacity (BCmin ): 250 ml Erlenmeyer flasks were added with 2.5 g of dietary fiber sample and 500 μmol/L heavy metal solutions (Pb(NO3)2, CuSO4, NaAsO2), respectively. Other conditions were the same as those in the determination of BCmax .
Electron microscopic observation of soluble dietary fiber
After pulverizing the SDF obtained by extrusion-expansion modification, it was sieved through a 0.5 mm sieve and gold plated by ion sputtering. The prepared sample was analyzed and observed through a scanning electron microscope to obtain the corresponding scanning electron microscope graph.
Data processing
Experimental design and data processing adopted Design-Expert 7.0.
Results and Analysis
Process conditions of peanut shell SDF modified by extrusion-expansion technology
Extrusion-expansion technology integrates various unit operations such as transportation, mixing, heating and pressurization. The materials are subjected to high temperature and high shear action in an extruder barrel, and a part of the macromolecular polymer can be directly or indirectly converted into soluble fiber in a short time. As shown in Fig. 1, the extraction rate of SDF showed a trend of increasing first and then decreasing with the increase of screw speed and extrusion temperature; and with the increase of the moisture content in the material, the trend was first to increase and then to be gentle. With the conditions of the screw speed of 200 rpm, the temperature at 130 ℃ and the material moisture content of 20%, the SDF extraction rate was the highest (22.3%).
Analysis of heavy metal adsorption capacity of soluble dietary fiber
Lead, arsenic and copper are chemical substances that endanger human health. Due to environmental pollution, these heavy metal elements can be detected in many foods. They are not easy to be discharged, and there is a potential crisis of enrichment in organisms. They can lead to poisoning and cancer after the amounts increasing to certain extents. The binding of dietary fiber to heavy metal ions mainly depends on chemical adsorption, while physical adsorption also exists. Chemical adsorption mainly relies on the binding of carboxyl group from uronic acid and phenolic acid of lignin in the fiber to heavy metal ions. Therefore, it is greatly affected by pH. As the pH value increases, the dissociation of these groups increases, and they can be ionically bonded to positively charged heavy metal cations. On the contrary, the dissociation of carboxyl groups is reduced, which may reduce the adsorption effect, that is, the acidic environment is not conducive to the absorption of heavy metal ions by dietary fibers. Physical adsorption is the result of van der Waals force, and is affected by temperature, and the reaction rate is generally very fast［19］. Table 1 shows the effects of extrusion-expansion treatment on the maximum binding capacity and minimum binding concentration of peanut shell SDF to heavy metals. It can be seen from Table 1 that in the adsorption process of the same heavy metal ion, the adsorption effect was better at pH 7.0 than at pH 2.0, indicating that the small intestine environment is more suitable for the absorption of heavy metal ions by dietary fiber. Thompson et al. ［20］ found that most dietary fibers have the strongest binding capacity for heavy metals at pH 6.8, and will desorb heavy metals at pH 0.65.
In this study, under the in-vitro simulated intestinal environment (pH 7.0), Control-SDF had a strong adsorption effect on Pb, As and Cu, with BCmax values of (239.5 ± 2.5), (198.5 ± 2.9) and (100.4 ± 3.1) μmol/g and BCmin of (90.5 ± 1.6), (126.4 ± 4.1) and (155.3 ± 4.2) μmol/L for the three heavy metals, respectively. Among the heavy metals, the adsorption of Pb was the strongest, followed by As, and the adsorption of Cu was weaker. After extrusion modification on SDF, the adsorption of heavy metal ions was enhanced, and the modified SDF showed BCmax values of (378.5 ± 5.3), (278.3 ± 3.2) and (167.2 ± 2.5) μmol/g and BCmin values of (30.4 ± 1.3), (63.4 ± 3.7) and (71.3 ± 4.2) μmol/L for the three heavy metals, respectively. Under the gastric environment (pH 2.0), SDF had a weaker adsorption effect on Cu, but the adsorption effects on Pb and As were significantly higher than that on Cu. Under the condition of pH 2.0, extrusion and expansion enhanced the adsorption of heavy metals. After peanut shell SDF is extruded and expanded, the sample particles may be highly fragmented, the specific surface area increases, and the tissue is loosened, so the SDF fully contacts heavy metal ions and acquires enhanced adsorption of heavy metal ions［21］.
Electron microscope observation of soluble dietary fiber
Scanning electron microscope (SEM) is a new type of electron optical instrument that has developed rapidly in the past three decades. It is characterized by its three-dimensional and realistic images, which can show the shape and size of particles. It has strong adaptability to samples and is suitable for intuitive research on dietary fiber particles, and can take photos of representative particle morphology.
Fig. 2 shows the scanning electron microscopy results of two dietary fibers with the scanning multiple of 3 000 times. It can be seen from the figure that the particles of the two dietary fibers were tiny, and had a slightly wrinkled surface and a clear loose flaky structure, but the peanut shell SDF particles after extrusion and expansion modification were finer and the surface had an obvious honeycomb structure, evenly distributed. The reason might be that after the peanut shell SDF was modified, the macromolecular material was degraded, and the molecular chain was cut, which led to relatively low molecular weight and reduced degree of polymerization, so the particles became smaller, and the microstructure and molecular size changed. The microstructure of SDF is closely related to its functional characteristics, so it is speculated that the modified SDF may have good adsorption capacity of cholesterol, sodium cholate and nitrite［22］. Conclusions
The optimal process conditions for the peanut shell SDF modified by extrusion and expansion were as follows: screw speed 200 rpm , extrusion temperature 130 ℃ and material moisture 20%, and the SDF extraction rate reached 22.3%.
The results showed that the adsorption of the same heavy metal ion was better at pH 7.0 than at pH 2.0. After modification by extrusion and expansion, under both the conditions of pH 7.0 and pH 2.0, the adsorption capacity of peanut shell SDF to heavy metal ions was improved.
Observed by scanning electron microscope, the particles of Control-SDF and E-SDF were tiny, and had a slightly wrinkled surface and clear loose flaky structure with pores for adsorbing other substances, but the peanut shell SDF modified by extrusion and expansion had denser particles, smaller pores, and an obvious honeycomb surface structure with a uniform distribution.
Xishuang XING et al. Study on Functional and Structure Properties of Soluble Dietary Fiber Modified by Extrusion-expansion Technology from Peanut Shells
References
［1］ YANG F, DUAN YF. Preparation, properties and application of the dietary fiber from Pyracantha fortuneana ［J］. Food Science and Technology, 2007(5): 79-81. (in Chinese)
［2］ ZHANG JF. Study on Preparation and properties of corn enzyme dietary fiber by double-enzyme method［J］. Food Research and Development, 2007, 28(4): 97-101. (in Chinese)
［3］ WANG L, YUAN F, XIANG J, et al. Functional properties and rheological behavior of soluble dietary fiber from ponkan residue［J］. Journal of Chinese Institute of Food Science and Technology, 15(3): 24-31. (in Chinese)
［4］ ZHANG AX, LU C, MA M. Dietary fiber and human health［J］. Food and Nutrition in China, 2005(3): 53-54. (in Chinese)
［5］ CHAWLA R, PATIL GR. Soluble dietary fiber［J］. Comprehensive Reviews in Food Science and Food Safery, 2010, 9(2): 178-196. (in Chinese)
［6］ SHI XM, LEI J, LIANG AH, et al. Comparison of antioxidant properties among three dietary fibers［J］. Food Science and Technology, 2013, 38(1): 71-75. (in Chinese)
［7］ LI Y, XIONG MZ, YIN CL, et al. Ultra-high pressure modification of sweet potato residue insoluble dietary fiber［J］. Transactions of the Chinese Society of Agricultural Engineering, 2012, 28(19): 270-278. (in Chinese)
［8］ BENITEZ V, MOLLA ESPERANZA, MATIN-CABREJAS MA, et al. Effect of sterilization on dietary fiber and physicochemical properties of onion by-products［J］. Food Chemistry, 2011, 27(2): 501-507. ［9］ WANG L, YUAN F, GAO YX, et al. Optimization of high-pressure homogenization extraction of soluble dietary fiber from ponkan residue using response surface methodology and its antioxidant activities［J］. Journal of Chinese Institute of Food Science and TEchnology, 2015, 15(5): 83-89. (in Chinese)
［10］ SUN P, LIU KZ, ZHAO F. Study on extraction of Lycium barbarum polysaccharide and its residue treatment［J］. The Food Industry, 2013, 4(1): 48-50. (in Chinese)
［11］ CAO ZX, LIU F, XIONG L, et al. Study on the Extraction of Bran Dietary Fiber by Double Enzyme Method［J］. Journal of Chinese Institute of Food Science and Technology, 2010, 0(2): 138-141. (in Chinese)
［12］ YAN H, WANG ZJ, XIONG J, et al. Development of the dietary fiber functional food and studies on its toxicological and physiologic properties ［J］. Food and Chemical Toxicology, 2012, 50(9): 3367-3374.
［13］ WANG L, XU HG, YUAN F, et al. Preparation and physicochemical properties of soluble dietary fiber from orange peel assisted by steam explosion and dilute acid soaking ［J］. Food Chemistry, 2015(185): 90-98.
［14］ CHEN L, GUO XH, LI FH, et al. Research progress on the function and application of dietary fiber from edible fungi［J］. Food Science, 2012, 33(11): 303-307. (in Chinese)
［15］ ZHANG XQ, REN LL, HE XL, et al. Study on technology for extraction of dietary fiber from red jujube by enzymatic hydrolysis［J］. Journal of Anhui Agricultural Sciences, 2012, 40(1): 113-115. (in Chinese)
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Editor: Yingzhi GUANGProofreader: Xinxiu ZHU