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Preparation and characterization of carboxymethyl starch from cadmium-contaminated rice
Journal Pre-proofs Preparation and Characterization of Carboxymethyl Starch from Cadmiumcontaminated Rice Yingjie Wang, Zhengfei Xie, Qian Wu, Wanying Song, Liang Liu, Yongning Wu, Zhiyong Gong PII: DOI: Reference:
S0308-8146(19)31801-1 https://doi.org/10.1016/j.foodchem.2019.125674 FOCH 125674
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
4 April 2019 7 September 2019 7 October 2019
Please cite this article as: Wang, Y., Xie, Z., Wu, Q., Song, W., Liu, L., Wu, Y., Gong, Z., Preparation and Characterization of Carboxymethyl Starch from Cadmium-contaminated Rice, Food Chemistry (2019), doi: https:// doi.org/10.1016/j.foodchem.2019.125674
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Preparation
and
Characterization
of
Carboxymethyl
Starch
from
Cadmium-contaminated Rice
Yingjie Wanga, Zhengfei Xiea, Qian Wua, Wanying Songa, Liang Liua, Yongning Wub, Zhiyong Gonga* aKey
Laboratory for Deep Processing of Major Grain and Oil of Ministry of Education, Wuhan Polytechnic University, Wuhan 430023, Hubei, China bKey Laboratory of Food Safety Risk Assessment, Ministry of Health, China National Centre for Food Safety Risk Assessment, Beijing 100021, China
ABSTRACT: Cadmium-contaminated rice has been a serious food safety issue in China. In this research, carboxymethyl rice starch (CMS) from cadmium-contaminated rice and native rice was prepared to remove the cadmium in rice. The preparation of native rice starch (NRS) and starch from cadmium-contaminated rice (Cd-CRS) was similar, and carboxymethyl starch was prepared following the same steps. A single factor experiment was performed to obtain the carboxymethyl starch prepared under the optimal conditions. Cadmium content was reduced from 0.38 mg/kg to 0.04 mg/kg after alkalization. The physical properties, including particle size, degree of crystallinity, water absorption and freeze-thaw stability, of NRS and Cd-CRS and their carboxymethyl starches were studied. The results showed that the cadmium was significantly removed after extracting starch from cadmium-contaminated rice by alkalization and carboxymethylation. Then, starch samples and carboxymethyl starch samples were characterized. All results showed no obvious difference between Cd-CMS
and
NCMS,
indicating
that
carboxymethyl
starch
from
cadmium-contaminated rice could be widely used in both food and nonfood industries. Keywords: Cadmium-contaminated rice; Rice starch; Carboxymethyl starch
1. Introduction Cadmium is widely found in the environment and subsequently in food and individuals who consumed cadmium-contaminated foods and can cause kidney disease, bone disease and even cancer (Ikeda, Nakatsuka, Watanabe, & Shimbo, 2018; Li, Yang, Xu, Li, & Wang, 2018). In recent decades, with the rapid development of industrialization in China, the soil and water in some areas have been contaminated, leading to food safety concerns. A safety issue related to rice is the occurrence of heavy metal, such as cadmium (Cd), in the rice (Järup & Åkesson, 2009). The most exposure route of cadmium exposure to nonsmoking individuals involves consuming food; thus, the consumption of rice leads to the intake of cadmium (Chen, Tang, Wang, & Zhao, 2018; Pastorelli, Angeletti, Binato, Mariani, Cibin, Morelli, et al., 2018). Rice is a staple food for approximately 3.5 billion people worldwide and contains lipids, protein and carbohydrate as nutrients. Rice is the main source for residents in south Asian countries, where severe contamination of heavy metals has been observed (Alvaro, Lanier, & Greg, 2018). Unlike other heavy metals, cadmium is more easily absorbed by the root and transported to the edible parts of crops, leading the cadmium contamination in many crops (Yang, Chen, Huang, Tang, Wang, Hu, et al., 2018). Rice is contaminated by Cd when irrigated via wastewater that contains cadmium or the plant absorbs Cd-contaminated soil (Clemens, Aarts, Thomine, & Verbruggen, 2013). Cultivated land contaminated by Cd in China includes greater than 13 million hectares, which is 65% of the total available cultivated lands (Hu, Cheng, & Tao, 2016). Therefore, taking measures to minimize cadmium levels in cadmium-contaminated rice is meaningful for the usage of wasted resources. Cadmium in rice binds to protein; however, due to special and complex structure of rice protein, the protein sites that bind to Cd2+ remain unclear (Feng, Dong, Li, Wang, Chen,
& Wang, 2019). Nevertheless, proteins and starch in rice can be separated under alkaline conditions. Cadmium bound to proteins will be eliminated after alkalization. Therefore, processing rice into starch is one of the most effective measures for contaminated rice. Starch is the most essential compound of rice and influences the processability and crystallization of rice. In general, starch is stored in plants as a primary carbohydrate and is an agricultural product for man that is used in food or nonfood industries (Amagliani, O'Regan, Kelly, & O'Mahony, 2016; Souza, Sbardelotto, Ziegler, Marczak, & Tessaro, 2016). Rice starch has smaller granules and higher acid resistance than other starches (Zhu, Zhang, Guo, Ke, Dai, Wei, et al., 2017). As an basic food component, starch mainly affects the texture, viscosity, shelf life and digestion of processed foods (Chao, Yu, Wang, Copeland, & Wang, 2017). However, native starch has limitations in the food industries given its weak shear, high retrogradation, poor process tolerance, thermal decomposition and high viscosity (Gani, Jan, Shah, Masoodi, Ahmad, Ashwar, et al., 2016). These limitations can be solved by chemical modification, physical modification and enzymatic methods. Chemical modification is a frequently used approach to enhance starches’ functions by introducing a new group into the starches’ structure (Ashwar, Gani, Shah, & Masoodi, 2017; Masina, Choonara, Kumar, Toit, Govender, Indermun, et al., 2016). Carboxymethylation is a chemical modifications whereby the carboxymethyl groups are substituted for the hydroxy groups in the starch molecule (Tatongjai & Lumdubwong, 2010). After carboxymethylation, many functions of native starch are improved, such as water solubility, paste and gel storage stability and paste clarity, thus facilitating use of the modified starch in various applications, such as pharmaceutical excipients, food additives, construction, and medical and paper industries. The objective of this research was to study methods to reduce cadmium levels in
contaminated rice by processing the rice into starch and subsequently preparing a more functional carboxymethyl starch by carboxymethylation. A method to reduce cadmium content in cadmium-contaminated rice could be widely applied, and carboxymethyl starch could be used in food, medicine and other industries. To eliminate the possible confounding effects of different varieties, all the rice samples in this study were obtained from the same variety.
2. Materials and methods 2.1. Materials and chemicals. Two rice grains were used in the present research. Cadmium-contaminated rice (Indica hybrid rice) was obtained from the Jinxia grain group (Hunan, China), and normal rice (Indica hybrid rice) was obtained from Wuhan (Hubei, China). Cadmium standard solution was purchased from Guobiao Testing & Certification Co. Ltd. (Beijing, China). Amylo-glucosidase (AMG) was provided by Megazyme (Wicklow, Ireland). The remaining chemicals were analysis grade with the exception that nitric acid was a guarantee reagent.
2.2. Preparation of starch. The extraction of Cd-contaminated rice starch was performed according to the method of Wang and Park (Park & Han, 2016) with few alterations as follows. Rice flour was weighed and soaked in hexane at room temperature for two hours. The supernatant was removed, and then the sediment was steeped in a NaOH solution (0.4%, w/w) at 40 °C for 2 h in a magnetic stirring apparatus. The mixture was centrifuged at 3000 r/min. The supernatant was discarded, and the sediment was washed with distilled water several times. After washing with ethanol (95%, v/v), the precipitate was transferred to a 1000-mL beaker and neutralized with 1 mol/L HCl to pH 7.0 and then centrifuged at
3000 r/min for 15 min. The residue was washed with distilled water thrice and dried in an oven at 40 °C for 48 h. All the waste liquor produced during the extraction was collected in the waste liquid tank contained active carbon to prevent the environment pollution.
2.3 Carboxymethylation of starch samples. Cd-CMS and NCMS was prepared by the method of Lv (Lv, Ye, Li, Ming, & Zhao, 2016) and Wang (Y. Wang, Li, & Yang, 2017) with slight modifications. Briefly, 16.2 g starches samples and 7.0 g NaOH were dissolved by ethanol (85%, v/v) in a 1000-mL three-necked flask and vortexed by magnetic stirring under 30 °C for 30 min. The entire mixture was stirred until white and homogenous. After alkalization, 9.5 g chloroacetic acid was added, and the temperature was increased to 40 °C for 1.5 h. The mixture was transferred to a beaker, neutralized with acetic acid, and washed with ethanol (90%, v/v) until the chloride ion was not detected. Purified CMS sediment was dried at 40 °C for 24 h.
2.4 Cadmium determination of starch samples. The cadmium content of all samples was determined by atomic absorption spectrometer (AA600, Perkin Elmer, USA). The samples were digested by a microwave digestion system (Multiwave PRO, Anton-Paar, Austria). After digestion, the solution was transferred to a 50-mL centrifuge tube and diluted to 20 mL. The diluted solution was homogenized by a vortex, and 20 μL of the slurry was injected into the graphite furnace for atomic absorption analysis.
2.5 Chemical composition of starch samples. The protein content of the samples was measured using an elemental analyzer (Vario EL Cube, Elementar Analysensysteme, Germany). The moisture and ash content were determined using the official AOAC method. The total starch content was determined
using a Megazyme total starch kit (Megazyme International Ireland Ltd, Co. Wicklow, Ireland).
2.6 Determination of degree of substitution. The degree of substitution (DS) of Cd-CMS and NCMS was determined based on the titration method of Liu (Liu, Ming, Li, & Zhao, 2012). Carboxymethyl samples were dissolved in 30 mL of 2 mol/L HCl and then stirred with a magnetic stirrer for 1 h. The mixture was rinsed with ethanol (80%) until the chloride ion was obliterated by filtration. The sediment dissolved by 40 mL of 0.1 M NaOH was titrated by standard 0.1 M HCl using phenolphthalein as an indicator. A blank experiment was included. The DS was calculated using the following equation: A= (C_NaOH×V_NaOH-C_HCl×V_HCl) /M×100% DS= (0.162×A) /(1-0.058×A) where A (mmol) is the amount of NaOH consumed to neutralize 1 g CMS; CNaOH (mol/L) and CHCl (mol/L) are the concentrations of NaOH and HCl, respectively; VNaOH (mL) is the NaOH volume; VHCl is the volume of HCl titrated for the samples; 0.162 g/mmol is the molar mass of the anhydroglucose unit (AGU); and 0.058 g/mmol is the molar mass of the carboxymethyl group.
2.7 Scanning electron microscopy. The microstructure of the samples was observed using a scanning electron microscope (SEM, S-3000N, Hitachi, Japan) at an accelerating voltage of 15 kV. The samples were fixed on the metal holder and then sprayed with gold. The magnification was set at 2000 X.
2.8 X-ray diffraction. The X-ray diffraction patterns of the samples were obtained using an X-ray diffractometer (Empyrean, PANalytical, Netherland) operating at 40 kV with Cu K.
The angle ranged from 3 ° to 40 ° at a scan rate of 2.0 °/min.
2.9 Fourier transform infrared spectroscopy. FTIR spectra of all the samples were detected using a Fourier infrared spectrometer (NEXUS-670, Thermo Nicolet Corp, USA). The dried samples were mixed with KBr powder and pressed into a tablet. For each sample, 225 scans were recorded with a resolution of 8 cm-1 from 4000 cm-1 to 400 cm-1.
2.10 Freeze-thaw stability. The freeze-thaw stability of Cd-CRS, NRS, NCMS and Cd-CMS was tested at a concentration of 5% (w/v) in ultrapure water. NRS and Cd-CRS paste were prepared from aqueous starch in boiling water for 30 min with stirring and subsequent cooling to room temperature (25 °C). Carboxymethyl starches were dissolved in ultrapure water and homogeneously mixed for 12 h at room temperature (25 °C). Then, 30 grams of paste were added to the preweighed 50-mL centrifuge tubes and frozen at -20 °C in a freezer for 24 h. All tubes were removed from the freezer and thawed at room temperature (25 °C). All tubes from each thawing circle were centrifuged at 4500 g for 20 min. The supernatant was discarded, and the deposit was weighed and recorded. The dehydration rate (%) was calculated as the ratio of the weight of discarded water to the total weight of the paste and multiplied by 100. Seven freeze-thaw cycles were performed.
2.11 Swelling power and solubility. The swelling power and solubility of NRS, Cd-CRS, NCMS and Cd-CMS were determined by adding 1.0 g of sample into preweighed 50-mL centrifuge tubes containing 50 mL of distilled water. The solution was mixed sufficiently for 30 seconds and heated at 50, 60, 70 and 80 °C for 30 min, separately. Then, the tube was centrifuged at 10000 g for 15 min, and the supernatant was removed to a glass culture
dish and dried to a constant weight at 120 °C. The dried culture dish and sediment were weighed and recorded to calculate the swelling power and solubility, respectively. Swelling power was calculated as the ratio of wet sediment weight to the dry sample weight, and solubility was represented as the percentage of the dry supernatant weight and the dry sample weight.
2.12 Statistical analysis. All experiments were performed at least in triplicate. The experimental data were analyzed using analysis of variance (ANOVA) and expressed as the mean ± standard deviation (SD). The significance level (P<0.05) was assessed using the Duncan test.
3. Results and discussion 3.1 Chemical composition and cadmium content. Composition of rice and starch is shown in Table 1. Isolation starch samples had very low protein and ash content, whereas cadmium-contaminated rice and native rice contained 7.76% and 7.20% protein, respectively. Carboxymethyl starches had lower lipid, ash and protein levels. After alkalization, the cadmium content in rice starch was dramatically decreased compared with rice flour. The cadmium in rice was mostly bound to protein to which metal ions could easily bind (Yu, Wei, Yang, Ding, Wang, Zhao, et al., 2018). Most proteins were eliminated during the alkalization of rice. Thus, cadmium levels in rice decreased.
3.2 Degree of carboxymethylation. The DS values of Cd-CMS and NCMS were 0.54 and 0.56, respectively, which was higher than that noted in other reports (Liu, Ming, Li, & Zhao, 2012; Tatongjai & Lumdubwong, 2010). Carboxymethyl samples were prepared under the same
conditions, but the DS was slightly different, which likely due to differences in rice breeds. Under lower chloroacetic acid conditions, the DS of carboxymethyl starches exhibited no evident changes. As the chloroacetic acid concentration increases, the DS of Cd-CMS was reduced from 0.54 to 0.22, and the DS of NCMS decreased from 0.56 to 0.23. This finding could be responsible for the increasing proportion of chloroacetic acid due to the usage of NaOH (Liu, Ming, Li, & Zhao, 2012). The more chloroacetic acid added, the less the NaOH could react with starch molecules. Moreover, the addition of more chloroacetic acid can lead to the formation of the side products. The DS is reduced with excessive chloroacetic acid according to the reports by Manal (El-Sheikh, 2010) and Sangseethong (Sangseethong, Chatakanonda, Wansuksri, & Sriroth, 2015).
3.3 Granule morphology. Scanning electron microscopy (SEM) was used to observe the microcosmic structure of the samples. The micrographs of NRS, Cd-RS, NCMS and Cd-CMS were shown in Figure 1. SEM revealed that carboxymethylation changed the structure of starch granules compared with the starch samples. NRS and Cd-RS granules were smooth, irregular polygons with sizes ranging from 3 to 6 µm (Zhang, Zhao, & Xiong, 2010). Although the granule structure significant changed after carboxymethylation, the integrity of starch granules was maintained. After carboxymethylation, there were not any evident changes to the shape of granules. SEM images showed that both Cd-CMS and CMS granules were located in close proximity to form a large group, and the starch surface became coarse and porous. The change of the surface of Cd-CMS and NCMS potentially contributed to the deletion of the crystalline structure and cleavage of chemical bonds upon heat treatment in the strong alkaline environment. These findings demonstrated that the reaction occurred not only on the surface but also within the
molecule. Similar observations were reported in previous research. In addition, the molecular weight of carboxymethyl starch decreased, which contributed to molecular degradation upon alkali treatment (Liu, Ming, Li, & Zhao, 2012; L.-F. Wang, Pan, Hu, Miao, & Xu, 2010). As observed, the introduction of carboxymethyl groups in starch markedly altered the structure of carboxymethyl starch. The structural changes were attributed to the strong alkaline environment during the etherification period. Due to the heavy use of chloroacetic acid and sodium hydroxide, the cracks appeared on the granule surface, which weakened the shear properties and stability of the products. To avoid this situation, the reaction system requires more moderate conditions.
3.4 XRD data analysis. X-ray diffraction patterns of NRS, Cd-CRS, NCMS and Cd-CMS are shown in Figure 2. NRS and Cd-CRS samples displayed the typical A-type diffraction pattern with strong peaks at 15.2 °, 17.0 °, 18.1 ° and 23.0 ° (Zhu, et al., 2017). After carboxymethylation, the main peaks of starch samples became smooth and weak, suggesting loss of starch kernel crystallization. Hydrogen bonds were responsible for forming and maintaining the crystal structure. The cracks in the starch granule might also be responsible for the loss of crystallinity. After carboxymethylation, the carboxymethyl groups in both NCMS and Cd-CMS on the AGU decreased the crystallinity as shown in Figure 2. The strong alkaline environment and heat conditions were responsible for the decomposition of starch molecules during the reaction (Liu, Ming, Li, & Zhao, 2012). Consistent with SEM images, it was rational that strong alkaline and high temperature condition destroyed the structure of starch granule during the period of modification.
3.5 FT-IR spectroscopy. The infrared spectra of NRS, Cd-CRS, NCMS, and Cd-CMS are presented in Figure 3. FT-IR spectroscopy results of Cd-CRS and NRS were similar. The characteristic peak at 3420 cm-1 was attributed to the stretching vibration of the hydroxyl on the starch molecules. The bond at 2920 cm-1 showed the -C-H stretching. The bond from 1030 to 1150 cm-1, which is typical for a starch molecular, was preserved in the rice starch and carboxymethyl starch. In CMS samples, the intense bonds ranging from 1440 cm-1 to 1620 cm-1 were assigned to carbonyl functional groups, indicating the introduction of carboxylic bond on the starch molecule (Čížová, Koschella, Heinze, Ebringerová, & Sroková, 2007; Lv, Ye, Li, Ming, & Zhao, 2016). The appearance of these new bonds explained that carboxymethylation occurred on the starch granules.
3.6 Freeze-thaw stability. The freeze-thaw stability of NRS, Cd-RS, NCMS and Cd-CMS is shown in Figure 4. The syneresis values of NCMS and Cd-CMS were generally unchanged in the first two cycles and slightly increased in the remaining five cycles. Similar results were reported in previous studies (Bhattacharyya, Singhal, & Kulkarni, 1995; Liu, Ming, Li, & Zhao, 2012). Compared with CMS samples with higher DS values, CMS samples with lower DS possessed improved freeze-thaw stability, which supported the findings of CMS samples in our study given their lower freeze-thaw stability and high DS. The freeze-thaw stability of the starch samples increased the first four cycles and remained stable in the last three cycles. Due to the numerous hydrophilic groups -COO- in CMS samples, the syneresis values of starch samples were considerably increased compared with CMS samples (Gong, Zhang, Cheng, & Zhou, 2015). Rice starch pastes showed a lack of freeze-thaw stability as the freeze-thaw cycles increased. When the syneresis values reached to 50%, the freeze-thaw stability maintained stable. In contrast, the
freeze-thaw stability of the CMS samples was considerable improved compared with starch samples. This finding could be attributed to the carboxymethyl groups introduced to the starch chains, which blocked hydrogen bonding during storage. Thus, the carboxymethyl group introduced to the starch chain interrupted the ordered structure and disrupted the derangement of gelatinized starch. Therefore, the Cd-CMS could be a better stabilizer under low temperature conditions. Syneresis is a phenomenon of water separation from the gel structure due to reorganization of the leaching starch molecules. However, differences in syneresis values were noted between NRS and Cd-CRS. As shown in Figure 4, all the starch samples and carboxymethyl starch samples exhibited increased syneresis after consecutive freeze thaw cycles. Phase formation occurred between starch molecules and ice crystals during the freezing of starch gels, which weakened the gel network of starch and reduced the water holding ability (Charoenrein & Preechathammawong, 2012; Zhao, Yu, Yang, Wang, Wang, & Bai, 2018). It is also possible that the starch particles were less decomposed, resulting in reduced leaching of starch in the matrix and weaker gel networks (Hsieh, Liu, Whaley, & Shi, 2019; Kumar, Brennan, Zheng, & Brennan, 2018).
3.7 Swelling power and solubility. As shown in Table 2, the swelling power of Cd-CMS was similar to the NRS, whereas the solubility of NRS was slightly higher than Cd-CRS samples. As the temperature increases, the solubility of NRS and Cd-CRS gradually increases. The same results were observed in the swelling power test. The CMS with DS 0.2 is completely soluble in cold water. Cd-CMS and NCMS samples in this study were mainly dissolved in water and formed a stable and clarified paste. NCMS and Cd-CMS possessed such favorable solubility because the hydroxyl groups on the starch molecule were replaced
by carboxyl groups during carboxymethylation (El-Sheikh, 2010). As the temperature increases, the solubility of starch samples increased. The swelling power of starch was mostly related to amylopectin, and the chain length was an effective factor on the swelling power. The longer the chain, the weaker the swelling power (Demirkesen-Bicak, Tacer-Caba, & Nilufer-Erdil, 2018). There was a significant difference in the solubility between the rice starch and carboxyethyl starch. The strengthen solubility of carboxymethyl starch reflected the variety of chemical bonds in starch molecular. The solubility of CMS samples increased with the cracks in hydrogen bonds and the introduction of the carboxyl group. This finding is the similar to the findings in previous studies (El-Sheikh, 2010; Liu, Ming, Li, & Zhao, 2012).
4. Conclusions NRS and Cd-CRS showed no difference in physicochemical properties and structure. After alkalization, cadmium was significantly reduced in contaminated rice, which proved that the rice starch prepared from cadmium-contaminated rice can be used as native starch. The modification changed the structure and physicochemical properties of the Cd-CRS. Cadmium levels in cadmium-contaminated rice were 0.38 mg/kg, whereas the cadmium content was reduced sharply to 0.02 mg/kg after alkalization. This finding demonstrated that most cadmium-binding proteins in rice were eliminated, which directly resulted in the remarkable reduction in cadmium levels. The FT-IR and XRD patterns demonstrated that carboxymethylation occurred. All the CMS samples exhibited improved freeze-thaw stability and solubility. The swelling power and the solubility of the starch samples increased as the temperature increased. All of the results revealed similar physicochemical properties between Cd-CRS and NRS. In addition, Cd-CMS could be widely used in various fields.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFC1600500) and National Science and Technology Project for Grain Industry of China (201513006). The experiments conducted in this research comply with the current laws of the People’s Republic of China.
Conflicts of interest There are no conflicts of interest.
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Figure 1. Microstructure of starch granules from (A) NRS, (B) Cd-CRS, (C)NCMS, and (D) Cd-CMS.
Figure 2. XRD spectra of NRS, Cd-CRS, NCMS and Cd-CMS.
Figure 3. FTIR spectra of starch and carboxymethyl starch.
Figure 4. Freeze-thaw stability of NRS, Cd-RS, NCMS and Cd-CMS.
Table 1. Chemical composition of rice, starch and carboxymethyl starch samples
Samples
Protein (%)
Moisture (%)
Ash (%)
Cadmium (mg/kg)
NR
7.20±0.16b
10.59±0.03a
0.65±0.03a
0.021±0.002bc
Cd-R
7.76±0.03a
10.06±0.05b
0.61±0.06a
0.381±0.023a
NRS
0.99±0.03c
7.25±0.03d
0.17±0.02c
0.003±0.001c
Cd-RS
0.64±0.12d
7.22±0.19d
0.16±0.01c
0.037±0.006b
NCMS
0.40±0.03e
8.06±0.02c
0.25±0.04b
0.005±0.003c
Cd-CMS
0.35±0.06e
8.02±0.03c
0.23±0.01b
0.015±0.001c
Data was displayed as mean values ± standard deviations. Values with different letters in the same column differ significantly (P<0.05)
Table 2. Water solubility and swelling power of starch samples Samples
Solubility (%)
Swelling power (g/g)
50℃
60℃
70℃
80℃
50℃
60℃
70℃
80℃
NRS
2.77±0.07
3.11±0.26
4.16±0.17
9.01±0.65
5.88±0.11
6.22±0.03
8.29±0.18
10.71±0.27
Cd-CRS
2.47±0.06
2.78±0.01
3.19±0.11
8.64±0.24
5.78±0.19
6.06±0.10
8.12±0.08
10.89±0.42
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this research.
Highlights · ·
·
Carboxymethyl starch from cadmium-contaminated rice were prepared and characterized. Cadmium content in cadmium-contaminated rice removed under alkaline condition. The properties and structure of starch and carboxymethyl starch were evaluated.
S0308-8146(19)31801-1 https://doi.org/10.1016/j.foodchem.2019.125674 FOCH 125674
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
4 April 2019 7 September 2019 7 October 2019
Please cite this article as: Wang, Y., Xie, Z., Wu, Q., Song, W., Liu, L., Wu, Y., Gong, Z., Preparation and Characterization of Carboxymethyl Starch from Cadmium-contaminated Rice, Food Chemistry (2019), doi: https:// doi.org/10.1016/j.foodchem.2019.125674
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Preparation
and
Characterization
of
Carboxymethyl
Starch
from
Cadmium-contaminated Rice
Yingjie Wanga, Zhengfei Xiea, Qian Wua, Wanying Songa, Liang Liua, Yongning Wub, Zhiyong Gonga* aKey
Laboratory for Deep Processing of Major Grain and Oil of Ministry of Education, Wuhan Polytechnic University, Wuhan 430023, Hubei, China bKey Laboratory of Food Safety Risk Assessment, Ministry of Health, China National Centre for Food Safety Risk Assessment, Beijing 100021, China
ABSTRACT: Cadmium-contaminated rice has been a serious food safety issue in China. In this research, carboxymethyl rice starch (CMS) from cadmium-contaminated rice and native rice was prepared to remove the cadmium in rice. The preparation of native rice starch (NRS) and starch from cadmium-contaminated rice (Cd-CRS) was similar, and carboxymethyl starch was prepared following the same steps. A single factor experiment was performed to obtain the carboxymethyl starch prepared under the optimal conditions. Cadmium content was reduced from 0.38 mg/kg to 0.04 mg/kg after alkalization. The physical properties, including particle size, degree of crystallinity, water absorption and freeze-thaw stability, of NRS and Cd-CRS and their carboxymethyl starches were studied. The results showed that the cadmium was significantly removed after extracting starch from cadmium-contaminated rice by alkalization and carboxymethylation. Then, starch samples and carboxymethyl starch samples were characterized. All results showed no obvious difference between Cd-CMS
and
NCMS,
indicating
that
carboxymethyl
starch
from
cadmium-contaminated rice could be widely used in both food and nonfood industries. Keywords: Cadmium-contaminated rice; Rice starch; Carboxymethyl starch
1. Introduction Cadmium is widely found in the environment and subsequently in food and individuals who consumed cadmium-contaminated foods and can cause kidney disease, bone disease and even cancer (Ikeda, Nakatsuka, Watanabe, & Shimbo, 2018; Li, Yang, Xu, Li, & Wang, 2018). In recent decades, with the rapid development of industrialization in China, the soil and water in some areas have been contaminated, leading to food safety concerns. A safety issue related to rice is the occurrence of heavy metal, such as cadmium (Cd), in the rice (Järup & Åkesson, 2009). The most exposure route of cadmium exposure to nonsmoking individuals involves consuming food; thus, the consumption of rice leads to the intake of cadmium (Chen, Tang, Wang, & Zhao, 2018; Pastorelli, Angeletti, Binato, Mariani, Cibin, Morelli, et al., 2018). Rice is a staple food for approximately 3.5 billion people worldwide and contains lipids, protein and carbohydrate as nutrients. Rice is the main source for residents in south Asian countries, where severe contamination of heavy metals has been observed (Alvaro, Lanier, & Greg, 2018). Unlike other heavy metals, cadmium is more easily absorbed by the root and transported to the edible parts of crops, leading the cadmium contamination in many crops (Yang, Chen, Huang, Tang, Wang, Hu, et al., 2018). Rice is contaminated by Cd when irrigated via wastewater that contains cadmium or the plant absorbs Cd-contaminated soil (Clemens, Aarts, Thomine, & Verbruggen, 2013). Cultivated land contaminated by Cd in China includes greater than 13 million hectares, which is 65% of the total available cultivated lands (Hu, Cheng, & Tao, 2016). Therefore, taking measures to minimize cadmium levels in cadmium-contaminated rice is meaningful for the usage of wasted resources. Cadmium in rice binds to protein; however, due to special and complex structure of rice protein, the protein sites that bind to Cd2+ remain unclear (Feng, Dong, Li, Wang, Chen,
& Wang, 2019). Nevertheless, proteins and starch in rice can be separated under alkaline conditions. Cadmium bound to proteins will be eliminated after alkalization. Therefore, processing rice into starch is one of the most effective measures for contaminated rice. Starch is the most essential compound of rice and influences the processability and crystallization of rice. In general, starch is stored in plants as a primary carbohydrate and is an agricultural product for man that is used in food or nonfood industries (Amagliani, O'Regan, Kelly, & O'Mahony, 2016; Souza, Sbardelotto, Ziegler, Marczak, & Tessaro, 2016). Rice starch has smaller granules and higher acid resistance than other starches (Zhu, Zhang, Guo, Ke, Dai, Wei, et al., 2017). As an basic food component, starch mainly affects the texture, viscosity, shelf life and digestion of processed foods (Chao, Yu, Wang, Copeland, & Wang, 2017). However, native starch has limitations in the food industries given its weak shear, high retrogradation, poor process tolerance, thermal decomposition and high viscosity (Gani, Jan, Shah, Masoodi, Ahmad, Ashwar, et al., 2016). These limitations can be solved by chemical modification, physical modification and enzymatic methods. Chemical modification is a frequently used approach to enhance starches’ functions by introducing a new group into the starches’ structure (Ashwar, Gani, Shah, & Masoodi, 2017; Masina, Choonara, Kumar, Toit, Govender, Indermun, et al., 2016). Carboxymethylation is a chemical modifications whereby the carboxymethyl groups are substituted for the hydroxy groups in the starch molecule (Tatongjai & Lumdubwong, 2010). After carboxymethylation, many functions of native starch are improved, such as water solubility, paste and gel storage stability and paste clarity, thus facilitating use of the modified starch in various applications, such as pharmaceutical excipients, food additives, construction, and medical and paper industries. The objective of this research was to study methods to reduce cadmium levels in
contaminated rice by processing the rice into starch and subsequently preparing a more functional carboxymethyl starch by carboxymethylation. A method to reduce cadmium content in cadmium-contaminated rice could be widely applied, and carboxymethyl starch could be used in food, medicine and other industries. To eliminate the possible confounding effects of different varieties, all the rice samples in this study were obtained from the same variety.
2. Materials and methods 2.1. Materials and chemicals. Two rice grains were used in the present research. Cadmium-contaminated rice (Indica hybrid rice) was obtained from the Jinxia grain group (Hunan, China), and normal rice (Indica hybrid rice) was obtained from Wuhan (Hubei, China). Cadmium standard solution was purchased from Guobiao Testing & Certification Co. Ltd. (Beijing, China). Amylo-glucosidase (AMG) was provided by Megazyme (Wicklow, Ireland). The remaining chemicals were analysis grade with the exception that nitric acid was a guarantee reagent.
2.2. Preparation of starch. The extraction of Cd-contaminated rice starch was performed according to the method of Wang and Park (Park & Han, 2016) with few alterations as follows. Rice flour was weighed and soaked in hexane at room temperature for two hours. The supernatant was removed, and then the sediment was steeped in a NaOH solution (0.4%, w/w) at 40 °C for 2 h in a magnetic stirring apparatus. The mixture was centrifuged at 3000 r/min. The supernatant was discarded, and the sediment was washed with distilled water several times. After washing with ethanol (95%, v/v), the precipitate was transferred to a 1000-mL beaker and neutralized with 1 mol/L HCl to pH 7.0 and then centrifuged at
3000 r/min for 15 min. The residue was washed with distilled water thrice and dried in an oven at 40 °C for 48 h. All the waste liquor produced during the extraction was collected in the waste liquid tank contained active carbon to prevent the environment pollution.
2.3 Carboxymethylation of starch samples. Cd-CMS and NCMS was prepared by the method of Lv (Lv, Ye, Li, Ming, & Zhao, 2016) and Wang (Y. Wang, Li, & Yang, 2017) with slight modifications. Briefly, 16.2 g starches samples and 7.0 g NaOH were dissolved by ethanol (85%, v/v) in a 1000-mL three-necked flask and vortexed by magnetic stirring under 30 °C for 30 min. The entire mixture was stirred until white and homogenous. After alkalization, 9.5 g chloroacetic acid was added, and the temperature was increased to 40 °C for 1.5 h. The mixture was transferred to a beaker, neutralized with acetic acid, and washed with ethanol (90%, v/v) until the chloride ion was not detected. Purified CMS sediment was dried at 40 °C for 24 h.
2.4 Cadmium determination of starch samples. The cadmium content of all samples was determined by atomic absorption spectrometer (AA600, Perkin Elmer, USA). The samples were digested by a microwave digestion system (Multiwave PRO, Anton-Paar, Austria). After digestion, the solution was transferred to a 50-mL centrifuge tube and diluted to 20 mL. The diluted solution was homogenized by a vortex, and 20 μL of the slurry was injected into the graphite furnace for atomic absorption analysis.
2.5 Chemical composition of starch samples. The protein content of the samples was measured using an elemental analyzer (Vario EL Cube, Elementar Analysensysteme, Germany). The moisture and ash content were determined using the official AOAC method. The total starch content was determined
using a Megazyme total starch kit (Megazyme International Ireland Ltd, Co. Wicklow, Ireland).
2.6 Determination of degree of substitution. The degree of substitution (DS) of Cd-CMS and NCMS was determined based on the titration method of Liu (Liu, Ming, Li, & Zhao, 2012). Carboxymethyl samples were dissolved in 30 mL of 2 mol/L HCl and then stirred with a magnetic stirrer for 1 h. The mixture was rinsed with ethanol (80%) until the chloride ion was obliterated by filtration. The sediment dissolved by 40 mL of 0.1 M NaOH was titrated by standard 0.1 M HCl using phenolphthalein as an indicator. A blank experiment was included. The DS was calculated using the following equation: A= (C_NaOH×V_NaOH-C_HCl×V_HCl) /M×100% DS= (0.162×A) /(1-0.058×A) where A (mmol) is the amount of NaOH consumed to neutralize 1 g CMS; CNaOH (mol/L) and CHCl (mol/L) are the concentrations of NaOH and HCl, respectively; VNaOH (mL) is the NaOH volume; VHCl is the volume of HCl titrated for the samples; 0.162 g/mmol is the molar mass of the anhydroglucose unit (AGU); and 0.058 g/mmol is the molar mass of the carboxymethyl group.
2.7 Scanning electron microscopy. The microstructure of the samples was observed using a scanning electron microscope (SEM, S-3000N, Hitachi, Japan) at an accelerating voltage of 15 kV. The samples were fixed on the metal holder and then sprayed with gold. The magnification was set at 2000 X.
2.8 X-ray diffraction. The X-ray diffraction patterns of the samples were obtained using an X-ray diffractometer (Empyrean, PANalytical, Netherland) operating at 40 kV with Cu K.
The angle ranged from 3 ° to 40 ° at a scan rate of 2.0 °/min.
2.9 Fourier transform infrared spectroscopy. FTIR spectra of all the samples were detected using a Fourier infrared spectrometer (NEXUS-670, Thermo Nicolet Corp, USA). The dried samples were mixed with KBr powder and pressed into a tablet. For each sample, 225 scans were recorded with a resolution of 8 cm-1 from 4000 cm-1 to 400 cm-1.
2.10 Freeze-thaw stability. The freeze-thaw stability of Cd-CRS, NRS, NCMS and Cd-CMS was tested at a concentration of 5% (w/v) in ultrapure water. NRS and Cd-CRS paste were prepared from aqueous starch in boiling water for 30 min with stirring and subsequent cooling to room temperature (25 °C). Carboxymethyl starches were dissolved in ultrapure water and homogeneously mixed for 12 h at room temperature (25 °C). Then, 30 grams of paste were added to the preweighed 50-mL centrifuge tubes and frozen at -20 °C in a freezer for 24 h. All tubes were removed from the freezer and thawed at room temperature (25 °C). All tubes from each thawing circle were centrifuged at 4500 g for 20 min. The supernatant was discarded, and the deposit was weighed and recorded. The dehydration rate (%) was calculated as the ratio of the weight of discarded water to the total weight of the paste and multiplied by 100. Seven freeze-thaw cycles were performed.
2.11 Swelling power and solubility. The swelling power and solubility of NRS, Cd-CRS, NCMS and Cd-CMS were determined by adding 1.0 g of sample into preweighed 50-mL centrifuge tubes containing 50 mL of distilled water. The solution was mixed sufficiently for 30 seconds and heated at 50, 60, 70 and 80 °C for 30 min, separately. Then, the tube was centrifuged at 10000 g for 15 min, and the supernatant was removed to a glass culture
dish and dried to a constant weight at 120 °C. The dried culture dish and sediment were weighed and recorded to calculate the swelling power and solubility, respectively. Swelling power was calculated as the ratio of wet sediment weight to the dry sample weight, and solubility was represented as the percentage of the dry supernatant weight and the dry sample weight.
2.12 Statistical analysis. All experiments were performed at least in triplicate. The experimental data were analyzed using analysis of variance (ANOVA) and expressed as the mean ± standard deviation (SD). The significance level (P<0.05) was assessed using the Duncan test.
3. Results and discussion 3.1 Chemical composition and cadmium content. Composition of rice and starch is shown in Table 1. Isolation starch samples had very low protein and ash content, whereas cadmium-contaminated rice and native rice contained 7.76% and 7.20% protein, respectively. Carboxymethyl starches had lower lipid, ash and protein levels. After alkalization, the cadmium content in rice starch was dramatically decreased compared with rice flour. The cadmium in rice was mostly bound to protein to which metal ions could easily bind (Yu, Wei, Yang, Ding, Wang, Zhao, et al., 2018). Most proteins were eliminated during the alkalization of rice. Thus, cadmium levels in rice decreased.
3.2 Degree of carboxymethylation. The DS values of Cd-CMS and NCMS were 0.54 and 0.56, respectively, which was higher than that noted in other reports (Liu, Ming, Li, & Zhao, 2012; Tatongjai & Lumdubwong, 2010). Carboxymethyl samples were prepared under the same
conditions, but the DS was slightly different, which likely due to differences in rice breeds. Under lower chloroacetic acid conditions, the DS of carboxymethyl starches exhibited no evident changes. As the chloroacetic acid concentration increases, the DS of Cd-CMS was reduced from 0.54 to 0.22, and the DS of NCMS decreased from 0.56 to 0.23. This finding could be responsible for the increasing proportion of chloroacetic acid due to the usage of NaOH (Liu, Ming, Li, & Zhao, 2012). The more chloroacetic acid added, the less the NaOH could react with starch molecules. Moreover, the addition of more chloroacetic acid can lead to the formation of the side products. The DS is reduced with excessive chloroacetic acid according to the reports by Manal (El-Sheikh, 2010) and Sangseethong (Sangseethong, Chatakanonda, Wansuksri, & Sriroth, 2015).
3.3 Granule morphology. Scanning electron microscopy (SEM) was used to observe the microcosmic structure of the samples. The micrographs of NRS, Cd-RS, NCMS and Cd-CMS were shown in Figure 1. SEM revealed that carboxymethylation changed the structure of starch granules compared with the starch samples. NRS and Cd-RS granules were smooth, irregular polygons with sizes ranging from 3 to 6 µm (Zhang, Zhao, & Xiong, 2010). Although the granule structure significant changed after carboxymethylation, the integrity of starch granules was maintained. After carboxymethylation, there were not any evident changes to the shape of granules. SEM images showed that both Cd-CMS and CMS granules were located in close proximity to form a large group, and the starch surface became coarse and porous. The change of the surface of Cd-CMS and NCMS potentially contributed to the deletion of the crystalline structure and cleavage of chemical bonds upon heat treatment in the strong alkaline environment. These findings demonstrated that the reaction occurred not only on the surface but also within the
molecule. Similar observations were reported in previous research. In addition, the molecular weight of carboxymethyl starch decreased, which contributed to molecular degradation upon alkali treatment (Liu, Ming, Li, & Zhao, 2012; L.-F. Wang, Pan, Hu, Miao, & Xu, 2010). As observed, the introduction of carboxymethyl groups in starch markedly altered the structure of carboxymethyl starch. The structural changes were attributed to the strong alkaline environment during the etherification period. Due to the heavy use of chloroacetic acid and sodium hydroxide, the cracks appeared on the granule surface, which weakened the shear properties and stability of the products. To avoid this situation, the reaction system requires more moderate conditions.
3.4 XRD data analysis. X-ray diffraction patterns of NRS, Cd-CRS, NCMS and Cd-CMS are shown in Figure 2. NRS and Cd-CRS samples displayed the typical A-type diffraction pattern with strong peaks at 15.2 °, 17.0 °, 18.1 ° and 23.0 ° (Zhu, et al., 2017). After carboxymethylation, the main peaks of starch samples became smooth and weak, suggesting loss of starch kernel crystallization. Hydrogen bonds were responsible for forming and maintaining the crystal structure. The cracks in the starch granule might also be responsible for the loss of crystallinity. After carboxymethylation, the carboxymethyl groups in both NCMS and Cd-CMS on the AGU decreased the crystallinity as shown in Figure 2. The strong alkaline environment and heat conditions were responsible for the decomposition of starch molecules during the reaction (Liu, Ming, Li, & Zhao, 2012). Consistent with SEM images, it was rational that strong alkaline and high temperature condition destroyed the structure of starch granule during the period of modification.
3.5 FT-IR spectroscopy. The infrared spectra of NRS, Cd-CRS, NCMS, and Cd-CMS are presented in Figure 3. FT-IR spectroscopy results of Cd-CRS and NRS were similar. The characteristic peak at 3420 cm-1 was attributed to the stretching vibration of the hydroxyl on the starch molecules. The bond at 2920 cm-1 showed the -C-H stretching. The bond from 1030 to 1150 cm-1, which is typical for a starch molecular, was preserved in the rice starch and carboxymethyl starch. In CMS samples, the intense bonds ranging from 1440 cm-1 to 1620 cm-1 were assigned to carbonyl functional groups, indicating the introduction of carboxylic bond on the starch molecule (Čížová, Koschella, Heinze, Ebringerová, & Sroková, 2007; Lv, Ye, Li, Ming, & Zhao, 2016). The appearance of these new bonds explained that carboxymethylation occurred on the starch granules.
3.6 Freeze-thaw stability. The freeze-thaw stability of NRS, Cd-RS, NCMS and Cd-CMS is shown in Figure 4. The syneresis values of NCMS and Cd-CMS were generally unchanged in the first two cycles and slightly increased in the remaining five cycles. Similar results were reported in previous studies (Bhattacharyya, Singhal, & Kulkarni, 1995; Liu, Ming, Li, & Zhao, 2012). Compared with CMS samples with higher DS values, CMS samples with lower DS possessed improved freeze-thaw stability, which supported the findings of CMS samples in our study given their lower freeze-thaw stability and high DS. The freeze-thaw stability of the starch samples increased the first four cycles and remained stable in the last three cycles. Due to the numerous hydrophilic groups -COO- in CMS samples, the syneresis values of starch samples were considerably increased compared with CMS samples (Gong, Zhang, Cheng, & Zhou, 2015). Rice starch pastes showed a lack of freeze-thaw stability as the freeze-thaw cycles increased. When the syneresis values reached to 50%, the freeze-thaw stability maintained stable. In contrast, the
freeze-thaw stability of the CMS samples was considerable improved compared with starch samples. This finding could be attributed to the carboxymethyl groups introduced to the starch chains, which blocked hydrogen bonding during storage. Thus, the carboxymethyl group introduced to the starch chain interrupted the ordered structure and disrupted the derangement of gelatinized starch. Therefore, the Cd-CMS could be a better stabilizer under low temperature conditions. Syneresis is a phenomenon of water separation from the gel structure due to reorganization of the leaching starch molecules. However, differences in syneresis values were noted between NRS and Cd-CRS. As shown in Figure 4, all the starch samples and carboxymethyl starch samples exhibited increased syneresis after consecutive freeze thaw cycles. Phase formation occurred between starch molecules and ice crystals during the freezing of starch gels, which weakened the gel network of starch and reduced the water holding ability (Charoenrein & Preechathammawong, 2012; Zhao, Yu, Yang, Wang, Wang, & Bai, 2018). It is also possible that the starch particles were less decomposed, resulting in reduced leaching of starch in the matrix and weaker gel networks (Hsieh, Liu, Whaley, & Shi, 2019; Kumar, Brennan, Zheng, & Brennan, 2018).
3.7 Swelling power and solubility. As shown in Table 2, the swelling power of Cd-CMS was similar to the NRS, whereas the solubility of NRS was slightly higher than Cd-CRS samples. As the temperature increases, the solubility of NRS and Cd-CRS gradually increases. The same results were observed in the swelling power test. The CMS with DS 0.2 is completely soluble in cold water. Cd-CMS and NCMS samples in this study were mainly dissolved in water and formed a stable and clarified paste. NCMS and Cd-CMS possessed such favorable solubility because the hydroxyl groups on the starch molecule were replaced
by carboxyl groups during carboxymethylation (El-Sheikh, 2010). As the temperature increases, the solubility of starch samples increased. The swelling power of starch was mostly related to amylopectin, and the chain length was an effective factor on the swelling power. The longer the chain, the weaker the swelling power (Demirkesen-Bicak, Tacer-Caba, & Nilufer-Erdil, 2018). There was a significant difference in the solubility between the rice starch and carboxyethyl starch. The strengthen solubility of carboxymethyl starch reflected the variety of chemical bonds in starch molecular. The solubility of CMS samples increased with the cracks in hydrogen bonds and the introduction of the carboxyl group. This finding is the similar to the findings in previous studies (El-Sheikh, 2010; Liu, Ming, Li, & Zhao, 2012).
4. Conclusions NRS and Cd-CRS showed no difference in physicochemical properties and structure. After alkalization, cadmium was significantly reduced in contaminated rice, which proved that the rice starch prepared from cadmium-contaminated rice can be used as native starch. The modification changed the structure and physicochemical properties of the Cd-CRS. Cadmium levels in cadmium-contaminated rice were 0.38 mg/kg, whereas the cadmium content was reduced sharply to 0.02 mg/kg after alkalization. This finding demonstrated that most cadmium-binding proteins in rice were eliminated, which directly resulted in the remarkable reduction in cadmium levels. The FT-IR and XRD patterns demonstrated that carboxymethylation occurred. All the CMS samples exhibited improved freeze-thaw stability and solubility. The swelling power and the solubility of the starch samples increased as the temperature increased. All of the results revealed similar physicochemical properties between Cd-CRS and NRS. In addition, Cd-CMS could be widely used in various fields.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2017YFC1600500) and National Science and Technology Project for Grain Industry of China (201513006). The experiments conducted in this research comply with the current laws of the People’s Republic of China.
Conflicts of interest There are no conflicts of interest.
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Figure 1. Microstructure of starch granules from (A) NRS, (B) Cd-CRS, (C)NCMS, and (D) Cd-CMS.
Figure 2. XRD spectra of NRS, Cd-CRS, NCMS and Cd-CMS.
Figure 3. FTIR spectra of starch and carboxymethyl starch.
Figure 4. Freeze-thaw stability of NRS, Cd-RS, NCMS and Cd-CMS.
Table 1. Chemical composition of rice, starch and carboxymethyl starch samples
Samples
Protein (%)
Moisture (%)
Ash (%)
Cadmium (mg/kg)
NR
7.20±0.16b
10.59±0.03a
0.65±0.03a
0.021±0.002bc
Cd-R
7.76±0.03a
10.06±0.05b
0.61±0.06a
0.381±0.023a
NRS
0.99±0.03c
7.25±0.03d
0.17±0.02c
0.003±0.001c
Cd-RS
0.64±0.12d
7.22±0.19d
0.16±0.01c
0.037±0.006b
NCMS
0.40±0.03e
8.06±0.02c
0.25±0.04b
0.005±0.003c
Cd-CMS
0.35±0.06e
8.02±0.03c
0.23±0.01b
0.015±0.001c
Data was displayed as mean values ± standard deviations. Values with different letters in the same column differ significantly (P<0.05)
Table 2. Water solubility and swelling power of starch samples Samples
Solubility (%)
Swelling power (g/g)
50℃
60℃
70℃
80℃
50℃
60℃
70℃
80℃
NRS
2.77±0.07
3.11±0.26
4.16±0.17
9.01±0.65
5.88±0.11
6.22±0.03
8.29±0.18
10.71±0.27
Cd-CRS
2.47±0.06
2.78±0.01
3.19±0.11
8.64±0.24
5.78±0.19
6.06±0.10
8.12±0.08
10.89±0.42
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this research.
Highlights · ·
·
Carboxymethyl starch from cadmium-contaminated rice were prepared and characterized. Cadmium content in cadmium-contaminated rice removed under alkaline condition. The properties and structure of starch and carboxymethyl starch were evaluated.