Effect of oxidation and esterification on functional properties of mung bean (Vigna radiata (L.) Wilczek) starch
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- Bushra, M., Yun, X.X., Pan, S.Y. et al. Eur Food Res Technol (2013) 236: 119. doi:10.1007/s00217-012-1857-x
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The chemical and physicochemical properties of mung bean starch oxidized by sodium hypochlorite and esterified with succinic anhydride were studied. Mung bean starch was modified by oxidation with sodium hypochlorite and esterified with succinic anhydride. The native mung bean starch (NMBS) granules were shown to have an irregular shape, which varied from oval to round to bean shape with a smooth surface. Succinylation led to partial rupture of the granule integrity while oxidation converted the smooth surface of the native granules to a surface with fissures. Swelling capacity improved through succinylation but was reduced after oxidation. Oxidation enhanced solubility; however, succinylation showed no uniform effect throughout the temperature range studied. Both modifications increased hydrophilic tendency and demonstrated decreased gelatinization temperature compared to the NMBS. Oil absorption capacity and syneresis of native starch was enhanced after oxidation but was reduced after succinylation. Both starch types, native and modified, exhibited non-Newtonian behavior, but to a different extent. The gel formation of oxidized starch revealed the highest storage modulus followed by native starch and then succinated starch.
KeywordsMung bean starch Oxidation Succinylation
Native starches are excellent raw materials; however, they possess some undesirable properties limiting their use in industrial food applications. Often the desired functional characteristic (texture stability, solubility in cold water, thickening power after cooking) cannot be achieved by using a native starch. These limitations can be overcome by physically, chemically, or enzymatically producing modified starches with improved functional properties.
Chemical modifications of starches can be performed by oxidation, esterification, etherification, acid hydrolysis, and cross-linking. Oxidized starches are commonly used in food applications, where neutral tasting, clarity, binding properties, and low viscosity are required. Among oxidizing reagents, sodium hypochlorite is the most popular for commercial use. It is suggested that oxidized starch, especially having a low degree of substitution (DS), is an important component in basic food grade modified starch .
Succinated starch which can be produced by the esterification of native starch with succinic anhydride provides a number of desirable properties such as high viscosity, low gelatinization temperature, high thickening power, ability to swell in cold water, freeze–thaw stability, and ability to form good films.
Comprehensive research has been conducted on cassava, cereal, potato, and sweet potato starches because they are readily available and widely used in food and non-food applications [2, 3, 4, 5]. The growing demand for starches in the food industry has created interest in finding new sources for these polysaccharides, such as legume seeds and fruits . Legumes are widely grown and consumed throughout the world owing to their high protein and carbohydrate contents [7, 8].
Among legumes, variations in native starch properties have been reported for cultivars of lentil , beans , field pea , dietary mung bean [12, 13], and different cultivars of black bean, chick pea, lentil, navy bean, smooth pea, and pinto bean . The effects of different chemical modifications on the physicochemical and thermal properties of a starches isolated from jack bean, (Canavalia ensiformis) [15, 16], yam bean (Sphenostylis sternocarpa) , and sword bean, (Canavalia gladiata)  have been studied. Huang  studied the effect of acetylation on function–structure relationship of cowpea, chickpea, and yellow pea starches.
Mung bean (Vigna radiata (L.) Wilczek) or green gram is native to the northeastern India–Burma (Myanmar) region of Asia. Owing to its high protein content, edible seeds of mung bean are now also widely cultivated in Africa, South and North America, Australia, and the United States. Mung bean has a similar chemical composition to the other members of the legume family, with 24 % protein, 1 % fat, 63 % carbohydrate, and 16 % dietary fiber . It is commonly eaten as bean sprouts, and extruded mung bean starch is used in the production of vermicelli or glass noodles, and traditionally, it has been used in the preparation of soup, pancake, and pyeon (Korean jelly-type dessert).
Mung bean, on the other hand, has been primarily looked upon as a protein source rather than as a carbohydrate source, although carbohydrate is the major component of the dry seeds.
Compared to other starch types, there is no reported literature about oxidized and succinated mung bean starch and their functional properties. Limited research has been conducted on native mung bean starch (NMBS), relating mostly to its physicochemical properties and the improvement of noodle quality. The molecular structure and physical properties of mung bean starch isolated by two different techniques, namely sour liquid processing and centrifugation, have been investigated . Yield and recovery, chemical composition, microscopic analysis, and physicochemical properties of starch isolated from whole and dehulled mung bean were determined . Cowpea and mung bean starches were reported to have high intrinsic viscosity and high degree of polymerization of amylose and amylopectins . The effects of a heat-moisture treatment on the formation and physicochemical properties of resistant starch from mung bean (Phaseolus radiatus) have been studied . Physicochemical properties of sonicated mung bean starch were investigated by Chung .
The effects of oxidation and succinylation on the physicochemical rheological and thermal properties of mung bean starch have not been studied .
The aim of this study was to chemically modify mung bean starch in order to broaden its application in the food industry.
The specific objectives of this research were to obtain basic information on the physicochemical, morphological, thermal, and rheological properties of oxidized and succinated mung bean starch.
Materials and methods
Mung beans were purchased from local market in Wuhan-Hubei province, China. The seeds were screened manually to remove the damaged ones and other impurities. All chemicals used in this research were of reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.
Mung bean starch isolation
Mung beans (5 kg) were soaked in water (1:3, v/v) at 30 °C for 18 h and then manually peeled. The peeled seeds were blended using 0.05 N NaOH (1:15 w/v) in a Warring blender. The resulting slurry was screened using a 200-μm sieve and left overnight at 4 °C for starch sedimentation. The starch was mixed with water and centrifuged at 3,500 rpm for 15 min. This washing step was repeated for five times; then, the starch was dried overnight in an air drier at 40 °C, then ground to a powder, passed through 75-μm sieve and stored in a tightly closed polythene bag for further analysis.
The procedure of Forssel et al.  was followed with a slight modification to prepare the oxidized mung bean starch (OMBS). Starch solution (50 % w/v) was heated in a beaker at 35 °C using a heating mantle and mechanical stirrer. The pH was adjusted to 9.5 with 2 M NaOH. Oxidation was achieved by slowly adding 25 ml of an aqueous solution of sodium hypochlorite (NaOCl) (5 % active chlorine) into the starch slurry within 30 min while maintaining the pH at 9.5 with 1 M H2SO4 and temperature at 35 °C. The starch slurry was held at the same pH and temperature for an additional 3 h with stirring. The slurry was then neutralized to a pH of 7.0 with 1 M H2SO4, filtered, washed four times with deionized water, oven-dried at 40 °C for 48 h, ground, passed through 75-μm sieve, and then stored under dry condition for further analysis.
Succinated mung bean starch (SMBS) was prepared according to the method of Trubiano . Mung bean starch (50 g, dry weight) was added to 3.3 % w/v sodium carbonate solution with shaking; then, 3.3 % w/v of succinic anhydride was added, and shaking was continued for 14 h at 25 °C. HCl 0.2 N was used to adjust the pH to 7.0; then, sample was centrifuged for 3 min at 3,500 rpm, washed four times with distilled water, oven-dried for 48 h at 40 °C, ground, passed through 75-μm sieve, and then stored under dry condition for further analysis.
Degree of succinylation
Scanning electron micrographs of starch samples were obtained using a scanning electron microscope (SEM; JEOL, JSM-6390/LV; Japan). The samples were sprinkled on double-sided tape, fixed to an aluminum stub, and then coated with gold. The images were taken at an accelerating voltage of 10 kV .
Functional properties of native and modified starches
Swelling power and solubility
Water and oil absorption
The oil and water absorption capacities of the NMBS and its derivatives were determined following the method of Beuchat . A starch sample (1 g) was mixed with 10 ml of distilled water and/or oil (soybean oil, density 0.9 g/ml) using Variwhirl mixer for 1 min and left for 2 h at room temperature. The volume of the supernatant was recorded. The mass of oil or water absorbed was expressed as g/g starch on a dry weight basis.
Effect of freeze–thaw cycles on gel syneresis
The method of Huang  was used to assess freeze–thaw stability. Starch mixtures (5 % w/v) prepared using distilled water were equilibrated at 25 °C for 30 min; then, 2 ml starch slurry was poured into a 4 ml tube, vigorously stirred and heated in a boiling water bath for 30 min with stirring. The slurry was cooled in ice water to improve starch retrogradation, kept at −20 °C for 20 h and thawed at 30 °C for 3 h. Free water was removed from the gel by placing it on several layers of tissue paper for 10 min. Five freeze–thaw cycles were performed. Syneresis was calculated as the ratio of free water to original paste weight.
A Differential Scanning Calorimeter (DSC; NETZSCH 204F1; Germany) equipped with a thermal analysis data station was used to study the gelation properties [onset temperature (To), peak temperature (Tp), conclusion temperature (Tc], and enthalpy of gelatinization (ΔHg, J/g)) of the native and modified starches.
Starch sample was weighed into aluminum DSC pans (ratio of 1:3, starch to distilled water). The aluminum pans were hermetically sealed, reweighed and left for 24 h at room temperature to ensure equilibration, and then tested at a rate of 5 °C/min from 30 to 110 °C. An empty aluminum pan was used as a reference.
Preparation of starch pastes
Starch–water dispersion (5 % w/v) was placed in a tightly closed beaker to minimize evaporation and then heated in a boiling water bath for 30 min with continuous stirring.
Determination of mechanical spectra of starch gels
The mechanical spectra of the native and modified mung bean starch gels were determined by placing hot pastes of the samples in the plate and allowing to stand for 15 min at 30 °C with the edges covered with paraffin oil to minimize water evaporation. Measurements were taken within the linear viscoelastic range at a constant strain of 0.03 % and a frequency range from 0.1 to 12 Hz.
Excel software package (MS-Office XP) was used for data analysis. Analysis of variance was performed to calculate the significant differences among means, and LSD (p < 0.05) was used to separate means (SPSS 17) . Analyses were done in triplicate.
Results and discussion
The amylose content of NMBS (34.7 %) is in agreement with the range of values reported in other studies on NMBS [12, 21, 34]. Succinylation reduced amylose content to 30.5 % probably due to structural disintegration and losses during modification processes. Similar observation has previously been reported for succinylation of hybrid maize starch .
Amylose content increased after oxidation to 36 % due to the depolymerization of high-molecular-weight amylose molecules during hypochlorite oxidation which result in short amylose chain that could still be identified in the measurement [36, 37].
Degree of modification
Carboxyl content and DS as calculated from Eqs. 1 and 3 provided by oxidation and succinylation were found to have average values of 0.05 and 0.1, respectively. The low carboxyl content of OMBS might be due to the relatively high amylose content of NMBS. Kuakpetoon and Wang  claimed that the oxidizing agent was consumed first to depolymerize the amylose present in the amorphous lamella of the outer layers of the starch granules before the formation of carboxyl group. Similar observation was reported by Gonzalez-Soto et al.  in banana starch with 37 % amylose content.
Effect of temperature on swelling power and solubility of NMBS and its derivatives
Oxidation increased the solubility of NMBS from 5 to 10 % at 90 °C (Fig. 2b). Starch solubility was enhanced by structural disintegration which probably weakens the starch granules, thus increasing solubility after modifications.
SMBS solubility was reduced, following the increase in temperature (1 % at 90 °C). A similar trend for succinated starch heated to 80 °C was shown by Olayinka et al. .
Water and oil absorption capacity
The water-holding capacity of the starch gel could be enhanced through the reduction of starch retrogradation by the introduction of the succinyl group during the esterification reaction. This in turn prevents alignment and movement of the starch chain via reducing the interchain bonding potential. In this study, mainly because of the high amylose content of the starch and low DS associated with the modified starches, the relatively high syneresis compared to others research, ranging from nil to 37.41 % w/w, was observed [48, 49, 50, 51]. The retrogradation of the starch molecules can either be increased or decreased by oxidation through two different mechanisms . The degradation of the long-chain amylopectin or even amylose molecules in the amorphous lamellae could produce dextrin with an appropriate length for re-association which could boost starch retrogradation. In contrast, the formation of carboxyl or carbonyl groups on oxidized starch molecules would hinder the chain association that results in less affinity to retrogradation. Because of a low carboxyl content and high amylose content, the OMBS showed the highest gel syneresis among all native and modified starches.
Gelatinization characteristics of native (NMBS), oxidized (OMBS), and succinylated mung bean starches (SMBS)
Type of starch
TC − To (°C)b
58.5 ± 0.1a
66.1 ± 0.1a
71.7 ± 0.1a
13.2 ± 0.1a
17.5 ± 0.005a
57.1 ± 0.1b
66 ± 0.05a
68.0 ± 0.1b
10.9 ± 0.1b
16.9 ± 0.005b
53.7 ± 0.1c
57.3 ± 0.1b
60.9 ± 0.1c
7.2 ± 0.1c
12.3 ± 0.1c
Meanwhile, oxidation showed no significant reduction (p > 0.05). The decline in the gelatinization temperature following succinylation could be attributed to the weakness of the biopolymer structure that develops with the introduction of the bulky groups, with the simultaneous structural rearrangement that results in a weakening of intragranular and intergranular binding forces within the starch molecules; thus, less energy would be required for gelatinization.
Rheological properties of mung bean starch and its derivatives were investigated using a TA-rheometer.
All starches (native and modified) exhibited non-Newtonian, shear-thinning flow behavior, where the viscosity decreased when shear rate increased, with the tendency to yield stress.
Parameters of the power law and Casson models describing the flow curves of native mung bean starch (NMBS), oxidized mung bean starch (OMBS), and succinated mung bean starch (SMBS). Pastes (5 % concentration) at 50 °C
Consistency coefficient (k) (Pa sn)
Flow behavior index (n) (−)
Casson yield stress (koc, pa)
Casson plastic viscosity(Pa s) (ηc)
20.73 ± 0.01a
0.37 ± 5.005a
4.9 ± 0.1a
0.54 ± 0.01a
2.49 ± 0.01b
0.47 ± 0.01b
1.62 ± 0.01b
0.30 ± 0.05b
9.11 ± 0.01c
0.44 ± 0.005c
3.24 ± 0.01c
0.48 ± 0.01c
Low viscosity associated with OMBS (0.30 Pa s) compared to the native starch can be attributed to the starch–starch interactions that are already reduced by thermal and shear forces, not allowing the short amylose chain, depolymerized by oxidation, to re-associate.
Chemical modifications of mung bean starch resulted in different properties with respect to the native starch. Reduction of the gelatinization temperature was apparent in modification by succinylation; thus, modified mung bean starch can be used as thickening and stabilizing agents in food product including ice creams, fruit jellies, and baked products. Succinated mung bean starch can also have application as carbohydrate-based fat replacer because of its high binding capacity of water that can add volume, thicken, and stabilize foods. Oxidized starch showed an apparent increase in starch solubility which enhances its digestibility. All starches, native and modified, showed a non-Newtonian behavior, in which viscosity decreases with increased stress (breakdown of structural units in a food due to the hydrodynamic forces generated during shear). Thus, it can be used in several food products like salad dressing, ketchup, and some concentrated fruit juices. Owing to their different physicochemical and rheological properties, the modified starches may act to improve the textural characteristics of food products, thus improving the use of mung bean starch in food processing.
The first author is grateful to Professor Pan Si Yi, the head of the collage of Food Science and Technology, Huazhong Agricultural University, Wuhan, China, for offering the financial support and required facilities to conduct this research. Thanks were also due to Dr. Xu Xiao Yun for her support and suggestion. Acknowledgements with gratitude are due to all members of food analysis laboratory in Huazhong Agricultural University-Wuhan-Hubei, China, for their enthusiastic help during the period of the study.
Conflict of interest
The authors declare that they have no conflict of interest.