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Meng, F., Tian, S., Wang, Y., Lu, J., Liu, Z., and Niu, Y. (2024). “Ultrasound-assisted extraction and physicochemical properties of starch from Cyperus esculentus tubers,” BioResources 19(3), 4264-4277.


The purpose of this study was to use ultrasound-based extraction to prepare starch from the tubers of Cyperus esculentus. Ultrasonic treatment of Cyperus esculentus powder with a medium of alkaline-treated water can effectively improve the starch extraction efficiency. Box-Behnken design was used to optimize the extraction process, and the results showed that the optimal parameters were ultrasound time of 30 minutes, pH value of 9.0, ultrasound temperature of 40 °C, and solid-liquid ratio of 10:1. The extraction percentage under these conditions was 90.1%. The physicochemical properties of C. esculentus starch were compared with those of cassava, potato, and corn starch. The particle size of C. esculentus starch was approximately 2 to 15 μm. The gelatinization temperature was 70.5 °C, and the peak viscosity was similar to cassava but with better thermal stability. Like other tuber starches, C. esculentus starch had higher swelling power and solubility at 85 °C.

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Ultrasound-assisted Extraction and Physicochemical Properties of Starch from Cyperus esculentus Tubers

Fanhao Meng,a,b Shuangqi Tian,a,b,c,* Ya’nan Wang,a,b Jing Lu,d Zehua Liu,a,b,c and Yongwu Niu a,b,c

The purpose of this study was to use ultrasound-based extraction to prepare starch from the tubers of Cyperus esculentus. Ultrasonic treatment of Cyperus esculentus powder with a medium of alkaline-treated water can effectively improve the starch extraction efficiency. Box-Behnken design was used to optimize the extraction process, and the results showed that the optimal parameters were ultrasound time of 30 minutes, pH value of 9.0, ultrasound temperature of 40 °C, and solid-liquid ratio of 10:1. The extraction percentage under these conditions was 90.1%. The physicochemical properties of C. esculentus starch were compared with those of cassava, potato, and corn starch. The particle size of C. esculentus starch was approximately 2 to 15 μm. The gelatinization temperature was 70.5 °C, and the peak viscosity was similar to cassava but with better thermal stability. Like other tuber starches, C. esculentus starch had higher swelling power and solubility at 85 °C.

DOI: 10.15376/biores.19.3.4264-4277

Keywords: Starch; Cyperus esculentus; Ultrasound-assisted extraction; Physicochemical Properties

Contact information: a: National Engineering Research Center of Wheat and Corn Further Processing, Henan University of Technology, Zhengzhou 450001, China; b: College of Food Science and Technology, Henan University of Technology, Zhengzhou, 450001, China; c: Food Laboratory of Zhongyuan, Luohe, 462300, China; d: Department of Molecular Sciences, Swedish University of Agricultural Sciences, PO Box 7015, SE-75007 Uppsala, Sweden; *Corresponding author:



Cyperus esculentus belongs to the perennial herbaceous plants of the Cyperaceae family in the Poaceae order. Cyperus esculentus is native to the Mediterranean region and widely distributed in tropical and subtropical regions around the world. In many countries, C. esculentus is present as harmful weeds, often competing with crops and horticultural plants for land resources (Manek et al. 2012). In fact, C. esculentus tubers contain many beneficial ingredients, and are rich in oil and starch. Studies have reported that their approximate contents of ash, crude fat, crude protein, crude fiber, and carbohydrates were 2.6%, 22.1%, 4.3%, 15.5%, and 48.1%, respectively. Starch is the main component of carbohydrates and its dry basis yield was 20.5% (Adel et al. 2015; Codina-Torrella et al. 2015). Making C. esculentus tubers into beverages is one of the interesting and mainstream application methods. In Spain, C. esculentus tubers are used to produce a plant-based milk substitute called “horchita de chufa” (Sánchez‐Zapata et al. 2012). Aydar et al. (2020) conducted a detailed analysis on the production process, bioavailability, and market share of C. esculentus and other plant-based dairy substitutes. Recent studies have shown that adding polysaccharide extracts from C. esculentus to protein drinks exhibits better stability than regular protein drinks (Yu et al. 2023). Chukwuma et al. (2010) observed the presence of alkaloids, cyanosides, resins, tannins, sterols, and saponins in the raw C. esculentus tubers, confirming that the tubers contain important nutrients and are essential nutrients for human health.

The starch content of C. esculentus tubers is high. Wu et al. (2024) analyzed the starch composition of C. esculentus from six different regions in China, with the highest starch content reaching 34.8%. Sometimes people use cold pressing to obtain the oil of C. esculentus tubers, while the remaining oil residue cake is used for feed, and the starch in it is not well utilized. Extracting starch from the oil residue cake of C. esculentus tubers is a potential way to improve its utilization efficiency. In recent years, ultrasound treatment has been increasingly applied in starch extraction, resulting in varying degrees of improvement in starch yield (Liu et al. 2020; Wang et al. 2022; Mieles-Gómez et al. 2023). The mechanism of ultrasound-assisted extraction is explained as the interaction between ultrasound and sound fields, causing cavitation bubbles in the liquid. The generation and disappearance of cavitation bubbles cause local pressure changes, resulting in turbulence, microjets, and shear effects (Cárcel et al. 2012). These effects can cause cell wall rupture, enhance the separation and dissolution of various biochemical components, such as promoting the separation of starch from lipids, proteins, and fibers, thereby improving the starch extraction yield.

This study used ultrasound-assisted extraction of starch and explored the optimal process conditions with extraction yield as the evaluation index. Through conducting scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and rapid viscosity determination (RVA) analyses, as well as examining the solubility and swelling power, the extracted C. esculentus starch, commodity cassava, potato, and corn starch were studied to gain a deeper understanding of the physicochemical properties of C. esculentus starch.



Cyperus esculentus tubers were produced in Shaoyang City, Hunan Province (Hunan, China). N-hexane and anhydrous ethanol were purchased from Fuyu Chemical Co., Ltd. (Tianjin, China). Hydrochloric acid was purchased from Kermel Chemical Co., Ltd. (Tianjin, China). Cassava, potato, and corn starch were purchased from Xinxiang Xinliang Grain and Oil Processing Co., Ltd. (Henan, China).

Defatting Treatment of C. esculentus Tubers

Fresh C. esculentus tubers were cleaned and oven-dried at 45 °C for 24 h to reduce moisture content. A universal crusher (YIRUI FW100, Tianjin Taisite Instrument Co., Ltd., Tianjin, China) was used to crush them to a size that could pass through a 20-mesh sieve. Material was processed through a hydraulic oil press (6YY-220, Luofeng Hydraulic Technology Co., Ltd., Henan, China) to obtain C. esculentus oil residue cake. The cake was crushed to pass through a 40-mesh sieve, and then n-hexane was used as a defatting treatment with solid-liquid ratio 6:1 for 5 h. After the n-hexane was filtered out, clean n-hexane was added for secondary defatting of C. esculentus. The solid-liquid ratio at this time was 3:1 for 5 h. Then, the n-hexane was filtered out again. The obtained secondary defatted C. esculentus was dried at 45 °C for 12 h and processed by a crushing method once more to a particle size of about 60-mesh and the C. esculentus defatted powder was obtained.

Extraction of Starch

Extraction of starch was achieved using the ultrasound-assisted alkaline method. Sodium hydroxide solution with a certain pH was mixed with C. esculentus defatted powder at a certain solid-liquid ratio and placed in a conical flask. The conical flask was placed into an ultrasonic water bath device with temperature control function (KQ5200DE, Kun Shan Ultrasonic Instruments Co., Ltd., Jiangsu, China), and the ultrasonic power was 200 W. After treatment at a certain temperature for a desired period of time, the material structure became loose under the ultrasonic effect, and the degree of binding between different components was reduced, thereby improving the efficiency of starch separation. After ultrasonic treatment, magnetic stirring (RCT basic, IKA Instrument and Equipment Co., Ltd., Germany) was performed for 30 min to fully dissolve the protein and other soluble substances in the C. esculentus. Afterwards, it was centrifuged (5810-R, Eppendorf AG, Germany) at 4000 rpm for 15 min, the supernatant was poured out. The precipitate was washed with distilled water, passed through a 200-mesh sieve, and the sieved material was centrifuged again and washed with water. After three repetitions, the precipitate was labelled as the extracted starch. It was washed with 80% alcohol and dried at 45 °C for 24 h. The obtained starch was crushed to pass through a 100-mesh sieve to obtain C. esculentus starch powder. Optical rotation of C. esculentus starch was determined by a polarimeter (W22-1S, Suoguang Optoelectronic Technology Co., Ltd., Shanghai, China). The starch extraction yield was calculated as follows,



where α is the optical rotation of starch, L (dm) is the length of the optical rotation tube, W (g) is the quantity of starch, H (%) denotes the moisture content of starch, M1 (g) and P1 (%) are the quality and purity of C. esculentus starch, respectively, and M2 (g) and P2 (%) are the quality and purity of C. esculentus defatted powder, respectively.

Single-Factor Test

Only one factor was changed at a time, while other factors remained unchanged (pH value of 10, ultrasound temperature of 45 ℃, ultrasound time of 30 min, solid-liquid ratio of 1:9). The factor levels were set as ultrasound time (10, 20, 30, 40, 50, and 60 min), pH value (8, 9, 10, 11, 12, and 13), ultrasound temperature (water bath temperature during ultrasound, set to 30, 35, 40, 45, 50, and 55 ℃), and solid-liquid ratio (1:5, 1:7, 1:9, 1:11, 1:13, and 1:15), with starch extraction yield as the evaluation index.

Box-Behnken Design

According to the Box-Behnken design (BBD) of response surface methodology (RSM), three points around the optimal level of factors were selected based on the results of the single-factor test. The starch extraction yield was taken as the evaluation index, and a four factor, three level of BBD was performed using Design Expert 8.0 software. The experimental level and coding are shown in Table 1.

Table 1. Factor Levels and Encoding of Box-Behnken Design


For capturing scanning electron micrographs (JSM-6490 SEM, JEOL, Peabody, MA, USA), a small amount of starch sample was added onto double-sided conductive adhesive and dispersed into a thin layer. An ion sputtering instrument was used to spray platinum on the surface of the sample for approximately 1.0 mm, with a voltage of 3 kV and an amplification factor of 3000X.


A rapid viscosity analyzer (RVA 4800, Perten, Sweden) was used to analyze the viscosity change curves of four types of starch during gelatinization and retrogradation processes. The operation method and testing procedure of RVA are based on GB/T 24853 (2010). Approximately 25 mL of water was taken and placed in a sample cylinder. A total of 3 g of starch (corrected on a 14% wet basis) was accurately added in the cylinder, gently stirred well, and placed in a rapid viscosity analyzer to measure the viscosity change curve of the sample.


A differential scanning calorimeter (TA Q20, Perkin Elmer Limited, Hopkinton, MA, USA) was used to determine the thermal stability of starch. Approximately 2.5 mg of starch and 7.5 μL distilled water were added into an aluminum plate in proper order. It was allowed to equilibrate at room temperature for 24 h. The scanning temperature range was 30 to 120 ℃, and the heating rate was 10 ℃/min.


Swelling Power and Solubility

The determination of starch swelling power and solubility was based on a modified method of Akonor et al. (2019). About 150 mg of starch and 10 mL of distilled water were added into a centrifuge tube. The tube was vortex mixed (Vortex 6, Kylin-Bell, Jiangsu, China) and reciprocating vibrated for 30 min in a water bath shaker (250 r/min, SHA-C, Zhiborui Instrument Manufacturing Co., Ltd., Jiangsu, China) at temperatures of 25 ℃, 60 ℃, and 85 ℃, respectively. The sample was allowed to cool to room temperature, and the tube was centrifuged at 4000 r/min for 40 min. The centrifuged supernatant was poured into a dry aluminum box and dried at 130 ℃ to a constant weight. The sediment quality and the quantity change of the aluminum box were weighed.



Statistical Analysis

The Design Expert 8.0 software (Stat-Ease, Inc, Minneapolis, MN, USA) was used for the BBD experiment and data analysis. All experiments were performed twice or more. The final results were evaluated using the statistical analysis software IBM SPSS Statistics 21 (IBM Corp., Armonk, NY, USA). The significant levels were established at p < 0.05. Statistical analysis was performed using the software Origin 2018 (Origin Lab Co., Northampton, MA, USA).


The Influence of Single Factors on Starch Extraction Yield

The results of the single-factor test are shown in Fig. 1. The effects of four factors, namely ultrasound time, pH, ultrasound temperature, and solid-liquid ratio, on the starch extraction yields from C. esculentus were studied. As shown in Fig. 1A, when the ultrasound time was 10 to 30 min, the starch extraction yield increased with the prolongation of ultrasound time, indicating that ultrasound had a promoting effect on starch extraction. Extraction yield began to decrease at 30 to 60 min, which may indicate that excessive ultrasound treatment damaged the structure of starch particles. The effect of ultrasound on starch structure and mechanical effects generated by ultrasound cavitation could degrade starch particles (BeMiller and Huber 2015).

Fig. 1. Single-factor test results (Error bars are used to represent the magnitude of uncertainty, and the height of the error bar is ± standard error)

Figure 1B shows that the optimal extraction effect was achieved when the pH of the alkaline solution was 9. However, as the OH concentration continues to increase, starch is hydrolyzed, leached, and oxidized, leading to a decrease in extraction yield (Karim et al. 2008). Figure 1C shows the effect of temperature on starch extraction yield. As the temperature increased, the starch yield showed an upward trend. However, when the temperature exceeded 45 ℃, the yield began to decrease. Perhaps the decrease in extraction rate is due to the swelling of starch particles, making it difficult to separate from components such as fibers and proteins (Ozturk et al. 2021). Figure 1D shows that the optimal solid-liquid ratio was 1:11. Insufficient addition of alkaline solution could not fully dissolve the protein, while excessive addition of alkaline solution might enhance the hydrolysis of starch.

Optimization of Starch Extraction

The Box-Behnken experiment and results are shown in Table 2.

Table 2. Box-Behnken Design Experiment and Results

Design Expert software was used to fit the quadratic multiple regression equation between ultrasound time (A), pH value (B), ultrasound temperature (C), solid-liquid ratio (D), and starch extraction yield:

Extraction yield (%) = 89.24 – 0.19A + 0.27B – 0.84C – 0.52D – 0.10AB – 0.78AC – 0.35AD + 0.28BC – 0.55BD + 0.43CD – 1.73A2 – 1.43B2 + 0.13C2 – 0.83D2

From Table 3, it can be seen that the P-value of the model was extremely significant, while the lake-of-fit was not significant. Among the four influencing factors, ultrasound temperature had an extremely significant impact on extraction yield, solid-liquid ratio had a significant impact, and ultrasound time and pH value had no significant impacts on extraction yield. From the F values of the four factors and the coefficients of the regression equation, the order of influence on starch yield was: ultrasound temperature > solid-liquid ratio > pH value > ultrasound time. The R2 of the model was 0.8768, indicating that the model could explain the experimental results well. The coefficient of variation (CV) was 0.81%, and a small CV value indicated that the regression equation had a good fit. The signal-to-noise ratio was 9.263, which was greater than four, indicating that the model was reliable.

Through the analysis and optimization of Design Expert 8.0, the optimal process conditions were obtained, which were ultrasound time of 32.32 min, pH value of 9.11, ultrasound temperature of 40 ℃, and solid-liquid ratio of 9.69:1. The predicted extraction percentage under these conditions was 90.60%. To verify the reliability of these results, an actual extraction of 90.08 ± 0.48 (%) was obtained by selecting an ultrasound time of 30 min, pH value of 9.00, ultrasound temperature of 40 ℃, and a solid-liquid ratio of 10:1. The obtained result was close to the predicted values, and the plan was judged to be feasible.

Table 3. Box-Behnken Design Analysis of Variance

The power of the ultrasound instrument is 200 W, and the electrical energy consumption for 30 min is 0.1 kWh. This device has a capacity of 10 L and theoretically can handle up to approximately 1 kg of C. esculentus powder. The energy consumption of 1 t of C. esculentus powder is estimated to be 100 kWh.

Microstructure of Starch Granules

The SEM images in Fig. 2 indicate that the microstructure of C. esculentus starch was similar to potato starch and cassava starch, with smooth circular or elliptical particles on the surface. Cyperus esculentus starch has a similar appearance to tropical tuber starch such as sweet potato starch (Zhu and Wang 2014; Akonor et al. 2019). Cassava starch also contains many truncated granules (Waterschoot et al. 2015). The shape of corn starch was significantly different from the other three types of starch, presenting a polygonal or irregular shape.

Fig. 2. Scanning electron microscope photographs of starch (A: Cassava; B: Corn; C: Potato; D: C. esculentus)

In terms of starch particle size, potato starch had the largest particle size among these types of starch, and its long axis size can reach up to 50 μm. There are reports indicating that the particle size distribution range of potato starch is large, ranging from 5 to 100 μm (Tester et al. 2004). In the pictures, the particle sizes of cassava starch and corn starch were relatively close. According to reports, the particle size of corn starch ranged from 5 to 20 μm. The particle size of C. esculentus starch was approximately 2 to 15 μm, it could be divided into small and medium-sized starch (Lindeboom et al. 2004).

Pasting Properties

The pasting properties of starch are one of the most explored aspects. Starch suspension undergoes particle expansion and rupture, polysaccharide leaching, and viscosity increase with starch gelatinization at a certain temperature (Juhász and Salgó 2008; Balet et al. 2019). The RVA curves of cassava, potato, corn, and C. esculentus starch suspensions at a concentration of 9.2% are shown in Fig. 3. The corresponding characteristic values of the RVA curves are shown in Table 4. The pasting temperature refers to the temperature at which the viscosity begins to rise. The pasting temperature of rhizome starch is lower than that of grain starch (Jane et al. 1999; Kaur et al. 2007). The C. esculentus starch had a higher pasting temperature than potato starch and cassava starch. The peak viscosity of C. esculentus starch was very close to that of cassava starch. Corn starch had the lowest peak viscosity. Reports have shown that the viscosity of grain starch is often low due to its low swelling force (Zhu and Corke 2011; Mieles-Gómez et al. 2023).

Fig. 3. RVA curve of 9.2% starch suspension

After reaching its maximum, the viscosity of starch began to decrease, which is attributed to the melting of the starch crystallization zone and the rapid entry of water into the particles (Balet et al. 2019). The collapse value represents the difference between peak viscosity and valley viscosity. A high collapse value usually indicates poor thermal stability. The viscosity of C. esculentus starch decreased more slowly than cassava, and its collapse value was also lower.

During the cooling stage, the four types of starch exhibited varying degrees of retrogradation, which is mainly reflected by their setback value and final viscosity. In the short term, retrogradation is mainly related to the recrystallization of amylose (Chen et al. 2015; Fu et al. 2017). From Fig. 3, the set-back trend of potato starch was the most obvious. Corn starch was the least noticeable. Cyperus esculentus starch had similar retrogradation values, but higher final viscosity compared to cassava starch.

Table 4. RVA Curve Characteristic Values of Starch