2020 Volume 26 Issue 4 Pages 469-478
An ultrasound-assisted laboratory scale wet-milling process was explored and the yield, purity and properties of corn starch were studied. The highest starch yield was obtained when the steeping time was 32 h, the concentration of sulfur dioxide was 0.05 % (w/w), the ultrasonic duration time was 15 min, the ultrasonic power was 200 W and the solid-liquid ratio of the slurry was 1:1 (g/mL). Starch yield increased by 10 % using the ultrasound-assisted wet milling (68.96 %) compared to traditional wet milling (62.48 %). The purity of starch remained unchanged. The results of X-ray diffraction, particle size, optical microscopy, scanning electron microscopy and differential scanning calorimeter revealed that the granular and crystalline structure and thermal properties were not statistically different for the starches isolated by two methods. The starch isolated using ultrasound-assisted wet milling exhibited lower yellowness, higher peak viscosity, similar thermal properties, and larger moduli compared to starch extracted by traditional wet milling.
Corn wet milling includes steps for steeping, milling to free germ, germ separation, milling to starch, fiber recovery, and separation of protein and starch (Dowd, 2003). The most important step is steeping. Corn kernels are soaked in a sulfurous acid solution at 50–52 °C for 48 h approximately (Singh and Eckhoff, 1996). Disulfide bonds of protein matrix in the corn endosperm can be cleaved by sulfur dioxide (SO2) during steeping. Nevertheless, the main problem of wet milling is the use of SO2 in the whole technology, which is harmful to environment and human health. It leads to a need for alternatives to produce corn starch that consume less water and energy, reduce emission of SO2, and help to develop sustainable processes and positive environmental impact.
Many researchers have tried kinds of methods of pilot- and laboratory-scale wet-milling to decrease SO2 use and steep time as well as estimate the mill ability of corn samples. Some studies were investigated to decrease sample amounts (1 kg to 10 g) and labor time requirements for determining the milling properties of corn samples (Eckhoff et al., 1996). We also developed a 50-g laboratory wet-milling procedure. Corn was steeped in the solution containing 0.20 % (W/W) SO2 and 0.50 % lactic acid at 52 °C for variable time and wet-milled. Various fractions (germ, fiber, protein, starch) were recovered. The optimal yield of starch was 62.56 % when corn samples were steeped for 48 h (Wang et al., 2017). Recently improved wet-milling procedures were developed including enzyme-assisted steeping and milling (Ramirez et al., 2009), alkali wet milling and ultrasonic assisted extraction (Chemat et al., 2017). The purposes of these improved methods were to reduce pollution and increase production efficiency.
Ultrasound is popular in food engineering, such as sterilizing, degassing, drying, filtrating and increasing the efficiency of extraction (Shirsath et al., 2010). Wang and Wang (2004a) applied the high-intensity ultrasound and surfactants (0.5 % sodium dodecyl sulfate) to improve the yield and purity of rice starch. Zhang et al. (2005) developed an improved 100-g laboratory process (without addition of SO2) by using sonication treatment at diverse phases in the milling process. Starch yields of the ultrasonic samples ranged from 66.93 % to 68.72 %, which was similar to that of traditional wet milling samples (68.92 %) and higher than that of milling-only (40.12 %) samples. Sonication treatment decreased the yellowness and increased the whiteness of isolated starch. Karaman et al. (2017) suggested that the yield of pulse starches reduced significantly (p < 0.05) and the amount of damaged starch developed significantly (p < 0.05) with the increasing power of ultrasonic. The hydrating speed was enhanced and the hydrating time of corn samples was reduced by ∼35 % when ultrasound (frequency 25 kHz and power 41 W/L) was applied in the hydrating process of corn. The hydrated corn kernels (with and without ultrasound) showed no significant differences in starch properties (Mino et al., 2017). Bernardo et al. (2018) used ultrasound to treat yam tubers after the grinding stage. Starch yield increased from 29.85 % to 32.09 %, while the amplitude and time of sonication will affect the physiochemical properties of yam starch.
Power ultrasound is sound waves in the frequency range of 20–100 kHz with a sound intensity from 10 to 1 000 W/cm2 (Tiwari, 2015). It is widely accepted that the mechanism of sonication is cavitation. When ultrasound is applied to a liquid medium, it induces rarefactions and compressions in the molecules of the medium. The cavitation bubbles from gas nuclei will be brought about within the fluid when refraction cycle surpasses the attraction of the liquid molecules. These bubbles become unsteady and collapse fiercely once they reach the critical volume over the period of several cycles. The application of ultrasound in the slurry was assumed to loosen the interaction between protein matrix and starch granules by ultrasonic cavitation. Collapsing of the cavitation bubbles might lead to the disruption of the noncovalent bonds between starch and protein, thus starch isolation was improved (Wang and Wang, 2004a).
A cleaner approach is demonstrated to be useful for development of sustainable products, such as the minimization of the emissions of processes and the use of resources (Berghout et al., 2015). The goal of this work was to enhance the efficiency of laboratory scale corn wet-milling by utilizing ultrasound. The evaluation on the physical and chemical properties of isolated corn starch was used to validate the feasibility of ultrasound-assisted corn wet-milling. A clean and efficient process for the production of corn starch would be beneficial to resource conservation and human health.
Materials The content of starch, protein, coarse fiber and fat were (71.45 ± 0.09) %, (8.19 ± 0.2) %, (2.70 ± 0.06) %, (4.22 ± 0.03) % in corn samples. Other chemicals and solvents were analytical grade and all of them were purchased from Kermel Chemical reagent CO. Ltd. of Tianjin, China.
Starch isolation using ultrasound Corn kernels (50 g) were steeped in 150 mL steepwater containing different concentration of SO2 at 53 °C by a water bath for different time. The steepwater was collected and dried at 45 °C to get soluble content. A knife blender (Guohua Instrument Co., Ltd, Changzhou, Jiangsu, China) was used to coarsely grind steeped corn with 150 mL water for 5 s. The slurry passed through a #7 (2.80 mm) sieve. The larger germs were moved to a #16 (1.25 mm) sieve, washed by 200 mL distilled water, dried and weighed successively. After free germ was removed, the pulped corn was ground secondly for 5 min and passed through a #50 (355 µm) sieve and #200 (75 µm) sieve in order. Accordingly, the material retained on the sieve was coarse fiber and fine fiber, respectively. A mortar was used to pestle the crude fibers and fine fibers for 10 min and 15 min, respectively. The fibers were transferred to the sieve, washed, and dried. The compound without fibers was washed with 900 mL water, filtered by vacuum and adjusted to various ratios with distilled water. Ultrasound was applied to sample suspensions using a sonication processor (Bioruptor UCD-200, Cat. No. UCD-pack 50, Diagenode, Liège, Belgium) and the frequency was fixed (20 kHz). Variable sonication power and sonication time were investigated during the process. A jacket cooling system in a procedure of the ultrasonic processor (1 min on and 5 s interval) was used in order to keep the sonication temperature at 0–9 °C. The starch slurry was centrifuged at a force of 12 857×g for 10 min (Eppendorf 5810R, Hamburg, Germany) with the help of a sucrose density solution (65 % w/v) for the final step of starch separation. The supernatant was decanted, the upper protein layer was scraped off, and the sediment was starch. The steps including centrifugation and washing were operated for four times. The fractions of corn sample (germ, coarse fiber, fine fiber, protein, starch) were dried at 45 °C for 24 h in an oven as well as weighed. Different steeping time (8, 16, 24, 32, 40 h), concentration of SO2 (0.05, 0.10, 0.15, 0.20, 0.25 %), ultrasonic power (130, 160, 200 W), ultrasonic duration time (5, 10, 15, 20, 25, 30 min), and solid to water ratio (1:1, 1:10, 1:20, 1:30 (g/mL)) were applied to optimize the conditions to isolate corn starch. In order to evaluate the feasibility of the method in this study, a traditional laboratory wet-milling procedure was used to isolate starch. Corn grains (50 g) were steeped in 150 mL steepwater containing 0.20 % of SO2 in a water bath (53 °C) for 48 h. The following process was same to the procedure used in the ultrasound-assisted isolation without the use of ultrasound.
Proximate analysis Protein content of the isolated starch was determined according to AACC Approved Method 46-30 (AACC, 2000). Moisture content was measured using Approved Method 44-15A (AACC, 2000). Yield of starch was obtained by calculating the amount of starch (dry basis) recovered from 100 g of corn samples.
Color analysis Color of isolated starch was analyzed in triplicate using a Konica Minolta colorimeter CR-400 (Osaka, Japan). L* and b* values were reported in this study (L* represents whiteness, b* represents yellowness-blueness).
Particle size The starch was dispersed in distilled water to form a suspension (2 %, w/v) and a laser diffraction particle size analyzer (SALD-301V, SHIMADZU, Kyoto, Japan) was used to determine the mean particle size of starch.
Light microscopy (LM) Starch suspension (2 %, w/v) was sprinkled onto a glass glide. Then a glass cover slip was used to cover the starch suspension. The morphology and birefrigence of starch granules could be observed clearly in a digital camera of the optical microscope (ECLIPSE 50i POL, Nikon, Kanagawa, Japan).
Scanning electron microscopy (SEM) The starch sample was fixed onto double-sided adhesive tap attached to aluminum stubs and then coated with a thin layer of gold. SEM micrographs of isolated starches were recorded using a high vacuum bench top scanning electron microscope (Quanta250 FEG, FEI, Hillsboro, USA).
X-ray diffraction (XRD) Crystalline structure of isolated starch granules were determined by an X-ray diffractometer (Rigaku MiniFlex600, Japan). Working voltage and current were 40 kV and 30 mA, and a divergence slit width of 1 mm was used during the process of measurement. The scans were operated in the diffraction angle of 5–45° with a step size of 0.02° and the rate of scans was 4°/min. The degree of relative crystallinity was calculated as follows: Rc=Ac / (Ac + Aa), where Rc is relative crystallinity, Ac is crystallized area, and Aa is amorphous form area on the X-ray diffractogram. It was analyzed by MDI Jade 6.0 software (Material Date, Inc., Livermore, CA, USA).
Thermal properties A differential scanning calorimeter (DSC) (Q20, TA Instruments, USA) was used to analyze the thermal properties of starch samples. Distilled water was mixed with 2 mg (dry basis) starch powder in an aluminum pan to make the concentration of 20 % of starch slurry. The pans were sealed and rested for 24 h at room temperature. An empty pan was used as a reference and the scan temperature ranged from 30 °C to 150 °C at a rate of 10 °C/min. DSC data were obtained including onset (To), peak (Tp), and final (Tc) temperatures. The range of gelatinization (Tc-To) and enthalpy of gelatinization (ΔH) can be calculated in this process.
Pasting properties A Rapid Visco Analyser (RVA) (Perten Instruments Ltd., Sydney, Australia) was used to analyze pasting properties of starch samples. Distilled water (25 mL) was mixed with starch (2 g) to get starch slurry (8 %, w/v). A heating and cooling cycle was used where the samples were heated from 50 to 95 °C in 2.5 min, held at 95 °C for 20 min, cooled from 95 to 50 °C in 3 min, and held at 50 °C for 9 min. The evaluated parameters were the peak viscosity (PV), trough viscosity (TV), final viscosity (FV), breakdown (BD) and setback viscosity (SV).
Starch rheological properties Rheological experiments were carried out using a physical HAAKE 400 rheometer (Thermo Fisher scientific, America) with a cone and plate geometry (5 cm diameter, 2° angle, 205 µm gap size). The temperature was maintained and regulated accurately through a physical circulating thermostated water-bath and a peltier-plate system during the experiments. Starch sample (6 g, dry basis) was dispersed in distilled water (35 mL). The starch was gelatinized by a Rapid Visco Analyser in order to obtained starch paste and the experiment method was described in section above. The starch paste was then standing 20 min to cool down to room temperature. About 1.5 mL of sample was loaded on a peltier-plate carefully. A thin layer of low viscosity silicon oil around the edge of sample was used to prevent water evaporation. Flow ramp tests were carried out at 25 °C and the shear rate ranged 0.1 s−1 to 100 s−1. The relationship between apparent viscosity and shear rate was recorded. For frequency sweep experiments, the samples were prepared according to the method described in steady shear part. The frequency dependence was determined at 25 °C with the frequency from 0.1 to 10 Hz at 1 % strain (within the linear viscoelastic region). The viscoelastic properties of samples were presented by storage (G′).
Orthogonal experimental design and statistical analysis The orthogonal experiments were designed according to the index of starch yield. Orthogonal experiments were carried out using four factors: steeping time (16, 24 and 32 h), sonication power (130, 160 and 200 W), sonication duration time (10, 15 and 20 min) and solid to water ratio (1:1, 1:10 and 1:20 g/mL). All measurements were done at least in three and data were reported as mean ± standard deviation. Significant differences among mean values of samples were analyzed at a confidence level of 95 % using one-way analyses of variance (ANOVA) with SPSS 19.0 (SPSS Inc. Chicago, IL, USA).
Effect of steeping time on starch recovery Steeping is a key process in corn wet-milling. The main purpose of this step is to soften the corn kernels and then the soluble carbohydrates and protein diffused into the steeping water. The steeping step mainly includes lactic fermentation phase followed by a SO2 treatment phase. The steeping time usually varies from 24 h to 40 h in laboratory corn wet-milling and it ranges from 36 h to 60 h in industrial corn wet-milling (Roushdi et al., 1981). The 50 g dent corn kernels were steeped at a temperature of 53 °C for different steeping time (8, 16, 24, 32, 40 h), the yields of starch and by-products were listed in Table 1. The yields of coarse fibers, fine fibers and total fibers were 7.32–8.01 %, 1.39–2.16 % and 9.15–9.72 %, respectively. The yields of germ were not significantly different (p > 0.05) when steeping time ranged from 8 h to 40 h. The concentration of soluble increased significantly (p < 0.05) when the steeping time increased from 8 h to 40 h. There were no significant increase in the yield of starch (p > 0.05) when steeping time ranged from 24 h to 40 h.
| Fraction | Steeping time (h) | Concentration of SO2 (%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 8 | 16 | 24 | 32 | 40 | 0.05 | 0.1 | 0.15 | 0.2 | 0.25 | |
| Soluble content | 2.23±0.07e | 3.74±0.05d | 5.53±0.05c | 6.09±0.00b | 6.40±0.06a | 5.09±0.05b | 5.37±0.05a | 5.49±0.03a | 5.53±0.05a | 5.59±0.11a |
| Germ | 6.74±0.21a | 6.38±0.04a | 6.16±0.38a | 6.40±0.33a | 6.24±0.07a | 6.30±0.11a | 6.62±0.21a | 6.60±0.03a | 6.16±0.38a | 6.51±0.32a |
| Coarse fiber | 7.32±0.12a | 7.57±0.02a | 8.01±0.29a | 7.58±0.30a | 7.89±0.39a | 7.78±0.23ab | 7.33±0.03b | 7.27±0.03b | 8.01±0.29a | 7.37±0.13ab |
| Fine fiber | 1.84±0.01a | 2.16±0.03a | 1.54±0.09a | 1.77±0.54a | 1.39±0.11a | 1.52±0.07a | 1.57±0.22a | 1.84±0.02a | 1.54±0.09a | 1.80±0.06a |
| Total fiber | 9.15±0.11a | 9.72±0.01a | 9.55±0.38a | 9.35±0.24a | 9.28±0.50a | 9.30±0.16a | 8.89±0.19a | 9.11±0.05a | 9.55±0.38a | 9.17±0.19a |
| Protein | 12.49±0.06a | 9.08±0.03b | 7.83±0.38bc | 6.58±0.82cd | 5.09±0.46d | 10.17±0.90a | 10.19±0.80a | 9.47±0.34ab | 7.83±0.38b | 8.67±0.10ab |
| Starch | 65.42±0.12c | 68.37±0.51b | 69.62±0.29ab | 70.11±0.48a | 70.35±0.35a | 68.59±0.38a | 68.76±0.35a | 68.77±0.17a | 68.91±0.12a | 69.01±0.14a |
| TDMR | 96.01±0.45b | 97.28±0.57ab | 98.67±0.71a | 98.53±0.23a | 97.36±0.27ab | 99.44±0.51a | 99.82±0.01a | 99.42±0.12a | 98.11±0.15b | 98.94±0.45ab |
TDMR represents the total dry matter recovery; Values with the same letter in the same row of one factor are not significantly different (P < 0.05)
Effect of so2 concentration in steeping water on starch recovery Starch granules are entrapped within the protein matrix. The disulfide linkages can be cleaved by SO2 to relax the combination between starch and protein during steeping. Vignaux et al. (2006) used the steep water containing the amount of 0.2 % SO2 and a little amount of lactic acid to isolate the starch of 10 g corn. Ramirez et al. (2008) reported that 2 200 ppm SO2 and 2 % sodium chloride in steep water could disrupt the insoluble protein matrix effectively. The effect of different contents of SO2 (0.05, 0.10, 0.15, 0.20, 0.25 %) on the yields of fractions at the temperature of 53 °C was presented in Table 1. The yields of fiber, germ, protein, starch and total dry matter recovery (TDMR) showed insignificant difference (p > 0.05). The yield of soluble enhanced significantly (p < 0.05) with the growth of SO2 content from 0.05 to 0.1. The SO2 dissolved in the water increased the rate of water infiltration into the corn and then increased the permeability of the pericarp of corn. The concentration of SO2 of 0.05 % was an optimum parameter as even at the lowest SO2 concentration, starch was obtained without loss of yield and more SO2 would cause the experimental facilities to be corroded and the water and air to be polluted.
Effect of ultrasonic power on starch recovery Some researchers (Hu et al., 2015; Sujka and Jamroz, 2013) reported that high intensity ultrasonic might corrode the surface of corn starch and generate some cracks during the ultrasonic process. Discussion on the effect of ultrasonic power on starch recovery is meaningful. The obtained results were presented in Table 2. The yield of soluble, germ, fiber and protein showed no significant increase (p > 0.05) with the increasing ultrasonic power. The yield of starch ranged from 67.00 % to 69.10 % when the sonication power changed from 130 W to 160 W. The increase of the power of sonication (from 160 W to 200 W) had no statistically effect on the yield of starch. The cavitation effect from ultrasonic in the aqueous medium would break the associations of protein and starch. The results were in agreement with previous reports (Benmoussa and Hamaker, 2011; Park et al., 2006).
| Fraction | Ultrasonic power (W) | Ultrasonic duration time (min) | Solid to water ratio (g/mL) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 130 | 160 | 200 | 5 | 10 | 15 | 20 | 25 | 30 | 1:01 | 1:10 | 1:20 | 1:30 | |
| Soluble content | 4.96±0.01a | 4.99±0.06a | 4.99±0.02a | 5.03±0.01ab | 4.94±0.01b | 5.00±0.05ab | 5.00±0.07ab | 5.09±0.00a | 5.10±0.01a | 5.01±0.06a | 5.08±0.01a | 5.04±0.09a | 5.04±0.07a |
| Germ | 6.53±0.06a | 6.55±0.38a | 6.23±0.04a | 6.70±0.03a | 6.42±0.09a | 6.71±0.08a | 6.83±0.21a | 6.49±0.15a | 6.62±0.06a | 6.62±0.14a | 6.51±0.27a | 6.28±0.12a | 6.20±0.05a |
| Coarse fiber | 8.13±0.12a | 7.48±0.37a | 7.43±0.01a | 7.83±0.01a | 7.74±0.29a | 7.59±0.33a | 7.58±0.33a | 7.54±0.18a | 7.72±0.39a | 7.20±0.14a | 7.90±0.18a | 7.82±0.52a | 8.11±0.23a |
| Fine fiber | 1.18±0.09b | 1.25±0.09ab | 1.54±0.05a | 1.52±0.12a | 1.54±0.11a | 1.39±0.17a | 1.61±0.15a | 1.72±0.10a | 1.52±0.17a | 1.64±0.14a | 1.50±0.48a | 1.36±0.13a | 1.68±0.02a |
| Total fiber | 9.31±0.21a | 8.73±0.46a | 8.97±0.04a | 9.35±0.11a | 9.28±0.18a | 8.98±0.16a | 9.19±0.18a | 9.26±0.28a | 9.24±0.22a | 8.84±0.01a | 9.39±0.30a | 9.18±0.39a | 9.79±0.25a |
| Protein | 10.53±0.62a | 9.23±0.06a | 9.73±0.05a | 10.95±0.10a | 9.78±0.02b | 9.97±0.32b | 8.63±0.11c | 8.63±0.06c | 8.67±0.46c | 9.95±0.22a | 10.70±1.21a | 10.85±0.13a | 9.82±0.02a |
| Starch | 67.00±0.16b | 69.10±0.38a | 69.24±0.41a | 67.05±0.10b | 68.64±0.10a | 68.92±0.20a | 68.96±0.11a | 69.00±0.06a | 68.98±0.03a | 68.65±0.16a | 66.41±0.03b | 66.14±0.01b | 66.16±0.02b |
| TDMR | 98.33±0.62a | 98.59±0.35a | 99.14±0.28a | 99.06±0.00b | 99.06±0.10b | 99.57±0.25a | 98.59±0.23c | 98.47±0.01c | 98.61±0.28c | 99.06±0.01a | 98.08±0.61ab | 97.49±0.07b | 97.00±0.37b |
TDMR represents the total dry matter recovery; Values with the same letter in the same row of one factor are not significantly different (p < 0.05)
Effect of ultrasonic duration time on starch recovery If the corn starch was exposed to the sonication field for a long time, the quality of starch might decrease and the cost will increase. Different sonication time was tested up to 30 min. The yield of starch increased from 67.05 % to 68.64 % when the sonication time increased from 5 min to 10 min (Table 2). The yield of starch showed no significant increase (p > 0.05) when sonication time increased from 15 min to 30 min. Though the yield of starch did not show significant difference when sonication time ranged from 10 min to 30 min, the TDMR increased when sonication time was 15 min. Thus the ultrasonic time of 15 min was used as an optimum parameter.
Effect of solid to water ratio on starch recovery The ratio of solid to water will influence the yield of starch, process efficiency and post-treatment of waste water in practical application. The highest yield of starch was reached when the solid to water ratio was 1:1 (g/mL) (Table 2). The obtained results were similar to the previous report of Kong et al. (2014). Thus the solid to water ratio (1:1 (g/mL)) was optimal.
Optimal conditions using orthogonal experimental design (OED) Based on the single factor experiments, the experiment parameters of starch extraction were optimized using orthogonal experiments (L34) (Table 3). The highest yield of corn starch (68.96 %) was obtained with the optimal conditions (steeping time 32 h, the concentration of SO2 0.05 %, ultrasonic power 200 W, ultrasonic duration time 15 min, and the solid to water ratio 1:1 (g/mL)). This result was higher than that obtained with the conditions listed in Table 3. The ANOVA for orthogonal experimental (results are not listed) revealed that the steeping time, ultrasonic duration time, ultrasonic power and the solid to water ratio had significant effect (p < 0.01) on the yield of corn starch. The yield of ultrasonic isolated starch varied from 64.35 % (Table 3) to 68.96 % and was higher than the yield of starch isolated by the traditional laboratory wet-milling (62.48 %). Wang and Wang (2004b) reported that combination of high-intensity ultrasound and neutral protease led the higher rice starch yield and unchanged properties of isolated starch. Park et al. (2006) used sonication and buffers to isolate sorghum starch and obtained a higher yield of starch. Shear forces and turbulences from the evolution of cavitation bubbles are generated within the liquid and at the vicinity of solid materials during ultrasonic irradiation. Acoustic cavitation disrupted the protein matrix to release starch granules so the starch yield improved.
| Sequence | Steeping time (h) | Ultrasonic duration time (min) | Ultrasonic power (W) | Solid to water ratio (g/mL) | Yield of starch (%) |
|---|---|---|---|---|---|
| 1 | 1(16) | 1(10) | 1(130) | 1(1:1) | 65.82±0.17 |
| 2 | 1 | 2(15) | 2(160) | 2(1:10) | 66.42±0.15 |
| 3 | 1 | 3(20) | 3(200) | 3(1:20) | 66.54±0.23 |
| 4 | 2(24) | 1 | 2 | 3 | 64.91±0.14 |
| 5 | 2 | 2 | 3 | 1 | 68.46±0.26 |
| 6 | 2 | 3 | 1 | 2 | 64.35±0.30 |
| 7 | 3(32) | 1 | 3 | 2 | 66.69±0.39 |
| 8 | 3 | 2 | 1 | 3 | 65.47±0.42 |
| 9 | 3 | 3 | 2 | 1 | 68.58±0.22 |
| Mean value 1 | 66.26 | 65.81 | 65.21 | 67.62 | |
| Mean value 2 | 65.91 | 66.78 | 66.24 | 65.82 | |
| Mean value 3 | 66.91 | 66.49 | 67.23 | 65.64 | |
| Range | 1 | 0.97 | 2.02 | 1.98 |
The protein content in starch isolated using ultrasonic-assisted wet-milling ranged from 0.55 % to 0.59 % and was comparable to that in starch isolated without ultrasonic treatment (0.55 %). They did not show significant difference (p > 0.05). Park et al. (2006) reported that the combination of ultrasonic and chemical reactants could increase the yield of sorghum starch and decrease the residual protein in starch. Cereal starches extracted from wheat, sorghum, corn, rice and barley using sonication showed low residual protein content in starch granules.
Color and particle size of the isolated starches The starches isolated with the assistance of ultrasound had lower b* value (yellowness) and comparable L* value (whiteness) with that isolated by tradition wet-milling method (Table 4). Zhang et al. (2005) reported that lower protein content of isolated starches corresponded to a lower b* value and hence a less yellowish color. Wang and Wang (2004a) reported high-intensity ultrasonic combined with surfactant was used to isolate rice starch and the starch showed lower b* value and higher L* value. However, Bernardo et al. (2018) reported that yam starch using ultrasound-assisted extraction showed a higher b* value than that of yam starch extracted using traditional method. It was in conflict with the result in this study and the difference might be attributed to the different parameters of ultrasonic during extraction and the different natural features of cereal and tuber endosperm. Mean particle size of the starch granules isolated using the ultrasonic assisted method was 16.58 µm and showed no significant difference compared to those isolated using traditional corn wet milling (p > 0.05, Table 4).
| Samples | b* | L* | Particle size (µm) | Crystallinity(%) | To (°C) | Tp (°C) | Tc (°C) | Tc-To (°C) | ΔH (J/g) | PV (cP) | TV (cP) | BD (cP) | FV (cP) | SB (cP) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Traditional wet milling | 3.69±0.01a | 98.46±0.02a | 16.60±0.03a | 30.56±0.17 a | 67.25±0.10a | 72.05±0.12a | 79.33±0.29a | 12.08±0.26a | 13.16±0.39a | 1235.66±15.03b | 856.67±17.02b | 379.00±6.11a | 1341.00±38.00b | 486.00±33.71a |
| Ultrasonic assisted isolation | 2.92±0.01b | 98.43±0.04a | 16.58±0.04a | 30.44±0.09 a | 67.56±0.16a | 72.05±0.23a | 79.15±0.18a | 11.59±0.21a | 11.64±0.81a | 1515.66±5.70a | 1309.33±5.04a | 206.33±1.20b | 1930.33±3.48a | 621.00±2.08a |
b * yellowness; L * whiteness; To Onset temperature; Tp Peak temperature of gelatinization; Tc Final temperature of gelatinization; Tc-To Range of gelatinization temperature; ΔH Enthalpy of gelatinization; PV Peak viscosity; TV Trough viscosity; BD Breakdown; FV Final viscosity; SB Setback; Values with the same letter in the same column are not significantly different (p < 0.05)
Granule morphology The starch granule morphology and surface was presented using LM with polarized light and SEM. As presented in Fig. 1-A and Fig. 1-a, the corn starch granules had typical round-shaped and angular-shaped. Besides, the maltese cross can be observed (Fig. 1-B and Fig. 1-b). It indicated that ultrasonic used in this study did not disrupt the granular and crystalline structure of the isolated starch.
Morphological photographs of isolated starches. A: Light microscopy of starch from traditional wet-milling, B: Microscopy with polarized light of starch from traditional wet-milling, C: Scanning electron microscopy of starch from traditional wet-milling. a: Light microscopy of starch from ultrasound-assisted wet-milling, b: Microscopy with polarized light of starch from ultrasound-assisted wet-milling, c: Scanning electron microscopy of starch from ultrasound-assisted wet-milling.
There were no clearly fissures and holes observed on the surface of isolated starch granules in the SEM micrographs (Fig. 1-C and Fig. 1-c). Miano et al. (2017) reported the similar results with this study. Many researchers found that ultrasonic caused damage on the surface of free starch granules (Amini et al., 2015; Sujka and Jamroz, 2013). The different results were due to the different objects in ultrasonic field. Corn starch in endosperm with the protection of protein matrix in this study was different from the free starch granules in other studies.
X-ray diffraction (XRD) Relative crystallinity of isolated starch can be observed in Table 4. Starch isolated by both methods with or without ultrasound exhibited characteristic A-type X-ray pattern with three typical peaks of scattering angles (2θ) of 15.3, 17.1 and 23.5° (Fig. 2).
X-ray pattern of isolated starch. a: Traditional wet milling, b: Ultrasound-assisted wet-milling.
The diffraction patterns suggested that ultrasonic did not change the crystalline structure of isolated starch. The size of amylopectin chains, density, packaging and amount of water present in the starch granule influence the relative crystallinity (Zhu et al., 2012). The relative crystallinity of corn starch isolated with and without ultrasound was 30.44 % and 30.56 %, respectively. The result suggested that ultrasound did not change the relative crystalline of isolated starch.
Thermal properties of the isolated starches DSC was used to evaluate the thermal properties of the isolated starches (Fig. 3) and the parameters of gelatinization temperature and gelatinization enthalpy were depicted in Table 4. The degree of heterogeneity of crystallites within the isolated starch granules can be estimated by difference in Tc-To. The values of Tc-To of isolated starch using two different methods testified that sonication did not distort the crystalline region in the starch granules. Bernardo et al. (2018) reported that the values of Tc-To decreased when high intensity ultrasonic was used in yam starch isolation and it might be due to high intensity ultrasonic destroying the weakest crystals and then leaving the strongest crystals in the starch granules. The value of gelatinization enthalpy (ΔH) could reflect the loss of double helical order (Liu et al., 2018). The values of ΔH of isolated starch also showed no significant difference (p > 0.05).
Thermograms of isolated starch. a: Traditional wet milling, b: Ultrasound-assisted wet-milling.
Pasting properties of the isolated starches During heating in aqueous suspension, starch granules absorb water and swell to larger size, resulting in granule disruption and changes in viscosity of the system. The starch isolated using two different methods exhibited different pasting properties (Table 4). The peak viscosity, trough viscosity, breakdown viscosity, final viscosity and setback viscosity of the starch isolated with ultrasound were 1 515.66 cP, 1 309.33 cP, 206.33 cP, 1 930.33 cP and 621 cP, respectively. These pasting properties of the starch isolated with ultrasound significantly were higher than those of starch isolated by traditional wet-milling, except for breakdown viscosity. The results obtained were in consistent with some other studies (Bernardo et al., 2018; Sit et al., 2014). The peak viscosity during heating is linked to granular swelling power. Final viscosity shows the stability of starch paste during cooling and the reaggregation of gelatinized starch molecules (Xu et al., 2018). Ultrasound would destroy the restriction of the protein molecules around the starch granules and then more water molecules could enter starch granules to increase viscosity. The lower breakdown of the starch isolated with ultrasound indicated that sonication decreased steric hindrance from the residue protein and enhanced the combination among starch molecules so the system was stable during heating.
Rheological properties of the isolated starches During gelatinization, the starch granules swell to many times than their original size, rupture and amylose leaches out of the granule then three dimensional network is formed. The evolution of the apparent viscosity of starch paste with shearing was in Fig. 4-A. It revealed that the apparent viscosity of two different starch pastes decreased with shear rate (from 0.1 to 100 s−1). It indicated that two starch pastes showed shear-thinning behavior. The result was in accordance with that of native corn starch (Silva et al., 2017). The ramifications degree and chain length of starch determine the apparent viscosity of gelatinized starch dispersions. Higher apparent viscosity for starch isolated with ultrasound was observed in Fig. 4-A.
Rheological properties of isolated starch: A: apparent viscosity and shear rate relationship; B: G′ of isolated starch; C: G′ of isolated starch. ●: Starch isolated by traditional wet-milling; ■: starch isolated by ultrasound-assisted wet-milling.
The typical mechanical spectrum of the isolated starch by a frequency sweep from 0.1 to 10 Hz was in Fig. 4-B and Fig. 4-C. The changes in G′ and G″ occurred in a similar pattern and the G′ values were higher than the G″ values throughout the frequency range. The storage modulus (G′) is used to represent the stored energy as a result of elastic changes in the network and the loss modulus (G″) is a measure to evaluate the viscous properties. The results indicated that the isolated starch produced a well-formed elastic gel structure. The two moduli of starch isolated with ultrasound were larger than that of starch isolated by traditional method, which indicated a much stronger viscoelastic behavior. When the frequency was less than 5 Hz, the G′ and G″ of isolated starches increased sharply. When the frequency ranged from 5 Hz to 10 Hz, the G′ and G″ of isolated starches tended stable. According to the results, the higher G′ and G″ of starch isolated with ultrasound were in accordance with that of commercial corn starch (Niu et al., 2018).
There is a developing interest in designing a sustainable process that enables benefits in the isolation efficiency of important products from crops without harmful impacts on the human health and environment. In order to use fewer resources and produce less waste during starch production, an ultrasonic-assisted laboratory scale wet-milling was developed. Use of SO2 and steeping time were reduced by 75 % and 33 %, the starch yield and separation efficiency were enhanced, and the purity of the isolated starch was maintained in the ultrasound assisted isolation compared with traditional laboratory wet-milling. The properties of isolated starch revealed that ultrasound had a great potential in corn wet-milling. Ultrasound possessed the capability to effectively enhance starch-protein separation by acoustic cavitation. The mechanism of acoustic cavitation will be discussed in the future studies. Moreover, the development of industrial ultrasound equipment is badly needed for the application of this method in plant scale wet-milling to reduce water pollution and energy consumption.
Acknowledgements This work was supported by the foundation for key teacher sponsored by Henan University of Technology (21420004) and the Education Department of Henan Province (2015GGJS-035), the China Scholarship Council (201908410077), the Natural Science Foundation of Hainan Province (817142), and the Finance Science and Technology Project of Hainan Province (ZDYD2019031).
