A method for preparing corn starch nanocrystals by pulsed electric field-assisted acid hydrolysis

Abstract

The present invention discloses a method for preparing corn starch nanocrystals by pulsed electric field-assisted acidolysis. First, the starch solution is treated with a pulsed electric field of 1-12 kV / cm, and then acidolysis is carried out to prepare starch nanocrystals. It is simple and efficient, which can not only significantly reduce the reaction time and prepare corn starch nanocrystals in a short time, but also has higher safety performance, low energy consumption due to mild reaction conditions, and avoids the adverse effects of instantaneous temperature rise on starch acidolysis. While improving the crystallinity, corn starch nanocrystals with smaller sizes are obtained, which can be used as fillers for composite materials to improve their mechanical properties and barrier properties, and are suitable for use as emulsion or embedding materials in the food or daily chemical fields.

Description

Technical Field:

[0001] The present invention relates to the technical field of preparing corn starch nanocrystals, and particularly relates to a method for preparing corn starch nanocrystals by pulsed electric field-assisted acidolysis. Background Art:

[0002] Starch is a polysaccharide polymerized from glucose molecules. Acidolysis below the gelatinization temperature can obtain modified starch. Under the action of acid, the glycosidic bond of starch molecules undergoes moderate hydrolysis, and the molecular weight becomes smaller. The viscosity of the denatured starch obtained by acid hydrolysis decreases, and the fluidity becomes stronger. After modification, starch can be widely used in textile, food, papermaking and other fields.

[0003] Currently, the most common methods for starch modification are acidolysis method, deep eutectic solvent method and enzymatic hydrolysis method. As the most basic method, the modified starch prepared by acidolysis method has a lower gelatinization degree and higher gel performance, and is simple to operate and low in cost, which is more suitable for mass production. The disadvantage is that the acidolysis reaction time is too long.

[0004] Pulsed electric field is a non-thermal treatment technology, which uses a method of applying a high-intensity pulsed electric field in a short time. Foods or substances contain various ions and have a certain electrical conductivity. When an electric field is applied between electrodes, the current will pass through the liquid food. Since the molecules of the food carry charges, these molecules will rearrange under the action of the electric field, resulting in the transfer of current at every point in the food. Chinese Patent CN 114805616 A discloses a modified method for preparing acid-hydrolyzed starch by electric field assistance. After mixing starch and water to prepare a starch emulsion, it is then mixed evenly with an acid solution to form a mixed material liquid, and then the mixed material liquid is placed in an electric field environment for treatment, with an electric field strength of 700 - 1000 V / cm, realizing rapid, efficient and high-yield modification of starch acidolysis, but the obtained product needs to be further improved.

[0005] Currently, people's research on polymer composites processed with nanoscale rigid particles is becoming more and more in-depth. Due to the size effect of nanometers, these composites have some unique and outstanding properties compared with traditional micro-composites. Summary of the Invention:

[0006] The purpose of the present invention is to provide a method for preparing corn starch nanocrystals by pulsed electric field-assisted acidolysis.

[0007] The present invention is realized through the following technical solutions:

[0008] A method for preparing corn starch nanocrystals by pulsed electric field-assisted acidolysis, the method comprising the following steps:

[0009] (1) Mix starch and water evenly and stir to form a uniform starch suspension with a mass fraction of 5-30%; subject the starch suspension to pulsed electric field treatment at a constant temperature of 5-30 °C through a constant-speed peristaltic pump; the specific conditions for the pulsed electric field treatment are: electric field strength 1-12 kV / cm, preferably 8-12 kV / cm, frequency 1-10 Hz, preferably 5-10 Hz, pulse width 1-10 μs, preferably 1-5 μs, treatment time 5-30 min;

[0010] (2) Mix the starch suspension after pulsed electric field treatment with an equal volume of 1-3.16 mol / L sulfuric acid solution, mix and stir at a certain rotation speed at 30-50 °C for several days. After the reaction is completed, wash with alcohol and then with water, then add a certain volume of water and homogenize to obtain a uniform starch nanocrystal suspension, and freeze-dry to obtain corn starch nanocrystals.

[0011] In step (1), the constant-speed peristaltic pump makes the starch suspension pass through the pulsed electric field at a constant flow rate, and the flow rate range is 100-400 mL / min.

[0012] In step (2), the preferred temperature for the acidolysis reaction by adding the sulfuric acid solution is 30-40 °C.

[0013] The specific steps of washing with alcohol and then with water in step (2) are: wash 4-6 times with anhydrous ethanol, discard the upper centrifuged liquid, let it stand for more than 12 h until the ethanol volatilization is completed, then wash 4-6 times with distilled water by centrifugation, and discard the upper filtrate; finally, add distilled water to make the total volume reach 45 mL, homogenize at 10,000 rpm for 5 min, and let it stand for more than two days to obtain the corn starch nanocrystal suspension.

[0014] In step (2), the conditions for freeze-drying are: freeze-dry at -75 to -80 °C for 24-34 h.

[0015] The beneficial effects of the present invention are as follows:

[0016] 1) The present invention first treats the starch solution with a pulsed electric field of 1-12 kV / cm and then performs acidolysis to prepare starch nanocrystals, which is simple and efficient. It can not only significantly reduce the reaction time and prepare corn starch nanocrystals in a short time, but also has higher safety performance and low energy consumption due to mild reaction conditions (pulsed electric field of 1-12 kV / cm at a constant temperature of 5-30 °C), and avoids the adverse effects of instantaneous temperature rise on starch acidolysis. While improving the crystallinity, corn starch nanocrystals with smaller sizes are obtained, which can be used as fillers for composite materials to improve their mechanical properties and barrier properties, and are suitable for use as emulsion or embedding materials in the food or daily chemical fields.

[0017] 2) In the present invention, the starch solution is first treated with pulsed electric fields, which can assist in opening the starch channels to facilitate the entry of sulfuric acid during the subsequent acid hydrolysis process, helping to improve the yield of corn starch nanocrystals. At the same time, it also helps to reduce resource consumption, does not require high temperatures, and increases the added value of the product.

[0018] 3) The present invention does not require the addition of extra reagents or expensive enzymes. The starch solution is first treated with pulsed electric fields and then acid hydrolyzed, reducing costs. Brief Description of the Drawings:

[0019] Figure 1 It is the scanning electron micrograph of Example 1 of the present invention.

[0020] Figure 2 It is the scanning electron micrograph of Example 2 of the present invention.

[0021] Figure 3 It is the scanning electron micrograph of Example 3 of the present invention.

[0022] Figure 4 It is the scanning electron micrograph of Example 4 of the present invention.

[0023] Figure 5 It is the scanning electron micrograph of Example 5 of the present invention.

[0024] Figure 6 It is the scanning electron micrograph of Example 6 of the present invention.

[0025] Figure 7 It is the scanning electron micrograph of Comparative Example 1 of the present invention.

[0026] Figure 8 It is the scanning electron micrograph of Comparative Example 2 of the present invention. Detailed Description of the Invention:

[0027] The following is a further description of the present invention, rather than a limitation thereof.

[0028] All pulsed electric field equipment in the examples is the PEF-SY-2 type pulsed electric field equipment of Guangzhou Paihu Technology Co., Ltd.

[0029] Example 1:

[0030] (1) Mix corn starch and distilled water evenly and stir until a uniform corn starch suspension with a mass fraction of 10% is formed; subject the corn starch suspension to pulsed electric field treatment under constant temperature conditions through a constant speed peristaltic pump, with the flow rate controlled at 300 mL / min, the temperature controlled at room temperature, and the pulsed electric field treatment conditions being: electric field strength 4 kV / cm, frequency 10 Hz, pulse width 10 μs, and treatment time 10 min;

[0031] (2) Mix the corn starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, and mix and stir at 200 r / min and 35 °C for 1.5 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water. Add a certain volume of water and homogenize at 10000 rpm for 5 min to obtain a uniform corn starch nanocrystal suspension, and freeze-dry to obtain corn starch nanocrystals.

[0032] The crystallinity of the treated starch was measured using a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was measured using a Malvern nano particle size and Zeta potential analyzer, and the results are shown in Table 1. The microstructure of the treated starch nanocrystals was observed using a Hitachi field emission scanning electron microscope. Figure 1 This is the scanning electron micrograph of Example 1 of the present invention. It can be clearly seen that there are a large number of nano-scale channels on the surface of the starch granules. After pulsed electric field treatment, the surface of the starch begins to break.

[0033] Example 2

[0034] Refer to Example 1, the difference is that the pulsed electric field strength is higher and the frequency is lower.

[0035] (1) Mix corn starch with distilled water evenly and stir until a uniform corn starch suspension with a mass fraction of 10% is formed; pass the corn starch suspension through a constant-speed peristaltic pump for pulsed electric field treatment under constant temperature conditions, with the flow rate controlled at 300 mL / min, the temperature controlled at room temperature, and the pulsed electric field treatment conditions being: electric field strength 8 kV / cm, frequency 1 Hz, pulse width 10 μs, and treatment time 10 min;

[0036] (2) Mix the corn starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, and mix and stir at 200 r / min and 35 °C for 1.5 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water. Add a certain volume of water and homogenize at 10000 rpm for 5 min to obtain a uniform starch nanocrystal suspension, and freeze-dry to obtain starch nanocrystals.

[0037] The crystallinity of the treated starch was measured using a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was measured using a Malvern nano particle size and Zeta potential analyzer, and the microstructure of the treated starch nanocrystals was observed using a Hitachi field emission scanning electron microscope. Figure 2 This is the scanning electron micrograph of Example 2 of the present invention. After high-field electric field-assisted treatment, starch nanocrystals are formed.

[0038] Example 3

[0039] Referring to Example 1, the difference is that the electric field strength of the pulsed electric field is higher.

[0040] (1) Mix corn starch and distilled water evenly and stir until a homogeneous corn starch suspension with a mass fraction of 10% is formed; pass the corn starch suspension through a constant-speed peristaltic pump for pulsed electric field treatment under constant temperature conditions, control the flow rate at 300 mL / min, control the temperature at room temperature, and the pulsed electric field treatment conditions are: electric field strength 12 kV / cm, frequency 10 Hz, pulse width 10 μs, treatment time 10 min;

[0041] (2) Mix the starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, mix and stir at 200 r / min and 35 °C for 1.5 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water, add a certain volume of water, and homogenize at 10000 rpm for 5 min to obtain a homogeneous starch nanocrystal suspension, and freeze-dry to obtain starch nanocrystals.

[0042] Use a Bruker X-ray colorimeter to measure the crystallinity of the treated starch, use a Malvern nanoparticle size and Zeta potential analyzer to measure the particle size of the treated starch nanocrystals, and use a Hitachi field emission scanning electron microscope to observe the microstructure of the treated starch nanocrystals. Figure 3 This is the scanning electron micrograph of Example 3 of the present invention, and individual starch granules can be clearly seen.

[0043] Example 4

[0044] Referring to Example 1, the differences are that the electric field strength of the pulsed electric field in step (1) is higher and the acid treatment temperature in step (2) is higher.

[0045] (1) Mix corn starch and distilled water evenly and stir until a homogeneous corn starch suspension with a mass fraction of 10% is formed. Pass the corn starch suspension through a constant-speed peristaltic pump for pulsed electric field treatment under constant temperature conditions, control the flow rate at 300 mL / min, control the temperature at room temperature, and the pulsed electric field treatment conditions are: electric field strength 8 kV / cm, frequency 10 Hz, pulse width 10 μs, treatment time 10 min;

[0046] (2) Mix the starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, mix and stir at 200 r / min and 50 °C for 1.5 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water, add a certain volume of water, and homogenize at 10000 rpm for 5 min to obtain a homogeneous starch nanocrystal suspension, and freeze-dry to obtain starch nanocrystals.

[0047] The crystallinity of the treated starch was determined by a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was determined by a Malvern nanoparticle sizer and Zeta potential analyzer, and the microstructure of the treated starch nanocrystals was observed by a Hitachi field emission scanning electron microscope. Figure 4 This is a scanning electron microscope image of Example 4 of the present invention. It can be clearly seen that the high temperature causes the starch to become gelatinous, and agglomeration occurs on the surface of starch nanocrystals, but the particles are still clearly visible.

[0048] Example 5

[0049] Referring to Example 1, the difference is that the pulse electric field in step (1) has a higher electric field intensity and a smaller pulse width.

[0050] (1) corn starch and distilled water were mixed uniformly and stirred to form a uniform corn starch suspension with a mass fraction of 10%, and the corn starch suspension was subjected to pulse electric field treatment under constant temperature conditions by a constant speed peristaltic pump, with the flow rate controlled at 300 mL / min and the temperature controlled at room temperature. The pulse electric field treatment conditions were: electric field intensity 8 kV / cm, frequency 10 Hz, pulse width 1 μs, and treatment time 10 min;

[0051] (2) The starch suspension treated by the pulse electric field in step (1) was mixed with an equal volume of 3.16 mol / L sulfuric acid solution, and the mixture was stirred at 200 r / min and 35° C. for 1.5 d. After the reaction was completed, the reaction solution was washed with alcohol 5 times and then with water 5 times. A certain volume of water was added and homogenized at 10,000 rpm for 5 min to obtain a uniform starch nanocrystal suspension, which was freeze-dried to obtain starch nanocrystals.

[0052] The crystallinity of the treated starch was determined by a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was determined by a Malvern nanoparticle sizer and Zeta potential analyzer, and the microstructure of the treated starch nanocrystals was observed by a Hitachi field emission scanning electron microscope. Figure 5 This is a scanning electron microscope image of Example 5 of the present invention, in which a large number of starch nanocrystals can be clearly seen.

[0053] Example 6

[0054] Refer to Example 1, the difference is that the pulse electric field intensity in step (1) is higher.

[0055] (1) corn starch and distilled water were mixed uniformly and stirred to form a uniform corn starch suspension with a mass fraction of 10%, and the corn starch suspension was subjected to pulse electric field treatment under constant temperature conditions by a constant speed peristaltic pump, with the flow rate controlled at 300 mL / min and the temperature controlled at room temperature. The pulse electric field treatment conditions were: electric field intensity 8 kV / cm, frequency 10 Hz, pulse width 10 μs, and treatment time 10 min;

[0056] (2) Mix the starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, and mix and stir at 200 r / min and 35 °C for 1.5 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water. Add a certain volume of water and homogenize at 10000 rpm for 5 min to obtain a uniform starch nanocrystal suspension, and then freeze-dry to obtain starch nanocrystals.

[0057] The crystallinity of the treated starch was measured using a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was measured using a Malvern nanoparticle size and Zeta potential analyzer, and the microstructure of the treated starch nanocrystals was observed using a Hitachi field emission scanning electron microscope. Figure 6 This is the scanning electron micrograph of Example 6 of the present invention, and a large number of starch nanocrystals can be clearly seen.

[0058] Comparative Example 1

[0059] Referring to Example 1, the difference is that the electric field strength of the pulsed electric field in step (1) is 0, that is, no pulsed electric field treatment is carried out.

[0060] (1) Mix corn starch and distilled water evenly and stir until a uniform corn starch suspension with a mass fraction of 10% is formed; pass the corn starch suspension through a constant-speed peristaltic pump for pulsed electric field treatment under constant temperature conditions, with the flow rate controlled at 300 mL / min and the temperature controlled at room temperature. The pulsed electric field treatment conditions are: electric field strength 0 kV / cm, frequency 10 Hz, pulse width 10 μs, and treatment time 10 min;

[0061] (2) Mix the starch suspension after pulsed electric field treatment in step (1) with an equal volume of 3.16 mol / L sulfuric acid solution, and mix and stir at 200 r / min and 35 °C for 3 d. After the reaction is completed, wash the reaction solution 5 times with alcohol and then 5 times with water. Add a certain volume of water and homogenize at 10000 rpm for 5 min to obtain a uniform starch nanocrystal suspension, and then freeze-dry to obtain starch nanocrystals.

[0062] The crystallinity of the treated starch was measured using a Bruker X-ray colorimeter, the particle size of the treated starch nanocrystals was measured using a Malvern nanoparticle size and Zeta potential analyzer, and the microstructure of the treated starch nanocrystals was observed using a Hitachi field emission scanning electron microscope. Figure 7 This is the scanning electron micrograph of Comparative Example 1 of the present invention. Starch nanocrystals can be seen, but the particle size is large.

[0063] Comparative Example 2:

[0064] Referring to Example 6, the difference is that the order of acid hydrolysis and pulsed electric field treatment is different.

[0065] (1) Mix corn starch and distilled water evenly and stir until a uniform corn starch suspension with a mass fraction of 10% is formed; mix the corn starch suspension with an equal volume of 3.16 mol / L sulfuric acid solution, stir the starch acidolysis solution at a speed of 200 r / min, and after mixing and stirring at 35 °C for 1.5 d, then perform pulsed electric field treatment on the starch acidolysis solution through a constant-speed peristaltic pump under constant temperature conditions. The flow rate is controlled at 300 mL / min, the temperature is controlled at room temperature, and the pulsed electric field treatment conditions are: electric field strength 8 kV / cm, frequency 10 Hz, pulse width 10 μs, and treatment time 10 min;

[0066] (2) After the treatment is completed, wash the reaction solution 5 times with alcohol and then 5 times with water, add a certain volume of water, and homogenize at 10000 rpm for 5 min to obtain a uniform starch nanocrystal suspension, which is freeze-dried to obtain starch nanocrystals.

[0067] Table 1 Particle size / crystallinity and yield of different samples

[0068]

[0069] The particle sizes, crystallinities, and yields of Examples 1-6 and Comparative Examples 1-2 are shown in Table 1.

[0070] Comparing the original corn starch and Comparative Example 1, it can be seen that corn starch nanocrystals can be prepared only through acid hydrolysis.

[0071] Comparing Example 6 and Comparative Example 1, it can be seen that the corn starch nanocrystals prepared by pulsed electric field treatment have smaller particle sizes, higher crystallinities, and higher yields.

[0072] Comparing Example 6 and Comparative Example 2, it can be seen that adding a pulsed electric field before acid hydrolysis in Example 6 can make the starch become loose and acid hydrolysis is more likely to enter, and it is more easily acid hydrolyzed. The obtained corn starch nanocrystals have smaller particle sizes, higher crystallinities, and higher yields; while in Comparative Example 2, the starch is acid hydrolyzed first and then a pulsed electric field is added, which will significantly damage the crystal structure of the starch nanocrystals, resulting in a decrease in crystallinity and a decrease in yield.

[0073] Comparing Example 1, Example 3 and Example 6, it can be seen that as the electric field strength increases from 4 kV / cm to 8 kV / cm, the particle size and crystallinity of corn starch nanocrystals decrease significantly, and the yield of corn starch nanocrystals increases significantly. This shows that pulsed electric fields with a certain field strength can promote the acid hydrolysis of starch. However, when the electric field strength increases from 8 kV / cm to 12 kV / cm, the field strength of the pulsed electric field is too high, the crystallinity of corn starch nanocrystals does not decrease significantly, the particle size does not decrease significantly either, and the yield even decreases. Therefore, the electric field strength is preferably 8 kV / cm - 12 kV / cm.

[0074] Comparing Example 2 and Example 6, it can be seen that the corn starch nanocrystals prepared at a pulse frequency of 10 Hz have a smaller particle size, higher crystallinity and higher yield than those prepared at a pulse frequency of 1 Hz. Comparing Example 5 and Example 6, it can be seen that the corn starch nanocrystals prepared with a pulse width of 1 μs have a smaller particle size, higher crystallinity and higher yield than those prepared with a pulse width of 10 μs.

[0075] Comparing Example 4 and Example 6, it can be seen that the preferred acid hydrolysis reaction temperature for adding sulfuric acid solution in step (2) is 35 °C. Temperature significantly affects the particle size, crystallinity and yield of corn starch nanocrystals. The reason for this phenomenon is that when the temperature is relatively high or close to the gelatinization temperature of starch, sulfuric acid quickly enters the interior of starch, destroying the crystalline region of starch molecules and reducing the crystallinity instead; only when the electric field strength is appropriate, sulfuric acid just enters the pores of starch granules, breaking corn starch into smaller particle size granules, and significantly improving the yield of corn starch nanocrystals.

Claims

1. A method for preparing corn starch nanocrystals by pulsed electric field assisted acid hydrolysis, characterized in that: The method comprises the following steps: (1) Mix starch and water uniformly and stir to form a uniform starch suspension with a mass fraction of 5 to 30%; subject the starch suspension to pulse electric field treatment at a constant temperature of 5 to 30° C. by a constant speed peristaltic pump; the specific conditions of the pulse electric field treatment are: electric field intensity 1 to 12 kV / cm, frequency 1 to 10 Hz, pulse width 1 to 10 μs, and treatment time 5 to 30 min; (2) Mixing the starch suspension treated by the pulse electric field with an equal volume of 1-3.16 mol / L sulfuric acid solution, stirring at a certain speed and 30-50° C. for several days, washing with alcohol and then with water after the reaction is completed, and then adding a certain volume of water and homogenizing to obtain a uniform starch nanocrystal suspension, which is freeze-dried to obtain corn starch nanocrystals.

2. The method according to claim 1, characterized in that The electric field strength is 8-12 kV / cm, the frequency is 5-10 Hz, and the pulse width is 1-5 μs.

3. The method according to claim 1, characterized in that Step (1) A constant speed peristaltic pump allows the starch suspension to pass through the pulsed electric field at a constant flow rate, and the flow rate range is 100 to 400 mL / min.

4. The method according to claim 1, characterized in that: The acid hydrolysis reaction temperature of adding sulfuric acid solution in step (2) is 30-40°C.

5. The method according to claim 1, characterized in that The specific steps of the alcohol washing and then water washing in step (2) are: washing with anhydrous ethanol for 4 to 6 times, discarding the upper centrifuge liquid, standing for more than 12 hours until the ethanol is completely volatilized, then centrifugally washing with distilled water for 4 to 6 times, discarding the upper filtrate; finally, adding distilled water to make the total volume reach 45 mL, homogenizing at 10000 rpm for 5 minutes, and standing for more than two days to obtain a corn starch nanocrystal suspension.

6. The method according to claim 1, characterized in that In step (2), the freeze-drying conditions are: freeze-drying at -75 to -80°C for 24 to 34 hours.

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