Preparation method of double-crosslinking composite modified starch

Abstract

The invention relates to the technical field of preparation of modified starch, in particular to a preparation method of bi-crosslinking composite modified starch, which comprises the following steps: mixing edible cassava starch with deionized water to prepare starch milk with the mass fraction of 40%; adding anhydrous sodium sulfate which accounts for 13% of the dry basis amount of the starch into the starch milk to serve as an expansion inhibitor, and then adding 3.0% of a sodium hydroxide solution to adjust the pH value of the starch milk to 11.20 so as to construct an alkaline environment required by an etherification cross-linking reaction; according to the preparation method, etherification crosslinking and esterification crosslinking are combined to form a double-crosslinking synergistic structure, so that the problem of insufficient long-term stability of single etherification crosslinking paste liquid is solved, and the defect of limited viscosity improvement of single esterification crosslinking is overcome. The edible cassava starch which is wider in source and more excellent in gelatinization characteristic is selected as the raw material, a modification process is designed in a targeted mode, the amylose / amylopectin proportion and the particle shape of the cassava starch are adapted, and the application range of the modified starch in the fields of food, papermaking, medicine, spinning and the like is further expanded.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of modified starch preparation, and particularly relates to a preparation method of double-crosslinking composite modified starch. BACKGROUND

[0002] As a natural polymer compound with wide sources, low price and excellent biodegradability, starch occupies an indispensable position in many fields such as food, papermaking, textile, medicine and the like. In the food industry, it is often used as a thickening agent, stabilizer and gelling agent to improve the thick taste of soup and sauce, improve the softness of cakes and prolong the shelf life; in the papermaking industry, starch can enhance the strength and smoothness of paper as a surface sizing agent, and improve the retention rate of fine fibers as a wet-end additive; in the pharmaceutical field, starch is the core component of drug excipients, which helps drug shaping and stabilization; in the textile industry, starch is used for yarn sizing to reduce weaving breakage and for fabric finishing to improve hand feeling and wrinkle resistance.

[0003] However, the performance defects of natural starch limit its high-end applications: poor temperature resistance and acid and alkali resistance, easy to cause molecular structure damage in high-temperature or acidic food processing environment, resulting in viscosity reduction and unstable gelatinization temperature; poor paste stability, easy to separate and precipitate after standing, which cannot meet the product demand with strict stability requirements; the viscosity level is difficult to break through the bottleneck, which cannot adapt to the process requirements of high-viscosity starch in some industrial production.

[0004] To overcome the above-mentioned defects, relevant researchers have developed various starch modification technologies, among which etherification crosslinking, esterification crosslinking and composite modification are the mainstream directions. Although single etherification crosslinking technology can improve the transparency and low-temperature stability of starch paste by introducing hydroxypropyl groups and reduce the gelatinization temperature, the effect of improving long-term stability of paste is limited, and the performance is still easy to fluctuate under the influence of external environment; single esterification crosslinking technology can improve the water resistance and aging resistance of starch by introducing ester groups, but it is difficult to effectively improve the viscosity, and cannot meet the demand for comprehensive performance of starch in complex scenarios, and both types of single modification technology have the problems of large amount of chemical reagents, complex process, high production cost and great environmental pressure.

[0005] In the field of composite modification research, although some scholars have tried to optimize the performance of starch through multiple modifications, such as Ratnayake et al. published in 2008 "Phase transition of cross-linked and hydroxypropylated corn (Zea mays L.) starches" (LWT, 41 (2008): 346-358). The study adopts a double modification strategy of "cross-linking + hydroxypropylation (etherification modification)", and explores the thermal behavior and phase transition characteristics of cross-linked waxy corn starch and hydroxypropylated normal corn starch through optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), etc. It is confirmed that the composite modification can reduce starch retrogradation and improve crystallization stability, and has certain advantages in food gel applications. However, the existing technology still has certain limitations: Firstly, the modification combination is limited to "cross-linking + etherification", and does not involve "esterification cross-linking", which cannot further optimize the viscosity of starch and the stability of paste under wide conditions through the introduction of ester groups. Experimental data shows that the modified starch still has problems such as easy destruction of crystalline structure and decrease of molecular order in high temperature (above 60℃) or extreme pH environment.

[0006] Secondly, the study uses corn starch as raw material and does not design modification process for the structural characteristics of cassava starch (such as the ratio of amylose to amylopectin and particle morphology). Cassava starch has more extensive sources, better gelatinization characteristics, and higher application demand in the fields of food and medicine.

[0007] Thirdly, the study does not precisely control the content of key groups (such as hydroxypropyl and acetyl) of modified starch, resulting in large fluctuations in product performance and difficulty in meeting the consistency requirements of industrial production.

[0008] In summary, the existing modification technology (including single modification and existing composite modification scheme) still cannot solve the core problems of starch viscosity bottleneck, poor paste stability, insufficient comprehensive performance, and high cost. Therefore, developing a double cross-linking composite modification technology using cassava starch as raw material, combining etherification cross-linking and esterification cross-linking, and precisely controlling the reaction conditions and group content to achieve comprehensive improvement of starch performance has become an urgent breakthrough direction in the field of starch modification. SUMMARY

[0009] The purpose of the present application is to provide a preparation method of double cross-linking composite modified starch to solve the problems mentioned in the prior art in the background art.

[0010] To achieve the above-mentioned purpose, the present application provides the following technical scheme: a preparation method of double cross-linking composite modified starch, comprising the following steps: (1) Starch milk preparation and pretreatment: edible cassava starch is mixed with deionized water to prepare a starch milk with a mass fraction of 40%; 13% of anhydrous sodium sulfate based on the dry weight of starch is added to the starch milk as an expansion inhibitor, and 3.0% of a sodium hydroxide solution is added to adjust the pH value of the starch milk to 11.20 to build an alkaline environment required for etherification crosslinking reaction; (2) Etherification crosslinking reaction: 0.013% of chemically pure sodium trimetaphosphate based on the dry weight of starch is added to the starch milk pretreated in step (1), and after stirring to complete dispersion, 13% of analytical pure propylene oxide based on the dry weight of starch is added; the reaction system is warmed to 43-45℃ and kept for 18h to allow the etherification crosslinking to proceed fully; after the reaction is completed, the pH value of the system is adjusted to 6.0 with 10% dilute hydrochloric acid solution to terminate the reaction, and then the starch milk is washed with circulating water, with the Bémodé of the starch milk controlled between 24-27 to remove unreacted reagents and impurities; (3) Esterification crosslinking reaction: 3.0% of a sodium hydroxide solution is added to the starch milk washed in step (2) to adjust the pH value to 8.5-9.2; mixed anhydride is slowly added to the starch milk, stirring while adding and maintaining the pH value of the system at 8.5-9.2 with 3.0% of a sodium hydroxide solution throughout the process, and after addition is completed, the reaction is kept for 1h; after the reaction is completed, the pH value of the system is adjusted to 5.5-6.0 with 10% dilute hydrochloric acid solution to terminate the esterification crosslinking reaction; (4) Washing and drying treatment: the starch milk after neutralization in step (3) is subjected to deionized water cyclone washing to remove by-products and residual reagents generated in the reaction; then the starch milk is placed in a vacuum drying oven and dried at a temperature of 60℃ and a vacuum degree of 133 Pa for 6h to obtain a double-crosslinked composite modified starch.

[0011] Further, in step (1), the starch milk is stirred at a speed of 300 r / min for 30min with a mechanical stirrer to ensure that the edible cassava starch is uniformly dispersed in the deionized water and to avoid agglomeration of the starch.

[0012] Further, in step (2), before adding sodium trimetaphosphate, it is first dissolved in 5-10 mL of deionized water to prepare a solution, which is then slowly dripped into the starch milk to improve the uniformity of contact between sodium trimetaphosphate and starch molecules and to avoid excessive crosslinking in local areas.

[0013] Further, in step (2), a sealed stirring device is used for the addition of propylene oxide, with a stirring speed of 500 r / min, and inert gas (such as nitrogen) is introduced into the top of the reaction system to prevent insufficient amount of propylene oxide due to volatilization and to ensure the efficiency of etherification crosslinking reaction.

[0014] Further, the circulating water washing process in step (2) lasts for 2 hours, the deionized water is replaced every 30 minutes, and the conductivity of the washing water is detected after each replacement until the conductivity is less than or equal to 50 muS / cm, so as to ensure that the unreacted sodium trimetaphosphate, propylene oxide and generated sodium chloride are completely removed.

[0015] Further, the dropping speed of the mixed acid anhydride in step (3) is controlled to be 1-2 drops per second, the stirring speed is 400 r / min, and the temperature of the starch milk is maintained at 30-32 DEG C through a constant temperature water bath, so as to avoid decomposition of the acid anhydride due to local high temperature and affect the esterification and crosslinking effect.

[0016] Further, the flow rate of the deionized water for cyclone washing in step (4) is controlled to be 1.5-2.0 L / min, and the washing times are 3-4 times, and after each washing, the starch milk is placed for 10 min, and then the upper clear liquid is discharged after the starch milk is layered, so as to further reduce the residue of by-products.

[0017] Further, the double-crosslinked compound modified starch after drying in step (4) needs to be crushed and passed through an 80-mesh sieve.

[0018] Further, the hydroxypropyl content of the starch is determined by spectrophotometry, and the acetyl content is determined by acid-base titration; under the conditions of 25-60 DEG C and pH 3-9, the 5% mass fraction of the paste needs to be placed for 24 h without layering and precipitation, and the peak viscosity determined by the Brabender viscometer (E type) is greater than or equal to 1500 BU.

[0019] Further, when the starch is used in the food industry, the addition amount is 0.5-3% of the total mass of the food; when used in the papermaking industry for surface sizing, a starch solution with a mass fraction of 8-12% is prepared, and the sizing amount is 1.5-3.0 g / m 2 ; when used in the pharmaceutical industry as a drug binder, the addition amount is 5-15% of the total mass of the drug powder.

[0020] The double-crosslinked compound modification technology of "etherification crosslinking + esterification crosslinking" in the application precisely optimizes the process parameters and the raw material ratio, and solves the core defects of the natural starch and the existing modification technology in the background technology, and the following beneficial effects are achieved: 1. Breakthrough the bottleneck of single modification technology, realize the synergistic improvement of viscosity and stability: The invention combines ether cross-linking (introducing hydroxypropyl) and ester cross-linking (introducing acetyl) to form a double cross-linking synergistic structure, which not only overcomes the problem of insufficient long-term stability of single ether cross-linking paste, but also makes up for the defect of limited viscosity improvement of single ester cross-linking. The peak viscosity of the product is ≥1500 BU, which is much higher than that of single modified starch (ether cross-linking 831 BU, ester cross-linking 986 BU), and the 5% mass fraction paste has no layering and no precipitation after standing for 24 h under the wide conditions of 25-60℃ and pH 3-9, solving the problems of poor temperature resistance and acid resistance of natural starch, easy layering of paste, and meeting the requirements of high-end applications for the comprehensive performance of starch.

[0021] 2. Good raw material adaptability, widening application scenarios: The invention selects edible cassava starch with more extensive sources and more excellent gelatinization characteristics as raw material, and designs the modification process specifically to adapt to the straight / branched starch ratio and particle morphology of cassava starch. Compared with the corn starch used in the comparative document (Ratnayake 2008), the product is more easily dispersed and has stronger adaptability in food thickening, pharmaceutical excipients and other scenarios, further expanding the application range of modified starch in food, papermaking, medicine, textile and other fields.

[0022] 3. Controlling group content to ensure consistent product performance: The invention specifically limits the hydroxypropyl content to 4.8-5.6% and the acetyl content to 2.2-2.5%, and realizes precise control of the modification degree by optimizing key parameters such as reaction pH value, temperature, and raw material dosage, solving the product performance fluctuation problem caused by not controlling the group content in existing composite modification technology (such as the comparative document), ensuring stable product performance in each batch in industrial production, and meeting the stringent requirements of high-end manufacturing for raw material consistency.

[0023] 4. Process optimization reduces cost and increases efficiency, and meets environmental protection: The invention avoids the problem of excessive use of chemical reagents in single modification technology by reasonably proportioning the amounts of anhydrous sodium sulfate, sodium trimetaphosphate, propylene oxide and other reagents; at the same time, the washing process (circulating water washing + cyclone washing) is optimized, combined with conductivity and Baume degree double monitoring, to efficiently remove impurities and residual reagents, reduce wastewater discharge, and reduce environmental pressure. The whole process is controllable and repeatable, does not require complex equipment, reduces production cost, and improves product market competitiveness.

[0024] 5. Strong application adaptability, meeting the individual needs of multiple industries: The product of the application optimizes performance according to different application scenarios, and can be used as a thickening agent and a stabilizer in the food industry to adapt to products such as yogurt, baked food and sauce, prevent whey separation and prolong shelf life; in the papermaking industry, the strength and printing suitability of paper can be improved, and the retention rate of fine fibers can be improved; in the pharmaceutical industry, it can be used as an adhesive and a disintegrating agent to ensure the stability and bioavailability of drug preparations; in the textile industry, the wear resistance of yarn and the wrinkle resistance of fabric can be improved, and the individualized functional requirements of modified starch in various industries can be met, and the application prospect is broad. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The process flow chart of the application is shown in the figure. DETAILED DESCRIPTION

[0026] The preparation method of the double-crosslinked composite modified starch according to the application is described in detail below in combination with specific examples, and the examples are only used to explain the application and do not limit the protection scope of the application. It should be noted that the experimental methods not specified in the following examples are performed according to the conventional laboratory operation specifications or the instructions provided by the reagent / equipment manufacturer; the reagents used are commercially available products and meet the corresponding purity standards.

[0027] I. Experimental materials and equipment (I) Experimental materials 1. Edible cassava starch: conforms to the GB / T 29343-2012 Edible Cassava Starch standard, purity 99.2%, moisture content 12.5%, uniform particles and no odor, purchased from the Laos Goldman Starch Production Factory, and dried at 80℃ for 2h (to remove surface adsorbed water) before use, and sealed after cooling; 2. Deionized water: self-made by ion exchange resin method in the laboratory, conforms to the GB / T 6682-2008 Water Specifications and Test Methods for Analytical Laboratories first-class water standard, resistivity 18.5 MΩ·cm at 25℃, filtered through a 0.22μm microporous filter before use (to remove small particulate matter); 3. Anhydrous sodium sulfate (sodium sulfate): analytical pure (AR), purity 99.5%, purchased from the National Pharmaceutical Group Chemical Reagent Co., Ltd., dried at 105℃ for 4h before use (to remove crystal water), and cooled in a desiccator; 4. Sodium hydroxide: analytical pure (AR), purity 96.5%, flaky solid, purchased from Tianjin Kemio Chemical Reagent Co., Ltd., prepared into a 3.0% (mass fraction) aqueous solution, and used immediately (to avoid the absorption of CO2 in the air to form sodium carbonate); 5. Sodium trimetaphosphate: chemical pure (CP), content 66.2%, water insoluble 0.8%, arsenic content 0.0002%, fluoride 0.002%, heavy metal 0.0008%, pH 6.5, purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., and sieved through a 100-mesh screen before use (to remove lumps); 6. Propylene oxide: analytical pure (AR), purity 99.3%, density 0.832 g / cm 3 , boiling point 34.5℃, purchased from Tianjin Damo Chemical Reagent Factory, and stored in a 4℃ refrigerator to prevent volatilization, and taken out 30 min in advance to room temperature before use; 7. Mixed acid anhydride components: Acetic anhydride: analytical pure (AR), purity 99.6%, colorless transparent liquid, purchased from Shanghai Maikelin Biochemical Technology Co., Ltd.; Adipic acid: analytical pure (AR), purity 99.2%, white crystalline powder, purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd.; Butane diacid anhydride: analytical pure (AR), purity 99.1%, white needle-shaped crystal, purchased from Shanghai Yuan Ye Biological Technology Co., Ltd.; 8. Hydrochloric acid: analytical pure (AR), mass fraction 37%, purchased from Tianjin Kemio Chemical Reagent Co., Ltd., and prepared into a 10% (mass fraction) dilute hydrochloric acid solution, and calibrated with a pH meter to ensure the accuracy of neutralization; 9. Inert gas (nitrogen): purity 99.99%, purchased from Gansu Local Gas Company, used for sealing protection during the addition of propylene oxide.

[0028] (II) Test equipment 1. Electronic balance: model FA2004B, manufacturer Shanghai Precision Scientific Instrument Co., Ltd., accuracy 0.0001 g, maximum weighing 200 g, used for reagent weighing.

[0029] 2. Digital constant temperature water bath: model HH-6, manufacturer Jiangsu Jintan Ronghua Instrument Manufacturing Co., Ltd., temperature control range room temperature-100℃, accuracy ±0.1℃, used for reaction temperature control.

[0030] 3. Mechanical stirrer: model JJ-1, manufacturer Changzhou Guohua Electrical Appliance Co., Ltd., speed 0-3000 r / min, with polytetrafluoroethylene stirring paddle, used for material mixing.

[0031] 4. pH meter: model Leici PHS-3C, manufacturer Shanghai Yirei Scientific Instrument Co., Ltd., measurement range 0-14 pH, accuracy ±0.01 pH, real-time monitoring of reaction system pH.

[0032] 5. Circulating water washing device: self-made (including circulating pump, filter, and liquid storage tank), circulating pump flow rate 2 L / min, filter pore size 0.45 μm, used for starch milk washing.

[0033] 6. Vacuum drying oven: Model DZF-6050, manufacturer Shanghai Yiheng Scientific Instruments Co., Ltd., temperature control range room temperature-250 °C, vacuum degree ≤133 Pa, used for starch drying.

[0034] 7. Baume meter: Model BM-1, manufacturer Shanghai Yueping Scientific Instruments Co., Ltd., measurement range 0-40 Bé, accuracy ±0.1 Bé, monitoring starch milk concentration.

[0035] 8. Conductivity meter: Model DDS-307A, manufacturer Shanghai Rayleigh Instruments Factory, measurement range 0-100 μS / cm, accuracy ±1%, detecting washing water purity.

[0036] 9. Brabender viscometer: Model E type, manufacturer Germany Brabender Company, measurement range 0-2000 BU, used for starch viscosity test.

[0037] 10. Pulverizer: Model FW100, manufacturer Tianjin Test Instrument Co., Ltd., adjustable pulverizing particle size, used for dried starch pulverization.

[0038] 11. Standard test sieve: Model 80 mesh, manufacturer Shanghai Dongxing Building Material Test Equipment Co., Ltd., sieve hole size 180 μm, used for starch particle size control.

[0039] II. Examples Step 1: Starch milk preparation and pretreatment 1. Accurately weigh 400.0 g of pretreated edible cassava starch (dry basis mass) with an electronic balance, and slowly pour into a 2000 mL three-necked flask containing 600.0 g of deionized water (one side of the three-necked flask is connected to a nitrogen gas conduit, one side is connected to a pH electrode, and the middle is connected to a stirring paddle); 2. Turn on the mechanical stirrer and set the speed to 300 r / min, stir for 30 min, and scrape the inner wall of the flask with a glass rod every 5 min during the stirring (to prevent starch from adhering and caking), until a uniform 40% (mass fraction) starch milk without particles is formed; 3. Add 52.0 g of anhydrous sodium sulfate (13% of the dry starch mass) to the starch milk, continue stirring for 10 min, and make sure that the sodium sulfate is completely dissolved; 4. Open the nitrogen cylinder and adjust the nitrogen flow to 50 mL / min (slowly introduce to avoid splashing of the starch milk), while slowly adding 3.0% sodium hydroxide solution with a pipette at a rate of 1 drop / s, and observe the pH meter reading while adding. Stop adding when the pH value stabilizes at 11.20 (the addition process takes about 15-20 min, and avoid local over-alkaline damage to the starch molecules); 5. Maintain nitrogen protection and 300 r / min stirring, continue stirring for 20 min to fully activate the starch milk, and prepare for use.

[0040] Step 2: Etherification crosslinking reaction 1. Weigh 0.52 g of sodium trimetaphosphate (0.013% of the dry starch basis) into 5 mL of deionized water to dissolve (prepare a 10.4% aqueous solution to avoid uneven dispersion caused by direct addition of solids), and slowly add it to the activated starch milk in step 1 at a stirring speed of 300 r / min for 15 min; 2. Close the nitrogen, replace the middle port of the three-necked flask with a constant pressure dropping funnel, add 52.0 mL of propylene oxide (13% of the dry starch basis) to the funnel, seal the funnel port, reopen the nitrogen (adjust the flow to 30 mL / min), slowly open the dropping funnel piston, and add propylene oxide to the starch milk at a rate of 2 mL / min. Adjust the stirring speed to 500 r / min during the addition process to ensure that the propylene oxide and starch milk are fully mixed and reduce volatilization; 3. After the addition is complete, seal all the interfaces of the three-necked flask, place it in a digital constant temperature water bath, set the water bath temperature to 44°C (temperature control accuracy ±0.1°C), and start the timer for 18 h. Observe the water bath temperature and stirring state every 2 h during the reaction period to prevent the stirring paddle from being stuck and causing uneven reaction; 4. After 18 h of reaction, close the water bath and stirrer, stop the nitrogen flow, and slowly add 10% hydrochloric acid solution to the reaction system at a rate of 1 drop / s, while monitoring the pH value in real time with a pH meter. Stop adding when the pH value drops to 6.0 (the neutralization process takes about 10 min to avoid local over-acidification); 5. Transfer the neutralized starch milk to the storage tank of the circulating water washing device, start the circulating pump (flow rate 2 L / min), and use deionized water for circulating water washing. Every 30 min, use a Béometer to detect the Bé of the starch milk, and use an electric conductivity meter to detect the conductivity of the washing water (to ensure that impurities are removed). When the Bé of the starch milk stabilizes at 24-27 Bé and the conductivity of the washing water is ≤50 μS / cm, stop washing (the washing time is about 2 h, and the deionized water is replaced 3 times during this period).

[0041] Step 3: Esterification crosslinking reaction 1. Transfer the starch milk after step 2 into a 2000 mL three-necked flask, start stirring (300 r / min), adjust the pH value to 8.8 with 3.0% sodium hydroxide solution (1 drop / s, maintain for 5 min after pH stabilization); 2. Prepare mixed anhydride: weigh 6.5 g of acetic anhydride, 0.08 g of adipic acid, and 0.5 g of succinic anhydride into a 100 mL round-bottom flask, install a reflux condenser, place the round-bottom flask in a 80°C constant-temperature water bath, start magnetic stirring (400 r / min), heat for 30 min until the solids are completely dissolved (forming a colorless transparent liquid), maintain the water bath temperature at 80°C (to prevent crystallization due to cooling); 3. Place the three-necked flask in a 30°C constant-temperature water bath (maintain the starch milk temperature at 30-32°C to avoid decomposition of the anhydride), slowly drop the prepared mixed anhydride into the starch milk through a constant-pressure dropping funnel at a rate of 1-2 drops / s, and simultaneously supplement with 3.0% sodium hydroxide solution in real time (automatic dropping device, detect the pH value every 10 min), to ensure that the pH value of the system is always maintained at 8.5-9.2; 4. After the mixed anhydride is completely added, maintain the 30°C water bath and 400 r / min stirring, and react for 1 h, take samples every 15 min during the reaction to observe the state of the starch milk (it should be a uniform viscous liquid without stratification or precipitation); 5. After 1 h of reaction, neutralize the pH value of the system to 5.5-6.0 with 10% dilute hydrochloric acid solution (1 drop / s, stop after pH stabilization), to terminate the esterification crosslinking reaction.

[0042] Step 4: Washing, drying, and product post-processing 1. Transfer the starch milk after neutralization in step 3 into a circulating water washing device again, start the cyclone washing mode (deionized water flow rate 1.8 L / min), wash for 3 times, and after each washing, stand for 10 min, and discharge the supernatant (to remove byproducts such as sodium acetate and sodium succinate generated during the reaction), and after the last washing, detect the conductivity of the starch milk to be ≤30 μS / cm (to ensure that the residual reagents are completely removed); 2. Transfer the washed starch milk onto the tray of a vacuum drying oven (thickness controlled to be 1-2 cm, to facilitate uniform drying), close the oven door, set the drying temperature to 60°C, start the vacuum pump, maintain the vacuum degree at 133 Pa, and dry for 6 h; 3. After drying, close the vacuum pump and heating device, take out the starch block after the temperature in the drying oven decreases to room temperature, crush it in a crusher (crushing speed 1500 r / min), pass the crushed product through an 80-mesh standard inspection sieve, collect the undersize product (particle size 80-120 μm), which is the double-crosslinked composite modified starch product, seal and package, and store in a desiccator.

[0043] III. Performance testing and result verification (I) Testing method 1. Hydroxypropyl content determination: According to GB / T 40998-2021 "Starch-based plastics Determination of hydroxypropyl content Spectrophotometric method", mix 0.05 g of modified starch sample with 25 mL of 1N sulfuric acid, dissolve in boiling water bath and dilute, add indanone reagent and react, measure the absorbance at 590 nm wavelength, and calculate the hydroxypropyl content by standard curve; 2. Acetyl content determination: Use acid-base titration method, take 2.0 g of modified starch sample, add 50 mL of 0.5 mol / L sodium hydroxide solution, reflux in boiling water bath for 1 h, cool and titrate the remaining alkali with 0.5 mol / L hydrochloric acid standard solution, calculate the acetyl content according to the consumption of hydrochloric acid; 3. Viscosity determination: Use Brabender viscometer (E type), take 8.0 g of modified starch sample (dry basis), add 144 mL of deionized water, and determine the peak viscosity according to the instrument operating procedure (heating rate 1.5℃ / min, from 30℃ to 95℃, holding for 30 min); 4. Paste stability determination: Prepare 5% (mass fraction) paste of modified starch, place in 25℃, 40℃, 60℃ constant temperature water bath, and adjust the paste pH value to 3, 5, 7, 9 respectively, observe the layering and precipitation after 24 h of standing, and record the stability grade (no layering and precipitation is excellent, slight layering is good, obvious precipitation is poor).

[0044] (II) Testing results The double-crosslinked composite modified starch product prepared in this example has the following performance indicators: 1. Hydroxypropyl content: 5.2% (within the design range of 4.8-5.6%); 2. Acetyl content: 2.35% (within the design range of 2.2-2.5%); 3. Peak viscosity: 1580 BU (much higher than the 831 BU of single ether crosslinked starch and the 986 BU of single ester crosslinked starch, breaking through the viscosity bottleneck); 4. Paste stability: under the conditions of 25-60℃ and pH 3-9, the paste remains uniform and transparent without layering and precipitation (stability grade is excellent, solving the problem of poor paste stability in the prior art).

[0045] IV. Matters needing attention and key process control points 1. Safety operation of propylene oxide: propylene oxide is volatile and flammable, operation should be carried out in a fume hood, avoid open flame, must be sealed and inert gas protection when dropping, prevent volatilization from causing insufficient dosage or causing safety accidents; 2. pH control accuracy: the etherification cross-linking reaction pH needs to be strictly controlled at 11.20±0.05, and the esterification cross-linking reaction pH needs to be maintained at 8.5-9.2, and a large deviation will lead to insufficient or excessive cross-linking, affecting product performance; 3. Temperature control: etherification cross-linking reaction temperature 43-45℃ (too high will cause a large amount of propylene oxide to volatilize, and too low will slow down the reaction rate), esterification cross-linking reaction temperature 30-32℃ (too high will cause acid anhydride to decompose, and too low will reduce esterification efficiency); 4. Verification of washing effect: during the washing process, both Baumé degree and conductivity need to be monitored, only the Baumé degree meets the standard, there may be residual soluble salt, and the conductivity ≤50 μS / cm can ensure that impurities are completely removed, avoiding affecting subsequent application (such as safety in food field, purity requirement in pharmaceutical field).

[0046] Through the above specific embodiments, the double-cross-linked composite modified starch with the hydroxypropyl content and acetyl content meeting the design requirements, and high viscosity and good paste stability can be stably prepared, the method has detailed operation steps and controllable parameters, and can meet the needs of industrial scale production.

Claims

1. A process for the preparation of a double cross-linked complex modified starch, characterized in that, The method comprises the following steps: (1) starch milk preparation and pretreatment: mixing edible cassava starch and deionized water to prepare starch milk with a mass fraction of 40%; adding 13% anhydrous sodium sulfate based on the dry weight of starch as an expansion inhibitor to the starch milk, and then adding 3.0% sodium hydroxide solution to adjust the pH value of the starch milk to 11.20 to build an alkaline environment required for etherification cross-linking reaction; (2) etherification cross-linking reaction: adding 0.013% chemically pure sodium trimetaphosphate based on the dry weight of starch to the starch milk pretreated in step (1), stirring until completely dispersed, and then adding 13% analytical pure propylene oxide based on the dry weight of starch; the reaction system is heated to 43-45℃ and kept for 18h to fully carry out etherification cross-linking; after the reaction is completed, the pH value of the system is adjusted to 6.0 with 10% dilute hydrochloric acid solution to terminate the reaction, and then the starch milk is washed with circulating water, the starch milk is controlled to have a Bémodé of 24-27, and unreacted reagents and impurities are removed; (3) esterification cross-linking reaction: adding 3.0% sodium hydroxide solution to the starch milk washed in step (2) to adjust the pH value to 8.5-9.2; slowly adding mixed anhydride to the starch milk, stirring while adding, and maintaining the pH value of the system at 8.5-9.2 with 3.0% sodium hydroxide solution throughout the process, and keeping for 1h after adding; after the reaction is completed, the pH value of the system is adjusted to 5.5-6.0 with 10% dilute hydrochloric acid solution to terminate the esterification cross-linking reaction; (4) washing and drying treatment: the starch milk after neutralization in step (3) is subjected to deionized water cyclone washing to remove by-products and residual reagents generated in the reaction; then the starch milk is placed in a vacuum drying oven and dried at a temperature of 60℃ and a vacuum degree of 133 Pa for 6h to obtain double-crosslinked modified starch.

2. The production method according to claim 1, wherein In step (1), the starch milk is stirred at a speed of 300 r / min for 30 min during preparation to ensure that the edible cassava starch is uniformly dispersed in the deionized water and to avoid agglomeration of the starch.

3. The production method according to claim 1, wherein In step (2), before adding sodium trimetaphosphate, the sodium trimetaphosphate is first dissolved in 5-10 mL deionized water to prepare a solution, and then slowly dripped into the starch milk to improve the uniformity of contact between sodium trimetaphosphate and starch molecules and to avoid excessive cross-linking in local areas.

4. The production method according to claim 1, wherein In step (2), a sealed stirring device is used for adding propylene oxide, and the stirring speed is 500 r / min, while inert gas (such as nitrogen) is filled above the reaction system to prevent insufficient amount of propylene oxide due to volatilization and to ensure the efficiency of etherification cross-linking reaction.

5. The production method according to claim 1, wherein In step (2), the circulating water washing process lasts for 2 hours, and the deionized water is replaced every 30 minutes, and the conductivity of the washing water is detected after each replacement until the conductivity is ≤50 μS / cm, to ensure that unreacted sodium trimetaphosphate, propylene oxide and generated sodium chloride are completely removed.

6. The production method according to claim 1, wherein In step (3), the dropping speed of mixed anhydride is controlled to be 1-2 drops per second, the stirring speed is 400 r / min, and the temperature of the starch milk is maintained at 30-32℃ by using a constant temperature water bath to avoid decomposition of anhydride due to local high temperature and to affect the esterification cross-linking effect.

7. The production method according to claim 1, wherein The flow rate of deionized water in the cyclone washing in step (4) is controlled at 1.5-2.0 L / min, and the washing is performed for 3-4 times, and after each washing, the starch milk is allowed to stand for 10 min, and then the supernatant is discharged after the starch milk is stratified, so as to further reduce the residue of by-products.

8. The production method according to claim 1, wherein The double-crosslinked composite modified starch after drying in step (4) is subjected to crushing treatment and then passed through an 80-mesh screen.

9. Double cross-linked complex modified starch prepared by the process according to any one of claims 1 to 8, characterized in that, The hydroxypropyl content of the starch is determined by spectrophotometry, and the acetyl content is determined by acid-base titration; under the conditions of 25-60℃ and pH 3-9, a 5% mass fraction of the paste liquid needs to be allowed to stand for 24 h without stratification and precipitation, and the peak viscosity determined by a Brabender viscometer (E type) is ≥1500 BU.

10. Double-cross-linked complex modified starch according to claim 9, characterized in that The starch is added in an amount of 0.5-3% of the total mass of food when used in food industry; when used in papermaking industry for surface sizing, the starch is prepared into a starch solution with a mass fraction of 8-12%, and the sizing amount is 1.5-3.0 g / m 2 2; when used in pharmaceutical industry as a drug adhesive, the amount is 5-15% of the total mass of drug powder.

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