Method for preparing starch citrate through solvent-free continuous stirring

Abstract

The invention belongs to the technical field of starch modification, and discloses a method for preparing starch citrate through solvent-free continuous stirring, which comprises the following steps: step a, a drying stage: firstly, crushing citric acid particles, sieving for the first time, uniformly mixing the citric acid powder sieved for the first time with starch, drying at high temperature, and cooling to room temperature, after crushing, sieving for the second time to obtain a citric acid-starch mixture; and b, a continuous stirring reaction stage: stirring the citric acid-starch mixture at high temperature while reacting to obtain starch citrate, cooling, washing with ethanol, drying, crushing, and sieving for the third time to obtain the starch citrate. The method does not use any solvent and catalyst, does not generate waste water and waste gas, is green and environment-friendly, and is short in preparation time and high in efficiency; by means of high-temperature drying pretreatment, the caking problem caused when stirring and reacting are conducted in the follow-up process is greatly solved, stirring and reacting are conducted in the follow-up process, the contact opportunity of starch and citric acid is increased, and the content yield of resistant starch is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of starch modification and relates to a method for preparing starch citrate by continuous stirring without solvent. Background Art

[0002] Starch, as the main component of grains, is a renewable resource with abundant reserves in nature and the main energy source for human diet. However, due to the special molecular structure of starch, its properties are insufficient. For example, starch is easy to decompose at high temperatures, easy to precipitate at low temperatures, has poor thermal stability, is easy to age, and has poor processing performance. These defects limit the application of starch in production and processing. In order to make starch meet specific needs and increase the added value of natural starch, physical, chemical or enzymatic methods are often used to process starch to improve its natural properties and meet more application needs.

[0003] Starch citrate, as a chemically modified starch, refers to the product obtained by the esterification reaction of starch and citric acid under certain conditions. It can hinder the gelatinization and swelling of starch molecules and inhibit enzymatic hydrolysis, thus having strong resistance to digestion. Compared with other modified starches, citric acid is cheap and safe and non-toxic. Resistant starch not only has unique food processing properties, but also has physiological functions such as preventing intestinal diseases, regulating blood sugar levels, promoting mineral absorption, and controlling body weight. Therefore, starch citrate can be widely used in food and medicine-related fields as a functional food substrate.

[0004] At present, the synthesis method of starch citrate is mainly extended from the method reported by Klaushofer et al., and its technical characteristics are: usually water or ethanol is used as a dispersant, starch and citric acid solution are mixed evenly, the mixture is treated for a long time (28h to 60h) (the treatment includes standing, drying, crushing, etc.), and then a high-temperature reaction is carried out at a low moisture content (<10%). The high-temperature reaction is generally carried out in a static state (oven, etc.)

[0005] The invention patent with publication number CN111234042A discloses a method for preparing citric acid modified starch with anti-digestion properties. Water is used as a dispersant, starch is added to a citric acid aqueous solution for reaction for 10 to 40 hours, dried and crushed, and then subjected to infrared radiation. After that, a dry heat reaction is carried out in an oven to prepare citric acid modified starch, and finally a product with a degree of substitution of 0.156 is obtained. The preparation time of the modified starch obtained by this method is long and the production process is complicated. In addition, the uniformity of the starch treated with infrared radiation is poor.

[0006] The invention patent with publication number CN110386989A discloses "a method for preparing low-viscosity and high-resistant starch". Water is used as a dispersant, and cassava starch milk is activated with glycine as an activator and sodium hydroxide to adjust the pH to 9. After activation, a citric acid aqueous solution is added, and the pH is adjusted to 3 with sodium hydroxide. The mixture is stirred at 40°C for 30 minutes, allowed to stand at room temperature for 4 hours, and then dried in a blower (50°C, 10 hours). The mixture is crushed and sieved, and esterified in a 120°C blower drying oven for 5 hours to prepare resistant starch. The resistant starch content of the obtained sample is as high as 86%.

[0007] The invention patent with publication number CN110981981A discloses "a starch citric acid ester, its preparation method and its application in corrosion and scale inhibition". The starch is pretreated by mechanical activation solid phase method (100-300r / min, 30-80℃, 10-60min); then water is used as a dispersant, the mechanically activated starch, citric acid and catalyst are mixed evenly in water, and dried after standing (2-8h); the dried mixture is subjected to secondary mechanical activation grinding reaction (30-100℃, 10-60min) to obtain starch citric acid ester. The starch pretreatment time is more than 12h, and the degree of substitution of the final sample is 0.29. This method takes a long time to prepare starch citrate, and the preparation process is cumbersome, involving water and catalysts.

[0008] The invention patent with publication number CN110372802A discloses "a citrate starch and its preparation method", which uses subcritical basic equipment to prepare citrate starch. Starch, citric acid, distilled water and a catalyst are mixed, sealed and placed for 4 to 12 hours, and then subcritically extruded, dried, crushed and sieved to obtain citrate starch with a degree of substitution ≥ 0.01.

[0009] Ning Yuqi et al. (Ning Yuqi, Wang Jiwei, Hu Huayu, et al. Preparation and characterization of citric acid resistant starch [J]. Food Science and Technology, 2021, 46(05): 213-218.) used cassava starch as raw material, firstly treated the starch in a mechanical activation solidification reactor; then the mechanically activated starch was mixed with citric acid solution, and ethanol was used as a dispersant. After standing at room temperature (12h) and drying (45°C), starch citrate was prepared in an oven by dry heat method (120-150°C, 2-6h). Finally, a product with a degree of substitution of 0.225 and a resistant starch content of 64.61g / 100g was obtained.

[0010] Singh et al. (Singh H, Punia R, Ganesh A, et al. Modification of moth beanstarch using mixture of organic acids under dry heating[J]. 2019, 71(9-10):1900061.) reported the preparation of starch citrate by solvent-free dry heat reaction. Starch and citric acid were mixed in solid phase without the participation of solvent. After mixing evenly, the mixture was directly placed in an oven at 110°C for 8 hours to prepare starch citrate. The final sample had a maximum resistant starch content of 61.3%, which was 15.2% higher than the resistant starch content of natural starch (46.1%). Compared with the preparation process involving solvents, this method greatly shortens the preparation time, but the raw materials are in a static reaction, the materials are not fully mixed, and the reaction efficiency is low.

[0011] The above disclosed related technologies require water or ethanol as a dispersant during the entire preparation process, and the starch citrate preparation time is long, the reaction efficiency is low, and more waste liquid is generated, which is not conducive to actual industrial production. In addition, the above technologies have the disadvantages of high equipment requirements, high production costs, and possible environmental pollution. Summary of the invention

[0012] The invention aims at the technical problems that the existing preparation of starch citrate requires solvents and catalysts, takes a long time, and has low efficiency. The invention provides a method for preparing starch citrate without solvent and continuous stirring. The method does not use any solvent and catalyst, does not generate wastewater and waste gas, is environmentally friendly, has a short preparation time, and has high efficiency. The high-temperature drying pretreatment greatly reduces the agglomeration problem during subsequent stirring and reacting, shortens the starch pretreatment time, and subsequently adopts stirring while reacting to increase the contact opportunity between starch and citric acid, thereby improving the yield of resistant starch content.

[0013] To achieve the above object, the present invention adopts the following technical solutions:

[0014] The present invention provides a method for preparing starch citrate by solvent-free continuous stirring, comprising the following steps:

[0015] Step a, drying stage:

[0016] First, the citric acid particles are crushed and sieved for the first time, the citric acid powder after the first sieve is mixed evenly with starch, dried at 110-130° C. for 15-60 minutes, cooled to room temperature, crushed and sieved for the second time to obtain a citric acid starch mixture;

[0017] Step b, continuous stirring reaction stage:

[0018] The citric acid starch mixture is reacted at 110-150° C. and 100-200 rpm while being stirred for 2-7 hours to obtain starch citrate, which is then washed with 80% ethanol for 3-5 times after cooling, dried at 45° C., crushed, and sieved for the third time to obtain starch citrate.

[0019] In the above technical solution, the mesh size of the first screening in step a is 100 to 300 meshes.

[0020] The mass ratio of the citric acid powder to the starch after the first sieving in the above technical solution is 0.1-0.5:1.

[0021] In the above technical solution, the mesh size of the second screening in step a and the third screening in step b is 60 to 100 meshes.

[0022] The starch described in the above technical solution is one or more of wheat starch, corn starch and potato starch.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) The present invention uses citric acid as a modifier, which is safe and non-toxic, has mild preparation process conditions, is relatively safe, and has a short overall preparation time.

[0025] 2) The present invention does not use any solvent or catalyst during the reaction process, and directly mixes starch and citric acid powder in a solid phase for reaction, thus not generating waste water or waste gas, and is environmentally friendly.

[0026] 3) The present invention uses high-temperature drying pretreatment to greatly reduce the agglomeration problem during subsequent stirring and reaction, and also shortens the starch pretreatment time. The subsequent reaction and stirring are adopted to increase the contact opportunity between starch and citric acid, thereby improving the yield of resistant starch content. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the agglomeration phenomenon in the initial stage of the reaction and after the reaction in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The following examples are used to illustrate the present invention, but are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. The test methods in the following examples are conventional methods unless otherwise specified.

[0029] Example 1

[0030] After citric acid is crushed through a 200-mesh sieve, it is mixed evenly with wheat starch in a mass ratio of 0.3:1. Then 20g is placed in a beaker and placed in an oven at 130°C for 30 minutes. After cooling to room temperature, it is crushed through a 60-mesh sieve to obtain a citric acid starch mixture. The citric acid starch mixture is placed in a beaker and reacted at 130°C and 150rpm while stirring for 4.5 hours. After cooling, it is washed three times with 80% ethanol to remove the unreacted citric acid, dried at 45°C for 14 hours, and crushed through an 80-mesh sieve to obtain starch citrate.

[0031] Example 2

[0032] After citric acid is crushed through a 200-mesh sieve, it is mixed evenly with corn starch at a mass ratio of 0.3:1. Then 25g is placed in a beaker and placed in an oven at 130°C for 30 minutes. After cooling to room temperature, it is crushed through a 60-mesh sieve to obtain a citric acid starch mixture. The citric acid starch mixture is placed in a beaker and reacted at 130°C and 150rpm while stirring for 4 hours. After cooling, it is washed with 80% ethanol 3 times to remove the unreacted citric acid, dried at 45°C for 14 hours, and then crushed through an 80-mesh sieve to obtain starch citrate.

[0033] Example 3

[0034] This embodiment is basically the same as Embodiment 1, except that the mass ratio of citric acid to wheat starch is 0.2:1.

[0035] Example 4

[0036] This embodiment is substantially the same as embodiment 1, except that the reaction was carried out at 120° C. and 150 rpm with stirring for 4.5 h.

[0037] Example 5

[0038] This embodiment is substantially the same as embodiment 1, except that the reaction was carried out at 130° C. and 150 rpm with stirring for 3.5 h.

[0039] Example 6

[0040] This embodiment is basically the same as Embodiment 2, except that the mass ratio of citric acid to corn starch is 0.2:1.

[0041] Example 7

[0042] This embodiment is substantially the same as embodiment 2, except that the reaction was carried out at 120° C. and 150 rpm with stirring for 4 h.

[0043] Example 8

[0044] This embodiment is substantially the same as embodiment 2, except that the reaction was carried out at 130° C. and 150 rpm with stirring for 3 h.

[0045] Comparative Example 1

[0046] After citric acid was crushed and passed through a 200-mesh sieve, it was mixed evenly with wheat starch at a mass ratio of 0.3:1, and then 20 g was placed in a beaker and reacted in an oven at 130°C for 4.5 hours. After cooling, it was washed three times with 80% ethanol to remove the unreacted citric acid, dried at 45°C for 14 hours, and then crushed and passed through an 80-mesh sieve to obtain starch citrate.

[0047] The resistant starch content and whiteness of the starch citrate obtained in Examples 1 to 8 and Comparative Example 1 were measured. The digestibility of glucose was measured by using enzyme hydrolyzed starch, the glucose content was measured by 3,5-dinitrosalicylic acid method, and the L*, a*, and b* values ​​were measured by a colorimeter and the whiteness was calculated. The results are shown in Table 1.

[0048] Table 1 Resistant starch content and whiteness of starch citrate obtained in Examples 1 to 8 and Comparative Example 1

[0049] Example Resistant starch content, % Whiteness, % Example 1 71.43 83.77 Example 2 74.37 90.93 Example 3 60.71 89.52 Example 4 62.76 91.24 Example 5 67.99 84.82 Example 6 63.22 94.75 Example 7 65.74 97.06 Example 8 69.36 94.21 Comparative Example 1 57.78 88.69

[0050] In addition, in Comparative Example 1, after the citric acid particles were mixed with wheat starch, when the esterification reaction was carried out in a direct reaction mode, at the initial stage of the reaction (reaction time of about 10 min, see Figure 1 -a), due to the trace amount of water contained in the starch and citric acid in the mixed system, the reaction mixture shows a macroscopic phenomenon of agglomeration, and these agglomerates still exist after the reaction is completed (see Figure 1 -b). In Examples 1 to 8, starch and citric acid are subjected to high temperature pretreatment to remove a small amount of water in the raw materials themselves, and then the raw materials are crushed and sieved again after high temperature drying, which greatly reduces the agglomeration problem during the subsequent stirring and reaction, and shortens the starch pretreatment time. The subsequent reaction and stirring are adopted to increase the contact opportunity between starch and citric acid, thereby improving the yield of resistant starch content.

[0051] The embodiments described above are only preferred embodiments of the present invention and are only used to explain the present invention, not to limit the scope of implementation of the present invention. For those skilled in the art, other implementation methods can certainly be easily made by replacement or modification based on the technical contents disclosed in this specification. Therefore, all changes and improvements made on the principles of the present invention should be included in the scope of the patent application of the present invention.

Claims

1. A method for preparing starch citrate by solvent-free continuous stirring, characterized in that: The following steps are involved: Step a, drying stage: First, the citric acid particles are crushed and sieved for the first time, the citric acid powder after the first sieve is mixed evenly with starch, dried at 110-130° C. for 15-60 min, cooled to room temperature, crushed and sieved for the second time to obtain a citric acid starch mixture; Step b, continuous stirring reaction stage: The citric acid starch mixture is reacted at 110-150°C and 100-200 rpm while stirring for 2-7 hours to obtain starch citrate. After cooling, it is washed with 80% ethanol for 3-5 times, dried at 45°C, crushed and sieved for the third time to obtain starch citrate.

2. The method according to claim 1, characterized in that The mesh size of the first sieving in step a is 100 to 300 meshes.

3. The method according to claim 1, characterized in that The mass ratio of the citric acid powder to the starch after the first sieving is 0.1-0.5:

1.

4. The method according to claim 1, characterized in that: The mesh size of the sieves used in the second screening in step a and the third screening in step b is 60 to 100 meshes.

5. The method according to claim 1, characterized in that: The starch is one or more of wheat starch, corn starch and potato starch.

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