A method for improving the quality of hot-pressed starch films

Abstract

The application discloses a method for improving the quality of hot-pressed starch film and belongs to the technical field of starch film processing and preparation. The method for improving the quality of hot-pressed starch film is specifically as follows: a plasticizer is mixed with water, a hydrophilic colloid is added and stirred to be uniformly mixed, then starch is added and stirred to be uniformly mixed, a dough-like starch mixture is obtained, and the dough-like starch mixture is placed in a sealed bag and balanced in moisture content under the condition of 20-30 DEG C and 55-80% RH; the balanced starch mixture is evenly spread on a mold, and hot pressing is carried out on the mold in a hot pressing plate, and balancing is performed, so that the hot-pressed starch film is obtained. The thickness of the starch film prepared by the method is greatly reduced, the surface is more flat, and the quality is better, and the problems of the starch film prepared by the traditional hot pressing method, such as too large thickness and too many surface wrinkles, are solved; compared with the traditional hot pressing method for preparing the starch film, the process treatment process such as double-screw extrusion is omitted, the process is simple, the cost is easy to control, the operability is strong, the energy consumption is low, and the method is suitable for industrialized production.

Description

Technical Field

[0001] This invention relates to a method for improving the quality of hot-pressed starch films, belonging to the field of starch film processing and preparation technology. Background Technology

[0002] Traditional plastics are mainly made of structurally stable polymers such as polystyrene, polypropylene, and polyvinyl chloride. These traditional plastic products take 200 to 400 years to degrade, which can easily cause environmental pollution. Therefore, it is particularly important to develop alternative plastic products to alleviate ecological pollution.

[0003] Starch, with its low price, wide availability, ease of film formation, and pollution-free degradation, is considered a biomaterial with the greatest potential to replace petroleum-based plastics. In recent years, the main dry and wet processes for preparing starch films include casting, blow molding, and hot pressing. Casting, due to its simplicity and low cost, is currently the most commonly used method for preparing starch-based composite films in laboratories. Since no pressure is applied during the process, the resulting starch films have good integrity, high transparency, and good flexibility. However, it has high energy consumption and low production efficiency, making large-scale production impossible. In industrial production, high-level and high-efficiency preparation of starch-based composite films is required, such as through blow molding and injection molding. However, these processing methods require expensive equipment, large production lines, and excessively high investment costs, hindering effective small-batch production.

[0004] Compared with the two production methods mentioned above, the hot pressing method has the advantages of high pressure per unit area, simple operation, short time, low cost, and low labor intensity. It can be used not only for small-scale laboratory research, but also for large-scale continuous industrial production, thus enabling rapid transformation from laboratory research to industrial production applications.

[0005] However, most existing hot-pressing methods for preparing starch films involve mixing starch with plasticizers and other substances to form powdered or granular film-forming materials before hot pressing. For example, Chinese patent CN114773687A discloses a method for preparing a heat-sealable, biodegradable composite starch film. This method uses an extrusion hot-pressing process to prepare the composite starch film. The process involves first mixing the starch film-forming materials, then extruding the mixture into mixed granules using a twin-screw extruder, and finally hot-pressing the mixed granules into a film using a hot press. In this process, the film-forming materials have a low moisture content, and the high temperature and strong shear force of the twin-screw extruder must be used to break down and reconstruct the crystalline structure of the starch to obtain thermoplastic starch granules before using a hot press to produce the starch film.

[0006] For example, Chinese patent CN109438772B discloses a method for preparing an edible packaging film. This method involves mixing and stirring starch film-forming raw materials in a constant-temperature water bath, and then using a two-stage hot-pressing process to prepare a thermoplastic starch film. This process requires uniformly heating the starch film-forming raw materials in a constant-temperature water bath at 70-100℃ for at least 1 hour, which is more cumbersome than the laboratory casting method and does not effectively solve the problem of industrial application of the hot-pressing method.

[0007] Although existing technologies all employ hot pressing, they all require complex processing of the starch film-forming raw materials before hot pressing, resulting in long processing times and high energy consumption. However, if the starch film-forming raw materials are directly mixed and then hot pressed, the mixed raw materials are mostly in powder or granular form, which has high hardness, poor flowability under pressure, and difficulty in controlling the dispersion state, thus increasing the difficulty of product quality control.

[0008] Therefore, there is an urgent need to develop a one-step hot pressing production process that is simple to operate, low in cost, and suitable for industrial production, so as to improve the quality of hot-pressed starch films while enhancing product quality stability and production efficiency. Summary of the Invention

[0009] To address the problems existing in the prior art, this invention provides a method for improving the quality of hot-pressed starch films. This method, by adjusting the types and proportions of raw materials, forms a starch film with good fluidity, easy dispersion, and direct applicability to hot presses. This not only improves the quality of hot-pressed starch films but also simplifies the operation, eliminates intermediate raw material processing, simplifies the process, makes cost control easy, enhances operability, reduces energy consumption, and is suitable for industrial production.

[0010] The purpose of this invention is to provide a method for improving the quality of hot-pressed starch films, the method comprising:

[0011] (1) Mix the plasticizer with water, then add the hydrophilic colloid and stir until well mixed, then add the starch and stir until well mixed to obtain a dough-like starch mixture.

[0012] (2) Place the dough-like starch mixture obtained in step (1) in a sealed bag and equilibrate the moisture at 20-30℃ and 55-80%RH.

[0013] (3) After the hot press is preheated, the starch mixture after equilibrium in step (2) is evenly spread on the mold and placed in the hot press plate for hot pressing and equilibrium to obtain a hot-pressed starch film; wherein, the mass ratio of starch, hydrophilic colloid, water and glycerin in step (1) is 100:(3-5):(50-55):(30-40), and the total mass of water and glycerin accounts for 80-90% of the mass of starch.

[0014] In one embodiment, the starch in step (1) is a naturally sourced starch or a modified starch prepared by physical, chemical, or enzymatic methods; the natural starch includes one or more combinations of cassava starch, corn starch, potato starch, wheat starch, rice starch, and mung bean starch; the modified starch includes one or more combinations of hydroxypropyl starch, oxidized starch, acetic acid starch, oxidized hydroxypropyl starch, and hydroxypropyl distarch phosphate starch.

[0015] In one embodiment, the plasticizer in step (1) is one or more of glycerol, sorbitol, citric acid, urea, polyethylene glycol, maltitol, erythritol, xylitol, fructose, and trehalose.

[0016] In one embodiment, the hydrophilic colloid in step (1) is a high-viscosity hydrophilic colloid that is soluble in cold water, including one or more of gellan gum, xanthan gum, locust bean gum, and guar gum.

[0017] In one embodiment, the plasticizer in step (1) accounts for 3-5% of the dry weight of starch.

[0018] In one embodiment, the stirring rate in step (1) is 10,000 to 20,000 rpm.

[0019] In one embodiment, the mass ratio of starch, hydrophilic colloid, water and glycerin in step (1) is 100:4:(50-55):(30-35).

[0020] In one embodiment, the mass ratio of starch, hydrocolloid, water and glycerin in step (1) is 100:4:50:30.

[0021] In one embodiment, the mass ratio of starch, hydrocolloid, water and glycerin in step (1) is 100:4:55:30.

[0022] In one embodiment, the mass ratio of starch, hydrocolloid, water and glycerin in step (1) is 100:4:55:35.

[0023] In one embodiment, the preheating time in step (3) is 20 to 30 minutes.

[0024] In one embodiment, the parameters of the hot pressing in step (3) are: temperature 80-120°C, pressure 2-10 MPa, and time 4-12 min.

[0025] In one embodiment, the balancing step (3) refers to placing the hot-pressed starch film in a constant temperature and humidity chamber for balancing; the conditions are: 25-30℃, relative humidity of 50-55%, and time of 48-72 hours.

[0026] Another object of the present invention is to provide an application of the method described above in the preparation of hot-pressed starch films.

[0027] Beneficial effects of the present invention

[0028] (1) Compared with the direct hot-pressed starch film with no or little hydrophilic colloid, the hot-pressed starch film prepared by the present invention has a thickness of 0.05-0.25 mm, a tensile strength greater than 9 MPa, an elongation at break greater than 25%, a brightness of not less than 85, and a color difference of not more than 10. The thickness of the starch film is greatly reduced, the surface is smoother, and the quality is better, which solves the problems of excessive thickness and excessive surface wrinkles of starch film prepared by traditional hot pressing method.

[0029] (2) The starch film of the present invention is prepared directly by hot press. Compared with the traditional hot pressing method for starch film preparation, it eliminates the process of double helix extrusion and other processes. The process is simple, the cost is easy to control, the operability is strong, the energy consumption is low, and it is suitable for industrial production. Detailed Implementation

[0030] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0031] The following is the detection method involved in this invention:

[0032] 1. Thickness Measurement Method

[0033] The starch film was measured at random points using an electronic digital caliper. The thickness of the starch film was the average of 30 repeated measurements. This value was used to calculate the mechanical properties and other related properties of the starch film.

[0034] 2. Methods for determining mechanical properties

[0035] The mechanical properties of the starch film were determined using a physical property analyzer. The prepared composite film was cut into 60mm × 10mm (length × width) sample strips, with an initial clamping distance of 40mm and a probe moving speed of 8.3mm / s. The mechanical properties (tensile strength MPa and elongation at break %) of the starch film samples were tested, and the test was repeated 5 times to ensure the feasibility of the measurement results.

[0036]

[0037] In the formula, TS is the tensile strength (MPa), and F is the tensile strength. max The maximum force (N) that causes the sample to fracture; A is the sample area (mm²). 2 ).

[0038]

[0039] In the formula, E is the elongation at break (%), L is the test length of the sample (mm), and L′ is the length of the sample after stretching (mm).

[0040] 4. Determination of color difference in starch film

[0041] The luminance L and chromaticity parameters a and b of the composite film were measured using a portable colorimeter. During testing, a standard white plate (L* = 97.13, a* = -0.20, b = 0.38) was placed under the film. The total color difference was represented by ΔE. Five points were randomly selected on each film surface for measurement, and the average value was taken.

[0042] ΔE=(LL * ) 2 +(aa * ) 2 +(bb * ) 2

[0043] Wherein, L, a, and b represent the color parameter values ​​of the starch film sample, and L*, a*, and b* represent the color parameter values ​​of the white standard plate. The hot press involved in this embodiment is model TH-XC601-H, manufactured by Shenzhen Xutaiheng Technology Co., Ltd.

[0044] Example 1

[0045] A method for improving the quality of hot-pressed starch films, the method comprising:

[0046] (1) Take 15g of glycerol (30% of the starch mass) and 25g of water (50% of the starch mass) and mix them in a grinder. Add 2.0g of xanthan gum (4% of the starch mass) and dissolve it completely at 18000rpm. Then gradually add 50g of tapioca starch and mix well to obtain a starch mixture.

[0047] (2) Place the starch mixture obtained in step (1) in a sealed bag and equilibrate the moisture at 25°C and 60% RH.

[0048] (3) Preheat the hot press for 30 minutes, take 10g of the "dough-like" mixture after step (2) and spread it evenly on the mold, then put it into the hot press and prepare a starch-based composite film at 90℃, 6Mpa, 6min.

[0049] (4) Place the starch-based composite film prepared in step (4) in a constant temperature and humidity chamber (temperature 25℃, relative humidity 53%) for equilibration for 72 hours.

[0050] Example 2

[0051] The only difference from Example 1 is that the mass percentage of xanthan gum in step (1) is adjusted to 3% and 5% of the starch mass, respectively, while the other conditions remain the same as in Example 1, to obtain a starch-based composite film.

[0052] Example 3

[0053] The only difference from Example 1 is that the total mass ratio of glycerol and water in step (1) is adjusted to 85% (glycerol 30%, water 55%) and 90% (glycerol 35%, water 55%) of starch mass, while the rest remains the same as in Example 1, to obtain a starch-based composite film.

[0054] Example 3

[0055] The only difference from Example 1 is that the total mass ratio of glycerol and water in step (1) is adjusted to 85% of the starch mass (glycerol 30% and water 55%), while the rest remains the same as in Example 1, resulting in a starch-based composite film.

[0056] Comparative Example 1

[0057] The only difference from Example 1 is that step (1) of adding xanthan gum is omitted, while other conditions remain unchanged, resulting in a starch-based composite film.

[0058] Comparative Example 2

[0059] The only difference from Example 1 is that the mass percentage of xanthan gum in step (1) is adjusted to 2% and 7% of the starch mass, respectively, while the other conditions remain the same as in Example 1, to obtain a starch-based composite film.

[0060] Comparative Example 3

[0061] The only difference from Example 1 is that the total mass ratio of glycerol and water in step (1) is adjusted to 75% (glycerol 30%; water 45%) and 95% (glycerol 40%, 45%; water 55%, 50%) of starch mass, while other conditions remain unchanged, to obtain a starch-based composite film.

[0062] Comparative Example 4

[0063] The difference from Example 1 is that step (1) of adding xanthan gum is omitted, the mass of water is adjusted to 30% of starch, and the film-forming mixture is made into a powdered starch mixture to obtain a starch-based composite film.

[0064] Comparative Example 5

[0065] The difference from Example 1 is that the preparation method of the starch-based composite film is changed, but the proportions of each component, cassava starch, glycerol, and xanthan gum, remain the same as in Example 1. Specifically, cassava starch and xanthan gum are stirred evenly in a mixer at room temperature, and then glycerol is slowly added and mixed evenly to obtain a starch mixture. This mixture is placed in a sealed bag and equilibrated for 24 hours. Then, the equilibrated cassava starch mixture is extruded through a twin-screw extruder at a temperature of 130°C to prepare an extrudate. The thermoplastic starch masterbatch that has been granulated is then placed on a hot press mold (parameters: temperature 130°C, pressure 4 MPa, time 4 min) to obtain a thermoplastic starch composite film.

[0066] Results Analysis

[0067] 1. The performance of the starch-based composite films prepared in Examples 1-2 and Comparative Examples 1-2 was measured, and the results are shown in Table 1:

[0068] Table 1. Mechanical properties and color difference data of starch-based composite films

[0069]

[0070] As shown in Table 1, when a small amount of xanthan gum is added, the mixture system becomes too hard. During hot pressing, the material flow rate is low, and moisture and heat cannot quickly penetrate the starch, resulting in severely uneven moisture distribution on the surface and inside the starch film. This leads to significant differences in the degree of plasticization across different parts of the starch film, resulting in a thick starch film. The excessive thickness of the starch film in Comparative Example 1 results in a small sample area, making it impossible to cut 1cm × 6cm sample strips, thus preventing the acquisition of relevant mechanical property indicators. When the xanthan gum content is 3-5%, the starch film exhibits better performance. With increasing xanthan gum content, the tensile strength of the starch film gradually increases, while the elongation at break initially increases and then decreases, indicating that xanthan gum and starch interpenetrate to form a more compact structure. When the xanthan gum content is 7%, due to its strong hydrophilicity, the movable water in the system decreases, which is detrimental to starch plasticization, resulting in poor overall performance of the starch film.

[0071] 2. The performance of the starch-based composite films prepared in Example 1 and Comparative Example 3 was measured, and the results are shown in Table 2:

[0072] Table 2. Mechanical properties and color difference data of starch-based composite films

[0073]

[0074] As shown in Table 2, with the increase of water or glycerol content, the tensile strength of the starch film decreases while the elongation at break increases. This is because water and glycerol have a plasticizing effect, promoting the molecular fluidity of the polymer chains and improving the flexibility of the film. When the total mass of glycerol and water is less than 80% of the starch mass, the material is difficult to achieve overall flow in the mold, resulting in a starch film with large thickness, low tensile strength and elongation at break, low brightness, large total color difference, and poor overall performance. When the total content of water and glycerol accounts for 80%-90% of the starch weight, the overall performance of the starch film improves. This is because the dough-like starch mixture at this point is a continuous system with a certain degree of fluidity. Therefore, under the action of pressure and heat, the flow rate of the material increases, the contact area with the hot press plate increases, and the starch absorbs more heat, making it easier to plasticize. However, when the total mass of water and starch exceeds 90% of the starch mass, the ability of xanthan gum in the material system to bind water and glycerol is greatly weakened, making it impossible for water and glycerol to be effectively retained in the starch network, resulting in a "fragmented" film.

[0075] Table 3. Comparison of comprehensive performance of starch-based composite films prepared by different processes

[0076]

[0077] The results from Tables 1, 2, and 3 show that:

[0078] Based on the method of this invention, starch is mixed with a high-concentration hydrophilic colloid, water, and glycerol to obtain a dough-like starch mixture, which is then hot-pressed to obtain a composite starch film. Only when the following specific preparation parameters are met simultaneously: the mass ratio of starch, hydrophilic colloid, water, and glycerol is 100:(3-5):(50-55):(30-40), wherein the total mass of water and glycerol accounts for 80-90% of the starch mass, can a composite starch film with the following comprehensive properties be successfully prepared: film thickness of 0.05-0.25 mm, tensile strength greater than 9 MPa, elongation at break greater than 25%, brightness not less than 85, and color difference not higher than 10.

[0079] Comparing the performance data of the above embodiments and comparative examples, it is evident that when any of the following conditions are not met: the mass ratio of cold water-soluble high-viscosity hydrophilic colloid to starch, the total mass ratio of glycerol and water to starch, the mass ratio of glycerol to starch, or the mass ratio of water to starch, the resulting starch-based composite film cannot achieve a balanced overall performance and thus fails to achieve the objective of this invention. Furthermore, Examples 1 and 4 are starch films obtained with different film-forming raw material mixture states under the same one-step hot-pressing process parameters, while Examples 1 and 5 are starch films obtained with the same proportions but different preparation methods. This demonstrates that the improved one-step hot-pressing process of this invention plays a crucial role in enhancing the overall performance of the starch-based composite film.

Claims

1. A method for improving the quality of hot-pressed starch films, characterized in that, The method includes: (1) Mix the plasticizer with water, then add the hydrophilic colloid and stir until well mixed, then add starch and stir until well mixed to obtain a dough-like starch mixture; The plasticizer is glycerin; (2) Place the dough-like starch mixture obtained in step (1) in a sealed bag and equilibrate the moisture at 20-30℃ and 55-80%RH. (3) After the hot press is preheated, the starch mixture after step (2) is evenly spread on the mold and placed in the hot press plate for hot pressing and balancing to obtain the hot-pressed starch film. In step (1), the mass ratio of starch, hydrocolloid, water and glycerin is 100:(3-5):(50-55):(30-40), wherein the total mass of water and glycerin accounts for 80-90% of the mass of starch. The balancing step (3) refers to placing the hot-pressed starch film in a constant temperature and humidity chamber for balancing under the following conditions: 25~30℃, relative humidity of 50~55%, and time of 48~72 hours.

2. The method according to claim 1, characterized in that, The hydrophilic colloid in step (1) is a high-viscosity hydrophilic colloid that is soluble in cold water, including one or more of gellan gum, xanthan gum, locust bean gum, and guar gum.

3. The method according to claim 1, characterized in that, The mass ratio of starch, hydrocolloid, water and glycerin in step (1) is 100:4:(50~55):(30~35).

4. The method according to claim 1, characterized in that, The stirring speed in step (1) is 10,000 to 20,000 rpm.

5. The method according to claim 1, characterized in that, The preheating time in step (3) is 20~30 minutes.

6. The method according to claim 1, characterized in that, The parameters for hot pressing in step (3) are: temperature 80~120℃, pressure 2~10Mpa, and time 4~12min.

7. The application of the method according to any one of claims 1 to 6 in the preparation of hot-pressed starch films.

Images