A water- and heat-resistant starch-based straw and its preparation method

Abstract

The present invention provides a water- and heat-resistant starch-based straw and a preparation method thereof, belonging to the technical field of straws. The present invention uses starch as a raw material and glycerol as a plasticizer, and prepares a starch straw by an extrusion method. First, the straw is subjected to retrogradation treatment to inhibit the swelling of the straw in water, and then sodium trimetaphosphate is used as a crosslinking agent to carry out crosslinking modification on the starch straw, so as to obtain a starch-based straw with good water resistance, heat resistance and excellent mechanical properties. The present invention adopts a method of combining retrogradation and crosslinking treatment. After crosslinking, the peak viscosity and attenuation value of the straw are reduced, and the thermal stability is improved, so that the mechanical strength, water resistance and heat resistance of the starch straw are improved. Even in a liquid with a relatively high temperature, the straw can still be used for a long time.

Description

Technical Field

[0001] The present invention relates to the technical field of straws, and in particular to a water- and heat-resistant starch-based straw and a preparation method thereof. Background Art

[0002] For many years, researchers have made great efforts in exploring new materials to replace plastic straws and solve environmental problems. After continuous screening, the current alternatives to plastic straws mainly include biodegradable polymers and natural plant-based straws (Timshina A, Aristizabal-Henao J J, Da Silva B F, et al. The last straw: Characterization of per- and polyfluoroalkyl substances in commercially-available plant-based drinking straws[J]. Chemosphere, 20 21, 277:130238.). As a typical example of biodegradable polymers, polylactic acid straws still have many problems, especially the high cost of raw materials (such as lactic acid in corn). In addition, an important disadvantage of PLA is its poor heat resistance, which is not suitable for temperatures above 60 °C, and compared with plant-based materials, it requires more stringent degradation conditions, limiting its application as a disposable straw (Farah eta l., 2016)(Farah S, Anderson D G, Langer, R. Physical and mechanical prop ertiesof PLA, and their functions in widespread applications-Acomprehensi ve review[J]. Advanced Drug Delivery Reviews, 2016, 107:367-392). As for plant-based straws such as paper straws, although they are the most commonly used alternatives to plastic straws, their mechanical properties are still relatively limited. Moreover, adhesives and additional hydrophobic coatings are needed to compensate for the poor waterproof ability, which will increase the cost. Nevertheless, paper straws still quickly become soft and easy to collapse in beverages, resulting in an unpleasant user experience. In addition, a "rice straw" has currently emerged on the market. This straw is mainly made by compounding rice flour with other starches (such as corn starch and tapioca starch). Since starch is a polyhydroxy molecule, it has strong water absorption and poor wet strength; moreover, it has poor heat resistance, so the water resistance and heat resistance of the "rice straw" need to be improved. Therefore, there is an urgent need for a straw with excellent mechanical properties, heat resistance and water stability. Summary of the Invention

[0003] The purpose of the present invention is to provide a water- and heat-resistant starch-based straw and a preparation method thereof. The prepared starch-based straw has excellent mechanical properties, water resistance and heat resistance.

[0004] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0005] The present invention provides a preparation method of a water- and heat-resistant starch-based straw, comprising the following steps:

[0006] Mix starch, glycerol and water, and extrude the obtained mixture to obtain a semi-finished straw.

[0007] After adjusting the moisture content of the semi-finished straw, carry out an aging treatment to obtain a strengthened starch straw.

[0008] Mix the strengthened starch straw with a sodium trimetaphosphate solution and an alkali solution, and carry out a cross-linking reaction under the condition that the pH is 10.5 - 11.5 to obtain a water- and heat-resistant starch-based straw.

[0009] Preferably, the starch includes corn starch; the mass of the glycerol is 7 - 15% of the dry basis mass of the starch, and the mass of the water accounts for 8 - 15% of the dry basis mass of the starch.

[0010] Preferably, the extrusion molding equipment used for the extrusion molding is a QL32 twin-screw extruder; the temperature of the extrusion molding is 80°C - 110°C, the main shaft frequency is 18 - 21 Hz, and the feeding frequency is 10 - 20 Hz.

[0011] Preferably, the wall thickness of the semi-finished straw is 0.7 mm - 2 mm.

[0012] Preferably, after adjusting the moisture content, the moisture mass content of the obtained straw is 20 - 40%.

[0013] Preferably, the temperature of the aging treatment is 4°C, and the time is 3 - 24 h.

[0014] Preferably, the mass ratio of sodium trimetaphosphate in the strengthened starch straw to the sodium trimetaphosphate solution is 2:1.

[0015] Preferably, the temperature of the cross-linking reaction is 40°C, and the time is 1 - 3 h.

[0016] Preferably, the wall thickness of the water- and heat-resistant starch-based straw is 1 mm.

[0017] The present invention provides a water- and heat-resistant starch-based straw prepared by the preparation method described in the above technical solutions.

[0018] The present invention provides a method for preparing a water- and heat-resistant starch-based straw. Using starch as the raw material and glycerol as the plasticizer, the starch straw is prepared by an extrusion method. First, the straw is subjected to retrogradation treatment to make the straw network denser, inhibit the swelling of the straw in water, and reduce the water absorption capacity. Then, under alkaline conditions, with sodium trimetaphosphate as the cross-linking agent, the starch straw is cross-linked and modified. The hydroxyl groups on the starch molecules react with the phosphate groups of the sodium trimetaphosphate molecules to form starch phosphodiester; cross-linking points are formed between the starch molecules and the sodium trimetaphosphate molecules. The cross-linked starch granule structure is dense, and the water resistance and heat resistance are improved, thereby obtaining a starch-based straw with good water resistance, heat resistance, and excellent mechanical properties. The present invention adopts a method combining retrogradation and cross-linking treatment. After cross-linking, the peak viscosity and attenuation value of the straw are reduced, and the thermal stability is improved, which improves the mechanical strength, water resistance, and heat resistance of the starch straw. Even in a liquid with a relatively high temperature, the straw can still be used for a long time.

[0019] The results of the examples show that the cross-linked straws prepared by the present invention have better mechanical properties and water stability whether in water at 25°C or in water at 60°C. After soaking in water at 25°C for 4 h, the rigidity and breaking force of the starch straw cross-linked for 2 h are 612.03 g / sec and 2420.91 g, which are 61 times and 16 times that of the uncross-linked starch straw, respectively. After soaking in water at 60°C for 30 min, the rigidity and breaking force of the starch straw cross-linked for 3 h are 289.04 g / sec and 3823.7 g, which are 19 times and 9 times that of the uncross-linked starch straw, respectively. Cross-linking improves the water resistance and heat resistance of the straw. Even after soaking in water for 24 h, the straw still remains straight and can be used normally. Moreover, the prepared starch-based straw has general adaptability and can be used normally in cola, coffee, milk, green tea, juice, water at 0°C, water at 90°C, etc. Description of the Drawings

[0020] Figure 1 is a flow chart for preparing the water- and heat-resistant starch-based straw of the present invention, a is a schematic flow diagram, and b is a schematic cross-linking reaction diagram;

[0021] Figure 2 is a viscosity characteristic diagram of CS, CSstraw, CSstraw30%-6h, STMP-1h, STMP-2h, and STMP-3h;

[0022] Figure 3 is a surface and cross-section scanning electron microscope diagram of CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h;

[0023] Figure 4 is an infrared spectrum diagram of CS, CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h;

[0024] Figure 5 Thermogravimetric analysis diagrams of CS, CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h;

[0025] Figure 6 Pictures of CSstraw, STMP-3h, and Ricestraw soaked in water for different times;

[0026] Figure 7 Three-point bending comparison diagrams of CSstraw, CSstraw30%-6h, STMP-3h, Paperstraw, and Ricestraw;

[0027] Figure 8 Phenomenon of bubble induction during stirring of paper straw (left) and crosslinked starch straw (right) in carbonated beverages (a), and photos of the straws in different beverages and temperatures (b). Detailed implementation mode

[0028] The present invention provides a method for preparing a water- and heat-resistant starch-based straw, comprising the following steps:

[0029] Mix starch, glycerol, and water, and extrude the resulting mixture to obtain a semi-finished straw;

[0030] After adjusting the moisture content of the semi-finished straw, perform an aging treatment to obtain a strengthened starch straw;

[0031] Mix the strengthened starch straw with a sodium trimetaphosphate solution and an alkali solution, and carry out a crosslinking reaction under the condition of pH 10.5-11.5 to obtain a water- and heat-resistant starch-based straw.

[0032] In the present invention, unless otherwise specified, the required preparation raw materials or reagents are all commercially available products well-known to those skilled in the art.

[0033] The present invention mixes starch, glycerol, and water, and extrudes the resulting mixture to obtain a semi-finished straw.

[0034] In the present invention, the starch preferably includes corn starch.

[0035] In the present invention, the mass of the glycerol is preferably 7-15% of the dry basis mass of the starch, and the mass of the water preferably accounts for 8-15% of the dry basis mass of the starch; more preferably, the mass ratio of the starch, glycerol, and water is 2.5:0.25:0.25. The present invention uses glycerol as a plasticizer to make the surface of the straw smooth.

[0036] The present invention preferably mixes the starch, glycerol and water uniformly under stirring conditions. The present invention has no special limitation on the stirring rate, and the materials can be mixed uniformly according to the processes well-known in the art.

[0037] In the present invention, the extrusion molding equipment used for extrusion molding is preferably a QL32 twin-screw extruder.

[0038] In the present invention, the temperature of the extrusion molding is preferably 80°C to 110°C, more preferably 90°C; the spindle frequency is preferably 18 to 21 Hz, more preferably 20 Hz; the feeding frequency is preferably 10 to 20 Hz, more preferably 15 Hz. During the extrusion molding process, the mixture of starch, glycerol and water is heated, pressurized, sheared, etc., causing the starch to gelatinize.

[0039] The present invention preferably cuts the straw after extrusion molding through a cooling conveyor belt with a length of 2 m to obtain a semi-finished straw. The present invention has no special limitation on the cutting process and the length of the obtained straw, which can be adjusted according to actual needs.

[0040] After completing the cutting, the present invention preferably dries the obtained straw in an oven at 40°C to obtain a semi-finished straw.

[0041] In the present invention, the wall thickness of the semi-finished straw is preferably 0.7 mm to 2 mm, more preferably 1.0 mm.

[0042] After obtaining the semi-finished straw, the present invention adjusts the moisture content of the semi-finished straw and then performs an aging treatment to obtain a reinforced starch straw.

[0043] In the present invention, the process of adjusting the moisture content is preferably soaking the semi-finished straw in water; the present invention has no special limitation on the specific soaking time, and it can reach the required moisture mass content.

[0044] In the present invention, after adjusting the moisture content, the moisture mass content of the obtained straw is preferably 20 to 40%, more preferably 30%. The present invention controls the moisture content of the straw to facilitate the aging of the starch straw, thereby improving the water resistance and mechanical properties of the starch straw.

[0045] In the present invention, the temperature of the aging treatment is preferably 4°C, and the time is preferably 3 to 24 h, more preferably 6 to 12 h. During the aging process, the hydroxyl groups on the starch chains are associated with each other through hydrogen bond interactions and re-arranged to form many low-energy ordered crystalline structures. The starch chains restrict and constrain each other, and the free space becomes smaller, resulting in a decrease in water absorption capacity and viscosity and an increase in strength.

[0046] After completing the aging treatment, the present invention preferably dries the obtained straws in an oven at 40°C to obtain strengthened starch straws.

[0047] After obtaining the strengthened starch straws, the present invention mixes the strengthened starch straws with a sodium trimetaphosphate solution and an alkali solution, and performs a cross-linking reaction under the condition that the pH is 10.5-11.5 to obtain a water- and heat-resistant starch-based straw.

[0048] In the present invention, the mass ratio of the strengthened starch straws to sodium trimetaphosphate in the sodium trimetaphosphate solution is preferably 2:1.

[0049] In the present invention, the preparation process of the sodium trimetaphosphate (STMP) solution is preferably to dissolve it in distilled water at 40°C to obtain an STMP solution; the concentration of the STMP solution is preferably 15 wt%.

[0050] In the present invention, mixing the strengthened starch straws with the sodium trimetaphosphate solution and the alkali solution is preferably to adjust the pH value of the STMP solution to 11.3 with a 3 mol / L NaOH solution, and put the strengthened starch straws (converted by the number of roots into mass) into the obtained mixed solution.

[0051] In the present invention, the temperature of the cross-linking reaction is 40°C, the time is preferably 1-3 h, more preferably 2 h; the present invention preferably adds a 3 mol / L NaOH solution during the cross-linking reaction to adjust the pH to 10.5-11.5, more preferably 10.8, to facilitate the cross-linking reaction.

[0052] After completing the cross-linking reaction, the present invention preferably soaks the obtained straws in distilled water until no free phosphorus is detected in the water, and dries the cross-linked straws at room temperature to obtain a water- and heat-resistant starch-based straw. The present invention has no special limitation on the process of detecting phosphorus and drying, and it can be carried out according to the processes well-known in the art.

[0053] In the present invention, the wall thickness of the water- and heat-resistant starch-based straw is preferably 1 mm.

[0054] The present invention provides a water- and heat-resistant starch-based straw prepared by the preparation method described in the above technical solution.

[0055] Figure 1 is a flow chart for preparing a water- and heat-resistant starch-based straw of the present invention, a is a schematic flow diagram, and b is a schematic cross-linking reaction diagram; as Figure 1As shown, taking corn starch as an example, in the present invention, corn starch is formed by a twin-screw extruder, retrograded and crosslinked with SRMP to prepare a water- and heat-resistant starch-based straw. Due to the hydrophilicity of starch, water molecules will quickly penetrate into the corn starch straw network, causing it to swell and resulting in a decrease in mechanical strength, that is, the wet strength is insufficient. In a high-concentration sodium trimetaphosphate solution, the corn starch straw will still swell greatly, and even if crosslinked, it will crack during the drying process. In order to inhibit the swelling degree of the straw during the crosslinking process, the straw is pre-retrograded before crosslinking to make the straw network more dense and reduce the water absorption capacity. Then, under alkaline conditions, the hydroxyl groups on the starch molecules react with the phosphate groups of the sodium trimetaphosphate molecules to form starch phosphate diester. Crosslinking points are formed between the starch molecules and the sodium trimetaphosphate molecules, and the crosslinked starch granule structure is dense, with improved water resistance and heat resistance.

[0056] The technical solutions in the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0057] Example 1

[0058] Mix 2.5 kg of corn starch, 0.25 kg of glycerol and 0.25 kg of water, stir evenly, place the obtained mixture in a QL32 twin-screw extruder, and extrude under the extrusion conditions of 90 °C, a main shaft frequency of 20 Hz and a feeding rate of 15 Hz (feeding frequency). After passing through a cooling conveyor belt with a length of 2 m, it is cut to obtain a straw with a wall thickness of 1.0 mm and a length of 20 cm, and placed in an oven at 40 °C for drying to obtain a semi-finished corn starch straw, denoted as CSstraw;

[0059] Soak the semi-finished straw in water to make its moisture content 30%, place it in an environment at 4 °C for aging treatment for 6 h, and then place it in an oven at 40 °C for drying to obtain a starch straw, denoted as CSstraw30%-6h;

[0060] Dissolve 150 g of STMP in 1000 mL of distilled water (40 °C) to obtain an STMP solution (15 wt%). Adjust the pH of the STMP solution to 11.3 with 3 mol / L NaOH solution, then put 60 CS straw 30%-6h (300 g) into the resulting mixture and react at 40 °C in a constant temperature water bath for 1 h, 2 h, and 3 h respectively. During the reaction, add 3 mol / L NaOH solution to keep the system pH at 10.8. After the reaction, soak the resulting straws in distilled water until no free phosphorus is detected in the water. Then dry the cross-linked straws at room temperature to obtain water-resistant and heat-resistant starch-based straws with a wall thickness of 1 mm.

[0061] Among them, the straw cross-linked with STMP for 1 h is named STMP-1h, the straw cross-linked with STMP for 2 h is named STMP-2h, and the straw cross-linked with STMP for 3 h is named STMP-3h.

[0062] Characterization and performance testing

[0063] 1) Use a rapid viscosity analyzer to analyze the effect of STMP modification on the pasting properties of the straws. Grind the prepared straws and pass them through a 100-mesh sieve to obtain straw powder. Accurately weigh 3.0 g of the sample and 25.0 g of distilled water and place them in the viscosity meter measuring cup, and stir evenly. During the test, two stages are experienced, first heating and then cooling: start heating from 50 °C to 90 °C, and then cool down to 50 °C. The time required for this process is 13 min to obtain the viscosity curve of the sample, as shown in Figure 2 .

[0064] Figure 2 are the viscosity characteristic diagrams of CS, CSstraw, CSstraw30%-6h, STMP-1h, STMP-2h, and STMP-3h; among them, CS is corn starch, CSstraw is a corn starch straw, CSstraw30%-6h is a corn starch straw aged for 6 h under 30% moisture content, STMP-xh is a cross-linked corn starch straw, and X represents the cross-linking time;

[0065] The degree of cross-linking is measured by the viscosity of the cross-linked starch. The viscosity-temperature change curves of CS, CSstraw, CSstraw30%-6h and STMP-1h, STMP-2h, STMP-3h are as shown in Figure 2 shown. From Figure 2It can be seen that the attenuation values of STMP-1h, STMP-2h, and STMP-3h are 437 cp, 199 cp, and 1 cp respectively. As the cross-linking time increases, the thermal stability of the cross-linked straws increases. Compared with CSstraw30%-6h (attenuation value of 272 cp), the attenuation value of STMP-1h does not decrease, while the attenuation values of STMP-2h and STMP-3h decrease significantly, indicating that short-term cross-linking cannot improve the thermal stability of starch straws. The cross-linked molecules bind to each other, increasing the mechanical strength of the molecules, thereby preventing viscosity loss and providing resistance to mechanical shear. As the cross-linking time extends, the straws exhibit lower peak viscosities, indicating an increase in cross-linking density. The cross-linking of STMP improves the integrity of starch granules and reduces the swelling force of starch granules.

[0066] 2) Use an S-3400N scanning electron microscope to observe the cross-section and surface morphology of the straws (the surface scale is 20 μm for all, the cross-section scale is 500 μm for all, and the cross-section (magnified) scale is 50 μm for all); Figure 3 Figures are the surface and cross-section scanning electron micrographs of CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h; among them, CSstraw is a corn starch straw, CSstraw30%-6h is a corn starch straw aged for 6 h under 30% moisture content, and STMP-xh is a cross-linked corn starch straw, where X represents the cross-linking time.

[0067] From Figure 3 it can be seen that the surface of CSstraw is rough with cracks; the surface of CSstraw30%-6h is relatively smooth, and the protruding parts may be corn starch granules. Compared with CSstraw30%-6h, the surface of the cross-linked straws is rough, and cracks appear in the cross-section of the cross-linked straws, which may be caused by external force when breaking the straws. Compared with the non-cross-linked straws, the cross-linked straws have a denser structure, which may be because the sodium trimetaphosphate molecules form cross-linking points with starch molecules, enhancing the bonding between starch chains and making the network structure denser.

[0068] 3) Grind the prepared straws and pass them through a 100-mesh sieve to obtain starch straw powder, and use a Fourier transform infrared spectrometer to analyze the changes in the properties of the starch straws. Figure 4 Figures are the infrared spectra of CS, CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h; among them, CS is corn starch, CSstraw is a corn starch straw, CSstraw30%-6h is a corn starch straw aged for 6 h under 30% moisture content, and STMP-xh is a cross-linked corn starch straw, where X represents the cross-linking time.

[0069] As Figure 4As shown, the absorption peaks at 3400 cm -1 and 2900 cm -1 are caused by the stretching vibration of -OH and the asymmetric stretching vibration of -CH2. Compared with the uncrosslinked starch straws, the peak at 3400 cm -1 of the crosslinked starch straws becomes broader, which may be due to the formation of more hydrogen bonds between the phosphate groups on the sodium trimetaphosphate molecules and the hydroxyl groups on the starch straws. The absorption peak at 1248 cm -1 corresponds to the bending vibration of O-H. The absorption peak at approximately 1160 cm -1 can be attributed to C-O bending. However, the characteristic absorption peaks inherent to P-O and P-O-C do not appear in the crosslinked starch straws, probably because the degree of crosslinking of these crosslinked starch straws is low.

[0070] 4) Figure 5 TG analysis diagrams of CS, CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h are shown; among them, CS is corn starch, CSstraw is corn starch straw, CSstraw30%-6h is the corn starch straw aged for 6 h under 30% moisture content, STMP-xh is the crosslinked corn starch straw, and X represents the crosslinking time.

[0071] The present invention uses TG analysis to judge the influence of crosslinking on the thermal stability of the straws; as Figure 5 shown, all samples exhibit two weight loss stages. The first weight loss stage is from approximately 40°C to 160°C, and the main reason is the volatilization of free water. The weight loss occurring in this stage is very low. The weight loss rate of CS is approximately 8%, and the weight loss rates of CSstraw, CSstraw30%-6h, STMP-2h, and STMP-3h are approximately 7%. The second weight loss stage is from 260°C to 330°C, and the main reason is the degradation of starch. In this stage, the weight loss rates of STMP-2h and STMP-3h are 55% and 52%, respectively, which are significantly lower than the weight loss rates of CSstraw and CSstraw30%-6h. This is because the crosslinking reaction increases the compactness of the starch structure. The compact structure restricts the movement of molecular chains and increases the anti-degradation ability. At 600°C, the residues of STMP-2h and STMP-3h are 30% and 32%, respectively, which are significantly higher than the residues of CSstraw and CSstraw30%-6h. In summary, after crosslinking with sodium trimetaphosphate molecules, the thermal stability of the starch straws is significantly improved, and with the increase of the crosslinking time, the thermal stability is improved, which is consistent with the results of the viscosity characteristics of the RVA test.

[0072] 5) Use a texture analyzer to analyze the bending performance of the straw in the dry state and the mechanical properties in the wet state; use a shear experiment to evaluate the mechanical properties of the straw after soaking in water at 25 °C for different times, and the results are shown in Table 1.

[0073] Table 1 Mechanical properties of starch straws treated with different cross-linking times after soaking in water at 25 °C for different times

[0074]

[0075] The values in the table are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

[0076] CS is corn starch, CSstraw is corn starch straw, CSstraw30%-6h is corn starch straw aged for 6 h at 30% moisture content, STMP-2h is corn starch straw cross-linked for 2 h, STMP-3h is corn starch straw cross-linked for 3 h, and Ricestraw is a commercially available rice straw.

[0077] As shown in Table 1, after soaking in water for 0.5 h, the rigidity and breaking force of the cross-linked starch straws are significantly higher than those of other straws. After soaking for 1 h, the rigidity and breaking force of the non-cross-linked and commercially available rice straws of the same type decreased sharply, while the changes in the rigidity and breaking force of the cross-linked straws were relatively small. At this time, the rigidity and breaking force of the starch straw cross-linked for 2 h are approximately 32 times and 6 times that of the corn starch straw, respectively. After soaking in water for 4 h, the rigidity and breaking force of the starch straw cross-linked for 2 h are 612.03 g / sec and 2420.91 g, which are approximately 61 times and 16 times that of the corn starch straw. This indicates that the cross-linked straws have greatly improved their water resistance and maintain good mechanical properties. This is because the cross-linking of starch molecules and STMP molecules increases the strength of intermolecular hydrogen bonds, inhibits the water absorption and swelling of the cross-linked straws, and has good mechanical properties. At this time, the rigidity and breaking force of the commercially available rice straw are 193.66 g / sec and 1733.30 g, which are significantly worse than those of the cross-linked starch straws. It should be noted that the rigidity and breaking force of the starch straws cross-linked for 2 h and 3 h basically did not change significantly at each time period when soaked in water at 25 °C.

[0078] Use a shear experiment to evaluate the mechanical properties of the straw after soaking in water at 60 °C for different times, and the obtained results are shown in Table 2.

[0079] Table 2 Mechanical properties of starch straws treated with different cross-linking times after soaking in water at 60 °C for different times

[0080]

[0081]

[0082] The values in the table are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

[0083] CS is corn starch, CSstraw is corn starch straw, CSstraw30%-6h is corn starch straw aged for 6 h under 30% moisture content, STMP-2h is corn starch straw cross-linked for 2 h, STMP-3h is corn starch straw cross-linked for 3 h, and Ricestraw is a commercially available rice straw.

[0084] As shown in Table 2, similar to the change trend in soaking in water at 25°C, the mechanical properties of the straws showed a similar downward trend with the extension of soaking time. After soaking in water at 60°C for only 5 min, the rigidity of the corn starch straw and the commercially available rice straw was 1519.46 g / sec and 2276.97 g / sec, while the rigidity of the starch straw cross-linked for 2 h and the starch straw cross-linked for 3 h was 4984.40 g / sec and 5371.89 g / sec, indicating that the hardness of the cross-linked starch straw was significantly higher than that of the corn starch straw and the commercially available starch-based straws. After soaking in water at 60°C for 30 min, the rigidity and breaking force of all straws decreased rapidly, but the rigidity and breaking force of the starch straw cross-linked for 2 h and the starch straw cross-linked for 3 h were significantly higher than those of the corn starch straw and the commercially available rice straw. At this time, the rigidity and breaking force of the starch straw cross-linked for 2 h were about 12 times, 5 times, 3 times, and 2 times that of the corn starch straw and the commercially available rice straw; the rigidity and breaking force of the starch straw cross-linked for 3 h were 289.04 g / sec and 3823.7 g, about 19 times, 9 times, 5 times, and 3 times that of the corn starch straw and the commercially available rice straw. The rigidity and breaking force of the starch straw cross-linked for 3 h soaked in water at 60°C were higher than those of the starch straw cross-linked for 2 h at each time period, indicating that with the extension of the cross-linking time, the water resistance and heat resistance of the straws increased. Starch molecules and sodium trimetaphosphate molecules formed cross-linking points through cross-linking reactions, increasing the relative molecular mass, increasing the strength of intermolecular hydrogen bonds, and making the structure of starch granules more compact. Even in hot water, the water absorption and swelling ability of starch would be inhibited.

[0085] Figure 6 Pictures of CSstraw, STMP-3h, and Ricestraw soaked in water for different times; among them, CSstraw is corn starch straw, STMP-3h is corn starch straw cross-linked for 3 h, and Ricestraw is a commercially available rice straw. By Figure 6It can be seen that the uncrosslinked corn starch straws and rice straws undergo obvious bending deformation after being soaked in water for 1 h, while the crosslinked straws remain in good condition. Although the rigidity and breaking force of the crosslinked straws gradually decrease with the increase of the soaking time in water, it does not affect their normal use. Even after being soaked in water for 24 h, the crosslinked straws still maintain a straight state and can be used normally.

[0086] Figure 7 Figure 4 is a three-point bending comparison diagram of CSstraw, CSstraw30%-6h, STMP-3h, Paperstraw, and Ricestraw; among them, CSstraw is a corn starch straw, CSstraw30%-6h is a corn starch straw aged for 6 h under 30% moisture content, STMP-3h is a corn starch straw crosslinked for 3 h, Paperstraw is a commercially available paper straw, and Ricestraw is a commercially available rice straw. It can be Figure 7 seen that the flexural strength of the crosslinked starch straw is 12,595.47 g, which is 3.2 times and 1.1 times that of the paper straw and the rice flour straw respectively. The flexural strength of the crosslinked straw is superior to that of the commercial paper straw and the rice flour straw.

[0087] Combined with Tables 1-2 and Figures 6 - 7 it can be seen that the crosslinked straws have better mechanical properties and water stability, and crosslinking improves the water resistance and heat resistance of the straws.

[0088] Figure 8 Figure 5 shows the bubble induction phenomenon (a) of a paper straw (left) and a crosslinked starch straw (right) during the stirring of carbonated beverages, and the photos of the straws in different beverages and temperatures (b); the surface of the paper straw is relatively rough. When the paper straw is immersed in carbonated beverages such as cola, a lot of large bubbles will be generated, and a large number of bubbles will be generated during stirring. Compared with the paper straw, the surface of the straw of the present invention is smooth, and when immersed in the same beverage and stirred, fewer bubbles are generated, which has less impact on the beverage ( Figure 8 as shown in a).

[0089] As Figure 8 shown in b, the crosslinked starch-based straws prepared in the present invention can be used normally in cola, coffee, milk, green tea, juice, water at 0°C, and water at 90°C, which proves that the crosslinked straws can be used in water at different beverages and different temperatures, and have universal applicability.

[0090] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a water-resistant and heat-resistant starch-based straw, characterized in that, The steps are as follows: Mix starch, glycerol and water, and extrude the obtained mixture to obtain a semi-finished straw; the mass of water accounts for 8-15% of the dry starch mass; After adjusting the moisture content of the semi-finished straw, perform an aging treatment to obtain a reinforced starch straw; the aging treatment time is 6-12 h; Mix the reinforced starch straw with a sodium trimetaphosphate solution and an alkali solution, and carry out a cross-linking reaction under the condition of pH 10.5-11.5 to obtain a water- and heat-resistant starch-based straw; the mass ratio of the reinforced starch straw to sodium trimetaphosphate in the sodium trimetaphosphate solution is 2:

1.

2. The preparation method according to claim 1, characterized in that, The starch includes corn starch; the mass of glycerol is 7-15% of the dry starch mass.

3. The preparation method according to claim 1, characterized in that, The extrusion molding equipment used for the extrusion molding is a QL32 twin-screw extruder; the extrusion molding temperature is 80°C - 110°C, the main shaft frequency is 18 - 21 Hz, and the feeding frequency is 10 - 20 Hz.

4. The preparation method according to claim 1, wherein The wall thickness of the semi-finished straw is 0.7 mm - 2 mm.

5. The preparation method according to claim 1, characterized in that, After adjusting the moisture content, the moisture mass content of the obtained straw is 20 - 40%.

6. The preparation method according to claim 1 or 5, characterized in that, The temperature of the aging treatment is 4°C.

7. The preparation method according to claim 1, characterized in that, The temperature of the cross-linking reaction is 40°C, and the time is 1 - 3 h.

8. The preparation method according to claim 1, wherein The wall thickness of the water- and heat-resistant starch-based straw is 1 mm.

9. A water- and heat-resistant starch-based straw prepared by the preparation method according to any one of claims 1 - 8.

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