Dry heat denatured cassava starch, and preparation method and application thereof

Abstract

This invention discloses a dry-heat modified cassava starch, its preparation method, and its application. The preparation method involves adding cassava starch to distilled water and stirring evenly at room temperature to obtain a starch slurry. The pH of the starch slurry is adjusted to 8-10, and the mixture is thoroughly stirred and mixed evenly at room temperature. Subsequently, it is dehydrated to obtain solid alkaline starch, which is then crushed and spread evenly in a dish-shaped container. This container is then placed in an oven and dried until the moisture content is between 5% and 10%, obtaining pre-dried starch. The pre-dried starch is then pulverized through an 80-120 mesh sieve, spread into a thin layer, and subjected to intermittent microwave irradiation for 0.5-1.5 min followed by a 15-45 s pause. Finally, the starch is placed in an oven for dry-heat treatment to obtain dry-heat modified cassava starch. This invention improves upon the low viscosity stability and low shear stability of cassava starch through the physical treatment of dry-heat modification. The resulting modified starch is clean, harmless, and highly adaptable, and the operation is simple with low equipment investment.

Description

Technical Field

[0001] This invention relates to starch modification, specifically to a dry heat modified cassava starch and its preparation method and application; it belongs to the field of clean label food technology. Background Technology

[0002] As living standards improve, people's demands for food have moved beyond simply eating enough and eating well; they now desire safer and healthier food. When purchasing food, people pay closer attention to ingredient lists, choosing products with fewer food additives. Therefore, products with "cleaner" ingredient lists are more popular with consumers. Natural starch, due to its poor viscosity stability and other drawbacks, is often limited in its food applications. Currently, natural starch is commonly modified to achieve higher viscosity, thermal stability, and shear resistance, making it more suitable for the food industry. Modified starches used in the food industry are generally chemically modified. However, because chemical modification introduces some chemicals, they are often considered food additives, resulting in less "clean" food labels. In contrast, modified starches obtained through physical or enzymatic methods, as well as natural starches, are classified as "starch" rather than additives, leading to cleaner food labels.

[0003] Cassava is one of the three major tuber crops (cassava, sweet potato, and potato), growing in tropical and subtropical regions worldwide. In my country, it is widely cultivated south of the Yangtze River, possessing excellent characteristics such as being easy to grow, easy to cultivate, high-yielding, and harvestable year-round. Its tubers are rich in starch, earning it the reputation of "King of Starch," "Underground Granary," and "Energy Organism." Cassava starch is rich in amylopectin, accounting for 70%-80% of its total starch content, exhibiting low gelatinization temperature, high viscosity, and high transparency. Furthermore, cassava starch contains a large number of hydroxyl groups, making it chemically reactive and widely used in the modified starch field of the food industry. Waxy cassava starch, on the other hand, is a relatively new starch variety, containing almost no amylose. Unmodified waxy cassava starch, after gelatinization, produces a thick, transparent starch paste with good freeze-thaw stability, showing broad development prospects in the field of clean-label starch for frozen / refrigerated foods. Therefore, developing a modification method that enhances the functional properties of cassava starch without introducing chemical reagents and solely through dry heat physical means has become a key technological path to meet the requirements of clean labeling.

[0004] Chinese invention patent CN101319060B discloses a method for preparing dry-heat modified rice starch. Using rice starch as raw material, different types of ionomer gums are added, and after uniform mixing, the mixture is dry-heat treated at 130°C for 4 hours under conditions where the moisture content is less than 10%, thus obtaining dry-heat ionomer gum modified rice starch. However, the ionomer gums used in this technology are selected from xanthan gum, sodium alginate, or guar gum; these substances are chemical substances introduced during the modification process, and the chemical groups introduced during modification do not meet the requirements for starch used in clean labeling. Summary of the Invention

[0005] The purpose of this invention is to improve the shortcomings of cassava starch, such as low viscosity stability and low shear stability, by using microwave combined with dry heat modification physical treatment. The invention provides a modified starch with significantly improved freeze-thaw stability, excellent gelatinization characteristics and water retention, and a safe and harmless dry heat modified cassava starch and its preparation method, without introducing other chemical groups into the modified starch molecules.

[0006] Another object of the present invention is the application of the aforementioned dry heat-modified cassava starch as a thickening and stabilizing agent for cleaning labels in the preparation of yogurt.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for preparing dry heat modified cassava starch includes the following steps:

[0009] 1) Add tapioca starch to distilled water and stir evenly at room temperature to obtain starch milk;

[0010] 2) Adjust the pH of the starch milk obtained in step 1) to 8-10, and mix thoroughly at room temperature;

[0011] 3) Dehydrate the alkaline starch milk obtained in step 2) to obtain solid alkaline starch, crush it and spread it evenly in a disc container, and then put it in an oven to dry until the moisture content is 5%-10% to obtain pre-dried starch;

[0012] 4) After pulverizing the preliminarily dried starch obtained in step 3) through an 80-120 mesh sieve, spread the starch in a thin layer and treat it with microwave intermittently. Irradiate for 0.5-1.5 min and then stop for 15-45 s. The treatment time is controlled at 5-8 min and the microwave power is controlled at 500-600W.

[0013] 5) Place the microwave-treated starch obtained in step 4) into an oven for dry heat treatment to obtain dry heat modified cassava starch; control the dry heat treatment temperature at 110-170℃ and the treatment time at 0.5-7 h.

[0014] To further achieve the purpose of this invention, preferably, in step 1), the cassava starch is waxy cassava starch or ordinary cassava starch.

[0015] Preferably, in step 1), the mass ratio of cassava starch to distilled water is 1:2.5-5.

[0016] Preferably, in step 1), the stirring speed is 200-500 r / min and the stirring time is 10-60 min.

[0017] Preferably, in step 2), the alkalinity adjusting agent is one or more of sodium carbonate, sodium bicarbonate, and sodium hydroxide.

[0018] Preferably, in step 2), the stirring speed is 200-500 r / min and the stirring time is 60-120 min.

[0019] Preferably, in step 3), the starch milk is dehydrated by centrifugation or vacuum filtration; the drying oven is a vacuum drying oven or a forced-air drying oven; and the drying temperature is 40-45℃.

[0020] Preferably, in step 4), the thickness of the starch layer is 0.5-1.0 cm.

[0021] A dry heat modified cassava starch, prepared by the above method, has a gelatinization temperature 1-3℃ lower than that of the original starch, and a final viscosity 160-350 BU higher than that of the original starch.

[0022] The application of the aforementioned dry heat modified cassava starch in the preparation of clean label food products.

[0023] Milk, sucrose, and starch are mixed evenly in a mass ratio of 100:7:0.1-1. The mixture is heated and stirred in a 95°C water bath for 10 minutes. After cooling to about 40°C, 0.1% of the mass of the mixture is added as yogurt starter. The mixture is fermented at a constant temperature of 40°C for 8 hours. After fermentation, the mixture is refrigerated at 4°C for 12 hours to obtain a clean label yogurt product.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] 1) This invention combines alkaline pretreatment, microwave intermittent irradiation and dry heat treatment, which can precisely control the degree of cross-linking and rearrangement of starch molecules. The resulting dry heat modified starch has significantly improved gelatinization characteristics, water retention and gel stability. The advantages of waxy cassava starch after branch chain structure modification are even more prominent.

[0026] 2) The present invention uses microwave pretreatment to achieve rapid and uniform heating inside starch granules, improve the efficiency of subsequent dry heat denaturation, make starch molecule modification more gentle and structure more regular, and significantly improve the freeze-thaw stability of modified starch.

[0027] 3) This invention uses physical methods to process native starch, and the resulting modified starch molecules do not introduce other chemical groups, making them safe and harmless. As a clean label starch, it has broad application prospects in the fields of food thickeners, stabilizers, and water-retaining agents.

[0028] 4) This invention uses cassava starch as raw material, which is widely available and inexpensive. Compared with corn and potato starch, the supply of raw materials is stable, and there are few by-products in the processing of cassava starch, resulting in high raw material utilization.

[0029] 5) The dry heat modified cassava starch obtained by this invention has better acid resistance and shear resistance than unmodified starch and conventional dry heat modified starch in acidic, low-temperature food systems (such as yogurt), and is less likely to lose its stability due to pH and temperature changes during food processing and storage.

[0030] 6) This invention is simple to operate and requires low equipment investment. Attached Figure Description

[0031] Figure 1 The images show the starch gelatinization viscosity characteristics of native starch, Comparative Example 1, Comparative Example 3, and Examples 1-3.

[0032] Figure 2 Images of the yogurt samples (blank, comparative example 2, comparative example 3, and examples 1-3) observed under an optical microscope. Detailed Implementation

[0033] To better understand this invention, the objectives, technical solutions, and effects of this invention are explained below, and further description is provided in conjunction with the accompanying drawings and specific embodiments. However, the implementation of this invention is not limited thereto. The described embodiments are some, but not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0034] This invention relates to dry heat modification, a chemical-free, green, and safe physical modification method. The process is roughly divided into two stages: preliminary drying and high-temperature treatment. Before preliminary drying, the native starch needs to undergo alkaline pretreatment. Alkaline pretreatment not only provides a chemical environment for molecular rearrangement but also enhances the stability of the particle structure by inducing weak interactions or trace cross-linking between starch molecular chains during the subsequent dry heat treatment. In this invention, the starch moisture content is controlled at 5%-10% during the preliminary drying stage. Tests have shown that excessive moisture content easily leads to starch gelatinization and clumping, while insufficient moisture content is detrimental to molecular rearrangement. This invention has found that using intermittent microwave irradiation in conjunction with subsequent dry heat treatment during the high-temperature treatment stage yields significant results. Microwave pretreatment enables rapid and uniform heating of the starch granules, improving the efficiency of subsequent dry heat modification, resulting in gentler starch molecular modification, a more regular structure, and significantly improved freeze-thaw stability of the modified starch. Tests have shown that intermittent microwave treatment, with irradiation for 0.5-1.5 min followed by a 15-45 s pause, and a treatment time controlled at 5-8 min, with a microwave power of 500-600 W, yields the best results. The high-temperature treatment stage following microwave treatment controls the processing temperature to 110-170℃ and the processing time to 0.5-7 h. Therefore, the present invention proposes a method for preparing dry heat-modified cassava starch, comprising the following steps:

[0035] 1) Add tapioca starch to distilled water and stir evenly at room temperature to obtain starch milk;

[0036] 2) Adjust the pH of the starch milk obtained in step 1) to 8-10, and mix thoroughly at room temperature;

[0037] 3) Dehydrate the alkaline starch milk obtained in step 2) to obtain solid alkaline starch, crush it and spread it evenly in a disc container, and then put it in an oven to dry until the moisture content is 5%-10% to obtain pre-dried starch;

[0038] 4) After pulverizing the preliminarily dried starch obtained in step 3) through an 80-120 mesh sieve, spread the starch in a thin layer and treat it with microwave intermittently. Irradiate for 0.5-1.5 min and then stop for 15-45 s. The treatment time is controlled at 5-8 min and the microwave power is controlled at 500-600W.

[0039] 5) Place the microwave-treated starch obtained in step 4) into an oven for dry heat treatment to obtain dry heat modified cassava starch; control the dry heat treatment temperature at 110-170℃ and the treatment time at 0.5-7 h.

[0040] In this invention, the cassava starch can be either waxy cassava starch or ordinary cassava starch. Ordinary cassava starch has an amylose content of approximately 18%–20% and an amylopectin content of approximately 80%–82%. Waxy cassava starch has an amylose content ≤5% (typically 0%) and an amylopectin content ≥95%.

[0041] In the above method, the mass ratio of cassava starch to distilled water can be obtained according to conventional dry heat treatment, preferably 1:2.5-5. The stirring speed and time in step 1) are also conventionally selected, preferably a stirring speed of 200-500 r / min and a stirring time of 10-60 min.

[0042] In the above method, it should be noted that although chemical substances such as sodium carbonate, sodium bicarbonate and sodium hydroxide are required to adjust the pH value of starch milk to 8-10 in step 2), these pH-adjusting chemicals are food safety substances. More importantly, these substances do not enter the starch as chemical groups for starch modification.

[0043] In the above method, the stirring speed and time in step 2) are also standard practices.

[0044] In the above method, the starch milk dehydration method in step 3) can be centrifugation or vacuum filtration; the drying oven can be a vacuum drying oven or a forced-air drying oven; the drying temperature is preferably 40-45℃.

[0045] In the above method, the thickness of the starch layer in step 4) is preferably controlled to be 0.5-1.0 cm.

[0046] Example 1

[0047] (1) Add cassava starch to distilled water, control the mass ratio of cassava starch to distilled water to be 1:3, stir at 200 r / min for 60 min at room temperature to obtain starch milk.

[0048] (2) Add sodium carbonate solution to the starch milk obtained in step (1) and adjust the pH of the system to 8. Stir at 300 r / min for 60 min at room temperature.

[0049] (3) The alkaline starch milk obtained in step (2) is centrifuged and dehydrated to obtain solid alkaline starch, which is crushed and spread evenly in a disc container, and then placed in a vacuum drying oven and dried at 40°C until the moisture content is 5% to obtain preliminary dried starch.

[0050] (4) After pulverizing the pre-dried starch obtained in step (3) through an 80-mesh sieve, spread the starch thin layer with a thickness of 1 cm and irradiate it with microwave intermittently for 0.5 min, 30 s, with a microwave power of 500 W and a processing time of 8 min.

[0051] (5) Place the microwave-treated starch obtained in step (4) into a 130℃ oven for dry heat treatment for 1 h to obtain dry heat modified cassava starch.

[0052] (6) Mix milk, sucrose and dry heat-modified cassava starch in a mass ratio of 100:7:0.5, heat and stir in a 95°C water bath for 10 min, and then cool to 40°C.

[0053] (7) Add the yogurt starter to the cooled milk mixture obtained in step (6) at a mass ratio of 1:1000. Ferment at a constant temperature of 40°C for 8 hours. After fermentation, place the mixture in a refrigerator at 4°C for 12 hours to obtain a clean label yogurt product.

[0054] Example 2

[0055] (1) Add waxy cassava starch to distilled water at a mass ratio of 1:4 and stir at 300 r / min for 30 min at room temperature to obtain starch milk.

[0056] (2) Add sodium bicarbonate solution to the starch milk obtained in step (1) to adjust the pH of the system to 9, and stir at 200 r / min for 80 min at room temperature.

[0057] (3) The alkaline starch milk obtained in step (2) is filtered and dehydrated to obtain solid alkaline starch, which is crushed and spread evenly in a dish-shaped container, and then placed in a forced-air drying oven and dried at 42°C until the moisture content is 8% to obtain preliminary dried starch.

[0058] (4) After crushing the pre-dried starch obtained in step (3) through a 100-mesh sieve, spread the starch thin layer with a thickness of 1 cm and irradiate it with microwave intermittently for 1 min, 15 s, with a microwave power of 550 W and a processing time of 7 min.

[0059] (5) Place the microwave-treated starch obtained in step (4) into a 150℃ oven for dry heat treatment for 3 h to obtain dry heat modified cassava starch.

[0060] (6) Mix milk, sucrose and dry heat-modified cassava starch in a mass ratio of 100:7:0.1, heat and stir in a water bath at 95°C for 10 min, and then cool to 40°C.

[0061] (7) Add the yogurt starter to the cooled milk mixture obtained in step (6) at a mass ratio of 1:1000. Ferment at a constant temperature of 40°C for 8 hours. After fermentation, place the mixture in a refrigerator at 4°C for 12 hours to obtain a clean label yogurt product.

[0062] Example 3

[0063] (1) Add cassava starch to distilled water at a mass ratio of 1:5 and stir at 500 r / min for 10 min at room temperature to obtain starch milk.

[0064] (2) Add sodium hydroxide solution to the starch milk obtained in step (1) to adjust the pH value of the system to 10, and stir at 400 r / min for 100 min at room temperature.

[0065] (3) The alkaline starch milk obtained in step (2) is filtered and dehydrated to obtain solid alkaline starch, which is crushed and spread evenly in a disc container, and then placed in a vacuum drying oven and dried at 45°C until the moisture content is 10% to obtain preliminary dried starch.

[0066] (4) After pulverizing the preliminarily dried starch obtained in step (3) through a 120-mesh sieve, spread the starch thin layer with a thickness of 1 cm and irradiate it with microwave intermittently for 1.5 min, pause for 45 s, microwave power of 600 W, and processing time of 6 min.

[0067] (5) Place the microwave-treated starch obtained in step (4) into a 170℃ oven for dry heat treatment for 1 h to obtain dry heat modified cassava starch.

[0068] (6) Mix milk, sucrose and dry heat-modified cassava starch in a mass ratio of 100:7:0.8, heat and stir in a 95°C water bath for 10 min, and then cool to 40°C.

[0069] (7) Add the yogurt starter to the cooled milk mixture obtained in step (6) at a mass ratio of 1:1000. Ferment at a constant temperature of 40°C for 8 hours. After fermentation, place the mixture in a refrigerator at 4°C for 12 hours to obtain a clean label yogurt product.

[0070] Comparative Example 1

[0071] (1) Add waxy cassava starch to distilled water at a mass ratio of 1:3 and stir at 300 r / min for 30 min at room temperature to obtain starch milk.

[0072] (2) Add sodium bicarbonate solution to the starch milk obtained in step (1) to adjust the pH of the system to 4, and stir at 500 r / min for 70 min at room temperature.

[0073] (3) The alkaline starch milk obtained in step (2) is filtered and dehydrated to obtain solid alkaline starch, which is crushed and spread evenly in a disc container, and then placed in a vacuum drying oven at 43°C to dry to a moisture content of 10% to obtain pre-dried starch.

[0074] (4) After pulverizing the pre-dried starch obtained in step (3) through an 80-mesh sieve, spread the starch thin layer with a thickness of 1 cm and irradiate it with microwave intermittently for 1 min, 30 s, with a microwave power of 500 W and a processing time of 5 min.

[0075] (5) The microwave-treated starch obtained in step (4) is placed in a 150℃ oven for dry heat treatment for 2 h to obtain dry heat modified cassava starch.

[0076] (6) Mix milk, sucrose and dry heat-modified cassava starch in a mass ratio of 100:7:1.0, heat and stir in a 95°C water bath for 10 min, and then cool to 40°C.

[0077] (7) Add the yogurt starter to the cooled milk mixture obtained in step (6) at a mass ratio of 1:1000. Ferment at a constant temperature of 40°C for 8 hours. After fermentation, place the mixture in a refrigerator at 4°C for 12 hours to obtain a clean label yogurt product.

[0078] Comparative Example 2

[0079] (1) Mix milk, sucrose and waxy cassava starch in a mass ratio of 100:7:0.5, heat and stir in a 95°C water bath for 10 min, and then cool to 40°C.

[0080] (2) Add the yogurt starter to the cooled milk mixture obtained in step (1) at a mass ratio of 1:1000, and ferment at a constant temperature of 40℃ for 8 hours. After fermentation, refrigerate at 4℃ for 12 hours to obtain a regular yogurt product.

[0081] Comparative Example 3

[0082] (1) Add waxy cassava starch to distilled water at a mass ratio of 1:3 and stir at 400 r / min for 30 min at room temperature to obtain starch milk.

[0083] (2) Add sodium bicarbonate solution to the starch milk obtained in step (1) to adjust the pH of the system to 9, and stir at 400 r / min for 60 min at room temperature.

[0084] (3) The alkaline starch milk obtained in step (2) is filtered and dehydrated to obtain solid alkaline starch, which is crushed and spread evenly in a disc container, and then placed in a vacuum drying oven at 43°C to dry to a moisture content of 8% to obtain preliminary dried starch.

[0085] (4) After the preliminary dried starch obtained in step (3) is pulverized and passed through a 120-mesh sieve, it is placed in a 170℃ oven for dry heat treatment for 0.5 h to obtain dry heat modified cassava starch.

[0086] (5) Mix milk, sucrose and dry heat-modified cassava starch in a mass ratio of 100:7:1.0, heat and stir in a 95°C water bath for 10 min, and then cool to 40°C.

[0087] (6) Add the yogurt starter to the cooled milk mixture obtained in step (5) at a mass ratio of 1:1000. Ferment at a constant temperature of 40°C for 8 hours. After fermentation, place the mixture in a refrigerator at 4°C for 12 hours to obtain a clean label yogurt product.

[0088] Comparative Example 1 uses waxy cassava starch as raw material, and adjusts the pH value to 4 before dry heat treatment to prepare dry heat modified waxy cassava starch; Comparative Example 2 uses waxy cassava native starch as raw material, without alkaline pretreatment and a series of drying treatments, to obtain ordinary yogurt products; Comparative Example 3 uses waxy cassava native starch as raw material, and after alkaline pretreatment but without microwave pretreatment, prepares dry heat modified waxy cassava starch, and then prepares yogurt products.

[0089] (a) Determination of starch gelatinization viscosity characteristics:

[0090] The gelatinization characteristics of the samples were determined using a Brabender viscometer. 6 g of starch (dry basis) sample was weighed into the sample cup of the Brabender viscometer, and distilled water was added to a total mass of 100 g. The mixture was thoroughly mixed to prepare a starch slurry with a mass percentage of 6%. The Brabender viscometer measurement program was set as follows: initial temperature 30℃, temperature increased to 95℃ at a rate of 7.5℃ / min, held at that temperature for 5 min; then decreased to 50℃ at a rate of 7.5℃ / min, held at that temperature for 5 min; stirring speed was 200 r / min throughout the process. The viscosity change curve was recorded, and the gelatinization temperature, peak viscosity, disintegration value, and final viscosity were read. The viscosity unit was BU. Each sample was measured in triplicate, and the average value was taken.

[0091] Experimental results are as follows Figure 1As shown in Table 1, the specific values ​​of gelatinization temperature, peak viscosity, disintegration value, and final viscosity are as follows. It can be observed that the overall viscosity of Comparative Example 1 is significantly lower than that of native starch, while the overall viscosity of Examples 1-3 is higher than that of native starch. This may be because starch molecules decompose under acidic conditions, while under alkaline conditions, starch molecules are more stable and can promote the dry heat denaturation reaction process. The gelatinization temperature of native starch is 69.10℃, while the gelatinization temperatures of the dry heat-modified starches of Comparative Example 3 and Examples 1-3 are 66.95℃, 67.63℃, 66.91℃, and 66.34℃, respectively. This indicates that dry heat treatment can significantly reduce the gelatinization temperature of starch, making starch granules easier to gelatinize. Peak viscosity is the viscosity corresponding to the equal expansion rate and disintegration rate of starch granules during gelatinization, reflecting the expansion and water absorption capacity of starch granules. Compared to native starch, the disintegration values ​​of Examples 1-3 all decreased, while the final viscosity increased significantly, indicating that the viscosity stability of the dry heat-modified starch was enhanced, and the thickening properties of the starch were improved. Compared to Comparative Example 3, Example 1 exhibited a higher peak viscosity, indicating that microwave pretreatment induced specific molecular structure rearrangements, enabling the starch to absorb water and swell more vigorously in the early stages of gelatinization, making it suitable for scenarios requiring rapid thickening. Examples 2 and 3 showed lower disintegration values ​​and higher final viscosities, indicating that microwave pretreatment promoted the formation of a more stable three-dimensional network. Although it reduced the initial peak viscosity, it enhanced long-term storage stability, making it suitable for scenarios requiring anti-aging and shape retention. The introduction of microwave pretreatment resulted in a richer diversity of gelatinization characteristics in the dry heat-modified starch, demonstrating that microwave pretreatment can effectively intervene in the assembly behavior of starch molecules, breaking the performance limitations of single dry heat treatment. This provides a theoretical basis and data support for the customized application of the product of this invention in different food processing scenarios.

[0092] Table 1. Results of Gelatinization Viscosity Measurement of Native Starch, Comparative Example 1, Comparative Example 3, and Examples 1-3

[0093] (II) Determination of starch solubility and swelling degree:

[0094] Weigh 0.50 g of starch (dry basis) and add distilled water to 50 g to prepare a 1% starch slurry. Heat and stir in a water bath at 80℃ for 10 min, then cool to room temperature. Transfer 25 g of starch paste to a 50 mL centrifuge tube and weigh the tube (m1). Centrifuge the tube at 3000 r / min for 20 min. Weigh the precipitate and the centrifuge tube after centrifugation (m2). Dry the aluminum box to constant weight, cool it in a desiccator, and weigh it (m3). Then, transfer the supernatant from the centrifugation process to the aluminum box, dry it at 105℃ to constant weight, cool it in a desiccator, and weigh it (m4). Perform three parallel measurements for each sample and take the average value.

[0095]

[0096]

[0097] Solubility and swelling are important indicators reflecting the compactness of the internal structure of starch granules and the interactions between molecular chains. During dry heat denaturation, some crystalline regions dissociate, enhancing the hydration capacity of starch segments and allowing more small molecular segments to dissolve from the granule surface, thus significantly increasing solubility. Swelling, however, decreases significantly because dry heat treatment causes rearrangement and local cross-linking of the internal structure of starch granules, forming a denser granular skeleton that limits the space for water absorption and swelling during heating. As shown in Table 2, compared to the native starch and Comparative Example 3, the solubility and swelling of starch in Examples 1-3 all increased. This indicates that microwave pretreatment followed by dry heat denaturation can achieve precise control of starch solubility and swelling. Compared to dry heat treatment alone, it can significantly improve starch solubility, enhance the molecular compatibility of starch with food systems, and avoid a grainy texture. Simultaneously, it can controllably reduce starch swelling, improving the structural stability of starch under thermal processing and shearing conditions, and effectively inhibiting aging, sedimentation, and water separation.

[0098] Table 2. Results of solubility and swelling degree determination of native starch, comparative example 3, and starches from Examples 1-3.

[0099] (III) Determination of freeze-thaw stability of starch paste:

[0100] Weigh 5 g of starch (dry basis) and add distilled water to 100 g to prepare a 5% starch slurry. Gelatinize in a boiling water bath for 30 min and cool to room temperature for 1.5 h. Place approximately 10 g of the starch slurry into a 15 mL centrifuge tube and record the mass of the centrifuge tube as m1 and the mass of the starch slurry as m2. Freeze the centrifuge tube at -20℃ for 24 h and thaw in a 30℃ water bath for 2 h. Centrifuge the centrifuge tube at 5000 r / min for 10 min, discard the water that separates from the supernatant, and weigh the centrifuge tube and the precipitate, recording the weight as m3. Place the centrifuge tubes back into the -18°C freezer. After 24 hours, remove them to thaw, centrifuge, and weigh them. Repeat this cycle 5 times.

[0101] Freeze-thaw stability is an important indicator for evaluating the anti-aging and anti-water separation ability of starch-based foods under freezing conditions. It is usually evaluated by the water separation rate after multiple freeze-thaw cycles. The lower the water separation rate, the more stable the starch gel network structure and the stronger the freeze-thaw resistance. As shown in Table 3, the native starch, the comparative example, and the dry heat modified starch in each example all exhibited relatively good freeze-thaw stability, with no water separation in the first two cycles, and only less than 2% water separation in the third cycle. With increasing freeze-thaw cycles, the water separation rates of each sample gradually showed differences, and these differences became increasingly significant with each cycle. Compared to native starch, the water separation rates of Comparative Example 3 in the 3rd-5th cycles were 1.62%, 3.12%, and 5.60%, respectively, slightly lower than native starch, indicating that simple dry heat modification has limited effect on improving the freeze-thaw stability of starch. In contrast, the water separation rates of Examples 1-3 were significantly lower than those of Comparative Example 3 and native starch in each cycle. Taking the fifth cycle as an example, the water separation rates of Examples 1-3 were 4.87%, 4.53%, and 4.70%, respectively, which were reduced by 13.0%, 19.1%, and 16.1% compared to Comparative Example 3, and the reduction was more significant than that of the original starch. This result indicates that the introduction of microwave pretreatment significantly improves the freeze-thaw stability of starch, giving its gel network a stronger resistance to water migration. During yogurt refrigeration, this stable network structure can effectively inhibit whey separation and maintain the uniformity of the product texture. At the same time, the dense three-dimensional network induced by microwave pretreatment endows the starch with higher water-holding capacity and gel strength, enabling the yogurt to maintain good firmness and adhesion during storage, avoiding coarse texture or deterioration in taste.

[0102] Table 3. Results of freeze-thaw stability tests on native starch, Comparative Example 3, and Starch Pastes from Examples 1-3

[0103] (iv) Optical microscopic observation of yogurt:

[0104] Place a small amount of yogurt on a glass slide, add an appropriate amount of iodine solution for staining, disperse evenly, cover with a coverslip, and observe the morphology of the yogurt and starch granules under an optical microscope.

[0105] Starch, added as a thickener to yogurt, requires high-temperature sterilization and agitation / shearing processes; therefore, it must possess good heat and shear stability. The observation results of the yogurt samples from the comparative examples and embodiments under an optical microscope are as follows: Figure 2As shown in the figure, the starch granules in Comparative Example 2 were almost completely broken, with severe structural damage. While starch granules were still observed in Comparative Example 3 and Examples 1-3, the edges of the granules in Comparative Example 3 were blurred, showing obvious signs of breakage. In contrast, the starch granules in Examples 1-3 had clear outlines and maintained a more complete granular structure. These results indicate that while single dry heat treatment can enhance the heat and shear stability of starch to some extent, the effect is limited. Starch samples pretreated with microwaves followed by dry heat modification exhibit more stable granular structures and significantly better resistance to breakage than samples treated with dry heat alone. The intact starch granules can continuously play a thickening role in the yogurt system, avoiding viscosity decrease and textural degradation caused by granule breakage, ensuring stable consistency and texture throughout the product's shelf life. Therefore, the starch prepared in this invention is beneficial for promoting the formation of a denser and more uniform protein gel network structure in yogurt, maintaining a good texture.

[0106] (v) Determination of apparent viscosity of yogurt:

[0107] Adopting Anton Paar Visco QC TM The apparent viscosity of yogurt samples was determined using a 100 rotational viscometer. A rotor model of L4 was selected, the rotation speed was set to 60 r / min, and the measured values ​​were recorded after stirring for 20 s.

[0108] The apparent viscosity of the yogurt is shown in Table 4. The apparent viscosity of the yogurt samples with added starch was higher than that of the blank yogurt group, indicating that starch gelatinization and expansion enhanced the gel structure of the yogurt and increased its viscosity. With increasing storage time, the apparent viscosity of the blank yogurt samples, comparative examples 2 and 3, and examples 1-3 generally showed a trend of first decreasing and then increasing. Under low-temperature storage conditions, the proteolytic enzymes secreted by microorganisms in the yogurt still act on the proteins, causing the protein network to loosen and resulting in a decrease in yogurt viscosity. Subsequently, as the protein micelles in the yogurt continuously associate, a denser three-dimensional network gel structure is formed. At the same time, the dry heat-modified starch particles play a good skeletal support role in the protein network, assisting the association process to form a more robust structure, and the yogurt viscosity gradually increases again. The viscosity of Example 1 was slightly lower than that of Comparative Example 3 in the early stage, but after long-term refrigeration, the viscosity was higher than that of Comparative Example 3. The viscosity of yogurt in Examples 2 and 3 was always higher than that of Comparative Example 3. This indicates that the dry heat modified starch after microwave pretreatment has a better effect on enhancing the viscosity stability of yogurt, which is significantly better than starch treated by dry heat alone. It can effectively improve the texture stability and anti-aging properties of yogurt.

[0109] Table 4. Results of apparent viscosity determination of yogurt in blank, comparative example 2, comparative example 3 and examples 1-3

[0110] As can be seen from the above examples and comparative examples, the present invention, by microwave pretreatment followed by dry heat treatment of cassava starch at a certain temperature for a certain period of time, can rearrange and recombine the starch molecular structure inside the starch granules, enhancing the heat resistance and shear resistance of the starch granules and ensuring their integrity during processing; at the same time, the induced dense network structure delays viscosity decay; particularly in terms of freeze-thaw stability, the combination of microwave and dry heat treatment shows significant advantages. The modified starch of the present invention can effectively maintain the consistency stability of yogurt during its shelf life, reduce whey separation, and improve product quality. The process of the present invention is simple and easy to operate, green, safe, and harmless, with diverse applications and adaptability to various food systems.

[0111] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing dry-heat modified cassava starch, characterized in that... Includes the following steps: 1) Add tapioca starch to distilled water and stir evenly at room temperature to obtain starch milk; 2) Adjust the pH of the starch milk obtained in step 1) to 8-10, and mix thoroughly at room temperature; 3) Dehydrate the alkaline starch milk obtained in step 2) to obtain solid alkaline starch, crush it and spread it evenly in a disc container, and then put it in an oven to dry until the moisture content is 5%-10% to obtain pre-dried starch; 4) After pulverizing the preliminarily dried starch obtained in step 3) through an 80-120 mesh sieve, spread the starch in a thin layer and treat it with microwave intermittently. Irradiate for 0.5-1.5 min and then stop for 15-45 s. The treatment time is controlled at 5-8 min and the microwave power is controlled at 500-600 W. 5) Place the microwave-treated starch obtained in step 4) into an oven for dry heat treatment to obtain dry heat modified cassava starch; control the dry heat treatment temperature at 110-170℃ and the treatment time at 0.5-7 h.

2. The method for preparing dry heat-modified cassava starch according to claim 1, characterized in that, In step 1), the cassava starch is waxy cassava starch or ordinary cassava starch.

3. The method for preparing dry heat modified cassava starch according to claim 1, characterized in that, In step 1), the mass ratio of cassava starch to distilled water is 1:2.5-5.

4. The method for preparing dry heat-modified cassava starch according to claim 1, characterized in that, In step 1), the stirring speed is 200-500 r / min and the stirring time is 10-60 min.

5. The method for preparing dry heat modified cassava starch according to claim 1, characterized in that, In step 2), the alkalinity adjusting agent is one or more of sodium carbonate, sodium bicarbonate, and sodium hydroxide.

6. The method for preparing dry heat modified cassava starch according to claim 1, characterized in that, In step 2), the stirring speed is 200-500 r / min and the stirring time is 60-120 min.

7. The method for preparing dry heat-modified cassava starch according to claim 1, characterized in that, In step 3), the starch milk is dehydrated by centrifugation or vacuum filtration; the drying oven is a vacuum drying oven or a forced-air drying oven; and the drying temperature is 40-45℃.

8. The method for preparing dry heat-modified cassava starch according to claim 1, characterized in that, In step 4), the thickness of the starch layer is 0.5-1.0 cm.

9. A dry-heat modified cassava starch, characterized in that, The product is prepared by any one of claims 1-8, and its gelatinization temperature is 1-3°C lower than that of the original starch, and its final viscosity is 160-350 BU higher than that of the original starch.

10. The application of the dry heat modified cassava starch according to claim 9 in the preparation of clean label food.

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