A clean modification process of high slowly digestible starch content yam starch

Abstract

This invention discloses a clean modification process for yam starch with high slow-digestible starch content, belonging to the field of clean starch modification technology. It aims to solve the problems of existing modification processes relying on exogenous additives, high costs, and difficulty in balancing starch digestibility and stability. This invention uses fresh yam as raw material and obtains a co-precipitation system retaining endogenous mucoprotein through isoelectric point sedimentation. Based on this, vacuum alternating pulsed steam treatment is used, utilizing pressure difference and thermal effects to promote the migration of endogenous proteins at the starch particle interface and construct a stable composite structure. Subsequently, annealing is performed under high humidity conditions to induce ordered rearrangement of molecular chains. This process, without adding exogenous substances, precisely controls the starch structure, significantly increases the slow-digestible starch content, and improves the processing stability of the product. It has advantages such as being green and clean, highly safe, and highly functional.

Description

Technical Field

[0001] This invention belongs to the field of starch clean modification technology, specifically relating to a clean modification process for yam starch with high slow-digestible starch content. Background Technology

[0002] In recent years, with the increasing incidence of chronic diseases such as diabetes, obesity, and metabolic syndrome, low glycemic index (GI) foods have become a research hotspot in the health food field. Starch, as a major source of carbohydrates in the human diet, can be classified into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) based on its digestion rate in the small intestine. Compared to resistant starch, which is not digested by the small intestine at all, slowly digestible starch releases glucose slowly and persistently in the small intestine, providing the energy needed by the body, effectively stabilizing postprandial blood sugar fluctuations, and maintaining a lasting feeling of fullness. Therefore, it has irreplaceable nutritional value in the fields of medical foods and healthy meal replacements. Yam, due to its natural content of mucoprotein, polysaccharides, and other active substances, is widely considered a low-GI food with both medicinal and edible properties. However, after conventional heating and cooking, the internal structure of natural yam starch is easily destroyed, significantly reducing the content of slowly digestible starch, resulting in a higher actual GI value for its processed products. To increase the content of slowly digestible starch in starch, those skilled in the art have conducted extensive research.

[0003] Existing technology CN106519048A proposes a method to increase the content of slowly digestible starch in starch. This method mainly involves adding α-amylase to modify the starch through enzymatic hydrolysis, followed by centrifugation, washing, and cold refrigeration. This enzymatic modification introduces exogenous biological enzyme preparations; however, the enzymatic hydrolysis process is difficult to precisely control the depth of hydrolysis and easily produces a large amount of indigestible starch (RS), which can easily cause gastrointestinal bloating and discomfort after large-scale consumption. Patent CN101880331A proposes a method to extract slowly digestible starch from broken rice using ultra-high pressure technology, which promotes starch rearrangement by applying ultra-high pressure of 600 MPa. However, the equipment cost for industrially achieving 600 MPa ultra-high pressure is extremely high, making large-scale mass production difficult; moreover, this method must first remove the natural proteins in the raw grain through alkaline methods. This pursuit of pure starch strips away the natural beneficial byproducts of the raw material and breaks the physical synergy between natural matrices.

[0004] In summary, there is an urgent need to develop a clean modification process that does not require the addition of exogenous substances or high-intensity acid-base purification, but purely utilizes the natural components in the raw grains and achieves a significant increase in the content of slowly digestible starch through low-cost physical environmental field mediation. Summary of the Invention

[0005] The present invention aims to solve the problems of existing modification processes that rely on exogenous additives, are costly, and are difficult to balance starch digestibility and stability.

[0006] The specific technical solution is as follows: A clean modification process for yam starch with high slow-digestible starch content includes the following steps: S1: Cut the peeled and washed fresh yam into chunks, add purified water, and homogenize at high speed to obtain a uniform slurry; filter the slurry, add acid to adjust its pH value, let it stand, centrifuge to discard the supernatant and obtain the precipitate, resuspend the precipitate with water, readjust the pH value and centrifuge again. Repeat the above process twice, collect the precipitate, filter by pressure to obtain yam starch / mucoprotein coprecipitate; S2: The obtained yam starch / mucoprotein coprecipitate is sent into a closed alternating pulse steam modification reaction chamber, and the vacuum is maintained under negative pressure for exhaust treatment. S3: High-heat dry saturated steam is introduced into the alternating pulse steam modification reaction chamber that is maintaining a negative pressure state, so that the chamber is reversed to a positive pressure state and the pressure is maintained; after the pressure is maintained, vacuum is drawn and exhaust is performed, so that the chamber returns to a negative pressure state and the pressure is maintained; the above positive pressure state and negative pressure state are regarded as a complete pressure difference alternating cycle, and the cycle is continuously performed. S4: After the cycle ends, the alternating pulse steam modification reaction chamber is adjusted to normal pressure, the temperature and humidity inside the chamber are adjusted, and continuous annealing treatment is carried out to obtain solidified material. S5: The solidified material is dried to the target moisture content by low-temperature hot air circulation, and then pulverized and sieved to obtain the yam starch with high slow-digestible starch content.

[0007] Furthermore, the fresh yam mentioned in step S1 is a winter fresh yam in its dormant period, and its natural starch contains 17wt%-25wt% amylose; the weight ratio of the fresh yam chunks to the purified water is 1:2; the high-speed homogenization process is performed at 10000 rpm for 3 minutes; and the filtration is carried out using a 100-mesh nylon filter.

[0008] Furthermore, the acid solution mentioned in step S1 is a food-grade citric acid solution with a concentration of 1 mol / L; the pH value is adjusted to 4.0-4.5; and the water content of the yam starch / mucoprotein co-precipitate is 35wt%-45wt%.

[0009] Furthermore, the vacuum degree of the exhaust treatment in step S2 is -0.08 MPa, and the maintenance time is 15 minutes.

[0010] Furthermore, the temperature of the high-temperature dry saturated steam in step S3 is 105-115°C.

[0011] Furthermore, in step S3, the positive pressure state is 0.15-0.25 MPa, and the pressure holding time is 5-8 minutes; the negative pressure state is -0.08 MPa, and the pressure holding time is 3-5 minutes.

[0012] Furthermore, the number of cycles in step S3 is 4-6 times.

[0013] Furthermore, in step S4, the temperature and humidity inside the chamber are adjusted to 50-55℃ and 85%-90%, respectively; the duration of the continuous annealing treatment is 12-16 hours.

[0014] Furthermore, the drying temperature in step S5 is 45°C; the target moisture content is 10 wt%.

[0015] Furthermore, the crushing and screening process in step S5 involves placing the dried, lumpy material into an air jet mill for crushing, and then screening it through a 100-mesh stainless steel vibrating screen.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention utilizes the natural properties of endogenous mucin in yam to construct a stable protein-starch interface composite structure through the synergistic effect of pressure difference driving and heat treatment. This structure can form a certain physical barrier effect on the surface of starch granules, reducing the direct contact efficiency of digestive enzymes with starch, thereby effectively increasing the SDS ratio, while avoiding the problem of excessive RS generation in traditional modification methods.

[0017] (2) This invention achieves controlled disturbance of starch particle structure through vacuum alternating pulse steam treatment. Under the premise of avoiding complete gelatinization and destruction, it adjusts the internal moisture distribution and amorphous region structure of the particles. Combined with subsequent annealing treatment, it promotes orderly rearrangement of starch molecular chains, improves crystal region stability, and makes the obtained starch exhibit relatively stable gelatinization characteristics and shear resistance during processing.

[0018] (3) The process of the present invention achieves regulation of starch structure through physical field regulation and synergistic effect of endogenous components, and has good clean label properties and food safety, and is applicable to the fields of functional food and special medical food. Attached Figure Description

[0019] Figure 1 This is a flowchart of the process for cleaning and modifying slow-digesting yam starch according to the present invention; Figure 2 The solid nuclear magnetic resonance spectra of starch samples from Example 1 and Comparative Examples 1-3 of this invention are shown. Figure 3 These are electron microscope morphology images of starch samples from Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0020] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0021] The present invention discloses a clean modification process for yam starch with high slow-digestible starch content, the process flow diagram of which is attached. Figure 1 The detailed steps are as follows: S1. Initial Proposal Peeled and washed fresh yam was cut into chunks and added to purified water at a weight ratio of 1:2. The mixture was homogenized at 10,000 rpm for 3 minutes to obtain a uniform slurry. The homogenate was then filtered through a 100-mesh nylon filter to remove coarse fiber residue. A 1 mol / L food-grade citric acid solution was slowly added dropwise to the slurry while mechanically stirring. The pH of the slurry was precisely adjusted to 4.0-4.5 using a pH meter, which corresponds to the isoelectric point threshold of yam mucilage glycoproteins. At the isoelectric point, the slurry was allowed to stand for 15 minutes, then centrifuged at 3,000 rpm for 10 minutes, and the supernatant was discarded. A suitable amount of purified water was then added to the resulting precipitate for thorough resuspending and dispersion. The pH was adjusted again to 4.0-4.5, and the mixture was allowed to stand before centrifugation. This process was repeated twice, and the precipitate was finally collected. Low-pressure filtration was employed to control the material's moisture content within the range of 35wt%-45wt%. This ensured good plasticity and water migration capacity of the system while preventing excessive moisture content from affecting the differential pressure transmission efficiency of subsequent vacuum pulse processing. The final product was a wet yam starch / mucoprotein co-precipitate.

[0022] The fresh yam mentioned is a winter fresh yam in its dormant period, and its natural starch contains 17wt%-25wt% amylose.

[0023] The above steps utilize the isoelectric point sedimentation characteristics of endogenous mucin in yam, allowing it to co-settle with starch granules during separation and preferentially accumulate on the surface of the granules and in the surrounding microenvironment. Through adsorption and preliminary interaction, co-retention is achieved, thus forming a yam starch / mucin co-precipitation system, which provides a basis for subsequent interface structure regulation.

[0024] S2. Vacuum Exhaust The wet yam starch / mucoprotein co-precipitate obtained in step S1 is spread evenly in a porous stainless steel reaction dish, with the material thickness controlled at 2-3 cm. It is then placed into a pressure-resistant, sealed alternating pulse steam modification reaction chamber. A high-power vacuum pump is activated to rapidly extract air from the reaction chamber, bringing the pressure inside to -0.08 MPa, and maintaining this deep negative pressure for 15 minutes. This process, through deep vacuuming, forcibly removes the micropores on the surface of the starch granules and the microbubbles accumulated in the amorphous regions inside.

[0025] S3. Vacuum Alternating Pulse Steam Treatment In the reaction chamber maintaining the negative pressure state of step S2, high-temperature dry saturated steam at a temperature of 105-115℃ is introduced into the chamber through an instantaneous high-pressure steam valve. With the instantaneous influx of the high-temperature steam, the pressure inside the reaction chamber instantly reverses from negative to positive, reaching a pressure of 0.15-0.25 MPa, while the temperature inside the chamber rapidly rises. This high-temperature, high-pressure state is maintained for 5-8 minutes; this is the positive pressure period. During this phase, protein molecular chains unfold and enhance their interaction with starch molecules, while the steam pressure promotes the entry of moisture into the surface region of the starch granules. After the pressure maintenance period ends, the steam source is cut off, and the vacuum pump unit is activated at full power to exhaust the air, returning the reaction chamber to a negative pressure state of -0.08 MPa. This negative pressure state is maintained for 3-5 minutes; during this negative pressure period, the sudden pressure drop causes some moisture in the system to flash evaporate, lowering the local temperature and inhibiting the continued expansion and complete gelatinization of the starch granules, thus achieving controlled disturbance rather than complete destruction of the starch structure.

[0026] The positive and negative pressure periods described above are combined to form a complete pressure differential alternation cycle. This cycle is repeated for 4-6 cycles. During the repeated pressure differential driving process, a significant pressure gradient and water migration channels are formed within the system. This causes the endogenous mucin attached to the surface of starch granules to expand its molecular chains after thermal denaturation. Under the synergistic effect of water, the mucin migrates and locally embeds itself into the surface of the starch granules and its amorphous regions, gradually constructing a stable mucin-starch interfacial complex structure.

[0027] S4. Annealing and curing After completing step S3, stop vacuuming and steam injection, adjust the modified reaction chamber to atmospheric pressure, and simultaneously activate the built-in humidity and temperature control module. Precisely lower and maintain the chamber temperature at 50-55℃, strictly controlling it within the annealing range above the glass transition temperature of the modified yam starch system but below its initial gelatinization temperature. Simultaneously, supply atomized water vapor into the chamber to ensure the relative humidity is maintained at 85%-90%. In this specific microenvironment of high humidity and constant temperature, anneal the material for 12-16 hours. During this process, the water content of the starch system increases, and the amorphous molecular chains transform from a glassy state to a highly elastic state with some mobility. Chain segments can undergo limited-scale orientation and rearrangement, thereby gradually increasing structural order. Simultaneously, the mucoproteins enriched on the surface and interface of the starch granules, having undergone conformational unfolding during the previous heat treatment, form a stable interfacial composite structure with the starch molecular chains through non-covalent interactions such as hydrogen bonds. This interface structure exerts a certain spatial constraint on the migration and rearrangement of starch molecular chains, causing them to tend to form a relatively regular and dense double helix crystalline structure under restricted conditions.

[0028] The above annealing process achieves orderly rearrangement and stabilization of the internal structure of starch, improves its structural density and enzymatic tolerance, thereby promoting the formation of SDS and inhibiting the excessive generation of RS.

[0029] S5. Drying and pulverizing The annealed and cured material is removed from the modification chamber and spread evenly in a low-temperature hot air circulating drying oven. The drying temperature is set to 45°C, and a large amount of cold air circulation is used to remove surface free moisture until the absolute moisture content inside the material drops to 10 wt%. The dried lumps of material are then placed in an air jet mill for pulverization, and sieved through a 100-mesh stainless steel vibrating screen to collect the fine powder, thus obtaining the finished yam starch product with a high slow-digestible starch content. Store in airtight, light-proof packaging for later use.

[0030] Example 1: 1000g of fresh yam with an amylose content of 17wt%-25wt% was washed, peeled, and cut into chunks. 2000g of purified water was added, and the mixture was homogenized at 10000rpm for 3 minutes. The homogenate was then filtered through a 100-mesh nylon filter. While stirring, a 1mol / L food-grade citric acid solution was slowly added dropwise to the filtrate to adjust the pH to 4.2. After standing for 15 minutes, the mixture was centrifuged at 3000rpm for 10 minutes. The supernatant was discarded, and purified water was added to the resulting precipitate for thorough resuspending and dispersion. The pH was adjusted to 4.2 again, and the mixture was centrifuged after standing. This process was repeated twice. The precipitate was collected and further filtered to obtain a wet yam starch / mucoprotein coprecipitate with a water content of 40wt%. The obtained yam starch / mucoprotein coprecipitate was spread evenly on a stainless steel tray, with a thickness controlled at 2-3cm, and placed in an alternating pulse steam modification reaction chamber. The chamber was evacuated to -0.08 MPa and maintained for 15 minutes. High-temperature dry saturated steam at 110℃ was then instantaneously introduced through a controlled valve, bringing the chamber pressure to 0.20 MPa. This pressure and temperature were maintained for 6 minutes. The chamber was then instantly evacuated back to -0.08 MPa and maintained for 4 minutes. This pressure differential cycle was repeated 5 times. After the cycle was complete, the pressure was depressurized to atmospheric pressure, and the chamber temperature was set at 52℃ with humidity controlled at 88%. Annealing was then performed for 14 hours. After annealing, the sample was removed and placed in a 45℃ hot air dryer until the moisture content dropped to 10 wt%. Subsequently, the sample was air-jet pulverized and sieved through a 100-mesh sieve to obtain the finished yam starch sample.

[0031] Example 2 follows the same process as Example 1, except that during the vacuum alternating pulse steam treatment stage, 105°C high-temperature dry saturated steam is introduced during the positive pressure period, with an internal pressure of 0.15MPa and maintained for 8 minutes; the negative pressure is -0.08MPa and maintained for 5 minutes; the pressure difference is cyclically repeated 6 times.

[0032] Example 3 follows the same process as Example 1, except that during the vacuum alternating pulse steam treatment stage, 115°C high-temperature dry saturated steam is introduced at 0.25 MPa and held for 5 minutes under positive pressure; then -0.08 MPa is introduced and held for 3 minutes under negative pressure; this cycle is repeated 4 times.

[0033] Example 4 follows the same process as Example 1, except that in the initial extraction stage, the pH of the slurry is adjusted to 4.5 with citric acid solution, and the water content of the resulting wet yam starch / mucoprotein coprecipitate is controlled at 35wt%.

[0034] Example 5 follows the same process as Example 1, except that during the initial extraction process, the pH of the slurry is adjusted to 4.0 with citric acid solution, and the water content of the resulting wet yam starch / mucoprotein coprecipitate is controlled at 45 wt%.

[0035] Example 6 follows the same process as Example 1, except that during the annealing and curing stage, the temperature inside the chamber is kept constant at 55°C, the humidity inside the chamber is controlled at 90%, and the annealing process lasts for 16 hours.

[0036] Example 7 follows the same process as Example 1, except that during the annealing and curing stage, the temperature inside the chamber is kept constant at 50°C, the humidity inside the chamber is controlled at 85%, and the annealing process lasts for 12 hours.

[0037] Comparative Example 1: This comparative example uses the traditional alkaline extraction method and atmospheric pressure moist heat extraction method for starch extraction. 1000g of fresh yam was homogenized and filtered, and the supernatant was collected. The supernatant was soaked and washed three times with a 0.1mol / L sodium hydroxide solution using the alkaline method, and centrifuged at high speed until no protein reaction was observed in the supernatant, obtaining high-purity yam starch free of protein. The moisture content of the starch was adjusted to 45wt%, and the starch was spread evenly in a conventional high-temperature and high-pressure autoclave at 110℃ for continuous constant pressure moist heat treatment (HMT) for 1 hour. After the treatment, the starch was removed and directly dried in a 45℃ drying device to obtain powdered starch.

[0038] Comparative Example 2: This comparative example did not undergo vacuum alternating pulsed steam treatment. Instead, a conventional method was used, in which the yam starch / mucoprotein co-precipitate was placed in a conventional constant temperature and pressure reaction chamber and continuously steam-fumigated at 110°C and corresponding constant positive pressure for 40 minutes. After fumigation, it was immediately transferred to a chamber at 52°C and 90% humidity for annealing for 14 hours. The remaining steps were the same as in Example 1.

[0039] Comparative Example 3 did not undergo an annealing and curing process. Instead of the 14-hour gentle annealing and crystal curing operation at 52°C after vacuum alternating pulsed steam treatment, it was directly subjected to rapid hot air forced drying at 80°C to quickly dehydrate it to a moisture content of 10 wt%. The remaining steps were the same as in Example 1.

[0040] Product testing: 1. Solid-state nuclear magnetic resonance (NMR) 13 CCP / MAS NMR characterization (1) Parameter setting and detection process: Starch samples from Example 1 and Comparative Examples 1-3 were analyzed using solid-state nuclear magnetic resonance spectroscopy. Cross-polarized magic angle rotation (CP / MAS) was used, with the resonance frequency set to 150.9 MHz and the rotor rotation speed to 10 kHz. The contact time was set to 1 ms, the delay time to 5 s, and the cumulative number of scans was no less than 2400.

[0041] (2) Characterization results: As attached Figure 2As shown, S1, S2, S3, and S4 correspond to Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3, respectively. All four groups of yam starch samples exhibited characteristic signals of the starch carbon skeleton within the range of 58-105 ppm. Among them, the C1 carbon resonance signal located at 94-105 ppm is extremely sensitive to the spatial conformation of starch molecules. The sharpness of its multiple splitting peak and the width of its base directly reflect the relative content and density of the short-range ordered double helix structure inside the starch granules. This dense structure can effectively slow down the enzymatic hydrolysis rate of digestive enzymes and is the core internal structural basis determining high SDS content. Observation shows that the C1 region of Comparative Example 1 almost completely merges into a broad and gentle amorphous envelope, and the triplet characteristic essentially disappears. This indicates that the traditional high-temperature and high-humidity process lacks interfacial protection, leading to irreversible damage to the original double-helix structure within the starch. While the C1 region of Comparative Examples 3 and 2 faintly shows a triplet outline, the peak shape is relatively blunt, and the area of ​​the broad peak representing the amorphous region at the bottom is relatively large, indicating that the double-helix chain segment arrangement is still loose and crystal growth is incomplete. In contrast, Example 1 exhibits extremely sharp and towering triplet splitting peaks at chemical shifts of 99.5 ppm, 101.0 ppm, and 102.5 ppm in the C1 region, and the area of ​​the broad amorphous peak at the bottom is significantly reduced. The spectra show that this invention effectively promotes the ordered rearrangement of starch molecular chains, constructing a more compact and complete double-helix structure, thus exhibiting a higher SDS level.

[0042] 2. Morphological characterization by scanning electron microscopy (SEM) (1) Parameter setting and detection process: The powder morphology of Example 1 and Comparative Example 1 was analyzed using scanning electron microscopy. A very small amount of dried yam starch sample was taken and allowed to fall freely onto a conductive tape on a standard aluminum sample stage. Free particles were removed by blowing with low-pressure nitrogen to achieve monolayer dispersion. Subsequently, the sample was placed in an ion sputtering instrument, and a 10-15 nm thick gold conductive film was uniformly sputtered onto the sample surface under vacuum at a current of 15 mA to eliminate the charging effect. During operation, the accelerating voltage was set in the low-loss range of 5.0-10.0 kV to prevent thermal damage, the working distance was set to 8.0-10.0 mm, a secondary electron detector was used for morphology capture, and the magnification was controlled between 2000× and 3000× for imaging scanning.

[0043] (2) Characterization results: As attached Figure 3As shown, Figures A and B correspond to the yam starch samples of Example 1 and Comparative Example 1, respectively. The surface topology and interfacial deposits of starch granules directly determine the spatial accessibility of digestive enzyme invasion and are the core external physical defense basis for high SDS content. Observation shows that Comparative Example 1 (Figure B) exhibits a destructive collapse and disintegration morphology. The originally clear starch granule boundaries have completely merged into one, and the field of view is filled with an amorphous gel network and sponge-like huge pores. This indicates that traditional continuous high-temperature and humid heat treatment, without pre-protection, leads to violent water absorption, swelling, and excessive gelatinization inside the starch. Its huge specific surface area exposure greatly accelerates the hydrolysis by digestive enzymes. In contrast, Example 1 (Figure A) exhibits a typical controlled micro-damage morphology. The starch granule group still maintains a relatively complete natural elliptical or polygonal three-dimensional boundary and has not undergone gelatinization and fusion. At the same time, the granule surface is covered with physical roughening features and a large number of nanoscale exhaust micropores, and high-density flocculent mucoprotein complex polymers are clearly attached around the micropores and at the granule interface. The graphs visually demonstrate that the alternating pressure difference process of this invention achieves perfect preservation of the starch skeleton and promotes the formation of a robust interfacial composite coating layer on the outer surface of the particles by endogenous denatured proteins, thus constructing a strong three-dimensional spatial steric barrier and endowing the embodiments with excellent high SDS characteristics.

[0044] 3. Detection of starch components in vitro Sample preparation: Accurately weigh each powder sample to make up 200 mg of anhydrous starch base, and place it in a centrifuge tube with glass beads for later use.

[0045] Detection Procedure: An in vitro simulated digestion method was used. Simulated gastric juice containing pepsin was added to the sample, and the mixture was pretreated at 37°C with shaking for a certain period to simulate gastric digestion. The pH of the system was then adjusted to neutral, and a proportionally prepared mixed enzyme solution of porcine pancreatic α-amylase and amyloglucosidase was added. The enzymatic reaction was carried out in a 37°C constant-temperature shaking water bath. Small amounts of the reaction solution were pipetted at 20 minutes and 120 minutes of digestion, and the reaction was terminated with hot anhydrous ethanol. The supernatant was obtained by centrifugation, and the glucose production was determined using a GOD-POD colorimetric kit. The hydrolyzed fraction within the first 20 minutes of digestion was defined as RDS, the hydrolyzed fraction between 20 and 120 minutes as SDS, and the unhydrolyzed fraction after 120 minutes as RS. Results were expressed as a percentage by dry weight. Each sample was tested in triplicate, and the average value was taken.

[0046] 4. Gelatinization characteristics test Sample preparation: Weigh a certain amount of dry basis sample, mix it with deionized water at a certain mass ratio (e.g., 1:3), seal it in an aluminum crucible, and equilibrate overnight at room temperature.

[0047] Test procedure: Differential scanning calorimetry (DSC) was used to heat the sample from room temperature to 120°C at a certain heating rate (e.g., 10°C / min). The gelatinization initiation temperature (T0) and peak temperature (T) were recorded. p The enthalpy change (ΔH, J / g) and the average value were used for each sample group.

[0048] Table 1. Results of starch in vitro digestion fraction detection in examples and comparative examples.

[0049] Table 2. Results of starch gelatinization characteristics test in the examples and comparative examples.

[0050] Results analysis: (1) As shown in Table 1, Examples 1-7 all exhibited high SDS content, while the RDS content decreased significantly, indicating that the process of the present invention can effectively regulate the starch digestion rate distribution and shift the system towards slow digestion. Combined with Table 2, it can be seen that the gelatinization initiation temperature and peak temperature of the samples in each example generally shifted upwards, and the enthalpy change level was high, indicating that their internal structure was more ordered and stable. In comprehensive comparison, Example 1 showed the most balanced performance in all indicators, indicating that under moderate pressure differential cycling intensity and annealing conditions, the starch structure rearrangement was more complete, and the interface structure regulation effect was more significant. In Examples 2 and 3, under conditions where the vacuum alternating steam treatment parameters were too high or too low, the SDS content and gelatinization characteristics decreased slightly compared to Example 1, indicating a decrease in the orderedness of the starch structure, suggesting that there is a reasonable optimization range for these parameters. In Examples 4 and 5, by adjusting the pH and water content of the coprecipitate in the initial extraction stage, the enrichment of mucoprotein and interfacial interactions were affected, resulting in certain fluctuations in the SDS content. Examples 6 and 7 show that the SDS and RS contents vary slightly under different annealing conditions, but they have a direct impact on the gelatinization temperature and enthalpy change, indicating that they play a key role in perfecting the crystal structure.

[0051] (2) The SDS content of Comparative Example 1 was significantly reduced, while the RDS content was significantly increased, indicating that under the conditions of removing endogenous proteins and using conventional moist heat treatment, the starch structure tends to form a structure that is easily acted upon by digestive enzymes, while lacking the ability to effectively regulate the digestion rate. As shown in Table 2, its gelatinization temperature and enthalpy change were both at a low level, reflecting insufficient structural stability and low orderliness. Although the SDS content of Comparative Example 2 was increased under the condition of retaining proteins but not using pressure differential treatment, it was still lower than that of the Example, indicating that simple steam treatment is difficult to achieve effective structural regulation. Its gelatinization characteristic parameters were improved compared to Comparative Example 1, but still did not reach the level of the Example, indicating that the pressure differential alternation process plays an important role in promoting uniform structural rearrangement.

[0052] (3) The SDS content of Comparative Example 3 was at a moderate level, while the ratios of RDS and RS were not effectively controlled, indicating that the lack of an annealing process would affect the stabilization of the starch structure. Table 2 shows that although its gelatinization temperature and enthalpy change were higher than those of Comparative Example 1, they were still significantly lower than those of the Example, indicating that structural order and thermal stability were not fully established. This demonstrates that the annealing process plays a crucial role in promoting the ordered rearrangement of molecular chains and stabilizing the crystal structure, and is an important step in achieving efficient SDS generation.

[0053] In summary, this invention achieves effective control of starch structure by retaining the endogenous mucoprotein of yam and combining vacuum alternating pulse steam treatment with subsequent annealing process, so that it can maintain a high SDS content while having good structural stability and thermal properties.

Claims

1. A clean modification process for yam starch with high slow-digestible starch content, characterized in that, Includes the following steps: S1: Cut the peeled and washed fresh yam into chunks, add purified water, and homogenize at high speed to obtain a uniform slurry; filter the slurry, add acid to adjust its pH value, let it stand, centrifuge to discard the supernatant and obtain the precipitate, resuspend the precipitate with water, readjust the pH value and centrifuge again. Repeat the above process twice, collect the precipitate, filter by pressure to obtain yam starch / mucoprotein coprecipitate; S2: The obtained yam starch / mucoprotein coprecipitate is sent into a closed alternating pulse steam modification reaction chamber, and the vacuum is maintained under negative pressure for exhaust treatment. S3: High-heat dry saturated steam is introduced into the alternating pulse steam modification reaction chamber that is maintaining a negative pressure state, so that the chamber is reversed to a positive pressure state and the pressure is maintained; after the pressure is maintained, vacuum is drawn and exhaust is performed, so that the chamber returns to a negative pressure state and the pressure is maintained; the above positive pressure state and negative pressure state are regarded as a complete pressure difference alternating cycle, and the cycle is continuously performed. S4: After the cycle ends, the alternating pulse steam modification reaction chamber is adjusted to normal pressure, the temperature and humidity inside the chamber are adjusted, and continuous annealing treatment is carried out to obtain solidified material. S5: The solidified material is dried to the target moisture content by low-temperature hot air circulation, and then pulverized and sieved to obtain the yam starch with high slow-digestible starch content.

2. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The fresh yam mentioned in step S1 is a winter fresh yam in its dormant period, and its natural starch contains 17wt%-25wt% amylose; the weight ratio of the fresh yam chunks to the purified water is 1:2; the high-speed homogenization process is performed at 10,000 rpm for 3 minutes; and the filtration is done using a 100-mesh nylon filter.

3. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The acid solution mentioned in step S1 is a food-grade citric acid solution with a concentration of 1 mol / L; the pH value is adjusted to 4.0-4.5; and the water content of the yam starch / mucoprotein coprecipitate is 35wt%-45wt%.

4. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The vacuum level of the exhaust treatment in step S2 is -0.08 MPa, and the maintenance time is 15 minutes.

5. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The temperature of the high-temperature dry saturated steam in step S3 is 105-115℃.

6. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, In step S3, the positive pressure is 0.15-0.25 MPa, and the pressure holding time is 5-8 minutes; the negative pressure is -0.08 MPa, and the pressure holding time is 3-5 minutes.

7. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The number of cycles in step S3 is 4-6.

8. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, In step S4, the temperature and humidity inside the chamber are adjusted to 50-55℃ and 85%-90%, respectively; the duration of the continuous annealing treatment is 12-16 hours.

9. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The drying temperature in step S5 is 45°C; the target moisture content is 10 wt%.

10. The clean modification process for yam starch with high slow-digestible starch content as described in claim 1, characterized in that, The crushing and screening process described in step S5 involves placing the dried, lumpy material into an air jet mill for crushing, and then screening it through a 100-mesh stainless steel vibrating screen.

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