Method for improving edible quality of brown rice by using microwave in combination with GABA

Abstract

This invention relates to a method for improving the edible quality of brown rice using GABA combined with microwave heating, comprising the following steps: hulling paddy rice to obtain brown rice; screening the brown rice to remove germless grains, discolored grains, immature grains, and shriveled grains, then washing and draining; milling the brown rice to remove part of the bran; immersing the brown rice in a GABA solution at a constant temperature; and microwave-heating the GABA-immersed brown rice to gelatinize it, obtaining cooked brown rice. Compared with existing technologies, this invention accelerates the softening of the brown rice bran, utilizes the biochemical properties of GABA to increase the water penetration channels while retaining the nutritional components of the brown rice, thus altering its texture; and leverages the core advantages of microwave heating, which offers high heating efficiency and rapid overall temperature rise, eliminating the need for prolonged contact with high-temperature water media, reducing the destruction of heat-sensitive nutrients and the loss of volatile flavor compounds, improving the texture of brown rice while shortening heating time and increasing production efficiency.

Description

Technical Field

[0001] This invention relates to the field of grain processing technology, specifically to a method for improving the edible quality of brown rice using microwaves in conjunction with GABA. Background Technology

[0002] Brown rice refers to whole-grain rice that has only had its outer husk removed, retaining the endosperm, germ, and most of the bran. Compared to refined white rice, brown rice retains the complete nutritional structure of the rice grain, with significantly higher levels of protein, dietary fiber, vitamins (especially B vitamins), minerals (such as magnesium, phosphorus, and potassium), and bioactive substances such as gamma-aminobutyric acid (GABA), oryzanol, and γ-oryzanol. Studies have shown that replacing refined white rice with brown rice as a staple food helps reduce the risk of type 2 diabetes, cardiovascular disease, and obesity, playing an important role in promoting human health. With the increasing public awareness of nutrition and health, the application prospects of brown rice as a high-nutrition alternative to refined white rice are attracting increasing attention.

[0003] However, the high fiber content and dense bran structure of brown rice significantly negatively impact its edible quality. During cooking, brown rice has difficulty allowing water to penetrate, resulting in hard, coarse grains, low viscosity, poor palatability, and longer cooking time with undesirable flavor. These inherent defects in edible quality make brown rice difficult for consumers to accept, severely restricting its market promotion and large-scale application as a staple food. Therefore, effectively improving the edible quality of brown rice while preserving its original nutritional components and whole-grain nutritional characteristics as much as possible is a key technical problem that urgently needs to be solved in the current brown rice processing industry.

[0004] Currently, to improve the edible quality of brown rice, the industry and academia have explored various processing methods, mainly including high-milling refining, freeze-thaw treatment, ultra-high pressure soaking, and microwave-assisted processing. High-milling refining removes the outer layers, including the bran, by increasing the degree of milling. While this can improve the taste to some extent, it severely damages dietary fiber, vitamins, minerals, and various bioactive substances, completely negating the nutritional advantages of brown rice as a whole grain and contradicting current dietary health concepts.

[0005] Freeze-thaw processing involves repeatedly freezing and thawing brown rice to disrupt its outer layer structure and promote water penetration. However, this method often results in large ice crystals formed during freezing, distributed in the intercellular spaces. These unevenly distributed ice crystals can cause irreversible damage to the brown rice cell structure during thawing, leading to significant nutrient loss and a general decline in food quality, failing to meet the nutritional preservation requirements of food processing. Chinese patent CN108094841A discloses a technical solution for improving the texture of brown rice using freeze-thaw processing, which involves soaking enzyme-deactivated brown rice in water, treating it with ultrasound, and then repeatedly freezing and thawing it. However, this method still cannot overcome the inherent defects of freeze-thaw processing, such as tissue damage and nutrient loss, thus significantly limiting its industrial application.

[0006] Ultra-high pressure soaking utilizes ultra-high pressure to rapidly penetrate and evenly distribute water into brown rice, which can shorten cooking time and improve color to some extent. Chinese patent CN110916087A discloses a method for improving the whiteness of parboiled rice by ultra-high pressure soaking followed by microwave cooking. However, this method relies on ultra-high pressure processing equipment, which is expensive to manufacture, has high maintenance costs, limited production capacity, and stringent requirements for production sites and operating conditions. Small and medium-sized enterprises cannot afford the costs of widespread application, resulting in significant limitations in industrial application.

[0007] Microwave heating technology has been widely used in food processing. Its heating principle utilizes the alternating electric field of microwaves to induce high-speed rotational friction among polar molecules (mainly water molecules) within food materials, converting microwave electromagnetic energy into heat energy, thus achieving simultaneous heating of the entire material. Compared to traditional external heat source conduction heating, microwave heating offers numerous advantages, including rapid heating, high heat conversion efficiency, good heating uniformity, and ease of precise control. Currently, some research has applied microwave heating to the cooking process of brown rice, aiming to shorten processing time by improving heating efficiency and by leveraging the penetrating power of microwaves to shorten the cooking cycle. However, existing microwave brown rice processing technologies still have limited effectiveness in improving texture. Simple microwave heating cannot effectively soften the dense outer layer structure of brown rice, failing to fundamentally solve the core problem of water penetration difficulties during brown rice cooking. Furthermore, under high-power microwave conditions, localized overheating may affect the product's flavor and nutritional quality. Therefore, how to effectively overcome the technical bottleneck of microwave processing alone in improving the quality of brown rice while preserving the whole grain nutrition of brown rice, and develop a comprehensive processing method that can significantly improve the edible quality of brown rice, remains a key problem that urgently needs to be solved in the field of brown rice processing technology. Summary of the Invention

[0008] The inventors discovered that the root cause of the deterioration in the edible quality of brown rice lies not only in the dense and tough dietary fiber structure of the bran layer, but also in the fact that the residual bran layer after the dietary fiber is removed still contains a large amount of protein. During conventional processing and heating, these proteins easily aggregate disorderly on the surface of starch granules, forming a dense physical barrier that severely hinders the penetration and diffusion of water into the endosperm during cooking. This results in insufficient starch gelatinization, prolonged cooking time, and an undercooked texture—soft on the outside and hard on the inside. Simultaneously, the disorderly aggregated proteins can also non-specifically bind to starch granules, damaging the gelatinization properties of starch and affecting the palatability and processing suitability of brown rice. Therefore, regulating the aggregation state of brown rice bran proteins, inducing orderly protein aggregation, and breaking down the water penetration barrier are key to fundamentally improving the edible quality of brown rice.

[0009] Further research revealed that existing thermally induced regulation methods, such as conventional water bath heating and steam heating, suffer from slow heating rates, insufficient temperature control precision, and uneven temperature distribution. The varying degrees of heating in different regions of the protein molecules make it difficult to target and regulate the aggregation process, easily leading to disordered protein aggregation and the formation of barrier protein films. Chemical modification regulation methods are limited by stringent reaction conditions and a limited range of food-grade safe reagents; most exogenous chemical reagents pose residue risks, highlighting significant food safety concerns. Therefore, there is an urgent need to develop a brown rice processing method that is safe and residue-free, can precisely regulate the orderly aggregation of bran proteins, and simultaneously maintains both edible quality and nutritional retention. This invention proposes a method for improving the edible quality of brown rice using microwave combined with GABA (γ-aminobutyric acid) treatment. This invention pre-treats brown rice with a small-molecule GABA solution, followed by cooking and gelatinization in a microwave alternating electric field. The orderly aggregation of brown rice bran proteins is achieved through the synergistic regulation of GABA dielectric buffering and microwave-induced orderly aggregation.

[0010] The objective of this invention can be achieved through the following technical solutions: One objective of this invention is to provide a method for improving the edible quality of brown rice using GABA combined with microwave heating, comprising the following steps: (1) Hull the rice to obtain brown rice; (2) Screen the brown rice obtained in step (1) to remove grains without germ, discolored grains, immature grains and shriveled grains, and wash and drain it. (3) The brown rice obtained in step (2) is milled to remove part of the bran; (4) Place the brown rice obtained in step (3) into a GABA solution and let it soak and penetrate at a constant temperature; (5) Brown rice soaked in GABA is heated and cooked in microwave to gelatinize it, and brown rice is obtained.

[0011] Furthermore, the rice hulling described in step (1) refers to removing only the outer husk of the rice grains while retaining the bran layer, germ, and endosperm.

[0012] Further, the degree of milling in step (3) is 0%, 2%, 4%, 6% or 8%. The degree of milling refers to the percentage of the weight of brown rice lost during the milling process relative to the weight of brown rice before milling. The calculation formula is: (weight of brown rice before milling - weight of brown rice after milling) / weight of brown rice before milling × 100%.

[0013] Furthermore, in step (4), the mass concentration of the GABA solution is 0.04%~0.16%.

[0014] Furthermore, in step (4), the immersion temperature of the GABA solution is 50℃~60℃, and the immersion time is 1 h~1.5 h.

[0015] Further, in step (4), the ratio of brown rice to GABA solution is 1:1.5 to 1:2.5.

[0016] Furthermore, in step (5), the power of microwave heating is 10 W / g to 27.5 W / g.

[0017] Furthermore, in step (5), the temperature for cooking and gelatinizing is 80℃~90℃.

[0018] Furthermore, in step (5), the cooking and gelatinization time is 25 min to 45 min.

[0019] The second objective of this invention is to produce brown rice using the method described above.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention effectively shortens the optimal cooking time of brown rice through the synergistic effect of milling treatment and GABA solution soaking. Experimental results show that as the milling degree gradually increases from 0% to 8%, the optimal cooking time of brown rice is significantly shortened from 33.67 min to 13.67 min, a reduction of up to 59.4%. GABA solution soaking can also gently shorten the optimal cooking time, with a reduction of 10% to 20%. The improvement effect is even better when GABA treatment is combined with microwave heating. The water absorption rate is significantly negatively correlated with the optimal cooking time, with a correlation coefficient of -0.89 and a significance level of P less than 0.01, indicating that this invention effectively eliminates the barrier effect of the dense bran layer on the surface of brown rice on water penetration and greatly improves processing efficiency.

[0021] (2) After treatment by the method of the present invention, the hardness and viscoelasticity of brown rice were significantly improved. Compared with the blank control group, the hardness of brown rice in the microwave synergistic GABA treatment group decreased from 1708.53±113.01g to 849.41±96.78g, a decrease of 50.3%; the viscoelasticity increased from 158.33±0.71g to 222.97±4.41g, an increase of 40.9%. The hardness of the water bath synergistic GABA treatment group decreased from 1860.91±143.72g to 945.47±91.01g, a decrease of 49.2%; the viscoelasticity increased from 136.95±0.93g to 168.21±1.87g, an increase of 22.8%. It can be seen that the synergistic improvement effect of microwave and GABA is significantly better than that of traditional water bath heating, which makes the brown rice form a uniform and dense gel network, significantly improving the palatability.

[0022] (3) The treatment of this invention caused all the gelatinization parameters of brown rice to show a significant upward trend. As the GABA concentration increased, the peak viscosity, disintegration value, trough viscosity, final viscosity and recovery value all steadily increased, and the increase in parameters was more obvious in the microwave treatment group. GABA treatment can effectively relax the dense protein barrier on the surface of brown rice, promote the rapid penetration of water molecules into the interior of the rice grain, make the starch granules more likely to absorb water and swell and dissociate from the rice grain matrix, thereby significantly improving the water absorption and swelling capacity and gel structure stability of brown rice.

[0023] (4) The milling process of this invention does not damage the internal crystal structure of rice starch, while the GABA concentration has a significant regulatory effect on the thermal properties of brown rice, and this effect is particularly prominent in the microwave treatment group. After soaking in different concentrations of GABA, the gelatinization enthalpy of the samples increased significantly and was positively correlated with the GABA concentration. Compared with traditional water bath heating, the starch gelatinization enthalpy of the microwave treatment group was significantly higher, indicating that microwaves, with their rapid and uniform endogenous heating advantages, effectively preserve the structural integrity of starch crystals, while enhancing the complexation between starch and matrix components, further stabilizing the starch structure.

[0024] (5) After treatment according to the present invention, the storage modulus and loss modulus of brown rice are significantly improved, and the storage modulus is significantly greater than the loss modulus, indicating that the system exhibits strong gel properties. With the increase of milling degree and GABA concentration, both the storage modulus and loss modulus show an upward trend, which is attributed to the fact that the removal of bran layer reduces physical obstacles, allowing starch particles to fully contact and form a denser cross-linked network. The storage modulus and loss modulus of brown rice after microwave treatment are higher than those of the water bath treatment group, indicating that microwave treatment can improve the cross-linking degree of the gel network, allowing starch to absorb energy from both the inside and outside, and significantly improving the hydration rate and uniformity of starch.

[0025] (6) X-ray diffraction and Fourier transform infrared spectroscopy analysis showed that the treatment of brown rice starch in this invention did not change the type A crystal structure, nor did it introduce new active functional groups, and no significant chemical modification was produced. However, as the GABA concentration increased, the 3380 cm⁻¹ crystal structure of brown rice starch decreased. -1 The absorption peak shifts towards higher wavenumbers and its intensity increases, indicating enhanced hydrogen bonding interactions between starch molecules. Microwave treatment further disrupts the hydrogen bonds within the starch chain, causing the helical structure to dissociate from its entangled state, optimizing the ordered arrangement of starch molecules while maintaining the integrity of the crystal structure.

[0026] (7) Scanning electron microscopy observation showed that the high-milled brown rice treated according to the present invention exhibited a uniform and dense network structure after steaming, with increasingly dense pore distribution on the surface of starch granules. After soaking in GABA solution, a suitable amount of pores appeared in the cross-section of the brown rice, and the starch granules were fully gelatinized, forming a fluffy structure of flocculent and small lumps. After microwave treatment, a large number of pits formed by the detachment of starch granules could be seen inside the brown rice, and the opening size of the pits was relatively large, indicating that the starch granules had undergone significant separation, which promoted the uniformity of starch gelatinization and was highly consistent with the characteristic of increased viscosity.

[0027] (8) Low-field nuclear magnetic resonance analysis showed that the higher the milling degree and the higher the GABA concentration, the darker the color of the water distribution inside the brown rice grains, and the more fully the water penetrated. The improvement effect of the microwave-assisted GABA treatment group was significantly better than that of the water bath-assisted GABA treatment group. This is because the endogenous heating of microwaves can raise the temperature of the brown rice inside and out simultaneously, accelerating the regulatory effect of GABA on protein structure, and promoting the rapid movement of water molecules, allowing them to penetrate more efficiently and be evenly distributed inside the grains. When high milling degree is combined with high concentration of GABA and microwave treatment, the brown rice has the most uniform water distribution and the most sufficient water penetration.

[0028] (9) By appropriately controlling the milling degree, this invention significantly improves the cooking quality and taste of brown rice while maximizing the retention of its nutritional components. When the milling degree is 2%, the dietary fiber retention rate in brown rice is 97.20%, and the protein retention rate is 96.20%; when the milling degree is 4%, the dietary fiber retention rate is still 93.50%, and the protein retention rate is 95.53%. Compared to the quality improvement brought about by high milling, the low milling degree range selected in this invention achieves a good balance between nutrient retention and quality improvement.

[0029] In summary, this invention combines GABA solution soaking with microwave heating and cooking, supplemented by appropriate milling, to significantly improve the edible quality of brown rice from multiple dimensions, including cooking characteristics, textural quality, gelatinization characteristics, thermodynamic properties, rheological properties, molecular structure, microstructure, and moisture distribution, while retaining the main nutrients of brown rice. This provides a novel, efficient, controllable, and nutrient-retention-rich processing method for high-quality processing of brown rice. Attached Figure Description

[0030] Figure 1 A graph showing the combined effects of GABA treatment and milling on the cooking quality of brown rice; Figure 2 The hardness of the sample after cooking; Figure 3 The viscoelasticity of the sample after cooking; Figure 4 The storage modulus (G′) characteristics of brown rice under microwave cooking conditions; Figure 5 The loss modulus (G″) characteristics of brown rice under microwave cooking conditions; Figure 6 The storage modulus (G′) characteristics of brown rice under water bath cooking conditions; Figure 7 The loss modulus (G″) characteristics of brown rice under water bath cooking conditions; Figure 8 The image shows the FTIR spectrum of brown rice after milling. Figure 9 The image shows the FTIR spectrum of brown rice after soaking. Figure 10 The crystal structure of brown rice was determined by XRD. Figure 11 Microscopic morphological changes of brown rice grains under water bath cooking conditions; Figure 12 Microscopic morphological changes of brown rice grains under microwave cooking conditions; Figure 13 Microscopic morphological changes of brown rice grain cross-section under water bath cooking conditions; Figure 14 Microscopic morphological changes of brown rice grain cross-section under microwave cooking conditions; Figure 15 The effect of different processing methods on the moisture distribution of brown rice. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0032] Unless otherwise stated, the following experimental materials and general methods were used in the following examples and comparative examples: Experimental materials: Huruan 1212 brown rice, produced in Chongming, Shanghai, and harvested in October 2023.

[0033] Preparation of Dosage of Milling (DOM): 100g of purified brown rice was weighed and milled for different durations using an experimental rice milling machine to prepare a series of samples with milling degrees of 0% (unmilled), 2%, 4%, 6%, and 8%. The degree of milling was calculated using the formula DOM = (M - M0) / M × 100%, where M is the total mass of brown rice before milling and M0 is the mass of brown rice after milling. After sieving through a 2.00mm round-hole sieve, the samples were sealed in polyethylene bags and stored at 4℃ until use.

[0034] GABA solution soaking: Soak brown rice in a GABA solution of appropriate concentration at a constant temperature of 60℃ for 1 hour. After soaking, remove the rice, drain the surface water, and set aside.

[0035] Aging process: Water bath (WB) ripening: Pretreated brown rice was evenly spread in a clean aluminum box at a rice-to-water ratio of 1:2 and placed in a constant-temperature water bath for ripening. Throughout the water bath heating process, a high-precision thermocouple (accuracy ±0.1℃) was inserted into the brown rice system inside the aluminum box in real time to continuously record the changes in heating time and corresponding system temperature. The time-temperature curve of the water bath heating was plotted to clarify key parameters such as the heating rate and isothermal stage of the brown rice system during the water bath heating process, thus constructing an isothermal heating mode. To ensure that the heating characteristics of subsequent microwave treatment and water bath heating are consistent, the operating program of the microwave (MW) solid-phase synthesis / extraction instrument was precisely set based on the plotted time-temperature curve, strictly matching key parameters such as the heating rate and isothermal duration of the water bath heating to ensure that the heating rate of the brown rice system during microwave treatment is synchronized with that of the water bath heating.

[0036] Microwave (MW) cooking: Pre-treated brown rice is evenly spread in a clean aluminum box at a rice-to-water ratio of 1:2, ensuring that the brown rice is completely submerged in water. A microwave solid-phase synthesis / extraction instrument operating program based on an isothermal heating mode is used to precisely set microwave power and time parameters, ensuring that the heating rate of the brown rice system during microwave treatment is synchronized with the water bath heating.

[0037] The quality indicators of the produced brown rice are defined and analyzed using the following methods: (1) Texture (hardness and viscosity): Texture is the most important indicator for evaluating the palatability of rice and can be used to determine the palatability of cooked rice. The texture index of cooked brown rice was determined according to the method described by Jie Yan et al. (A combination of germination and moderate milling improves the textural quality and altersstarch digestion characteristics of brown rice[J]. International Journal of Biological Macromolecules, 2025 (4): 320). The specific operation is as follows: The rice grains were cooked according to the previous study. In short, 20 g of rice grains were mixed with 26 g of distilled water, cooked for 30 min, and kept warm for 20 min. The texture characteristics were analyzed by a texture analyzer (TA-XT2i, Shanghai Boxin Industrial Development Co., Ltd., Shanghai, China), following the procedure described in the previous study with some modifications. Three pieces of rice were placed in the middle of the test platform and measured using a 36 mm probe. Each group contained 10 replicates for measurement. The test conditions were as follows: the speed before and after the test was set to 5.0 mm / s, and the test speed was adjusted to 0.5 mm / s.

[0038] (2) Optimal cooking time: The white core method was used, and the method described by Thirumdas et al. (Influence of low pressure cold plasma on cooking and textural properties of brown rice[J].Innovative Food Science & Emerging Technologies, 2016 (37), 53-60) was used to determine the optimal cooking time. A brown rice sample (7 g) was added to boiling water and steamed for 20 min. Subsequently, at least 10 grains of brown rice were removed every 1 minute and compressed and observed between two glass slides (75 mm × 25 mm). The time when all rice grains were fully cooked (without a white core in the center of the grain) was recorded as the optimal cooking time.

[0039] (3) Water absorption rate: Take 5 g (denoted as m0) of sample and place it in a beaker, add 10 mL of distilled water, and heat to the optimal cooking time. Blot the surface moisture of the brown rice with absorbent paper, cool to room temperature, and weigh (denoted as m1). The water absorption rate during the brown rice processing is calculated according to the following formula: Water absorption rate (%) = (m1-m0) / m0×100%.

[0040] Determination of the basic nutrient content of brown rice: The method for determining protein content refers to the national standard GB 5009.5-2025; The method for determining starch content refers to the national standard GB 5009.9-2016; The method for determining ash content follows the national standard GB 5009.4-2016; The dietary fiber content was determined according to the national standard GB 5009.88-2014, which specifies the total dietary fiber content.

[0041] Example 1 This embodiment provides a method for improving the edible quality of brown rice using GABA combined with microwave heating, including the following steps: ①The newly harvested rice is cleaned of impurities and then hulled to obtain brown rice as a raw material for processing. ② The obtained brown rice is screened to remove grains without germ, discolored grains, immature grains and shriveled grains, and then washed and drained. ③ Weigh the purified brown rice and use a rice milling machine to prepare samples with milling degrees of 0%, 2%, 4%, 6% and 8% respectively; ④ Prepare GABA solutions with mass concentrations of 0%, 0.04%, 0.08%, 0.12%, and 0.16% as soaking solutions for later use; ⑤ Place the brown rice with different milling degrees obtained in step ③ into the GABA solution with different mass concentrations prepared in step ④ at a material-to-liquid ratio of 1:15, and soak and penetrate at a constant temperature of 60℃ for 1 hour. After the soaking and penetration, wash and drain the rice for later use. ⑥ Add distilled water to the brown rice soaked in GABA at a rice-to-water ratio of 1:2, spread it evenly in a clean aluminum box, place it in a microwave heating device, heat it for 30 minutes at a power of 20 W / g, and cook it at a gelatinization temperature of 80℃~90℃ to obtain brown rice.

[0042] Comparative Example 1 This comparative example is basically the same as the embodiment, except that step ⑥ is different. In this comparative example, step ⑥ involves heating the aluminum box in a constant temperature water bath.

[0043] Results and Analysis: (1) Effects of different milling precision on the nutrients in brown rice Table 1 reflects the effect of different milling degrees on the content of basic nutrients in brown rice. The main component of brown rice is starch, accounting for approximately 60%–80% of the total. As the milling reduction rate increases, the content of nutrients such as protein and dietary fiber in brown rice gradually decreases. At a milling degree of 2%, the retention rates of dietary fiber and protein in brown rice are 97.20% and 96.20%, respectively. At a milling degree of 4%, the retention rates of dietary fiber and protein remain at 93.50% and 95.53%, respectively. When the milling degree reaches 6%, the retention rates are only 83.82% and 85.91%, respectively. As the degree of milling gradually increases, more of the bran and germ are removed. Studies have confirmed that during the milling process of brown rice (MER P, BARUAH C, NEERAJA CN, et al. Comprehensive nutritional profile of biofortified and non-biofortified rice subjected to processing and different degrees of milling [J]. Journal of Cereal Science, 2025, 126: 104258.), with the increase in milling precision, the pericarp, seed coat, and nucellus of the bran are removed in sequence, and the aleurone layer is gradually exposed and eventually removed, resulting in a significant decrease in the content of major nutrients in brown rice (P<0.05). The bran and germ are removed during milling, and these parts are rich in fat and protein. The starch content gradually increases with the increase of milling degree because other components of the bran layer are continuously reduced during milling, increasing the proportion of starch.

[0044] Table 1. Changes in basic nutritional components of brown rice under different milling degrees. (2) Effects of different processing methods on the cooking characteristics of brown rice Figure 1 This study demonstrates the combined effects of GABA treatment and milling on the cooking quality of brown rice. Figure 1 a to Figure 1 The samples e represent milling degree (DOM) values ​​of 0%, 2%, 4%, 6%, and 8%, respectively. The main evaluation indicators are optimal cooking time (min) and water absorption rate (%). Optimal cooking time reflects the shortest time required for brown rice to reach the appropriate level of doneness, while water absorption rate characterizes its ability to absorb water during cooking. These two indicators are core evaluation metrics and are directly related to processing efficiency and eating experience.

[0045] As the milling degree gradually increases, the optimal cooking time for brown rice significantly decreased from 33.67 min to 13.67 min, a reduction of 59.4% (P<0.05). The dense bran layer on the surface of brown rice is rich in dietary fiber, lipids, and proteins, forming a natural physical barrier that hinders water molecule penetration and delays the binding of starch and water. Water absorption rate directly determines cooking characteristics; the higher the water absorption rate, the shorter the cooking time. The dense bran layer on the surface of brown rice significantly hinders water penetration during cooking, and water absorption rate, as a core indicator, directly determines the cooking characteristics of brown rice. Therefore, the significant reduction in the optimal cooking time is mainly due to the effective removal of the bran layer structure during milling, thereby eliminating the main barrier to water penetration.

[0046] Soaking in GABA solution can also shorten the optimal cooking time, but the reduction is mild (10%-20%) and gradually decreases with increasing GABA concentration. The mechanism is speculated to be that GABA, as a small molecule active substance, can interact with proteins on the surface of brown rice, breaking hydrogen bonds, relaxing the dense structure, increasing the specific surface area, promoting the adsorption and penetration of water molecules, and at the same time weakening the inhibition of starch gelatinization by proteins, thus helping to shorten the cooking time.

[0047] MW treatment, with its advantages of rapid heating and uniform heat transfer, exhibits superior improvement effects when synergistically combined with GABA. Endogenous heating by MW avoids uneven heat transfer, enhances the relaxation effect of GABA, and accelerates water penetration and starch swelling. Water absorption rate and optimal cooking time show a significant negative correlation (r=-0.89, P<0.01), increasing with the degree of treatment. During cooking, starch swelling and gelatinization depend on water diffusion efficiency and the motion state of water molecules, thus affecting the cooking rate and eating quality. These three factors are interconnected and jointly determine the cooked quality of brown rice. (3) Effects of different processing methods on the textural properties of brown rice Figure 2-3 The textural properties of the cooked samples are shown. Hardness and viscoelasticity are core indicators for evaluating the textural characteristics of cooked rice, directly determining palatability. Hardness reflects the force required for chewing, while viscoelasticity reflects the elastic recovery and viscosity during chewing; together, they constitute the core evaluation criteria for the eating experience. Milling degree, as a key parameter in brown rice processing, has a significant regulatory effect on textural characteristics. As the milling degree increases, the surface bran, germ, and dietary fiber of brown rice are gradually removed, effectively promoting the hydration of starch and water molecules, thereby improving the granular crystal structure, promoting full gelatinization, and forming a uniform and dense gel network inside the rice, ultimately significantly improving viscoelasticity and reducing hardness. In addition, low-milled brown rice has high hardness and poor viscoelasticity because its rich fiber, protein, and other non-starch components have strong water-holding capacity, competing with starch for water during cooking and inhibiting starch swelling and gelatinization.

[0048] Besides milling degree, increased GABA concentration also significantly alters the texture of brown rice, exhibiting a synergistic effect with heating method. Compared to the blank control group, the hardness of brown rice in the MW synergistic GABA group decreased from 1708.53±113.01g to 849.41±96.78g (a decrease of 50.3%), while the viscoelasticity increased from 158.33±0.71g to 222.97±4.41g (an increase of 40.9%). In the WB synergistic GABA group, the hardness decreased from 1860.91±143.72g to 945.47±91.01g (a decrease of 49.2%), while the viscoelasticity increased from 136.95±0.93g to 168.21±1.87g (an increase of 22.8%). This demonstrates that the synergistic improvement effect of microwave and GABA is superior, providing data support for process optimization. This phenomenon is closely related to starch gelatinization. GABA can regulate the aggregation state of proteins on the surface of brown rice, break intermolecular hydrogen bonds, loosen the dense protein barrier, promote full water penetration, and thus promote starch swelling and gelatinization and improve texture.

[0049] Compared to the WB treatment group, the MW treatment group exhibited superior texture improvement. This difference is likely primarily due to the unique non-thermal effect of microwave heating. This effect can break the glycosidic bonds between starch molecules without significantly increasing the system temperature, causing long-chain starch to degrade into short-chain fragments. The short-chain structure significantly reduces the thermal stability of starch and weakens the intermolecular forces. During the gelatinization process of brown rice, the thermally unstable linear starch molecules can absorb water, swell, and stretch more fully, forming more hydrogen bonds. This increases the total viscosity of the starch gel, making the gel network structure inside the rice denser and more uniform, ultimately resulting in lower hardness and higher viscoelasticity, further improving the eating quality of brown rice.

[0050] (4) Effects of different processing methods on the gelatinization characteristics of brown rice Gelatinization characteristics are one of the core indicators for evaluating the cooking quality of brown rice. Specifically, it reflects the complete process of starch granules transforming from an ordered crystalline state to a disordered gel state under the combined influence of moisture, temperature, and shear force during cooking. This process encompasses a series of physicochemical changes, including melting of the starch microcrystalline structure, granule water absorption and swelling, structural rupture, and subsequent recombination. This characteristic is usually closely related to the texture, stickiness, and elasticity of cooked rice, directly determining the consumer's eating experience. With increasing milling degree, all gelatinization parameters of brown rice show a significant upward trend. The core reason for this phenomenon is that the milling process removes the dense bran layer on the surface of brown rice. This bran layer acts as a physical barrier, hindering water penetration into the grain, and its lipids and proteins bind with starch molecules, jointly inhibiting starch swelling and gelatinization. When the bran layer is removed, this inhibitory effect is significantly weakened, and the gelatinization characteristics of brown rice are significantly improved. However, as shown in Table 2, compared to the effect of milling degree, the regulatory effect of GABA concentration and heating treatment method on the gelatinization characteristics of brown rice is more significant: with the gradient increase of GABA concentration, all gelatinization parameters of the MW treatment group and the WB heating treatment group showed a stable upward trend, with the increase of parameters in the microwave treatment group being more obvious. Peak viscosity directly reflects the water absorption and swelling potential and water holding capacity of starch granules during gelatinization. The higher the peak viscosity, the greater the swelling degree of the starch granules and the stronger the water holding capacity. Disintegration value is the difference between peak viscosity and trough viscosity. Trough viscosity is the lowest viscosity that starch granules can maintain under continuous high-temperature shear after reaching maximum swelling. Therefore, disintegration value is usually closely related to the stability, swelling and rupture tendency of starch granules under high-temperature shear environment. The higher the disintegration value, the easier it is for starch granules to rupture during gelatinization. The significant increase in the disintegration value of brown rice in this invention indicates that GABA treatment effectively relaxes the dense protein barrier on the surface of brown rice, promoting the rapid penetration of water molecules into the rice grain. This makes it easier for water molecules to form hydrogen bonds with starch molecules, thereby significantly improving the water absorption and swelling capacity of brown rice and promoting the dissociation of starch granules from the rice matrix. Final viscosity reflects the ability of brown rice to form a gel structure after gelatinization, as well as the degree of interaction between amylose and amylopectin. A higher final viscosity indicates a more stable starch gel structure. The retrogradation value is essentially the result of short-term retrogradation and rearrangement of starch molecules after gelatinization, forming a new microcrystalline structure. An increase in the retrogradation value indicates an enhanced degree of recrystallization of amylose molecules. In addition, the difference in heating treatment methods also significantly affected the performance of brown rice gelatinization characteristics: Compared with traditional water bath heating, microwave heating, with its advantage of "endogenous heating", can make the inside and surface of rice grains heat up rapidly at the same time, and the heat transfer is uniform, thereby promoting the uniform and rapid absorption and expansion of starch granules by water. This difference in the behavior of starch granules under different heating methods is the main reason for the significant difference in the gelatinization characteristics of brown rice between the microwave treatment group and the water bath treatment group.

[0051] Table 2. Effects of different processing methods on the gelatinization characteristics of brown rice Note: Different letters in the same column indicate significant differences (P< 0.05). 0%+0%MW means that brown rice with 0% milling degree was soaked in a solution with 0% GABA concentration and then cooked by microwave heating. WB means that it was cooked by water bath.

[0052] (5) Effects of different processing methods on the thermodynamic properties of brown rice Gelatinization temperature and enthalpy change (ΔH) are core indicators for evaluating the thermal properties of starch. They directly reflect the structural stability of starch granules and the energy required for gelatinization, and are closely related to the density and quantity distribution of starch crystallites. Their variation patterns indirectly reflect the integrity of the starch crystal structure. Table 3 shows that there was no statistically significant difference in peak starch temperature (Tp) and enthalpy change (ΔH) among different milling treatment groups (P>0.05). Tp did not show significant fluctuations, indicating that milling only removes the bran layer and some non-starch components from the surface of brown rice, without destroying the internal crystal structure of rice starch, thus preserving the ordered arrangement of starch crystallites. ΔH, as an important quantitative indicator of starch crystallinity, directly represents the energy required for gelatinization. Higher crystallinity indicates tighter bonding between starch molecules, resulting in higher energy requirements for gelatinization. However, unlike the effect of milling degree, GABA concentration has a significant regulatory effect on the thermal properties of brown rice, and this effect is particularly pronounced in the microwave treatment group. Samples treated with different concentrations of GABA all showed a significant increasing trend in ΔH values, and the GABA concentration was positively correlated with the ΔH value. This increase in enthalpy is presumably related to the disruption of the integrity of brown rice cell structure: GABA, as a small-molecule active substance, can relax the protein barrier on the surface of brown rice, causing the rice cells to rupture and releasing more starch granules encased within the cells. With the increase in the number of released starch granules and the expansion of the exposed area, the contact area between the starch granules and water molecules also increases significantly. The swollen starch granules form a physical barrier, hindering further penetration of water molecules into the rice grain, resulting in starch gelatinization requiring more heat and ultimately causing an increase in the enthalpy of starch gelatinization. Furthermore, microwave heating also significantly affected the thermal properties of brown rice starch. Microwave heating, with its rapid and uniform endogenous heating advantage, can facilitate the rapid penetration of moisture into the starch crystallization region while effectively preserving the structural integrity of the starch crystals and preventing their destruction due to localized overheating. Simultaneously, microwave heating enhances the complexation between starch and matrix components, improving the starch's resistance to gelatinization. Its unique non-thermal effects may also strengthen the hydrogen bond network between starch molecules, further stabilizing the starch structure. Ultimately, compared to traditional water bath heating, the starch gelatinization enthalpy (ΔH) in the MW treatment group was significantly higher, indicating the synergistic effect of microwaves and GABA in regulating the thermal properties of brown rice starch.

[0053] Table 3. Effects of different processing methods on the thermodynamic properties of brown rice Note: Different letters in the same column indicate significant differences (P < 0.05). 0%+0%MW means that brown rice with 0% milling degree was soaked in a solution with 0% GABA concentration and then cooked by microwave heating. WB means that it was cooked by water bath.

[0054] (6) Effects of different processing methods on the rheological properties of brown rice Dynamic rheological properties refer to the mechanical response of a material under alternating stress. This property can be used to characterize the strength of the gel formed by the material and is one of the important indicators for evaluating the processing characteristics and quality of brown rice. Figure 4-7 The rheological properties of brown rice under different treatment conditions were demonstrated. The rheological spectra showed that both the storage modulus (G′) and the loss modulus (G″) changed with the angular frequency (ω, rad / s), and G′ was significantly greater than G″, indicating that the system exhibited strong gel properties, which is a typical characteristic of brown rice starch forming a gel network.

[0055] With the increase of DOM (Matrices Oxide Content) in brown rice, both G′ and G″ showed an upward trend, which is attributed to the enhanced interaction between starch and other molecules due to the removal of the bran layer. Specifically, bran removal reduced physical barriers, allowing starch granules to fully contact and form a cross-linked network, thereby improving gel strength (G′ and G″). The changes in G′ and G″ indicate that the degree of hydration and gelatinization of brown rice starch varies significantly under different milling degrees due to different retained aleurone layer contents, thus affecting the density of the gel network. Soaking in solutions with different GABA concentrations increased both G′ and G″ of brown rice, because GABA enhances starch hydration, thereby strengthening the intermolecular binding forces and ultimately constructing a denser gel network, giving the system higher elasticity and viscosity. The viscoelasticity of starch is mainly affected by the volume fraction of starch granules; this observation is consistent with subsequent scanning electron microscopy (SEM) analysis, further verifying the correlation between gel network structure and rheological properties.

[0056] After microwave (MW) treatment, the G′ and G″ values ​​of brown rice were higher than those of the water bath treatment group, indicating that MW treatment can improve the cross-linking degree of the gel network. Since the heat transfer mechanism of microwaves does not depend on temperature gradient conduction, starch granules can absorb energy from both the inside and outside simultaneously, promoting rapid hydrogen bond breakage and allowing water to penetrate evenly and quickly into the granule interior. This process effectively avoids the hydration hindrance caused by overflow gelatinization, significantly improving the hydration rate and uniformity of starch. Furthermore, the results from a rapid viscosity analyzer showed that the retrogradation value of brown rice increased with increasing GABA concentration. These results indicate that the synergistic treatment of GABA and microwaves can accelerate the retrogradation process of brown rice gel and enhance its gel-forming ability.

[0057] (7) Effects of different processing methods on the crystal structure of brown rice FTIR can reveal structural changes in samples at the molecular level and determine the types of functional groups contained in substances by analyzing characteristic absorption peaks. It is an important means of studying the evolution of molecular structure during brown rice processing. Figure 8-9 The FTIR spectra of brown rice after milling and soaking are shown. All samples exhibit typical infrared absorption peaks of starch, particularly in the 3000-3700 cm⁻¹ range. -1 The absorption peak at 2900-2750 cm⁻¹ corresponds to the stretching vibration of the hydroxyl group (OH), reflecting the strength of the hydrogen bonds between starch molecules; -1 The absorption peak at that point corresponds to the stretching vibration of the methylene group (CH), characterizing the structural features of the hydrocarbon chains in the starch molecule.

[0058] For milling, the absorption intensity at specific wavelengths varied among samples under different DOM conditions, which is closely related to the degree of bran removal during the milling process. Notably, GABA treatment of the brown rice samples did not introduce any new active functional groups, indicating that this treatment did not alter the fundamental chemical properties of the brown rice. Although the brown rice underwent some structural or physical changes after treatment, no significant chemical modifications were produced, and no new chemical functional groups were formed.

[0059] As can be seen from the spectrum, with increasing GABA concentration, all samples showed a decrease at 3380 cm⁻¹. -1 The absorption peaks at these locations all shifted towards higher wavenumbers, and their peak intensities significantly increased, indicating that the hydrogen bonding interactions between starch molecules are the main driving force behind this change. (1047 / 1022 cm⁻¹) -1 The absorbance ratio at a certain point is a recognized indicator for measuring the short-range molecular order of starch. With the increase of milling degree, the order of starch decreased by 10.83%. This is because after the outer protective structure (bran) is removed, the starch granules are directly exposed to mechanical force and gradually break into irregular small granules or fragments. A large number of "structural defect areas" are formed on the surface of the granules, the starch molecules are arranged in a disordered manner, the proportion of amorphous areas increases significantly, and thus the overall order is reduced.

[0060] When the GABA concentration increased, the orderliness of brown rice starch with 0% milling decreased by 5.83%. This is because the amorphous regions inside the starch granules first absorb water and swell. The swelling pressure generated by the increased moisture content disrupts the double-helix structure of the crystalline regions. The insertion of water molecules loosens the originally regular lattice, and some double helices unwind into disordered single chains. Notably, the 3380 cm⁻¹ in the FTIR spectrum... -1 The increased peak intensity indicates that microwave treatment enhances hydrogen bond interactions while simultaneously disrupting intra-chain hydrogen bonds in starch, leading to a decrease in orderliness. The heat generated during microwave treatment can further disrupt intra- and inter-chain hydrogen bonds in starch molecules, causing dissociation of the helical structure and entanglement. This is the main reason for the difference between the microwave-treated group and the water bath-treated group.

[0061] The crystal structure of brown rice was determined using XRD. Figure 10 The results showed that, based on the strong diffraction peaks of rice starch at 15°, 17°, 18°, and 23°, all samples exhibited a typical type A crystal structure (YANG X, MA L, YU P, et al. The comparative evaluation of the quality of brown rice by plasma treatment and milling treatment: Appearance, cooking characteristics, texture characteristics, and nutrient composition [J]. Journal of Cereal Science, 2025, 122: 104127.). The position and intensity of the diffraction peaks did not change with increasing milling degree, indicating that milling treatment did not affect the crystal structure type of brown rice starch. After cooking, the characteristic diffraction peaks of starch basically disappeared, with only a diffraction peak around 20° remaining, indicating that heating severely damaged the starch crystalline region and expanded the amorphous region; the crystallization peak at 20° may originate from the complexation of starch and lipids during the heating process. Even at high concentrations, the position and shape of the diffraction peaks remained consistent after treatment with different concentrations of GABA, indicating that GABA treatment does not change the existing crystal type of starch, which is consistent with the FTIR analysis results.

[0062] (8) The effect of different processing methods on the crystal structure of brown rice Figure 11-14 The microscopic morphological changes of brown rice (BR) particles under different processing methods are presented. Compared with unmilled natural brown rice (DOM0%), highly milled brown rice exhibits a uniform and dense network structure after cooking. With increasing milling degree, the diameter of the pores on the surface of starch granules gradually decreases, but the distribution becomes more dense. This phenomenon is mainly attributed to the fact that milling removes the bran barrier on the outer layer of brown rice, improves the fluidity of free water within the starch system, and allows water to penetrate more fully into the interior of the starch granules, ultimately forming a microstructure with smaller pores and a more compact structure after cooking. At the same time, the pore distribution and layered structure of brown rice starch also become denser with increasing milling degree, further confirming the optimizing effect of milling on the starch gel network.

[0063] After soaking in a GABA solution, brown rice developed pores of varying sizes in its cross-section. As the GABA concentration increased, the starch granules underwent full gelatinization, forming a fluffy structure of flocculent and small lumps, while amylose slowly dissolved from the brown rice grains. GABA treatment effectively promoted the dispersion of starch granules in cooked brown rice and improved the uniformity of the starch gelatinization process, which aligns perfectly with the increased viscosity of the cooked brown rice after treatment. During gelatinization, the presence of GABA facilitated the penetration of water from the surface of the brown rice into its interior, causing the internal starch granules to swell fully and separate from each other, thus improving both the degree of starch gelatinization and the edible quality of the brown rice.

[0064] Inside the MW-treated brown rice, numerous pits formed by the detachment of starch granules were visible, with relatively large openings, indicating significant starch granule separation. The steamed brown rice treated with hydrothermal energy also exhibited abundant starch detachment pits, similar in morphology to the microwave-treated samples, but with significantly smaller pit pore sizes. This is because microwave energy is directly absorbed by the polar molecules (mainly water molecules) within the starch granules, rather than being transferred from the surface to the interior through traditional heat conduction. This unique heating method directly causes damage to the physical structure of the starch granules. These results clearly demonstrate a close correlation between changes in the microscopic morphology of starch granules and processing methods and GABA concentration, providing direct microscopic evidence for elucidating the mechanism by which different treatments improve brown rice quality.

[0065] (9) The effect of different processing methods on the crystal structure of brown rice Combination Figure 15 The moisture distribution characteristics during the continuous soaking process of brown rice are shown. Further analysis of the regulatory effects of milling degree and GABA concentration on moisture distribution reveals that both significantly influence the color of the moisture distribution, exhibiting a consistent pattern: the higher the milling degree, the redder the color of the brown rice grains. Simultaneously, as the GABA soaking concentration increases, the color of the brown rice grains gradually deepens and tends towards red. This color difference directly reflects the degree of water penetration and the uniformity of moisture distribution within the brown rice. Notably, the heating method significantly affects the effect of GABA in regulating moisture distribution, with microwave-assisted GABA treatment showing a significantly better improvement than water bath-assisted GABA treatment.

[0066] From a mechanistic perspective, a redder color indicates a higher internal moisture content and more thorough water penetration in brown rice, consistent with the previous results regarding water absorption rate. Higher milling fineness leads to more thorough removal of the outer bran layer. As a natural physical barrier to water penetration, the greater the removal of bran, the weaker the water-blocking effect, allowing water to penetrate the brown rice grains more quickly and fully during soaking. This results in a shorter optimal cooking time and a redder color, consistent with the previous conclusion that milling improves the free water flow of starch and promotes water penetration. The higher the GABA concentration, the redder the color, primarily due to GABA's effect on the surface proteins of brown rice. Increased GABA concentration further disrupts protein distribution, making the surface structure of brown rice more ordered and reducing water penetration resistance. Compared to the WB synergistic treatment, the endogenous heating of MW allows for simultaneous heating inside and outside the brown rice, accelerating the regulatory effect of GABA on protein structure and promoting rapid water molecule movement, enabling more efficient penetration and uniform distribution within the grains. Therefore, the MW synergistic GABA group exhibits a deeper and more uniform water distribution color, significantly superior to the water bath synergistic group.

[0067] Furthermore, the synergistic effect of milling degree, GABA concentration, and microwave treatment can further optimize moisture distribution. The brown rice exhibits the darkest color in terms of moisture distribution when high milling degree is combined with high GABA concentration and microwave treatment, indicating the most complete water penetration. This result not only confirms the regulatory effect of each treatment factor on moisture distribution but also provides intuitive experimental evidence at the moisture distribution level for further elucidating the mechanism by which milling, GABA, and microwave treatment synergistically improve the cooking quality of brown rice.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for improving the edible quality of brown rice by using GABA in combination with microwave heating, characterized in that, Includes the following steps: (1) Hull the rice to obtain brown rice; (2) Screen the brown rice obtained in step (1) to remove grains without germ, discolored grains, immature grains and shriveled grains, and wash and drain it. (3) The brown rice obtained in step (2) is milled to remove part of the bran; (4) Place the brown rice obtained in step (3) into a GABA solution and let it soak and penetrate at a constant temperature; (5) Brown rice soaked in GABA is heated and cooked in microwave to gelatinize it, and brown rice is obtained.

2. The method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, The rice hulling process described in step (1) refers to removing only the outer husk of the rice grains while retaining the bran layer, germ, and endosperm.

3. The method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, The grinding degree of the grinding process in step (3) is 0%, 2%, 4%, 6% or 8%.

4. The method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (4), the mass concentration of the GABA solution is 0.04%~0.16%.

5. A method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (4), the immersion temperature of the GABA solution is 50℃~60℃.

6. The method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (4), the soaking time of the GABA solution is 1 h to 1.5 h.

7. The method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (4), the ratio of brown rice to GABA solution is 1:1.5 to 1:2.

5.

8. A method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (5), the power of microwave heating is 10 W / g to 27.5 W / g.

9. A method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (5), the temperature for cooking and gelatinization is 80℃~90℃.

10. A method for improving the edible quality of brown rice using GABA combined with microwave heating according to claim 1, characterized in that, In step (5), the cooking and gelatinization time is 25 min to 45 min.

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