Method for improving digestion resistance of preboiled rice and reducing glycemic index by regulating and controlling activity of endogenous enzyme of unhulled rice

Abstract

The invention provides a method for regulating and controlling the activity of an endogenous enzyme of rice to improve the digestion resistance of preboiled rice and reduce the glycemic index (eGI). Segmented heat shock is carried out on the rice before soaking and in the soaking process to activate the activity of endogenous protease and amylase, the water absorption rate of the soaked rice is controlled, the germination stage of rice germination is controlled to enable the activated endogenous enzyme to carry out in-situ enzymolysis, and protein and starch in the rice are modified by regulating and controlling the activity of the endogenous enzyme before pre-gelatinization; when the endogenous enzyme activity is extinguished, starch is gelatinized and dried to form starch recrystallization, the structure of the starch is obviously changed, the obtained unhulled rice is made into preboiled rice for cooking rice, the digestion resistance of the preboiled rice is obviously improved, and eGI is reduced. The proper heat shock treatment activates the activity of endogenous amylase and protease of the rice, and by regulating and controlling the relationship between the heat shock strength and time and the water content in the rice, the advantage of the digestion resistance of the endogenous enzyme system in-situ enzyme method modified rice starch is fully played, and the low glycemic index preboiled rice product with good taste and quality is prepared.

Description

Technical Field

[0001] The invention belongs to the technical field of food processing, and in particular relates to a method for regulating the activity of endogenous enzymes in rice to improve the digestibility resistance of parboiled rice and reduce the glycemic index. Background Art

[0002] With the improvement of residents' living standards, chronic metabolic diseases (such as diabetes, obesity, hyperlipidemia and hypertension) have gradually had a significant impact on people's quality of life. Such diseases are closely related to residents' eating habits. The World Health Organization (WHO) pointed out that refined rice, a traditional staple food, is more likely to cause a rapid rise in blood sugar, which poses a potential threat to health. Rice is the main food source for Chinese residents, and refined rice is the main product on the market. Refined rice contains a large amount of starch, and starch is quickly converted into glucose in the body, leading to drastic fluctuations in blood sugar, which is particularly evident in diabetic patients. In recent years, nutritious rice products have gradually become popular among consumers. Parboiled rice is made by washing, soaking, steaming and drying rice, removing the husk and grinding it. In the preparation process of parboiled rice, rice starch undergoes gelatinization and retrogradation, and some nutrients in the rice bran layer are transferred to the inner layer of the rice endosperm, thereby retaining more nutrients and promoting human health. Compared with white rice, parboiled rice has the advantages of low broken rice rate and slow digestion. Studies have shown that the content of resistant starch (RS) in parboiled rice is about 10% higher than that in polished rice, and the glycemic index (GI value) is lower than that in polished rice. These results indicate that parboiled rice has the potential to significantly reduce postprandial blood sugar and can be considered as a slow-digesting staple food option. Therefore, parboiled rice processing technology not only improves the storage stability of rice, but also significantly improves its nutritional quality and processing quality, providing new possibilities for residents' healthy diet.

[0003] The soaking process of parboiled rice is usually considered to allow the rice to fully absorb water and expand itself, creating the necessary conditions for the starch to be fully gelatinized in the cooking process. However, the activation of the rice's own endogenous proteases and amylases during the soaking stage and the effects of the interaction of enzymatic products on the digestion and quality of rice are ignored. Seed germination includes three stages: imbibition, germination and sprouting. The imbibition stage is characterized by rapid water absorption, and the seeds immediately start the recovery and activation of basal metabolism after absorbing water. At present, the use of endogenous enzyme activation during seed germination to change the direction of food application is mainly studied in the germination stage of brown rice. With the extension of the germination time of brown rice, the increase in the activity of a large number of α-amylases and β-amylases is significantly correlated with the germination ability of rice seeds. At present, there are no reports on the changes in the activity of proteases and amylases during rice germination, especially during the imbibition process, and the effects of their enzymatic products on rice digestion.

[0004] Therefore, providing a new digestion-resistant parboiled rice with low processing cost is a problem that those skilled in the art need to solve urgently. Summary of the invention

[0005] The object of the present invention is to provide a method for improving the digestibility of parboiled rice and reducing the glycemic index (eGI) and the digestibility-resistant parboiled rice prepared thereby. The digestibility-resistant parboiled rice prepared by the present invention is more nutritious and has a low glycemic index, and can effectively reduce the risk of hyperglycemia and diabetes.

[0006] The method for improving the digestibility resistance and reducing the glycemic index of parboiled rice provided by the present invention comprises the following steps:

[0007] (1) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0008] ①The first heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0009] The dry rice is blanched in hot water of 80-85° C. for 2-3 minutes at a ratio of dry rice: hot water = 1 g: 10-30 mL, put into running water at room temperature and rinse, the temperature is reduced to below 50° C. within 1 minute, then soaked in water of 40-50° C. for 60-75 minutes at a ratio of dry rice: water = 1 g: 10-30 mL, taken out and drained to obtain soaked rice 1, wherein the water absorption rate is ≥ 20%, the protease activity is ≥ 123.53 U / g dry rice, and the amylase activity is ≥ 17.77 U / g dry rice;

[0010] ②The second heat shock treatment during the soaking process increases the water absorption rate and further increases the endogenous enzyme activity

[0011] The soaked rice 1 is blanched at 80-85° C. for 60-80 seconds at a ratio of soaked rice 1: hot water = 1 g: 10-30 mL, put into room temperature tap water and rinse, the temperature is reduced to below 50° C. within 1 minute, then soaked in 40-50° C. water at a ratio of soaked rice 1: water = 1 g: 10-30 mL for 90-100 minutes, taken out and drained to obtain soaked rice 2, the water absorption rate of which is 45-50%, the protease activity is ≥148.76 U / g dry rice, and the amylase activity is ≥37.86 U / g dry rice;

[0012] ③ Keep the seeds warm and moist and continue in-situ enzymolysis

[0013] The soaked rice 2 is kept warm in a 70%-75% humidity insulation box at 25-28° C. for 5-8 hours to enter the middle stage of the germination of seeds, and the germination initial stage rice 3 with high protease and amylase activities is obtained, wherein the protease activity of the rice 3 is ≥160.33 U / g dry rice, and the amylase activity is ≥53.12 U / g dry rice;

[0014] ④Inactivate endogenous enzymes and steam the rice

[0015] Put rice 3 into a steamer and steam it to obtain rice 4;

[0016] Steaming conditions: temperature 100℃~120℃, steaming time 10min~20min;

[0017] (2) Rice drying and starch recrystallization

[0018] Drying the cooked rice 4 and cooling it to room temperature to obtain rice 5;

[0019] The drying temperature is 30℃~40℃, the drying time is 24~36h, and the moisture content is ≤10%;

[0020] (3) Husking, milling and color sorting

[0021] The dried rice 5 is husked and milled by a rice mill to obtain parboiled rice, and a small amount of black rice, black-headed rice and yellow rice mixed in the finished rice are removed to obtain the finished parboiled rice.

[0022] The estimated glycemic index (eGI) of the obtained finished steamed rice is <64, and the taste quality score is greater than 76.

[0023] In step (1) of the above method, the dry rice is obtained by drying rice with full grains, uniform size, no damage, and no pests and diseases.

[0024] The parboiled rice prepared by the above method also belongs to the protection scope of the present invention.

[0025] The use of the parboiled rice in preparing products having the function of lowering the glycemic index also falls within the protection scope of the present invention.

[0026] The present invention has the following beneficial effects:

[0027] The present invention performs segmented heat shock on rice before and during soaking, controls the water absorption rate of soaked rice, controls the germination stage of rice germination, modifies the protein and starch in rice by regulating endogenous enzyme activity before pre-gelatinization, forms starch recrystallization by drying after gelatinization, significantly changes the structure of starch, and makes the rice obtained into parboiled rice for steaming rice, significantly improving the digestibility of parboiled rice. In addition, the appropriate heat shock treatment activates the endogenous amylase and protease activity of rice to the maximum extent, and by regulating the relationship between the heat shock intensity and time and the water content in rice, gives full play to the advantages of endogenous enzyme system in situ enzymatic modification of rice starch digestibility, and obtains a low glycemic index parboiled rice product with good taste quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the standard curve of casein content;

[0029] Figure 2is the standard curve of maltose content;

[0030] Figure 3 The moisture distribution diagram of rice in Examples 1-2 and Comparative Examples 1-5;

[0031] Figure 4 The hydrolysis curves of parboiled rice starch of Examples 1-2 and Comparative Examples 1-5;

[0032] Figure 5 These are laser confocal images of rice starch protein of Examples 1-2 and Comparative Example 1; wherein A, B, and C represent Comparative Example 1, Example 1, and Example 2, respectively.

[0033] Figure 6 The SDS-gel electrophoresis diagram of rice protein of Examples 1-2 and Comparative Examples 1-5; from left to right are marker, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5, Example 1, and Example 2. DETAILED DESCRIPTION

[0034] The present invention is further described in detail below in conjunction with specific embodiments, and the examples provided are only for illustrating the present invention, rather than for limiting the scope of the present invention. The examples provided below can be used as a guide for further improvements by those of ordinary skill in the art, and do not constitute a limitation of the present invention in any way.

[0035] The experimental methods in the following examples, unless otherwise specified, are all conventional methods, and are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial channels.

[0036] The rice used in the following examples and comparative examples is early indica rice.

[0037] Example 1: Preparation of parboiled rice by two-stage heat shock and heat preservation in situ enzymatic hydrolysis

[0038] (1) Raw material selection: Select rice with full grains, uniform size, no damage, and no pests and diseases as raw materials;

[0039] (2) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0040] ①The first heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0041] The dry rice is immersed in hot water for heat shock treatment, wherein the specific operation of the heat shock is dry rice: hot water = 1g: 20mL; the water temperature is 80°C, the cleaned rice is added, the hot water is blanched for 2min, and the room temperature tap water is rinsed, the temperature drops to below 50°C within 1min, and then the rice is soaked in 50°C water at a ratio of dry rice: 50°C water = 1g: 10mL for 60min, and then the soaked rice 1 is fished out and the water is drained to obtain the soaked rice 1, which has a water absorption rate of 21%, a protease activity of 129.28U / g dry rice, and an amylase activity of 18.36U / g dry rice;

[0042] ②The second heat shock treatment during the soaking process increases the water absorption rate and further increases the endogenous enzyme activity

[0043] The soaked rice 1 was blanched in 80°C hot water (soaked rice 1: hot water = 1 g: 20 mL) for 60 seconds, put into room temperature tap water and rinsed, the temperature dropped to below 50°C within 1 minute, then soaked in 50°C water for 100 minutes (soaked rice 1: water = 1 g: 10 mL), taken out and drained to obtain soaked rice 2, whose water absorption rate was 45%, whose protease activity was 157.38 U / g dry rice, and whose amylase activity was 37.95 U / g dry rice;

[0044] ③ Keep the seeds warm and moist and continue in-situ enzymolysis

[0045] The soaked rice 2 was kept warm at 28°C for 6 hours in a 70% humidity incubator until the seeds germinated in the middle of the germination stage, and soaked rice 3 with higher protease and amylase activities was obtained. The protease activity in the soaked rice 3 was 170.92 U / g dry rice, and the amylase activity was 63.63 U / g dry rice.

[0046] ④Inactivate endogenous enzymes and steam the rice

[0047] Put rice 3 into a steamer for steaming. Steaming conditions: temperature 110°C, steaming time 15 min, obtain rice 4;

[0048] (3) Rice drying and starch recrystallization

[0049] The cooked rice 4 is dried in a dryer at a temperature of 40° C. for 24 hours and a moisture content of ≤10% to obtain rice 5;

[0050] (4) Husking, milling and color sorting

[0051] The dried rice 5 was husked and milled by a rice mill to obtain parboiled rice. A small amount of black rice, black-headed rice and yellow rice mixed in the finished rice were removed to obtain the finished parboiled rice. The eGI of the parboiled rice was 63.71 and the taste quality score was 80.

[0052] Example 2: Preparation of parboiled rice by two-stage heat shock and heat preservation in situ enzymatic hydrolysis

[0053] (1) Raw material selection: same as in Example 1.

[0054] (2) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0055] ①The first heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0056] The dry rice is immersed in hot water for heat shock treatment, wherein the specific operation of the heat shock is dry rice: hot water = 1g: 20mL; the water temperature is 85°C, the cleaned rice is added, the hot water is blanched for 2min, and the tap water is rinsed, the temperature drops to below 50°C within 1min, and then soaked in 50°C water at a ratio of dry rice: 50°C water = 1g: 10mL for 60min, and then the soaked rice 1 is fished out and the water is drained to obtain the soaked rice 1, which has a water absorption rate of 22%, a protease activity of 123.53U / g dry rice, and an amylase activity of 17.77U / g dry rice;

[0057] ②The second heat shock treatment during the soaking process increases the water absorption rate and further increases the endogenous enzyme activity

[0058] The soaked rice 1 was blanched in 85°C hot water (soaked rice 1: hot water = 1g: 20mL) for 60s, put into room temperature tap water and rinsed, the temperature dropped to below 50°C within 1min, then soaked in 50°C water for 90min (soaked rice 1: water = 1g: 10mL), taken out and drained to obtain soaked rice 2, whose water absorption rate was 45%, so that its protease activity was 148.76U / g dry rice, and its amylase activity was 37.86U / g dry rice;

[0059] ③ Keep the seeds warm and moist and continue in-situ enzymolysis

[0060] The soaked rice 2 was kept warm at 28°C for 6 hours in a 70% humidity incubator until the seeds germinated in the middle of the germination stage, and soaked rice 3 with higher protease and amylase activities was obtained. The protease activity in the soaked rice 3 was 160.33 U / g dry rice, and the amylase activity was 53.12 U / g dry rice.

[0061] ④Inactivate endogenous enzyme activity and steam the rice

[0062] The rice 3 was placed in a steamer for steaming. The steaming conditions were as follows: temperature of 110°C, steaming time of 15 min, and the rice 4 was obtained.

[0063] (3) Rice drying and starch recrystallization: Same as Example 1

[0064] (4) Husking, milling and color sorting: Same as in Example 1, the eGI of the parboiled rice was 63.96, and the taste quality score was 76.

[0065] Comparative Example 1 (Traditional parboiled rice process, no heat shock, no heat preservation, in-situ enzymatic hydrolysis)

[0066] (1) Raw material selection: same as in Example 1

[0067] (2) Rice soaking: soak the dry rice in (dry rice: 50℃ water = 1g: 10mL) in a 50℃ water bath until the water absorption rate of the rice is 45%, take it out and drain the water, and measure the protease activity in the soaked rice to be 72.51U / g dry rice, and the amylase activity to be 17.75U / g dry rice; then put the rice into a steamer for steaming. Steaming conditions: temperature 110℃, steaming time 15min.

[0068] (3) Drying and starch recrystallization: same as in Example 1;

[0069] (4) Husking, milling and color sorting: Same as in Example 1, the eGI of the parboiled rice was 73.01, and the taste quality score was 68.

[0070] Comparative Example 2 (single heat shock before soaking with in-situ enzymolysis)

[0071] (1) Raw material selection: same as in Example 1.

[0072] (2) Heat shock treatment and soaking of dry rice regulate endogenous protease and amylase activities

[0073] ① Single heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0074] The dry rice was immersed in hot water for heat shock treatment, wherein the specific operation of the heat shock was dry rice: hot water = 1 g: 20 mL; the water temperature was 80° C., the cleaned rice was added, blanched with hot water for 2 minutes, rinsed with tap water, the temperature dropped to below 50° C. within 1 minute, and then immersed in 50° C. water at a ratio of dry rice: 50° C. water = 1 g: 10 mL until the water absorption rate was 45%, and rice 1 was obtained, with a protease activity of 124.55 U / g dry rice and an amylase activity of 18.63 U / g dry rice;

[0075] ② Keep the seeds warm and moist and continue in-situ enzymolysis:

[0076] The soaked rice 1 was kept warm at 28°C for 8 hours in a 70% humidity incubator until the seeds entered the middle germination stage, and rice 2 was obtained. The protease activity in rice 2 was 140.26 U / g dry rice, and the amylase activity was 33.28 U / g dry rice.

[0077] (3) Drying and starch recrystallization: same as in Example 1;

[0078] (4) Husking, milling and color sorting: Same as in Example 1, the eGI of the parboiled rice was 68.21, and the taste quality score was 72.

[0079] Comparative Example 3 (Secondary heat shock without heat preservation and in-situ enzymolysis)

[0080] (1) Raw material selection: same as in Example 1

[0081] (2) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0082] ①The first heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0083] The dry rice was immersed in hot water for heat shock treatment, dry rice: hot water = 1g: 20mL; the water temperature was 80°C, the cleaned rice was added, the hot water was blanched for 2min, and the tap water was rinsed. The temperature dropped to below 50°C within 1min, and then soaked in 50°C water at a ratio of dry rice: 50°C water = 1g: 10mL for 60min, and then the soaked rice 1 was taken out and the water was drained to obtain the soaked rice 1, which had a water absorption rate of 21%, a protease activity of 129.44U / g dry rice, and an amylase activity of 18.16U / g dry rice;

[0084] ②The second heat shock treatment during the soaking process increases the water absorption rate and further increases the endogenous enzyme activity

[0085] The soaked rice 1 was blanched in 80°C hot water (soaked rice 1: hot water = 1g: 20mL) for 60s, rinsed with room temperature tap water, the temperature dropped to below 50°C within 1min, then soaked in 50°C water for 100min (soaked rice 1: water = 1g: 10mL), taken out and drained to obtain soaked rice 2, whose water absorption rate was 45%, protease activity was 157.12U / g dry rice, and amylase activity was 37.86U / g dry rice;

[0086] ③ Kill endogenous enzyme activity and steam the rice

[0087] Put rice 2 into a steamer for steaming. Steaming conditions: temperature 110°C, cooking time 15 min, to obtain rice 3.

[0088] (3) Drying and starch recrystallization: same as in Example 1;

[0089] (4) Hulling, milling and color sorting: Same as in Example 1, the eGI of the steamed rice was 66.66, and the taste quality score was 73.

[0090] Comparative Example 4 (single heat shock before soaking without heat preservation and in-situ enzymatic hydrolysis)

[0091] (1) Raw material selection: same as in Example 1;

[0092] (2) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0093] ① Single heat shock treatment before soaking increases the activity of endogenous enzymes in rice

[0094] The dry rice was immersed in hot water for heat shock treatment, wherein the specific operation of the heat shock was dry rice: hot water = 1 g: 20 mL; the water temperature was 80° C., the cleaned rice was added, blanched with hot water for 2 minutes, rinsed with tap water at room temperature, the temperature dropped to below 50° C. within 1 minute, and then immersed in 50° C. water at a ratio of dry rice: 50° C. water = 1 g: 10 mL until the water absorption rate was 45%, obtaining rice 1, with a protease activity of 136.14 U / g dry rice and an amylase activity of 25.25 U / g dry rice;

[0095] (3) Drying and starch recrystallization: same as in Example 1;

[0096] (4) Husking, milling and color sorting: Same as in Example 1, the eGI of the parboiled rice was 70.13, and the taste quality score was 70.

[0097] Comparative Example 5 (single heat shock during soaking without heat preservation and in-situ enzymatic hydrolysis)

[0098] (1) Raw material selection: same as in Example 1;

[0099] (2) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages

[0100] ① Soaking dry rice

[0101] Soak dry rice in 50°C water at a ratio of 1 g:10 mL for 90 min, remove and drain water to obtain soaked rice 1, which has a water absorption rate of 20%, a protease activity of 62.37 U / g dry rice, and an amylase activity of 11.27 U / g dry rice, to obtain soaked rice 1;

[0102] ② Heat shock treatment during soaking increases the activity of endogenous enzymes in rice

[0103] The rice 1 was immersed in hot water for heat shock treatment, wherein the specific operation of the heat shock was dry rice: hot water = 1 g: 20 mL; the water temperature was 80° C., the cleaned rice was added, blanched in hot water for 2 minutes, rinsed with tap water at room temperature, the temperature dropped to below 50° C. within 1 minute, and then soaked in 50° C. water at a ratio of dry rice: 50° C. water = 1 g: 10 mL until the water absorption rate was 45%, obtaining rice 2, with a protease activity of 106.56 U / g dry rice and an amylase activity of 22.54 U / g dry rice;

[0104] (3) Drying and starch recrystallization: same as in Example 1;

[0105] (4) Husking, milling and color sorting: Same as in Example 1, the eGI of the parboiled rice was 71.57, and the taste quality score was 70.

[0106] The relevant detection methods and results of the present invention are as follows:

[0107] (1) Determination of endogenous protease and amylase activities

[0108] ① Extraction of crude enzyme solution: Take the rice of the embodiment and comparative example, add 50mM phosphate buffer solution with pH=7.0 at a ratio of 1:6 (based on the mass of dry rice) for pulping, centrifuge the slurry at 2000r / min for 30min, filter the supernatant through filter paper and store it at -20°C until further use.

[0109] ②Determination of protease activity

[0110] Dilute the crude enzyme solution by an appropriate multiple, take 2mL and add 2mL of 0.5% casein solution, shake well and react in a 37℃ water bath for 20min. Add 10% trichloroacetic acid solution to terminate the reaction, centrifuge at 3000r / m for 10min, and measure the absorbance of the supernatant at 275nm. In the blank experiment, first add 10% trichloroacetic acid solution and then casein solution, keep warm and centrifuge, and measure the absorbance. The calculation method is: enzyme activity (U / g) = (H×F) / (15×0.05), H is the absorbance value from the standard curve ( Figure 1 ) is the casein content found in the sample, μg / mL; F is the final dilution multiple of the enzyme solution.

[0111] ③ Determination of amylase activity

[0112] Preparation of 3,5-dinitrosalicylic acid reagent: Weigh 5 g of 3,5-dinitrosalicylic acid, dissolve it in a small amount of deionized water, add 10 g of NaOH, 100 g of potassium sodium tartrate, 1.0 g of phenol and 0.25 g of anhydrous sodium sulfite, make up to 500 mL, store in a brown bottle, and keep at room temperature away from light for 7 days before use.

[0113] Drawing of the standard curve: Accurately transfer 0, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.80, and 1.00 mL of a 2% maltose standard solution into a 25 mL test tube, and add deionized water to 1 mL. After preheating in a 50°C water bath for 3 minutes, add 2 mL of a 1% starch solution. React in a 50°C constant temperature water bath for 5 minutes, immediately add 4 mL of 3,5-dinitrosalicylic acid reagent that has been preheated in a boiling water bath, shake well, and react in a boiling water bath for 10 minutes. After taking out and cooling, measure the absorbance of the solution at a wavelength of 500 nm, and draw a standard curve with maltose concentration as the horizontal axis and absorbance as the vertical axis ( Figure 2 ).

[0114] Determination of amylase activity in samples: dilute the crude enzyme solution 10 times, take 1 mL of the diluted enzyme solution, place it in a 50°C water bath for 3 minutes, and add 2 mL of 1% starch solution. The remaining steps are the same as the standard solution reaction steps for drawing the calibration curve. The measured absorbance is used to obtain the corresponding maltose amount based on the standard curve. Using the boiled inactivated enzyme solution as a control, 1 unit of enzyme activity is defined as the amount of enzyme required to hydrolyze starch and release 1 mg of maltose in 5 minutes at 50°C (U). The calculation formula is as follows:

[0115] Amylase activity (U) = C × V × n

[0116] Wherein, C represents the concentration of maltose in the enzyme solution; V represents the total volume of the extracted enzyme solution; nenzyme represents the dilution multiple of the enzyme solution.

[0117] (2) Taste characteristics determination

[0118] Weigh 30g of steamed rice, add 1.4 times the water, use an induction cooker + steamer + 6cm diameter 304 stainless steel small pot to steam the rice, and use the Satake rice taste meter (model STA1B) for measurement. The sampling personnel of the testing instrument use a plastic spoon to fluffily pick up 30g of each rice sample, and make sure that there is no rice grain residue in the plastic spoon before picking each sample. The test uses a taste meter, a computer, a rice former, a rice shaper, a plastic spoon, and an electronic balance. The taste meter is a device for testing the taste quality of the shaped rice sample.

[0119] The taste meter is connected to a computer, the electronic balance and the taste meter are turned on and preheated for 30 minutes, and the taste meter is calibrated using a black and white reference plate. Weigh (8.0±0.1)g of rice and put it into the shaper. Be careful to press it gently so that it is all in the auxiliary shaping ring and does not overflow, and remove the auxiliary ring. Move the shaper close to the shaper platform, quickly tilt it upside down in the middle, press the shaper handle to the bottom for 10 seconds and then lift it up. Take out the shaper and reverse the auxiliary shaping ring embedded inside, and repeat the above steps. The shaping of the rice sample is completed. Check that there is no rice paste adhering to the measurement groove, and the auxiliary shaping ring can be placed. The taste meter measures the auxiliary shaping ring in the sample mouth twice, both forward and reverse, and the above operation is one repetition. Each rice sample is repeated 3 times, and a total of 6 measurements are made. The final taste, appearance and taste value data of the sample are displayed on the computer.

[0120] Evaluation method The rice taste meter has a full score of 10 for both appearance and taste indicators, and is divided into 5 levels: 8 points or more for good, 5 points or less for bad, and 1 point for each level in between. The taste value is the comprehensive evaluation value of the rice obtained through analysis, with a full score of 100 points, and is also divided into 5 levels: 80 points or more for good, 50 points or less for bad, and 10 points for each level in between. The taste meter tests the rice sample, and the final taste quality obtained is based on the taste value.

[0121] (3) Determination of digestion characteristics

[0122] Weigh 5 g of rice, grind it into a uniform paste, dilute it to 50 mL with sodium acetate buffer (pH 5.2), mix the enzyme solution with 10 mL (300 U / mL porcine pancreatic α-amylase and 150 U / mL saccharifying enzyme solution), and shake the reaction in a 37°C water bath. Take samples at 0, 10, 20, 30, 60, 90, 120, and 180 min.

[0123] The contents of rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) were determined by 3,5-dinitrosalicylic acid method (DNS). The calculation formulas are shown in equations (1) to (3).

[0124] RDS=(m 20 -m 0 )×0.9 / m×100%(1)

[0125] SDS=(m 120 -m 20 )×0.9 / m×100%(2)

[0126] RS=100%-RDS-SDS(3)

[0127] Where: m 0 is the mass of reducing sugar in the sample before enzymatic hydrolysis, mg; m 20 is the mass of reducing sugar after 20 minutes of enzymatic hydrolysis of the sample, mg; m 120 is the mass of reducing sugar after 120 min of enzymatic hydrolysis of the sample, mg; 0.9 is the conversion factor; m is the mass of total starch, mg.

[0128] The starch hydrolysis curve is drawn with the starch hydrolysis rate as the ordinate and the time as the abscissa. The curve follows the first-order reaction equation. The calculation formula of the starch hydrolysis rate is as follows.

[0129] Starch hydrolysis rate / % = glucose content in digestive juice at sampling time (g) × 0.9 / total starch content (g) × 100 The hydrolysis index (HI) of the sample is the percentage of the area under the hydrolysis curve of each sample to the area under the hydrolysis curve of white bread. eGI = 39.71 + 0.549 × HI

[0130] Table 1 Taste rating of parboiled rice

[0131]

[0132]

[0133] As can be seen from Table 1, when the dry rice absorbs water and swells, the dormant enzymes are activated, resulting in increased metabolic activity, a significant increase in water absorption rate, increased hydration of starch and non-starch polysaccharides, and increased bulk water content. These conclusions are confirmed by the water distribution results ( Figure 3 ). Heat shock can destroy the outer seed coat. At the same time, heat shock treatment during soaking effectively activates endogenous enzymes in rice to hydrolyze. Endogenous enzymes are activated to hydrolyze proteins and starch in rice. The enzymatic products produced affect the properties of starch and protein. Affects the binding structure of starch and protein ( Figure 5 ), the taste quality of the parboiled rice also changed. Compared with the traditional parboiled rice (Comparative Example 1), the taste comprehensive scores of Examples 1 and 2 were significantly increased to 80.0 and 76.0 respectively. The comparative group treated with heat shock effectively activated the endogenous enzymes, so that the taste quality was improved compared with the traditional parboiled rice. The heat shock of rice improved the taste quality of parboiled rice; the endogenous protease amylase continued to enzymolyze during soaking and heat preservation, and the production of parboiled rice was completed without the addition of exogenous additives.

[0134] Table 2 Digestive properties of parboiled rice

[0135] Group Fast digesting starch Slowly digestible starch Resistant starch Glycemic Index Example 1 Secondary heat shock and in situ enzymatic hydrolysis 1 29.31 18.96 51.73 63.71 Example 2 Secondary heat shock and in situ enzymatic hydrolysis 2 30.78 19.03 50.19 63.96 Comparative Example 1 No heat shock, no heat preservation, in situ enzymatic hydrolysis (traditional) 48.45 20.11 31.44 73.01 Comparative Example 2 Single heat shock and in situ enzymatic hydrolysis before soaking 39.52 18.19 42.29 68.21 Comparative Example 3 Secondary heat shock and no incubation in situ enzymatic hydrolysis 38.9 15.51 45.59 66.66 Comparative Example 4 Single heat shock before soaking without heat preservation enzymatic hydrolysis 43.41 17.43 39.16 70.13 Comparative Example 5 Single heat shock without heat preservation during immersion 45.02 21.55 33.43 71.57

[0136] Protein or its hydrolysate is a key food source component compounded with starch, and is used to adjust the digestibility of starch-based food systems. The digestibility of starch is related to the changes in the starch hierarchy caused by the interaction between starch molecules and protein or its hydrolysate in the binary system. As can be seen from Table 2, the starch hydrolysis rate of the steamed rice in the embodiment is lower than that in Comparative Example 1 (traditional steamed rice), and the estimated glycemic index (eGI) is reduced from 73.01 to 63.71 and 63.96, from the original high GI value to a medium GI value, and the resistant starch content is increased by about 20%. In the embodiment, in the soaking after heat shock combined with the heat preservation in situ enzymatic hydrolysis process, the protease amylase with higher activity in situ enzymatic hydrolysis of rice grains, and the rate and degree of particle expansion caused by enzymatic hydrolysis of protein decreases. After the starch is wrapped by the network structure of the protein, it effectively hinders the combination of starch and starch hydrolyzing enzymes. Therefore, after the rice grains are heat-shocked, the protease activity increases, and some enzymes hydrolyze the protein ( Figure 6 ), the binding force with starch is enhanced during pre-gelatinization and recrystallization, the amount of extracted protein decreases, making the color in SDS-PAGE lighter, and at the same time, in the example, 55kDa and 50kDa proteins are decomposed to make the band lighter or disappear. In addition, the new starch structure formed by the enzymatic protein and starch after gelatinization and recrystallization increases the resistance between the amylase that digests starch and the recrystallized starch, which reduces the starch hydrolysis rate of steamed rice ( Figure 4 ), the estimated glycemic index also decreased.

[0137] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that the present invention can be further improved. In a word, according to the principles of the present invention, the application is intended to include any changes, uses or improvements to the present invention, including departure from the disclosed scope in the application, and changes made with conventional techniques known in the art.

Claims

1. A method for improving the digestibility of parboiled rice and reducing the glycemic index, comprising the following steps: (1) Regulating endogenous protease and amylase activities by heat shock treatment and soaking of dry rice grains in different stages ①The first heat shock treatment before soaking increases the activity of endogenous enzymes in rice The dry rice is blanched in hot water of 80-85° C. for 2-3 minutes at a ratio of dry rice: hot water = 1 g: 10-30 mL, put into running water at room temperature and rinse, the temperature is reduced to below 50° C. within 1 minute, then soaked in water of 40-50° C. for 60-75 minutes at a ratio of dry rice: water = 1 g: 10-30 mL, taken out and drained to obtain soaked rice 1, wherein the water absorption rate is ≥ 20%, the protease activity is ≥ 123.53 U / g dry rice, and the amylase activity is ≥ 17.77 U / g dry rice; ②The second heat shock treatment during the soaking process increases the water absorption rate and further increases the endogenous enzyme activity The soaked rice 1 is blanched at 80-85° C. for 60-80 seconds at a ratio of soaked rice 1: hot water = 1 g: 10-30 mL, put into room temperature tap water and rinse, the temperature is reduced to below 50° C. within 1 minute, then soaked in 40-50° C. water at a ratio of soaked rice 1: water = 1 g: 10-30 mL for 90-100 minutes, taken out and drained to obtain soaked rice 2, the water absorption rate of which is 45-50%, the protease activity is ≥148.76 U / g dry rice, and the amylase activity is ≥37.86 U / g dry rice; ③ Keep the seeds warm and moist and continue in-situ enzymolysis The soaked rice 2 is kept warm in a 70%-75% humidity insulation box at 25-28° C. for 5-8 hours to enter the middle stage of the germination of seeds, and the germination initial stage rice 3 with high protease and amylase activities is obtained, wherein the protease activity of the rice 3 is ≥160.33 U / g dry rice, and the amylase activity is ≥53.12 U / g dry rice; ④Inactivate endogenous enzymes and steam the rice Put rice 3 into a steamer and steam it to obtain rice 4; Steaming conditions: temperature 100℃~120℃, steaming time 10min~20min; (2) Rice drying and starch recrystallization Drying the cooked rice 4 and cooling it to room temperature to obtain rice 5; The drying temperature is 30℃~40℃, the drying time is 24~36h, and the moisture content is ≤10%; (3) Husking, milling and color sorting The dried rice 5 is husked and milled by a rice mill to obtain parboiled rice, and black rice, black-headed rice and yellow rice mixed in the finished rice are removed to obtain finished parboiled rice; The estimated glycemic index (eGI) of the obtained finished steamed rice is <64, and the taste quality score is greater than 76.

2. Parboiled rice obtained by the method of claim 1.

3. Use of the parboiled rice according to claim 2 in preparing a product having the function of lowering the glycemic index.

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