Method for improving resistance of corn starch through microwave and hot air combined drying

Abstract

The invention relates to the field of grain drying, in particular to a method for improving corn starch resistance through hot air and microwave combined drying. The method comprises the following steps: drying corn kernels with the moisture content of 22-30% by using microwaves and hot air at the same time; the method comprises the following steps: soaking dried corn kernels, extracting starch through a wet grinding procedure, and drying to obtain the high-resistance corn starch. The microwave enables the interior of the material to rise quickly, and hot air is combined for synergistic heating, so that drying uniformity and heat and mass transfer efficiency can be improved. Meanwhile, due to the combined action of heat and moisture, the starch structure is changed, so that the resistance of the starch is improved. The resistance of the corn starch is improved through drying processing, the subsequent tedious processing flow can be simplified, the production cost is reduced, and the method has remarkable energy-saving and environment-friendly advantages.

Description

Technical Field

[0001] The invention relates to the field of grain drying, in particular to a method for improving the resistance of corn starch by combined microwave and hot air drying. Background Art

[0002] Drying is an important processing link after corn harvest and a prerequisite for safe storage of corn. Existing drying technology mainly uses hot air drying, which has a long drying time, high energy consumption and poor corn quality. Microwave is an efficient, energy-saving and environmentally friendly drying technology that has been widely used in fruit and vegetable drying, but there is a lack of research on microwave in grain drying.

[0003] Starch is the main carbohydrate in corn kernels, which can be hydrolyzed into glucose by amylase to provide energy for the human body. According to the release and absorption rate of glucose in the gastrointestinal tract, starch can be divided into: rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). RS cannot be digested and absorbed by the small intestine of a healthy human body, and can be fermented in the colon to produce short-chain fatty acids and other degradation products that are beneficial to the human body. Studies have shown that RS can enhance the body's sense of fullness, delay the rise in blood sugar levels after meals, and improve sugar and lipid metabolism. Therefore, it is of great significance to develop corn starch with a high resistant starch content using corn as raw material, which is suitable for consumption by people with diabetes and obesity.

[0004] RS can be divided into five types, namely, physically embedded starch (RS1), resistant starch granules (RS2), retrograded starch (RS3), chemically modified starch (RS4) and amylose-lipid complex (RS5). Existing technologies usually use starch as raw material, and adopt physical modification (extrusion, wet heat, microwave, ultrasound), chemical modification (acidification, oxidation, acetylation), enzyme modification (pullulanase, starch branching enzyme) and composite modification to obtain resistant starch. For example, the Chinese invention patent with application number 201811492438.0 uses pullulanase to debranch wheat starch, and then compounds it with monostearate glycerol, and finally wet heat treatment to increase the content of wheat resistant starch; the Chinese invention patent with application number 202010119096.9 uses high-temperature dry heat amorphization, low-frequency ultrasonic emulsification, high-pressure homogenization combined with spray drying to obtain a highly resistant corn starch. However, the above preparation process is relatively cumbersome, and increases equipment and labor costs and may cause environmental pollution. Summary of the invention

[0005] In view of the defects existing in the prior art, the purpose of the present invention is to provide a method for improving the resistance of corn starch by combining microwave and hot air drying, so as to solve the problems existing in the current process such as complicated process, high pollution and high energy consumption.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a method for simultaneously treating corn kernels with hot air and microwaves to improve their starch resistance, characterized in that it comprises the following steps:

[0007] S1. Select freshly harvested corn kernels with a moisture content of 22-30%, then use hot air and microwaves to dry the corn kernels to a moisture content of 14% to improve corn starch resistance.

[0008] S2. Soak the dried corn kernels and extract starch through a wet grinding process, and then obtain high-resistant corn starch after drying.

[0009] As a further optimization of the above technical solution, in step S1, the microwave power density is 0.6-2.4 W / g, and the hot air temperature is 80-100°C.

[0010] As a further optimization of the above technical solution, the microwave frequency in step S1 is 2.45 GHz, an intermittent control mode is adopted, and the microwave intermittent ratio is 1:3.

[0011] As a further optimization of the above technical solution, the drying process in step S1 uses multiple optical fiber temperature sensors and infrared moisture sensors to monitor the temperature and moisture changes of the material during the drying process, and adjusts and monitors the drying process and equipment operating status in real time through a human-computer interaction interface.

[0012] As a further optimization of the above technical solution, the drying process in step S1 dynamically adjusts the microwave power by setting a PID control circuit.

[0013] As a further optimization of the above technical solution, the corn kernels in step S2 are firstly cleaned before wet grinding.

[0014] As a further optimization of the above technical solution, the corn starch in step S2 is dried in an oven and then passed through a 80-120 mesh sieve.

[0015] The high-resistant corn starch can be used as a raw material in food processing to produce food suitable for people with diabetes and obesity.

[0016] The method for improving corn starch resistance by microwave and hot air combined drying of the present invention has the following beneficial effects:

[0017] 1. The present invention introduces microwaves into the hot air drying of corn. Microwaves can rapidly increase the internal temperature of the material. The synergistic effect of hot air can reduce the temperature difference between the inside and outside of the material, improve the uniformity of drying and the efficiency of heat and mass transfer. At the same time, PID combined with human-machine interface monitoring and feedback adjustment of drying regulation can reduce energy loss and improve drying efficiency.

[0018] 2. The present invention denatures corn starch by changing the drying process and drying conditions of corn kernels, thereby improving the resistance of corn starch, simplifying the subsequent cumbersome processing procedures, reducing production costs, and having the advantages of energy saving and environmental protection. Adding it to food can effectively inhibit the rise in blood sugar after a meal and improve intestinal health. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention has the following accompanying drawings:

[0020] Figure 1 This is the XRD diagram of corn starch treated by Examples 1-4 and Comparative Examples 1-3 of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with the accompanying drawings. It should be understood that these embodiments are only for illustrative purposes and do not limit the scope of protection of the present invention.

[0022] In the following embodiments and comparative examples, the experimental corn variety is Zhengdan 958.

[0023] Example 1

[0024] Corn kernels were dried using a microwave and hot air combined dryer. The initial moisture content of the corn was 29.1%. The hot air temperature was set to 80°C, the microwave power density was 0.6 W / g, and preheating was performed for 30 minutes. The microwave was in intermittent control mode, working for 1 minute with an interval of 3 minutes. Drying was stopped when the moisture content of the corn dropped to 14%, and corn starch was extracted by wet grinding.

[0025] Example 2

[0026] Corn kernels were dried using a microwave and hot air combined dryer. The initial moisture content of the corn was 29.1%. The hot air temperature was set to 80°C, the microwave power density was 1.2 W / g, and preheating was performed for 30 minutes. The microwave was in intermittent control mode, working for 1 minute with an interval of 3 minutes. Drying was stopped when the moisture content of the corn dropped to 14%, and corn starch was extracted by wet grinding.

[0027] Example 3

[0028] Corn kernels were dried using a microwave and hot air combined dryer. The hot air temperature was set to 100°C, the initial moisture content of the corn was 29.1%, the microwave power density was 0.6 W / g, and the preheating was 30 minutes. The microwave was in intermittent control mode, with the microwave working for 1 minute and an interval of 3 minutes. Drying was stopped when the moisture content of the corn dropped to 14%, and corn starch was extracted by wet grinding.

[0029] Example 4

[0030] Corn kernels were dried using a microwave and hot air combined dryer. The initial moisture content of the corn was 29.1%. The hot air temperature was set to 100°C, the microwave power density was 1.2 W / g, and preheating was performed for 30 minutes. The microwave was in intermittent control mode, with the microwave working for 1 minute and an interval of 3 minutes. Drying was stopped when the moisture content of the corn dropped to 14%, and corn starch was extracted by wet grinding.

[0031] Comparative Example 1

[0032] The corn was dried using hot air, the hot air temperature was set to 80°C, the initial moisture content of the corn was 28.9%, and the drying was stopped when the moisture content of the corn was reduced to 14%, and the corn starch was extracted by wet grinding.

[0033] Comparative Example 2

[0034] The corn was dried using hot air, the hot air temperature was set to 100°C, the initial moisture content of the corn was 28.9%, and the drying was stopped when the moisture content of the corn was reduced to 14%, and the corn starch was extracted by wet grinding.

[0035] Comparative Example 3

[0036] The corn was laid out flat and dried under natural sunlight. The initial moisture content of the corn was 28.1%. The drying was stopped when the moisture content of the corn dropped to 14%. Corn starch was extracted by wet milling.

[0037] The resistant starch content in Examples 1 to 4 and Comparative Examples 1 to 3 is analyzed below:

[0038] Weigh 200 mg of starch, add 10 mL of acetic acid-sodium acetate buffer (pH 5.2), and gelatinize in boiling water for 30 minutes, shaking continuously. After cooling to 37 ° C, balance in a water bath shaker (speed of 150r / min) for 10 minutes. Add 5 mL of a mixed enzyme solution containing porcine pancreatic α-amylase (290U / mL) and saccharifying enzyme (50U / mL), start timing, take 0.5 mL of the sample hydrolyzed for 20min and 120min, add 4.5 mL of anhydrous ethanol to inactivate the enzyme, and then centrifuge. The DNS method is used to determine the glucose content.

[0039] %RDS=G20*0.9*100 / S; %SDS=%RDS=(G120-G20)*0.9*100 / S; %RS=1-%RDS-%SDS

[0040] S represents starch mass; RDS is fast-digestible starch; SDS is slowly-digestible starch; RS is resistant starch; G20 and G120 are the glucose contents at 20 min and 120 min of digestion, respectively; 0.9 is the stoichiometric constant of starch in glucose.

[0041] The test results are shown in Table 1:

[0042] Table 1 Digestion characteristics of starch in comparative examples and embodiments

[0043]

[0044] Note: Different lowercase letters in each column represent significant differences between the values ​​(p<0.05)

[0045] As shown in Table 1, compared with sun drying and hot air drying, the resistant starch content in corn starch obtained by microwave hot air combined drying is significantly increased. Compared with sun drying (Comparative Example 3), the resistant starch content in corn starch obtained by combined drying with hot air temperature of 100°C and microwave power density of 1.2W / g is increased by 39.42%. The increase in resistant starch content is due to the reorganization of starch molecules in corn kernels under the synergistic effect of microwaves and hot air, and the gelatinization temperature of the reorganized crystals increases, thereby reducing the swelling power of starch, making it difficult for starch to be hydrolyzed by enzymes.

[0046] Table 2 Double helix and crystalline structure of starch in comparative examples and embodiments

[0047]

[0048]

[0049] Note: Different lowercase letters in each column represent significant differences between the values ​​(p<0.05)

[0050] The double helix of corn starch was analyzed by DSC, as shown in Table 2. Compared with hot air drying (Comparative Examples 1-2) and sun drying (Comparative Example 3), the gelatinization transition temperature (T o , T p and T c ) increases, and the gelatinization enthalpy (ΔH) decreases. With the increase of hot air temperature and microwave power, the decrease of ΔH and the increase of transition temperature are more obvious. The increase of gelatinization transition temperature indicates that the thermal stability of starch is improved. The decrease of ΔH indicates that the destruction of hydrogen bonds in the crystalline region in the microwave field leads to the loss of double helical order in starch.

[0051] Under the action of microwaves, starch molecular chains further interact with proteins and lipids to form complexes. Figure 1 As shown in the figure, under the combined action of microwaves and hot air, there is a V-type crystal peak (2θ=20°) in the XRD diagram of starch. This shows that microwaves lead to the formation of straight-chain starch-lipid complex, which has high thermal stability. Starch-lipid complex is also called RS5 resistant starch. This complex reduces the degree of swelling of the granules, so it is difficult for enzymes to reach the inside of the starch granules.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Anyone familiar with this technology can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the definition of the claims.

Claims

1. A method for improving corn starch resistance by microwave hot air combined drying, characterized in that: The steps include: Hot air and microwaves are used simultaneously to dry corn kernels to improve the resistance of corn starch.

2. The method for improving corn starch resistance by microwave and hot air combined drying as claimed in claim 1, characterized in that: The hot air temperature is 80-100°C.

3. The method for improving corn starch resistance by microwave and hot air combined drying as claimed in claim 2, characterized in that: The microwave power density is 0.6-2.4 W / g.

4. The method for improving corn starch resistance by microwave and hot air combined drying as claimed in claim 2, characterized in that: The microwave frequency is 2.45 GHz, an intermittent control mode is adopted, and the microwave intermittent ratio is 1:

3.

5. Application of the high-resistant starch corn obtained according to the method of claim 1 in food processing.

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