Yolk moon cake anti-cracking baking process based on microwave targeted preheating

Abstract

The invention relates to the field of food processing, and particularly discloses an anti-cracking baking process for yolk mooncakes based on microwave targeted preheating, which comprises the following steps: carrying out targeted preheating on mooncake green bodies by using microwaves, selectively acting microwave energy on yolk areas according to the difference of microwave absorption capacities of yolk, stuffing and mooncake crust, preheating the yolk areas to 45-60 DEG C, and carrying out microwave targeted preheating on the mooncake green bodies; meanwhile, the outer layer temperature rise is controlled not to exceed 10 DEG C; then overall hot air baking is performed, and preferential curing and overall uniform heating of the egg yolk are achieved through cooperative regulation and control of microwave frequency, power and time in combination with infrared real-time temperature measurement feedback. According to the method, the yolk in the yolk mooncake is pre-heated in a targeted manner, so that the thermal stress of internal and external components of the mooncake in the baking process is balanced, the problem of cracking of the mooncake crust caused by asynchronous curing is fundamentally solved, and the yield and the product quality are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, and in particular to a microwave-targeted preheating process for preventing cracking in egg yolk mooncakes. Background Technology

[0002] Egg yolk mooncakes are a traditional seasonal food. Their classic structure consists of an outer crust, a filling layer of lotus seed paste or red bean paste, and a whole salted egg yolk at the core. In industrial production, this multi-layered composite structure is highly susceptible to cracking under traditional whole-piece baking processes, severely impacting product quality and commercial value. The root cause lies in the significant differences in the thermophysical properties of the components: the egg yolk, rich in protein and fat, has a high specific heat capacity and slow thermal conductivity, making it a component with high thermal inertia; while the outer filling and crust are primarily composed of starch and sugar, exhibiting rapid thermal response. In a single, high-temperature baking process, to ensure the egg yolk is fully cooked, the outer structure must withstand a longer heating time. This leads to excessive evaporation and shrinkage of moisture in the crust and filling, while the egg yolk expands due to heat. The resulting immense thermal stress and drying shrinkage stress are concentrated at the weakest point of the crust, causing cracking.

[0003] To improve heating efficiency, microwave preheating technology has been applied in numerous industrial fields, such as preheating slabs before hot pressing of engineered wood products and preheating mixtures before metallurgical sintering. However, existing microwave preheating technologies share several limitations: First, they are applied to relatively homogeneous industrial materials, not to heterogeneous foods with stringent requirements for texture and shape. Second, their heating methods involve overall, uniform heating of the material, aiming to achieve overall temperature rise to shorten the processing time of subsequent main processes; the core objective is to improve efficiency and save energy. Third, their technical approach has never involved utilizing the differences in microwave absorption capacity between components to achieve spatially selective energy deposition, nor has it been used to address the structural damage to products caused by thermal stress. In the very few microwave pretreatments involving food, the heating is only for overall drying purposes. Therefore, existing technologies have failed to provide any effective insights into solving the structural cracking problem in egg yolk mooncakes caused by the mismatch of component thermophysical properties.

[0004] In summary, there is an urgent need to develop an innovative baking process that can actively balance the heat and stress state of the inner and outer layers of mooncakes through precise energy control, thereby solving the problem of crust cracking during the processing of egg yolk mooncakes. Summary of the Invention

[0005] The purpose of this invention is to provide a microwave-targeted preheating-based baking process for preventing cracking of egg yolk mooncakes, which can effectively solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A microwave-targeted preheating-based baking process for preventing cracking in egg yolk mooncakes includes the following steps: Step S1: Targeted Microwave Preheating: Microwave heating is applied to the raw egg yolk mooncake, which includes an egg yolk, a filling layer in direct contact with the egg yolk, and a crust layer outside the filling layer. Utilizing the difference in microwave absorption capacity among the egg yolk, filling layer, and crust layer at different microwave frequencies, microwave energy is selectively deposited in the egg yolk region. During the microwave heating process, heating parameters are controlled to preheat the temperature of the egg yolk region to 45°C to 60°C, while the temperature rise of the filling layer and crust layer is controlled within 10°C. Step S2: Whole-body hot air baking: Place the mooncakes processed in step S1 in a hot air environment for whole-body baking until the mooncakes are fully cooked.

[0007] Furthermore, the microwave heating uses a microwave frequency of 2450MHz or 915MHz; during the microwave heating process, the raw mooncake is rotated at a constant speed around its central axis.

[0008] Preferably, the microwave frequency is selected based on the characteristic parameters of the raw mooncake, including total mass, size, number of egg yolks, and type of filling.

[0009] Preferably, the method for selecting the microwave frequency includes: When the total mass of the raw mooncake is greater than 150 grams, and / or contains two or more egg yolks, and / or the maximum radial dimension is greater than 8 cm, a heating frequency of 915 MHz is preferred. When the total mass of the raw mooncake is no more than 150 grams, and it is a single-yellow mooncake with a maximum radial dimension of no more than 8 centimeters, a heating frequency of 2450MHz is preferred. For fillings with low dielectric loss, a frequency of 915MHz is preferred for heating.

[0010] Furthermore, in step S1, the heating parameters include microwave frequency and heating time; by selecting the microwave frequency and controlling the heating time, the temperature rise rate of the egg yolk region is higher than the average temperature rise rate of the filling layer and the crust layer.

[0011] Preferably, the temperature rise rate of the egg yolk region is at least twice the average temperature rise rate of the filling layer and the crust layer.

[0012] Furthermore, in step S1, the heating parameters include microwave power and treatment time; the microwave power and treatment time are dynamically adjusted according to the total mass of the raw mooncake and the number and size of the egg yolks it contains.

[0013] Furthermore, step S1 also includes a temperature monitoring and feedback control step: real-time non-contact monitoring of the temperature of the egg yolk area, the filling layer, and the crust layer, and adjusting the heating parameters according to the monitoring results; the real-time non-contact monitoring is performed using infrared thermal imaging.

[0014] Furthermore, in step S1, the microwave heating is performed collaboratively by at least two independent microwave sources, and the relative phase difference between the microwaves output by each microwave source is dynamically adjusted.

[0015] Furthermore, in step S2, the hot air baking temperature is 180°C to 220°C, and the baking time is 12 minutes to 20 minutes.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes the inherent differences in microwave absorption capacity of egg yolks, fillings, and crusts to pioneer the process of "microwave-targeted preheating of egg yolks," which selectively deposits energy on the high-thermal-inertia egg yolks. This balances the thermal stress during baking from the source, fundamentally inhibiting crust cracking caused by asynchronous ripening, and significantly improving the yield.

[0017] 2. By employing a technology that dynamically controls the phase difference of multiple microwaves, this invention achieves more precise and uniform heating of the yolk region, avoiding localized overheating or uneven heating that may occur inside the yolk during targeted preheating, thereby further improving process stability and product quality consistency.

[0018] 3. This invention creatively combines microwave targeted preheating with subsequent hot air baking, which not only significantly shortens the overall baking time and reduces the moisture loss of the crust and filling, making the mooncakes softer and more moist, but also achieves a uniform golden coloring effect through parameter coordinated control.

[0019] 4. The target temperature, frequency selection rules, and phase control methods described in this process form clear and quantifiable operating standards, and can be integrated into existing production lines through modular equipment, realizing large-scale, efficient, and stable continuous production of high-quality egg yolk mooncakes. Attached Figure Description

[0020] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0021] Figure 1 This is a schematic diagram of the overall process of the egg yolk mooncake anti-crack baking process provided in the embodiments of the present invention.

[0022] Figure 2 This is a decision-making flowchart for selecting microwave frequencies based on product characteristic parameters provided in an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0024] Example 1

[0025] like Figure 1 As shown, Embodiment 1 of the present invention discloses a microwave-targeted preheating-based anti-cracking baking process for egg yolk mooncakes, comprising the following steps: Step S1: Targeted Microwave Preheating Step: Microwave heating is performed on the raw egg yolk mooncake, which includes an egg yolk, a filling layer in direct contact with the egg yolk, and a crust layer outside the filling layer. Utilizing the difference in microwave absorption capacity of the egg yolk, filling layer, and crust layer in the microwave frequency band, microwave energy is selectively deposited in the egg yolk area. During the microwave heating process, heating parameters are controlled so that the temperature of the egg yolk area is preheated to 45°C to 60°C, and the temperature rise of the filling layer and crust layer is controlled within 10°C.

[0026] Specifically, step S1 includes the following refinement steps to ensure the precise achievement of the targeted preheating effect: Step S11: Placement of Raw Mooncake and Selection of Microwave Frequency: Place the raw egg yolk mooncake blank to be processed on a special rotating tray inside the microwave equipment cavity. Select a microwave frequency of 2450MHz or 915MHz. During the microwave heating process, rotate the raw mooncake blank at a uniform speed around its central axis.

[0027] Specifically, the rotating tray is made of microwave-resistant, low-loss material (such as polytetrafluoroethylene) and is driven by a bottom motor with an adjustable rotation speed, for example, ranging from 5 to 20 revolutions per minute. The rotation aims to ensure uniform microwave energy deposition in the egg yolk area, avoiding localized overheating or heating dead zones caused by uneven microwave field distribution. The microwave frequency is selected based on the characteristic parameters of the raw mooncake, including total mass (M, unit: grams), dimensions (mainly referring to the maximum radial dimension D, unit: centimeters), number of egg yolks (N), and type of filling.

[0028] Furthermore, step S11 includes: Step S111: Feature parameter acquisition step: Obtain the feature parameters of the current batch or a single raw mooncake through a visual recognition system or preset production parameters, including total mass M, maximum radial dimension D, number of egg yolks N, and filling type identifier.

[0029] Specifically, weighing sensors and vision measurement units can be integrated into the production line to automatically acquire M and D. N and filling types, which can be issued by the production management system (MES) according to the production formula or identified through QR code / RFID tags.

[0030] Step S112: Microwave frequency decision step: Select microwave frequency according to the feature parameters obtained in step S111 and the preset decision logic.

[0031] like Figure 2 As shown, the decision logic for step S112 includes: 1) If the total mass M of the raw mooncake is greater than 150 grams, and / or contains two or more egg yolks (N≥2), and / or the maximum radial dimension D is greater than 8 cm, a frequency of 915 MHz should be preferred for heating. This is because the microwave wavelength of 915 MHz is longer (approximately 33 cm), allowing for deeper penetration into materials, making it more suitable for heating larger and heavier mooncakes, and ensuring that microwave energy effectively reaches the central egg yolk area.

[0032] 2) If the total mass M of the raw mooncake is no more than 150 grams, and it is a single-yolk mooncake (N=1), and the maximum radial dimension D is no more than 8 cm, a frequency of 2450 MHz should be preferred for heating. This is because the microwave wavelength of 2450 MHz is relatively short (about 12 cm), and the heating rate is usually faster. For small mooncakes, it can achieve rapid targeted preheating and improve efficiency.

[0033] 3) For fillings with low dielectric loss (i.e., weak microwave absorption) (e.g., certain sugar and oil-based fillings with very low water content, or certain dried fruit fillings), a heating frequency of 915MHz is preferred. This is because 915MHz microwaves have slightly weaker selective heating of water compared to 2450MHz, but their response to ionic conductivity and interfacial polarization effects may be more conducive to energy penetration through the low-loss filling layer to reach the yolk, avoiding excessive energy deposition on the outer layer. The dielectric loss properties of filling types can be established through preliminary experiments to create a database, or classified according to their water activity and main components.

[0034] For example, for a double-yolk white lotus seed paste mooncake with M=180 grams, N=2, and D=8.5 centimeters, the system will automatically select 915MHz as the operating frequency.

[0035] Step S12: Dynamic adjustment of heating parameters: Based on the microwave frequency determined in step S11, the microwave power and action time are dynamically adjusted according to the total mass of the raw mooncake and the number and size of the egg yolks contained therein, so that the egg yolk area reaches the target preheating temperature.

[0036] Specifically, the heating parameters include microwave power P (in watts, W) and treatment time t (in seconds, s). The system pre-stores a power-time matching model based on extensive experimental data. The core of this model is to ensure that the total input energy E matches the heat capacity of the egg yolk to be preheated, while also considering the targeted deposition efficiency of the microwave energy. (Empirical value, related to frequency and mooncake structure). A simplified example of the dynamic adjustment formula is as follows:

[0037] in, The total weight of the egg yolk (in grams) can be estimated based on nitrogen and the standard egg yolk weight. The specific heat capacity of the egg yolk (approximately 3.5 J / (g·℃)); Set the target preheating temperature for the egg yolks (e.g., 50°C). The initial temperature of the egg yolk (usually room temperature, such as 25°C); Estimated weight (in grams) of the filling material adjacent to the egg yolk; The specific heat capacity of the filling (approximately 3.0 J / (g·℃)); For the allowable temperature rise of the filling layer (e.g., <5°C); Microwave energy targeted deposition efficiency (empirical coefficient, e.g., 0.6–0.8).

[0038] The system estimates the required energy E based on the above formula, and calculates and sets the initial power based on the device's power level (e.g., 500W, 800W, 1000W) and efficiency. and time .

[0039] Step S13: Temperature Monitoring and Feedback Control Step: The temperature of the egg yolk area, the filling layer, and the crust layer is monitored in real time using non-contact monitoring, and the heating parameters are adjusted according to the monitoring results. The real-time non-contact monitoring is performed using infrared thermal imaging.

[0040] Specifically, an infrared transparent window (such as a germanium window) is provided above or to the side of the microwave cavity, and an infrared thermal imager is installed there, its field of view covering the mooncakes on the rotating tray. During microwave heating, the infrared thermal imager acquires images of the temperature distribution on the surface of the mooncake at a certain frequency (e.g., 1-5 frames per second). Through image processing algorithms, the temperature of the crust surface (representing the temperature rise of the crust layer) can be identified and tracked, and the temperature change trend of the internal egg yolk region can be estimated by using model inversion or based on the relative position of the egg yolk in the mooncake, combined with the surface temperature gradient. When the estimated temperature of the egg yolk region reaches a preset target value (e.g., 50℃±2℃), or the temperature rise of the crust surface approaches the upper limit (e.g., 8℃), the control system immediately cuts off the microwave power, ending step S1. If a feedforward-feedback composite control is used, the microwave power can be dynamically fine-tuned according to the real-time monitored temperature curve to achieve more precise temperature control.

[0041] In a preferred embodiment, step S1 can also be achieved by selecting the microwave frequency and controlling the heating time, such that the temperature rise rate of the egg yolk region is at least twice the average temperature rise rate of the filling layer and the crust layer.

[0042] Specifically, when selecting the microwave frequency (step S112) and setting the initial power / time (step S12), the differences in dielectric properties between egg yolk (rich in moisture, salt, and fat, with a high dielectric constant and loss factor) and pastry / filling (mainly composed of starch and sugar, with relatively low dielectric loss) are fully considered. Selective heating can be achieved using microwaves at frequencies of 915MHz and 2450MHz. The estimated temperature rise rate of the egg yolk region can be calculated using real-time monitoring data from infrared thermal imaging. (unit: ℃ / s) and the average temperature rise rate of the crust surface ( (Unit: ℃ / s). One of the goals of a control system is to ensure... ≥ This further enhances the effect of targeted preheating, making the energy more concentrated in the yolk.

[0043] Through the specific implementation of step S1 (including its detailed steps S11 to S13), precise, rapid, and controllable preheating of the egg yolk area is achieved, raising its temperature to the critical range of 45℃-60℃ (preferably 50℃±2℃), while effectively suppressing the temperature rise of the outer crust and filling (preferably controlled within 5℃). Before the subsequent baking begins, the initial temperature difference between the egg yolk and the crust / filling is reduced in advance, allowing the egg yolk to enter the initial stage of protein denaturation and coagulation earlier. This fundamentally changes the heat transfer pattern and stress development path during the baking process, laying a decisive foundation for completely preventing crust cracking. Simultaneously, integrated frequency selection, dynamic parameter adjustment, and temperature feedback control ensure the strong adaptability and stability of this process for different mooncake specifications and recipes.

[0044] In a further preferred embodiment, to more precisely control the microwave field distribution, enhance targeting, and improve heating uniformity (especially for irregularly shaped or multi-yolk mooncakes), the microwave heating in step S1 can be performed collaboratively by at least two independent microwave sources. These microwave sources (e.g., magnetrons) feed microwave energy into the cavity in a specific geometric layout (e.g., vertically opposed, multi-sided surround). The system is equipped with a phase controller capable of dynamically adjusting the relative phase difference between the output microwaves of each microwave source. By changing the phase difference, regions of constructive or destructive interference can be constructed within the cavity, thereby "guiding" the hotspot region of the microwave energy field distribution in three-dimensional space to better match the spatial position of the egg yolks inside the mooncake. For example, for a double-yolk mooncake, the phase can be adjusted so that the region of maximum field strength is aligned with the positions of the two egg yolks respectively. This active field matching technology can further improve targeted preheating efficiency, reduce energy waste, and reduce heating of non-target areas.

[0045] Step S2: Overall hot air baking step: Place the mooncakes processed in step S1 in a hot air environment for overall baking until the mooncakes are fully cooked.

[0046] Specifically, step S2 includes: Step S21: Setting hot air baking parameters: Set the hot air baking temperature to 180℃ to 220℃ and the baking time to 12 minutes to 20 minutes.

[0047] Specifically, the microwave-preheated mooncakes are quickly transferred, either via conveyor belt or manually, to a preheated hot air circulating oven (tunnel oven or layer oven) at the set temperature. The hot air temperature ( ) and baking time ( The precise setting of the baking process requires comprehensive consideration of the initial state of the mooncake after step S1 (the egg yolks have been preheated), the size of the mooncake, the type of filling, and the desired degree of browning of the crust. Since the egg yolks have been preheated, the required baking time can be shortened by about 20%-30% compared to traditional methods. Bake until the center temperature of the mooncake reaches the safe ripening temperature (usually >85℃) and the crust has a uniform golden-yellow color.

[0048] Through the specific implementation of step S2, the final overall maturation, shaping, coloring, and sterilization of the mooncake are completed. Since the egg yolk with high thermal inertia has been effectively pretreated in step S1, the maturation process of the internal and external components in step S2 is more synchronized, the thermal stress is significantly reduced, and the crust can maintain its structural integrity during rapid dehydration and Maillard reaction, ultimately resulting in a high-quality, crack-free finished product.

[0049] The technical effects of the present invention are further verified through specific embodiments and comparative examples below.

[0050] In one specific embodiment, a standard single-yolk lotus seed paste mooncake (120g, 7cm in diameter, with a yolk diameter of 2.2cm) was selected for processing. First, the raw mooncake was placed in a microwave preheating cavity using a frequency of 2450MHz, a microwave power of 500W, a heating time of 30 seconds, and a rotating tray speed of 4 revolutions per minute. Real-time monitoring with an infrared thermal imager showed that the highest surface temperature of the mooncake at the end of 30 seconds was 27℃ (initial 22℃, temperature rise of 5℃), and the estimated center temperature of the yolk was 50.3℃. Subsequently, the mooncake was transferred to a tunnel oven within 4 seconds and baked at 190℃ for 16 minutes. After baking, the measured center temperature of the mooncake was 86.5℃, the crust was a uniform golden yellow, and there were no visible cracks.

[0051] In another embodiment, a double-yolk red bean paste mooncake (weighing 160 grams, with a diameter of 9 cm and two symmetrically distributed egg yolks) was processed. A microwave frequency of 915 MHz, a power of 850 W, a processing time of 42 seconds, and a rotation speed of 3 rpm were used. Infrared monitoring showed that the crust temperature rose by 7°C, and the center temperature of the egg yolks reached 51.1°C. It was then baked at 185°C for 19 minutes, with a center temperature of 87.2°C, and the crust remained intact without cracks.

[0052] As a comparative example, single-yolk mooncakes of the same size were processed using traditional methods: they were directly baked in a 190℃ tunnel oven for 22 minutes. After baking, the center temperature was 85.8℃, but obvious longitudinal cracks appeared on the surface of the crust, with an average crack length of 8 mm and a cracking rate of 68% (test sample of 100).

[0053] To quantify the evaluation results, the following comparative experiment was conducted:

[0054] Data shows that the process of this invention significantly reduces the cracking rate of the crust while preserving better color and texture. Higher crust moisture content indicates better moisture retention; a lower L* value indicates a more complete Maillard reaction, resulting in a deeper and more appealing color; sensory evaluation, conducted by a professional judging panel based on four indicators—appearance, aroma, texture, and flavor consistency—shows that the sample from this invention has significant advantages in terms of integrity and moisture content.

[0055] The foregoing has shown and described the basic principles, main features, and advantages of this invention. Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A yolk mooncake anti-cracking baking process based on microwave targeted preheating, characterized in that, Includes the following steps: Step S1: Targeted microwave preheating: Microwave heating is performed on the raw egg yolk mooncake, which includes an egg yolk, a filling layer in direct contact with the egg yolk, and a crust layer outside the filling layer; by utilizing the difference in microwave absorption capacity of the egg yolk, filling layer and crust layer in the microwave frequency band, microwave energy is selectively deposited on the egg yolk area. During the microwave heating process, the heating parameters are controlled so that the temperature of the egg yolk area is preheated to 45°C to 60°C, and the temperature rise of the filling layer and the crust layer is controlled within 10°C. Step S2: Whole-body hot air baking: Place the mooncakes processed in step S1 in a hot air environment for whole-body baking until the mooncakes are fully cooked.

2. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 1, characterized in that, In step S1, the microwave heating uses a microwave frequency of 2450MHz or 915MHz; during the microwave heating process, the raw mooncake is rotated at a constant speed around its central axis.

3. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 2, characterized in that, The microwave frequency is selected based on the characteristic parameters of the raw mooncake, including total mass, size, number of egg yolks, and type of filling.

4. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 3, characterized in that, The method for selecting the microwave frequency includes: When the total mass of the raw mooncake is greater than 150 grams, and / or contains two or more egg yolks, and / or the maximum radial dimension is greater than 8 cm, a heating frequency of 915 MHz is preferred. When the total mass of the raw mooncake is no more than 150 grams, and it is a single-yellow mooncake with a maximum radial dimension of no more than 8 centimeters, a heating frequency of 2450MHz is preferred. For fillings with low dielectric loss, a frequency of 915MHz is preferred for heating.

5. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 1, characterized in that, In step S1, the heating parameters include microwave frequency and heating time; by selecting the microwave frequency and controlling the heating time, the temperature rise rate of the egg yolk region is higher than the average temperature rise rate of the filling layer and the crust layer.

6. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 5, characterized in that, The temperature rise rate of the egg yolk region is at least twice the average temperature rise rate of the filling layer and the crust layer.

7. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 1, characterized in that, In step S1, the heating parameters include microwave power and treatment time; the microwave power and treatment time are dynamically adjusted according to the total mass of the raw mooncake and the number and size of the egg yolks it contains.

8. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 1, characterized in that, Step S1 also includes a temperature monitoring and feedback control step: real-time non-contact monitoring of the temperature of the egg yolk area, the filling layer, and the crust layer, and adjusting the heating parameters according to the monitoring results; the real-time non-contact monitoring is performed using infrared thermal imaging.

9. The egg yolk moon cake anti-cracking baking process based on microwave target preheating according to claim 1, characterized in that, In step S1, the microwave heating is performed by at least two independent microwave sources working together, and the relative phase difference between the microwaves output by each microwave source is dynamically adjusted.

10. The egg yolk moon cake anti-cracking baking process based on microwave targeted preheating according to claim 1, characterized in that, In step S2, the temperature of the hot air baking is 180°C to 220°C, and the baking time is 12 minutes to 20 minutes.

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