Method for predicting hot air drying characteristics of pitaya based on drying dynamics

Abstract

The invention relates to a method for predicting hot air drying characteristics of pitaya based on drying dynamics, and belongs to the technical field of agricultural product drying. Comprising the following steps: preprocessing pitaya, testing, averaging, and analyzing data; determining and calculating drying characteristic indexes; solving an effective moisture diffusion coefficient; solving the activation energy; a common dry model example; a color determination method; determining the content of total flavonoids; a rehydration determination method; an antioxidant activity determination method; and the data is sorted and drawn through Excel software. The invention provides a method for predicting hot air drying characteristics of dragon fruits based on drying dynamics, which comprises the following steps of: optimally fitting experimental data by using an existing mathematical model for describing thin-layer hot air drying of fruits, and converting the experimental data into an equation expressed by parameters; therefore, a reliable basis is provided for analysis and process optimization of the hot air drying process, and the influence on the hot air drying characteristics and the main quality of the red pitaya at the specific temperature and different thicknesses is better studied.

Description

Technical Field

[0001] The invention relates to a method for predicting hot-air drying characteristics of pitaya based on drying dynamics, and belongs to the technical field of agricultural product drying. Background Art

[0002] Pitaya (Hylocereus undatus) is the fruit of the Cactaceae family and the Hylocereus genus. It is mainly divided into two varieties: white heart and red heart. Pitaya is juicy, delicious, nutritious, and contains a variety of vitamins and minerals. It has good nutritional and health benefits such as anti-oxidation and prevention of dementia. However, since fresh pitaya is soft and contains a lot of water, it will undergo normal metabolism after picking. Too high a water content can easily cause corruption and deterioration, while too low a water content can affect the sensory quality. The water content can significantly affect the preservation of fruits and vegetables. Therefore, processing pitaya can solve the problem of short storage period and help extend its storage period and shelf life. Pitaya processed products include dried pitaya, pitaya ice cream, pitaya yogurt, etc. Among them, the process of dried pitaya is relatively simple, and drying can reduce water activity, inhibit microbial growth, extend shelf life, and double the added value. For the drying of fruits and vegetables, the atmospheric pressure hot air drying method is often used. It is a drying method that uses hot air as a medium, heats through convection conduction and other methods, and removes the moisture of the material within a limited time. Hot air drying is currently the most common drying technology. There are three main types of hot air drying: the first type is to use hot air drying as one of the methods for studying pitaya drying and compare the effects of several drying methods on the quality or physical and chemical properties of pitaya; the second type is to combine hot air drying with another drying method to analyze the impact on pitaya quality or optimize the process; the third type is the least studied type, which involves pretreatment before hot air drying and hot air drying kinetic model. According to the different research parts of pitaya, the optimal drying method selected after comparison will also be different, but hot air drying is always the better treatment method.

[0003] However, the above research on the characteristics and quality changes of pitaya hot air drying is still rare. The incompleteness and lag of the research content in this area and the ambiguity of the characteristics of pitaya drying will limit the industrial application and promotion of pitaya drying, so pitaya hot air drying was selected for research.

[0004] In the process of pitaya drying, it is most important to control the moisture content. In drying dynamics, the key factor of drying rate can be used to better describe the speed of moisture content change. [2] At the same time, with the growing demand for dried products around the world, traditional drying technology alone is no longer in short supply, which has attracted great attention to drying dynamics at home and abroad. Summary of the invention

[0005] In view of this, the present invention provides a method for predicting the hot air drying characteristics of pitaya based on drying dynamics. The method utilizes the existing mathematical model describing the thin layer hot air drying of fruits to perform optimal fitting on the experimental data, regress it, and convert it into an equation represented by parameters, thereby providing a reliable basis for the analysis and process optimization of the hot air drying process, so as to better study the influence of specific temperature and different thickness on the hot air drying characteristics and main qualities of red pitaya, thereby reasonably optimizing the drying process and accurately predicting some changes in drying conditions.

[0006] The present invention provides a method for predicting hot air drying characteristics of pitaya based on drying dynamics, comprising the following steps:

[0007] 1) Pre-treat the dragon fruit, conduct experiments, and analyze the data after taking the mean;

[0008] 2) Determination and calculation of drying characteristic index: Material dry basis moisture content) is determined by direct drying method and calculated according to C t =(m r -m) / m; moisture (MR) ratio is calculated according to

[0009] MR=(C t -C e ) / (C0-C e ) is calculated; the drying rate (DR / (g / (g·h))) is calculated according to

[0010] DR = (C1-C2) / (t2-t1) calculation;

[0011] m r is the mass of the sample at a certain moment / g;

[0012] m is the mass of the sample after drying / g;

[0013] C t , C0, C e They are the dry basis moisture content of the material at drying time t, initial time, and equilibrium time / (g / g);

[0014] C1 and C2 are the moisture content of the material on dry basis at drying time t1 and t2 / (g / g);

[0015] 3) Solution of effective moisture diffusion coefficient: The effective moisture diffusion coefficient can be obtained by measuring the drying curve, using the Fick and Arrhenius equations, combining the two equations, making a ln MR-t relationship curve, fitting the linear equation, and obtaining the slope k0 according to k0 = -π 2 ·D eff / 4L 2 calculate;

[0016] Deff is the effective water diffusion coefficient / (m 2 / s);

[0017] L is 1 / 2 slice thickness / m; t is drying time / s;

[0018] 4) Solution of activation energy: Material D eff与 The relationship between temperature and

[0019] D eff =D0exp[-Ea / R / (T+273.15)], take Ln on both sides to get ln D eff =lnD0-E a / R / (T+273.15), we get ln D eff It is linearly related to 1 / (T+273.15), and the slope is -E by fitting the linear equation. a / R, thus obtaining E a

[0020] D0 is the material diffusion constant / (m 2 / s);

[0021] E a is the drying activation energy / (kJ / mol);

[0022] R is the gas molar constant (8.314 47 J / (mol·K));

[0023] 5) Examples of commonly used drying models: By establishing drying models to study drying variation patterns and predict process parameters;

[0024] 6) Color measurement method: The chromaticity of the dried pitaya was measured using a CR-400 handheld colorimeter. After the colorimeter was calibrated, the lens was aimed at the surface to record the L*, a*, and b* values. Three pieces were measured as a group, and the average of the measured results was finally taken;

[0025] 7) Total flavonoid content determination method: The flavonoid content was determined by aluminum nitrate colorimetry;

[0026] 8) Rehydration assay: by I Rh =(m Rh -m d ) / m d / t Rh ×100% to determine the rehydration value;

[0027] I Rh is the rehydration index / (g·(g·min) -1 );

[0028] m Rh is the mass of the sample after water bath / (g);

[0029] m d is the mass of the absolute dry sample / (g);

[0030] t Rh is the water bath duration / (min);

[0031] 9) Antioxidant activity assay: The DPPH scavenging rate was determined by the DPPH method, and the reducing capacity was determined by the potassium hexacyanoferrate method;

[0032] 10) The data were organized and plotted using Excel software and expressed as mean ± standard deviation. The model was fitted using Origin2021, and finally data statistical analysis was performed using SPSS20.

[0033] Preferably, in the step 1), fresh red pitaya is taken, washed and peeled, and processed into pitaya slices with slice thickness of 2, 4, 6, and 8 mm respectively by a slicer, a total of 24 slices, of which 15 slices are 4 mm, and the remaining 3 slices are uniformly pressed into pitaya discs by a circular mold with a diameter of 5.5 cm;

[0034] A single-factor test was conducted with slice thickness as the experimental factor: pitaya discs were divided into four thicknesses, three slices of each thickness were weighed, the weights were averaged, and then three identical slices were dried in each group, and the average was finally taken for data analysis;

[0035] A single-factor experiment was conducted with drying temperature as the experimental factor: three slices of 4 mm thick pitaya slices were weighed at each of the four temperatures, the mass was averaged, and the slices were dried in a constant temperature forced air drying oven at 50, 60, 70, and 80°C. The data were analyzed after taking the average.

[0036] Preferably, in step 3), the Fick and Arrhenius equations are used to combine the two equations, and the former is simplified to the linear equation ln MR = ln (8 / π 2 )-t·π 2 D eff / 4L 2 ;

[0037] D eff is the effective water diffusion coefficient / (m 2 / s);

[0038] L is 1 / 2 slice thickness / m;

[0039] t is the drying time / s.

[0040] Preferably, the food drying model in step 5) includes Page model, Logarithmic model and Newton model.

[0041] Preferably, in the step 7), 0.6 g of pitaya powder is weighed and transferred to a centrifuge tube, 15 mL of 80% ethanol is poured in, ultrasound is applied for 30 min, ultrasonic power is 400 W, extraction temperature is 40 ° C, and centrifugation is performed at 4000 r / min for 10 min. The supernatant is collected, 1 mL is drawn, and placed in a volumetric flask. 60% ethanol is added to 2 mL, 0.5 mL of 5% sodium nitrite solution is added and shaken, and the mixture is placed for 6 min. 0.5 mL of 10% aluminum nitrate solution is added. After 6 min, 4 mL of 4% sodium hydroxide solution is added, shaken, and 60% ethanol is added to make the volume 10 mL. After 15 min, the absorbance is measured at 510 nm, and the total flavonoid content is calculated according to the rutin standard curve.

[0042] Preferably, in the step 8), 3 pieces of dried pitaya are taken out according to different temperatures and thicknesses, the initial mass is weighed and averaged, and the pieces are put into a large beaker filled with distilled water, and then put into a 60° C. water bath, and taken out after 10 minutes. The surface moisture is removed with absorbent paper and kitchen paper, and the final mass is weighed and averaged, and the rehydration value is determined by the formula.

[0043] Preferably, in step 9), the supernatant is used, and the DPPH clearance rate is determined using the DPPH method.

[0044] Preferably, in the step 9), the DPPH clearance rate is measured by respectively preparing 24 samples into 0.5, 1, 2, and 4 mg / mL sample solutions and aspirating 2 mL into a test tube, adding 2 mL of DPPH solution that has been ultrasonically treated and placed away from light, shaking well, reacting at room temperature for half an hour away from light, and measuring the absorbance at 517 nm. The calculation formula is clearance rate (%) = [1-(A1-A2) / A0] × 100%; A1-absorbance of sample and DPPH;

[0045] Absorbance of A0-DPPH and 80% ethanol;

[0046] A2- Absorbance of sample and 80% ethanol.

[0047] Preferably, in the step 9), the reducing ability is measured by configuring 24 samples into 96 sample solutions according to four concentrations of 0.5, 1, 2, and 4 mg / mL, taking 1 mL of each sample solution in a test tube, adding 1 mL of phosphate buffer (0.2 mol / L, pH=6.6), 1 mL of 1% K3[Fe(CN)6] solution, mixing, placing in a 50°C water bath for 20 min, cooling, adding 1 mL of 10% trichloroacetic acid, mixing, taking 2 mL of supernatant, adding 1 mL of 0.1% FeCl3 and 2 mL of distilled water, placing at room temperature for 10 min, and measuring the absorbance at 700 nm.

[0048] Beneficial effects of the present invention:

[0049] The invention provides a method for predicting hot air drying characteristics of pitaya based on drying dynamics. The method utilizes an existing mathematical model for describing thin-layer hot air drying of fruits to perform optimal fitting on experimental data, regress the data, and convert the data into an equation represented by parameters, thereby providing a reliable basis for analysis and process optimization of a hot air drying process, so as to better study the influence of specific temperature and different thicknesses on the hot air drying characteristics and main qualities of red pitaya, thereby reasonably optimizing the drying process and accurately predicting some changes in drying conditions.

[0050] Instruction manual drawings:

[0051] Figure 1 Schematic diagram of the drying curve of pitaya slices at different drying temperatures of the present invention;

[0052] Figure 2 Schematic diagram of drying rate curve of pitaya slices at different drying temperatures of the present invention;

[0053] Figure 3 Another schematic diagram of drying curves of pitaya slices at different drying temperatures of the present invention;

[0054] Figure 4 This is another schematic diagram of drying rate curves of pitaya slices at different drying temperatures of the present invention;

[0055] Figure 5 It is a schematic diagram of the (-ln MR)-t relationship curve of red pitaya slices with different thicknesses of the present invention;

[0056] Figure 6 Schematic diagram of the relationship curve of (ln(-ln MR))-ln t of red pitaya slices of different thicknesses of the present invention;

[0057] Figure 7 Schematic diagram of the MR-t relationship curve of red pitaya slices with different thicknesses of the present invention;

[0058] Figure 8 Schematic diagram of the (-ln MR)-t relationship curve of red pitaya slices at different drying temperatures of the present invention;

[0059] Fig. 9 It is a schematic diagram of the relationship curve of (ln(-ln MR))-ln t of red pitaya slices at different drying temperatures of the present invention;

[0060] Fig.10 Schematic diagram of the MR-t relationship curve of red pitaya slices at different drying temperatures of the present invention;

[0061] Fig.11 Schematic diagram of experimental value and predicted value of MR of red pitaya slices under the same conditions of the present invention;

[0062] Fig.12 Schematic diagram of flavonoid extraction rate of red pitaya slices at different temperatures of the present invention;

[0063] Fig.13 Schematic diagram of DPPH clearance rate of red pitaya slices at different temperatures of the present invention;

[0064] Fig.14 Schematic diagram of the reducing power of red pitaya slices at different temperatures of the present invention;

[0065] Fig.15 Schematic diagram of flavonoid extraction rate of red pitaya slices with different thicknesses of the present invention;

[0066] Fig.16 Schematic diagram of DPPH clearance rate of red pitaya slices with different thicknesses of the present invention;

[0067] Fig.17 Schematic diagram of the reducing power of red pitaya slices with different thicknesses according to the present invention; DETAILED DESCRIPTION

[0068] The preferred embodiments of the present invention will be described in detail below.

[0069] The present invention provides a method for predicting hot air drying characteristics of pitaya based on drying dynamics, which comprises

[0070] Take fresh red pitaya, wash and peel, and use a slicer to process into 24 pitaya slices with slice thickness of 2, 4, 6, and 8 mm, of which 15 slices are 4 mm, and the rest are 3 slices each. Then use a circular mold with a diameter of 5.5 cm to uniformly press pitaya discs for experiments. Take slice thickness as the experimental factor, and conduct a single factor experiment: the pitaya discs are divided into 4 thicknesses, three slices of each thickness are weighed, and the mass is averaged. Then, they are marked in order and laid flat on a clean and dry circular steaming rack at a certain distance. Dry in a constant temperature blast drying oven at 70 ° C, take samples every 1h, and continue drying after measuring the mass until the dry basis moisture content of the material drops to 1% or below, and then stop. Dry three identical slices in each group, and finally take the average value for data analysis.

[0071] A single factor test was conducted with drying temperature as the test factor: three slices of 4 mm thick dragon fruit slices were weighed at each of the four temperatures, and the mass was averaged. Then, they were marked in order and laid flat on a clean and dry round steaming rack at a certain distance. They were dried in a constant temperature blast drying oven at 50, 60, 70, and 80°C. Samples were taken every 1 hour to measure the mass and recorded. The data were analyzed after taking the average.

[0072] Material dry basis moisture content (C t / (g / g)) was determined by direct drying method and t =(m r-m) / m calculation

[0073] Where: m r is the mass of the sample at a certain moment / g; m is the mass of the sample after drying / g;

[0074] The moisture ratio (MR) is calculated as follows: MR = (C t -C e ) / (C0-C e )calculate;

[0075] Where: C t , C0, C e They are the dry basis moisture content of the material at drying time t, initial time, and equilibrium time / (g / g);

[0076] The drying rate (DR / (g / (g·h))) is calculated according to the formula DR = (C1-C2) / (t2-t1);

[0077] Where: C1 and C2 are the moisture content of the material on a dry basis at drying time t1 and t2 / (g / g);

[0078] The effective moisture diffusion coefficient is obtained by measuring the drying curve and combining the Fick and Arrhenius equations. The former is simplified to the linear equation ln MR = ln(8 / π 2 )

[0079] -t·π 2 D eff / 4L 2 , draw a ln MR-t relationship curve, fit the linear equation, and get the slope k0, according to k0 = -π 2 ·D eff / 4L 2 calculate;

[0080] Where: D eff is the effective water diffusion coefficient / (m 2 / s); L is 1 / 2 slice thickness / m; t is drying time / s;

[0081] Material D eff与 The relationship between temperature can be expressed by the Arrhenius equation D eff =D0exp[-Ea / R / (T+273.15)]; take Ln on both sides to get n D eff =ln D0-E a / R / (T+273.15) = ln D eff It is linearly related to 1 / (T+273.15), and the slope is -E by fitting the linear equation. a / R, thus obtaining E a ;

[0082] Where: D0 is the material diffusion constant / (m 2 / s); E a is the drying activation energy / (kJ / mol); R is the gas molar constant (8.314 47J / (mol·K)); T is the drying temperature / ℃;

[0083] By establishing a drying model, the drying variation law is studied and the process parameters are predicted. The models describing food drying include Page model, Logarithmic model, Newton model, etc. Table 1 Five drying models and expressions

[0084]

[0085] In the formula: t is the drying time / h; k, n, a, b, c are all unknown coefficients in the expression.

[0086] The chromaticity of the dried pitaya was measured with a CR-400 handheld colorimeter. After the colorimeter was calibrated, the lens was aimed at the surface to record the L*, a*, and b* values. The a* value represents the red-green color from green (-) to red

[0087] (+); b* value indicates the yellow-blue degree from blue (-) to yellow (+); L* value indicates the brightness from black (L*=0) to white (L*=100). Three pieces are measured as a group, and the average value of the measured results is finally taken;

[0088] The flavonoid content was determined by aluminum nitrate colorimetry. Weigh 0.6g of pitaya powder and transfer it to a centrifuge tube, pour in 15mL of 80% ethanol, ultrasonicate for 30min, ultrasonic power 400W, extraction temperature 40℃, centrifuge at 4000r / min for 10min, collect the supernatant, aspirate 1mL, put it in a volumetric flask, add 60% ethanol to 2mL, add 0.5mL of 5% sodium nitrite solution and shake well, let it stand for 6min, add 0.5mL10% aluminum nitrate solution, after 6min, add 4mL of 4% sodium hydroxide solution. Shake well, add 60% ethanol to make up to 10mL, after 15min, measure the absorbance at 510nm, and calculate the total flavonoid content according to the rutin standard curve;

[0089] According to different temperatures and thicknesses, 3 pieces of dried pitaya were taken out, the initial mass was weighed and the average was taken, and the pieces were put into a large beaker filled with distilled water, and then put into a 60°C water bath. After 10 minutes, the pieces were taken out, the surface moisture was removed with absorbent paper and kitchen paper, and the final mass was weighed and the average was taken;

[0090] Use I Rh =(m Rh -m d ) / m d / tRh ×100% to determine the rehydration value;

[0091] Where: I Rh is the rehydration index / (g·(g·min) -1 );m Rh is the mass of the sample after water bath / (g); m d is the mass of the absolutely dry sample / (g); t Rh is the water bath duration / (min);

[0092] The supernatant obtained by the above method was used to determine the scavenging rate of DPPH using the DPPH method.

[19] , the reducing power was determined by potassium hexacyanoferrate method;

[0093] Determination of DPPH scavenging rate: Prepare 24 samples into 0.5, 1, 2, 4 mg / mL sample solutions (96 in total), draw 2 mL into a test tube, add 2 mL of DPPH solution that has been ultrasonically treated and kept away from light, and shake well. Keep away from light and react at room temperature for half an hour, and measure the absorbance at 517 nm;

[0094] Clearance rate (%) = [1-(A1-A2) / A0] × 100%

[0095] Where: A1-absorbance value of sample and DPPH; A0-absorbance value of DPPH and 80% ethanol; A2-absorbance value of sample and 80% ethanol;

[0096] Measure the reducing ability: Prepare 96 sample solutions of 24 samples at 4 concentrations of 0.5, 1, 2, and 4 mg / mL, take 1 mL of each solution in a test tube, add 1 mL of phosphate buffer (0.2 mol / L, pH=6.6). Add 1 mL of 1% K3[Fe(CN)6] solution, mix well, and place in a 50°C water bath for 20 min. Cool, add 1 mL of 10% trichloroacetic acid, and mix well. Take 2 mL of supernatant, add 1 mL of 0.1% FeCl3 and 2 mL of distilled water. Place at room temperature for 10 min, and measure the absorbance at 700 nm.

[0097] The data were sorted and plotted using Excel software, expressed as mean ± standard deviation, and the model was fitted using Origin 2021. Finally, SPSS20 was used for data statistical analysis;

[0098] Depend on Figure 1 It can be seen that as the drying progresses, the dry basis moisture content of the material decreases rapidly in the initial stage, and then basically proceeds in a decreasing stage. The moisture content does not decrease significantly in the later stage, and the drying time shortens with the decrease in thickness. The time required for the dry basis moisture content of 8mm pitaya to reach 1g / g is much longer than the time required for 2mm.

[0099] Depend on Figure 2It can also be seen that a peak appears in the early stage of drying, and the drying rate is the fastest. After that, as the dry basis moisture content of the material decreases, the drying rate slowly decreases, which means a slowly decelerating drying process, similar to the drying stage of lotus seeds, Pleurotus eryngii and bamboo shoots.

[0100] In the early stage of drying, the temperature of the water rises after absorbing heat energy, and steam is quickly generated inside the pitaya, forming a pressure gradient, which causes the water to be transported to the surface. The evaporation rate of the water on the surface of the pitaya is accelerated, and the drying rate increases rapidly. After the water content is reduced to a certain level, the diffusion of water inside the material becomes the main factor affecting drying. The thinner the pitaya slice, the shorter the migration distance of water from the inside to the outside, the faster the surface water flows, and the faster the drying rate. Under certain volume conditions, the thinner the thickness of the red pitaya slice, the larger the relative surface area in contact with the air, the faster the drying, and the shorter the time required;

[0101] from Figure 3 The dry basis moisture content decreases as the drying process progresses, and Figure 1 The trend in the curve is similar. The higher the temperature, the faster the rate of decrease of the dry basis moisture content of the material, and the shorter the time required to reach the same moisture content; Figure 4 It is known that in the early stage of drying, the drying rate quickly reaches the peak value and then begins to decrease. Except for the initial stage, the whole process can be regarded as a deceleration drying process. The higher the temperature rises, the more obvious the deceleration is. In the later stage of drying, due to the decrease in the water content of pitaya, the internal mass transfer and heat transfer resistance increase, so that the speed from the surface to the atmosphere and the speed from the inner layer to the surface are reduced, thereby reducing the drying rate. The drying rate at 80℃ is higher than that at other temperatures, mainly because during the drying process from 50 to 80℃, as the temperature increases, the surface moisture content of the red pitaya slices will decrease, forming a humidity gradient between the inner and outer layers, which is conducive to moisture transport; among the five models, the —lnMR-t of the Logarithmic, Newton, Handerson and Pabis models is a linear relationship, the ln(—lnMR)-lnt of the Page model is a linear relationship, and the MR-t of the Wang and Singh model is a linear relationship, so the above three curves at different thicknesses and temperatures are made to establish the optimal hot air drying model for red pitaya slices.

[0102] from Figure 5 , Figure 7 , Figure 8 , Fig.10 It can be seen that -ln MR and t, MR and t are both nonlinear relationships; Figure 6 , Fig. 9 It can be seen that (ln(-ln MR)) and ln t are linearly related, so the Wang and Singh model is not suitable for establishing the current model. The R 2=0.90747. The other three models are also not suitable. The R of the Logarithmic model is 2 =0.99862, R of Handerson and Pabis model 2 =0.99839, R of Newton model 2 =0.99839. In the model in Table 1, only (ln(-ln MR)) and ln t are linearly related, and the R of the Page model fitting equation is 2 =0.99995 is better than the other models mentioned above, indicating that the Page model is more suitable for drying conditions, so the Page model is selected as the drying kinetic model in this study. ln k and n are respectively calculated according to ln k=a+bT+cT 2 +dH+eH 2 ; n=f+gT+hT 2 +iH+jH 2 ;calculate

[0103] Where: H is the slice thickness / mm; T is the drying temperature / ℃; a, b, c, d, e, f, g, h, i, j are unknown coefficients. The linear expression is converted to ln(-ln MR)=a+bT+cT 2 +dH+eH 2 +(f+gT+hT 2 +iH+jH 2 )ln t;

[0104] n and k under different conditions can be obtained by drawing ln(-ln MR)-ln t curves.

[0105] Table 2 Page model coefficients of pitaya slices at different thicknesses and drying temperatures

[0106] Table 2Page equation coefficients at different slice thickness and

[0107] drying temperatures

[0108]

[0109] Substitute the coefficients in Table 2 into ln k=a+bT+cT 2 +dH+eH 2 ; n=f+gT+hT 2 +iH+jH 2; The undetermined coefficients are calculated as follows: a=7.861、b=-0.080、c=3.347、d=-0.318、e=-0.039、f=0.781、g=0.019、h=-1.879、i=0.048、j=-0.751. Substitute the undetermined coefficients into the formula ln(-ln MR)=a+bT+cT 2 +dH+eH 2 +(f+gT+hT 2 +iH+jH 2 )ln t, the model regression equation is ln(-ln MR) = 1.149 + 2.189 - 0.016T + 0.433T 2 -0.331H+0.605H 2 +(0.414-0.553+0.019T-1.879T 2 +0.037H-0.914H 2 )ln t;

[0110] After testing, the model r = 0.988, F = 81.226, P = 0.011 < 0.05. This method can well reflect the hot air drying process of red pitaya slices at different drying temperatures and slice thicknesses. The fitting results are shown in Table 2. R 2 The closer to 1, the 2 The closer it is to 0, the better the model fit is. Comprehensive analysis shows that the correlation coefficient of the Page model is close to 0.999, and χ 2 Close to 0, the model fit is very good;

[0111] from Fig.11 It can be seen that the test and the model prediction have a high fit, and the Pearson correlation coefficient is 0.998, which has a very significant correlation (P<0.05), indicating that the Page model can well reflect the change of moisture ratio during the drying process, indicating that the model is suitable for the hot air process under the experimental conditions;

[0112] Table 3 D of red pitaya slices during drying process eff and E a

[0113] Table 3Moisture effective diffusion coefficients and activation energy

[0114] during drying of Hylocereus undatus slices

[0115]

[0116] D eff and E a It can be used to measure the dehydration capacity of materials during the drying process. eff The higher the value, the stronger its dehydration ability. a The lower;

[0117] It can be seen from Table 2 that the D corresponding to different thicknesses and different temperatures eff Different, its value is 0.7036×10 －10 ~6.4268×10 －10 m 2 / s increases with the increase of drying temperature. When the thickness is the same, D eff Greater than 50℃. When the temperature is fixed, the thickness increases, D eff When dried at 70℃, the D eff About 9 times of 2mm

[0118] Four quality characteristics, namely, total flavonoid content, antioxidant activity, rehydration and color, were selected to analyze the Figures 11 to 13 It can be seen that when the thickness remains unchanged, the drying temperature has a significant effect on the total flavonoids and antioxidant capacity of the red pitaya slices. The total flavonoid content and antioxidant activity increase with increasing temperature, and the total flavonoid content of pitaya slices at 80°C is significantly higher than that at 50°C. It may be that the oxidase is not inactivated at low temperatures, and more flavonoids are decomposed. Within a certain temperature range, the red pitaya slices obtained by high-temperature drying have relatively high antioxidant properties, which is related to their total antioxidant content. Moreover, as the sample concentration continues to increase, the antioxidant activity also tends to increase, and the above law is always maintained. It can be seen from Table 4 that the drying temperature has no significant effect on the color of the pitaya slices, and there is no significant difference in the product color. It can be seen from Table 5 that when the slice thickness is constant, the drying temperature has a certain effect on the rehydration property, and the rehydration value of the pitaya slices shows a trend of first increasing and then decreasing with increasing temperature.

[0119] Table 4 Effect of drying temperature on the color of Hylocereus undatus slices

[0120]

[0121] Table 5 Effect of drying temperature on rehydration value of Hylocereus undatus tablets

[0122]

[0123] Note: The lowercase letters in the same column are all a, which means there is no significant difference.

[0124] from Figure 14-16 It can be seen that when the drying temperature remains unchanged, the total flavonoids and antioxidant capacity of the red pitaya slices are significantly affected by the thickness. There is no significant difference in the total flavonoid content and antioxidant activity of the red pitaya slices with a slice thickness difference of 2mm, but the difference is more obvious when the difference is 4-6mm, and the total flavonoid content and antioxidant activity of 8mm are better than those of 2mm. This is probably because during the high-temperature drying process with the same drying time, antioxidant substances undergo decomposition, polymerization and other reactions, but the thicker the red pitaya slices, the higher the total amount of antioxidant substances than the thinner red pitaya slices. Moreover, as the sample concentration continues to increase, the antioxidant activity also tends to increase, and the above rule is always maintained. As can be seen from Table 6, when dried at 70℃, the slice thickness has no significant effect on the color (L*, a*, b* values) of the red pitaya slices, and there is no significant difference in color under the four slice thicknesses. As can be seen from Table 7, under the condition of constant drying temperature, the slice thickness has a certain influence on the rehydration of pitaya slices. In the case of 2mm and 4mm, the rehydration value is higher than that of 6mm and 8mm pitaya slices, indicating that water is easier to penetrate when the slice thickness is low. This may be because under the conditions of a certain volume and the same time, the smaller the thickness of the red pitaya slice, the larger the relative surface area in contact with water, and the faster the penetration.

[0125] Table 6 Effect of slice thickness on the color of Hylocereusundatus slices

[0126]

[0127] Table 7 Effect of slice thickness on rehydration of Hylocereusundatus slices

[0128]

[0129] At different drying temperatures and thicknesses, the drying rate curves of red pitaya slices are basically consistent. Under the temperature and thickness conditions within the scope of this application, the moisture ratio of pitaya drops very quickly in the early stage and slowly in the later stage; the drying time decreases with increasing temperature and increases with increasing thickness. The Page model can well reflect the change in moisture ratio of pitaya slices at any time during hot air drying, indicating that the model is suitable for the hot air process under the experimental conditions. The fitting equation is ln(-ln MR) = 1.149 + 2.189 - 0.016T + 0.433T 2 -0.331H+0.605H 2 +(0.414-0.553+0.019T-1.879T 2 +0.037H-0.914H 2 )ln t. D of red pitaya slices eff At 0.7036×10 －10 ~6.4268×10 －10 m 2 / s; the total flavonoids content and antioxidant activity of pitaya slices were significantly affected by slice thickness and drying temperature and showed the same pattern with the increase of sample concentration: within the thickness and temperature range studied in this paper, the thicker the pitaya slices, the higher the total flavonoids content and antioxidant activity; the higher the drying temperature, the higher the total flavonoids content and antioxidant activity, but the effect on its color and rehydration property was not significant.

[0130] The present invention and its embodiments are described above, and such description is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. In short, if ordinary technicians in the field are inspired by it, without departing from the purpose of the invention, they can design a structure and embodiment similar to the technical solution without creativity, which should belong to the protection scope of the present invention.

Claims

1. A method for predicting hot air drying characteristics of pitaya based on drying dynamics, characterized in that: The following steps are involved: 1) Pre-treat the dragon fruit, conduct experiments, and analyze the data after taking the mean; 2) Determination and calculation of drying characteristic index: Material dry basis moisture content) is determined by direct drying method and calculated according to C t =(m r -m) / m; moisture (MR) ratio is calculated according to MR=(C t -C e ) / (C0-C e ) is calculated; the drying rate (DR / (g / (g·h))) is calculated according to DR=(C1-C2) / (t2-t1); m r is the mass of the sample at a certain moment / g; m is the mass of the sample after drying / g; C t , C0, C e They are the dry basis moisture content of the material at drying time t, initial time, and equilibrium time / (g / g); C1 and C2 are the moisture content of the material on dry basis at drying time t1 and t2 / (g / g); 3) Solution of effective moisture diffusion coefficient: The effective moisture diffusion coefficient can be obtained by measuring the drying curve, using the Fick and Arrhenius equations, combining the two equations, making a ln MR-t relationship curve, fitting the linear equation, and obtaining the slope k0 according to k0 = -π 2 ·D eff / 4L 2 calculate; D eff is the effective water diffusion coefficient / (m 2 / s); L is 1 / 2 slice thickness / m; t is drying time / s; 4) Solution of activation energy: Material D eff与 The relationship between temperature and eff =D0exp[-Ea / R / (T+273.15)], take Ln on both sides to get ln D eff =ln D0-E a / R / (T+273.15), we get ln D eff It is linearly related to 1 / (T+273.15), and the slope is obtained by fitting the linear equation - E a / R, thus obtaining E a D0 is the material diffusion constant / (m 2 / s); E a is the drying activation energy / (kJ / mol); R is the gas molar constant (8.314 47 J / (mol·K)); 5) Examples of commonly used drying models: By establishing drying models to study drying variation patterns and predict process parameters; 6) Color measurement method: The chromaticity of the dried pitaya was measured using a CR-400 handheld colorimeter. After the colorimeter was calibrated, the lens was aimed at the surface to record the L*, a*, and b* values. Three pieces were measured as a group, and the average of the measured results was finally taken; 7) Total flavonoid content determination method: The flavonoid content was determined by aluminum nitrate colorimetry; 8) Rehydration assay: by I Rh =(m Rh -m d ) / m d / t Rh ×100% to determine the rehydration value; I Rh is the rehydration index / (g·(g·min) -1 ); m Rh is the mass of the sample after water bath / (g); m d is the mass of the absolute dry sample / (g); t Rh is the water bath duration / (min); 9) Antioxidant activity assay: The DPPH scavenging rate was determined by the DPPH method, and the reducing capacity was determined by the potassium hexacyanoferrate method; 10) The data were organized and plotted using Excel software and expressed as mean ± standard deviation. The model was fitted using Origin 2021, and finally data statistical analysis was performed using SPSS20.

2. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In the step 1), fresh red pitaya is taken, washed and peeled, and sliced ​​into pitaya slices with a slice thickness of 2, 4, 6, and 8 mm, respectively, for a total of 24 slices, of which 15 slices are 4 mm, and the remaining 3 slices are uniformly pressed into pitaya discs using a circular mold with a diameter of 5.5 cm; A single-factor test was conducted with slice thickness as the experimental factor: pitaya discs were divided into four thicknesses, three slices of each thickness were weighed, the weights were averaged, and then three identical slices were dried in each group, and the average was finally taken for data analysis; A single-factor experiment was conducted with drying temperature as the experimental factor: three slices of 4 mm thick pitaya slices were weighed at each of the four temperatures, the mass was averaged, and the slices were dried in a constant temperature forced air drying oven at 50, 60, 70, and 80°C. The data were analyzed after taking the average.

3. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In step 3), the Fick and Arrhenius equations are used to combine the two equations, and the former is simplified to the linear equation ln MR = ln (8 / π 2 )-t·π 2 D eff / 4L 2； D eff is the effective water diffusion coefficient / (m 2 / s); L is 1 / 2 slice thickness / m; t is the drying time / s.

4. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: The food drying models in step 5) include Page model, Logarithmic model and Newton model.

5. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In the step 7), 0.6 g of pitaya powder is weighed and transferred to a centrifuge tube, 15 mL of 80% ethanol is poured in, ultrasonication is performed for 30 min, the ultrasonic power is 400 W, the extraction temperature is 40° C., and centrifugation is performed at 4000 r / min for 10 min. The supernatant is collected, 1 mL is drawn, and placed in a volumetric flask, 60% ethanol is added to 2 mL, 0.5 mL of 5% sodium nitrite solution is added and shaken, and the mixture is allowed to stand for 6 min. 0.5 mL of 10% aluminum nitrate solution is added. After 6 min, 4 mL of 4% sodium hydroxide solution is added, shaken, and 60% ethanol is added to make the volume to 10 mL. After 15 min, the absorbance is measured at 510 nm, and the total flavonoid content is calculated according to the rutin standard curve.

6. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In the step 8), 3 pieces of dried pitaya are taken out according to different temperatures and thicknesses, the initial mass is weighed and averaged, and the pieces are put into a large beaker filled with distilled water, and then put into a 60° C. water bath. After 10 minutes, the pieces are taken out, and the surface moisture is removed with absorbent paper and kitchen paper. The final mass is weighed and averaged, and the rehydration value is determined by the formula.

7. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 5, characterized in that: In the step 9), the supernatant is used, and the DPPH clearance rate is measured using the DPPH method.

8. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In the step 9), the DPPH clearance rate is measured by preparing 24 samples into sample solutions of 0.5, 1, 2, and 4 mg / mL, respectively, and 2 mL of the sample solution is drawn into a test tube, and 2 mL of the DPPH solution that has been ultrasonically treated and kept away from light is added, shaken well, and reacted at room temperature for half an hour away from light, and the absorbance is measured at 517 nm, and the calculation formula is clearance rate (%) = [1-(A1-A2) / A0]×100%; A1- absorbance value of sample and DPPH; Absorbance of A0-DPPH and 80% ethanol; A2- Absorbance of sample and 80% ethanol.

9. The method for predicting hot air drying characteristics of pitaya based on drying dynamics according to claim 1, characterized in that: In the step 9), the reducing ability was measured by preparing 96 sample solutions of 24 samples at four concentrations of 0.5, 1, 2, and 4 mg / mL, and 1 mL was taken from each sample solution in a test tube. 1 mL of the sample solution was added to a phosphate buffer solution (0.2 mol / L, pH=6.6) and 1 mL of a 1% K3[Fe(CN)6] solution, and the solution was mixed. The solution was placed in a 50°C water bath for 20 min, cooled, and 1 mL of 10% trichloroacetic acid was added, and the solution was mixed. 2 mL of the supernatant solution was taken, and 1 mL of 0.1% FeCl3 and 2 mL of distilled water were added. The solution was placed at room temperature for 10 min, and the absorbance was measured at 700 nm.

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