Combined drying method of pumpkin pulp and application

Abstract

The invention discloses a combined drying method and application of pumpkin pulp. The combined drying method comprises vacuum freeze drying and hot air drying. By means of the combined drying method, the drying efficiency is improved, and the defect that single vacuum freeze drying time is too long is overcome. In addition, by means of the combined drying method, the yellow blue degree, the beta-carotene content and the total sugar content of the dried pumpkin pulp can be improved. The pumpkin powder prepared by the combined drying method disclosed by the invention is well reserved in structure, high in active ingredient content, strong in oxidation resistance, golden in color and luster, fragrant, and improved in flowability and filling property.

Description

Technical Field

[0001] The invention relates to the technical field of plant drying, and in particular to a combined drying method for pumpkin pulp and application thereof. Background Art

[0002] Fresh pumpkin is rich in various nutritionally active ingredients, such as total phenolics, total flavonoids, β-carotene, and reducing sugars. It possesses excellent antioxidant, blood sugar, and lipid-lowering properties, and holds great research value and development potential. However, fresh pumpkin has a high moisture content and is susceptible to microbial decay and deterioration even under refrigerated conditions. Therefore, it is necessary to dry it to produce pumpkin powder to improve its storage and transportability, thereby increasing its added value.

[0003] Currently, common drying methods include hot air drying, microwave drying, vacuum freeze drying, and far-infrared drying. Vacuum freeze drying has the advantages of inhibiting most microbial activity and enzymatic reactions while preserving the active ingredients and the material's good color and appearance. However, compared to traditional drying methods, vacuum freeze drying takes significantly longer, several times or even dozens of times longer than other drying methods. Furthermore, vacuum freeze drying consumes a lot of energy, the equipment is expensive, and the drying efficiency is low. These production costs have limited the industrial application of vacuum freeze drying in fruit and vegetable processing.

[0004] To overcome the shortcomings of vacuum freeze-drying and improve drying efficiency, common drying methods such as hot air drying, microwave drying, and infrared drying can be combined with vacuum freeze-drying. However, different drying methods have different effects on pumpkin quality, and the effects of combined drying on pumpkin quality are unknown. Studies have shown that hot air drying significantly degrades the yellowness of pumpkin slices, attributed to the degradation and isomerization of carotenoids (Study on Color Change and Maillard Reaction Mechanism of Pumpkin Slices During Hot Air Drying, Publication Date: June 1, 2021). Furthermore, other reports indicate that both far-infrared drying and hot air drying are detrimental to the retention of β-carotene, resulting in a significant decrease in carotenoid content (PMID: 35985240). Therefore, determining the optimal process conditions for combined drying of pumpkin slices using different drying methods is crucial for improving the quality of pumpkin powder. Summary of the Invention

[0005] In order to overcome the above-mentioned defects and deficiencies in the prior art, the present invention provides a combined drying method for pumpkin pulp and its application.

[0006] The first object of the present invention is to provide a combined drying method for pumpkin pulp.

[0007] The second object of the present invention is the application of the above-mentioned combined drying method in the preparation of pumpkin products.

[0008] The present invention claims the following: The invention discloses a combined drying method for pumpkin pulp, which comprises the following steps: firstly subjecting the pumpkin pulp to a vacuum freeze drying treatment for 4.5 to 5.5 hours, and then subjecting the pumpkin pulp to a hot air drying treatment at 40 to 60°C.

[0009] Because hot air drying severely degrades the yellowness of pumpkin slices and significantly reduces the carotenoid content, and the yellowness and β-carotene content decrease significantly with increasing hot air drying temperature (Study on Color Change and Maillard Reaction Mechanism of Pumpkin Slices During Hot Air Drying, Publication Date: June 1, 2021), it is necessary to strictly control the hot air drying conditions during the combined drying process to ensure the quality of the pumpkin after combined drying. Therefore, preferably, the hot air drying temperature is 55°C. Under this condition, the yellow-blueness of the dried pumpkin powder is well maintained, which is 11.90% higher than that of single vacuum freeze drying. The β-carotene content and total sugar content are increased by 24.18% and 11.46% respectively compared to single vacuum freeze drying. At the same time, the total drying time is reduced by 23.53% compared to single vacuum freeze drying.

[0010] More preferably, the pumpkin pulp is first subjected to vacuum freeze-drying for 4.5 to 5.5 hours, and then the pumpkin pulp is subjected to hot air drying and far-infrared drying at 40 to 60°C, wherein the power of the far-infrared lamp in the far-infrared drying is 350 to 450 W. Under this condition, the total drying time is reduced by 24.71% to 50% compared with single vacuum freeze-drying.

[0011] Further preferably, the temperature of the hot air drying and far infrared drying treatment is 45° C. The present invention has found that by adding far infrared drying to vacuum freeze-hot air drying, the total sugar content of the dried pumpkin powder is increased by 6.72% compared with vacuum freeze-hot air drying, 28.98% compared with single vacuum freeze drying, and 16.46% compared with fresh pumpkin samples.

[0012] Further preferably, the power of the far-infrared lamp in the far-infrared drying is 400 W.

[0013] Preferably, the distance between the far-infrared lamp and the pumpkin pulp is 25 to 30 cm.

[0014] More preferably, the distance between the far-infrared lamp and the pumpkin pulp is 27 cm.

[0015] Preferably, the vacuum freeze-drying conditions are: cold trap temperature -45 to -55°C, and vacuum degree maintained at 15 to 20 Pa.

[0016] More preferably, the vacuum freeze-drying conditions are: cold trap temperature -50°C, and vacuum degree maintained at 17 Pa.

[0017] Preferably, the pumpkin pulp is pretreated before being subjected to vacuum freeze-drying treatment, and the pretreatment method is: blanching the pumpkin pulp, treating it with a color-protecting liquid after blanching, and pre-freezing the treated pumpkin pulp.

[0018] As an practicable manner, the pumpkin pulp is cut into pumpkin slices of 5 to 6 mm.

[0019] More preferably, the blanching temperature is 90-100° C., and the blanching time is 8-12 s.

[0020] More preferably, the blanching temperature is 95° C., and the blanching time is 10 s.

[0021] More preferably, the color protection solution comprises 0.1-0.2% by mass of ascorbic acid, 0.1-0.2% by mass of L-cysteine, and 0.5-1.5% by mass of citric acid.

[0022] Further preferably, the color protection solution contains 0.15% by mass of ascorbic acid, 0.15% by mass of L-cysteine and 1.0% by mass of citric acid.

[0023] More preferably, the pre-freezing temperature is -75 to -85°C, and the pre-freezing time is 22 to 26 hours.

[0024] More preferably, the pre-freezing temperature is -80°C, and the pre-freezing time is 24 hours.

[0025] Application of any of the above combined drying methods in the preparation of pumpkin products.

[0026] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a combined drying method for pumpkin pulp and its application. The combined drying method comprises vacuum freeze-drying and hot air drying. The combined drying method can improve the yellow-blueness, β-carotene content, and total sugar content of the dried pumpkin pulp. The pumpkin powder prepared using the combined drying method retains a well-preserved structure, has a high content of active ingredients, exhibits strong antioxidant capacity, has a golden color, a fragrant aroma, and exhibits improved flowability and filling properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Drying characteristic curve of single vacuum freeze-dried pumpkin slices; A: drying curve, B: drying rate curve.

[0028] Figure 2The following are the appearances of the pumpkin slices and pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1.

[0029] Figure 3 The effect of hot air drying temperature on the volatile components of pumpkin powder; A and C are radar charts, and B and D are PCA charts.

[0030] Figure 4 The following are scanning electron microscope images (6000 times) of the pumpkin powder prepared in Example 1, Example 6 and Comparative Example 1; A: pumpkin powder prepared in Comparative Example 1, B: pumpkin powder prepared in Example 1, C: pumpkin powder prepared in Example 6. DETAILED DESCRIPTION

[0031] The present invention is further described below with reference to specific examples, which, however, are not intended to limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0032] Unless otherwise specified, the reagents and materials used in the following examples were commercially available.

[0033] Example 1 A vacuum freezing-hot air combined drying method for pumpkin pulp This embodiment provides a method for combined drying of pumpkin, with a total drying time of 13 hours, comprising the following steps: S1. Select a fresh pumpkin of suitable size, regular shape, and free of rot or spoilage. Wash it, peel it, remove the seeds, and cut it into 5-6 mm slices using a slicer. S2. Pretreatment: Blanch the pumpkin slices in 95°C water for 10 seconds, then soak them in a color-protecting solution for 10 minutes. Remove the slices and blot dry with paper towels. The color protection solution is purified water containing 0.15% ascorbic acid, 0.15% L-cysteine and 1.0% citric acid; S3. Prefreeze: Prefreeze the pretreated pumpkin slices at -80°C for 24 h. S4. Vacuum freeze-drying: The pre-frozen pumpkin slices were vacuum freeze-dried for 5 h. The vacuum freeze-drying conditions were: a cold trap temperature of -50°C and a vacuum degree of approximately 17 Pa. S5. Hot air drying: After vacuum freeze drying, place the pumpkin slices in a forced air drying oven and dry them with hot air at 55°C. Dry the pumpkin slices until the moisture content drops to 5-6%. Grind the pumpkin slices into powder and store the powder in a refrigerator at 4°C.

[0034] Example 2 A vacuum freezing-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 1, with a total drying time of 20 h, except that the hot air drying temperature is set to 40°C.

[0035] Example 3 A vacuum freezing-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 1, with a total drying time of 18.3 h, except that the hot air drying temperature is set to 45°C.

[0036] Example 4 A vacuum freezing-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 1, with a total drying time of 16 h, except that the hot air drying temperature is set to 50°C.

[0037] Example 5 A vacuum freezing-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 1, with a total drying time of 11.8 h, except that the hot air drying temperature is set to 60°C.

[0038] Example 6 A vacuum freezing-far infrared-hot air combined drying method for pumpkin pulp This embodiment provides a method for combined drying of pumpkin, with a total drying time of 11.8 h, comprising the following steps: S1. Select a fresh pumpkin of suitable size, regular shape, and free of rot or spoilage. Wash it, peel it, remove the seeds, and cut it into 5-6 mm slices using a slicer. S2. Pretreatment: Blanch the pumpkin slices in 95°C water for 10 seconds, then soak them in a color-protecting solution for 10 minutes. Remove the slices and blot dry with paper towels. The color protection solution is purified water containing 0.15% ascorbic acid, 0.15% L-cysteine and 1.0% citric acid; S3. Prefreeze: Prefreeze the pretreated pumpkin slices at -80°C for 24 h. S4. Vacuum freeze-drying: The pre-frozen pumpkin slices were vacuum freeze-dried for 5 h. The vacuum freeze-drying conditions were: a cold trap temperature of -50°C and a vacuum degree of approximately 17 Pa. S5. Far-infrared and hot air drying: After vacuum freeze-drying, the pumpkin slices were placed in a forced air drying oven with two far-infrared quartz tubes (40 cm long, 400 W) positioned at the top of the oven, 27 cm from the slices. The pumpkin slices were simultaneously dried with far-infrared and hot air at 45°C. Drying was terminated when the moisture content of the pumpkin slices dropped to 5-6%. The pumpkin powder was then pulverized and stored in a refrigerator at 4°C.

[0039] Example 7 A vacuum freezing-far infrared-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 6, with a total drying time of 12.8 h, except that the hot air drying temperature is set to 40°C.

[0040] Example 8 A vacuum freezing-far infrared-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 6, with a total drying time of 10.3 h, except that the hot air drying temperature is set to 50°C.

[0041] Example 9 A vacuum freezing-far infrared-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 6, with a total drying time of 9 h, except that the hot air drying temperature is set to 55°C.

[0042] Example 10 A vacuum freezing-far infrared-hot air combined drying method for pumpkin pulp This embodiment is carried out in accordance with the embodiment 6, with a total drying time of 8.5 h, except that the hot air drying temperature is set to 60°C.

[0043] Comparative Example 1 A method for single vacuum freeze-drying pumpkin This comparative example provides a method for single vacuum freeze-drying pumpkin, with a total drying time of 17 h, comprising the following steps: S1. Select a fresh pumpkin of suitable size, regular shape, and free of rot or spoilage. Wash it, peel it, remove the seeds, and cut it into 5-6 mm slices using a slicer. S2. Pretreatment: Blanch the pumpkin slices in 95°C water for 10 seconds, then soak them in a color-protecting solution for 10 minutes. Remove the slices and blot dry with paper towels. The color protection solution is purified water containing 0.15% ascorbic acid, 0.15% L-cysteine and 1.0% citric acid; S3. Prefreeze: Prefreeze the pretreated pumpkin slices at -80°C for 24 h. S4. Vacuum freeze-drying: The pre-frozen pumpkin slices were vacuum-freeze-dried under the following conditions: the cold trap temperature was -50°C and the vacuum degree was maintained at approximately 17 Pa. Drying was terminated when the moisture content of the pumpkin slices dropped to 5-6%. The pumpkin powder was then ground into powder and collected, and stored in a refrigerator at 4°C.

[0044] Example 11 Effect of vacuum freeze-drying time on the moisture ratio of pumpkin 1. Experimental Methods According to Comparative Example 1, single vacuum freeze drying was performed. During the vacuum freeze drying process, the moisture content of the pumpkin slices was measured every 30 minutes. The moisture content was measured using the direct drying method of GB 5009.3-2016, and the results were calculated on a dry basis.

[0045] The moisture ratio and drying rate are calculated according to the following formulas, and the drying curve and drying rate curve are drawn according to the moisture ratio and drying rate:

[0046] Where M t represents the dry basis moisture content of the pumpkin slices at time t during the drying process (g / g), and M0 represents the dry basis moisture content of the pumpkin slices at the initial moment (g / g).

[0047]

[0048] Where M t1 and M t2 represent the dry basis moisture content (g / g) of pumpkin slices at t1 and t2, respectively. t1 and t2 are the corresponding drying times, min.

[0049] 2. Experimental Results Drying curve Figure 1 As shown in A, the drying rate curve is as follows Figure 1 As shown in Figure B, the moisture ratio gradually decreases as drying time increases. The drying rate initially increases, then decreases before leveling off. When the drying time reaches 300 minutes (5 hours), the moisture ratio of the pumpkin slices decreases slowly, indicating that with continued drying time, the drying efficiency decreases. Therefore, this time point (5 hours) was selected as the moisture conversion point for the pumpkin slices. This means setting the vacuum freeze-drying time to 5 hours before proceeding with subsequent drying to ensure rapid evaporation of the remaining moisture, shorten the drying time, and increase the drying efficiency.

[0050] Example 12 Effect of hot air drying temperature on the particle size and color of pumpkin powder 1. Experimental Methods The particle size of the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was measured using a Mastersizer 3000 laser particle size analyzer. The color difference of the pumpkin powder was measured using a San Enchi colorimeter, and the L value, a value, and b value were recorded. Each sample was measured three times and the average value was taken. The L value, a value, and b value represent the brightness, red-greenness, and yellow-blueness of the pumpkin powder, respectively.

[0051] 2. Experimental Results The particle size and color comparison results of the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 are shown in Table 1 and Table 2. The appearance of pumpkin slices and pumpkin powder is as follows: Figure 2As shown. Vacuum freezing - hot air - 55℃, vacuum freezing - far infrared - hot air - 55℃ ~ vacuum freezing - far infrared - hot air - 40℃ pumpkin powder D 50 Compared with the D of single vacuum freeze-dried pumpkin powder 50 Small, including vacuum freezing-far infrared-hot air-45℃ and vacuum freezing-far infrared-hot air-40℃ 50 Minimum.

[0052] The L value of the pumpkin powder dried with vacuum freeze-drying alone was the highest (86.44±0.18 and 82.20±6.17). The a values of the pumpkin powder dried with vacuum freeze-hot air were higher than those dried with vacuum freeze-drying alone, while the opposite was true for the pumpkin powder dried with vacuum freeze-far infrared and hot air. The b values of the pumpkin powder dried with vacuum freeze-hot air and far infrared were higher than those dried with vacuum freeze-drying alone, but the b values of the pumpkin powder dried with vacuum freeze-far infrared and hot air were closer to those of the pumpkin powder dried with vacuum freeze-drying alone.

[0053] The above shows that compared with single vacuum freeze-drying, the pumpkin powder particles become smaller after combined drying treatment, the powder has high brightness and golden color. Among them, the color of vacuum freezing-far infrared-hot air pumpkin powder is closest to that of single vacuum freeze-drying pumpkin powder, and can well retain the original color of pumpkin powder.

[0054] Table 1 Comparison of average particle size and color of pumpkin powder after single vacuum freeze drying and vacuum freeze-hot air combined drying

[0055] Table 2 Comparison of average particle size and color of pumpkin powder after single vacuum freeze drying and vacuum freeze-far infrared-hot air combined drying

[0056] Example 13 Effect of hot air drying temperature on the physical properties of pumpkin powder 1. Experimental Methods 1. Determination of water holding capacity Weigh the pumpkin powder prepared in Examples 1-10 and Comparative Example 1 into a centrifuge tube. Add 10 mL of distilled water and shake until evenly distributed. Place the centrifuge tube in a 45°C water bath and let it sit for 15 minutes. Then, centrifuge at 10,000 rpm for 10 minutes. Pour out the distilled water, turn the centrifuge tube upside down, and let it sit for 10 minutes. After ensuring that no water remains on the tube wall, weigh the tube and calculate the mass using the following formula:

[0057] Where m0 is the mass of the empty centrifuge tube, g; m1 is the mass of the pumpkin powder, g; and m2 is the total mass of the centrifuge tube after being inverted and allowed to stand, g.

[0058] 2. Determination of oil holding capacity The method of "Determination of water holding capacity" was followed, except that distilled water was replaced by peanut oil.

[0059] 3. Determination of Solubility Weigh the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1, add 10 mL of distilled water to a centrifuge tube, shake evenly, place the centrifuge tube in a 45°C water bath, let it stand for 15 minutes, and then centrifuge at 10,000 rpm for 10 minutes. Dry the empty weighing bottle at 105°C to constant weight. Open the centrifuge tube and pour the supernatant into the empty weighing bottle that has been dried to constant weight. Place the centrifuge tube in a 105°C oven again and dry to constant weight. The calculation formula is:

[0060] Where m0 is the mass of the empty weighing bottle dried to constant weight, g; m1 is the mass of the pumpkin powder, g; m2 is the total mass of the weighing bottle dried to constant weight, g.

[0061] 4. Determination of the Angle of Repose Fix a glass funnel on an iron stand with the tail end of the funnel 2 cm above the horizontal plane. Slowly pour the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 into the funnel. The powder falls vertically through the funnel onto the horizontal plane to form a cone. When the highest point of the cone touches the lowest end of the funnel, record the height and diameter of the cone at this moment. The calculation formula is:

[0062] Where h is the height of the cone, cm; r is the radius of the cone, cm.

[0063] 5. Determination of Slip Angle Weigh 0.5 g of the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 respectively on a glass plate, spread it gently and evenly, and slowly adjust the glass plate to tilt it. When the pumpkin powder is about to move, record the height of the glass plate from the horizontal plane and the length of the glass plate at this moment. The calculation formula is:

[0064] Where H is the vertical height of the glass plate from the horizontal plane, cm; L is the length of the glass plate, cm.

[0065] 6. Determination of bulk density The empty measuring cylinder was dried to a constant weight, and then the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was respectively poured into the empty measuring cylinder. The excess pumpkin powder was gently scraped off to make the volume flush with the mouth of the measuring cylinder. The volume was then weighed and the volume of the measuring cylinder was recorded. The calculation formula is:

[0066] Where m1 is the mass of the empty measuring cylinder, g; m2 is the mass of the pumpkin powder and the measuring cylinder, g; and V is the volume of the measuring cylinder, mL.

[0067] 2. Experimental Results The results are shown in Tables 3 and 4. The water holding capacity, oil holding capacity, solubility, angle of repose and slip angle of single vacuum freeze-dried pumpkin powder are the largest, and the bulk density is the smallest, indicating that single vacuum freeze-dried pumpkin powder has good adsorption and solubility, but poor fluidity and filling properties; in vacuum freezing-hot air and vacuum freezing-far infrared-hot air combined drying, the angle of repose and slip angle of pumpkin powder are smaller than those of single vacuum freeze-dried pumpkin powder, and the bulk density is greater than that of single vacuum freeze-dried pumpkin powder.

[0068] The above results show that after the combined drying treatment, the good adsorption and solubility of pumpkin powder can be retained, while the shortcomings of poor fluidity and filling properties of single vacuum freeze-dried pumpkin powder are improved.

[0069] Table 3 Physical properties of pumpkin powder after single vacuum freeze drying and vacuum freeze-hot air combined drying

[0070] Table 4 Physical properties of pumpkin powder after vacuum freeze drying alone and vacuum freeze-far infrared combined drying

[0071] Example 14 Effect of hot air drying temperature on the volatile components of pumpkin powder 1. Experimental Methods Weigh 1 g of each pumpkin powder prepared in Examples 1-10 and Comparative Example 1 into a sealed headspace vial. Heat the vial in a 60°C waterbath for 15 minutes. After the waterbath, let it rest for 15 minutes to allow the volatile components in the vial to reach equilibrium. Volatile components were measured using an electronic nose under the following test conditions: sampling interval of 1 second, purge time of 120 seconds, preparation time of 5 seconds, test time of 120 seconds, and an inlet flow rate of 300 mL / min. Radar plots and PCA plots were constructed using the sensor values corresponding to each volatile component.

[0072] 2. Experimental Results Radar chart Figure 3As shown in Figures A and C, the content of alkane compounds in the single vacuum freeze-dried pumpkin powder is the highest. After the combined drying treatment, the content of sulfide, organic sulfide and nitrogen oxide in the pumpkin powder increases. Among them, the vacuum freezing-far infrared-hot air-45℃ pumpkin powder has the highest overall content of various volatile components.

[0073] PCA diagram Figure 3 As shown in Figures B and D, the volatile components of pumpkin powder after combined drying treatment did not overlap with those of single vacuum freeze drying, indicating that the types of volatile components had changed greatly.

[0074] The above results show that compared with single vacuum freeze-drying, the content and type of volatile components of pumpkin powder have changed after combined drying treatment, which has a certain effect on enhancing the flavor of pumpkin powder. Among them, vacuum freezing-far infrared-hot air-45℃ has the best flavor-enhancing effect on pumpkin powder.

[0075] Example 15 Effect of hot air drying temperature on the active ingredients of pumpkin powder 1. Experimental Methods 1. Determination of Total Phenols The total phenol content (mg / g DW) in the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was calculated by using the Folin-phenol method and compared with the standard curve.

[0076] 2. Determination of Total Flavonoids The NaNO2-Al(NO3)3-NaOH colorimetric method was used for determination, and the total flavonoid content (mg / g DW) in the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was converted by reference to the standard curve.

[0077] 3. Determination of β-carotene 0.2 g of the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was respectively weighed, 20 mL of petroleum ether was added, and β-carotene was extracted by ultrasonication at 300 W for 10 min. Subsequently, the mixture was centrifuged at 10,000 r / min for 10 min. The supernatant was taken and diluted to volume with petroleum ether in a 100 mL brown volumetric flask. The absorbance was then measured at 450 nm, and the β-carotene content (mg / g DW) was converted against the standard curve.

[0078] 4. Determination of Reducing Sugar and Total Sugar The DNS method was used for determination, and the reducing sugar and total sugar contents (mg / g DW) in the pumpkin powders prepared in Examples 1 to 10 and Comparative Example 1 were converted by reference to the standard curve.

[0079] 5. Protein Determination The protein content (mg / g DW) in the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was calculated by using the Coomassie Brilliant Blue method in SN / T3926-2014 and referring to the standard curve.

[0080] 6. Determination of Vitamin C The 2,6-dichloroindophenol titration method in GB 5009.86-2016 was used for determination.

[0081] 2. Experimental Results The results are shown in Tables 5 and 6. In the vacuum freeze-hot air combined drying, the active ingredient content of the vacuum freeze-hot air-55℃ pumpkin powder was the highest overall, among which the contents of total flavonoids (5.59±0.02 mg / g DW), β-carotene (2.26±0.01 mg / g DW) and total sugar (493.48±0.65 mg / g DW) were higher than those of the corresponding active ingredients in the single vacuum freeze-dried pumpkin powder.

[0082] In the vacuum freezing-far infrared-hot air combined drying, the active ingredient content of the vacuum freezing-far infrared-hot air-45℃ pumpkin powder was the highest overall, among which the contents of total phenols (12.02±0.08 mg / g DW), total flavonoids (7.11±0.08 mg / g DW), and total sugars (526.62±1.47 mg / g DW) were higher than those of the corresponding active ingredients in the single vacuum freeze-dried pumpkin powder and the fresh sample.

[0083] Compared with the fresh sample, the retention rate of various active ingredients of the vacuum frozen-hot air-55℃ pumpkin powder is above 64%, while the total phenols, total flavonoids, total sugars and other contents of the vacuum frozen-far infrared-hot air-45℃ pumpkin powder are much higher than the fresh sample (over 100%). Therefore, in a comprehensive comparison, the vacuum frozen-far infrared-hot air-45℃ pumpkin powder has the highest activity, which increases the nutritional value of the pumpkin powder as a whole.

[0084] Table 5 Comparison of active ingredient content in pumpkin powder after vacuum freeze drying alone and vacuum freeze-hot air drying (mg / gDW)

[0085] Table 6 Comparison of active ingredient content in pumpkin powder after single vacuum freeze drying and vacuum freezing-far infrared-hot air drying (mg / g DW)

[0086] Example 16 Effect of hot air drying temperature on the antioxidant capacity of pumpkin powder 1. Experimental Methods 1 g of the pumpkin powder prepared in Examples 1 to 10 and Comparative Example 1 was weighed respectively, and 100 mL of anhydrous ethanol was added. The mixture was ultrasonically extracted at 500 W for 15 min, and the extract was filtered with gauze. The extract was stored in a refrigerator at 4°C away from light until testing.

[0087] 1. Determination of DPPH free radical scavenging rate Take 2 mL of the above extract and 2 mL of 0.2 mmol / L DPPH ethanol solution in a centrifuge tube and mix them evenly. Let them stand in the dark at room temperature for 30 min. Then measure the absorbance at 517 nm. The calculation formula is:

[0088] Where A i A is the absorbance value of 2 mL extract + 2 mL DPPH ethanol solution; j A is the absorbance value of 2 mL extract + 2 mL anhydrous ethanol; C It is the absorbance value of 2 mL of anhydrous ethanol + 2 mL of DPPH ethanol solution.

[0089] 2. Determination of ABTS Free Radical Scavenging Rate Take 0.5 mL of the above extract and 2.5 mL of ABTS working solution and mix them evenly in a centrifuge tube. Incubate the mixture in the dark at room temperature for 6 min and then measure the absorbance at 734 nm. The calculation formula is:

[0090] Where B i B is the absorbance value of 0.5 mL extract solution + 2.5 mL ABTS working solution. j B is the absorbance value of 0.5 mL extract + 2.5 mL anhydrous ethanol; c It is the absorbance value of 0.5 mL anhydrous ethanol + 2.5 mL ABTS working solution.

[0091] 3. Determination of total reducing capacity Take 1 mL of the above extract and add 3 mL of FRAP solution into a centrifuge tube. After shaking, stand it in the dark at room temperature for 10 min. Measure the absorbance at 593 nm and compare it with the FeSO4 standard curve to convert the reducing capacity, where 1 FRAP unit = 1 mmol / L FeSO4.

[0092] 2. Experimental Results The results are shown in Tables 7 and 8. In the vacuum freeze-hot air combined drying, the DPPH radical scavenging rate (83.99±0.16%), ABTS radical scavenging rate (71.69±1.27%) and total reducing capacity (33.28±0.05 μmol / L) of vacuum freeze-hot air-55℃ were the highest, and were close to the antioxidant capacity of single vacuum freeze-dried pumpkin powder.

[0093] In the vacuum freezing-far infrared-hot air combined drying, the DPPH and ABTS free radical scavenging rate at vacuum freezing-far infrared-hot air-45℃ was the highest (more than 98%), and the total reducing capacity was the strongest (93.47±0.11 μmol / L), far exceeding the antioxidant capacity of single vacuum freeze-dried pumpkin powder.

[0094] The antioxidant capacity is related to the content of active ingredients. Combined with the results of Example 15, it can be seen that the retention rate and content of total phenols, total flavonoids and total sugars in vacuum freezing-far infrared-hot air-45°C are very high, thus showing very strong antioxidant capacity.

[0095] Table 7 Comparison of antioxidant capacity of pumpkin powder under vacuum freeze drying and vacuum freeze-hot air drying

[0096] Table 8 Comparison of antioxidant capacity of pumpkin powder under vacuum freeze drying alone and vacuum freezing-far infrared-hot air drying

[0097] Example 17 Scanning electron microscopy characterization 1. Experimental Methods The pumpkin powder prepared in Example 1, Example 6, and Comparative Example 1 was evenly applied to a sample stage with conductive adhesive. Excess sample was purged, gold-sprayed, and observed using a scanning electron microscope. Featured areas were photographed. The test voltage was 10 kV and the magnification was 6000x.

[0098] 2. Experimental Results The results are as follows Figure 4 As shown in A to C, single vacuum freeze-dried pumpkin powder ( Figure 4 The surface of A) is smooth and has a complete pore structure inside. Vacuum freezing-hot air-55℃ ( Figure 4 B) and vacuum freezing-far infrared-hot air-45℃ ( Figure 4 C) The pumpkin powder still has a porous structure inside, but due to heating, it has slightly shrunk overall.

[0099] The above results show that the combined drying treatment can still better preserve the original structure of pumpkin powder, among which the vacuum freezing-far infrared-hot air-45℃ pumpkin powder has the largest pores.

[0100] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A combined drying method for pumpkin pulp, characterized in that: The pumpkin pulp is first subjected to vacuum freeze drying for 4.5 to 5.5 hours, and then subjected to hot air drying at 40 to 60°C.

2. The combined drying method according to claim 1, wherein The pumpkin pulp is first subjected to vacuum freeze drying for 4.5 to 5.5 hours, and then subjected to hot air drying and far-infrared drying at 40 to 60° C. The power of the far-infrared lamp in the far-infrared drying is 350 to 450 W.

3. The combined drying method according to claim 1 or 2, characterized in that: The vacuum freeze-drying conditions are: cold trap temperature -45 to -55°C, and vacuum degree maintained at 15 to 20 Pa.

4. The combined drying method according to claim 2, wherein: The distance between the far-infrared lamp and the pumpkin pulp is 25 to 30 cm.

5. The combined drying method according to claim 1 or 2, characterized in that: The pumpkin pulp is pretreated before being subjected to vacuum freeze-drying treatment. The pretreatment method comprises the following steps: blanching the pumpkin pulp, treating the pulp with a color-protecting liquid after blanching, and pre-freezing the treated pumpkin pulp.

6. The combined drying method according to claim 5, characterized in that The pumpkin pulp is cut into pumpkin slices of 5 to 6 mm.

7. The combined drying method according to claim 5, wherein: The blanching temperature is 90-100° C., and the blanching time is 8-12 s.

8. The combined drying method according to claim 5, characterized in that: The color protection liquid contains 0.1-0.2% by mass of ascorbic acid, 0.1-0.2% by mass of L-cysteine and 0.5-1.5% by mass of citric acid.

9. The combined drying method according to claim 5, wherein: The pre-freezing temperature is -75 to -85°C, and the pre-freezing time is 22 to 26 hours.

10. Use of the combined drying method according to claim 1 or 2 in the preparation of pumpkin products.

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