Method for preparing dried lychee by using air-permeable and moisture-permeable film to prevent contamination and maintain original flavor

Abstract

The application discloses a method for preparing dried litchi by using a breathable and moisture-permeable film, wherein fresh litchi is subjected to physical field sterilization treatment, wrapped in a self-made sulfonated polyether ether ketone preservative film, subjected to heat pump drying treatment by means of a blowing drying box, and the O2 and CO2 concentrations during drying are adjusted by means of a modified atmosphere packaging film, so that the nutritional components and aroma components of the litchi are retained to the maximum extent during the drying process, and the physical field sterilization is combined, so that the microbial pollution is reduced, and the shelf life of the dried fruit is further improved. The application is green and safe, one-step drying is adopted, the drying time is short, and the drying energy consumption is reduced. Finally, the data results show that the dried litchi obtained by using the application is superior to the dried litchi obtained by using common drying methods in terms of various physical and chemical properties and aroma components, and no microbial breeding and rotting phenomenon occurs after being placed for several months.

Description

TECHNICAL FIELD

[0001] The present application relates to food processing technology, in particular to a method for preparing dried litchi by using a breathable and moisture-permeable film to prevent pollution and maintain original flavor. BACKGROUND

[0002] At present, the main litchi processing products include dried litchi, canned litchi, litchi juice, litchi wine, litchi vinegar, litchi jam and litchi yogurt, etc. Due to the lagging development of processing technology, there is no large-scale industrialized production enterprise at present. Therefore, realizing large-scale industrialized production, improving the yield, quality and market competitiveness of litchi processing products is one of the important development prospects of litchi industry. Dried litchi is the main litchi processing product at present, accounting for 80% of all litchi processing products. Producing dried litchi can handle a large amount of fresh litchi in a short time, and the processing amount is large, the processing is rapid, the process conditions are simple, and the participation of farmers is high.

[0003] The existing technology mainly has the problems of complex steps, long time consumption and poor quality. For example, patent document CN109527459A discloses a preparation method of dried litchi, which includes picking, separation, fixation, initial baking, softening, secondary baking, softening, and three times of baking. The dried litchi prepared by this method has the characteristics of low shell breaking rate and high appearance excellent rate, but the preparation process is complex, the time is long, the energy consumption is high, and the multiple baking process also increases the loss of main aroma components of dried litchi, and reduces the taste. The preparation method of dried litchi disclosed in patent document CN109197989A mainly includes: raw material selection, steam treatment, color protection treatment, drying treatment, and post-treatment after drying. After being stored for 1 year, the good fruit rate reaches 74.9%. However, the color protection treatment in this method may cause the penetration of color protection reagent into the litchi after soaking, resulting in the residue of organic matter and safety hidden trouble.

[0004] Therefore, improving the quality of dried litchi is extremely important for the industrialized production and commercialized application of litchi. SUMMARY

[0005] In view of the problems of long processing time, high energy consumption, low energy utilization rate and poor quality of dried products in the existing dried litchi preparation industry, the present application provides a method for preparing dried litchi by using a breathable and moisture-permeable film to prevent pollution and maintain original flavor.

[0006] The technical scheme adopted by the present application is as follows:

[0007] A method for preparing dried litchi by using a breathable and moisture-permeable film to prevent pollution and maintain original flavor, characterized in that sulfonated polyether ether ketone film is used to wrap and seal the pre-sterilized litchi, and then the litchi is dried at 50-80℃ for 36-96h to obtain dried litchi.

[0008] Preferably, the pre-sterilization refers to sterilizing the lichis in ozone for 2-10 minutes.

[0009] Preferably, the sulfonated polyether ether ketone film is prepared by the following method:

[0010] (1) adding polyether ether ketone into concentrated sulfuric acid, heating in a water bath at 50-70 DEG C for 2-5 hours, and then cooling, taking the precipitate, drying, and obtaining sulfonated polyether ether ketone particles;

[0011] (2) dissolving the sulfonated polyether ether ketone particles in an organic solvent to obtain a casting solution, casting the casting solution on a flat plate, drying, immersing in water, taking out, and drying to obtain a sulfonated polyether ether ketone film.

[0012] Preferably, the mass ratio of the polyether ether ketone to the concentrated sulfuric acid in step (1) is 1:(15-25), and the concentration of the concentrated sulfuric acid is 98±2wt%.

[0013] Preferably, the stirring condition of the water bath heating in step (1) is 400±50r / min, the temperature is 60±5 DEG C, and the heating time is 4±0.5h.

[0014] Preferably, the drying temperature is 60±5 DEG C, and the time is 72±12h.

[0015] Preferably, the drying is performed using a blast drying oven.

[0016] The lichis prepared by the method of the present application.

[0017] The present application sterilizes fresh lichis by physical field sterilization, wraps them in self-made sulfonated polyether ether ketone preservative film, and performs heat pump drying treatment by a blast drying oven, and adjusts the O2 and CO2 concentrations during drying by a modified atmosphere packaging film, so that the nutritional components and aroma components of the lichis are retained to the greatest extent during the drying process, and the use of physical field sterilization reduces microbial pollution and further improves the shelf life of the dried fruits. The present application is green and safe, uses one-step drying method, has short drying time, and reduces drying energy consumption. The final data show that the dried lichis obtained by the present application are superior to those obtained by ordinary drying methods in terms of various physical and chemical properties and aroma components, and no microbial growth and decay occurs after being stored for several months.

[0018] Compared with the prior art, the present application has the following advantages:

[0019] (1) In the heat pump drying process, high concentration of carbon dioxide plays a key role in maintaining the key quality of dried litchi during the process, and can retain the main aroma components of dried litchi, and improve the commodity nature. The sulfonated polyether ether ketone membrane used in the application has good air and moisture permeability, good selectivity to gas, and can reasonably control the concentration of O2 and CO2 in the drying process, thereby significantly improving the water activity, polyphenol content, Vc content and content of various aroma components of dried litchi, and can well maintain the original color of litchi, greatly improving the quality of dried litchi.

[0020] (2) The application adopts high-concentration gaseous physical field sterilization method for sterilization pretreatment of fresh litchi, and the high-concentration ozone gas is subjected to deep sterilization in all directions under the action of the physical field, and the sterilization efficiency can reach more than 99%, so that the dried litchi is free from pollution and invasion of microorganisms and other molds during preservation, and the shelf life is greatly prolonged.

[0021] (3) The preparation method of the dried litchi is simple, and only needs to be wrapped with sulfonated polyether ether ketone membrane for drying, which greatly simplifies the preparation method of dried litchi and is suitable for industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 Example 1-2, comparative example 1, the change of moisture content of litchi in the heat pump drying process.

[0023] Figure 2 Example 1-2, comparative example 1, the change of water activity of litchi in the heat pump drying process.

[0024] Figure 3 Example 1-2, comparative example 1, the change of polyphenol content of litchi in the heat pump drying process.

[0025] Figure 4 Example 1-2, comparative example 1, the change of soluble solid content of litchi in the heat pump drying process.

[0026] Figure 5 Example 1-2, comparative example 1, the change of Vc content of litchi in the heat pump drying process.

[0027] Figure 6(a) is a gas chromatogram of litchi before drying; figures 6(b), (c), (d) are gas chromatograms of litchi after drying of example 1, example 2 and comparative example 1, respectively.

[0028] Figures 7(a), (b), (c) are photos of dried litchi prepared by example 1, example 2 and comparative example 1 respectively after 60 days of preservation. DETAILED DESCRIPTION

[0029] The following will be described in detail the embodiments of the present application, the embodiments are implemented on the premise of the technical solutions of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following embodiments.

[0030] Example 1

[0031] (1) 10 g of PEEK particles were weighed, washed with deionized water for 10 min, and then washed with anhydrous ethanol for 10 min. The washed PEEK was taken out and drained, and after the anhydrous ethanol was completely volatilized, it was loaded into a 500 mL three-necked round-bottom flask. 200 mL of 98 wt% concentrated sulfuric acid was injected into the flask, and the mixture was fully reacted under the condition of 400 r / min and 60°C water bath heating. Subsequently, the mixture was dropped into an ice water mixture at an extremely slow speed for cooling, and the precipitate was separated and washed with deionized water until the pH approached 7. The washed precipitate was placed in an oven and vacuum dried at 60°C for 24 h to obtain the final product SPEEK.

[0032] (2) The SPEEK was added to dimethylformamide and magnetically stirred at 60°C and 500 r / min for 2 h until completely dissolved, and then cooled for use to obtain a casting solution with a concentration of 1 g / 10 mL.

[0033] (3) The casting solution obtained in step (2) was cast into a film on a clean glass plate with a film thickness of 50 μm, and then placed in an oven and dried at 60°C for 12 h. After natural cooling to room temperature, it was immersed in distilled water at room temperature, taken out, and dried at room temperature to obtain a SPEEK film.

[0034] (4) Fresh litchi was placed in a physical field sterilization equipment for 5 min, and then wrapped with a SPEEK film. After the sealing machine was sealed, it was placed in a 60°C air drying oven for heat pump drying for 72 h. The changes in various physical and chemical indicators during the treatment process are shown in Table 1. Figures 1-5

[0035] Example 2

[0036] Fresh litchi was placed in a physical field sterilization equipment for 5 min, and then wrapped with a commercially available PE film. After the sealing machine was sealed, it was placed in a 60°C air drying oven for heat pump drying for 72 h. The changes in various physical and chemical indicators during the treatment process are shown in Table 2. Figures 1-5

[0037] Comparative Example 1

[0038] Fresh litchi was washed and sieved, and then directly placed in a 60°C air drying oven for heat pump drying for 72 h. The changes in various physical and chemical indicators during the treatment process are shown in Table 3. Figures 1-5

[0039] ​​​Detection method:

[0040] Detection of moisture content: Take the sample aluminum box and place it in the oven to dry at 100-105°C until constant weight, cool in a desiccator, number, then accurately weigh. Cut and mix when sampling, take 5-10g of sample and divide it into 3 parts, accurately weigh each part. Place the aluminum box containing the sample in the oven, first at 60-80°C for 2-3 hours until the sample becomes brittle and dry, then at 100-105°C for 1-2 hours, remove and cool in a desiccator, weigh, and then at 100-105°C for 0.5-1 hours, cool and weigh until the difference between the two weighings is not more than 0.002g.

[0041] Detection of water activity: Use HD-4 intelligent water activity meter to measure. Mix the litchi samples to be measured uniformly, randomly select 3 litchi, cut them into uniform particle size, and then lay them flat on the measuring dish. Place the measuring dish into the inductive probe for measurement. After 30 minutes of equilibrium, record the obtained data, which is the water activity of the corresponding sample. Repeat the measurement 3 times for each sample.

[0042] Detection of polyphenol content: Take 10g of litchi dried fruit pulp, add 50mL of 80% acetone solution, and homogenize for 3 minutes. Extract at room temperature for 3 times, 1 hour each time. Vacuum filter, combine the extract, and concentrate at 45°C until there is no residual organic phase. Collect the concentrated solution and dilute to 50mL. Prepare a standard curve with gallic acid, measure the absorbance at 760nm, and calculate the total phenol content.

[0043] Detection of soluble solids: Use a handheld refractometer to determine the content of soluble solids in litchi pulp according to GB / T 12295. Repeat 3 times for each sample and take the average value, expressed as a percentage.

[0044] Detection of Vc content: Use UV spectrophotometry.

[0045] a) Preparation of standard curve: Take Vc standard solution (2.5ug / mL) respectively,

[0046] (1) Preparation of standard solution: Accurately weigh 0.0250g of vitamin C, dissolve in distilled water, transfer to a 100mL volumetric flask, dilute to the mark, and shake well for use. Use a pipette to transfer 10.00mL to a 100mL volumetric flask, dilute to the mark with distilled water, and shake well. The Vc concentration is C=2.5ug / mL.

[0047] (2) Preparation of standard series solution: Take 0.50, 1.00, 2.00, 3.00, 4.00, and 5.00mL of the above standard solution into 6 10mL stoppered test tubes, dilute to the mark with distilled water, and shake well.

[0048] (3) Measure the absorbance at 267 nm, and draw the standard curve with the concentration as the horizontal coordinate and the absorbance as the vertical coordinate. Y = 0.518X - 0.017 (R = 0.9996) 2

[0049] b) Preparation of sample: 50 g of fruit was taken in a quartz grinder, and the juice was squeezed out. The juice was transferred to a 10 ml test tube and centrifuged at low speed for 10 min at 4000 r / min in a low-speed centrifuge to obtain the supernatant.

[0050] c) Determination of Vc content: the above supernatant was diluted 100 times, and the absorbance was measured at 267 nm with deionized water as the reference. The Vc content was calculated according to the following formula:

[0051] W = 100 x C

[0052] Wherein W is the content of Vc in Psidium guajava (μg / mL)

[0053] 100 is the dilution multiple

[0054] C is the content of Vc obtained from the standard curve (μg / mL)

[0055] Detection of soluble sugar: a handheld refractometer was used for measurement, 3 times for each group, and the average value was taken.

[0056] Determination of aroma components: GC-MS was used for analysis and determination, headspace solid phase microextraction (HS-SPME) was used to extract the volatile flavor components of litchi samples at each drying time, and Agilent 6890-5973 GC-MS mass spectrometer was used to determine and analyze the flavor components of litchi samples.

[0057] Gas chromatography conditions: HP-5MS quartz capillary column (30 m x 0.25 mm x 0.25 μm). Capillary column temperature program: initial temperature 40℃, hold for 3 min, increase to 110℃ at 5℃ / min, hold for 2 min, increase to 130℃ at 5℃ / min, hold for 5 min, increase to 200℃ at 10℃ / min.

[0058] Mass spectrometry conditions: transfer line temperature 250℃, ionization mode EI, ion source temperature 230℃, electron energy 69.9 eV, photomultiplier voltage 1671 V, scan mass range 50-550 amu.

[0059] Test results:

[0060] Moisture content: from Figure 1 ​It can be seen that the moisture content of the wet basis continuously decreases as the heat pump drying process proceeds. In Example 1, the moisture content of the wet basis decreased from 82% to about 28% after 96 hours of drying. In Comparative Example 1, the moisture content of the wet basis decreased from 82% to about 20% after 32 hours of drying, and remained basically unchanged thereafter. In the first 60 hours of drying, the moisture content of the whole lychee fruit decreased rapidly, and after 60 hours of drying, the moisture content decreased slowly.

[0061] Water activity: by Figure 2 It can be seen that the water activity of lychees gradually decreased as the heat pump drying process proceeded. In Example 1, the water activity decreased from 0.96 to 0.60 after 96 hours of drying, while in Comparative Example 1, it decreased from 0.96 to 0.50 after 20 hours of drying. In Example 1, the water activity decreased significantly during the 24–72 hour drying period, and decreased slowly in the remaining stages; while in Comparative Example 1, the water activity decreased significantly during the 4–16 hour drying period, and decreased more slowly in the remaining stages. Due to the membrane coating, the water activity of Example 1 after 96 hours of drying was still higher than that of Comparative Example 1 after 20 hours of drying, while the water activity of lychees coated with PE film was not significantly different from that of Comparative Example 1 after 72 hours of drying, indicating that the PE film was not effective. This reflects the differences in the physical changes and chemical reactions that occurred during lychee drying before and after membrane coating.

[0062] Polyphenol content: Polyphenol content such as Figure 3 As shown, in Example 1, the polyphenol content still reached about 2.6 mg / g after 72 hours of heat pump drying, while in Example 2 and Comparative Example 1, the polyphenol content was only about 2.1 mg / g after 72 hours. The carbon dioxide content during the heat pump drying process can affect the polyphenol content. The phenol content is significantly positively correlated with the antioxidant capacity. The higher the phenol content, the stronger the antioxidant capacity. This indicates that the lychee wrapped with a membrane in Example 1 has a high internal carbon dioxide content, which effectively reduces the loss of nutrients.

[0063] Soluble solids content: by Figure 4 It can be concluded that during the heat pump drying process of lychees, the soluble solids content in lychees will continuously increase due to the decrease in moisture content. Therefore, the sugar content of lychees will increase after drying. In Example 1, the soluble solids content of lychees increased from 18.3 to 27.3 during the drying process, while in Example 2 and Comparative Example 1, the increase in soluble solids content was only from 18.2 to 26. This indicates that the sugar content of lychees wrapped in film increased more after drying, making them sweeter.

[0064] Vitamin C content: from Figure 5 It can be concluded that... Figure 3 Similarly, the membrane-wrapped lychees, during the drying process, effectively prevented the decrease in ascorbic acid content due to the increased carbon dioxide content inside the bag, thus better preserving their nutrients and resulting in higher nutritional value.

[0065] Flavor test results:

[0066] Fig. 6(a), (b), (c), (d) are gas chromatograms of litchi before drying, after drying according to Example 1, Example 2, and Comparative Example 1, respectively. Analysis of the results obtained by GC-MS shows that the volatile components of the initial dried litchi sample are mainly: germacrene D, a- myrcenol, γ-elemene, β-piperitol, γ-myrcenol, δ-cadinene, a-piperitol, geraniol, β-elemene, cyclocommunol, etc.; the volatile components of the litchi pulp sample dried for 4h are mainly germacrene D, γ-elemene, a-myrcenol, β-piperitol, a-piperitol, δ-cadinene, γ-myrcenol, β-farnesene, γ-cadinene, geraniol, etc.; the volatile components of the dried litchi pulp sample are mainly germacrene D, γ-elemene, a-myrcenol, β-piperitol, δ-cadinene, γ-myrcenol, a-piperitol, etc.

[0067] In Comparative Example 1, the contents of terpenoids such as germacrene, myrcenol, elemene, piperitol, cadinene, and geraniol are greatly reduced after drying, indicating that the aroma of litchi is greatly lost after drying. In Example 1, the contents of germacrene, myrcenol, elemene, piperitol, cadinene, and geraniol are relatively high, indicating that litchi can effectively retain its aroma after drying after packaging, making its flavor more prominent.

[0068] Comparison of litchi dry preservation for 60 days:

[0069] The litchi dry prepared in Examples 1, 2, and Comparative Example 1 is shown in Figs. 7(a), (b), (c) after 60 days of storage. It can be seen that the shell of the litchi dry of Comparative Example 1 is white, and the litchi dry of Example 1 is redder than that of Example 2, maintaining better color. Since the SPEEK film of Example 1 has the function of adjusting the concentrations of oxygen and carbon dioxide, the shelf life of the litchi dry prepared in Example 1 is longer.

[0070] Membrane gas permeability data

[0071]

[0072] The requirement of carbon dioxide concentration is different in the process of conventional fruit and vegetable preservation and litchi drying. In the process of fruit and vegetable controlled atmosphere preservation, the carbon dioxide concentration is generally 1%-3%, and the carbon dioxide concentration is about 5% in the process of litchi drying, so the carbon dioxide permeation of the film is required to be low, and the carbon dioxide selectivity is required to be good. It can be obtained from the film permeability data that the carbon dioxide and oxygen permeation of SPEEK is lower than that of PE film, and the gas separation coefficient of SPEEK is higher than that of PE film, which indicates that the carbon dioxide content in the SPEEK bag will be higher than that in the PE bag during the drying process, so the physicochemical indexes of litchi in the SPEEK bag after drying are better than those in the PE bag.

[0073] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are included in the protection scope of the present application.

Claims

1. A method for preparing dried lychee by using a breathable and moisture-permeable film to prevent contamination and maintain original flavor, characterized in that, The pre-sterilized lychee is wrapped and sealed by using the sulfonated polyether ether ketone film, and is dried at 50-80℃ for 36-96h to obtain lychee dry fruit; The sulfonated polyether ether ketone film is prepared by the following method: (1) polyether ether ketone is added into concentrated sulfuric acid, and is heated in water bath at 50-70℃ for 2-5h, then is cooled, and the precipitate is washed and dried to obtain sulfonated polyether ether ketone particles; (2) the sulfonated polyether ether ketone particles are dissolved in organic solvent to obtain casting solution, the casting solution is cast on a flat plate to form a film, then is dried, and is immersed in water, and is taken out and dried to obtain sulfonated polyether ether ketone film; In step (1), the mass ratio of polyether ether ketone to concentrated sulfuric acid is 1:(15-25), and the concentration of concentrated sulfuric acid is 98wt%.

2. The method of claim 1, wherein, The pre-sterilization refers to sterilizing lychee in ozone for 2-10min.

3. The method of claim 1, wherein, In step (1), the stirring condition of water bath heating is 400±50r / min, the temperature is 60±5℃, and the heating time is 4±0.5h.

4. The method according to any one of claims 1 to 3, characterized in that, The drying temperature is 60±5℃, and the time is 72±12h.

5. The method of claim 4, wherein, The drying is performed by using a blast drying oven.

6. Lychee dry fruit prepared by the method of any one of claims 1-5.

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