Microwave-assisted cold pressing extraction process of citrus essential oil

Abstract

The invention discloses a microwave-assisted cold-pressing extraction process of citrus essential oil. The microwave-assisted cold-pressing extraction process comprises the following steps: crushing citrus peels into blocks; performing ultralow-temperature freezing treatment on the citrus peel fragments at the temperature of-60 DEG C to-100 DEG C for 12-36 hours; performing low-energy microwave treatment on the citrus peel fragments subjected to the ultralow-temperature freezing treatment, wherein the microwave power of the low-energy microwave treatment is 70-90W, and the time is 10-15 minutes; performing cold pressing and collecting an extracting solution; carrying out centrifugal treatment on the extracting solution; collecting upper oily liquid of the centrifuged extracting solution, and performing dehydration treatment to obtain the citrus essential oil. According to the method, ultralow-temperature freezing, microwaves and cold pressing are combined, the essential oil is effectively locked through ultralow-temperature freezing, and the loss of the essential oil in the pretreatment process is reduced; the microwave treatment can destroy membranes of citrus oil cells and form micropores and cracks, a mass transfer channel is established for continuous release of the essential oil in the subsequent cold pressing process, the yield of the essential oil is increased, and the application prospect is good.

Description

Technical Field

[0001] This invention relates to the field of essential oil extraction technology, and to a microwave-assisted cold pressing extraction process for citrus essential oil. In particular, it relates to a method for extracting citrus essential oil by combining ultra-low temperature freezing treatment and low-energy microwave treatment to achieve high cell wall breakage rate and high cold pressing yield. Background Technology

[0002] Currently, citrus essential oils are widely used in the pharmaceutical, perfume, food, and cosmetic industries due to their various benefits, including antibacterial properties, mood-soothing effects, flavor enhancement, and skin-firming properties. However, existing extraction processes generally suffer from low extraction efficiency and damage to aroma quality. The extraction efficiency of essential oils is closely related to the extraction method used. Currently, the main extraction methods for citrus essential oils include cold pressing, steam distillation, supercritical carbon dioxide extraction, and solvent extraction.

[0003] Aroma quality is a core indicator for evaluating the quality of essential oils, and its quality is determined by the chemical composition of the oil. The main chemical components of citrus essential oils are terpenes, esters, alcohols, and aldehydes, and different extraction processes significantly affect their composition and content. While steam distillation yields a high rate, prolonged high-temperature processing can cause terpenes in citrus to oxidize into off-flavor compounds such as carvone and carvacrol, resulting in the loss of heat-sensitive flavor substances and thus affecting the aroma quality of citrus essential oils. Solvent extraction, as a traditional method, suffers from long extraction times and difficulty in completely removing solvent residues, limiting its application in food and pharmaceutical fields. Supercritical CO2 extraction, while offering advantages such as being environmentally friendly and having a gentle extraction temperature, follows the principle of "like dissolves like," resulting in weak solubility for the polar oxygen-containing compounds that determine the core aroma of citrus essential oils. Its limited penetration also makes it difficult to efficiently penetrate the cellulose walls of the oil sacs in the peel. Impurities such as pectin and waxes further reduce extraction efficiency, ultimately leading to a limited variety of aroma components in the resulting essential oil, failing to fully express the unique flavor of citrus essential oils. In contrast, the cold-pressing process is simple and low-cost, and operates at low temperatures, which can preserve the types of terpenoids and their natural aromas in citrus essential oils to the greatest extent, thus helping to maintain the flavor integrity of the essential oils.

[0004] Cold pressing primarily uses physical compression to rupture oil cells, thereby releasing essential oils. To improve the essential oil yield of cold pressing, the raw materials are usually pretreated before cold pressing, such as crushing, soaking in lime water, or ultrasonic treatment. While crushing increases the exposed surface area of ​​oil cells, excessive crushing can lead to the absorption of essential oils by the white peel layer, resulting in losses. Soaking in lime water can disrupt cell structure and decompose pectin, helping to increase the oil yield of citrus fruits, but prolonged soaking can cause the loss of some aroma components, affecting aroma quality. Ultrasonic treatment utilizes the cavitation effect to promote the release of cell contents and is a non-high-temperature process, which is beneficial for preserving active ingredients. However, its equipment cost is high, and it generates significant noise, making industrial application difficult.

[0005] Therefore, developing a method for extracting citrus essential oils that is cold-pressed with high yield, high cell wall breakage rate, low cost, convenient for industrial production, and does not affect the quality of the essential oil is of great practical significance. Summary of the Invention

[0006] Due to the aforementioned deficiencies in existing technologies, this invention provides a method for extracting citrus essential oils that features high cold-pressing yield, high cell wall breakage rate, low cost, ease of industrial production, and no impact on essential oil quality. This method solves the problems of low essential oil yield and unsatisfactory cold-pressing effects caused by incomplete cell wall breakage during citrus cold pressing, thereby improving the yield of cold-pressed citrus essential oils. It also shortens the pretreatment time and reduces the difficulty of operation for citrus samples, further increasing the yield of cold-pressed citrus essential oils and achieving efficient extraction of solvent-free, green essential oils. Furthermore, it utilizes a combination of ultra-low temperature freezing and low-energy microwave treatment to obtain high-quality essential oils with excellent aroma. The ultra-low temperature freezing process locks in essential oils by instantly forming tiny ice crystals within cells, preventing excessive damage to cell structure. This not only increases the juice yield from cold pressing and maximizes the retention of volatile aroma substances, but also efficiently extracts high-quality citrus essential oils. Simultaneously, microwaves damage the lipid bilayer of the oil cell membrane, creating micropores and cracks, establishing continuous mass transfer channels for essential oil outflow. This improves the extraction efficiency of citrus essential oils and overcomes the shortcomings of existing cold pressing processes, such as low yield, low cell wall breakage, high cost, difficulty in industrial application, and impact on product quality.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A microwave-assisted cold-pressing extraction process for citrus essential oil includes the following steps:

[0009] (1) First, separate the citrus peel from the fruit, keeping the peel part, and break the citrus peel into pieces;

[0010] (2) The citrus peel fragments are subjected to ultra-low temperature freezing treatment. The ultra-low temperature freezing treatment temperature is -60 to -100℃ and the time is 12 to 36h. The ultra-low temperature freezing process can rapidly form tiny and uniform ice crystals in the citrus peel cells, ensuring the cell integrity and flavor of the citrus peel and preventing the essential oil from leaking due to the rupture of oil cells.

[0011] (3) The citrus peel fragments that have been subjected to ultra-low temperature freezing are subjected to low-energy microwave treatment. The microwave power of the low-energy microwave treatment is 70-90W and the time is 10-15min. The microwave damages the lipid bilayer of the oil cell membrane, creating micropores and cracks, which establishes a continuous mass transfer channel for the essential oil to flow out, which is beneficial to improving the extraction efficiency of citrus essential oil.

[0012] (4) The citrus peel fragments treated with low-energy microwaves were cold-pressed and the extract was collected;

[0013] (5) Centrifuge the extract;

[0014] (6) Collect the oily liquid on the top layer of the extract after centrifugation and dehydrate it to obtain citrus essential oil.

[0015] Microwaves, as an auxiliary extraction method, soften cell wall structures and reduce the hardness of raw materials. Microwave effects are divided into thermal and non-thermal effects: the thermal effect causes polar molecules within citrus cells to vibrate rapidly and alternately in the direction of the electric field. The intense collisions and friction between molecules are instantly converted into heat energy, creating an "internal heating" effect—heat is generated from inside the cell, rather than conducted from the outside to the inside. Non-thermal effects refer to the direct influence of microwaves on the physical structure and chemical properties of substances without causing a significant temperature increase.

[0016] This invention combines ultra-low temperature freezing, microwave technology, and cold pressing. Ultra-low temperature freezing effectively locks in essential oils, reducing oil loss during pretreatment. Simultaneously, microwave treatment disrupts the membranes of citrus oil cells to some extent, creating micropores and fissures. This establishes mass transfer channels for the continuous release of essential oils during subsequent cold pressing, thereby increasing the oil yield. In this application, the combined ultra-low temperature freezing and microwave treatment not only ensures the quality of the essential oils but also improves their yield. Microwave pretreatment offers advantages in terms of its short treatment time and high efficiency. Combining it with cold pressing technology not only helps maintain the high aroma quality of citrus essential oils but also significantly improves the extraction yield. Therefore, this process is particularly suitable for scenarios where the current cold pressing yield of citrus is poor, providing a feasible approach for achieving efficient and high-quality extraction of citrus essential oils, with promising application prospects.

[0017] As a preferred technical solution:

[0018] The microwave-assisted cold-pressing extraction process for citrus essential oil described above involves selecting fresh, unrotten, undamaged citrus fruits without any coating before separating the peel from the fruit. These fruits are then washed and air-dried naturally.

[0019] In the microwave-assisted cold pressing extraction process of citrus essential oil described above, in step (1), a mixer is used to stir the citrus peel for 2-4 seconds at 40-60Hz and 250-350W to break it into 2-4cm pieces. If the citrus peel pieces are too small (e.g., in a muddy state), the oil cells may rupture during the crushing process, and the essential oil may be absorbed by the white peel layer, resulting in essential oil loss. Furthermore, the machine heating will cause the essential oil to volatilize, thus reducing the essential oil yield. On the other hand, if the pieces are too large, it will be difficult for the material to enter the extrusion device, resulting in too many gaps in the extrusion device, which will affect the extraction efficiency and ultimately reduce the yield.

[0020] In the microwave-assisted cold pressing extraction process for citrus essential oil described above, step (4) involves cold pressing in a spiral extrusion cold press. Before cold pressing, the citrus peel fragments treated with low-energy microwaves are left to stand at room temperature for 5-10 minutes, and the pressing chamber temperature does not exceed 45°C during the cold pressing process. Excessive pressing chamber temperature not only accelerates essential oil volatilization, resulting in a reduced extraction yield, but also causes oxidation and enzymatic reactions of terpenoids in the citrus essential oil, generating off-flavor substances such as carvone and carvacrol, thus lowering the quality of the essential oil.

[0021] In the microwave-assisted cold pressing extraction process of citrus essential oil described above, in step (5), the centrifugation speed is 8000-10000 rpm, the temperature is 2-6℃, and the centrifugation time is 10-20 min.

[0022] In the microwave-assisted cold pressing extraction process of citrus essential oil described above, the dehydration treatment in step (6) is carried out using anhydrous sodium sulfate.

[0023] The microwave-assisted cold pressing extraction process for citrus essential oil described above involves dehydration followed by standing for 5–10 hours and then filtering to obtain the citrus essential oil.

[0024] As described above, in a microwave-assisted cold-pressing extraction process for citrus essential oil, the sample temperature after low-energy microwave treatment does not exceed 10°C. During the microwave process, the thermal effect of microwaves raises the temperature of the citrus peel. Excessive temperature not only accelerates essential oil volatilization and reduces the yield, but also causes oxidation and enzymatic reactions of aroma compounds in the citrus essential oil, producing off-flavors and thus degrading the quality of the essential oil. Therefore, it is necessary to control the sample temperature during the microwave process to avoid excessively high temperatures.

[0025] The present invention also provides a citrus essential oil obtained by a microwave-assisted cold pressing extraction process as described above.

[0026] Furthermore, the present invention also provides the use of citrus essential oils as described above in pharmaceuticals, perfumes, food, and cosmetics.

[0027] The above technical solution is only one feasible technical solution of the present invention. The scope of protection of the present invention is not limited thereto. Those skilled in the art can reasonably adjust the specific design according to actual needs.

[0028] The above invention has the following advantages or beneficial effects:

[0029] This invention relates to a microwave-assisted cold-pressing extraction process for citrus essential oil. The process involves ultra-low temperature freezing of citrus peel, followed by microwave-assisted extraction to enhance the essential oil extraction rate. Compared to conventional freezing, ultra-low temperature freezing forms tiny ice crystals, protecting the citrus peel cell structure, increasing the juice yield during cold pressing, and maximizing the preservation of flavor and nutrients. Microwave treatment utilizes both thermal and non-thermal effects to disrupt the citrus cell wall structure. Ultra-low temperature freezing better "locks in" the essential oil, while microwaves can, to some extent, disrupt the membranes of citrus oil cells and form micropores. This facilitates the continuous flow of essential oil through these micropores during cold pressing, resulting in lower pressing resistance and more thorough oil extraction. The combined use of these two treatments achieves a high cell wall breakage rate, thereby increasing the cold-pressing yield and producing citrus essential oil with high aroma and quality, showing promising application prospects. Attached Figure Description

[0030] The invention, its features, shape, and advantages will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings. Like reference numerals denote like parts throughout the drawings. The drawings are not drawn to scale; their focus is on illustrating the gist of the invention.

[0031] Figure 1 This is a flowchart of the microwave-assisted cold pressing extraction process for citrus essential oil according to the present invention.

[0032] Figure 2 The chromatogram of the citrus essential oil obtained in Example 1 is shown below.

[0033] Figure 3 for Figure 2 Enlarged graph of the vertical axis. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but these are not intended to limit the scope of the invention.

[0035] The following analytical identification methods for flavor compounds were employed: Gas chromatography-mass spectrometry (GC-MS, Agilent 8860-5977c). The initial column oven temperature was set at 60℃, held for 5 min, then programmed to 90℃ at a rate of 1℃ / min and held for 3 min. The temperature was then programmed to 120℃ at a rate of 2℃ / min and held for 1 min, followed by a final ramp to 240℃ at a rate of 6℃ / min and held for 3 min. Helium was used as the carrier gas at a flow rate of 1.5 mL / min. The injection port temperature was set to 250℃, using a split injection mode with a split ratio of 1:10. The quadrupole temperature and ion source temperature were set to 150℃ and 230℃, respectively. Mass spectrometry detection was performed using electron impact ionization (EI) mode with an ionization energy of 70 eV and a mass scan range of 30–300 m / z. The electron multiplier was set to auto-tuning mode, and the ionization current was 10 μA. Qualitative analysis of volatile organic compounds was performed by comparing their mass spectra with standard spectra in the NIST23 library.

[0036] Quantitative methods for flavor compounds: Qualitative and quantitative analysis was performed using the internal standard method, and the types and contents of aroma components in cold-pressed products with different pretreatment methods were determined by retention index calculation, NIST database retrieval, standard comparison and spectral analysis.

[0037] Methods for calculating essential oil yield:

[0038]

[0039] Wherein, mEO: mass of the extracted essential oil (g); mDM: mass of the citrus peel raw material (g).

[0040] Example 1

[0041] A microwave-assisted cold-pressing extraction process for citrus essential oil includes the following steps (flowchart shown). Figure 1 (as shown)

[0042] Step S1: First, separate the citrus peel from the fruit, keeping the peel. Then, break the citrus peel into pieces. Specifically, use a blender at 50Hz and 300W for 3 seconds to break the citrus peel into 3cm pieces.

[0043] Step S2: Place the citrus peel fragments at -80℃ for ultra-low temperature freezing treatment for 24 hours;

[0044] Step S3: Place the citrus peel fragments that have undergone ultra-low temperature freezing into a microwave device for low-energy microwave treatment. Set the power to 80W and the time to 12.5min. The temperature of the sample after low-energy microwave treatment should not exceed 10℃.

[0045] Step S4: Put the citrus peel fragments treated with low-energy microwave into a screw extrusion cold press for cold pressing, and collect the extract obtained from the outlet.

[0046] Step S5: Centrifuge the extract at 10,000 rpm, 4°C, for 15 minutes.

[0047] Step S6: Collect the oily liquid on the top layer of the extract after centrifugation, dehydrate it with anhydrous sodium sulfate, weigh it and calculate the yield. The final yield is 0.69±0.029%.

[0048] Example 2

[0049] A microwave-assisted cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the power of step S3 is 70W.

[0050] Example 3

[0051] A microwave-assisted cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the power of step S3 is 90W.

[0052] Example 4

[0053] A microwave-assisted cold pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the microwave treatment time in step S3 is 10 minutes.

[0054] Example 5

[0055] A microwave-assisted cold pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the microwave treatment time in step S3 is 15 minutes.

[0056] Comparative Example 1

[0057] A process that uses only ultra-low temperature freezing followed by direct cold pressing includes the following steps:

[0058] Step S1: First, separate the citrus peel from the fruit, keeping the peel. Then, break the citrus peel into pieces. Specifically, use a blender at 50Hz and 300W for 3 seconds to break the citrus peel into 3cm pieces.

[0059] Step S2: Place the citrus peel fragments at -80℃ for ultra-low temperature freezing treatment for 24 hours;

[0060] Step S3: Put the citrus peel fragments that have been subjected to ultra-low temperature freezing into a screw extrusion cold press for cold pressing, and collect the extract obtained from the outlet;

[0061] Step S4: Centrifuge the extract at 10,000 rpm, 4°C, for 15 minutes.

[0062] Step S5: Collect the oily liquid on the top layer of the extract after centrifugation, dehydrate it with anhydrous sodium sulfate, weigh it and calculate the yield. The final yield is 0.30±0.014%.

[0063] Comparative Example 2

[0064] A cold pressing process based on a combination of ultra-low temperature freezing and lime water soaking includes the following steps:

[0065] Step S1: Soak the whole citrus fruit in 3% lime water for 12 hours, then wash and dry it.

[0066] Step S2: Separate the soaked citrus peel from the fruit, keeping the peel, and break the citrus peel into pieces. Specifically, use a mixer at 50Hz and 300W for 3 seconds to break the citrus peel into 3cm pieces.

[0067] Step S3: Place the citrus peel fragments at -80℃ for ultra-low temperature freezing treatment for 24 hours;

[0068] Step S4: Put the citrus peel fragments that have been subjected to ultra-low temperature freezing into a screw extrusion cold press for cold pressing, and collect the extract obtained from the outlet.

[0069] Step S5: Centrifuge the extract at 10,000 rpm, 4°C, for 15 minutes.

[0070] Step S6: Collect the oily liquid on the top layer of the extract after centrifugation, dehydrate it with anhydrous sodium sulfate, weigh it and calculate the yield. The final yield is 0.38±0.045%.

[0071] Comparative Example 3

[0072] A cold pressing process based on a combination of ultra-low temperature freezing and ultrasonic pretreatment includes the following steps:

[0073] Step S1: First, separate the citrus peel from the fruit, keeping the peel. Then, break the citrus peel into pieces. Specifically, use a blender at 50Hz and 300W for 3 seconds to break the citrus peel into 3cm pieces.

[0074] Step S2: Place the citrus peel fragments at -80℃ for ultra-low temperature freezing treatment for 24 hours;

[0075] Step S3: Place the citrus peel fragments that have undergone ultra-low temperature freezing into a beaker containing 120ml of ultrapure water, and place it into an ultrasonic device for ultrasonic treatment. Set the power to 200W, the frequency to 35kHz, and the time to 25min.

[0076] Step S4: Put the ultrasonically treated citrus peel fragments into a screw extrusion cold press for cold pressing, and collect the extract obtained from the outlet.

[0077] Step S5: Centrifuge the extract at 10,000 rpm, 4°C, for 15 minutes.

[0078] Step S6: Collect the oily liquid on the top layer of the extract after centrifugation, dehydrate it with anhydrous sodium sulfate, weigh it and calculate the yield. The final yield is 0.55±0.043%.

[0079] Comparative Example 4

[0080] A low-temperature cold-pressing process based on a combination of low-temperature freezing and microwave pretreatment includes the following steps:

[0081] Step S1: First, separate the citrus peel from the fruit, keeping the peel. Then, break the citrus peel into pieces. Specifically, use a blender at 50Hz and 300W for 3 seconds to break the citrus peel into 3cm pieces.

[0082] Step S2: Place the citrus peel fragments at -20℃ for 24 hours for low-temperature freezing treatment;

[0083] Step S3: Place the citrus peel fragments that have undergone low-temperature freezing into a microwave device for low-energy microwave treatment. Set the power to 80W and the time to 12.5min. The temperature of the sample after low-energy microwave treatment should not exceed 10℃.

[0084] Step S4: Put the citrus peel fragments treated with low-energy microwave into a screw extrusion cold press for cold pressing, and collect the extract obtained from the outlet.

[0085] Step S5: Centrifuge the extract at 10,000 rpm, 4°C, for 15 minutes.

[0086] Step S6: Collect the oily liquid on the top layer of the extract after centrifugation, dehydrate it with anhydrous sodium sulfate, weigh it and calculate the yield. The final yield is 0.29±0.035%.

[0087] Comparative Example 5

[0088] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the power of step S3 is 60W.

[0089] Comparative Example 6

[0090] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the power of step S3 is 100W.

[0091] Comparative Example 7

[0092] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the microwave treatment time in step S3 is 5 minutes.

[0093] Comparative Example 8

[0094] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the microwave treatment time in step S3 is 7.5 min.

[0095] Comparative Example 9

[0096] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the microwave treatment time in step S3 is 17.5 min.

[0097] Comparative Example 10

[0098] A cold-pressing extraction process for citrus essential oil is basically the same as that in Example 1, except that the citrus peel is crushed into a paste.

[0099] Before cold pressing, accurately weigh the citrus peel, process the citrus peel according to the cold pressing process provided in Example 1 and Comparative Examples 1-4, weigh the essential oil obtained from cold pressing, and calculate the essential oil yield.

[0100] The above examples are compared as follows: Example 1 involves cold pressing after ultra-low temperature freezing and microwave treatment; Comparative Example 1 involves direct cold pressing after ultra-low temperature freezing treatment; Comparative Example 2 involves cold pressing after soaking in lime water and ultra-low temperature freezing treatment; Comparative Example 3 involves cold pressing after ultra-low temperature freezing and ultrasonic treatment; Comparative Example 4 is compared with Example 1, involving cold pressing after microwave and low temperature freezing treatment, the difference being the pre-cooling temperature, to explore the effect of ultra-low temperature freezing on the overall process. The yields of citrus peels from the five different pretreatment methods were calculated and the results are shown in Table 1.

[0101] Table 1

[0102] Essential oil yield of cold pressing process in Example 1 and Comparative Examples 1-4

[0103]

[0104] As shown in Table 1, comparing the cold-pressing yields of five different pretreatment methods, the cold-pressing yield of Example 1 after ultra-low temperature freezing and microwave treatment was significantly higher than that of the other four cold-pressing processes. Compared with Comparative Example 1, which was cold-pressed without microwave treatment, the microwave-assisted cold-pressing yield of Example 1 was significantly increased, reaching up to 2.3 times the yield of direct cold pressing in Comparative Example 1; compared with Comparative Example 4, which was not subjected to ultra-low temperature freezing treatment, the cold-pressing yield of Example 1 after ultra-low temperature freezing treatment was significantly increased, reaching up to 2.35 times the essential oil yield of Comparative Example 4.

[0105] The only difference between Examples 1-3 and Comparative Examples 5-6 is the processing power of the microwave treatment. The cold-pressed oil yield of the citrus peel is shown in Table 2.

[0106] Table 2

[0107] Comparison of yields of cold-pressed essential oils from citrus peels at different processing power levels

[0108]

[0109] As shown in Table 2, the yield of citrus essential oil at microwave treatments of 60W and 100W was significantly lower than that at 70W, 80W, or 90W. Furthermore, the yield gradually decreased after reaching 90W, possibly because the high-frequency microwaves caused the sample to heat up, leading to essential oil volatilization and a decrease in overall essential oil content. Therefore, it can be concluded that the 70-90W microwave treatment power is one of the important factors contributing to the high yield of the citrus peel cold-pressing process provided in this application.

[0110] The only difference between Examples 1, 4, 5 and Comparative Examples 7-9 is the processing time of microwave treatment. The cold-pressed oil yield of their citrus peels is shown in Table 3.

[0111] Table 3

[0112] Comparison of yields of cold-pressed essential oils from citrus peels at different processing times

[0113]

[0114] As shown in Table 3, the yield of citrus essential oil after microwave treatment for 5 min, 7.5 min, or 17.5 min was significantly lower than that after 10 min, 12.5 min, or 15 min. The yield gradually decreased after 15 min, possibly due to the natural dissipation of volatile compounds that had already been extracted during prolonged treatment, leading to a decrease in the overall essential oil content. Therefore, it can be concluded that a microwave treatment time of 10–15 min is one of the important factors contributing to the high yield of the citrus peel cold-pressing process provided by this invention.

[0115] The only difference between Example 1 and Comparative Example 10 is the size of the citrus peel powder. In Example 1, the citrus peel was powdered into blocks, while in Comparative Example 10, the citrus peel was powdered into a paste. The cold-pressed oil yield of the citrus peel is shown in Table 4.

[0116] Table 4

[0117] Comparison of yields of cold-pressed essential oils from citrus peels of different grades

[0118]

[0119] As shown in Table 4, the yield of citrus essential oil obtained by microwave cold pressing in Example 1, where the peel was processed into blocks, was significantly higher than that of citrus essential oil obtained in Comparative Example 10, where the peel was crushed into a paste. This may be because the oil cells rupture during the crushing process, leading to the absorption of essential oil by the white pith and subsequent loss of the oil. Furthermore, the increased machine temperature causes the essential oil to volatilize, resulting in a lower yield. Therefore, it can be concluded that crushing the peel into blocks is one of the factors contributing to the higher yield of the citrus cold pressing process provided by this invention.

[0120] Comparative Examples 1-3 and Example 1 were subjected to qualitative and quantitative analysis using gas chromatography-mass spectrometry (GC-MS) combined with internal standard method. The types and contents of aroma components of cold-pressed products with different pretreatment methods were determined by retention index calculation, NIST23 database search, standard comparison and spectral analysis. The analysis results are recorded in Table 4.

[0121] Table 4

[0122] GC-MS analysis of aroma compounds in citrus essential oils extracted using four cold-pressing processes

[0123]

[0124]

[0125] As shown in Table 4, the GC-MS analysis results indicate that terpenoids are the main aroma components of citrus essential oils extracted using the four cold-pressing processes. Figure 2 As shown in the chromatograms, the concentration of D-limonene was significantly higher than that of other compounds, making it a key substance determining the basic aroma of citrus essential oils; for example... Figure 3 As shown, magnifying the vertical axis of the chromatogram of Example 1 reveals that, in addition to D-limonene, terpenes such as α-pinene (333.06~381.61 mg / kg), β-phellandrene (215.63~275.33 mg / kg), myrcene (1187.93~1230.63 mg / kg), and juniperene (104.65~199.75 mg / kg) are also common auxiliary aroma components in all examples and have high concentrations, further enriching the terpene aroma profile of the essential oil.

[0126] Regarding the diversity of aroma compounds, Comparative Example 1 contained 50 aroma compounds, Comparative Example 2 contained 37 aroma compounds, Comparative Example 3 contained 51 aroma compounds, and Example 1 contained 51 aroma compounds. Among them, Comparative Example 2 had the fewest aroma compounds, possibly due to aroma loss caused by prolonged soaking in lime water; Comparative Example 3 showed better overall performance and a more complete range of aroma compounds; Example 1 had a high content and a complete range of core terpenoid aroma compounds. In summary, the microwave-assisted pretreatment method in Example 1 not only yielded a high yield but also produced citrus essential oil with good aroma quality.

[0127] Those skilled in the art should understand that variations can be implemented by combining existing technology with the above embodiments, which will not be elaborated here. Such variations do not affect the essence of the present invention, and will not be elaborated here either.

[0128] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and the devices and structures not described in detail should be understood as being implemented in a conventional manner in the art. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the present invention. This does not affect the essential content of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention's technical solutions still fall within the protection scope of the present invention.

Claims

1. A process for microwave assisted cold pressing extraction of citrus essential oils, characterized in that, The process comprises the following steps: (1) separating the citrus peel from the fruit, retaining the peel part, and crushing the citrus peel into pieces; (2) subjecting the citrus peel pieces to ultra-low temperature freezing treatment, the temperature of the ultra-low temperature freezing treatment being -60 to -100℃, and the time being 12 to 36 hours; (3) subjecting the citrus peel pieces subjected to the ultra-low temperature freezing treatment to low-energy microwave treatment, the microwave power of the low-energy microwave treatment being 70 to 90 W, and the time being 10 to 15 minutes; (4) subjecting the citrus peel pieces subjected to the low-energy microwave treatment to cold pressing and collecting the extraction liquid; (5) subjecting the extraction liquid to centrifugal treatment; (6) collecting the oil-like liquid on the upper layer of the extraction liquid after centrifugal treatment and subjecting the oil-like liquid to dehydration treatment, thereby obtaining the citrus essential oil.

2. A process for microwave assisted cold pressing extraction of citrus essential oil as claimed in claim 1 wherein, Before the citrus peel is separated from the fruit, fresh, non-rotten, non-broken and non-coated citrus fruits are screened, washed and naturally dried.

3. A process for microwave assisted cold pressing extraction of citrus oils as claimed in claim 1 wherein, In step (1), the citrus peel is crushed into pieces of 2 to 4 cm in size by using a blender under the conditions of 40 to 60 Hz and 250 to 350 W for 2 to 4 seconds.

4. A process for microwave assisted cold expression extraction of citrus oils as claimed in claim 1, wherein, In step (4), the cold pressing is performed in a spiral extrusion type cold pressing instrument, and the citrus peel pieces subjected to the low-energy microwave treatment are allowed to stand at room temperature for 5 to 10 minutes before the cold pressing, and the temperature of the extrusion chamber during the cold pressing is not higher than 45℃.

5. A process for microwave assisted cold pressing extraction of citrus oils as claimed in claim 1 wherein, In step (5), the centrifugal treatment is performed at a speed of 8000 to 10000 rpm and a temperature of 2 to 6℃ for 10 to 20 minutes.

6. A process for microwave assisted cold pressing extraction of citrus oils as claimed in claim 1 wherein, In step (6), the dehydration treatment is performed by using anhydrous sodium sulfate.

7. A process for microwave assisted cold expression extraction of citrus oils as claimed in claim 6 wherein, The citrus essential oil is obtained by filtering after the dehydration treatment and standing for 5 to 10 hours.

8. A process for microwave assisted cold expression extraction of citrus oils as claimed in claim 1, wherein, The temperature of the sample after the low-energy microwave treatment is not higher than 10℃.

9. The citrus essential oil obtained by the process according to any one of claims 1 to 8.

10. The citrus essential oil according to claim 9 for use in medicine, perfume, food and cosmetics.

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