Walnut oil and low-temperature squeezing method thereof

Abstract

The invention discloses walnut oil and a low-temperature squeezing method thereof, and aims to solve the technical problems of nutrient loss, solvent residue and environmental pollution in the existing hot squeezing method and leaching method. The method sequentially comprises the steps of raw material pretreatment, crushing, low-temperature squeezing, rough filtration, physical refining and filling storage. Walnuts are strictly screened in raw material pretreatment, and the raw material quality is ensured through multiple procedures; the low-temperature squeezing temperature is controlled to be less than or equal to 24 DEG C; the physical refining adopts film evaporation to avoid nutrition loss and pollution; high-purity nitrogen is used for deoxidation in filling storage, low-temperature storage is achieved, and the shelf life is prolonged. The obtained walnut oil is light yellow to golden yellow in color and luster and has the faint scent of walnuts, the acid value is smaller than or equal to 1.0, the peroxide value is smaller than or equal to 0.25 g / 100g, the shelf life is larger than or equal to 18 months, the walnut oil is rich in nutritional ingredients such as vitamin E and squalene, and the content of alpha-linolenic acid is larger than or equal to 9.54%.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and fat food processing, and particularly to a walnut oil and a low-temperature pressing method thereof. Background Art

[0002] The existing walnut oil production processes mostly adopt hot pressing or leaching methods.

[0003] Leaching method: Using an organic solvent (such as n-hexane) to dissolve the oil, and separating the solvent through evaporation and steam stripping. Process flow: 1) Pretreatment: Pre-pressing: First extract part of the oil through cold pressing or hot pressing, and the residual oil rate of the cake is 7%-9%. Crushing: Crush the cake to 40-60 mesh to increase the solvent contact area.

[0004] 2) Leaching: Solvent: n-hexane (boiling point 68.7°C), mixed with the cake in a ratio of 1:1.5 - 2. Temperature: 45 - 55°C to keep the solvent in a liquid state. Time: 60 - 120 minutes to fully dissolve the oil.

[0005] 3) Solvent recovery: Evaporation: Heat the mixed oil to 100°C to evaporate 90% of the solvent. Steam stripping: Pass the remaining mixed oil through direct steam to remove the residual solvent ≤50 mg / kg.

[0006] 4) Refining: It is necessary to degum, deacidify, decolorize, and deodorize, and the solvent residue in the refined oil ≤10 mg / kg (national standard requirement).

[0007] Core principle of the hot pressing method: Soften the walnut kernel structure by preheating (60 - 120°C) to improve the oil yield, but it will damage some heat-sensitive nutrients.

[0008] Process flow: 1) Raw material pretreatment: Screw press or hydraulic press, but adding a steaming and frying link: Steaming and frying temperature: 80 - 120°C, denaturing the protein, reducing the oil viscosity, possibly generating benzopyrene, and increasing the peroxide value. 2) High-temperature pressing: Pressing temperature: 80 - 120°C (some equipment can reach 150°C).

[0009] Pressure control: Raise the pressure to 10 - 15 MPa at one time to shorten the pressing time.

[0010] Oil yield: 40% - 45%, residual oil rate reduced to 7%. 3) Refining: It needs to be refined by alkali refining to remove free fatty acids, decolorize, and deodorize, which may cause about vitamin loss.

[0011] Product features: Nutrition: High temperature causes partial oxidation of unsaturated fatty acids, and the smoke point is increased to 180 °C. Sensory: The color ranges from dark yellow to brownish, with a strong flavor, suitable for cooking. Shelf life: 6 - 12 months (stored at room temperature). Product features: Nutrition: The refining process results in about [X]% loss of vitamins and may produce trans fatty acids. Sensory: The color is light yellow, with a light flavor, and the smoke point is above 200 °C.

[0012] Cost: The oil yield is as high as 98%, but the equipment investment and solvent recovery costs are high.

[0013] Although the hot pressing method has a relatively high oil yield, high temperature is likely to damage heat-sensitive nutrients in walnut oil, such as vitamin E, squalene, etc., and may also produce harmful substances such as benzo[a]pyrene, increasing the risk of oil rancidity.

[0014] The leaching method uses chemical solvents to extract oil. Although the oil yield is high, there is a risk of solvent residue, the refining process is complex, it is easy to cause loss of nutritional components, and the solvent recovery cost is high and it pollutes the environment. The low-temperature pressing method can effectively avoid these problems, but there is a lack of mature and standardized low-temperature pressing processes in the industry. Summary of the Invention

[0015] This application aims to solve the technical problems raised in the background art and provides a walnut oil and its low-temperature pressing method.

[0016] The technical solution adopted in the present invention is as follows: In the first aspect, a low-temperature pressing method for walnut oil includes the following steps: Screen, peel, wash, dry, air-screen, screen and shell the walnuts to obtain kernels; Crush the walnut kernels into particles with a particle size of 5 - 10 mm; use low-temperature pressing equipment, control the pressing temperature ≤ 24 °C, increase the pressure in stages to 5 MPa to extract oil, and then gradually increase it to 15 MPa for discharging; at the initial stage of pressing, first increase the pressure to 5 MPa at a slower speed, which takes about 1 - 2 minutes, to allow the walnut kernels to gradually adapt to the pressure change and make the oil start to flow out slowly; subsequently, according to the oil extraction situation and the equipment operation status, gradually increase the pressure to 15 MPa at a rate of 1 - 2 MPa per minute, and the entire pressure increase process is controlled within about 5 - 10 minutes.

[0017] The entire implementation process is controlled by a PLC, the oil output is obtained through a liquid level sensor and a flow rate sensor, and the overall control is digitally visualized. At each pressure stage, a certain pressure stabilization time should be maintained. Stabilize the pressure at 5 MPa for 3 - 5 minutes to allow the oil to flow out fully, and then continue to increase the pressure. The obtained walnut oil is successively filtered through a coarse filter, physically refined, and filled and stored. The physical refining process includes precipitation, low-temperature dehydration and physical deacidification technologies, including thin-film evaporation; For filling and storage, nitrogen with a purity of ≥98% is filled into the pipeline and storage tank for deoxidization and low-temperature storage, and floating substances are filtered by a high-pressure plate-frame paper filter before automatic filling is completed.

[0018] Preferably, in the raw material pretreatment step, walnuts are strictly screened to remove mildewed and insect-infested walnuts to ensure the quality of the raw materials.

[0019] Preferably, the oil yield during the pressing process is controlled at 30%-40%, and the residual oil rate is 12%-18%.

[0020] Preferably, during the filling and storage process, the storage temperature is 0℃-10℃, and the relative humidity ≤60%.

[0021] The walnut oil produced by the low-temperature pressing production process of walnut oil provided in the first aspect of the present invention has the following quality indicators: The color is light yellow to golden yellow; the smell has the fragrance of walnuts; the acid value KOH ≤1.0; the peroxide value (g / 100g) ≤0.25; the shelf life ≥18 months.

[0022] 6. The walnut oil according to claim 5, characterized in that: the walnut oil is rich in nutritional components such as vitamin E and squalene, and among them, the content of α-linolenic acid can reach ≥9.54%.

[0023] Compared with the prior art, the beneficial effects are as follows: The walnut oil produced by this process has the following advantages: Good nutrition retention: The low-temperature pressing and physical refining technologies enable the retention of nutritional components such as vitamin E and squalene in the walnut oil, and the content of α-linolenic acid can reach ≥9.54%.

[0024] High quality: The color is light yellow to golden yellow, with the fragrance of walnuts, the acid value KOH ≤1.0, the peroxide value ≤0.25 g / 100g), and the shelf life ≥18 months (requires light-proof refrigeration).

[0025] No pollutants: No auxiliary agents, additives and other substances need to be added during the whole production process, there is no chemical solvent pollution, and it is safe and environmentally friendly.

[0026] Strong market competitiveness: The product has high quality and is rich in nutrition, which can meet the needs of the high-end edible oil market and the infant food market. Specific embodiments

[0027] In the following description, specific details such as specific system structures and technologies are presented for the purpose of illustration rather than limitation, so as to thoroughly understand the embodiments of the present application. However, those skilled in the art should be clear that the present application can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known methods are omitted to avoid unnecessary details from interfering with the description of the present application.

[0028] This embodiment provides a low-temperature pressing method for walnut oil, including the following steps: Raw material pretreatment: Screen, peel, wash, dry, air-screen, screen and shell the walnuts to remove mildewed and insect-infested walnuts and ensure the quality of the raw materials.

[0029] Crushing: Crush the walnut kernels into particles with a particle size of 5 - 10 mm to increase the contact area with the pressing chamber.

[0030] Raw material pretreatment Screening method and equipment: In actual production, a multi-layer vibrating screen can be used to screen walnuts. Through the action of sieves with different mesh sizes and vibration force, this equipment separates unqualified walnuts such as mildewed and insect-infested ones. For example, using a sieve with a pore size of 10 mm, first pour the walnuts into the feed inlet. As the vibrating screen swings, the qualified walnuts will move forward along the sieve, while the smaller mildewed and insect-infested walnuts will fall through the holes of the sieve, thus achieving preliminary screening.

[0031] Peeling treatment: For the peeling process, there are special walnut peeling machines on the market currently. Its working principle is generally to remove the green peel of walnuts through mechanical friction and extrusion. When using the peeling machine, parameters such as rotation speed and pressure need to be adjusted according to the size of the walnuts and the thickness of the green peel. For example, for medium-sized walnuts, the rotation speed of the peeling machine can be set to 300 - 500 r / min, and the pressure can be adjusted to an appropriate level to ensure that the green peel can be effectively removed while avoiding excessive damage to the walnut kernels. After peeling, the walnuts also need to be manually selected to further remove the remaining green peel and damaged walnuts.

[0032] Washing and drying parameters: When washing walnuts, softened water that meets food processing requirements should be used to reduce the impact of impurities and hardness in the water on the walnuts. The washing time can be determined according to the degree of contamination of the walnuts, generally 10 - 15 minutes. During the drying process, in addition to controlling the temperature at 40°C - 60°C, attention also needs to be paid to the drying time. For example, when the drying temperature is 50°C, the drying time can be controlled within 2 - 3 hours. During this period, the walnuts need to be regularly turned over to ensure uniform drying and reduce the moisture content of the walnuts to an appropriate range (generally 5% - 8%).

[0033] Low-temperature pressing: Use low-temperature pressing equipment, control the pressing temperature ≤ 24°C, increase the pressure in stages. First, increase the pressure to 5 MPa to extract oil, and then gradually increase it to 15 MPa to discharge the material, obtaining walnut oil.

[0034] The low-temperature pressing equipment can be modified to achieve low-temperature pressing. During the pressing process, the increase in pressure should follow the principle of gradual progress. For example, at the initial stage of pressing, first increase the pressure to 5 MPa at a slower speed, which takes about 1 - 2 minutes, allowing the walnut kernels to gradually adapt to the pressure change and enabling the oil to start flowing out slowly; subsequently, according to the oil extraction situation and the equipment operation status, gradually increase the pressure to 15 MPa at a rate of 1 - 2 MPa per minute, and control the entire pressure increase process within about 5 - 10 minutes. At each pressure stage, a certain pressure stabilization time should be maintained. For example, stabilize the pressure at 5 MPa for 3 - 5 minutes to allow the oil to flow out fully, and then continue to increase the pressure. This can improve the oil extraction rate and avoid adverse effects on the equipment and the quality of walnut oil due to too rapid pressure changes.

[0035] This process can retain the nutritional components in walnut oil and avoid damage to the oilseed structure.

[0036] To ensure the stability of the oil extraction rate and the residual oil rate, first, strict control should be exercised over the raw materials to ensure that the quality and characteristics of each batch of walnuts are relatively consistent. During the production process, the pressing equipment should be regularly maintained and calibrated, and check whether all components of the equipment are operating normally, such as the wear condition of the screw shaft and the gap of the pressing chamber. Once problems are found, repair or replace them in a timely manner. At the same time, use advanced on-line detection technologies, such as near-infrared spectrometers, to monitor the walnut oil and cake during the pressing process in real time, obtain the oil content information therein, and adjust the pressing process parameters, such as pressure, temperature, and feeding speed, in a timely manner according to the detection results to achieve effective control of the oil extraction rate and the residual oil rate. Coarse filtration, refining, filling and storage: Coarse filtration: Remove solid impurities through separation tanks and pressure plate frame filtration and separation precipitation.

[0037] Physical refining: Adopt precipitation low-temperature dehydration and physical deacidification technologies, including thin-film evaporation, to prevent acidification, emulsification, and alkalization, and avoid nutritional loss caused by refining.

[0038] During the thin-film evaporation process, the evaporation temperature is generally controlled at 65°C - 95°C, the vacuum degree should reach -0.09 MPa to -0.1 MPa, the scraper rotation speed is 200 - 400 r / min, and the feeding speed is adjusted according to the flow rate of walnut oil to ensure the formation of a uniform thin film. For example, when the processing volume is small, the feeding speed can be controlled at 1 - 2 L / h, and the scraper rotation speed is selected to be about 300 r / min. This can improve the evaporation efficiency and avoid problems such as oxidation caused by the excessive residence time of the oil in the equipment. By precisely controlling these parameters, low-molecular-weight acidic substances, impurities, and harmful components in walnut oil can be effectively removed, improving the quality and stability of walnut oil.

[0039] Monitoring the changes in nutritional components during the refining process: During the physical refining process, various analytical methods can be used to monitor the changes in nutritional components in walnut oil. For example, high-performance liquid chromatography (HPLC) is used to determine the content change of vitamin E, gas chromatography (GC) is used to analyze the retention of unsaturated fatty acids such as α-linolenic acid, and ultraviolet-visible spectrophotometer is used to detect the content of squalene, etc. At the key nodes of the refining process, samples are taken for nutritional component analysis before and after thin-film evaporation respectively, and corresponding quality control standards and warning mechanisms are established. Once it is found that the content of nutritional components is lower than the specified standard, the refining process parameters should be adjusted in time or the equipment operation status should be checked to ensure the effective retention of nutritional components in walnut oil.

[0040] Filling and storage: Nitrogen with a purity of ≥98% is used to fill the pipeline and storage tank for deoxidation and low-temperature storage. A high-pressure plate-frame paper filter is used to filter floating substances, and automatic filling is completed after nitrogen filling the tank. The storage temperature is 0°C - 10°C, and the relative humidity ≤60%.

[0041] Low temperature I) Equipment requirements Low-temperature pressing equipment: The equipment needs to have precise temperature control and pressure regulation functions to ensure that the pressing temperature is stable at ≤24°C, and the pressure can be increased in stages to 5 MPa and 15 MPa. Equipment suitable for low-temperature pressing of walnut oil includes screw oil presses and hydraulic oil presses, etc. Taking the screw oil press as an example, its transformation methods can include optimizing the structure of the pressing chamber, installing an efficient cooling system and precise temperature sensors, etc. By improving the pitch and diameter of the pressing chamber, the walnuts are more evenly stressed during the pressing process, reducing the temperature rise caused by excessive local pressure; the cooling system can use circulating water cooling or air cooling methods to ensure that the pressing temperature is stable at ≤24°C; the installation positions of the temperature sensors should be reasonably distributed throughout the pressing chamber to monitor the temperature changes in real time and feed the data back to the automatic control system to achieve precise temperature control and pressure regulation.

[0042] Refining equipment: Equipped with an efficient thin-film evaporator for physical deacidification treatment.

[0043] Filling and storage equipment: Filling equipment with nitrogen filling function, and the storage tank should have good sealing and heat preservation performance.

[0044] II) Detailed process description Raw material pretreatment: Screening: Use a screening machine to remove moldy, insect-eaten and other unqualified walnuts.

[0045] Peeling: Use a peeling machine to remove the green skin of walnuts to reduce impurities and odors.

[0046] Cleaning: Thoroughly clean walnuts in a cleaning pool to remove impurities such as sediment and dust on the surface.

[0047] Drying: Put the cleaned walnuts into a dryer, control the drying temperature at 40°C - 60°C, and reduce the moisture content of walnuts to an appropriate range.

[0048] Air separation and screening: Further remove impurities and small particle walnuts in walnuts through an air separator and a screening machine.

[0049] Shelling and kernel extraction: Use a walnut shelling machine to completely extract the walnut kernels.

[0050] Crushing: Put the walnut kernels into a crusher, adjust the parameters of the crusher, and crush the walnut kernels into particles with a particle size of 5 - 10 mm.

[0051] Low-temperature pressing: Send the crushed walnut kernel particles into the low-temperature pressing equipment, start the equipment, and control the pressing temperature at ≤24°C.

[0052] In the initial stage of pressing, the pressure is increased to 5 MPa to gradually let out the walnut oil; as the pressing process progresses, the pressure is gradually increased to 15 MPa to discharge the remaining oil materials.

[0053] Coarse filtration, refining, filling and storage: Coarse filtration: Send the walnut oil obtained by pressing into a separation tank for preliminary precipitation, and then remove solid impurities through pressure plate-frame filtration separation precipitation.

[0054] Physical refining: The coarsely filtered walnut oil is successively subjected to precipitation and low-temperature dehydration treatment to remove moisture; then, thin-film evaporation technology is used to evaporate low-molecular acidic substances in the oil under the conditions of low temperature and high vacuum; finally, through thin-film evaporation, the effective components in the oil are further separated and purified.

[0055] Filling and storage: The refined walnut oil is sent to the filling workshop. First, nitrogen with a purity of ≥98% is filled into the pipeline and storage tank for deoxidation and low-temperature storage. Then, a high-pressure plate-frame paper filter is used to filter floating substances, and finally, automatic filling is carried out. The storage conditions are a temperature of 0°C - 10°C and a relative humidity of ≤60%.

[0056] (III) Key points of quality control Raw material inspection: Strictly control the quality of walnut raw materials to ensure that their quality meets the requirements. The detection indicators include appearance, taste, moisture content, degree of rancidity, etc.

[0057] Process monitoring: During the pressing and refining processes, key parameters such as temperature and pressure are monitored in real time to ensure the stability and controllability of the process conditions. Advanced sensors and automated control systems are used to monitor and adjust the operating status of the equipment in real time.

[0058] Finished product inspection: Comprehensive quality inspections are carried out on the produced walnut oil, including analysis of nutritional components, detection of pollutants, sensory evaluation, etc. The detection indicators cover color, smell, acid value, peroxide value, vitamin E, squalene, α-linolenic acid, etc.

[0059] (IV) Comparative examples Note: Low-temperature pressing method: Through low-temperature pressing technology, nutritional components in walnut oil such as vitamin E and squalene are retained, and there is no residue of pollutants. It is suitable for high-end edible oils and infant foods.

[0060] Hot pressing method: Although the oil yield is relatively high, due to high-temperature treatment, some heat-sensitive nutritional components will be damaged, and there may be residues of auxiliaries. It is suitable for ordinary edible oils and cooking oils.

[0061] Leaching method: It has the highest oil yield, but chemical solvents are used for extraction, there is a risk of solvent pollution, and a large amount of nutritional components are lost during the refining process. It is suitable for large-scale oil production and industrial oils.

[0062] Through the above comparison, it can be seen that the low-temperature pressing method has obvious advantages in terms of nutritional retention, environmental protection, and product quality, and is suitable for the production of high-quality walnut oil.

[0063] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included within the protection scope of the present application.

Claims

1. A low-temperature pressing method for walnut oil, the whole implementation process is controlled by a PLC, the oil output is obtained through a liquid level sensor and a flow rate sensor, and the overall control is digitally visualized for management. It is characterized in that, It includes the following steps: Screen, peel, wash, dry, winnow, screen and shell walnuts to obtain kernels; Crush the walnut kernels into particles with a particle size of 5-10 mm; Use low-temperature pressing equipment, control the pressing temperature ≤ 24°C, increase the pressure in stages to 5 MPa to extract oil, and then gradually increase it to 15 MPa for discharging. Gradually increase the pressure to 15 MPa at a rate of 1.2 MPa per minute, and control the entire pressure increase process within 10 minutes. Maintain the pressure at 10-15 MPa for 15 minutes.

2. The obtained walnut oil is successively subjected to coarse filtration, physical refining and filling and storage, The physical refining process includes precipitation, low-temperature dehydration and physical deacidification techniques, including thin-film evaporation; For the filling and storage, nitrogen with a purity ≥ 98% is filled into the pipeline and storage tank for deoxidation and low-temperature storage, and the floating substances are filtered by a high-pressure plate-frame paper filter and then automatic filling is completed.

3. The low-temperature pressing method of walnut oil according to claim 1, characterized in that: In the raw material pretreatment step, walnuts are strictly screened to remove mildewed and insect-infested walnuts to ensure the quality of the raw materials.

4. The low-temperature pressing method of walnut oil according to claim 1, characterized in that: Control the oil yield during the pressing process to be 30%-40%, and the residual oil rate to be 12%-18%.

5. The low-temperature pressing method of walnut oil according to claim 1, characterized in that: During the filling and storage process, the storage temperature is 0°C-10°C, and the relative humidity ≤ 60%.

6. The walnut oil obtained by the low-temperature pressing method according to any one of claims 1-4, characterized in that: The walnut oil has the following quality indicators: light yellow to golden in color; with a walnut fragrance; acid value KOH ≤ 1.0; peroxide value ≤ 0.25 g / 100 g; shelf life ≥ 18 months.

7. The walnut oil according to claim 5, characterized in that: The walnut oil is rich in nutrients such as vitamin E and squalene. Among them, the content of α-linolenic acid can reach ≥ 9.54%.

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