Stress relaxation microchannel guided low-fat high-oxidation-stability nut preparation method

Abstract

The invention relates to a food processing technology, and discloses a stress relaxation microchannel guided low-fat high-oxidation-stability nut preparation method. The method comprises the following steps: adjusting the content of nut meat in an aqueous solution containing an antioxidant to 8-12% (dry basis), and then carrying out phase change induced stress treatment to form a micro-fracture network communicated with the outer surface in the interior; then increasing the pressure to 3 to 10 MPa at the speed of 1.0 to 2.5 MPa / min, and maintaining the pressure, wherein the hot / trap is 1.5 to 1.8; and rapidly relieving pressure and discharging residual oil at (2.5-3.5) v1, and rapidly shaping by adopting high-energy drying for 2-10 minutes by adopting radio frequency / infrared / microwave vacuum and the like. According to the method, the degreasing efficiency and the size stabilization rate are improved under the low pressure intensity, surface residual oil is reduced, flavoring is promoted, and the storage oxidation stability is improved.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, and in particular to a method for preparing low-fat, highly oxidatively stable nuts guided by stress relaxation microchannels. Background Technology

[0002] Nuts and nut products are widely consumed for their flavor and nutritional value, but nut kernels typically have a high fat content, resulting in a high energy density. To meet the demand for low-fat products, existing technologies often employ methods such as mechanical pressing, solvent extraction, or supercritical fluid extraction to remove some of the oil from nuts. Compared to chemical extraction processes that require solvent recovery and residue control, mechanical pressing does not introduce organic solvents, thus offering advantages in terms of food safety and process acceptability. However, when targeting the "whole nut" product form, mechanical pressing often faces the dilemma of balancing degreasing efficiency with tissue integrity.

[0003] Specifically, during the pressing process, oil needs to migrate from the interior of the nut tissue to the exterior and be discharged under external force. To increase oil yield, the process usually involves increasing pressure or extending the pressing time. However, as compression deformation intensifies, the porosity of the nut tissue decreases, the seepage channels shrink, and the resistance to oil migration increases, leading to a decrease in oil discharge efficiency in the later stages of pressing. Simultaneously, higher pressure can also cause tissue collapse, crushing and breakage, textural hardening, and surface oil adhesion. Furthermore, the retention of residual oil at the pore outlet can lead to localized blockage, making the oil migration channels more prone to closure in the later stages. This not only affects the consistency of the defatting endpoint but also results in a greasy product surface, slow penetration of subsequent seasoning liquids, and uneven flavoring.

[0004] Furthermore, defatting nuts increases their exposed surface area, which may lead to a decrease in their oxidative stability during storage.

[0005] On the other hand, while solvent extraction and supercritical fluid extraction can achieve high degreasing rates, they are usually accompanied by increased equipment investment, energy consumption, and process complexity, and may introduce solvent residue control or regulatory compliance pressures. Therefore, there is an urgent need for a low-fat preparation method suitable for whole nuts: one that can maintain effective and interconnected oil drainage channels under lower pressure conditions, inhibiting channel closure and outlet blockage, thereby achieving a balance of high degreasing efficiency, high whole-nut yield, good oxidative stability, and good taste, while providing a favorable pore structure foundation for subsequent rapid flavor infusion. Summary of the Invention

[0006] Technical problems to be solved To address the problems existing in the mechanical degreasing process of whole nuts, such as easy breakage under high pressure, structural damage leading to residual oil oxidation, decreased oil discharge efficiency in the later stages of pressing, residual oil retention at the outlet of the duct resulting in greasy surface, and slow flavor absorption, the present invention aims to provide a method for preparing low-fat, highly oxidatively stable nuts. This method enables nuts to form stable oil migration channels under lower pressure and achieve high degreasing efficiency, while maintaining a high whole-nut ratio, good oxidative stability, and good texture and taste, and promoting rapid penetration and even flavor absorption of subsequent seasonings.

[0007] Technical solution To achieve the above objectives, this invention provides a method for preparing low-fat, highly oxidatively stable nuts, comprising the following steps S1 to S5; wherein, S1 provides the hydrated state required for phase transition induction in S2 and introduces antioxidant components; S2 provides the microchannel basis for directional migration in S3; S3 and S4 together inhibit channel closure and outlet blockage; and S5 is used to complete structural locking before secondary closure of the tissue, thereby forming a closed-loop process. S1 Moisture content control: The moisture content of the nut kernels is adjusted to 8% to 12% (dry basis) through heat pretreatment and conditioning treatment, so that the nut tissue has appropriate plasticity and elastic recovery ability in subsequent processing, and provides the necessary moisture conditions for phase change induced stress, while improving the oxidative stability of the final product.

[0008] S2 Phase Change Induced Stress Treatment and Microchannel Construction: Phase change induced stress treatment is performed in the water-containing state obtained in S1, so that the water inside the nut kernel undergoes at least one solid-liquid phase change temperature cycle, thereby forming microcracks inside the tissue and expanding into a microcrack network that connects with the outer surface; thus, the migration of oil is "directed" by microchannels from the inside to the outside, reducing the dependence of subsequent pressing on high pressure.

[0009] S3 Low-pressure boost-holding coupling and stress relaxation window: Boost-holding coupling control is implemented within a predetermined low-pressure range (3–10 MPa). The boost rate during the boost phase is v1, and the boost time is t. ramp After reaching the target pressure, the pressure holding stage begins, with a holding time of t. hold and satisfy t hold / t ramp =1.5~1.8. By setting a stress relaxation window, the oil is continuously migrated and discharged along the microchannels formed by S2 under the combined action of pressure drive and tissue viscoelastic relaxation, while reducing irreversible collapse and promoting elastic recovery after pressing.

[0010] S4 Rapid Depressurization and Oil Removal at the Outlet: After completing S3, rapid depressurization is performed at a depressurization rate of v2, where v2 is 2.5 to 3.5 times that of v1. Rapid depressurization reduces the residence time of residual oil at the outlet of the short channel, decreases the probability of residual oil adhesion and blockage, and helps maintain the openness of the channel during the depressurization rebound process, creating conditions for the subsequent entry of flavoring liquid into the tissue.

[0011] S5 High-energy physical drying for rapid shaping: High-energy physical drying is used to rapidly shape the nuts after depressurization in S4, so that they reach the target moisture content and complete the textural shaping before the secondary closure of the tissue. This locks the interconnected pore structure formed by S2 to S4, reduces the risk of pore shrinkage / closure caused by subsequent rehydration, and improves the stability of the production cycle.

[0012] Beneficial effects Compared with the prior art, the present invention has at least the following beneficial effects, and these effects are achieved by the synergistic effect of the above steps: By controlling the water content in S1 and inducing stress through phase change in S2, this invention pre-constructs a microcrack network connected to the outer surface before pressing, allowing oil migration to be guided by microchannels from the inside out. This enables the formation of an effective oil discharge path even at lower pressures, improving degreasing efficiency per unit pressure and reducing the risk of breakage and hardening caused by increased pressure.

[0013] Set t via S3 hold / t ramp With a stress relaxation window of 1.5 to 1.8, this invention achieves time-scale matching between oil permeation and tissue viscoelastic relaxation during the oil discharge process. This ensures oil discharge while inhibiting channel collapse and closure, thereby improving the whole grain rate and volume recovery level, reducing textural degradation, and alleviating the decline in oil discharge efficiency in the later stages of pressing.

[0014] By employing a rapid depressurization strategy in S4 where v2 is 2.5 to 3.5 times that of v1, this invention can reduce residual oil retention and blockage at the channel outlet, and alleviate surface greasiness. At the same time, keeping the channel open helps the seasoning liquid penetrate into the tissue more quickly, thereby shortening the flavoring time and improving flavor uniformity.

[0015] By using S5 high-energy physical drying for rapid shaping, this invention can lock the interconnected pore structure and textural features before the pore structure closes for the second time. This helps retain antioxidant components inside the nut kernel and inhibits oil oxidation, increases production cycle time, and reduces the risk of pore shrinkage / closure caused by moisture regain, thereby improving the consistency of product quality. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process for preparing low-fat, highly oxidatively stable nuts using a stress relaxation microchannel guided by the present invention.

[0017] Figure 2 This is a schematic diagram of the pressure-time curve for pressurization-pressure holding-rapid pressure release.

[0018] Figure 3 This is a schematic diagram of the implementation process of freeze-microwave phase transition stress treatment.

[0019] Figure 4 This is a schematic diagram of the pressing chamber and the rapid pressure relief and oil discharge structure.

[0020] Reference numerals in the attached diagram: 1-pressing chamber; 2-piston; 3-pressing disc; 4-filter structure; 5-oil drain groove; 6-oil drain port; 7-quick drain valve; 8-throttle valve; 9-oil collection container; 10-pressure sensor; 11-sealing ring. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, all percentages in this application are by mass percentages; moisture content is on a dry basis; and oil content is crude fat content on a dry basis.

[0022] Terminology Definitions and Measurement Methods Pressure increase rate v1: The amount of pressure increase in the pressing chamber per unit time, in MPa / min; Pressure release rate v2: The amount of pressure decrease in the pressing chamber per unit time, in MPa / min.

[0023] boost time t ramp The time required for the pressure to rise from atmospheric pressure to the target pressure; the pressure holding time t. hold The duration during which the pressure remains at the target pressure (within the allowable fluctuation range).

[0024] Low-pressure range: In order to balance the granulation rate and degreasing efficiency, the low-pressure range in this application preferably refers to a target pressure of no more than 10 MPa (e.g., 3 to 10 MPa).

[0025] Secondary closure: refers to the phenomenon that microcracks / channels shrink and stick together during cooling, rehydration, or slow drying after pressure relief, resulting in decreased connectivity.

[0026] Moisture content (dry basis) x db The moisture content was determined according to the direct drying method in GB 5009.3-2016 "National Food Safety Standard - Determination of Moisture in Food". The sample mass m0 was weighed and dried at 101–105℃ until constant weight (the difference between two weighings should not exceed 2 mg), yielding the dried mass m1.

[0027] Moisture content (dry basis) is calculated according to formula (1): x db (%) = (m0-m1) / m1×100%.

[0028] When conversion to wet basis is required, it can be done according to formula (2): x wb (%) = (m0 - m1) / m0 × 100% = x db / (100+x db ()×100%. The preferred graduation value of the weighing balance is 0.001g.

[0029] Oil content (crude fat on a dry basis) O: determined by Soxhlet extraction method according to GB 5009.6-2016 "National Food Safety Standard - Determination of Fat in Food". The sample is preferably dried to constant weight at 101–105℃ to remove moisture; extracted with petroleum ether (boiling range 30–60℃) for 8 hours, and the extracted oil mass m is recorded. fat and dry sample mass m dry The oil content is calculated according to formula (3): O(%) = m fat / m dry ×100%.

[0030] Degreasing rate R: represents the reduction in oil content before and after degreasing, calculated according to formula (4): R(%)=(O0-O1) / O0×100%, where O0 is the oil content before treatment and O1 is the oil content after treatment.

[0031] Whole kernel rate Y: The percentage of whole nut kernels in the total mass of the sample after processing. In this application, "whole nut kernels" preferably refers to kernels that retain their basic shape, have a length loss of less than 1 / 5 of their original length, and are not obviously crushed.

[0032] Volume recovery rate VR: Calculated according to formula (5): VR(%) = V after / V before ×100%, where V before V is the volume of the nut kernels before pressing. after This refers to the volume after depressurization and drying. Volume determination can be performed using the seed displacement method / volume displacement method. For irregularly shaped particles with multiple fissures, the fine powder filling method is preferred for indirect determination, with the following steps: a) Take a container with a fixed volume V and add sample mass M. s b) Fill the container with inert fine powder with a particle size of approximately 80 μm and compact it until the total volume of the powder and the sample is exactly V; weigh the total mass M at this point. t c) Take another container of the same volume, fill it with the same fine powder, and compact it to a volume of V. Weigh the powder to obtain its mass M. p d) Powder bulk density Powder occupies volume Sample volume V s =VV p e) Measure the sample volume V before and after pressing. before With V afterSubstitute into equation (5) to calculate the volume recovery rate. The sample amount is preferably about 10g, and the average value is taken after ≥5 repeated measurements.

[0033] Peroxide value: Determined by indicator titration method according to GB 5009.227-2023 "Determination of Peroxide Value in Food". After crushing the nut sample, the oil sample was extracted with petroleum ether. The filtrate filtered through a funnel containing anhydrous sodium sulfate was distilled under reduced pressure to obtain the sample, with a mass of m. After dissolving in chloroform-glacial acetic acid solution, it was titrated according to the determination procedure. The volume V of standard sodium thiosulfate solution with concentration c consumed in the titration was obtained, and the volume V0 of standard sodium thiosulfate solution consumed in the blank was obtained. The peroxide value was calculated according to the formula: X = (V - V0) × c × 0.1269 / m × 100.

[0034] Acid value: determined by cold solvent indicator titration method according to GB 5009.229-2025 "Determination of Acid Value in Food". Nut samples were pressed for oil using a screw press, filtered through filter paper, and immediately dissolved in a cold solvent. Phenolphthalein indicator was used, and titration with standard potassium hydroxide solution was performed until the solution changed from colorless to slightly red. The sample mass was m, and the volume V of standard potassium hydroxide solution with concentration c consumed in the titration was calculated. The volume V0 of standard potassium hydroxide solution consumed in the blank titration was calculated. The acid value was calculated using the formula: X = (V - V0) × c × 56.1 / m.

[0035] Surface oil content rating: Surface residual oil can be qualitatively / semi-quantitatively evaluated using filter paper adsorption. The rating criteria are shown in Table 7.

[0036] Effective specific surface area (SSA): can be determined using the BET nitrogen adsorption method. In the embodiments of this application, the sample was degassed for 10 hours under a vacuum limit of approximately 9 Pa, with the degassed temperature not exceeding 80°C, and nitrogen was used as the adsorbed gas.

[0037] 30s liquid absorption weight gain rate: The sample was immersed in a fixed concentration of flavoring solution (e.g., 5% saline solution) for 30s, drained, and the surface was lightly touched with filter paper to remove free liquid before weighing. Calculated according to formula (6): Δm30(%) = (m30-m0) / m0 × 100%, where m0 is the sample mass before immersion and m30 is the sample mass after immersion for 30s. The osmotic constant K can be obtained by measuring Δm(t) at multiple immersion time points and fitting it with a first-order osmotic model Δm(t) = Δm∞·(1-exp(-K·t)). Wherein, Δm∞ is the equilibrium weight gain rate (%), t is the immersion time (min), and K is in min. -1 .

[0038] Hardness and brittleness: measured using a texture analyzer, unit is N, test conditions are P / 2 probe, pre-test speed 2mm / s, test speed 2mm / s, post-test speed 5mm / s, compression ratio 50%, each sample is measured 30 times; Brittleness score: sensory evaluation using a 10-point scale, 10 points indicates extremely brittle and 1 point indicates not brittle; scores are given under the same sample temperature and chewing number conditions and the average value is taken.

[0039] Process steps Step S1 (Heat Pretreatment / Tempering): The nuts are subjected to one or more combinations of baking, hot air heating, microwave heating, radio frequency heating, or superheated steam heating to induce initial stress inside the nuts. Then, an aqueous solution containing antioxidants at 50–70°C is used for conditioning and hydration to improve oxidative stability. If the moisture content exceeds the target range, heating or ventilation drying can be continued to reduce moisture.

[0040] Step S2 (Phase Change Induced Stress Treatment): The nuts treated in S1 undergo a physical phase change treatment to induce thermo-mechanical coupling within the nuts, forming microcracks. These microcrack networks are at least partially connected to the outer surface of the nuts, creating microchannels for oil migration. A freeze-microwave treatment is preferred: the nuts are frozen at -10 to -30°C for 0.5 to 6 hours, followed by microwave treatment at 300 to 1000 W for 20 to 120 seconds.

[0041] Step S3 (pressing and stress relaxation): The nuts treated in S2 are placed in the pressing chamber for segmented controlled-pressure pressing. The pressure-time curve can be found in [reference needed]. Figure 2 The pressure is increased to 3–10 MPa at a rate v1 of 1.0–2.5 MPa / min and held for a duration t. hold With boost duration t ramp The ratio is 1.5–1.8:1. During the pressure holding stage, it is preferable to use a pressure sensor and PLC closed-loop control to keep the pressure fluctuation ≤ ±0.2 MPa. To reduce radial stress concentration and absorb exudated oil, an oil-absorbing protective medium can be placed around the nut kernel, such as food-grade porous elastic material, nut skin, nut shell, or food-grade fiber oil-absorbing paper. For details on the pressing chamber and rapid pressure relief and oil discharge structure, please refer to [reference needed]. Figure 4 The components that come into contact with the nuts or the discharged oil (such as the inner wall of the pressing chamber, the filter structure, the oil outlet, the sealing ring, and the oil collection container) are preferably made of materials that comply with relevant national standards for food contact materials and products (such as GB 4806 series), such as 304 / 316L stainless steel, food-grade silicone rubber, or PTFE. The oil-absorbing protective medium should preferably be a food-grade, low-shedding material to reduce the risk of foreign matter.

[0042] Step S4 (Rapid Pressure Relief and Oil Discharge): After the pressure holding period, rapidly depressurize to atmospheric pressure at a depressurization rate v2 and collect the discharged grease, where v2 is 2.5 to 3.5 times v1. Preferably, rapid pressure relief is achieved using a large-diameter quick-release valve (pressure relief valve) in conjunction with a throttle valve, with the depressurization time controlled to 10–90 seconds. Rapid pressure relief helps reduce residual oil retention and blockage at the microchannel outlet, and alleviates surface grease. The relative arrangement of the quick-release valve, throttle structure, and oil collection container can be found in [reference needed]. Figure 4 It is preferable to install a filter structure at the oil drain trough or oil drain port to prevent solids from entering the oil drain channel and reduce splashing.

[0043] Step S5 (High-energy physical drying for rapid shaping): After depressurization, the nut kernels are dried using high-energy physical methods such as radio frequency, infrared, microwave vacuum, or pulse jet drying for 2–10 minutes to achieve brittle shaping and lock the pore structure before the tissue closes again. For example, infrared drying at 120°C for 5 minutes can be used to bring the core temperature of the nut kernel to 120–150°C; after drying, air cooling or natural cooling is preferred to avoid moisture regain.

[0044] Optional seasoning steps: After obtaining low-fat, highly oxidatively stable nuts, seasoning treatments such as soaking in seasoning liquid, spraying, or roller coating can be applied according to product requirements. Under conditions of microcrack network and low surface residual oil, the seasoning liquid can penetrate into the nut tissue more quickly. Example 1 (Peanuts)

[0045] Raw material: Peanut kernels, initial moisture content of approximately 6% (dry basis), initial oil content of approximately 54%, sample size of approximately 40g.

[0046] S1 heat pretreatment: Bake at 120℃ for 15 min, then condition with an aqueous solution containing antioxidants (TBHQ and citric acid compound, with concentrations of 0.2 g / L and 0.1 g / L respectively) at 60℃ for 15 min to adjust the moisture content to approximately 9% (dry basis).

[0047] S2 phase transformation induced stress treatment: Peanut kernels were frozen at -20℃ for 4 hours; then microwaved at 600W for 45 seconds.

[0048] S3 pressing and stress relaxation: Pressure is increased from 1.6 MPa / min to 5 MPa (pressurization time approximately 3.1 min), followed by holding at that pressure for 5 min (t... hold / t ramp ≈1.6:1).

[0049] S4 Rapid Pressure Relief and Oil Discharge: After the pressure holding period ends, the pressure is reduced to normal pressure within 1 minute and the discharged grease is collected (v2 / v1≈3.1).

[0050] S5 rapid shaping: Infrared drying at 120℃ for 5 minutes is used to bring the center temperature of the nut kernel to about 135℃; then it is naturally cooled to obtain a low-fat peanut product with high oxidation stability.

[0051] Test results: Degreasing rate approximately 59%, oil content after treatment approximately 22%, granulation rate approximately 99%, and volume recovery rate approximately 99%. Key comparative indicators are shown in Table 1; degreasing and morphology indicators are summarized in Table 5; penetration and texture indicators are summarized in Table 6; acid value and peroxide value indicators are summarized in Tables 8 and 9. Example 2 (Pine Nuts)

[0052] Raw material: Pine nuts, initial moisture content approximately 6% (dry basis), initial oil content approximately 74%.

[0053] S1 heat pretreatment: Bake at 140℃ for 8 minutes, then condition with an aqueous solution containing antioxidants (TBHQ and citric acid compound, with concentrations of 0.2 g / L and 0.1 g / L, respectively) at 60℃ for 15 minutes to adjust the moisture content to approximately 12% (dry basis).

[0054] S2 phase transformation induced stress treatment: Pine nuts were frozen at -20℃ for 4 hours; then microwaved at 500W for 60 seconds.

[0055] S3~S5: The remaining steps are the same as in Example 1 (P=5MPa, v1=1.6MPa / min, t hold =5min, depressurization time 1min, infrared drying 120℃ 5min).

[0056] Test results: Degreasing rate was approximately 58%, oil content after treatment was approximately 31.08%, granulation rate was approximately 99%, and volume recovery rate was approximately 98%. A summary of degreasing and morphological indicators is shown in Table 5; a summary of permeability and texture indicators is shown in Table 6.

[0057] Comparative Example 1 (Conventional High-Pressure Pressing) Peanut kernels from the same batch as in Example 1 were processed using a conventional high-pressure pressing process. The pressing pressure was 20 MPa, the pressurization rate was approximately 10 MPa / min, and the pressure was maintained for 5 minutes, followed by slow depressurization as usual. Due to the severe damage to the nut tissue caused by the high pressure, a water-soaking and reshaping treatment was performed to reduce pulverization before drying (infrared drying at 120℃ for 5 minutes). Test results: defatting rate was approximately 82%, oil content after treatment was approximately 9.72%, whole kernel rate was approximately 3%, and volume recovery rate was approximately 5%. Key comparative indicators are shown in Table 1; defatting and morphological indicators are summarized in Table 5; and permeability and texture indicators are summarized in Table 6.

[0058] Comparative Example 2 (S2 phase transformation induced stress treatment omitted) Under the same raw materials and conditions as in Example 1, step S2 is omitted, and the remaining steps are performed as in Example 1 (P=5MPa, v1=1.6MPa / min, t hold / t ramp (Approximately 1.6:1, depressurization time 1 min, infrared drying at 120℃ for 5 min). Test results: degreasing rate approximately 15%, oil content after treatment approximately 45.9%, whole grain rate approximately 43%, volume recovery rate approximately 63%. Degreasing and morphological indicators are summarized in Table 5; permeability and texture indicators are summarized in Table 6.

[0059] Comparative Example 3 (slow pressure relief, v2 / v1 does not meet the 2.5~3.5 standard) Under the same raw materials and conditions S1, S2, and S3 as in Example 1, the depressurization rate was reduced to v2 / v1 ≤ 1.5 (e.g., extending the depressurization time to approximately 3 minutes); the remaining steps were the same as in Example 1. Test results: degreasing rate approximately 38%, oil content after treatment approximately 33.48%, granulation rate approximately 98%, volume recovery rate approximately 99%, surface oil content score 4, and 30s liquid absorption weight gain rate approximately 19.28%. A summary of degreasing and morphological indicators is shown in Table 5; a summary of penetration and texture indicators is shown in Table 6.

[0060] Comparative Example 4 (without pressure holding or t) hold / t ramp (Deviation of 1.5 to 1.8) Under the same raw materials, S1, and S2 conditions as in Example 1, the holding time was shortened to t hold / t ramp <1.0, the remaining steps are the same as in Example 1. Test results: degreasing rate is about 29%, oil content after treatment is about 38.34%, granulation rate is about 57%, and volume recovery rate is about 73%. Degreasing and morphological indicators are summarized in Table 5; permeability and texture indicators are summarized in Table 6.

[0061] Comparative Example 5 (omitting S5 rapid setting or using conventional slow drying) Using the same raw materials and conditions S1-S4 as in Example 1, step S5 was changed to hot air / oven drying (170°C, 3.5 min) to ensure the final moisture content was consistent with Example 1. Test results: degreasing rate approximately 51%, oil content after treatment approximately 26.4%, granulation rate approximately 99%, volume recovery rate approximately 71%, surface oil content score 4; 30s liquid absorption weight gain rate approximately 34.14%, permeability constant K approximately 1.722 min. -1 SSA is approximately 0.659 m 2 / g, hardness approximately 44.8 N, brittleness approximately 10.7 N (see Tables 5 and 6).

[0062] Comparative Example 6 (S1 moisture content deviates from the range of 8% to 12%) Using the same raw materials and under the same conditions (S2-S5) as in Example 1, the moisture content of the nut kernels obtained in step S1 was adjusted to 13% (dry basis), while keeping other parameters unchanged. Test results: defatting rate approximately 45%, oil content after treatment approximately 29.7%, whole kernel rate approximately 84%, volume recovery rate approximately 76%, surface oil content score of 2; 30s liquid absorption weight gain rate approximately 27.38%, osmotic constant K approximately 1.047 min. -1 SSA is approximately 0.579 m 2 / g, hardness approximately 24.72 N, brittleness approximately 8.46 N (see Tables 5 and 6).

[0063] Comparative Example 7 (antioxidant addition in step S1 omitted) Using the same raw materials and conditions S2-S5 as in Example 1, the moisture content of the nuts was adjusted to approximately 9% (dry basis) using pure water in step S1, without adding any antioxidants and keeping other parameters unchanged. Test results: defatting rate approximately 55%, oil content after treatment approximately 24.15%, whole nut rate approximately 97%, and volume recovery rate approximately 98%. Acid value and peroxide value during storage are shown in Tables 8 and 9; a summary of basic indicators for defatting, morphology, and oxidative stability is shown in Table 5; and a summary of permeability and texture indicators is shown in Table 6.

[0064] Comparative Example 8 (Antioxidant was added to the surface after step S5 by spraying). Using the same raw materials and conditions S2-S5 as in Example 1, the moisture content of the nuts was adjusted to approximately 9% (dry basis) using pure water in step S1. After the shaping treatment in S5, an equal amount of antioxidant was added by surface spraying. Test results: defatting rate approximately 56%, oil content after treatment approximately 23.6%, whole nut rate approximately 97%, volume recovery rate approximately 97%, hardness approximately 19.6 N, and brittleness approximately 13.2 N. Acid value and peroxide value during storage are shown in Tables 8 and 9; a summary of basic indicators for defatting, morphology, and oxidative stability is shown in Table 5; and a summary of permeability and texture indicators is shown in Table 6.

[0065] Experimental data and comparison results To verify the synergistic effect of the present invention's "phase change-induced microchannel + stress relaxation window + rapid pressure relief and oil removal + high-energy drying keyhole + introduction of antioxidant components during conditioning stage," key quality indicators, microstructure, and flavor infusion kinetics data are presented below through comparative experiments of examples and comparative examples. Table 1 compares the overall differences between the present invention and conventional high-pressure pressing; Tables 2-4 illustrate the influence of holding pressure on defatting, structural recovery, and permeability; Tables 5-6 summarize the comparisons of examples / comparative examples; Table 7 presents the surface oil content scoring criteria; and Tables 8-9 compare acid value and peroxide value during storage. Examples 1A and 1B represent boundary conditions where all other conditions are the same as in Example 1, only the v2 / v1 ratio is adjusted to 2.5 and 3.5, respectively. Unless otherwise stated, each indicator measurement was repeated at least three times, and the results were averaged.

[0066] Table 1 compares the key indicators of Example 1 and Comparative Example 1 (conventional high-pressure pressing) to verify the comprehensive effect of the present invention in balancing degreasing degree, granulation rate, surface residual oil and flavor infusion speed under lower pressure conditions.

[0067] Table 1 Comparison of key indicators between Example 1 and Comparative Example 1 As shown in Table 1, compared with conventional high-pressure pressing, the present invention can achieve a higher degreasing rate under lower pressures such as 5 MPa, while significantly improving the granulation rate and volume recovery rate; and the surface oil content score is lower, the permeability constant K is higher and the 30s liquid absorption weight gain rate is higher, indicating that rapid pressure relief and oil removal and pore structure locking can significantly improve surface dryness and promote flavor absorption.

[0068] Table 2 illustrates the situation when all other conditions are the same (S1~S2 treatment, t) hold / t ramp (The shaping methods of v2 / v1 and S5 remain unchanged). The influence of holding pressure on degreasing rate, granulation rate and macroscopic morphology, thereby supporting the "low pressure range" and preferred pressure window described in step S3.

[0069] Table 2 Comparison of basic quality indicators under different holding pressures As shown in Table 2, insufficient degreasing occurs when the holding pressure is too low, while excessive pressure leads to a decrease in the whole grain ratio and tissue cracking. A better balance can be achieved between degreasing efficiency and morphology preservation at around 5.0 MPa, which supports the pressure window setting of "3 to 10 MPa, preferably 4 to 7 MPa" in the instruction manual.

[0070] Table 3 is used to characterize the microstructure changes and elastic recovery ability under different pressures from the perspectives of volume recovery rate and specific surface area (SSA), in order to verify the role of "stress relaxation window + rapid pressure relief" in suppressing pore collapse and maintaining connectivity.

[0071] Table 3 Microstructure parameters and mechanical recovery performance test results As shown in Table 3, the volume recovery rate remains high and the SSA reaches a relatively large level near 5.0 MPa, indicating that the microcrack network is easier to form and remain open. When the pressure is further increased, the hardness increases and the SSA decreases, suggesting that the pore closure / collapse intensifies, which is detrimental to subsequent penetration and taste.

[0072] Table 4 is used to compare the permeation kinetics of the seasoning liquid under different holding pressures (taking 5% brine as an example), and to establish a corresponding relationship with the defatting / structural indicators, in order to support the description of the effect of "facilitating rapid flavor absorption" in this invention.

[0073] Table 4 Osmotic kinetics (using 5% saline solution as flavoring) As shown in Table 4, the 30s liquid absorption weight gain rate and permeability constant K are higher near 5.0 MPa, indicating that the interconnected pore structure formed under this pressure is more conducive to the rapid entry of flavoring liquid into the tissue. When the pressure is too high and the structure becomes dense, the permeability performance decreases, further illustrating the necessity of "low pressure + pore structure locking".

[0074] Table 5 summarizes the basic indicators of degreasing, morphology, and oxidative stability of the examples and comparative examples, which are used to compare the overall impact of different key steps / parameter settings on the degreasing endpoint, particle retention, and oxidative stability, and to provide implementation support for the parameter range in the claims.

[0075] Table 5 Summary of basic indicators of defatting, morphology and oxidative stability in the examples and comparative examples Table 5 shows that although Comparative Example 1 had a high degreasing rate, its particle size reduction and volume recovery rate were significantly reduced. Comparative Example 2 showed a significant decrease in degreasing rate and particle size reduction after omitting phase change induction. Comparative Example 3 showed an increase in surface oil content and a decrease in permeability after reducing the pressure relief ratio. Comparative Example 4 showed a decrease in degreasing rate and morphological degradation after shortening the holding time. Comparative Examples 5 and 6 indicate that rapid shaping and the S1 moisture window have significant impacts on pore structure maintenance and the degreasing endpoint. Comparative Examples 7 and 8 show that introducing antioxidants in the S1 conditioning stage is more beneficial for reducing post-treatment acid value and peroxide value. In summary, phase change-induced microchannel construction, stress relaxation window control, rapid pressure relief and oil removal, high-energy drying and pore locking, and the introduction of antioxidants in the conditioning stage have synergistic effects in improving degreasing efficiency, maintaining particle size, reducing residual oil, and enhancing oxidative stability. Examples 1A and 1B serve as boundary points for the depressurization ratio (v2 / v1 are 2.5 and 3.5 respectively), and their comprehensive indicators remain stable (see Tables 5 and 6).

[0076] Table 6 summarizes the permeation and texture indices of the embodiments and comparative examples, which are used to verify the overall effect of the present invention from the chain of "surface residual oil - pore structure openness - permeation rate - texture change".

[0077] Table 6 Summary of Permeability and Texture Indicators for Examples and Comparative Examples As shown in Table 6, compared with Comparative Examples 1 to 4, the 30s liquid absorption weight gain rate and permeability constant K of the embodiments of the present invention are higher, and good brittleness is maintained at a lower hardness level. This indicates that, based on the microchannel connectivity and outlet oil clearing, rapid shaping can lock the pore structure and accelerate the penetration of seasoning liquid, thereby improving surface dryness and flavor absorption speed.

[0078] Table 7 provides the scoring criteria and judgment basis for the surface oil content rating (filter paper adsorption method), which is used to conduct a semi-quantitative evaluation of surface residual oil and facilitates horizontal comparison between different batches and different nut varieties.

[0079] Table 7 Surface Oil Content Scoring Criteria (Filter Paper Adsorption Method) The surface oil content score can be evaluated in conjunction with the degreasing rate, depressurization ratio, and drying and shaping method. When the score is too high, it usually indicates residual oil retention at the outlet or closure of the channel. Key control points such as depressurization time, v2 / v1, and shaping speed should be verified first.

[0080] Tables 8 and 9 summarize the acid value and peroxide value of the baked samples, Example 1, Comparative Example 7, and Comparative Example 8 during storage, used to compare the effects of introducing antioxidants in the conditioning stage of step S1 and subsequent surface spraying on oxidative stability. As shown in Tables 8 and 9, Example 1 had lower acid value and peroxide value at all storage time points, indicating that introducing antioxidants in step S1 is more beneficial for improving storage oxidative stability. Storage conditions were 60±1℃, relative humidity 40%, unpackaged and laid flat on a stainless steel tray, with samples taken and measured every 2 days.

[0081] Table 8. Changes in acid value over storage time Table 9. Changes in peroxide value over storage time The present invention has been described above with reference to the embodiments. It should be understood that the above embodiments are only used to explain the technical ideas and implementation paths of the present invention, and are not intended to limit it. Those skilled in the art can make equivalent substitutions or reasonable modifications to the nut types, phase change induction methods (e.g., freeze-thaw cycles or freeze-microwave treatment), pressure boosting-holding-depressurization control methods, and high-energy physical drying methods (e.g., radio frequency drying, infrared drying, or microwave vacuum drying) within the parameter range disclosed in this specification without departing from the spirit and substance of the present invention, and all such substitutions or reasonable modifications should fall within the protection scope of the present invention.

Claims

1. A method for preparing low-fat, highly oxidatively stable nuts guided by stress relaxation microchannels, characterized in that, Includes the following steps: S1 Heat pretreatment: The nuts are heated and conditioned to adjust the moisture content of the nuts to 8% to 12% (on a dry basis), and antioxidants are introduced through the conditioning process. S2 Phase Transformation Induced Stress Treatment: The nuts processed in step S1 undergo a physical phase transformation treatment to form a microcrack network inside the nuts that communicates with the outer surface, thereby constructing microchannels for oil migration. S3 Pressing and Stress Relaxation: The nut kernels processed in step S2 are placed in the pressing chamber, and the pressure is increased to 3-10 MPa at a pressurization rate v1 of 1.0-2.5 MPa / min and maintained at that pressure for a duration t. hold With boost duration t ramp The ratio is 1.5 to 1.8:1; S4 Rapid Pressure Relief and Oil Discharge: After the pressure holding period ends, the pressure is rapidly reduced to normal pressure at a pressure relief rate v2 and the grease is collected and discharged. The pressure relief rate v2 is 2.5 to 3.5 times the pressure increase rate v1. S5 Rapid Shaping: The nuts processed in step S4 are subjected to high-energy physical drying for 2-10 minutes to achieve rapid shaping and obtain low-fat, highly oxidatively stable nuts.

2. The method as described in claim 1, characterized in that, The nut kernels are one or more of the following: peanut kernels, pine nuts, almonds, cashew kernels, walnut kernels, hazelnut kernels, or pecan kernels.

3. The method as described in claim 1, characterized in that, The heating process in step S1 is one or more combinations of baking, hot air, microwave, radio frequency, or superheated steam heating.

4. The method as described in claim 3, characterized in that, The heating temperature in step S1 is 100–160°C, and the heating time is 5–30 min.

5. The method as described in claim 1, characterized in that, In step S1, when the moisture content of the nut kernels is lower than the target range, the nut kernels are conditioned and moisturized with an aqueous solution containing antioxidants at 50-70℃, and then heated to bring the moisture content into the target range.

6. The method as described in claim 1, characterized in that, The physical phase change treatment in step S2 is a freeze-microwave treatment, which includes: freezing the nut kernels at -10 to -30°C for 0.5 to 6 hours; and then treating them under microwave conditions of 300 to 1000W for 20 to 120 seconds.

7. The method as described in claim 1, characterized in that, The holding pressure in step S3 is 4 to 7 MPa.

8. The method as described in claim 1, characterized in that, In step S3, an oil-absorbing protective medium is placed on the outer periphery of the nut kernel to disperse radial pressure and absorb the exuded oil; the oil-absorbing protective medium is one or more of the following: food contact grade porous elastic material, nut skin, nut shell, or food grade fiber oil-absorbing paper.

9. The method as described in claim 1, characterized in that, The high-energy physical drying in step S5 is one or more of the following methods: radio frequency drying, infrared drying, microwave vacuum drying, or pulse jet drying; when infrared drying is used, the temperature at the center of the nut kernel reaches 120-150°C.

10. The method as described in claim 1, characterized in that, The depressurization time in step S4 is 10–90 seconds.

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