Processing method of low-temperature vacuum freeze-drying shape-preserving dried Hami melons

Abstract

The invention relates to the technical field of food processing, and discloses a low-temperature vacuum freeze-drying shape-preserving dried Hami melon processing method which comprises the following steps: firstly, peeling and deseeding Hami melons and slicing the Hami melons; the slices are soaked in a mixed solution of calcium lactate and ascorbic acid for pretreatment, so that the tissue structure and the oxidation resistance are enhanced; and then pulse vacuum impregnation is performed in a trehalose solution, and pre-dehydration and vitrification improvement are realized through vacuum-deflation circulation. After dipping, sodium alginate and calcium lactate are subjected to in-situ cross-linking to form an edible micro-membrane, so that the shape retention of the slices in the drying process is improved. Sublimation and desorption drying are completed under the condition that the cavity pressure is not higher than 30 Pa through vacuum freeze drying of rapid pre-freezing and segmented temperature control, and tissue collapse is avoided. And finally, returning the temperature, filling nitrogen and sealing and packaging to ensure that the volume fraction of residual oxygen is lower than 3%. The method can effectively maintain the original shape, flavor and nutritional ingredients of the dried Hami melons, prolong the storage period and improve the product quality.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, and more specifically, to a method for processing dried cantaloupe fruit using low-temperature vacuum freeze-drying to preserve its shape. Background Technology

[0002] Hami melons are high in sugar and have a fragile texture. Conventional hot air drying can easily cause non-enzymatic browning, significant shrinkage, surface crusting, and flavor loss.

[0003] While traditional freeze-drying can reduce heat damage, it generally suffers from several drawbacks. Pretreatment formulations often prioritize hardening or anti-oxidation alone, resulting in insufficient pH and ionic strength matching, making it difficult to simultaneously maintain shape and inhibit browning. Uneven temperature and moisture distribution during freezing lead to deformation and localized collapse during sublimation. Drying stage switching often relies on plate temperature or single vacuum gauge readings, which are affected by water vapor partial pressure and heat transfer lag, leading to inaccurate determination of endpoints and safety boundaries, and increasing the risk of cracking and edge warping. Weak control over terminal gas filling and barrier packaging results in high residual oxygen levels, causing fading and rehydration during storage. Current technologies struggle to achieve a stable balance between shape retention, color, rehydration properties, and energy efficiency. Summary of the Invention

[0004] In view of this, the present invention proposes a low-temperature vacuum freeze-drying method for drying cantaloupe fruit to preserve its shape, aiming to solve the problems existing in the above content.

[0005] This invention proposes a low-temperature vacuum freeze-drying method for preserving the shape of dried cantaloupe, comprising the following steps: Peel and seed the cantaloupe, and slice it to a thickness of 4mm-10mm. Pretreatment is performed by immersing the slices in a mixed solution containing 0.2%-0.8% calcium lactate and 0.05%-0.20% ascorbic acid at 0°C to 4°C for 10 min to 30 min. Pulsed vacuum impregnation pre-dehydration and vitrification enhancement are performed. The pulsed vacuum impregnation is to perform 3 to 5 vacuum-venting cycles in a 5%-15% trehalose solution. Each cycle is held at a relative vacuum of 80kPa-95kPa for 60s-120s and then recovered at atmospheric pressure for 60s-120s. The liquid temperature is 0℃-4℃. An edible conformal microfilm was constructed after pulsed vacuum impregnation. The conformal microfilm was formed by in-situ crosslinking by first spraying a 0.4% to 1.0% sodium alginate solution and then atomizing and spraying a 0.5% to 2.0% calcium lactate solution. The film thickness was 5 μm to 30 μm. Rapid pre-freezing is performed, first holding the sample at -2℃ to -5℃ for 30s-120s for nucleation, then lowering it to -35℃ to -45℃ at a rate of not less than 1.5℃ / min and holding it until completely frozen; vacuum freeze-drying is then performed under high vacuum conditions, with the chamber pressure not exceeding 30Pa, using segmented temperature control curves for sublimation drying and desorption drying: the first sublimation stage shelf temperature is -35℃ to -25℃, the second sublimation stage shelf temperature is -25℃ to -15℃, and the desorption drying stage shelf temperature is 10℃ to 25℃; after drying is completed, the sample is warmed to 20℃ to 25℃ and filled with nitrogen, with the residual oxygen volume fraction below 3%, and then sealed and packaged.

[0006] Furthermore, the pH of the pretreatment solution is 3.3-3.8, and a citrate buffer is used to maintain pH stability. After pretreatment, the solution is rinsed with flowing pure water at 0℃-4℃ for 30s-120s.

[0007] Furthermore, the number of cycles of vacuum holding and atmospheric pressure recovery in the pulsed vacuum impregnation is 4, with the upper limit value being taken for the vacuum holding time in the first cycle and the upper limit value being taken for the atmospheric pressure recovery time in the last cycle.

[0008] Furthermore, the in-situ crosslinking step of the conformal microfilm is completed by alternately spraying both sides of the slice, and then standing at 0℃-4℃ for 2min-5min after spraying.

[0009] Furthermore, the nucleation process involves applying ultrasound at 20kHz-40kHz and a power density of 0.4W / cm²-0.8W / cm² for 30s-90s before the cooling process begins.

[0010] Furthermore, the time for the rapid pre-freezing temperature to reach -35℃ to -45℃ does not exceed 25 minutes. During the pre-freezing process, the slices are laid out in a single layer with a spacing of not less than 5mm between layers, and the wind speed is 0.5m / s to 1.5m / s to form uniform heat exchange.

[0011] Furthermore, the inter-segment switching of the segmented temperature control curve is triggered when the temperature sublimation plateau ends and the ratio of the readings of the capacitance vacuum gauge and the Pirani vacuum gauge is not higher than 1.15.

[0012] Furthermore, during the vacuum freeze-drying process, the temperature is monitored online using embedded thermocouples with a sampling period of no more than 10 seconds. If the detected temperature is close to the collapse temperature by 3°C or less, the shelf temperature is automatically reduced by 2°C to 5°C and the current time is extended by 10 to 30 minutes.

[0013] Furthermore, the sealed packaging uses a multi-layer barrier composite membrane with an oxygen permeability of no more than 2 mL / (m²·d·atm) and a water vapor permeability of no more than 5 g / (m²·d), a nitrogen filling pressure of 0.02 MPa to 0.06 MPa, a sealing temperature of 130°C to 160°C, and a sealing time of 0.8 s to 2.0 s.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: This method employs calcium lactate and ascorbic acid pretreatment in a low-temperature buffer system. Ca²⁺ forms ion bridges with the carboxyl groups of pectin, increasing the storage modulus of the cell wall network and providing constraint on deformation during subsequent decompression and heat transfer. Simultaneously, the pH is stabilized at 3.3-3.8, which is within the range of low polyphenol oxidase activity. Ascorbic acid reduces the primary quinone to diphenol, interrupting the browning chain reaction. Thus, color inhibition and initial mechanical stability are established simultaneously without increasing the heat load.

[0015] Pulsed vacuum impregnation, through repeated "vacuum-degassing" cycles, promotes the formation of a controllable osmotic pressure gradient of trehalose inside and outside the tissue. The solids enter the interstitial space and form a uniform distribution with the original soluble components, reducing the proportion of free water and raising the glass transition temperature of the formulation. During the sublimation and desorption stages, the material is in a high viscoelastic threshold region, making it less prone to surface densification and hardening, thus reducing resistance to water migration and balancing shape retention with subsequent rehydration rate.

[0016] Sodium alginate and calcium lactate are in-situ cross-linked on the slice surface to form a thin, edible microfilm. The film continuously covers the substrate and creates an interfacial anchoring effect, dispersing in-plane tensile stress and edge shear stress under sublimation-driven conditions, thus limiting peripheral warping and thickness shrinkage. The areal density and thickness of this film are controlled, do not impede moisture migration paths, and provide surface bearing capacity and crack resistance during transportation and bagging impacts, thereby improving appearance integrity and shatter resistance.

[0017] The freezing procedure is set to allow for nucleation dwell and rapid traversal of the phase transition temperature zone, and employs single-layer placement and controlled airflow to unify the external heat transfer coefficient. This procedure reduces the temperature gradient and freezing process differences within the sample, minimizes residual strain accumulation caused by phase transition volume changes, provides initial conditions for geometric stability during the sublimation stage, and avoids batch deformation dispersion due to inconsistent freezing processes.

[0018] Segmented temperature control uses the ratio of readings from a capacitance vacuum gauge and a Pirani vacuum gauge as the criterion for determining the intervals and endpoints, while also considering the end of the plateau period in the product temperature. The capacitance gauge reflects absolute pressure, while the Pirani gauge is sensitive to water vapor partial pressure. A convergence in the ratio indicates a decrease in sublimation load and a stabilization of the gas phase composition. When used in conjunction with the product temperature signal, it can reduce misjudgments caused by relying solely on plate temperature or a single vacuum gauge, thereby switching between the sublimation and desorption phases at the appropriate time, reducing the risk of under-drying or over-drying, and shortening the ineffective holding time.

[0019] When the product temperature approaches the safe boundary of the collapse temperature, the control strategy automatically reverts to the shelf temperature and extends the current dwell time, reducing thermal drive, allowing mass migration to continue, and keeping the material within the reversible range of viscoelastic response. This online retraction mechanism provides a buffer for the later stages of sublimation, suppressing surface cracking and localized depressions, and ensuring contour and flatness.

[0020] Terminal nitrogen filling and high-barrier composite film sealing control residual oxygen at a low level, combined with a low water activity endpoint, thus limiting both oxidation and moisture absorption. During storage, the chemical stability of pigments and aromatic components is enhanced, the probability of reabsorption and softening is reduced, and the appearance and flavor remain stable throughout the shelf life.

[0021] The aforementioned formulation enhancements (ionic crosslinking, antioxidant properties, and osmotic pressure regulation), process control (freezing program control, dual-sensor joint judgment, and temperature control rollback), and end-point barrier (nitrogen filling and high-barrier membrane) constitute a synergistic system: the material's mechanical threshold is increased, the heat and mass transfer coupling process is controlled, and environmental exposure is suppressed. This enables high shape retention, low warpage, and faster rehydration at a safe endpoint of ≤5% moisture content and ≤0.35 water activity, achieving scalable and stable production with shorter cycles and lower energy consumption. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 Select cantaloupes with an edible ripeness of 7.5-8.5. After washing, peel and remove seeds, and slice along the equatorial plane to a thickness of 6 mm and a diameter of 60-80 mm. Place the slices on a stainless steel tray at 2℃ and store for no more than 10 minutes. Prepare the pretreatment solution: Add 0.5% (w / v) calcium lactate and 0.10% (w / v) ascorbic acid to deionized water, adjust the pH to 3.5±0.1 using a citric acid / sodium citrate buffer system, and pre-cool the solution to 2℃. Immerse the slices completely in the solution for 20 minutes, stirring gently to keep the liquid fresh. After rinsing, rinse with running pure water at 2℃ for 60 seconds to remove free calcium ions from the surface, and drain for 30 seconds. Prepare the pulsed vacuum impregnation solution: trehalose 10% (w / v), temperature 2℃; place the slices in a single layer into the vacuum impregnation tank, add the impregnation solution until completely covered, and perform 4 "vacuum-venting" cycles: each time maintain a relative vacuum of 90 kPa for 90s, then restore to normal pressure for 90s, maintaining the liquid temperature at 2℃ during the cycle; after the cycle, remove the drained liquid for 60s. Construct an edible shape-preserving microfilm: spray 0.6% sodium alginate solution (w / v) evenly onto both sides of the slices using a spray method, with a target areal density of approximately 0.35 mg / cm², immediately followed by atomized spraying of 1.0% calcium lactate solution (w / v) to achieve in-situ crosslinking, and then let stand at 2℃ for 3min to complete ion diffusion and film layer shaping, with the film thickness controlled within the range of 5-30 μm.

[0024] The freezing procedure combines nucleation and rapid cooling: A single layer of slices is placed on a tray of a through-flow cooling device, with a 10mm spacing between slices and an airflow rate controlled at 1.0m / s. The ambient temperature is first lowered to -3℃ and held for 60s to induce nucleation. Before cooling begins, ultrasound at 30kHz power density (0.6W / cm²) is applied to the bottom of the tray for 60s to improve nucleation synchronization. The temperature is then lowered to -40℃ at a rate of 2.0℃ / min and maintained. A K-type thermocouple embedded in the geometric center of the thickest slice monitors the core temperature in real time. Complete freezing is determined by ensuring the core temperature reaches -40℃ and is maintained for 5 minutes. The total duration of this stage does not exceed 25 minutes. After freezing, the material is quickly transferred to a vacuum freeze dryer, with slices loaded in a single layer without overlap. The chamber door is closed, and a vacuum of 25Pa is applied. The segmented temperature control curve is activated, and the core temperature is collected at 5-second intervals throughout the process. Simultaneously, readings from the capacitance vacuum gauge and the Pirani vacuum gauge are recorded for inter-segment analysis.

[0025] In the first stage of sublimation drying, the shelf temperature is set to -30℃ and maintained until a product temperature plateau occurs and ends, while the ratio of the capacitance vacuum gauge reading to the Pirani vacuum gauge reading is ≤1.15. Once both conditions are met, the process switches to the second stage. In the second stage of sublimation drying, the shelf temperature is set to -20℃, maintaining the above two criteria again. If the product temperature is detected to be within 3℃ of the preset collapse temperature at any stage, the control system automatically lowers the shelf temperature by 3℃ and extends the current stage by 20 minutes before reassessing and proceeding to the next stage. In the analytical drying stage, the shelf temperature is raised to 20℃ and maintained, while the chamber pressure is maintained at ≤30Pa until the capacitance / Pirani ratio stabilizes and the product quality shows no continuous downward trend. Drying ends, and nitrogen is used to backfill to 0.03MPa. The product is then removed after it warms up to 22℃. The extracted samples were immediately sealed and packaged: a multi-layer barrier composite membrane with oxygen permeability ≤2mL / (m²·d·atm) and water vapor permeability ≤5g / (m²·d) was used. After nitrogen filling, the membrane was heat-sealed at a sealing temperature of 150℃ and a sealing time of 1.2s. The sealing pressure was performed according to the equipment calibration value.

[0026] Example 2 Compared with Example 1, the difference is that the pre-freezing temperature is adjusted to -38°C and liquid nitrogen is used for rapid freezing.

[0027] Example 3 Compared with Example 1, the difference is that in the vacuum freeze-drying stage, the temperature of the silo plate is raised in three stages: first, it is maintained at -25°C for stable sublimation, then raised to -15°C for desorption, and then slowly raised to -5°C to complete the structural fixation, so that the water sublimation process is more uniform.

[0028] Example 4 The difference compared to Example 1 is that nitrogen gas is used for purging and sealing in the post-processing stage.

[0029] Comparative Example 1 The difference compared to Example 1 is that the pre-freezing treatment only involves slow freezing to -18°C in a regular cold storage.

[0030] Comparative Example 2 Compared with Example 1, the difference is that: no segmented temperature control was set during vacuum freeze drying, but a constant low temperature (40°C) was maintained throughout the process.

[0031] Comparative Example 3 The difference compared to Example 1 is that the post-processing only uses room temperature polyethylene bags for direct sealing.

[0032] Experimental tests were conducted on all embodiments and comparative examples, and the test results are shown in Table 1.

[0033] Table 1

[0034] It should be noted that the moisture content (%) was calculated using the oven drying method, where the product was dried at 105℃ to constant weight, and the average of the three drying cycles was taken.

[0035] Water activity (aw): Measured using a water activity meter, and the average value is taken from three measurements.

[0036] Residual oxygen in packaging (%, volume fraction): measured by headspace oxygen analyzer within 1 hour after sealing.

[0037] Shape retention rate (%): The ratio of the overlapping areas of the binarized contours of the photos before and after drying, averaged from three points.

[0038] Rehydration time (min): Placed in pure water at 25℃, record the time required to restore to 80% of the original thickness, and take the median value from three measurements.

[0039] Rehydration rate (mass ratio): Weigh the sample 5 minutes after rehydration, and calculate the rehydrated mass / dry sample mass, taking the average of three measurements.

[0040] Sensory evaluation (out of 10): Five trained sensory officers rated the appearance, taste, aroma, and chewiness, and the average score was taken.

[0041] As shown in the table, the moisture content was tested first in the sample testing of Examples 1–4 and Comparative Examples 1–3. The results show that the moisture content of the sample samples in the examples was generally controlled within a low range, with small fluctuations between batches, indicating that the described preparation process can effectively reduce residual moisture in the product, thereby improving its storage stability. However, the moisture content of the comparative examples was relatively high, and the differences between repeated tests were large, indicating that the existing method has shortcomings in terms of the uniformity and stability of moisture removal.

[0042] In the water activity determination, the aw values ​​of the example samples were all below 0.30, which was significantly better than those of the comparative samples. This means that the products of the examples have a lower risk of microbial growth during storage and can significantly extend their shelf life; while the aw values ​​of the comparative samples were close to or exceeded 0.40, posing a potential risk of spoilage.

[0043] The results of residual oxygen detection inside the packaging showed that the residual oxygen level of the example sample was low 1 hour after packaging, indicating that the structure and packaging formed in the preparation process were better matched and could effectively inhibit oxygen residue and penetration; in contrast, the residual oxygen content in the comparative sample was significantly higher, which could easily cause oxidative flavor or color deterioration.

[0044] Regarding shape retention, the sample from the example exhibited higher overlap in its outer contours, indicating less tissue collapse after drying and better maintenance of its original shape. This is closely related to the density of its internal structure and the uniformity of drying. In contrast, the comparative sample showed significant collapse and warping, resulting in a decreased shape retention.

[0045] In the rehydration performance test, the sample from the example showed a shorter rehydration time and a significantly higher rehydration rate than the comparative sample, indicating that its porous structure has good permeability when water enters, and can quickly recover to a near-original state. The comparative sample showed a slow rehydration speed and a low rehydration rate, indicating that its structure was severely damaged during the drying process and the water absorption path was blocked.

[0046] The sensory evaluation results further confirmed the above physical indicators. The sample of the example scored highly in terms of appearance, color, taste recovery, and aroma retention, indicating that the above-described technical solution can maintain the original sensory quality to the maximum extent during the drying process; while the comparative sample scored lower, showing a dull color, poor rehydration taste, or weakened flavor.

[0047] Based on the above tests and analyses, it can be seen that the embodiment is significantly superior to the comparative example in terms of moisture control, structural stability, rehydration performance, and sensory quality.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for processing dried cantaloupe fruit using low-temperature vacuum freeze-drying to preserve its shape, characterized in that, include: Peel and seed the cantaloupe, and slice it to a thickness of 4mm-10mm. Pretreatment is performed by immersing the slices in a mixed solution containing 0.2%-0.8% calcium lactate and 0.05%-0.20% ascorbic acid at 0°C to 4°C for 10 min to 30 min. Pulsed vacuum impregnation pre-dehydration and vitrification enhancement are performed. The pulsed vacuum impregnation is to perform 3 to 5 vacuum-venting cycles in a 5%-15% trehalose solution. Each cycle is held at a relative vacuum of 80kPa-95kPa for 60s-120s and then recovered at atmospheric pressure for 60s-120s. The liquid temperature is 0℃-4℃. An edible conformal microfilm was constructed after pulsed vacuum impregnation. The conformal microfilm was formed by in-situ crosslinking by first spraying a 0.4% to 1.0% sodium alginate solution and then atomizing and spraying a 0.5% to 2.0% calcium lactate solution. The film thickness was 5 μm to 30 μm. Rapid pre-freezing is performed, first holding the sample at -2℃ to -5℃ for 30s-120s for nucleation, then lowering it to -35℃ to -45℃ at a rate of not less than 1.5℃ / min and holding it until completely frozen; vacuum freeze-drying is then performed under high vacuum conditions, with the chamber pressure not exceeding 30Pa, using segmented temperature control curves for sublimation drying and desorption drying: the first sublimation stage shelf temperature is -35℃ to -25℃, the second sublimation stage shelf temperature is -25℃ to -15℃, and the desorption drying stage shelf temperature is 10℃ to 25℃; after drying is completed, the sample is warmed to 20℃ to 25℃ and filled with nitrogen, with the residual oxygen volume fraction below 3%, and then sealed and packaged.

2. The method according to claim 1, characterized in that, The pH of the pretreatment solution is 3.3-3.8, and a citrate buffer is used to maintain pH stability. After pretreatment, the solution is rinsed with flowing pure water at 0℃-4℃ for 30s-120s.

3. The method according to claim 1, characterized in that, The number of cycles for vacuum holding and atmospheric pressure recovery in the pulsed vacuum impregnation is 4, with the upper limit of the vacuum holding time for the first cycle and the upper limit of the atmospheric pressure recovery time for the last cycle.

4. The method according to claim 1, characterized in that, The in-situ crosslinking step of the conformal microfilm is completed by alternately spraying both sides of the slice, and then standing at 0℃-4℃ for 2min-5min after spraying.

5. The method according to claim 1, characterized in that, The nucleation process involves applying ultrasound at 20kHz-40kHz and a power density of 0.4W / cm²-0.8W / cm² for 30s-90s before the cooling process begins.

6. The method according to claim 1, characterized in that, The time required for the rapid pre-freezing temperature to reach -35℃ to -45℃ shall not exceed 25 minutes. During the pre-freezing process, the slices shall be laid out in a single layer with a spacing of not less than 5mm between layers, and the wind speed shall be 0.5m / s to 1.5m / s to form uniform heat exchange.

7. The method according to claim 1, characterized in that, The segment switching of the segmented temperature control curve is triggered when the temperature sublimation plateau ends and the ratio of the readings of the capacitance vacuum gauge and the Pirani vacuum gauge is not higher than 1.

15.

8. The method according to claim 1, characterized in that, During the vacuum freeze-drying process, the temperature is monitored online using embedded thermocouples with a sampling period of no more than 10 seconds. If the detected temperature is close to the collapse temperature by 3°C, the shelf temperature is automatically reduced by 2°C to 5°C and the current time is extended by 10 to 30 minutes.

9. The method according to claim 1, characterized in that, The sealed packaging uses a multi-layer barrier composite film with an oxygen permeability of no more than 2 mL / (m²·d·atm) and a water vapor permeability of no more than 5 g / (m²·d). The nitrogen filling pressure is 0.02 MPa to 0.06 MPa, the sealing temperature is 130℃ to 160℃, and the sealing time is 0.8 s to 2.0 s.

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