Method for recovering grape drying wastewater based on low-temperature freeze drying and application

Abstract

The invention belongs to the technical field of fruits, vegetables and food, and discloses a method for recovering grape drying wastewater based on low-temperature freeze-drying and application, the method comprises the steps of selecting grapes, taking the grapes, cleaning, wiping and pre-freezing, drying the grapes by freeze-drying, and recovering water generated in the process to obtain grape freeze-drying recovered water. According to the method for recovering the moisture through low-temperature freeze drying, the lost moisture in the raisin processing process can be efficiently recovered, and the obtained condensate water keeps natural fragrance and nutritional activity, can be directly used for food and daily chemical industries and has remarkable economic and ecological benefits. According to the method, the comprehensive utilization rate of the fruit and vegetable drying process is increased, the application field of fruit and vegetable processing byproducts is expanded, a new solution is provided for green and sustainable development of the food industry, and the problems of waste water recycling and reutilization in the fruit and vegetable drying process are solved.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable and food technology, and in particular to a method and application for recovering wastewater from dried grapes based on low-temperature freeze drying. Background Technology

[0002] Grape drying is a dehydration process in which a large amount of water is removed through evaporation or sublimation. In traditional production, this water is often considered a "byproduct" and is frequently discharged or evaporated as unused process water. In recent years, with the deepening of the concepts of energy conservation, emission reduction, and resource utilization, low-temperature freeze-drying technology has been gradually applied to the fruit and vegetable processing industry. The water released during the drying process can be systematically recovered, realizing the transformation and value-added utilization of traditional waste into high-purity natural water resources. Unlike traditional wastewater treatment, the goal of this technology is not wastewater purification, but rather to preserve the original characteristics of the naturally dissolved and volatile components in the water, giving it potential high-value-added uses.

[0003] The water inside the grapes is pre-frozen and then rapidly frozen. During the low-temperature freeze-drying process, it sublimates directly from a solid to a gaseous state in a vacuum environment. It then naturally flows to the condenser, the area with the lowest temperature and pressure in the machine, where it re-sublimates back into solid ice. This process takes place at extremely low temperatures, typically between -40°C and -60°C, and avoids high-temperature oxidation or thermal degradation. Therefore, the water obtained after condensation still contains phenolic substances such as proanthocyanidins B2. Figure 1 Soluble sugars, organic acids, aromatic substances, and natural volatile components are present in the recycled water. These components endow the recycled water with a unique grape aroma and bioactivity potential, making it valuable for development as a natural additive, flavoring water, or functional raw material. After simple physical filtration and sterilization, this type of condensate can be directly used in food additives, beverage formulations, cosmetic base liquids, or aromatic extracts, realizing the "resource utilization of water" and the "re-value utilization of products." Related equipment for low-temperature freeze-drying technology includes... Figure 2 As shown.

[0004] Compared to traditional processes such as rotary evaporation, hot air drying, vacuum drying, and heated freeze-drying, low-temperature freeze-drying offers significant advantages in terms of moisture preservation and component protection. Hot air drying involves high temperatures, which easily leads to the loss of aromatic substances and heat-sensitive components in grapes. The condensed water contains almost no aroma components and is accompanied by high energy consumption and decreased product quality. While vacuum drying reduces oxidation to some extent, it still suffers from nutrient degradation and flavor loss due to heat conduction (heating), limiting the value of the condensed water. Heated freeze-drying, although able to maintain the rehydration and appearance of the product, promotes the release of some volatile organic compounds, reducing the natural components of the recycled water. In contrast, low-temperature freeze-drying systems operate without heating, and the sublimated gases almost completely preserve the natural volatile substances of the fruits and vegetables. After collection in the condenser, this yields natural water resources with original aroma, low pollution, and high purity.

[0005] Therefore, from a resource utilization perspective, the application of low-temperature freeze-drying in the raisin industry is no longer limited to improving the quality of dried products, but also lies in its pioneering approach to "natural moisture recovery and high-value utilization." The condensate recovered by this technology does not require complex purification processes; simple physical treatment is sufficient to meet standards for non-drinking, processing, and even cosmetic applications, making it a novel natural raw material for the food, fragrance, and daily chemical industries. Through this process, the moisture lost during drying is effectively captured and reintegrated into the product cycle, achieving true green production and resource recycling.

[0006] By comparison, the following patent publications related to this invention patent application were found: Comparison Patent 1: Patent "An Improved Hot Air Drying Device (CN210154278U)" proposes an improved hot air drying device aimed at solving the problem of excessively high steam condensate temperature in traditional drying devices. This device uses multiple heat exchangers connected in series and a moving chamber system to transport the generated high-temperature condensate to a preheater for cooling, thus avoiding scalding of materials from direct use of the high-temperature condensate. By driving a motor to move the moving plates reciprocating, utilizing the principles of negative pressure and water pressure, the condensate can be recovered and reused within a reasonable temperature range, improving steam utilization and reducing energy consumption.

[0007] Existing drawbacks: (1) Hot air drying may lead to loss of material quality, especially for heat-sensitive materials; (2) Hot air drying requires high heat energy; (3) The technology is also limited in operation and applicability, especially for extremely sensitive materials.

[0008] Comparison Patent 2: The aforementioned problem of hot air drying easily damaging heat-sensitive substances in raw materials is addressed by patent "A Novel Circulating Vacuum Dryer (CN103512323B)". This novel circulating vacuum dryer mainly consists of a vacuum drying cylinder, a heating device, a condensing device, circulating pipelines, and a vacuum pump. It removes moisture from the material by heating and evaporating it under vacuum conditions, and utilizes the condensing device to circulate and recover steam, thereby improving thermal efficiency and reducing energy consumption. The innovation of this device lies in the combination of a circulating structure and vacuum drying, which not only accelerates the drying process but also maintains the color and quality of the material to a certain extent. It is suitable for temperature-sensitive materials such as food, traditional Chinese medicine slices, and agricultural by-products.

[0009] Existing defects: (1) Since it still relies on heating to drive evaporation, the drying temperature is relatively high compared to freeze drying, resulting in limited protection for heat-sensitive substances and active ingredients; (2) The material is prone to collapse and shrinkage during the drying process, and the microstructure is difficult to maintain; (3) The circulation pipeline and condensation device may be blocked or scaled, affecting the long-term operational stability, while the drying uniformity depends on precise temperature and vacuum control.

[0010] A comparison reveals a fundamental difference between this invention and the aforementioned patent publications. The method of this invention utilizes low-temperature freeze-drying to recover moisture, achieving highly efficient recovery of moisture lost during the raisin-making process. Firstly, freeze-drying technology directly removes moisture at low temperatures, better protecting heat-sensitive materials and preventing high-temperature damage, while hot air drying and vacuum drying still rely on heating, potentially compromising material quality. Secondly, freeze-drying offers significant advantages in temperature control, maximizing the preservation of the material's color, flavor, and active ingredients, while also boasting high energy efficiency and reducing resource waste. Furthermore, this invention improves resource recycling efficiency and reduces environmental pollution by recovering raisin-making wastewater, demonstrating better environmental friendliness and sustainability. In contrast, the other two technologies are less effective in wastewater recovery and long-term stability. Overall, this invention offers significant advantages in material protection, energy recovery, and wastewater utilization, providing a more efficient and environmentally friendly solution. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and application for recovering dried grape wastewater based on low-temperature freeze drying.

[0012] The technical solution adopted by this invention to solve its technical problem is: A method for recovering grape drying wastewater based on low-temperature freeze drying, the method includes grape selection, grape washing, drying, pre-freezing, and freeze drying the grapes to recover the water generated during the process, to obtain grape freeze-dried water recovery.

[0013] Furthermore, the specific steps are as follows: (1) Selecting grapes: Select grapes that are free from rot, pests, and mold, and are uniform in shape and size, with a weight of 10-15 g / fruit. (2) Removing the grapes: Twist the grapes off the stems to remove the stems while keeping the grapes intact; (3) Washing: Use running water to wash the surface of the grapes clean; (4) Dry: After washing, remove excess moisture from the surface of the grapes until they are dry; (5) Pre-freezing: Place the dried grapes at -18 ℃, -40 ℃, -80 ℃ and -196 ℃ for 24 h to ensure that all the water in the grapes exists in the form of ice crystals; (6) Freeze-drying: Turn on the freeze dryer half an hour in advance to ensure that the cold trap temperature is maintained at -45±1℃; take out the pre-frozen grapes, place them on the sample tray and then place them on the sample rack, cover them with the protective cover to ensure sealing, start vacuuming, ensure that the vacuum degree is maintained at 10±2 Pa, control the freeze-drying time to 12~108 h, and obtain grape freeze-dried water recovery.

[0014] Furthermore, the grapes mentioned are Muscat grapes.

[0015] Furthermore, in step (4), kitchen paper towels are used to absorb excess moisture from the surface of the grapes.

[0016] Furthermore, in step (6), the freeze dryer is FD-1A-50+ with a power of 850 W and a freeze-drying efficiency of 2 L / 24 H.

[0017] Furthermore, in step (5), the dried grapes are pre-frozen at -80 ℃ for 24 h; in step (6), the freeze-drying time is controlled to be 60 h.

[0018] The application of the method described above in the recycling of grape drying wastewater.

[0019] The advantages and positive effects of this invention are as follows: 1. This invention utilizes a low-temperature freeze-drying method to efficiently recover moisture lost during the raisin drying process. The resulting condensate retains its natural aroma and nutritional activity, and can be directly used in the food and daily chemical industries, demonstrating significant economic and ecological benefits. This invention not only improves the comprehensive utilization rate of the fruit and vegetable drying process, providing a new solution for the green and sustainable development of the food industry, but also solves problems related to the resource utilization and reuse of wastewater during fruit and vegetable drying.

[0020] 2. Preservation of natural aroma and active ingredients: This invention achieves water sublimation under low temperature conditions of -40 ℃ to -60 ℃, avoiding high temperature oxidation and thermal degradation, and preserving the original aromatic substances, phenolic compounds, soluble sugars, organic acids and other bioactive substances in grapes to the maximum extent, so that the condensate has natural aroma and functional characteristics, and has high added value as a food and daily chemical raw material.

[0021] 3. Achieving high-value reuse of water: This invention effectively recovers the water that was originally emitted as steam during the drying process, transforming it into a reusable natural resource. The resulting condensate can meet non-drinking or processing standards after simple physical filtration and sterilization, without the need for complex chemical purification processes. This realizes the recycling of "waste water resources" and promotes the resource utilization and recycling of food processing waste.

[0022] 4. Maintaining the quality and structural integrity of dried products: Because the drying process of this invention is carried out under low-temperature vacuum conditions, it avoids the problems of dull color, loss of aroma, and tissue collapse caused by hot air drying. The dried products retain their natural color and good rehydration properties, while obtaining high-quality by-products of condensate, thus achieving a dual improvement in product quality and by-product value.

[0023] 5. Combining economic and ecological benefits: This invention improves the quality of dried raisins while achieving energy conservation and wastewater recycling, reducing waste emissions and environmental pollution, thus demonstrating excellent environmental performance and ecological sustainability. The resulting condensate can be used as a food flavoring, beverage formulation, and cosmetic raw material, exhibiting significant economic value and promising prospects for industrial promotion.

[0024] 6. Strong Technological Innovation and Application Expansion: Compared with existing drying equipment, this invention is the first to apply low-temperature freeze-drying technology to the recovery and utilization of by-product water from fruit and vegetable drying, realizing a transformation from "single dehydration" to "multi-resource utilization." This method can not only be used for grape drying, but can also be extended to the resource utilization of condensate from other fruits, vegetables, and traditional Chinese medicinal materials, showing broad industrial application potential.

[0025] 7. This invention is a method for recovering raisin drying wastewater based on low-temperature freeze-drying, which improves the comprehensive utilization rate of resources in the entire fruit and vegetable drying process, reduces raw material waste and environmental impact during production. The condensate, as a recovered product, not only has the potential for high-value utilization but also expands the application fields of by-products. It breaks the cycle of resource loss during raisin and fruit and vegetable drying, realizing the recovery and reuse of moisture during the drying process. This not only effectively improves resource utilization efficiency but also promotes ecological benefits, providing a new path for green production models. Attached Figure Description

[0026] Figure 1 The structural formula of proanthocyanidin B2 in the existing technology; Figure 2 This is a schematic diagram of the structural connection of an equipment for low-temperature freeze-drying technology in the prior art; wherein, the attached diagram is labeled as follows: 1. Shelf; 2. Vacuum hood; 3. Material tray; 4. O-ring seal; 5. Touch screen; 6. Heat dissipation vent; 7. Air extraction hole; 8. Moisture collection port / vent; 9. Vacuum pump; 10. Collection bottle; Figure 3 This is the standard curve of proanthocyanidin B2 in this invention; Figure 4 This is a graph showing the total phenol content of freeze-dried water at different pre-freezing temperatures in this invention. Figure 5 This is a graph showing the proanthocyanidin B2 content of freeze-dried water at different pre-freezing temperatures in this invention. Figure 6 This is a graph showing the total phenol content of water recovered at different freeze-drying times in this invention. Figure 7 This is a graph showing the proanthocyanidin B2 content in water recovered at different freeze-drying times in this invention. Figure 8 This is a graph showing the total phenol content of water recovered at different rotary evaporation times in this invention. Figure 9 This is a graph showing the proanthocyanidin B2 content in water recovered at different rotary evaporation times in this invention. Detailed Implementation

[0027] The present invention will be further described below with reference to the embodiments. The following embodiments are descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0028] The various experimental operations involved in the specific embodiments are all conventional techniques in the art. For parts not specifically annotated herein, those skilled in the art can refer to various commonly used reference books, scientific and technological literature, or related instructions and manuals prior to the filing date of this invention for implementation. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art, and the raw materials used are all commercially available products.

[0029] A method for recovering grape drying wastewater based on low-temperature freeze drying, the method includes grape selection, grape washing, drying, pre-freezing, and freeze drying the grapes to recover the water generated during the process, to obtain grape freeze-dried water recovery.

[0030] Preferably, the specific steps are as follows: (1) Selecting grapes: Select grapes that are free from rot, pests, and mold, and are uniform in shape and size, with a weight of 10-15 g / fruit. (2) Removing the grapes: Twist the grapes off the stems to remove the stems while keeping the grapes intact; (3) Washing: Use running water to wash the surface of the grapes clean; (4) Dry: After washing, remove excess moisture from the surface of the grapes until they are dry; (5) Pre-freezing: Place the dried grapes at -18 ℃, -40 ℃, -80 ℃ and -196 ℃ for 24 h to ensure that all the water in the grapes exists in the form of ice crystals; (6) Freeze-drying: Turn on the freeze dryer half an hour in advance to ensure that the cold trap temperature is maintained at -45±1℃; take out the pre-frozen grapes, place them on the sample tray and then place them on the sample rack, cover them with the protective cover to ensure sealing, start vacuuming, ensure that the vacuum degree is maintained at 10±2 Pa, control the freeze-drying time to 12~108 h, and obtain grape freeze-dried water recovery.

[0031] Preferably, the grapes are Muscat grapes.

[0032] Preferably, in step (4), kitchen paper towels are used to absorb excess moisture from the surface of the grapes; Alternatively, in step (6), the freeze dryer is FD-1A-50+ with a power of 850 W and a freeze-drying efficiency of 2 L / 24 H.

[0033] The present invention adopts the following technical solution: a freeze-drying process for Muscat grapes, including selecting Muscat grapes, cleaning the grapes, drying them, pre-freezing them, and freeze-drying them. The total phenols and proanthocyanidins B2 content of the recovered water are determined by the Folin-Ciocalteu method and high performance liquid chromatography, respectively. The content of the above substances is compared with that of the recovered water obtained by rotary evaporation of grapes.

[0034] Specifically, the relevant preparation and testing methods are as follows: Example 1 Select Muscat grapes from Tianjin Chadian that are free from rot, pests, and mold, and are uniform in shape and size, weighing 10-15 g each. Slowly twist the grapes off the stem, removing the stem while keeping the grape intact. Wash the grapes thoroughly with running water, and then use kitchen paper towels to absorb excess moisture until dry. Then proceed with the following treatment: Freeze-drying for 48 h: The dried grapes were pre-frozen for 24 h at -18 ℃, -40 ℃, -80 ℃, and -196 ℃ (liquid nitrogen treatment) to ensure that all the water in the grapes existed in the form of ice crystals. The freeze dryer (FD-1A-50+, power 850 W, freeze-drying efficiency 2 L / 24 H) was turned on half an hour in advance to ensure that the cold trap temperature was maintained at around -45 ℃. The pre-frozen grapes were taken out, placed in a sample tray, and then placed on a sample rack. The protective cover was closed, and a vacuum was drawn to ensure that the vacuum degree was maintained at around 10±2 Pa. Freeze-dried water was obtained after 48 h of freeze-drying.

[0035] The total phenolic content of the lyophilized water samples was determined using the Folin-Ciocalteu method. The specific procedure was as follows: 25 μL of the lyophilized water sample or gallic acid standard solution (concentration range 15.63-1000.00 μg / mL) was mixed with 125 μL of 0.2 M Folin-Ciocalteu reagent in a 96-well plate. After incubation at room temperature for 10 minutes, 125 μL of 0.71 M Na₂CO₃ solution was added, and the mixture was reacted in the dark for 2 hours. The absorbance was measured at 765 nm using a multi-functional microplate reader (Synergy HTX, BioTek Instruments Co., Ltd., Winooski, USA), with a 70% methanol solution containing 0.1% glacial acetic acid as a blank control. The total phenolic content was expressed as micrograms of gallic acid equivalent (μg GAE / L) per liter of lyophilized recycled water sample (regression equation: y = -0.2313x + 1.6203, R0). 2 =0.9991).

[0036] Chromatographic grade proanthocyanidin B2 standards were prepared into standard solutions with concentrations of 0.005, 0.01, 0.02, 0.04, 0.06, and 0.08 mg / mL, and the peak areas were measured. A standard curve was plotted with the standard concentration (mg / mL) on the x-axis and the peak area on the y-axis, and linear regression was performed: Proanthocyanidin B2 standard curve: y = 102094x - 166.15, R0 2 =0.9955 ( Figure 3 ).

[0037] The content of proanthocyanidin B2 was determined by high-performance liquid chromatography (HPLC). The specific procedure was as follows: The lyophilized and recovered water was filtered through a 0.22 μm membrane and injected into a 2 mL brown bottle. Compound identification and quantification were performed using an Agilent Technologies (USA) 1260 Infinity HPLC system equipped with a C18 column. The flow rate was 1 mL / min, and the gradient mobile phase consisted of solvent A (0.1% acetic acid in aqueous solution) and solvent B (0.1% acetic acid in methanol): Solvent B: 0-2 min 5%, 2-4 min 15%, 4-12 min 25%, 12-25 min 35%, 25-30 min 100%, 30-32 min 5%. The column temperature was set to 25 ℃. The detection wavelength was 245 nm, and the injection volume was 20 μL.

[0038] from Figure 4 The results showed that after freeze-drying at different pre-freezing temperatures, the total phenol content (expressed as μg GAE / L) in the recovered water exhibited a clear trend of first increasing and then decreasing, and the differences among the treatments reached a significant level. P<0.05). Specifically, the total phenol content was lowest at a pre-freezing temperature of -18 ℃, approximately 4132.98 μg GAE / L; when the pre-freezing temperature dropped to -40 ℃, the total phenol content significantly increased to approximately 4764.65 μg GAE / L; it reached its highest value at -80 ℃, approximately 5239.29 μg GAE / L; however, when the pre-freezing temperature was further reduced to -196 ℃, the total phenol content decreased compared to -80 ℃, to approximately 4837.94 μg GAE / L. Overall, lowering the pre-freezing temperature can increase the total phenol content of the recovered water, but this increase has an optimal range, with -80 ℃ showing the peak point for the release level of total phenols from the recovered water. This trend can be explained from the perspective of the influence of freezing kinetics on ice crystal structure, the degree of tissue damage, and phenol migration behavior. At higher pre-freezing temperatures (e.g., -18 °C), the freezing rate is relatively slow, and the sample spends a longer time in the temperature transition phase. Phenolic substances are more likely to undergo a certain degree of oxidation or enzymatic reaction, resulting in loss. Simultaneously, the ice crystal morphology formed under these conditions and its destructive effect on cell structure are relatively limited, which is not conducive to the release of phenols from the tissue matrix and their migration into the recycled water during subsequent freeze-drying. Therefore, the total phenol content in the recycled water is at a low level. As the pre-freezing temperature decreases to -40 °C and -80 °C, the freezing process accelerates. On the one hand, it can more quickly inhibit adverse reactions such as oxidation and improve the stability of polyphenols; on the other hand, it is also more likely to generate cracks and microstructural damage on the cell wall and cell membrane, thus providing more migration channels during freeze-drying sublimation and solute redistribution, promoting more complete entry of soluble phenols into the recycled water system. Therefore, the total phenol content increases significantly, reaching its maximum at -80 °C. However, when the pre-freezing temperature is further reduced to -196 °C (liquid nitrogen-level ultra-low temperature), freezing is almost instantaneous, often forming finer, more uniform microcrystalline structures or even localized vitrification. Although this ultra-rapid freezing method has a stronger inhibitory effect on chemical reactions and a better protective effect on the components themselves, in terms of the total phenol content entering the recycled water, excessively small ice crystals may have a weaker mechanical fracturing effect on tissues, and the development of pores and migration channels may not be optimal. Some phenols may be embedded or retained in the dry matrix, resulting in a lower migration rate to the recycled water compared to the -80 °C condition, manifested as a decrease in the total phenol content. Overall, -80 °C may achieve a better balance between reducing phenol degradation and promoting tissue damage and solute migration, thus corresponding to the highest level of total phenol content in the recycled water.

[0039] from Figure 5 The data show that after freeze-drying grapes at different pre-freezing temperatures, the content of proanthocyanidins B2 (μg / L) in the recovered water exhibited a significant non-linear change, accompanied by clear and significant differences between groups. P<0.05). In terms of numerical levels, the proanthocyanidin B2 content was lowest at -18 ℃, approximately 1520.43 μg / L; after the pre-freezing temperature dropped to -40 ℃, the content rose to approximately 1875.48 μg / L; when the pre-freezing temperature was further reduced to -80 ℃, a "jump" occurred, reaching the highest value of approximately 4210.37 μg / L; while at -196 ℃ (liquid nitrogen pre-freezing), the content decreased to approximately 3898.52 μg / L, but was still significantly higher than at -18 ℃ and -40 ℃. As the pre-freezing temperature decreased, the proanthocyanidin B2 content first gradually increased, reaching a peak at -80 ℃, and then slightly decreased at -196 ℃ (with a post-peak decline), showing a single-peak change of first increasing and then decreasing. Essentially, this is related to the coupling mechanism between the freezing rate, ice crystal morphology, degree of tissue microstructure damage, and solute release / migration caused by changes in pre-freezing temperature. At higher pre-freezing temperatures (-18 °C), the freezing rate is relatively slow, and the sample undergoes a longer phase transition and temperature transition phase. The rupture of cell structures and the formation of pore channels are relatively insufficient, which is detrimental to the desorption and migration of proanthocyanidin B2 from cell walls / vacuoles and other tissue structures. Simultaneously, the longer semi-freezing time before and after freezing may provide a window for oxidation-related processes (such as polyphenol oxidase or non-enzymatic oxidation), causing some B2 to oxidize, condense, or bind to the matrix, reducing its release proportion. Therefore, the proanthocyanidin B2 level in the recovered water is the lowest. Lowering the pre-freezing temperature to -40 °C accelerates freezing, more effectively inhibiting oxidative loss and enhancing cell membrane / wall damage and microcrack formation to a certain extent, thus improving the release and migration capacity of proanthocyanidin B2. Therefore, the content increases significantly compared to -18 °C, but remains relatively low. When the pre-freezing temperature reaches -80 °C, the freezing rate and ice crystal growth behavior often achieve a better balance between protecting component stability and generating sufficient structural damage / pore channels. Rapid freezing significantly shortens the time that phenols are exposed to an oxidizable environment, which is beneficial to maintaining the chemical stability of proanthocyanidin B2. Under these conditions, the ice crystals and phase interface effects formed are more likely to cause cell structure rupture, pore network and mass transfer channel establishment. As a result, during freeze-drying sublimation and subsequent water migration / condensation, more proanthocyanidin B2 or droplets / solutes containing this substance migrate into the recycled water system with the water, which is manifested as a significant jump in the proanthocyanidin B2 content in the recycled water and reaches a peak value. Conversely, when the pre-freezing temperature is further reduced to -196 °C, freezing is completed almost instantaneously, which is more likely to form finer and more uniform microcrystals or even localized vitrification structures. This ultra-rapid freezing has a stronger inhibitory effect on chemical reactions, but excessively fine ice crystals may not produce the strongest mechanical tearing effect. The development of tissue pores and migration channels may not be as sufficient as at -80 °C. Some proanthocyanidins B2 may be more easily embedded / retained in the freeze-dried solid matrix, resulting in a decrease in the net migration to the recycled water. Therefore, a peak and subsequent decline occur, but the overall level remains high and is significantly higher than at -18 °C and -40 °C.In summary, -80 ℃ is more likely to achieve the optimal balance between reducing degradation loss and promoting tissue damage and mass transfer release, thus obtaining the highest proanthocyanidin B2 content in the recovered water.

[0040] Example 2 Select Muscat grapes from Tianjin Chadian that are free from rot, pests, and mold, and are uniform in shape and size, weighing 10-15 g each. Slowly twist the grapes off the stem, removing the stem while keeping the grape intact. Wash the grapes thoroughly with running water, and then use kitchen paper towels to absorb excess moisture until dry. Then proceed with the following treatment: Freeze-drying for 12-108 h: Pre-freeze the dried grapes in a -80 ℃ freezer for 24 h to ensure that all the water inside the grapes exists in the form of ice crystals. Turn on the freeze dryer (FD-1A-50+, power 850 W, freeze-drying efficiency 2 L / 24 H) half an hour in advance to ensure that the cold trap temperature is maintained at about -45 ℃. Remove the pre-frozen grapes from the -80 ℃ freezer, place them in a sample tray and then on a sample rack, cover with a protective cover, and start vacuuming to ensure that the vacuum degree is maintained at about 10±2 Pa. Freeze-dried water was obtained after 12 h, 36 h, 60 h, 84 h, and 108 h of freeze-drying.

[0041] The methods for determining the total phenol content and proanthocyanidin B2 content are the same as in Example 1.

[0042] like Figure 6 As shown, different freeze-drying times have a significant effect on the total phenol content (in μg GAE / L) in grape recovery water. PThe total phenol content (<0.05) generally showed a trend of "increasing first and then decreasing." At 12 h of freeze-drying, the total phenol content in the recovered water was 4180.57 μg GAE / L, which then increased significantly with prolonged freeze-drying time, reaching approximately 4955.65 μg GAE / L at 36 h, and peaking at 5890.78 μg GAE / L at 60 h. Afterward, the total phenol content decreased, dropping to approximately 5069.16 μg GAE / L at 84 h, and further decreasing to approximately 4550.43 μg GAE / L at 108 h, but still higher than the initial 12 h level. Statistical analysis showed that the 60 h group was significantly higher than all other groups, while there was no significant difference between the 12 h and 108 h groups. The changes in total phenol content were mainly related to the degree of cell structure damage caused by the freeze-drying process and the mechanism of phenol release. At shorter freeze-drying times (e.g., 12 h), some cell walls and membrane structures rupture due to freezing stress, and soluble phenols begin to be released, but intracellular bound phenols are not yet fully dissociated. As the freeze-drying time is extended to 60 h, tissue water is removed more thoroughly, and the mechanical damage to cell structures caused by ice crystal sublimation intensifies, leading to a more complete release of bound phenols (including phenolic acids and flavonoids), resulting in a peak total phenol content. However, when the freeze-drying time is further extended to 84-108 h, the total phenol content decreases. This may be related to the oxidation, polymerization, or structural transformation of some phenolic substances during prolonged freeze-drying. Although freeze-drying usually inhibits oxidation reactions, prolonged exposure to low-temperature vacuum conditions may promote the oxidation of phenolic hydroxyl groups to quinone structures or condensation with other matrices, causing some phenols to become inactive or undetectable, ultimately leading to a decrease in the measured total phenol content.

[0043] like Figure 7 As shown, different freeze-drying times had a significant impact on the proanthocyanidin B2 content in grape recycled water. P<0.05), showing an overall trend of first increasing and then gradually decreasing in a single-peak pattern. Specifically, the proanthocyanidin B2 content in the freeze-dried water was approximately 2289.17 μg GAE / L at 12 h, rising to approximately 3298.55 μg GAE / L as the freeze-drying time was extended to 36 h, and reaching a peak of approximately 4480.84 μg GAE / L at 60 h, which was significantly higher than all other treatment groups. With further extension of the freeze-drying time, the proanthocyanidin B2 content began to decrease, slightly decreasing to 4093.96 μg GAE / L at 84 h, and further decreasing to approximately 3413.88 μg GAE / L at 108 h, but still higher than the initial 12 h level. This trend is mainly determined by the effect of the freeze-drying process on cell structure and the stability of phenolic substances. In the early and middle stages of freeze-drying (12-60 h), ice crystal formation continuously and significantly damaged the cell wall and cell membrane, allowing for a more complete release of the previously bound proanthocyanidins from the cell structure. Furthermore, the low temperature and low oxygen conditions of freeze-drying limit enzymatic browning and oxidation reactions, which is conducive to the retention of phenolic compounds, thus causing the proanthocyanidin B2 content to reach its peak at 60 h. Conversely, when the freeze-drying time is too long (60-108 h), the proanthocyanidin B2 content decreases, which may be related to the slow oxidation, polymerization, or structural transformation of some phenolic substances under long-term vacuum and drying conditions, reducing their detectable amount. Nevertheless, the proanthocyanidin B2 content in the later freeze-drying stage is still higher than the initial level, indicating that although the compound degrades to some extent, it still maintains a high overall retention ratio. Overall, the changes in proanthocyanidin B2 reflect the dynamic balance between the two effects of "structural destruction promoting release" and "excessive drying leading to degradation" during freeze-drying, with 60 h being the key time point where the release effect is most significant and the content is highest.

[0044] Meanwhile, by comparing Examples 1 and 2, it can be seen that steps (5) and (6) in the method of the present invention have a synergistic effect, which can synergistically improve the total phenol content and proanthocyanidin B2 content of the prepared freeze-dried grape water. In particular, the steps (5) "pre-freezing the dried grapes at -80 ℃ for 24 h" and (6) "controlling the freeze-drying time to 60 h" in the method of the present invention have a synergistic effect, which can synergistically improve the total phenol content and proanthocyanidin B2 content of the prepared freeze-dried grape water.

[0045] Example 3 Select Muscat grapes from Tianjin Chadian that are free from rot, pests, and mold, and are uniform in shape and size, weighing 10-15 g each. Slowly twist the grapes off the stem, removing the stem while keeping the grape intact. Wash the grapes thoroughly with running water, and then use kitchen paper towels to absorb excess moisture until dry. Then proceed with the following treatment: Rotary distillation of grape water: Place dried grapes (about 150 g) in a 500 mL round-bottom flask, install the rotary distillation system, and start vacuuming to ensure that the vacuum degree is maintained at about 10±2 Pa. Rotary distillation water is obtained at 3 h, 6 h, 9 h and 12 h respectively.

[0046] The methods for determining the total phenol content and proanthocyanidin B2 content are the same as in Example 1.

[0047] like Figure 8 As shown, different rotary evaporation times had a significant effect on the total phenol content (μg GAE / L) in the grape recovery water. P The total phenol content (GAE) showed a monotonically decreasing trend with increasing rotary evaporation time (<0.05). Specifically, the highest total phenol content was observed at 3 h of rotary evaporation, reaching approximately 4012.84 μg GAE / L. When the evaporation time was extended to 6 h, the total phenol content decreased to approximately 3346.65 μg GAE / L, further decreasing to approximately 2943.93 μg GAE / L at 9 h, and reaching its lowest point at 12 h, at approximately 2364.44 μg GAE / L. Statistical grouping also clearly showed significant differences between groups with increasing rotary evaporation time. This decreasing trend is mainly related to the thermal effects, volatilization losses, and stability of phenolic compounds during rotary evaporation. In the short-term rotary evaporation stage (e.g., 3 h), the total phenolic substances received limited heat exposure, resulting in a relatively stable structure and less loss, thus leading to a higher measured content. As rotary evaporation time increases, continuous heating of the system may promote the oxidation, degradation, or structural breakage of some phenolic compounds, especially temperature-sensitive phenolic acids and flavonoids, which are more likely to oxidize into quinone structures under prolonged heating conditions, thereby reducing the number of detectable phenolic hydroxyl groups. Furthermore, volatile or semi-volatile low-molecular-weight phenols may partially evaporate with the water vapor during rotary evaporation, causing a further decrease in their content. Prolonged reduced pressure may also promote condensation or polymerization of phenols with other matrices, causing some phenols to transform from soluble forms to insoluble bound forms, further reducing their detectable amount in the recycled water. Overall, the total phenol content in grape samples shows a significant decreasing trend with increasing rotary evaporation time, mainly due to the combined effects of multiple factors including thermal degradation, oxidative loss, volatilization migration, and structural transformation.

[0048] according to Figure 9The data shown indicate that the content of proanthocyanidin B2 in the recovered water decreased significantly as the rotary evaporation time increased from 3 h to 12 h. The content was highest at 3 h (approximately 2289.17 μg GAE / L), decreased to approximately 2098.55 μg GAE / L at 6 h, further decreased to approximately 1880.84 μg GAE / L at 9 h, and reached its lowest value at 12 h (approximately 1693.96 μg GAE / L). Overall, this showed a monotonically decreasing trend over time, with statistically significant differences between different time points. This trend reflects the instability of proanthocyanidin B2 during rotary evaporation, and its reduction is related to multiple chemical and physical mechanisms. First, proanthocyanidin B2 is a typical heat-sensitive polyphenol. Even under reduced pressure during rotary evaporation, it degrades due to continuous heating, including bond breaking, configurational changes, and oxidation into smaller molecules. Therefore, the longer the rotary evaporation time, the greater the degree of thermal degradation, leading to a continuous decrease in proanthocyanidin B2 content. Secondly, dissolved oxygen in the solution promotes the oxidation of polyphenols under heating conditions. Proanthocyanidin B2 easily generates free radicals and continues to be oxidized. Its oxidative condensation reaction with other phenols or organic components in the system also reduces the detectable content of B2 monomers. In summary, the decrease in proanthocyanidin B2 content with prolonged rotary evaporation time is the result of multiple factors, including thermal degradation, oxidation, and concentration effects.

[0049] In existing technologies, methods such as rotary evaporation, vacuum drying, reduced-pressure cold air drying, microwave vacuum drying, and infrared vacuum drying focus on "moisture removal," which is usually accompanied by the destruction of material structure and composition by heat energy. In contrast, vacuum freeze-drying technology, by directly sublimating water at low temperatures, not only preserves the original form, active ingredients, and flavor of the material to the greatest extent, but also efficiently condenses and recovers the moisture that has escaped from the material, achieving a dual leap in drying quality and resource recovery.

[0050] Although embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the contents disclosed in the embodiments.

Claims

1. A method for recovering grape drying wastewater based on low-temperature freeze-drying, characterized in that: The method includes selecting grapes, cleaning, drying, pre-freezing, and using freeze-drying to dry the grapes and recover the water generated during the process to obtain freeze-dried grape water.

2. The method according to claim 1, characterized in that: The specific steps are as follows: (1) Select grapes: Select grapes that are free from rot, pests, and mold, and are uniform in shape and size, with a weight of 10-15g / fruit; (2) Removing the grapes: Twist the grapes off the stems to remove the stems while keeping the grapes intact; (3) Washing: Use running water to wash the surface of the grapes clean; (4) Dry: After washing, remove excess moisture from the surface of the grapes until they are dry; (5) Pre-freezing: Place the dried grapes at -18 ℃, -40 ℃, -80 ℃ and -196 ℃ for 24 h to ensure that all the water in the grapes exists in the form of ice crystals; (6) Freeze-drying: Turn on the freeze dryer half an hour in advance to ensure that the cold trap temperature is maintained at -45±1℃; take out the pre-frozen grapes, place them on the sample tray and then place them on the sample rack, cover them with the protective cover to ensure sealing, start vacuuming, ensure that the vacuum degree is maintained at 10±2 Pa, control the freeze-drying time to 12~108 h, and obtain grape freeze-dried water recovery.

3. The method according to claim 1, characterized in that: The grapes mentioned are Muscat grapes.

4. The method according to claim 1, characterized in that: In step (4), kitchen paper towels are used to absorb excess moisture from the surface of the grapes.

5. The method according to claim 1, characterized in that: In step (6), the freeze dryer is FD-1A-50+ with a power of 850W and a freeze-drying efficiency of 2 L / 24 H.

6. The method according to any one of claims 1 to 5, characterized in that: In step (5), the dried grapes are pre-frozen at -80 ℃ for 24 h; in step (6), the freeze-drying time is controlled to be 60 h.

7. Application of the method described in any one of claims 1 to 6 in the recovery of grape drying wastewater.

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