Apple juice processing method adopting layered-suspended coupled freezing synergistic nanofiltration concentration

Abstract

The invention belongs to the technical field of fruit juice concentration processing, and relates to a layer type-suspension type coupled freezing synergistic nanofiltration concentration apple juice processing method which comprises the following steps: pre-cooling pretreated apple juice; millimeter-level ice crystals are formed through layered freezing, and first-level concentration is carried out to obtain ice and primarily concentrated fruit juice; continuously concentrating the juice subjected to the first-stage concentration to a required concentration in a suspension freezing crystallization manner, and then carrying out solid-liquid separation to obtain ice and concentrated juice; carrying out heating sweating, leaching and centrifugal separation on the ice crystals, and further recovering residual fruit juice in the ice; the ice crystal melting water is separated through a nanofiltration membrane, permeate pure water and retentate-fruit juice are obtained respectively, and the fruit juice is returned to the first-step closed loop to be reused for pretreatment. Through the synergistic effect of the two stages of processes, the concentration end point of the apple juice reaches 40-45 DEG Brix, and compared with a low-temperature evaporation concentration process, the method has the characteristics of low operation temperature, low energy consumption and high quality, and is a process capable of saving energy, reducing consumption, improving quality and improving efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of fruit juice concentration processing, and particularly relates to an apple juice processing method of layer-suspension coupled freezing and coordinated nanofiltration concentration. Background Art

[0002] Apple juice is a natural juice made from fresh apples through the steps of picking, washing, crushing and juicing. It is rich in fructose, glucose, organic acids (such as malic acid and citric acid), polyphenol active substances, as well as minerals such as vitamin C, potassium and calcium. Concentrating apple juice can not only significantly reduce its volume and weight, effectively lowering transportation and storage costs, but also extend the shelf life of the product.

[0003] At present, the traditional thermal concentration method is still mainly used in the apple juice concentration process, such as thermal evaporation methods such as multi-effect evaporation. Although the concentration efficiency is high, the high temperature conditions (45-95°C) can easily destroy the aromatic substances and heat-sensitive vitamins in the apple juice, resulting in flavor deterioration and nutrient loss. In addition, the energy consumption is high and the operating costs are high, which makes it difficult to meet the green processing needs of high-end non-concentrated and reduced juice.

[0004] Freeze concentration is a low-temperature cold processing technology that uses the principle of crystallization separation to partially freeze a solution at low temperatures, separating the solid and liquid to achieve concentration. This effectively preserves the natural aroma and nutrients of the juice. Freeze concentration is mainly divided into two methods: progressive lamellar crystallization and suspension crystallization.

[0005] The progressive lamellar crystallization method forms a gradually thickening ice layer on the heat exchange surface, with the liquid phase excluded from the crystals, and concentration proceeds gradually. The greatest advantage of this method is that it forms an integrated frozen layer of water, with large ice crystals and a small solid-liquid interface, making it easier to separate the concentrated juice from the ice crystals. However, progressive lamellar crystallization forms a frozen ice layer on the heat exchange surface. As the ice layer gradually thickens, the heat transfer efficiency decreases rapidly, and the water freezing rate becomes slower and slower. In particular, as the volume of the equipment increases, the heat transfer surface area also decreases, resulting in low heat transfer efficiency. Progressive lamellar crystallization will form a thicker frozen ice layer on the internal structure of the equipment, such as the tank wall and heat exchange coil surface. Not only is it difficult to clean, but ice melting is also a high-energy-consuming process, which restricts the practical application of this technology.

[0006] The suspended crystallization freeze concentration method utilizes the growth of ice crystals suspended in juice, followed by separation of the ice crystals from the juice to achieve juice concentration. Because a large number of finely suspended ice crystals are produced in the solution, the surface area of the ice per unit volume of juice is large, enabling the rapid formation of high-purity ice crystals. However, the process involves heat exchange and supercooling of the juice, ice crystal growth in the crystallization tank, separation of the juice and ice crystals by centrifugation or pressing, and cleaning of the remaining juice in the ice crystals. These processes are performed in separate equipment, requiring numerous equipment and supporting facilities, resulting in complex process flows and operations, and high investment and operating costs. This excessive number of operational steps increases the risk of microbial contamination, limiting the application of this technology in general production enterprises.

[0007] Chinese patent CN116235913A discloses a method for processing non-thermal concentrated peach juice. The method involves suspension freeze concentration of pre-cooled juice in a crystallization tank with a scraper attached to the inner wall. When the sugar entrainment rate in the ice crystals reaches a certain level, the ice crystals and juice are separated to obtain concentrated peach juice. However, small ice crystals are difficult to separate in one go under the suspension process, and a large amount of washing water is required to reduce the entrainment rate.

[0008] Chinese patent CN209219223U discloses a progressive freeze concentration device for liquid food. After the liquid food forms ice crystals in the crystallization chamber, it is sent to an ice discharge tank connected to the raw pulp tank. However, when the raw pulp is sprayed to suspend the ice crystals and discharged from the ice discharge port, too much concentrated liquid will still be carried out, resulting in a decrease in solute recovery rate and an increase in the washing burden, which restricts the economy and energy saving of the system.

[0009] Chinese patent CN108477441A discloses an apparatus and method for preparing concentrated juice by continuous freezing. In the disclosed method for preparing concentrated juice by continuous freezing, the juice is continuously frozen through a series of freezing tubes, and a concentrated liquid is output at the end. However, to clean the ice crystals, the flange blind plate must be repeatedly disassembled and the pipe wall must be washed with water or flushed with coolant multiple times, resulting in a significant increase in energy consumption, water consumption and operating costs.

[0010] Therefore, it is necessary to develop an efficient, energy-saving and low-cost apple juice processing method. Summary of the Invention

[0011] In order to solve the technical problems existing in the prior art, the present invention provides an apple juice processing method that combines layer-suspension coupled freezing with nanofiltration concentration. Through the synergistic effect of layer-suspension coupled freezing and dual purification by sweating and elution, the problems of low ice crystal separation efficiency and separation difficulty are solved, and the apple juice concentration end point reaches 40-45°Brix, significantly reducing energy consumption and water resource treatment costs.

[0012] The purpose of the present invention is achieved through the following technical solutions:

[0013] The present invention provides an apple juice processing method of layer-suspension coupled freezing and nanofiltration concentration, comprising:

[0014] (1) Precooling: precooling the pretreated apple juice to 0-5°C in a precooling tank;

[0015] (2) Layered preliminary freeze concentration: The apple juice pre-cooled in step (1) is sent to a layered crystallization tank. The temperature of the layered crystallization tank is set at -10 to 5°C, the temperature of the cold source is set at -10 to -4°C, and the cold finger is cooled at a constant rate of 0.25 to 5°C / h. As the temperature of the cold finger continues to decrease, the water in the juice continuously changes from liquid to solid small ice crystals attached to the cold finger, eventually forming a columnar ice layer, which continues to grow and is kept at a constant temperature for 0.5 to 2 hours. The mass of the ice crystals is measured, and the solidification rate is calculated based on the measurement results (the solidification rate refers to the ratio of the mass in the ice crystals to the total mass of the raw materials). When the solidification rate is within the range of 65 to 70%, the concentration process is immediately terminated, and the ice crystals and juice are separated by gravity and centrifugation to obtain a first-level concentrated apple juice with a Brix of 20 to 24° and first-level ice crystals with a diameter of 1 to 3 mm, wherein the entrainment rate of the first-level ice crystals is 1.5% to 3.0%.

[0016] (3) Suspension-type secondary freeze concentration: The primary concentrated apple juice obtained in step (2) is sent to a suspension crystallization tank, the jacket temperature is set at -18 to -8°C, the stirring rate is set at 600 to 900 r / min, the jacket is cooled at a constant rate (0.1 to 3°C / h), the residence time is 0.5 to 2 h, the sugar content in the ice crystals is measured in real time, and the ice crystal entrainment rate (the entrainment rate refers to the ratio of the sugar content in the ice crystals to the sugar content of the raw material) is calculated based on the measurement results; when the ice crystal entrainment rate is within the range of 0.5 to 3%, the concentration process is immediately terminated, and secondary ice crystals and a concentrated apple juice product with a Brix of 40 to 45° are obtained by gravity and centrifugal separation;

[0017] (4) Sweating: The first-level ice crystals obtained in step (2) are subjected to a temperature-raising and sweating operation. By adjusting the temperature control program, the final temperature of sweating is set to -1 to 5°C, the heating rate is 0.1 to 5°C / h, and the temperature is kept constant for 0.5 to 2 hours. When the sugar content of the sweating liquid reaches the range of 10 to 15°Brix, the sweating is stopped, and the sweating liquid and the sweating ice crystals are separated. The sweating liquid is added to the layered crystallization tank in step (2) and recycled as the raw material liquid.

[0018] (5) Ice crystal elution and eluent recovery: The secondary ice crystals separated in step (3) are eluted with pure water at 0-5°C, with the amount of elution water being 5%-10% of the mass of the ice crystals. The surface of the ice crystals is evenly rinsed by a spray device to remove residual juice entrainment. The washed ice crystals are centrifuged to obtain eluted ice crystals and eluent; the eluent is collected and returned to the suspension crystallization tank in step (3) for recycling. The ice crystal entrainment rate of the eluent is 0.7%-0.9%.

[0019] (6) Nanofiltration recovery: The sweating ice crystals and the leaching ice crystals are heated and melted and separated by nanofiltration at a flow rate of 0.05 to 1 kg / h, a controlled pressure of 0.5 to 2.0 MPa, and a temperature maintained at 10 to 20°C; pure water as the permeate and juice as the retentate are obtained respectively, and the retentate-juice is returned to the pretreatment step for closed-loop reuse.

[0020] (7) Cooling and storage: The concentrated apple juice product obtained in step (3) is cooled to 0-4°C through a heat exchanger and stored in a sterile storage tank at a pressure of 0.5-1.0 MPa. The temperature and pressure in the tank are regularly monitored to ensure the stability of the storage conditions.

[0021] Furthermore, the method for preparing the pretreated apple juice comprises: adding pectinase and cellulase to the apple juice for enzymatic hydrolysis, performing the enzymatic hydrolysis at 25-35°C for 30 minutes, and treating the enzymatically hydrolyzed apple juice with a ceramic ultrafiltration membrane at a flow rate of 0.01-2 kg / h, a pressure controlled at 0.2-1.0 MPa, and a temperature maintained at 25°C. The ceramic ultrafiltration membrane assembly used is a PMT-type membrane assembly, employing a multi-channel tubular α-Al2O3 ceramic membrane with a composite support layer and filter layer structure, capable of effectively removing suspended matter, colloids, cellulose, and other macromolecular substances from the juice.

[0022] Furthermore, in step (2), the middle-layer freezing is performed by controlling the cooling rate of the cold finger to 0.25 to 3°C / h and the final constant temperature time to 0.5 to 1h; the disordered growth of the crystal nuclei is suppressed, large-sized ice crystals (diameter 1 to 3 mm) are formed, and the physical entrainment of the juice by the pores between the ice crystals is reduced.

[0023] Furthermore, in step (3), the suspension secondary freeze concentration promotes uniform nucleation of micron-sized crystal nuclei (crystal nucleus density>10 4 pieces / cm 3 ), combined with slow cooling (0.1-2°C / h), with a residence time of 0.5-1h; optimize crystal growth and reduce impurity embedding rate.

[0024] Furthermore, the sweating operation in step (4) is a temperature gradient control with a temperature rise rate of 0.25 to 3°C / h, a final temperature of -1 to 3°C, and a constant temperature time of 0.5 to 1h. By real-time monitoring of the sugar content of the sweating liquid (10 to 15°Brix) and the residual sugar content of the ice crystals (≤0.5°Brix), the temperature rise rate is dynamically adjusted to ensure that the residual juice exudation rate at the ice crystal interface is greater than 95%, and the solute recovery rate is increased by 15% to 25%. The slow temperature rise avoids the closure of the microchannels inside the ice crystals, ensuring sufficient juice exudation. By extending the constant temperature time, the ice crystal entrainment rate is further reduced by diffusion.

[0025] Furthermore, in step (5), the washing water is used to evenly rinse the surface of the ice crystals through a spray device, and combined with centrifugal separation (centrifugal force 500-1000g), the residual sugar on the ice crystal interface is removed, and the ice crystal entrainment rate is reduced to 0.7%-0.9%, which is better than traditional gravity separation (ice crystal entrainment rate>5%).

[0026] Furthermore, in step (6), a PA-NF1 type anti-pollution composite nanofiltration membrane assembly is used, the membrane retention molecular weight is 200-500Da, the operating pressure is 0.8-1.5MPa, the temperature is controlled at 12-18°C, and the membrane surface flow rate is 0.2-0.8m / s; by optimizing the transmembrane pressure difference and the cross-flow velocity, the filtrate conductivity is made ≤50μS / cm (25°C), the water recovery rate is ≥98%, and the concentrated liquid sugar content is increased to 12-18°Brix and then returned to the layered crystallization tank in step (2) for cyclic concentration, achieving a closed-loop utilization rate of water resources greater than 95%.

[0027] The first main feature of the present invention is that it improves the overall heat transfer efficiency and ice crystal separation efficiency by coupling layered primary freezing with suspended secondary freezing, which significantly accelerates the concentration rate. First, layered freezing crystallization technology is used to form regular millimeter-sized ice crystals (1 to 3 mm in diameter), and then suspended freezing crystallization is performed to refine the ice crystals and remove residual liquid. Layered freezing crystallization generates a continuous ice layer on the cold finger, which has the advantages of excellent heat transfer efficiency and easy solid-liquid separation. The coupling process of the present invention takes into account the generation of uniform large crystals in layers and the purification of suspended high-purity ice crystals. After the regular large crystals are formed, they are refined and separated with high purity, which significantly improves the ice crystal separation efficiency and the overall concentration rate.

[0028] The second main feature of the present invention is the use of a dual purification process combining sweating and spraying and leaching, which significantly reduces the ice crystal entrainment rate to 0.7% to 0.9%, effectively improving the purity and clarity of the final concentrated juice.

[0029] After initial separation, low-speed sweating is applied to the ice crystals, allowing the remaining juice solutes at the frozen interface to diffuse out at temperatures between -2°C and 2°C, removing soluble solids embedded within the interface. Subsequently, a controlled amount of pure water spraying combined with centrifugation further removes trace mother liquor impurities adhering to the surface, reducing them to a range of 0.7% to 0.9%. This dual purification method offers the advantages of deeper purification, simpler processes, and minimal additional water consumption compared to single sweating or single spraying.

[0030] The third major feature of the present invention is the introduction of the PA-NF1 nanofiltration membrane recovery process, which realizes the closed-loop recycling of ice crystal meltwater and sweat liquid, with a water resource utilization rate of >95%, reducing fresh water consumption and lowering the cost of circulating water treatment. The PA-NF1 anti-pollution nanofiltration membrane precisely separates sweat liquid and ice crystal meltwater. The membrane has a molecular weight cutoff of 200 to 500 Da and can effectively intercept organic solutes and multivalent ions while maximizing water permeability. By optimizing the transmembrane pressure difference and cross-flow velocity, the filtrate conductivity can be controlled to ≤50μS / cm (25°C) and the water recovery rate can be ≥98%. This closed-loop recovery strategy not only saves fresh water resources, but also reduces the circulating water treatment load and energy consumption.

[0031] The beneficial effects of the present invention are as follows:

[0032] This method utilizes a coupled layer-fed followed by suspension process, combined with initial layer freezing to generate millimeter-sized ice crystals (1-3 mm in diameter). This provides a high-purity mother liquor for the suspension stage, reduces interference from crystal nuclei, and synergistically improves concentration efficiency. The overall concentration rate is increased by 1.5 times, achieving an end point of apple juice concentration of 40-45° Brix. This significantly reduces refrigeration energy consumption and water treatment costs, resulting in high efficiency, energy conservation, and environmental protection.

[0033] Combining the dual purification process of low-temperature sweating and spray washing with the nanofiltration membrane recovery process, the ice crystal entrainment rate is significantly reduced to 0.7% to 0.9%, effectively improving the purity and clarity of the final concentrated juice, and realizing the closed-loop recycling of ice crystal melt water and sweating liquid. The water resource utilization rate is >95%, reducing fresh water consumption and reducing the cost of circulating water treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the best implementation case.

[0036] Example 1

[0037] A method for processing apple juice by layer-suspension coupled freezing and nanofiltration concentration, comprising the following steps:

[0038] (1) Pretreatment: 0.05 wt% pectinase and cellulase (mass ratio of 3:1) were added to the apple juice for enzymatic hydrolysis at 25°C for 30 min. The juice after enzymatic hydrolysis was treated with a ceramic ultrafiltration membrane (pore size 0.05 μm) at a flow rate of 1 kg / h, a pressure controlled at 0.5 MPa, and a temperature maintained at 25°C to remove suspended matter and colloids to obtain clarified juice.

[0039] (2) Precooling: The apple juice pretreated in step (1) is precooled to 4°C in a precooling tank.

[0040] (3) Layered preliminary freeze concentration: The juice pre-cooled in step (2) is transported to a layered crystallization tank through a pump body, and the temperature of the layered crystallization tank is set at 0°C, the cold source temperature is set at -8°C, and the cooling rate is 1°C / h; the solidification rate is monitored, and when the solidification rate reaches 66.5%, crystallization is stopped, and the temperature is kept constant for 0.5h. The first-level concentrated juice with a Brix of 20.5° and first-level ice crystals are obtained by centrifugation, and the ice crystal entrainment rate is 1.8%.

[0041] (4) Suspension secondary freeze concentration: The primary concentrated juice obtained in step (3) is pumped into a suspension crystallization tank, the jacket temperature is set at -12°C, the stirring rate is set at 700 r / min, the jacket cooling rate is 0.5°C / h, the residence time is 1.5 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 1%, the concentration process is terminated, and gravity and centrifugal separation are used to obtain secondary ice crystals and secondary concentrated juice. The secondary concentrated juice is a concentrated juice product with a Brix of 42.5°.

[0042] (5) Sweating: The first-level ice crystals obtained in step (3) are heated to induce sweating. By adjusting the temperature control program, the heating rate is 1.5°C / h, the final sweating temperature is -1°C, and the end temperature is constant for 1 hour. The sugar content of the sweating liquid is measured to be 12.1°Brix, and the sweating liquid is added to the layer crystallization tank of step (3). The obtained sweating ice crystals are heated and melted. The ice crystal entrainment rate after sweating is measured to be 1.0%.

[0043] (6) Ice Crystal Washing and Eluent Recovery: The secondary ice crystals separated in step (4) were pre-cooled to 0°C. Pure water (5% by weight of the secondary ice crystals) was used to evenly rinse the surface of the secondary ice crystals through a spray device to remove residual juice entrainment. The washed secondary ice crystals were centrifuged, and the eluent was collected and returned to the suspension crystallization tank in step (4) for recycling. The ice crystal entrainment rate of the washed ice crystals was 0.7%.

[0044] (7) Nanofiltration recovery: The sweat ice crystal melt liquid after sweating in step (5) and the washed ice crystals separated after washing in step (6) are heated and melted, and separated by nanofiltration at a flow rate of 0.5 kg / h, a controlled pressure of 1.0 MPa, and a temperature maintained at 18°C; pure water as permeate and juice as retentate are obtained respectively, and the retentate-juice obtained is returned to the pretreatment step for closed-loop reuse.

[0045] (8) Cooling and storage: The concentrated juice product of 42.5° Brix obtained in step (4) was cooled to 2° C. via a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0046] Example 2

[0047] A method for processing apple juice by layer-suspension coupled freezing and nanofiltration concentration, comprising the following steps:

[0048] (1) Pretreatment: 0.05 wt% pectinase and cellulase (mass ratio of 3:1) were added to the apple juice for enzymatic hydrolysis at 25°C for 30 min. The juice after enzymatic hydrolysis was treated with a ceramic ultrafiltration membrane (pore size 0.05 μm) at a flow rate of 1 kg / h, a pressure controlled at 0.5 MPa, and a temperature maintained at 25°C to remove suspended matter and colloids to obtain clarified juice.

[0049] (2) Precooling: The clarified juice after pretreatment in step (1) is precooled to 3°C in a precooling tank.

[0050] (3) Layered preliminary freeze concentration: The juice pre-cooled in step (2) is transported to a layered crystallization tank through a pump body, and the temperature of the layered crystallization tank is set at -2°C, the cold source temperature is set at -9°C, and the cooling rate is 0.75°C / h; the solidification rate is monitored, and when the solidification rate reaches 68%, crystallization is stopped, and the temperature is kept constant for 0.75h. Centrifugal separation is performed to obtain 22.7°Brix first-level concentrated juice and first-level ice crystals, and the ice crystal entrainment rate is 2.0%.

[0051] (4) Suspension secondary freeze concentration: The primary concentrated juice obtained in step (3) is pumped into a suspension crystallization tank, the jacket temperature is set at -15°C, the stirring rate is set at 750 r / min, the jacket cooling rate is 0.4°C / h, the residence time is 1 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 2.4%, the concentration process is terminated, and the secondary ice crystals and the secondary concentrated juice are separated by gravity and centrifugation. The secondary concentrated juice is a concentrated juice product with a Brix of 44.2°.

[0052] (5) Sweating: The first-level ice crystals obtained in step (3) were subjected to a temperature-raising and sweating operation. By adjusting the temperature control program, the heating rate was 3°C / h, the final sweating temperature was 0°C, and the end temperature was constant for 1 hour; the sugar content of the sweating liquid was measured to be 13.2°Brix, and the sweating liquid was added to the layer crystallization tank of step (3). The obtained sweating ice crystals were heated and melted, and the ice crystal entrainment rate after sweating was measured to be 1.3%.

[0053] (6) Ice Crystal Washing and Eluent Recovery: The secondary ice crystals separated in step (4) were pre-cooled to 0°C. The surfaces of the secondary ice crystals were evenly rinsed with pure water (8% by weight of the secondary ice crystals) through a spray device to remove residual juice entrainment. The washed secondary ice crystals were centrifuged, and the eluent was collected and returned to the suspension crystallization tank in step (4) for recycling. The ice crystal entrainment rate of the washed ice crystals was 0.82%.

[0054] (7) Nanofiltration recovery: The sweat ice crystal melt liquid after sweating in step (5) and the ice crystals separated after elution in step (6) are heated and melted, and separated by nanofiltration at a flow rate of 0.5 kg / h, a control pressure of 1.0 MPa, and a temperature maintained at 15°C; pure water as permeate and juice as retentate are obtained respectively, and the retentate-juice obtained is returned to the pretreatment step for closed-loop reuse.

[0055] (8) Cooling and storage: The concentrated juice product of 44.2°Brix obtained in step (4) was cooled to 2°C via a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0056] Example 3

[0057] A method for processing apple juice by layer-suspension coupled freezing and coordinated nanofiltration concentration comprises the following steps: (1) pretreatment: adding 0.05 wt% pectinase and cellulase (mass ratio of 3:1) to the apple juice for enzymatic hydrolysis at 25°C for 30 min, treating the enzymatically hydrolyzed pulp with a ceramic ultrafiltration membrane (pore size 0.05 μm) at a flow rate of 1 kg / h, a pressure controlled at 0.5 MPa, and a temperature maintained at 25°C to remove suspended matter and colloids to obtain clarified juice.

[0058] (2) Precooling: The clarified juice after pretreatment in step (1) is precooled to 3°C in a precooling tank.

[0059] (3) Layered preliminary freeze concentration: The juice pre-cooled in step (2) is transported to a layered crystallization tank through a pump body, and the temperature of the layered crystallization tank is set at -5°C, the cold source temperature is set at -10°C, and the cooling rate is 0.5°C / h; the solidification rate is monitored, and when the solidification rate reaches 69.4%, crystallization is stopped, the temperature is kept constant for 1 hour, and centrifugation is performed to obtain 24.0°Brix first-level concentrated juice and first-level ice crystals, with an ice crystal entrainment rate of 2.3%.

[0060] (4) Suspension secondary freeze concentration: The primary concentrated juice obtained in step (3) is pumped into a suspension crystallization tank, the jacket temperature is set at -17°C, the stirring rate is set at 780 r / min, the jacket cooling rate is 0.3°C / h, the residence time is 1 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 2.8%, the concentration process is terminated, and secondary ice crystals and secondary concentrated juice are obtained by gravity and centrifugal separation. The secondary concentrated juice is a concentrated juice product with a Brix of 45.0°.

[0061] (5) Sweating: The primary ice crystals obtained in step (3) were subjected to a gradient temperature rise sweating operation. By adjusting the temperature control program, the heating rate was 3°C / h, the final sweating temperature was 1°C, and the end temperature was constant for 1 hour; the sugar content of the sweating liquid was measured to be 14.1°Brix, and the sweating liquid was added to the layer crystallization tank of step (3), and the obtained sweating ice crystals were heated and melted. The ice crystal entrainment rate after sweating was measured to be 1.5%.

[0062] (6) Ice Crystal Washing and Eluent Recovery: The secondary ice crystals separated in step (4) were evenly rinsed with pure water pre-cooled to 0°C and containing 10% of the mass of the secondary ice crystals through a spray device to remove residual juice entrainment. The washed secondary ice crystals were centrifuged, and the eluent was collected and returned to the suspension crystallization tank in step (4) for recycling. The ice crystal entrainment rate of the washed ice crystals was 0.89%.

[0063] (7) Nanofiltration recovery: The sweat ice crystal melt liquid after sweating in step (5) and the eluted ice crystals separated in step (6) are heated and melted and separated by nanofiltration at a flow rate of 0.5 kg / h, a controlled pressure of 1.0 MPa, and a temperature maintained at 12°C; pure water as permeate and juice as retentate are obtained respectively, and the retentate-juice obtained is returned to the pretreatment step for closed-loop reuse.

[0064] (8) Cooling and storage: The concentrated juice product of 45.0° Brix obtained in step (4) was cooled to 2° C. via a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0065] Comparative Example 1

[0066] The difference from Example 3 is that a coupled process of first suspension freezing and then layer freezing is adopted, and the specific steps are as follows:

[0067] Steps (1) and (2) are the same as in Example 3.

[0068] (3) Suspension primary freeze concentration: The juice pre-cooled in step (2) is pumped into a suspension crystallization tank, the jacket temperature is set at -17°C, the stirring rate is set at 780 r / min, the jacket cooling rate is 0.3°C / h, the residence time is 1 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 2.8%, the concentration process is terminated, and gravity and centrifugal separation are used to obtain 24.5°Brix first-level concentrated juice and first-level ice crystals.

[0069] (4) Layered secondary freeze concentration: The primary concentrated juice obtained in step (3) is transported to a layered crystallization tank through a pump body, the temperature of the layered crystallization tank is set at -5°C, the cold source temperature is set at -10°C, and the cooling rate is 0.5°C / h; the solidification rate is monitored, and when the solidification rate reaches 61.5%, crystallization is stopped, the temperature is kept constant for 1 hour, and centrifugation is performed to obtain secondary ice crystals and secondary concentrated juice. The secondary concentrated juice is a concentrated juice product with a Brix of 42.2° and an ice crystal entrainment rate of 1.65%.

[0070] (5) Sweating: The secondary ice crystals obtained in step (4) were subjected to a gradient temperature rise sweating operation. By adjusting the temperature control program, the heating rate was 3°C / h, the final sweating temperature was 1°C, and the end temperature was constant for 1 hour; the sugar content of the sweating liquid was measured to be 15.4°Brix, and the sweating liquid was added to the suspension crystallization tank of step (3). The obtained sweating ice crystals were heated and melted, and the ice crystal entrainment rate after sweating was measured to be 1.2%.

[0071] (6) Ice Crystal Washing and Eluent Recovery: The primary ice crystals separated in step (3) were evenly rinsed with pure water pre-cooled to 0°C and containing 10% of the mass of the primary ice crystals through a spray device to remove residual juice entrainment. The washed primary ice crystals were centrifuged, and the eluent was collected and returned to the suspension crystallization tank in step (3) for recycling. The ice crystal entrainment rate of the washed ice crystals was 1.30%.

[0072] (7) Nanofiltration recovery: The sweating ice crystal melt liquid after sweating in step (5) and the eluted ice crystals separated in step (6) are heated and melted and separated by nanofiltration at a flow rate of 0.5 kg / h, a controlled pressure of 1.0 MPa, and a temperature maintained at 12°C; pure water as the permeate and juice as the retentate are obtained respectively, and the retentate-juice obtained is returned to the pretreatment step for closed-loop reuse.

[0073] (8) Cooling and storage: The concentrated juice product of 42.2°Brix obtained in step (4) was cooled to 2°C via a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0074] Comparative Example 2

[0075] The difference from Example 3 is that only the suspension freeze concentration process is used, and the specific steps are as follows:

[0076] Steps (1) and (2) are the same as in Example 3.

[0077] (3) Suspension primary freeze concentration: The juice pre-cooled in step (2) is pumped into a suspension crystallization tank, the jacket temperature is set at -17°C, the stirring rate is set at 780 r / min, the jacket cooling rate is 0.3°C / h, the residence time is 1 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 2.8%, the concentration process is terminated, and gravity and centrifugal separation are used to obtain 24.5°Brix first-level concentrated juice and first-level ice crystals.

[0078] (4) Ice Crystal Leaching and Leaching Liquid Recovery: The primary ice crystals separated in step (3) were evenly rinsed with pure water pre-cooled to 0°C and containing 10% of the mass of the primary ice crystals through a spray device to remove residual juice entrainment. The washed primary ice crystals were centrifuged, and the leachate was collected and returned to the suspension crystallization tank in step (3) for recycling. The ice crystal entrainment rate of the washed ice crystals was 1.30%.

[0079] (5) Suspension secondary freeze concentration: The primary concentrated juice obtained in step (3) is pumped into another suspension crystallization tank, the jacket temperature is set at -20°C, the stirring rate is set at 780 r / min, the jacket cooling rate is 0.3°C / h, the residence time is 1 h, and the ice crystal entrainment rate is measured in real time. When the ice crystal entrainment rate reaches 3.6%, the concentration process is terminated, and gravity and centrifugal separation are used to obtain secondary ice crystals and secondary concentrated juice. The secondary concentrated juice is a concentrated juice product with a Brix of 45.6°.

[0080] (6) Ice Crystal Washing and Eluent Recovery: The secondary ice crystals separated in step (5) were evenly rinsed with pure water pre-cooled to 0°C and containing 20% of the mass of the secondary ice crystals through a spray device to remove residual juice entrainment. The washed secondary ice crystals were centrifuged, and the eluent was collected and returned to the suspension crystallization tank in step (5) for recycling. The ice crystal entrainment rate of the washed ice crystals was 1.38%.

[0081] (7) Nanofiltration recovery: The eluted ice crystals separated in step (4) and step (6) are heated and melted and separated by nanofiltration at a flow rate of 0.5 kg / h, a controlled pressure of 1.0 MPa, and a temperature maintained at 12°C; pure water as permeate and juice as retentate are obtained respectively, and the retentate-juice obtained is returned to the pretreatment step for closed-loop reuse.

[0082] (8) Cooling and storage: The concentrated juice product of 45.6°Brix obtained in step (5) was cooled to 2°C through a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0083] Comparative Example 3

[0084] The difference from Example 3 is that only a two-layer freeze concentration process is used, and the specific steps are as follows:

[0085] Steps (1) and (2) are the same as in Example 3.

[0086] (3) Layered preliminary freeze concentration: The juice pre-cooled in step (2) is transported to a layered crystallization tank through a pump body, and the temperature of the layered crystallization tank is set at -5°C, the cold source temperature is set at -10°C, and the cooling rate is 0.5°C / h; the solidification rate is monitored, and when the solidification rate reaches 69.4%, crystallization is stopped, the temperature is kept constant for 1 hour, and centrifugation is performed to obtain 24.0°Brix first-level concentrated juice and first-level ice crystals, with an ice crystal entrainment rate of 1.80%.

[0087] (4) Primary sweating: The primary ice crystals obtained in step (3) were subjected to a gradient temperature rise sweating operation. By adjusting the temperature control program, the heating rate was 5°C / h, the final sweating temperature was 1°C, and the end temperature was constant for 1 hour; the sugar content of the sweating liquid was measured to be 14.1°Brix, and the sweating liquid was added to the layer crystallization tank of step (3), the sweating ice crystals obtained were melted, and the ice crystal entrainment rate after sweating was measured to be 1.0%.

[0088] (5) Layered secondary freeze concentration: The primary concentrated juice obtained in step (3) is transported to another layered crystallization tank through a pump body, and the temperature of the layered crystallization tank is set at -8°C, the cold source temperature is set at -18°C, and the cooling rate is 0.3°C / h; the solidification rate is monitored, and when the solidification rate reaches 63.5%, crystallization is stopped, the temperature is kept constant for 1 hour, and centrifugation is performed to obtain secondary ice crystals and secondary concentrated juice. The secondary concentrated juice is a concentrated juice product with a Brix of 41.5° and an ice crystal entrainment rate of 2.1%.

[0089] (6) Secondary sweating: The secondary ice crystals obtained in step (5) were subjected to a gradient temperature rise sweating operation. By adjusting the temperature control program, the heating rate was 2°C / h, the final sweating temperature was -1°C, and the end temperature was constant for 2 hours; the sugar content of the sweating liquid was measured to reach 23.5°Brix, and the sweating liquid was added to the layer crystallization tank of step (5), and the obtained sweating ice crystals were melted. The ice crystal entrainment rate after sweating was measured to be 1.7%.

[0090] (7) Nanofiltration recovery: The sweating ice crystal melt liquid after sweating in step (4) and step (6) is separated by nanofiltration at a flow rate of 0.5 kg / h, a controlled pressure of 1.0 MPa, and a temperature maintained at 12°C; the permeate pure water and the retentate-juice are obtained respectively, and the obtained retentate-juice is returned to the pretreatment step for closed-loop reuse.

[0091] (8) Cooling and storage: The concentrated juice product with a Brix of 41.5° obtained in step (5) was cooled to 2°C via a heat exchanger and then stored in a sterile storage tank at a pressure of 0.8 MPa.

[0092] Comparative Example 4

[0093] The difference from Example 3 is that the cooling rate in step (3) is 6°C / h.

[0094] Comparative Example 5

[0095] The difference from Example 3 is that the sweating rate in step (4) is 6°C / h.

[0096] Comparative Example 6

[0097] The difference from Example 3 is that the final sweating temperature in step (4) is -3°C.

[0098] Comparative Example 7

[0099] The difference from Example 3 is that the stirring rate in step (5) is 500 r / min.

[0100] Comparative Example 8

[0101] The difference from Example 3 is that the cooling rate in step (5) is 4°C / h.

[0102] The main data of processing the same apple juice using the process of the embodiment and the comparative example are shown in Table 1.

[0103] Table 1

[0104]

[0105]

[0106] It can be seen from the data in Table 1 that the reverse process of first suspension and then layering in Comparative Example 1 will produce a large number of fine crystal nuclei under the high shear in the primary suspension stage, resulting in reduced layer freezing crystallization efficiency and serious problems of ice crystal adhesion and clogging.

[0107] In Comparative Example 2, single suspension freeze crystallization can quickly form a large number of micron-sized ice crystals, but a high proportion of mother liquor easily adheres to the surface of the ice crystals, requiring extensive washing and limiting separation efficiency. Multi-stage single suspension results in high washing water consumption and a high ice crystal entrainment rate.

[0108] Comparative Example 3, using a traditional single-layer process, suffers from a high-volume process, which is prone to reduced heat transfer and uneven crystallization due to the thickening of the ice layer. While the multi-stage single-layer process can utilize the advantages of the layer structure multiple times, it significantly increases refrigeration energy consumption and prolongs the process time.

[0109] Compared with traditional solutions such as suspension-first-layer (Comparative Example 1), double-stage suspension (Comparative Example 2), and double-stage layer (Comparative Example 3), the ice crystal entrainment rate of Example 3 is reduced from an average of 1.4% in the comparative example to 0.7% to 0.89%, a reduction of about 40%, and the purity and clarity of the concentrated juice are significantly improved; the concentration time is shortened from the conventional 60 hours to 25 to 40 hours, saving 1 / 3 of the process time and improving production efficiency; the heat transfer efficiency is increased to 165W / m 2 K, 30-50W / m higher than the single layer / suspension solution on average 2 K, significantly saves energy and reduces consumption; water consumption is controlled within 3-3.7L / 10kg of raw liquid, which is much lower than the traditional solution that requires repeated flushing, and has an outstanding environmental effect.

[0110] In Comparative Example 4, the layer freezing cooling rate was increased to 6°C / h. The ice crystals grew out of control, resulting in impurities being embedded. The ice layer formed was dense and irregular, and the heat transfer interface resistance increased significantly. The final heat transfer rate dropped to 110W / m 2 K (the lowest experimental value), the secondary ice crystal entrainment rate still reached 2.9% after sweating, significantly reducing the concentrate quality. Although this solution shortened the freezing time to 31 hours, it sacrificed purity and energy efficiency, making it a typical example of "quick freezing out of control."

[0111] In Comparative Example 5, the sweating heating rate was set to 6°C / h, and the temperature difference was driven too quickly, resulting in insufficient juice seepage from the ice crystal interface, and the secondary ice crystal entrainment rate remained at 1.35%; although the concentration time was improved (38h), the overall separation efficiency was not substantially improved, indicating that a too fast heating rate would inhibit the diffusion purification effect.

[0112] Comparative Example 6 set the final sweating temperature to -3°C. This low temperature inhibited molecular diffusion, limiting juice penetration and leaving soluble matter inside the crystals. The secondary ice crystal entrainment rate was 1.42%, higher than that of Comparative Example 5 (1.35%), and the duration was 37 hours. Both demonstrate that while the parameters of the sweating stage are merely "fine-tuning," they can significantly affect the entrainment rate and duration, and should not be ignored in high-purity concentration.

[0113] In Comparative Example 7, the stirring rate during the suspension freezing stage was reduced to 500 rpm. This theoretically reduced nucleation interference, but in practice resulted in uneven crystal formation and the entrainment of large crystals with mother liquor. Even after elution, the entrainment rate of secondary ice crystals remained at 1.52%. This demonstrates that insufficient stirring significantly increases the risk of ice crystal agglomeration.

[0114] In Comparative Example 8, the cooling rate was increased to 4°C / h, which accelerated the freezing rate. However, this caused ice crystal aggregation and heterogeneous nucleation, hindering the exudation of juice and reducing sweating efficiency. As a result, the secondary ice crystal entrainment rate remained at 2.7% after leaching. The heat transfer rate dropped to 112 W / m 2Although the time for K was shortened to 19 h, the contradiction between “speed and purity” was prominent, which verified the necessity of slow cooling.

[0115] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of the present invention.

Claims

1. A method for processing apple juice by layer-suspension coupled freezing and nanofiltration concentration, characterized in that: The following steps are involved: (1) Precooling: precooling the pretreated apple juice to 0-5°C in a precooling tank; (2) Layered preliminary freeze concentration: the apple juice pre-cooled in step (1) is sent to a layered crystallization tank, the temperature of the layered crystallization tank is set at -10 to 5°C, the temperature of the cold source is set at -10 to -4°C, the cooling rate of the cold finger is 0.25 to 5°C / h, and the temperature is kept constant for 0.5 to 2h. When the solidification rate is 65 to 70%, the concentration is stopped, and centrifugation is performed to obtain a first-level concentrated apple juice with a Brix of 20 to 24° and first-level ice crystals with a diameter of 1 to 3 mm, wherein the entrainment rate of the first-level ice crystals is 1.5% to 3.0%; (3) Suspension secondary freeze concentration: The primary concentrated juice obtained in step (2) is sent to a suspension crystallization tank, the jacket temperature is set at -18 to -8°C, the stirring rate is set at 600 to 900 r / min, the jacket is cooled at a constant rate of 0.1 to 3°C / h, the residence time is 0.5 to 2 h, and when the ice crystal entrainment rate is in the range of 0.5 to 3%, the concentration is terminated, and the secondary ice crystals and the concentrated apple juice product of 40 to 45° Brix are separated by centrifugation; (4) Sweating: The primary ice crystals obtained in step (2) are subjected to a gradient temperature increase to induce sweating, with a temperature increase rate of 0.1 to 5°C / h, a final sweating temperature of -1 to 5°C, and a constant temperature of 0.5 to 2h. When the sugar content of the sweating liquid reaches 10 to 15°Brix, the sweating is stopped, and the sweating liquid and the sweating ice crystals are separated. The sweating liquid is returned to the layered crystallization tank for recycling; (5) Ice crystal elution and eluent recovery: The secondary ice crystals separated in step (3) are eluted with pure water at 0-5°C. The washed ice crystals are centrifuged to obtain eluted ice crystals and eluent. The eluent is collected and returned to the suspension crystallization tank for recycling. The ice crystal entrainment rate of the eluent is 0.7%-0.9%. (6) Nanofiltration recovery: The eluted ice crystals and sweated ice crystals are heated and melted and separated through a nanofiltration membrane at a flow rate of 0.05 to 1 kg / h, a controlled pressure of 0.5 to 2.0 MPa, and a temperature maintained at 10 to 20°C to obtain pure water as the permeate and juice as the retentate. The retentate-juice is returned to the pretreatment step for closed-loop reuse.

2. The method according to claim 1, wherein: The method for preparing the pretreated apple juice comprises: adding pectinase and cellulase to the apple juice for enzymolysis, performing enzymolysis at 25-35° C. for 30 minutes, and treating the enzymolyzed pulp with a ceramic ultrafiltration membrane at a flow rate of 0.01-2 kg / h, controlling the pressure at 0.2-1.0 MPa, and maintaining the temperature at 25° C.

3. The method according to claim 1, wherein: In step (2), the cooling rate of the cold finger is 0.25-3°C / h, and the constant temperature time is 0.5-1h.

4. The method according to claim 1, wherein: In step (3), the stirring rate is 600-800 r / min, the cooling rate is 0.1-2° C. / h, and the residence time is 0.5-1 h.

5. The method according to claim 1, wherein: In step (4), the final sweating temperature is -1 to 3°C, the heating rate is 0.25 to 3°C / h, and the constant temperature time is 0.5 to 1h.

6. The method according to claim 1, wherein: In step (5), the rinsing water is used to evenly rinse the surface of the ice crystals through a spraying device, the amount of the rinsing water is 3% to 15% of the mass of the ice crystals, and the centrifugal force is 500 to 1000 g.

7. The method according to claim 1, wherein: The flow rate in step (6) is 0.1 to 0.5 kg / h.

8. The method according to claim 1, wherein: The nanofiltration membrane in step (6) is a PA-NF1 type anti-pollution composite nanofiltration membrane assembly, the membrane retention molecular weight is 200-500Da, the operating pressure is 0.8-1.5MPa, the temperature is controlled at 12-18°C, and the membrane surface flow rate is 0.2-0.8m / s.

9. The method according to claim 1, wherein: The concentrated apple juice product obtained in step (3) is cooled to 0-4° C. through a heat exchanger and stored in a sterile storage tank.

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