Method for efficiently concentrating and obtaining high-quality apple juice by using molecular distillation technology

Abstract

The invention relates to the field of food processing, in particular to a method for efficiently concentrating and obtaining high-quality apple juice by using a molecular distillation technology. According to the method, a molecular distillation technology is utilized, apple raw materials are subjected to pretreatment, vacuum crushing, juicing and centrifugation, then VKL70-5 molecular distillation equipment is adopted, the speed of a feeding pump is controlled to be 40-60 r / min, the distillation temperature is controlled to be 30-50 DEG C, the rotating speed of a scraper is controlled to be 80 r / min, and the vacuum degree is controlled to be-0.1 Mbar, and apple juice is concentrated to be 40-degree Brix. Wherein the retention rates of polyphenol, vitamin C and soluble protein respectively reach 88.64%, 86.28% and 86.37%, which are obviously superior to those of traditional heat concentration and membrane concentration; 35 volatile organic compounds (VOCs) can be detected; the browning index of the concentrated juice is as low as 0.77, and the antioxidant activity IC50 value is 4.31-4.87. Degradation of thermosensitive components is avoided through low-temperature operation, chemical reagents do not need to be added, energy consumption is low, and the green processing standard is met. A reliable technical scheme is provided for high-quality concentration of the apple juice, and the industry is promoted to be upgraded to the high-valued and nutritional direction.

Description

Technical Field

[0001] This invention relates to the field of food processing, specifically a method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology. Background Technology

[0002] The apple juice processing industry is an important part of the food manufacturing industry, with broad market demand and a large consumer base. Apple juice is rich in vitamin C, soluble protein, and polyphenolic compounds, giving it high nutritional value. However, in the apple juice processing, the concentration step is crucial, aiming to increase the solids content of the juice, extend its shelf life, and reduce transportation costs.

[0003] Traditional concentration methods mainly include thermal concentration and membrane concentration. Thermal concentration achieves concentration by evaporating water through heating, but high temperatures can lead to the degradation of nutrients (such as vitamin C), accelerated browning reactions, damage to microstructure, and loss of volatile aroma components, thus reducing the quality of apple juice. While membrane concentration can be carried out at lower temperatures, it is susceptible to membrane fouling, flux attenuation, and permeation selectivity limitations, resulting in low efficiency and component loss. Furthermore, existing concentration technologies often cannot simultaneously maintain the physicochemical properties, nutritional functions, and flavor characteristics of apple juice, hindering the transformation of the concentrated apple juice industry towards high-quality, green, and low-carbon practices.

[0004] Systematic research on molecular distillation technology in apple juice concentration is still lacking, and existing technologies lack systematic analysis and cannot comprehensively guide industrial applications. Therefore, developing a widely applicable method for molecular distillation concentration of apple juice that can efficiently concentrate the juice while more completely preserving key flavor compounds and maintaining its quality has significant practical and economic value. Summary of the Invention

[0005] The purpose of this invention is to solve the aforementioned technical problems and provide a method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology. This method achieves gentle concentration of apple juice by adjusting the process parameters of molecular distillation, maximizing the preservation of its physicochemical properties, nutritional components, microstructure, and flavor compounds.

[0006] This invention uses molecular distillation equipment to concentrate apple juice under low temperature and high vacuum conditions. With the feed pump speed at 50 r / min and the scraper speed at 80 r / min, the apple juice is concentrated to 40 °Brix to obtain molecularly distilled concentrated apple juice.

[0007] The technical solution of this invention is: A method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology includes the following steps: (1) Raw material processing: Apples are washed, cut into pieces, crushed (Stephan-UMC5 vacuum crusher), pressed (300 mesh gauze), and centrifuged to obtain unconcentrated and clarified apple juice; (2) Molecular distillation concentration: Clarified apple juice is transported to molecular distillation equipment via a peristaltic pump. The feed pump speed is controlled at 40~60 r / min, the distillation temperature at 30~50 ℃, the scraper speed at 80 r / min, and the vacuum degree at -0.1 Mbar. The juice is concentrated to 40 °Brix to obtain molecularly distilled concentrated apple juice. (3) Quality analysis: The physicochemical indicators, nutritional components, structural characteristics, antioxidant activity and VOCs of the molecularly distilled concentrated apple juice were analyzed; (4) The apple juice concentrated by molecular distillation retained 88.64% of its polyphenols, 78.23% of its total sugars, 86.37% of its protein, and 86.28% of its vitamin C. Furthermore, the concentrated apple juice by molecular distillation showed good retention of DPPH· and ABTS·. + · and reducing power IC 50 The values ​​were 4.87, 4.83 and 4.31, respectively, demonstrating a significant activity retention effect.

[0008] Preferably, the apple variety in (1) is “Ruixue” apple, and it is cut into 6 to 8 pieces on average.

[0009] Preferably, (2) the molecular distillation concentration conditions are: feed pump speed 50 r / min, distillation temperature 30 ℃, scraper speed 80 r / min, and vacuum degree -0.1 Mbar; Preferably, the determination conditions for the volatile compounds in apple juice in (3) are as follows: column type MXT-WAX (30 m × 0.53 mm, 1 μm), gas phase ion mobility spectrometry temperature 45℃, column temperature 60℃, injection needle temperature 80℃, carrier gas / drift gas 99.99% N2, analysis time 20 min, and automatic injection volume 500 μL.

[0010] The concentrated apple juice obtained above was analyzed by GC-IMS technology, which identified 35 types of VOCs.

[0011] Preferably, when the scraped-film molecular distillation equipment is in operation, an auxiliary cold trap device is added and set to 2 ℃; wherein the scraped-film molecular distillation equipment is preferably a VKL70-5 molecular distillation equipment.

[0012] The evaluation method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology includes the following steps: The physicochemical properties of concentrated apple juice samples prepared by molecular distillation were determined, including titratable acid (TA), browning index, and 5-hydroxymethylfurfural content. Nutritional content determination, including vitamin C, polyphenols, antioxidant activity, and volatile organic compound composition indicators.

[0013] The determination of apple juice obtained by molecular distillation concentration specifically includes the following indicators: Titrateable acid (TA), sugar-acid ratio, brightness value L* Value, Red Value a* Value, Blue Value b* Value, turbidity, browning index, 5-hydroxymethylfurfural content, vitamin C content, soluble protein content, polyphenol content, total sugar content, particle size, polydispersity index (Pdi), zeta potential, DPPH·, reducing power, ·OH, ABTS + IC 50 Value, VOCs with retention index calculated using n-ketone C4~C9 as external standard, appearance, color, flavor, taste, and acceptability.

[0014] The beneficial effects of this invention are as follows: (1) This invention, through a systematic analysis of the changes in quality indicators during apple juice concentration, clarifies the advantages of molecular distillation in preserving heat-sensitive nutrients. Compared with traditional thermal concentration and membrane concentration, molecular distillation concentrates apple juice with a polyphenol retention rate of 88.64%, a soluble protein retention rate of 86.4%, a browning index of only 0.77, and a transparency of 90.8%. Simultaneously, the apple juice treated by molecular distillation has a smaller particle size (46µm) and a smaller Pdi level (0.41), resulting in improved solution uniformity and stability, significantly superior to other concentration methods.

[0015] (2) This invention is the first to construct a spectrum of VOC composition changes during molecular distillation and concentration of apple juice. Based on gas chromatography-ion mobility spectrometry, a total of 47 VOCs were detected in all samples, 40 of which were detected in the original apple juice and 35 of which were retained in the molecular distillation concentrate. The flavor characteristics are closest to those of the original apple juice, which helps to maintain the flavor characteristics of fruit aroma, nutty aroma and fresh aroma.

[0016] (3) The process characteristics of combining high vacuum and low temperature in this invention effectively avoid the degradation of heat-sensitive components, promote the full retention of nutrients such as vitamin C and soluble protein, and effectively maintain the antioxidant level. At the same time, this technology also improves the solution uniformity and stability of the juice. (4) This invention achieves green and efficient processing of apple juice concentration, reducing energy consumption and environmental pollution. The molecular distillation equipment operates stably with low energy consumption and requires no chemical reagents, meeting the standards for green food processing. The quality of the apple juice concentrated by the molecular distillation equipment is effectively maintained, and energy saving and emission reduction are significantly achieved in the concentration process. This provides a systematic process practice and theoretical basis for the application of molecular distillation technology in high-quality concentration of apple juice, promoting the transformation and upgrading of concentrated apple juice towards high value, nutrition, and functionality. The concentrated apple juice can be widely used in beverages, edible flavorings, and other fields, enhancing product added value and market competitiveness. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a molecular distillation concentration device; Figure 2 Screening for different feed pump speeds and concentration temperatures; Figure 3 A comparative graph showing the effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation—on the clarity and turbidity of apple juice; Figure 4 A comparison of the effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation concentration—on BI and 5-HMF; Figure 5 This study compares the effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation—on the nutritional components of apple juice, including vitamin C, soluble protein, polyphenols, and total sugar content. Figure 6 The effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation concentration—on the results of Ds and Pdi in apple juice structure detection, as well as zeta potential, were compared. Figure 7 The three concentration methods—thermal concentration, membrane concentration, and molecular distillation concentration—are compared in terms of DPPH· scavenging rate, reducing power, ·OH, and ABTS. + • A comparison of the effects of various factors on the antioxidant activity of apple juice; Figure 8 The effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation concentration—on DPPH· scavenging rate, reducing power, OH·, and ABTS were investigated. + · of IC 50 Value impact; Figure 9 The effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation—on volatile organic compounds in apple juice were investigated. Figure 10 The effects of three concentration methods—thermal concentration, membrane concentration, and molecular distillation—on the sensory evaluation of apple juice were investigated. Figure 11 A schematic diagram of the process for obtaining apple juice through molecular distillation concentration. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present invention, the invention will now be further described in conjunction with specific embodiments. However, the scope of the present invention is not limited to the following embodiments.

[0019] The equipment used in this application has the following model number: Table 1 Equipment Name and Model Equipment Name factory VKL70-5 Molecular Distillation Equipment German VTA company Hei-VAP Core Automatic Rotary Evaporator Shanghai Yarong Biochemical Instrument Factory, China Membrane Concentration Filtration Equipment Mann+Hum Filtration Technology Co., Ltd. Flavour Spec® Gas Phase Ion Mobility Spectrometer GAS Company of Germany UV-1000 Ultraviolet Spectrophotometer Chengdu Lianjie Fluid Separation Technology Co., Ltd. WSF-1S Physical Spectrophotometer Nanjing Xiaoxiao Instrument Equipment Co., Ltd.

[0020] Example 1 This invention discloses a method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology, comprising the following steps: (1) Select fresh “Ruixue” apples with a regular shape, no cracks on the skin, and natural stems. Wash them and cut them into pieces. (2) Use a Stephan-UMC5 vacuum crusher to crush apples into a pulp; (3) After pressing with 300-mesh gauze and centrifuging at 4000 rpm / min, apple juice was obtained; (4) Take 1 L of apple juice and inject it into the VKL70-5 molecular distillation equipment. The concentration conditions are: feed pump speed 50 r / min, scraper speed 80 r / min, vacuum degree -0.1 Mbar, temperature 30 ℃. The apple juice is concentrated to 40 °Brix to obtain molecular distillation concentrated apple juice. The physicochemical indicators, nutrient content, antioxidant activity and VOCs content are determined.

[0021] Example 2 Example 2 is similar to Example 1 in steps, the main difference being that (4) the feed pump speed is 40 r / min and the temperature is 40 ℃ in the concentration conditions.

[0022] Example 3 Example 3 is similar to Example 1 in steps, the main difference being that (4) the feed pump speed is 60 r / min and the temperature is 50 ℃ in the concentration conditions.

[0023] Comparative Example 1 The traditional method for heat-concentrating apple juice includes the following steps: (1) Select fresh “Ruixue” apples with a regular shape, no cracks on the skin, and natural stems. Wash them and cut them into pieces. (2) Use a Stephan-UMC5 vacuum crusher to crush apples into a pulp; (3) After pressing with 300-mesh gauze and centrifuging at 4000 rpm / min, apple juice was obtained; (4) Take 1 L of apple juice and inject it into an automatic rotary evaporator. Use the rotary evaporator to concentrate the apple juice to 40°Brix at 60℃, 50 r / min and 0.005 MPa to obtain thermally concentrated apple juice. Then, determine its physicochemical properties, nutrient content, antioxidant activity and VOCs content.

[0024] Comparative Example 2 A method for concentrating apple juice using a membrane includes the following steps: (1) Select fresh “Ruixue” apples with a regular shape, no cracks on the skin, and natural stems. Wash them and cut them into pieces. (2) Use a Stephan-UMC5 vacuum crusher to crush apples into a pulp; (3) After pressing with 300-mesh gauze and centrifuging at 4000 rpm / min, apple juice was obtained; (4) Pour 5 L of apple juice into the membrane concentration equipment, select an ultrafiltration membrane with a pore size of 25 kDa, and perform ultrafiltration under the condition that the pressure difference across the membrane is 0.1 MPa. (5) After ultrafiltration, a reverse osmosis membrane with a pore size of 100 Da was used to filter and concentrate the juice under a pressure difference of 2.4 MPa across the membrane to obtain concentrated apple juice with 40°Brix. (6) The physicochemical properties, nutrient content, antioxidant activity and VOCs content of the membrane concentrated apple juice were determined.

[0025] Example 4 Screening of molecular distillation feed pump speed, scraper speed, and concentration temperature As shown in the figure, the feed pump speed, scraper speed, and concentration temperature have a significant impact on the molecular distillation concentration time and vitamin C content. Figure 2 As shown in (A) and (B), the lowest point in the figure corresponds to the minimum concentration time and the highest concentration efficiency of molecular distillation when the feed pump and scraper speeds are 50 rpm / min and 80 rpm / min, respectively. The concentration times are 104 min and 105 min, indicating that the mass transfer and evaporation processes in molecular distillation reach optimal balance at these speeds. Lower speeds, while extending the residence time of the material in the evaporator and promoting the separation of light components, limit the throughput. Higher speeds, while increasing the feed rate, may lead to insufficient evaporation due to insufficient material residence time, thus reducing the efficiency of a single distillation. As the most abundant nutrient in apple juice, vitamin C content significantly affects its nutritional value; however, as a heat-sensitive component, temperature control is crucial during concentration. Figure 2As shown in (B), the vitamin C content in apple juice decreases with increasing concentration temperature, reaching its highest level of 68.42 mg / 100 g at a concentration temperature of 30℃, demonstrating the best concentration effect. This indicates that low-temperature concentration avoids thermal degradation reactions. While higher temperatures accelerate solvent evaporation, they also exacerbate the oxidative loss of vitamin C, leading to a decline in the nutritional quality of the apple juice. The core issue in molecular distillation efficiency is optimizing mass transfer conditions while ensuring throughput, and balancing the evaporation rate with the retention of heat-sensitive substances through precise temperature control. It is worth noting that when the concentration temperature is 10℃, the concentration time increases significantly, leading to a substantial increase in actual processing costs, which contradicts the need for "energy saving, consumption reduction, and efficiency improvement" in processing production.

[0026] Table 2. Effects of feed pump speed and scraper speed on concentration time and effects of concentration temperature on vitamin C content. Concentration temperature / °C Vitamin C content (mg / 100g) Scraper rotation speed (rpm / min) Concentration time / min Feed pump speed (rpm / min) Concentration time / min 30 68.42±0.87 70 112±1.14 40 114±2.11 40 67.51±1.02 80 105±0.89 50 104±1.42 50 65.27±0.58 90 108±2.11 60 109±1.61

[0027] Example 5 The Effects of Different Concentration Methods on Basic Indicators of Apple Juice Table 3. Effects of different concentration methods on basic indicators of apple juice Concentrated sample Soluble solids (°Brix) pH Total acid Sugar-acid ratio brightness() Red value () Blue value () Apple juice <![CDATA[15.20±0.1 b ]]> <![CDATA[3.75±0.1 a ]]> <![CDATA[3.38±0.3 d ]]> <![CDATA[4.50±0.02 c ]]> <![CDATA[39.54±0.01 a ]]> <![CDATA[-2.17±0.02 d ]]> <![CDATA[7.61±0.05 d ]]> Hot concentrated apple juice <![CDATA[40.00±0.2 a ]]> <![CDATA[3.77±0.1 a ]]> <![CDATA[9.24±0.1 a ]]> <![CDATA[4.33±0.01 bc ]]> <![CDATA[37.73±0.01 d ]]> <![CDATA[-0.27±0.04 b ]]> <![CDATA[8.26±0.03 c ]]> Membrane concentrated apple juice <![CDATA[40.00±0.1 a ]]> <![CDATA[3.77±0.1 a ]]> <![CDATA[9.08±0.3 c ]]> <![CDATA[4.40±0.01 a ]]> <![CDATA[38.20±0.01 c ]]> <![CDATA[-0.09±0.01 a ]]> <![CDATA[9.03±0.02 b ]]> Molecular distillation to concentrate apple juice <![CDATA[40.00±0.2 a ]]> <![CDATA[3.76±0.1 a ]]> <![CDATA[9.11±0.2 b ]]> <![CDATA[4.39±0.01 b ]]> <![CDATA[38.75±0.06 b ]]> <![CDATA[-0.93±0.02 c ]]> <![CDATA[9.49±0.04 a ]]>

[0028] This application compared the effects of three different concentration methods on the basic indicators of apple juice. Table 3 shows that the concentration method significantly affected the basic indicators of apple juice. Compared to regular apple juice, the pH value of apple juice concentrated by heat, membrane concentration, and molecular distillation did not change much, but the titratable acid (TA) content was 2.73 times, 2.67 times, and 2.70 times that of regular apple juice, respectively. This is because the removal of water during concentration relatively concentrates organic acids, increasing the acidity of the juice. The sugar-acid ratio is an important indicator reflecting the flavor quality and maturity of fruit juice; the higher the value, the richer the taste. This invention found that the sugar-acid ratio of concentrated apple juice decreased, but the sugar-acid ratio of molecularly distilled concentrated apple juice remained at a high level of 4.39. Furthermore, the color changed significantly during the apple juice concentration process. Compared to other concentration methods, the brightness value of molecularly distilled concentrated apple juice (…) was significantly higher. L* ), Red Value ( a* It is closest to apple juice, while the blue value ( b* The concentration of ions increased significantly, resulting in a brighter yellow color in the apple juice obtained by molecular distillation concentration.

[0029] Example 6 The effect of concentration methods on the clarity and turbidity of apple juice Figure 3The changes in turbidity and clarity of apple juice during the concentration process are shown. The turbidity of apple juice increased after different concentration treatments, with thermally concentrated apple juice showing an 86.5% increase, while molecularly distilled apple juice showed only a 30.0% increase. This is because molecular distillation, conducted at low temperatures, for short periods, using physical separation, and under mild conditions, effectively reduces the increase in turbidity. Clarity reflects the degree of light transmission through apple juice, which is related to the concentration of solutes in the juice. Generally, higher turbidity corresponds to lower clarity. After different concentration treatments, the clarity of apple juice decreased, with thermal concentration decreasing by 9.2%, membrane concentration by 7.3%, and molecular distillation concentration by only 2.9%.

[0030] Example 7 Effects of Concentration Methods on Browning Index and 5-Hydroxymethylfurfural in Apple Juice During apple juice concentration, significant changes were observed in the browning index (BI) and the content of 5-hydroxymethylfurfural (5-HMF). For example... Figure 4 As shown, compared with apple juice, the BI values ​​of hot-concentrated apple juice, membrane-concentrated apple juice, and molecular distillation-concentrated apple juice increased by 0.47, 0.22, and 0.15, respectively, indicating that high temperature accelerates Maillard and caramelization reactions. The 5-HMF content increased most significantly during hot concentration, reaching 0.85 mg / L, indicating the highest degree of browning. The increase in 5-HMF during molecular distillation concentration was smaller, indicating less browning.

[0031] Example 8 The effect of concentration methods on the nutrient content of apple juice Different concentration methods have a significant impact on the nutritional composition of apple juice. For example... Figure 5 As shown, vitamin C decreased by 32.7% and 22.0% during heat concentration and membrane concentration, respectively, while it only decreased by 13.7% during molecular distillation concentration, as the low-temperature vacuum treatment avoided the oxidative decomposition of vitamin C. As a heat-sensitive component, the content of soluble protein decreased by 37.6% during heat concentration, but the retention rate in molecularly distilled apple juice was as high as 86.4%. Polyphenols decreased by 22.0% during membrane concentration, but only by 11.4% during molecular distillation. Total sugars decreased significantly during heat concentration (27.6%), but showed better retention during both membrane and molecular distillation concentrations. These results indicate that molecular distillation concentration can better preserve the nutrients in apple juice.

[0032] Example 9 The effect of concentration methods on the structure of apple juice Particle size, Pdi (polydispersity index), and zeta potential are key indicators of the stability of apple juice. Generally, smaller particle size, lower Pdi values, and larger absolute zeta potential values ​​indicate a more stable solution system. Figure 6It can be seen that after thermal concentration, the particle size change is small, the Pdi increases slightly, and the absolute value of the zeta potential decreases. This indicates that thermal treatment disrupts the cell structure of apple juice, releasing suspended matter such as pectin and particles, thus reducing stability. In contrast, membrane concentration significantly increases the particle size and Pdi, and also significantly decreases the absolute value of the zeta potential, further reducing stability. Molecular distillation concentration exhibits the best stability: the lowest particle size and Pdi, and the highest absolute value of the zeta potential. This means that molecular distillation effectively removes suspended particles and soluble salts, reduces electrostatic repulsion of the solution, and maintains high stability and homogeneity of the solution.

[0033] Example 10 The effect of concentration method on the antioxidant activity of apple juice Antioxidant activity is an important indicator for evaluating the functionality of apple juice. Commonly used free radical models include DPPH·, reducing power, ·OH, and ABTS. + ·.like Figure 7 As shown, the antioxidant activity of different concentrated apple juice samples increased with increasing solution concentration. Among the three concentrated apple juices, the DPPH· and ABTS of the molecularly distilled concentrated apple juice were... + Both the reducing power and the reducing power levels are at their highest. Specifically, such as... Figure 8 As shown, it affects DPPH·, reducing power and ABTS. + IC 50 The values ​​were the lowest, at 4.87, 4.31, and 4.83 respectively, indicating the strongest antioxidant capacity. The high antioxidant activity of molecularly distilled concentrated apple juice is closely related to its high content of vitamin C, polyphenols, soluble protein, and total sugar, as well as its smallest particle size and most uniform distribution.

[0034] Example 11 The impact of concentration methods on VOCs in apple juice The flavor of concentrated apple juice has a significant impact on consumer choice. VOCs in different apple juice samples were analyzed using GC-IMS technology. Retention indices were calculated using n-ketones (C4-C9) as external standards, and qualitative analysis was performed using a database. Results are as follows: Figure 9 As shown, a total of 47 VOCs were detected in all samples, including 16 esters, 11 aldehydes, 7 alcohols, 7 ketones, 3 heterocyclic compounds, 2 olefins, and 1 ether. All samples refer to the original juice plus 3 concentration methods, totaling 47 detected. The 40 VOCs detected in apple juice are included. Among the 35 detected VOCs, key components produced substances not present in the original juice, including propionaldehyde, isoamyl propionate, 4-methyl-3-penten-2-one, (E)-2-pentenal, and trans-2-hexenoic acid ethyl ester.

[0035] Among these VOCs, esters are the key flavor components of apple juice, contributing fruity, vegetable, floral, and sweet notes. Aldehydes, mainly derived from the hydrolysis of triglycerides and the oxidative degradation of unsaturated fatty acids, are easily oxidized to produce a sour and astringent taste, while also providing fatty, spicy, and cheese-like flavors. Alcohols and ketones provide a slight alcoholic flavor, a mild fruity flavor, and a fruity and woody flavor, respectively. Of the 47 VOCs detected, 40 were found in pure apple juice, 9 in hot concentrated apple juice, 28 in membrane concentrated apple juice, and 35 in molecularly distilled concentrated apple juice, indicating that it performed best in preserving the original flavor.

[0036] Analysis revealed the following VOCs in the molecularly distilled concentrated apple juice: methyl propionate (fresh fruit flavor), methyl butyrate (fruit flavor), ethyl acetate (fruity aroma), 2,4-heptadienal (citrus flavor), nonanal (pea flavor), 2-ethylfuran (nut flavor), 2-acetylpyrazine (popcorn flavor), 1-penten-3-one (pepper flavor), acetic acid (mild vinegar flavor), 2-methylbutanol (mild alcohol), α-pinene (woody flavor), 2-methyl-2-pentanal (green and fruity flavor), isoamyl acetate (banana flavor), furfuryl propionate (sweet fruit flavor), 1-butanol (alcoholic flavor), hex-2(E)-enal (cheese), methyl valerate (fruit flavor), 5-methyl-3- Heptanone (fruity, herbal), 3-methyl-2-butenal (sweet fruity), 4-heptanone (fruity), 1-penten-3-ol (tropical fruity), propyl butyrate (fruity), heptanal (fatty), (E)-2-octanal (banana waxy), pentan-2-one (fruity), butyl acetate (fruity), (E)-3-hexen-1-ol (floral), methyl acetate (sweet), methyl hexanoate (fruity, bacon), propionaldehyde (earthy and nutty), isoamyl propionate (tropical fruity), 4-methyl-3-penten-2-one (mint and honey), (E)-2-pentenal (spicy), ethyl trans-2-hexenoate (fruity and vegetable), and E-2-heptenal (vegetarian). Molecular distillation concentration is a concentration technique under low-temperature, high-vacuum conditions. Because its concentration temperature is much lower than the boiling point of apple juice and the processing time is short, it can minimize the loss of VOCs. During the concentration process, key esters (such as ethyl propionate and isobutyl acetate), aldehydes (such as (E)-2-hexenal) and ketones remain relatively intact, giving it a prominent fruity, floral and fresh flavor.

[0037] Example 12 The effect of concentration method on the sensory properties of apple juice Table 4 Sensory Evaluation Table for Concentrated Apple Juice

[0038] Using apple juice as a reference, the appearance, color, aroma, taste and acceptability of concentrated apple juice were evaluated, with weights of 15%, 20%, 30%, 25% and 10%, respectively, for a total of 100%. The specific evaluation criteria are shown in Table 4. Figure 10 This study demonstrates the impact of different concentration methods on the sensory evaluation of apple juice. After concentration, the sensory scores of all apple juices decreased. Based on appearance, color, flavor, taste, and acceptability, the overall score order was: apple juice > molecularly distilled apple juice > membrane-concentrated apple juice > heat-concentrated apple juice. Molecularly distilled apple juice, due to the mild conditions of molecular distillation, avoids the degradation of heat-sensitive components, Maillard reactions, and caramelization, thus preserving the natural sensory characteristics and solution stability of apple juice. In contrast, heat concentration leads to browning of apple juice and loss of flavor and taste; membrane concentration filters out some organic matter and pigments, affecting sensory quality. The sensory characteristics of concentrated apple juice prepared by different methods differ significantly, influencing consumer choice. Molecularly distilled apple juice achieved the highest sensory score and is of the best quality.

Claims

1. A method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology, characterized in that, The steps are as follows: (1) Wash and cut into pieces; (2) Broken; (3) Juicing and centrifugation; (4) Molecular distillation concentration: The apple juice obtained in (3) is concentrated using a scraped membrane molecular distillation device. The parameters of the distillation device are adjusted as follows: feed pump speed 40~60 r / min, distillation concentration temperature 20~50 ℃, scraper speed 70~90 r / min, vacuum degree -0.2~0.1 Mbar.

2. The method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology as described in claim 1, characterized in that, (4) The feed pump speed is 50 r / min.

3. The method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology according to claim 1, characterized in that, (4) The distillation and concentration temperature is 30 °C.

4. The method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology according to claim 1, characterized in that, (4) The scraper rotation speed is 80 r / min and the vacuum degree is -0.1 Mbar.

5. The method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology according to claim 1, characterized in that, The steps include the following: (1) Pre-treatment of apple raw materials: Select fresh apples with regular shape, no cracks on the skin and natural stems, wash and cut into pieces; (2) Apple crushing: Use a vacuum crushing mixer to crush apples into apple pulp; (3) Juicing and centrifugation: The apple pulp obtained in (2) was juiced using 300 mesh gauze and then centrifuged to obtain clear apple juice; (4) Molecular distillation concentration: The apple juice obtained in (3) was concentrated using a VKL70-5 molecular distillation equipment. The parameters of the distillation equipment were adjusted as follows: feed pump speed 50 r / min, distillation concentration temperature 30 ℃, scraper speed 80 r / min, and vacuum degree -0.1 Mbar.

6. The method for efficiently concentrating and obtaining high-quality apple juice using molecular distillation technology according to claim 1, characterized in that, (4) When the scraped membrane molecular distillation equipment is working, a cold trap system is set and the condensation temperature is set to 1~3 ℃.

7. A method for evaluating the quality of apple juice obtained as claimed in claim 1, characterized in that, The steps include the following: (1) The physicochemical properties of concentrated apple juice samples prepared by molecular distillation were determined, including the titratable acid TA, browning index, and 5-hydroxymethylfurfural content. (2) Determination of nutritional components, including vitamin C, polyphenols, antioxidant activity, and volatile organic compound composition indicators.

8. The method as described in claim 7, characterized in that, In the evaluation method, the chromatographic column used for the determination of volatile compounds in apple juice was an MXT-WAX column with specific parameters of 30 m × 0.53 mm and a liquid film thickness of 1 μm.

9. The method as described in claim 7, characterized in that, The gas phase ion mobility spectrometry (GC-IMS) temperature was 45℃, the column temperature was 60℃, and the injection needle temperature was 80℃; the carrier gas / drift gas was 99.99% N2; the analysis time was 20 min; and the automatic injection volume was 500 μL.

10. A concentrated apple juice prepared by the method according to any one of claims 1-6, characterized in that, The concentrated apple juice has a browning index of no more than 0.77 and a 5-hydroxymethylfurfural content of no more than 0.85 mg / L; and, by gas chromatography-ion mobility spectrometry, the concentrated apple juice contains no fewer than 35 kinds of volatile organic compounds, including isoamyl propionate and 4-methyl-3-penten-2-one.

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