A low-damage vacuum pulse drying processing method for red jujubes in the brittle-hardening stage

Abstract

The present application relates to the field of jujube drying processing, in particular to a low-damage vacuum pulsating drying method for jujubes in the crisp maturity period, which comprises the following steps: screening jujubes in the crisp maturity period, collecting appearance images and grading cracks, removing jujube fruits with bad quality or with crack area accounting for more than 15% or penetrating the pulp; cleaning the graded jujube fruits, removing surface free water, and removing the core along the jujube core axis; batching according to crack grade and initial moisture content; preheating under circulating airflow, first implementing rapid dehydration to reduce the moisture content to 38%-45%, and then implementing low-damage slow drying to reduce the moisture content to 23%-25%. In the low-damage slow drying stage, the pressure change rate of vacuum establishment and normal pressure recovery is not greater than 3 kPa / s, and the normal pressure recovery adopts a segmented air inlet mode. The method can reduce crack expansion, pulp eversion, sugar liquid precipitation and excessive shrinkage, and improve the drying efficiency of jujubes in the crisp maturity period, the appearance integrity of the product and the moisture content uniformity.

Description

Technical Field

[0001] This invention relates to the field of jujube drying and processing technology, specifically a low-damage vacuum pulse drying method for jujubes in the crisp ripe stage. Background Technology

[0002] Jujubes are an important specialty fruit product of Northwest my country. Jujubes in their crisp-ripe stage are characterized by high water content, crisp and tender flesh, and high levels of soluble sugars and vitamins. After entering the crisp-ripe stage, jujubes are susceptible to cracks of varying lengths and widths on the skin due to factors such as diurnal temperature variations, rainfall, irrigation conditions, and the fruit's own expansion. If cracked fruit continues to hang on the tree to dry, it is easily contaminated by dust, microorganisms, and pests, and may suffer from mold, browning, and nutrient loss. Timely harvesting and artificial drying of crisp-ripe jujubes, as well as cracked jujubes, helps shorten their exposure time in the field and provides conditions for processing and utilization of cracked jujubes.

[0003] Current methods for drying jujubes mainly include natural sun-drying, hot air drying, vacuum drying, infrared drying, and microwave-assisted drying. Natural sun-drying is constrained by climate conditions, has a long processing cycle, and results in poor consistency in product moisture content and appearance quality. Conventional hot air drying relies mainly on increasing temperature and extending processing time to remove moisture; however, high temperatures can easily cause hardening of the skin, darkening of color, and loss of vitamin C. Vacuum pulse drying, by alternately establishing vacuum and atmospheric pressure environments, can enhance the migration of moisture from the inside of the jujube to the surface. However, current processes typically process the entire batch of raw materials according to uniform parameters, rarely considering the differences in heat and mass transfer properties, structural strength, and crack propagation tendency among whole, slightly cracked, moderately cracked, and severely cracked fruits. When rapidly switching between vacuum and atmospheric pressure, the peel on both sides of the crack and the pulp at the crack are subjected to a large instantaneous pressure difference, which can easily lead to crack expansion, pulp eversion, sugar syrup precipitation, and localized excessive shrinkage. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a low-damage vacuum pulse drying method for jujubes at the crisp ripe stage. This method solves the problem that existing processes process a whole batch of raw materials according to uniform parameters, and rarely consider the differences between whole fruits, slightly cracked fruits, moderately cracked fruits, and severely cracked fruits.

[0005] To achieve the above objectives, the present invention provides a low-damage vacuum pulse drying method for ripe jujubes, comprising the following steps:

[0006] S1. Screen the jujubes in the crisp ripe period, remove diseased, insect-infested, moldy, and mechanically damaged jujubes, collect and retain the appearance images of the jujubes, and classify them according to the crack length, crack width and crack area ratio in the appearance images. Wash the graded jujubes and remove the surface free water.

[0007] The images of jujube fruit appearance were sequentially processed by background separation, jujube fruit contour extraction, crack region enhancement, and crack connected component recognition. The ratio of the pixel area of ​​the crack region to the pixel area of ​​the jujube fruit projection region was used as the crack area percentage. Jujube fruit with a crack area percentage of 0 was classified as intact fruit, jujube fruit with a crack area percentage greater than 0 but not exceeding 3% was classified as slightly cracked fruit, jujube fruit with a crack area percentage greater than 3% but not exceeding 8% was classified as moderately cracked fruit, jujube fruit with a crack area percentage greater than 8% but not exceeding 15% was classified as severely cracked fruit, and jujube fruit with a crack area percentage greater than 15% or cracks penetrating the flesh were removed.

[0008] S2. Align the central axis of the cylindrical pitting knife with the longitudinal center line of the jujube fruit, and guide the cylindrical pitting knife through the jujube fruit from the pedicel end to the calyx end to remove the pit, so as to obtain a whole pitted fruit with a complete shape and continuous pit holes.

[0009] S3. Determine the initial moisture content of the pitted whole fruit. Group pitted whole fruit of the same crack grade and with an initial moisture content difference not exceeding 5 percentage points into the same drying batch. Lay the pitted whole fruit of the same batch on a material tray with ventilation holes, with an air gap between adjacent pitted whole fruit and a vertical distance of 80-200 mm between adjacent material trays. The drying airflow flows along the upper and lower surfaces of the material trays, and the airflow direction is changed within a preset time interval to reduce the moisture content difference between different trays and different areas of the same tray.

[0010] S4. After loading, maintain the pressure inside the drying equipment at atmospheric pressure. Preheat the pitted and whole jujubes with a circulating airflow of 45–55℃ for 20–60 minutes, ensuring the temperature difference between the inside and surface of the jujubes does not exceed 8℃. Use a relatively gentle temperature during the preheating stage to gradually raise the temperature of the jujube surface and interior. The surface temperature of the jujubes can be detected by an infrared temperature sensor, and the internal temperature can be detected by a temperature probe inserted into the center of a representative sample. The circulating airflow speed during preheating is 0.5–2.0 m / s.

[0011] When the temperature difference between the inside and surface of the jujube fruit is no greater than 8℃, and the change in internal temperature within 5 minutes is no greater than 2℃, the preheating ends and the rapid dehydration stage begins.

[0012] S5. Perform the rapid dehydration stage, controlling the drying temperature at 55-68℃ and the vacuum stage pressure at 15-45kPa. Each vacuum pulsation cycle includes a vacuum holding process lasting 15-35 minutes and an atmospheric pressure holding process lasting 3-12 minutes. Repeat the vacuum pulsation cycle until the moisture content of the pitted and whole fruit is reduced to 38%-45%.

[0013] After the vacuum is established, the boiling point of the water inside the pitted fruit decreases, and the water migrates from the inside of the pulp to the pit pores, cracks, and the surface of the peel. After the pressure returns to normal, the circulating airflow carries away the water vapor from the surface of the jujube and the equipment cavity. The alternation between vacuum and normal pressure constitutes a vacuum pulsation cycle.

[0014] S6. When the moisture content of the pitted and whole fruit decreases to 38%–45%, a low-damage slow drying stage is implemented. The drying temperature is adjusted to 48–60℃, and the pressure during the vacuum stage is controlled at 25–60 kPa. Each vacuum pulsation cycle includes a vacuum holding process lasting 10–25 minutes and an atmospheric pressure holding process lasting 5–15 minutes. During the vacuum establishment and atmospheric pressure recovery processes, the pressure change rate is limited to no more than 3 kPa / s. The pressure is then restored to atmospheric pressure through a segmented air intake method: after the vacuum holding period, dry air with a temperature difference of no more than 5℃ from the current drying temperature is introduced into the equipment. The pressure inside the equipment is increased to 60–75 kPa at a pressure change rate of 0.5–1.5 kPa / s, maintained for 30–120 seconds, and then increased to 95–101 kPa at a pressure change rate of 1–3 kPa / s. At the start of the next vacuum holding process, the pressure inside the equipment is reduced at a rate of no more than 3 kPa / s until the moisture content of the pitted and whole fruit reaches 23% to 25%.

[0015] Segmented air intake can be achieved by setting a proportional intake valve, a buffer chamber, and a pressure feedback control loop. The programmable controller reads real-time data from the pressure sensor and adjusts the opening of the proportional intake valve according to the set pressure change curve, so that the pressure rises at the target rate. After the pressure reaches 60-75 kPa, it enters an intermediate holding range, allowing the internal pressure of the jujube fruit to gradually reach equilibrium with the pressure in the equipment cavity; then, atmospheric pressure recovery is completed, reducing the impact of a single pressure repressurization on the crack edges and the periphery of the kernel pores.

[0016] S7. After reaching the drying endpoint, stop heating and keep the circulating fan running to cool the pitted fruit under circulating airflow. During the cooling process, the pressure inside the equipment is maintained at 95-101 kPa, and the temperature of the circulating airflow is gradually reduced until the temperature of the pitted fruit is no higher than 5°C above the ambient temperature.

[0017] This invention provides a low-damage vacuum pulse drying method for ripe jujubes. It offers the following advantages:

[0018] 1. This invention addresses the differences in raw material damage, internal moisture migration conditions, and structural strength among ripe jujubes, especially cracked jujubes. Jujubes are categorized into intact, slightly cracked, moderately cracked, and severely cracked fruits. Fruits with excessively large crack areas, cracks penetrating the flesh, or those exhibiting disease, insect infestation, mold, or mechanical damage are discarded. Simultaneously, jujubes of different crack grades are washed, pitted, have their initial moisture content measured, and are grouped into batches of the same grade. This ensures that the degree of cracking within the same drying batch is similar, and the difference in initial moisture content does not exceed 5 percentage points. This method reduces the problems of under-drying, over-drying, and uneven quality caused by mixing jujubes with different structural states and moisture contents during drying. It also provides a basis for selecting corresponding drying temperatures, vacuum absolute pressures, and pressure change rates for different crack grades, thereby improving the consistency of moisture content, appearance, and quality of the dried product.

[0019] 2. This invention involves preheating the jujube fruit at 45–55℃ under normal pressure with circulating airflow for 20–60 minutes, ensuring the temperature difference between the inside and surface of the fruit is no greater than 8℃. Then, rapid dehydration is performed at 55–68℃ and 15–45 kPa, reducing the moisture content of the pitted fruit to 38%–45%. Finally, low-damage slow drying is carried out at 48–60℃ and 25–60 kPa, stabilizing the moisture content at the final drying point at 23%–25%. The continuous pit pores created by directional pitting also provide channels for moisture migration within the fruit pulp. Combined with the tray spacing and reversing of the circulating airflow, this promotes uniform moisture removal from the jujube fruit. Efficiency is improved by utilizing the rapid dehydration capacity at the higher moisture content in the early stages of drying, while the intensity of heat and vacuum is reduced in the later stages.

[0020] 3. This invention limits the pressure change rate during vacuum establishment and atmospheric pressure recovery to no more than 3 kPa / s during the low-damage slow drying stage, and employs a segmented air intake method to restore pressure. Simultaneously, it utilizes an image acquisition device, temperature sensor, pressure sensor, humidity sensor, and weighing device to acquire crack grade, real-time moisture content, water loss rate, and absolute humidity difference between air intake and exhaust. A programmable controller automatically corrects the drying temperature, absolute vacuum pressure, vacuum holding time, and atmospheric pressure holding time. By slowly and segmentedly repressurizing, the instantaneous pressure difference inside and outside the crack is reduced, and real-time feedback control avoids the use of high temperature, high vacuum, or inappropriate holding times in the later stages of drying. Attached Figure Description

[0021] Figure 1 The bar chart shows the properties of dried jujubes after different holding times at normal pressure; from top to bottom, the values ​​are: skin color, total acid content, vitamin C content, and reducing sugar content.

[0022] Figure 2 The drying characteristic curves of vacuum pulsating drying test of cracked jujube fruit under different atmospheric pressure holding times are shown.

[0023] Figure 3 The drying characteristic curves of vacuum pulse drying test of cracked jujube fruit under different vacuum holding times are shown.

[0024] Figure 4 The bar chart shows the properties of dried jujubes after different vacuum holding times; from top to bottom, the values ​​are reducing sugar content, vitamin C content, total acid content, and sample skin color.

[0025] Figure 5 The bar chart shows the properties of dried jujubes at different drying temperatures; from top to bottom, the values ​​are reducing sugar content, vitamin C content, total acid content, and sample skin color.

[0026] Figure 6 The drying characteristic curves of cracked jujube fruit under vacuum pulsed drying test at different drying temperatures are shown.

[0027] Figure 7 Comparison images of the appearance of dried jujubes under different drying conditions. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1:

[0030] This invention provides a vacuum pulse drying method for moderately cracked fruit, as detailed below:

[0031] After harvesting, diseased, insect-infested, moldy, and mechanically damaged fruits are removed. Moderately cracked fruits with a cracked area greater than 3% but not exceeding 8% are selected. The moderately cracked fruits are washed with running water and then clean airflow is used to remove surface water.

[0032] Identify the pedicel end and the calyx end, and use a cylindrical pitting knife to continuously remove the pit along the direction from the pedicel end to the calyx end. After removing the pit, discard any jujubes with obvious cracks at the edge of the pit hole.

[0033] The initial moisture content of the samples was determined, and pitted whole fruits with an initial moisture content difference of no more than 5 percentage points were grouped into the same batch. The pitted whole fruits were laid in a single layer on an open material tray, with a spacing of 10 mm between adjacent fruits and a vertical distance of 120 mm between adjacent trays.

[0034] The material was loaded into a vacuum pulse drying device and preheated for 40 minutes at 50°C using a circulating airflow under normal pressure. The circulating airflow speed was 1.2 m / s. The temperature difference between the inside and surface of the jujube fruit was monitored, and preheating was stopped when the temperature difference decreased to within 8°C.

[0035] The drying temperature for the rapid dehydration stage was set at 61℃, the absolute pressure for the vacuum stage was set at 35 kPa, the vacuum holding time was 25 min, and the atmospheric pressure holding time was 7 min. The equipment alternately performed vacuum holding and atmospheric pressure holding, and the program controller calculated the real-time moisture content of the wet basis based on the online weighing results. When the moisture content decreased to 42%, it switched to the low-damage slow drying stage.

[0036] The temperature for the low-damage slow drying stage was set at 55℃, the absolute pressure for the vacuum stage was set at 45 kPa, the vacuum holding time was 20 min, and the atmospheric pressure holding time was 10 min. After the vacuum holding period, the pressure inside the equipment was increased to 68 kPa at a pressure change rate of 1.0 kPa / s, held for 60 s, and then increased to 98 kPa at a pressure change rate of 2 kPa / s. At the start of the next vacuum stage, the pressure reduction rate was controlled at 2.5 kPa / s.

[0037] During the period of maintaining normal pressure, the relative humidity of the circulating airflow is controlled at 10%–20%, and the flow rate is 1.5 m / s. The airflow direction is changed every 20 minutes. When the real-time wet base moisture content reaches 24%, heating is stopped, and the circulating airflow is maintained for cooling until the temperature of the jujube fruit is no higher than the ambient temperature by 5°C.

[0038] Example 2:

[0039] This invention provides a vacuum pulse drying method for slightly cracked fruit, as detailed below:

[0040] Slightly cracked fruits with a crack area ratio greater than 0 and not exceeding 3% were selected, and the fruits were cleaned, surface free water was removed, and the pits were removed in a directional manner according to the method in Example 1.

[0041] Pitted whole jujubes with an initial wet basis moisture content difference of no more than 5 percentage points are grouped into the same batch, with an airflow gap of 8 mm between adjacent jujubes and a vertical distance of 100 mm between adjacent trays.

[0042] Preheating was performed at 52°C with circulating airflow for 30 minutes under normal pressure. The rapid dehydration stage employed a drying temperature of 64°C, a vacuum absolute pressure of 30 kPa, a vacuum holding time of 25 minutes, and a normal pressure holding time of 7 minutes. When the real-time wet basis moisture content reached 40%, the process transitioned to a low-damage, slow-drying stage.

[0043] The low-damage slow drying stage uses a drying temperature of 58℃, an absolute vacuum pressure of 38kPa, a vacuum holding time of 18min, and an atmospheric pressure holding time of 8min. During the re-pressure stage, the pressure is increased to 70kPa at 1.4kPa / s, held for 45s, and then increased to 98kPa at 2.5kPa / s.

[0044] During the drying process, the average water loss rate is calculated based on the mass data collected by the weighing device. When the difference in water loss rate between two adjacent vacuum pulsation cycles does not exceed 10%, and the absolute humidity difference between the inlet and outlet continuously decreases, the drying temperature is reduced by 3°C. Drying is stopped and cooled after the moisture content reaches 23%–25%.

[0045] Slightly cracked fruit has a lower degree of cracking, so a higher drying temperature and lower vacuum absolute pressure can be used to improve processing efficiency while maintaining low damage and pressure.

[0046] Example 3:

[0047] This invention provides a vacuum pulse drying method for severely cracked fruit, as detailed below:

[0048] Severely cracked fruits with a crack area greater than 8% but not exceeding 15% were selected. Jujube fruits with cracks penetrating the flesh, mold spots appearing at the crack opening, or soft rot in the cracked tissue were discarded.

[0049] After washing, draining, and directional pitting, lay the pitted whole dates on trays, leaving a 15mm gap between adjacent dates and a vertical distance of 150mm between adjacent trays. Increasing the spacing between dates helps maintain airflow exchange at lower drying temperatures.

[0050] The preheating temperature under normal pressure was 48℃, and the preheating time was 50 minutes. The rapid dehydration stage used a drying temperature of 58℃, a vacuum absolute pressure of 42 kPa, a vacuum holding time of 20 minutes, and a normal pressure holding time of 10 minutes. When the real-time wet basis moisture content reached 43%, the low-damage slow drying stage commenced.

[0051] The low-damage slow drying temperature was set at 50℃, the absolute vacuum pressure at 58 kPa, the vacuum holding time at 15 min, and the atmospheric pressure holding time at 12 min. After the vacuum holding period, the pressure was increased to 65 kPa at 0.6 kPa / s, held for 100 s, and then increased to 98 kPa at 1.2 kPa / s. The pressure reduction rate for the next vacuum stage was 1.5 kPa / s.

[0052] When the absolute humidity difference between the intake and exhaust gases decreases for two consecutive cycles, extend the atmospheric pressure holding time by 3 minutes to allow water vapor near the cracks and core pores to be fully expelled. Stop heating and begin cooling once the moisture content reaches 24%.

[0053] Example 4:

[0054] This invention provides a vacuum pulse drying method for whole fruits, as detailed below:

[0055] Select whole fruits with a crack area ratio of 0, and complete the cleaning, removal of surface free water, and directional pitting according to the method in Example 1. Group the pitted whole fruits with an initial wet basis moisture content difference of no more than 5 percentage points into the same batch, with an airflow gap of 5 mm between adjacent fruits and a vertical distance of 70 mm between adjacent trays.

[0056] Preheating was performed at 54°C for 25 minutes under normal pressure using circulating airflow. During the rapid dehydration stage, the drying temperature was 60°C, the absolute vacuum pressure was 20 kPa, the vacuum holding time was 27 minutes, and the normal pressure holding time was 8 minutes. When the real-time wet basis moisture content reached 40%, the process was switched to the low-damage slow drying stage.

[0057] The low-damage slow drying stage uses a drying temperature of 60℃, a vacuum absolute pressure of 30kPa, a vacuum holding time of 16min, and a normal pressure holding time of 6min. During the re-pressure stage, the pressure is increased to 73kPa at 1.8kPa / s and held for 30s, then increased to 98kPa at 2.8kPa / s. Heating is stopped and cooling is performed after the moisture content reaches 24%.

[0058] Comparative Example 1:

[0059] See Figure 1 , Figure 2 and Figure 7 This comparative example verifies the vacuum pulsating drying test under different atmospheric pressure holding times.

[0060] Pitted and ripened jujubes from southern Xinjiang were evenly spread on a tray and placed in a vacuum pulse dryer. The drying temperature was set to 65℃, and the vacuum holding time was set to 25 minutes, with timing starting after the absolute pressure inside the equipment dropped to 11 kPa. Three sets of atmospheric pressure holding times were set to 5 minutes, 7 minutes, and 9 minutes respectively, forming three sets of atmospheric pressure holding time experiments. After each set of experiments, the mass of the sample was recorded at different drying times, and the moisture ratio, dry basis moisture content, and drying rate were calculated. The reducing sugar, vitamin C, total acid, and color of the dried samples were also tested.

[0061] The experimental results show that, under the conditions of drying temperature of 65℃ and vacuum holding time of 25 min, the drying rate of cracked jujubes first increases and then decreases with the increase of atmospheric pressure holding time. When the atmospheric pressure holding time is increased from 5 min to 7 min, the drying efficiency of cracked jujubes increases; when the atmospheric pressure holding time is increased to 9 min, the drying efficiency decreases.

[0062] Under the experimental conditions, the drying times for cracked jujubes were 19.3 h, 17.8 h, and 20.5 h when held at atmospheric pressure for 5 min, 7 min, and 9 min, respectively. The drying time was shortest when held at atmospheric pressure for 7 min. The reducing sugar content was measured as follows: 32.20% and 30.87% for whole and cracked jujubes when held at atmospheric pressure for 5 min; 35.23% and 33.93% for whole and cracked jujubes when held at atmospheric pressure for 7 min; and 34.52% and 31.85% for whole and cracked jujubes when held at atmospheric pressure for 9 min.

[0063] The results show that as the holding time under normal pressure increased from 5 min to 9 min, the reducing sugar content in both intact and cracked fruits exhibited a trend of first increasing and then decreasing. When the holding time under normal pressure was 7 min, the reducing sugar content in both intact and cracked jujube fruits was relatively high; when the holding time was 9 min, the reducing sugar content in cracked fruits decreased significantly.

[0064] When the holding time at normal pressure is extended, the residence time of jujube fruit in the normal pressure heating environment increases, and polysaccharides inside the jujube fruit decompose to form reducing sugars during the drying process. When the holding time at normal pressure is too long, the contact time between cracked fruit pulp and oxygen increases, and the reducing sugars at the cracks are more easily oxidized and consumed, thus reducing the reducing sugar content.

[0065] One-way ANOVA was performed on the reducing sugar, vitamin C, and total acid content of whole and cracked jujube fruits. The results showed that the holding time under normal pressure had a highly significant effect on the reducing sugar, vitamin C, and total acid content of both whole and cracked jujube fruits. Based on the color measurement results, the brightness, red-green value, and blue-yellow value of the samples changed after holding times under normal pressure of 5 min, 7 min, and 9 min. The brightness value of the samples ranged from 32.30 to 32.93; the brightness value was lower at 9 min. With prolonged holding time under normal pressure, the overall color of the sample surface tended to darken, with increased redness and decreased yellowness.

[0066] Comparative Example 2:

[0067] See Figure 3 , Figure 4 and Figure 7 This comparative experiment verifies the vacuum pulsating drying test under different vacuum holding times.

[0068] The drying temperature was set to 65℃, and the atmospheric pressure holding time was set to 7 minutes. Vacuum holding times were set to 20 minutes, 25 minutes, and 30 minutes, with the starting point for the vacuum holding process being the absolute pressure inside the equipment decreasing to 11 kPa. When the vacuum holding time was 20 minutes, 25 minutes, and 30 minutes, the drying completion time for the cracked jujubes was 22.2 hours, 17.8 hours, and 22.8 hours, respectively. Increasing the vacuum holding time from 20 minutes to 25 minutes shortened the drying time for the cracked jujubes; increasing the vacuum holding time to 30 minutes lengthened the drying completion time.

[0069] The experimental results showed that under normal pressure for 7 minutes and a drying temperature of 65℃, the drying rate of cracked jujubes initially increased and then decreased with increasing vacuum holding time. The drying rate was highest and the drying time was shortest when the vacuum holding time was 25 minutes. At a vacuum holding time of 20 minutes, the total acid content of intact and cracked jujubes was 0.226% and 0.221%, respectively; at 25 minutes, it was 0.241% and 0.239%, respectively; and at 30 minutes, it was 0.210% and 0.206%, respectively.

[0070] It is evident that as the vacuum holding time increased from 20 min to 30 min, the total acid content of both intact and cracked fruits showed a trend of first increasing and then decreasing. The total acid content was highest when the vacuum holding time was 25 min, corresponding to a total drying time of 17.8 h; the total drying times were 22.2 h and 22.8 h for vacuum holding times of 20 min and 30 min, respectively.

[0071] When the vacuum holding time is 25 minutes, the moisture migration rate is faster, the total drying time is shortened, and the time the sample is exposed to high temperature is reduced, which is beneficial for the retention of total acid. If the vacuum holding time is too short, the moisture removal is insufficient, and the drying process is prolonged; if the vacuum holding time is too long, the processing time of the jujubes in the drying environment increases, and the dried quality is affected.

[0072] One-way ANOVA was conducted to investigate the effects of vacuum holding time on the reducing sugar content, vitamin C content, and total acid content of both whole and cracked jujubes. Color analysis revealed that prolonged vacuum holding time significantly decreased the brightness value and deepened the browning of the skin. The vacuum environment limited oxidation reactions, resulting in relatively gradual changes in red and green values. Longer vacuum holding times reduced the contact time between the sample and oxygen, leading to smaller changes in yellow and blue values. Combined with visual observation, prolonged vacuum holding time increased the wrinkling degree on the surface of dried cracked jujubes.

[0073] Comparative Example 3:

[0074] See Figure 5 , Figure 6 and Figure 7 This comparative experiment verifies the vacuum pulsating drying test under different drying temperatures.

[0075] The drying time was set to 7 minutes under normal pressure and 25 minutes under vacuum. Experiments were conducted at three drying temperatures: 60℃, 65℃, and 70℃. During the experiments, the moisture content of the cracked jujubes decreased with increasing drying time at all three temperatures (60℃, 65℃, and 70℃). The rate of decrease in moisture content accelerated with increasing drying temperature, indicating a higher drying rate.

[0076] At a drying temperature of 60℃, the drying time for cracked jujubes was 20.3 hours; at 65℃, it was 17.8 hours; and at 70℃, it was 14.2 hours. This indicates that, under the conditions of 7 minutes at normal pressure and 25 minutes under vacuum, increasing the drying temperature can shorten the drying time for cracked jujubes.

[0077] The drying rate of cracked jujubes under different temperature conditions showed an initial increase followed by a decrease in the dry basis moisture content. In the initial stage of drying, the moisture content in the samples was high, and the resistance to moisture migration was low. As the drying process progressed, the evaporable water in the cracked jujubes gradually decreased, the resistance to the migration of internal moisture to the surface increased, and the drying rate decreased.

[0078] Drying temperature significantly affected the reducing sugar content, vitamin C content, and total acid content of whole and cracked jujubes. One-way ANOVA results showed that the effects of drying temperature on these quality indicators were all highly significant. Color testing results indicated that as drying temperature increased, the browning of the dried samples deepened, and the brightness decreased. Higher temperatures promoted non-enzymatic browning reactions; the red-green value increased with increasing temperature, while the yellow-blue value gradually decreased, and the overall color of the sample surface tended to darken. At a drying temperature of 70℃, the sample color was darker, and surface wrinkling was more pronounced.

[0079] The reducing sugar content was determined by direct titration according to GB / T 5009.7—2016 "Determination of Reducing Sugars in Food". The skin of the jujubes was removed, and a specified weight of pulp was cut to prepare the sample. Each sample was tested in triplicate, and the average value was taken as the result. Vitamin C content was determined using the 2,6-dichlorophenolindophenol method. After removing the skin of the jujubes, an appropriate amount of pulp was weighed to prepare the test sample solution, which was then titrated with a standardized 2,6-dichlorophenolindophenol dye solution. The titration endpoint was reached when the test sample solution turned pink and did not fade within 15 seconds; a blank control was also included. Each sample was tested in triplicate, and the average value was taken. The total acid content was determined by acid-base titration according to GB / T 12456—2021 "Determination of Total Acids in Food". The main organic acid in jujubes is malic acid; therefore, the total acid determination result was calculated using a conversion factor (k=0.067). Each sample was tested in triplicate, and the average value was taken. Color was measured using a colorimeter, and the test indicators included brightness, red or green intensity, and yellow or blue intensity. Each sample was measured three times, and the average value was used as the color test result.

[0080] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage, characterized in that, Includes the following steps: S1. Screen the jujubes in the crisp ripe period, remove diseased fruits, insect-infested fruits, moldy fruits and fruits with mechanical damage, collect and retain the appearance images of the jujubes, and classify them according to the crack length, crack width and crack area ratio in the appearance images. Wash the graded jujubes and remove the surface free water. S2. Pit the jujube fruit in a directional manner according to the axis of the jujube pit to obtain a whole pitted fruit with an intact shape and continuous pit holes; S3. Determine the initial moisture content of the pitted whole fruit. Pitted whole fruit of the same crack grade and with an initial moisture content difference of no more than 5 percentage points shall be regarded as the same drying batch and loaded into the vacuum pulse drying equipment in single or multiple layers at intervals. S4. Preheat the pitted whole fruit with circulating air at 45-55℃ for 20-60 minutes under normal pressure, so that the temperature difference between the inside and the surface of the jujube fruit is no more than 8℃. S5. Perform the rapid dehydration stage, controlling the drying temperature at 55–68℃ and the vacuum stage pressure at 15–45 kPa. Each vacuum pulsation cycle includes a vacuum holding process lasting 15–35 minutes and a normal pressure holding process lasting 3–12 minutes. Repeat the vacuum pulsation cycle until the moisture content of the pitted and whole fruit is reduced to 38%–45%. S6. Perform a low-damage, slow-drying stage, adjusting the drying temperature to 48–60°C and controlling the pressure during the vacuum stage to 25–60 kPa. Each vacuum pulsation cycle includes a vacuum holding process lasting 10–25 minutes and an atmospheric pressure holding process lasting 5–15 minutes. During the vacuum establishment and atmospheric pressure recovery processes, the pressure change rate is limited to no more than 3 kPa / s, and the pressure is restored to atmospheric pressure through segmented air intake until the moisture content of the pitted and whole fruit reaches 23%–25%. S7. Stop heating and cool the pitted whole fruit to no more than 5°C above the ambient temperature under circulating airflow. The cumulative processing time from step S4 to step S6 is 12 to 18 hours. The drying process automatically adjusts the drying temperature, vacuum absolute pressure, vacuum holding time and atmospheric pressure holding time according to the crack grade of the jujube fruit, real-time moisture content and water loss rate.

2. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, In step S1, the jujube appearance image is sequentially processed by background separation, jujube contour extraction, crack region enhancement, and crack connected component recognition. The ratio of the crack region pixel area to the jujube projection region pixel area is used as the crack area percentage. Jujubes with a crack area percentage of 0 are classified as intact fruits, jujubes with a crack area percentage greater than 0 but not exceeding 3% are classified as slightly cracked fruits, jujubes with a crack area percentage greater than 3% but not exceeding 8% are classified as moderately cracked fruits, jujubes with a crack area percentage greater than 8% but not exceeding 15% are classified as severely cracked fruits, and jujubes with a crack area percentage greater than 15% or cracks penetrating the flesh are removed.

3. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, The specific operation of directional pitting in step S2 is as follows: Align the central axis of the cylindrical pitting knife with the longitudinal center line of the jujube fruit, and pass the cylindrical pitting knife through the jujube fruit from the stem end to the calyx end.

4. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, In step S3, the pitted whole fruit is laid on a material tray with ventilation holes. The airflow gap between adjacent pitted whole fruit and the vertical distance between adjacent material trays is 80-200mm. The drying airflow flows along the upper and lower surfaces of the material trays and changes the airflow direction within a preset time interval to reduce the moisture content difference between different trays and different areas of the same tray.

5. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, Differentiated drying parameters were adopted for jujubes with different crack levels: based on the drying parameters corresponding to intact fruit, the drying temperature of slightly cracked fruit was reduced by 2-4℃ and the pressure in the vacuum stage was increased by 5-10 kPa; the drying temperature of moderately cracked fruit was reduced by 4-8℃ and the pressure in the vacuum stage was increased by 10-15 kPa; and the drying temperature of severely cracked fruit was reduced by 6-10℃ and the pressure in the vacuum stage was increased by 15-20 kPa. The rate of pressure change during the recovery process under normal pressure decreased sequentially for slightly cracked, moderately cracked, and severely cracked fruit, thereby inhibiting crack propagation, fruit pulp eversion, and the precipitation of soluble substances from the cracks.

6. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, The segmented air intake in step S6 is as follows: After the vacuum holding period ends, dry air with a temperature difference of no more than 5°C from the current drying temperature is introduced into the vacuum pulse drying equipment. The pressure inside the equipment is increased to 60-75 kPa at a pressure change rate of 0.5-1.5 kPa / s, maintained for 30-120s, and then increased to 95-101 kPa at a pressure change rate of 1-3 kPa / s. At the beginning of the next vacuum holding process, the pressure inside the equipment is reduced at a pressure change rate of no more than 3 kPa / s.

7. The method for low-damage vacuum pulse drying of jujubes at the crisp ripe stage according to claim 1, characterized in that, During the atmospheric pressure maintenance process, the relative humidity of the circulating airflow is 5% to 25%, and the flow velocity is 0.5 to 2.5 m / s. The flow direction of the circulating airflow is changed every 10 to 30 minutes. When the absolute humidity difference between the exhaust airflow and the inlet airflow decreases for two consecutive vacuum pulsation cycles, and the difference in the average water loss rate between two adjacent vacuum pulsation cycles does not exceed 10%, the drying temperature is reduced by 2 to 5 °C or the atmospheric pressure maintenance time is extended by 2 to 5 minutes.

8. The low-damage vacuum pulse drying method for jujubes at the crisp ripe stage according to any one of claims 1-7, characterized in that, The drying process is automatically controlled by a programmable controller. The programmable controller is connected to an image acquisition device, a temperature sensor, a pressure sensor, a humidity sensor, a weighing device, a vacuum pump, a heating device, a circulating fan, and an intake and exhaust regulating valve. The programmable controller calls up the corresponding initial drying parameters according to the crack level, determines the switching point between the rapid dehydration stage and the low-damage slow drying stage according to the real-time moisture content, and corrects the drying temperature, vacuum absolute pressure, and vacuum holding time according to the change in water loss rate in adjacent vacuum pulsation cycles, so that the moisture content at the end of drying is stabilized at 23% to 25%.

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