Vacuum freeze-drying method for pre-treating jingan apricot and prepared jingan apricot food

Abstract

The present application belongs to the technical field of food processing, and particularly relates to a pretreatment method of vacuum freeze-drying Diaogan apricot and a prepared Diaogan apricot food. The pretreatment method comprises the following steps: S1, pretreatment: selecting Diaogan apricot and cutting the selected Diaogan apricot into slices with a thickness of 5-8 mm; S2, freeze-thaw treatment: freezing the Diaogan apricot and thawing the frozen Diaogan apricot, and sealing the thawed Diaogan apricot after the internal temperature of the Diaogan apricot is kept constant; and S3, ultrasonic treatment: ultrasonically treating the thawed and sealed Diaogan apricot. The pretreatment method of the present application combines ultrasonic treatment and freeze-thaw treatment, so that the cell walls of the Diaogan apricot are damaged, the heat and mass transfer rates are increased, the drying time is shortened, and the energy consumption is saved. Moreover, the sensory quality and antioxidant capacity of the Diaogan apricot product are significantly improved compared with traditional vacuum freeze-drying, and the contents of total phenols and total flavonoids are retained more. The prepared Diaogan apricot can shorten the drying time without loss of nutritional ingredients.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of food processing, and particularly relates to a pretreatment method for vacuum freeze-dried dried apricot and a dried apricot food prepared by the method. BACKGROUND

[0002] Apricots are nutritionally balanced and rich in carbohydrates and minerals, including sugars, organic acids, and sorbitol. The most abundant minerals are magnesium, calcium, iron, zinc, and copper. Apricots are also rich in vitamin A and carotenoids. In addition, chlorogenic acid, neochlorogenic acid, catechin, catechin, and rutin, as well as quercetin, are the main phenolic compounds in apricots. These substances have good biological activity functions, such as antioxidant, anti-inflammatory, anti-diabetic, brain protection, and prevention of heart disease, so apricots are widely favored by consumers.

[0003] However, apricots are harvested in summer, and high temperature and high moisture content lead to softening, browning, and even rotting after harvest, which severely limits the fresh eating value of apricots. In order to solve this problem, it is necessary to find a suitable processing method to maintain the quality of apricots and improve their market value. At present, the commonly used drying methods for apricots include natural air drying, hot air drying, microwave drying, and vacuum freeze-drying. Due to the differences in heat transfer mechanisms, each drying method has its advantages and disadvantages. Compared with other drying methods, vacuum freeze-drying can maximize the retention of color, aroma, taste, shape, and nutritional ingredients of the dried product due to the low temperature and vacuum environment. However, vacuum freeze-drying technology has the disadvantages of long drying time and high energy consumption, which limits its widespread application in industrial production. Therefore, it is necessary to further study how to maintain and improve the quality of vacuum freeze-dried dried apricots while reducing the drying time and energy consumption. SUMMARY

[0004] (I) Technical problems to be solved

[0005] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present application provides a pretreatment method for vacuum freeze-dried dried apricots and a dried apricot food prepared by the method. The dried apricots are pretreated by ultrasonic and freeze-thaw combination before vacuum freeze-drying, which greatly shortens the vacuum freeze-drying time of the dried apricots, improves the quality of the prepared dried apricots, and reduces the production cost and energy consumption.

[0006] (II) Technical solutions

[0007] In order to achieve the above-mentioned purposes, the main technical solutions adopted by the present application include:

[0008] In a first aspect, the present application provides a pretreatment method for vacuum freeze-dried dried apricots, which is as follows:

[0009] S1 Pretreatment: The dried apricots are screened and then cut into slices with a thickness of 5-8 mm;

[0010] S2 freeze-thaw treatment: freeze and thaw the dried apricot, and seal after the internal temperature of the dried apricot remains constant;

[0011] S3 ultrasonic treatment: ultrasonic treatment is performed on the dried apricot after thawing and sealing.

[0012] Optionally, in the pretreatment S1, the screening method comprises cleaning, de-stemming and de-stoning.

[0013] Optionally, the freeze-thaw treatment comprises S21 freezing treatment and S22 thawing treatment; wherein,

[0014] S21 freezing treatment: freeze the dried apricot at -18℃ to -22℃;

[0015] S22 thawing treatment: thaw the frozen dried apricot at 20℃ to 25℃ until the internal temperature of the dried apricot remains constant, and then seal.

[0016] Optionally, the freezing time of the freeze-thaw treatment is 20h to 30h, and the internal temperature of the dried apricot remains constant.

[0017] Optionally, the sealing method is bag sealing.

[0018] Optionally, in the ultrasonic treatment S3, the thawed and sealed dried apricot is immersed in water at 20℃ to 25℃ as a medium, and ultrasonic waves are applied to the water, the ultrasonic power is 180W to 220W, the ultrasonic frequency is 38kHz to 42kHz, and the ultrasonic treatment time is 20min to 30min.

[0019] During the ultrasonic process, the mutual mechanical vibration between the media produces thermal effect, mechanical action and cavitation effect on the dried apricot; at the same time, the ultrasonic waves can accelerate the mass transfer and heat transfer coefficient of the dried apricot in the later drying process, improve the drying rate, reduce the drying time, reduce the broken material, and improve the color of the freeze-dried dried apricot.

[0020] In a second aspect, the present application provides a dried apricot food, which is prepared by the above-mentioned pretreatment and vacuum freeze-drying method.

[0021] Optionally, the dried apricot is subjected to vacuum freeze-drying treatment after pretreatment.

[0022] Optionally, in the vacuum freeze-drying treatment, the freezing time is 2h to 4h, the freezing temperature is -25℃ to -40℃, and the drying temperature is 20℃ to 25℃.

[0023] Optionally, the dried apricot after pretreatment and vacuum freeze-drying has a drying time of ≤14h and a total energy consumption of ≤42kWh / kg.

[0024] Optionally, the FRAP content of the pretreated and vacuum freeze-dried dried apricot is ≥7.12 mg Trolox / g, the TFC content is ≥1.67 mg RE / g, and the TPC content is ≥7.70 mg GAE / g.

[0025] The dried apricot food is obtained by vacuum freeze-drying technology after freeze-thaw and ultrasonic combined pretreatment. Freeze-thaw causes the cell tissue structure to be destroyed, forming a porous network microstructure. Microscopic channels are formed under the action of ultrasonic, accelerating the migration of free water. Freeze-thaw and ultrasonic treatment change the water distribution inside the dried apricot. In addition, ultrasonic cavitation effect and freeze-thaw damage to cells are helpful to the extraction of phenolic and flavonoid compounds.

[0026] (Three) beneficial effects

[0027] The beneficial effects of the present application are: the vacuum freeze-drying pretreatment method of dried apricot of the present application adopts an ultrasonic and freeze-thaw combined pretreatment scheme, which causes the cell wall of dried apricot to be destroyed, changes the water distribution in the cell, increases the heat and mass transfer rate, and shortens the drying time to save energy consumption; improves the color of freeze-dried dried apricot to improve the sensory quality, while maximizes the retention of the mouthfeel of dried apricot and meets the hardness of consumer demand; retains more total phenol and total flavonoid content in dried apricot, and increases the antioxidant capacity of dried apricot.

[0028] Preferably, the drying time of the freeze-dried dried apricot sample without pretreatment is 21h, and the drying time of the freeze-dried dried apricot sample treated by freeze-thaw and ultrasonic combined treatment is 14h, which effectively shortens the vacuum freeze-drying time of dried apricot.

[0029] Preferably, compared with freeze-dried dried apricot samples without pretreatment or single treatment, freeze-dried dried apricot samples treated by freeze-thaw and ultrasonic combined treatment exhibit higher dehydration rate.

[0030] Preferably, the total energy consumption of the freeze-dried dried apricot sample without pretreatment is 64.33kWh / kg, and the total energy consumption of the freeze-dried dried apricot sample treated by freeze-thaw and ultrasonic combined treatment is 41.39kWh / kg, which reduces the energy consumption by 35.66%, and is more suitable for industrial production.

[0031] Preferably, on the microstructure, freeze-thaw-ultrasonic combined treatment causes the greatest damage to dried apricot cells, and the damage degree is not simply additive, which confirms the feasibility of freeze-thaw and ultrasonic combined treatment and the synergy of the two.

[0032] Preferably, freeze-thaw-ultrasonic combined treatment has the least effect on the color difference of freeze-dried dried apricot, and can better maintain the color of the fruit.

[0033] Preferably, the freeze-dried apricot treated by freeze-thaw-ultrasound combination process ensures the total phenol and total flavonoid content in the freeze-dried apricot, maximally retains the nutritional ingredients of the freeze-dried apricot, and improves the antioxidant capacity.

[0034] Preferably, the freeze-dried apricot treated by freeze-thaw-ultrasound combination process has similar taste to the freeze-dried apricot sample without pretreatment, ensuring the retention rate of taste.

[0035] Preferably, the freeze-dried apricot treated by freeze-thaw-ultrasound combination process can appropriately reduce the hardness of the freeze-dried apricot, prevent the sample from breaking during the drying process, maintain the integrity and forming rate of the apricot slice, and meet the consumer demand for hardness. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Comparison chart of the influence of different pretreatments on the moisture ratio of freeze-dried apricot;

[0037] Figure 2 Comparison chart of the influence of different pretreatments on the drying rate of freeze-dried apricot;

[0038] Figure 3 Water distribution chart of freeze-dried apricot under different pretreatments;

[0039] Figure 4 Chart of the influence of different pretreatments on the total energy consumption of freeze-dried apricot;

[0040] Figure 5 Microstructure chart of freeze-dried apricot under different pretreatments;

[0041] Figure 6 Comparison chart of the influence of different pretreatments on the color of freeze-dried apricot;

[0042] Figure 7 Radar chart of the influence of different pretreatments on the taste of freeze-dried apricot;

[0043] Figure 8 PCA chart of the influence of different pretreatments on the taste of freeze-dried apricot;

[0044] Figure 9 Chart of the influence of different pretreatments on the hardness of freeze-dried apricot DETAILED DESCRIPTION

[0045] In order to better explain the present application and facilitate understanding, the present application will be described in detail below in combination with specific embodiments.

[0046] Example 1

[0047] The present embodiment provides a preparation method of freeze-dried apricot, and the steps are as follows:

[0048] (1) Pretreatment: Select apricots with uniform size and color, no mechanical damage, no disease and insect pests, wash, remove the core, and cut into slices with a thickness of 6mm.

[0049] (2) Freezing: Dry apricots are placed at -18℃ for 30h until the internal temperature of the sample remains constant.

[0050] (3) Thawing: Dry apricots are placed at 20℃ for thawing until the internal temperature of the sample remains constant.

[0051] (4) Ultrasonic: The sealed dry apricots are placed in an ultrasonic device with 20℃ water as the medium, and ultrasonic treatment is performed at 180W (42kHz) for 30min.

[0052] (5) Vacuum freeze-drying: The pre-freezing time is 2h, the pre-freezing temperature is -25℃, the drying temperature is 20℃, and the vacuum pressure is 10Pa.

[0053] Example 2

[0054] The present example provides a preparation method of freeze-dried dry apricots, the steps of which are as follows:

[0055] (1) Pretreatment: Select apricots with uniform size and color, no mechanical damage, no disease and insect pests, wash, remove the core, and cut into slices with a thickness of 6mm.

[0056] (2) Freezing: Dry apricots are placed at -18℃ for 30h until the internal temperature of the sample remains constant.

[0057] (3) Thawing: Dry apricots are placed at 20℃ for thawing until the internal temperature of the sample remains constant.

[0058] (4) Ultrasonic: The sealed dry apricots are placed in an ultrasonic device with 20℃ water as the medium, and ultrasonic treatment is performed at 180W (42kHz) for 30min.

[0059] (5) Vacuum freeze-drying: The pre-freezing time is 2h, the pre-freezing temperature is -25℃, the drying temperature is 20℃, and the vacuum pressure is 10Pa.

[0060] Example 3

[0061] The present example provides a preparation method of freeze-dried dry apricots, the steps of which are as follows:

[0062] (1) Pretreatment: Select apricots with uniform size and color, no mechanical damage, no disease and insect pests, wash, remove the core, and cut into slices with a thickness of 6mm.

[0063] (2) Freezing: The dried apricot was frozen at -22°C for 20h until the internal temperature of the sample remained constant.

[0064] (3) Thawing: The dried apricot was thawed at 25°C until the internal temperature of the sample remained constant.

[0065] (4) Ultrasonic: The sealed dried apricot was treated with ultrasonic waves in a water medium at 25°C for 20min at 220W (38kHz).

[0066] (5) Vacuum freeze-drying: The pre-freezing time was 4h, the pre-freezing temperature was -40°C, the drying temperature was 25°C, and the vacuum pressure was 10Pa.

[0067] Comparative Example 1

[0068] The difference between this comparative example and Example 1 is that the dried apricot was directly subjected to step (5) vacuum freeze-drying treatment after pre-treatment to prepare the dried apricot.

[0069] Comparative Example 2

[0070] The difference between this comparative example and Example 1 is that the dried apricot was subjected to step (4) ultrasonic treatment after pre-treatment, and then directly subjected to step (5) vacuum freeze-drying treatment without being subjected to step (2) freezing and step (3) thawing, to prepare the dried apricot.

[0071] Comparative Example 3

[0072] The difference between this comparative example and Example 1 is that the dried apricot was subjected to step (2) freezing and step (3) thawing after pre-treatment, and then directly subjected to step (5) vacuum freeze-drying treatment without being subjected to step (4) ultrasonic treatment, to prepare the dried apricot.

[0073] Performance Test

[0074] The freeze-dried dried apricots prepared in Example 1 and Comparative Examples 1-5 were respectively tested, and the test items were respectively the influence of different pre-treatments on the moisture ratio of the freeze-dried dried apricot, the influence on the drying rate, the influence on the moisture distribution, the influence on the total energy consumption, the influence on the microstructure, the influence on the color, the influence on the total phenol and total flavonoid content and the antioxidant capacity, the influence on the taste, and the influence on the hardness.

[0075] (1) Influence of different pre-treatments on the moisture ratio of the freeze-dried dried apricot

[0076] The change of the moisture ratio (MR) of the dried apricot sample with time was as follows Figure 1As shown, the moisture ratio of the four groups of apricot samples decreased rapidly at the initial stage of drying, and the trend slowed down at the later stage of drying until the end of drying. The drying time of the comparative example 1, comparative example 2, comparative example 3 and the sample of example 1 was 21 h, 17 h, 15 h and 14 h respectively, which indicated that the freeze-thaw or ultrasonic pretreatment effectively shortened the vacuum freeze-drying time of apricot.

[0077] Compared with the results of example 1, the combination of freeze-thaw pretreatment and ultrasonic treatment for a suitable time could effectively reduce the drying time of freeze-dried apricot.

[0078] The rapid decrease of the moisture ratio at the initial stage was due to the rapid removal of free water in apricot at the initial stage of vacuum freeze-drying. The main reason for the slow decrease of the moisture ratio at the later stage of drying was that the bound water and the water that was not easy to flow in apricot was not easy to remove. At the same time, the freeze-thaw and ultrasonic combined treatment affected the water distribution, which led to the difference in drying time.

[0079] (2) Effect of different pretreatments on the drying rate of freeze-dried apricot

[0080] The change of the drying rate (DR) of apricot samples with time is shown in Figure 2 , in which the drying rate of the sample of example 1 was the fastest, followed by comparative example 3 and comparative example 2, and finally comparative example 1. This was because the mechanical effect and cavitation effect of ultrasonic treatment made the apricot repeatedly shrink and expand, forming micro-pores and accelerating the migration of water. Freeze-thaw treatment destroyed the cell structure and changed the cell permeability, which was beneficial to the diffusion of water. Individually, freeze-thaw and ultrasonic could be used as effective pretreatment methods to destroy the microstructure of apricot and accelerate the diffusion of water, but the sample with freeze-thaw-ultrasonic combined treatment showed higher dehydration efficiency, which indicated that there was a strong synergistic effect between the two pretreatments, and also meant that the combined treatment could have lower time cost.

[0081] (3) Determination of water distribution of freeze-dried apricot with different pretreatments

[0082] Low-field nuclear magnetic resonance (LF-NMR) analysis is an effective means to detect the water distribution of samples. The relaxation time and water distribution of different samples are shown in Figure 3 , in which the peaks of different relaxation times of freeze-dried apricot samples represent three states, respectively, T 21 (combination water combined with cell wall polysaccharides; 1ms-10ms), T 22 (water that is not easy to flow interacting with macromolecules such as proteins; 10ms-100ms), and T 23 (free water with higher mobility; 100ms-1000ms). The water distribution of the four groups of apricot samples was roughly the same, but the relaxation time and peak area were slightly different. T 21 and T 22The peak area ratio is relatively high (see Table 1), therefore bound water and non-flowing water are the main water components in the dried samples. Compared with Comparative Example 1, the pretreated dried apricot samples T from Comparative Example 2, Comparative Example 3, and Example 1 showed higher peak area ratios (see Table 1), indicating that bound water and non-flowing water are the main water components in the dried samples. 23 The relaxation time shifts to the left, and the peak area A 23 Decrease, A 22 The increase indicates that pretreatment accelerated the removal of free water. During ultrasound and freeze-thaw pretreatment alone, the dried apricot cells were damaged to varying degrees, forming a porous structure, which accelerated water migration during vacuum freeze-drying, but the effect was not particularly significant. The combination of ultrasound and freeze-thaw significantly shortened the drying time and had a positive effect on energy conservation.

[0083] Table 1. Transverse relaxation time and peak area ratio of freeze-dried and hung-dried apricots with different pretreatments.

[0084]

[0085] (4) The effect of different pretreatments on the total energy consumption of freeze-drying and hanging-drying apricots

[0086] Comparative Examples 1, 2, 3 and 1: TEC Figure 4 As shown, the energy consumption values ​​were 64.33 kWh / kg, 52.96 kWh / kg, 44.06 kWh / kg, and 41.39 kWh / kg, respectively. Compared to Comparative Example 1, the energy consumption values ​​of Comparative Example 2, Comparative Example 3, and Example 1 were relatively low, with Example 1 having the lowest energy consumption. This indicates that the pretreatment in Example 1 significantly reduced the energy consumption of freeze-dried apricots, while ultrasonic treatment or freeze-thaw treatment alone did not significantly reduce energy consumption.

[0087] (5) Effects of different pretreatments on the microstructure of freeze-dried apricots

[0088] The changes in the texture, color, rehydration properties, and other quality characteristics of freeze-dried and hang-dried apricots are closely related to the changes in their microstructure during processing. Scanning electron microscope images of hang-dried apricot samples are shown below. Figure 5The results showed that the pre-treatments changed the microstructure of apricots, which promoted water diffusion and thus improved the efficiency of vacuum freeze-drying. The observed porous structure in the ultrasonic pre-treatment group was probably due to the inertial flow and sponge effect caused by ultrasonic cavitation, thus forming micro-channels. The freeze-thaw pre-treatment group had a surface covered with holes, and the tissue arrangement was irregular. During the freeze-thaw pre-treatment process, the material was frozen to the freezing point, and the water in the cells was converted into ice crystals. Then, the material was thawed at high temperature, which caused damage to the cell structure, resulting in more intracellular space and cavities, and severely damaging the internal structure of the cells. After freeze-thaw-ultrasonic treatment, it was almost impossible to observe the structure of the complete cell wall, and even traces of cell wall breakage could be observed in the 200x magnification images, which indicated that freeze-thaw-ultrasonic treatment caused the most damage to the microstructure of apricots, and the damage was not simply additive, confirming the feasibility of freeze-thaw-ultrasonic treatment and the synergy between the two. In summary, we believe that the pre-treatments caused differences in the microstructure, which changed the dehydration efficiency of apricots. The increase in the number and size of micro-channels and pores was beneficial to improving the drying rate and shortening the drying time.

[0089] (6) Effect of different pre-treatments on the color of freeze-dried apricots

[0090] Color is an important sensory quality for evaluating apricots and their freeze-dried products. The L * (whiteness), a * (red / green), b * (yellow / blue) and ΔE (color difference) values of the samples of Example 1 and Comparative Examples 1-5 are shown in Table 2. Figure 6 The results showed that different pre-treatments had a significant effect on the color of apricots. Compared with the undried apricots, the L* value of the dried apricots increased (p<0.05), indicating that vacuum freeze-drying improved the brightness of apricots. In addition, the a* and b* values of the dried apricots were significantly lower than those of the undried apricots (p<0.05), and the order from large to small was: Example 1 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1, indicating that vacuum freeze-drying caused the apricots to change color, and freeze-thaw-ultrasonic treatment was more helpful in maintaining the original color of apricots. The ΔE value of Example 1 was the lowest (16.07), followed by Comparative Example 3 (17.10), Comparative Example 2 (28.24) and Comparative Example 1 (31.11). Compared with ultrasonic and freeze-thaw single treatments, freeze-thaw-ultrasonic pre-treatment had the smallest color difference for apricots, which resulted in less color change for Example 1. This was also related to the shorter drying time required after pre-treatment, which better preserved the color of the fruit.

[0091] (7) Effect of different pre-treatments on the total phenol and total flavonoid contents and antioxidant capacity of freeze-dried apricots

[0092] Total phenolic (TPC) and total flavonoid (TFC) contents are important bioactive components in apricot fruits. The TPC and TFC contents of different samples are shown in Table 2. The TPC content of Comparative Example 1 was 7.82 mg / g, the TPC content of Comparative Example 3 was 7.57 mg / g and the TPC content of Example 1 was 7.70 mg / g, which was not significantly different from Comparative Example 1, while the TPC content of Comparative Example 2 (7.01 mg / g) was significantly reduced, which was attributed to the release of phenolic compounds and the inactivation of enzymes related to the degradation of phenolic substances caused by the cavitation effect of ultrasound, but when ultrasound and freeze-thaw were combined, the TPC content actually increased, which was probably because freeze-thaw made the surface of the apricot fruit full of holes, the tissue arrangement was irregular, a maze was formed between the holes, which hindered the release of phenolic compounds and the enzymes related to the degradation of phenolic substances lost activity during the ultrasound process. At the same time, too long or too short ultrasound also affected the TPC content within a certain range.

[0093] For TFC, the total flavonoid content of Example 1 (1.67 mg / g) was increased compared to Comparative Example 1 (1.44 mg / g), which indicated that freeze-thaw-ultrasound had a more positive effect on the retention of flavonoids. Freeze-thaw, freeze-thaw-ultrasound pretreatment had a positive effect on the retention of flavonoids during the freezing and thawing process, and the pretreatment of freezing and thawing and ultrasound had a better effect on the retention of flavonoids.

[0094] Table 2, Effect of different pretreatments on total phenolic, total flavonoid and antioxidant capacity of freeze-dried apricots.

[0095]

[0096] The antioxidant capacity of the samples was represented by DPPH, ABTS and FRAP. The four groups of samples had no significant difference in the ability to scavenge DPPH free radicals (p>0.05). The ABTS (1.72 mg Trolox / g) and FRAP (7.12 mg Trolox / g) of Example 1 were significantly higher (p<0.05) than those of the other samples. Example 1 had higher retention rates of total phenols and total flavonoids, and thus maintained higher antioxidant activity.

[0097] (8) Effect of different pretreatments on the taste of freeze-dried apricots

[0098] As Figure 7As shown, the different pretreated freeze-dried apricot samples of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 1 were detected by electronic tongue technology, and the radar chart directly reflected the strength of the corresponding signals of different sensors on the taste of freeze-dried apricot. The freeze-dried apricot samples of Comparative Example 1 and Example 1 had lower acidity than the tasteless point, indicating that the acidity was not the taste index of the two groups of freeze-dried apricot samples, while a certain acidity was detected in the Comparative Example 2 and Comparative Example 3 groups. Compared with Comparative Example 1, the sweetness response value of the pretreatment groups of Comparative Example 2 and Comparative Example 3 decreased, and the sweetness response value of the pretreatment group of Example 1 increased. In contrast, the sweetness response value of the pretreatment group of Example 1 was higher due to the lower acidity than the tasteless point, so the taste of the freeze-dried apricot samples played a positive role.

[0099] The PCI was used to analyze the taste of different pretreated freeze-dried apricot samples. As shown in Figure 8 , the contribution rates of the four freeze-dried apricot samples in the principal components PC1 and PC2 were 51.9% and 29.5%, respectively, and the cumulative contribution rate reached 81.4%, which could better reflect the original information of the taste of freeze-dried apricot. The pretreated freeze-dried apricot samples of Comparative Example 2 and Comparative Example 3 had overlapping parts, indicating that the taste of the two kinds of pretreated freeze-dried apricot samples was similar. The pretreatment groups of Comparative Example 1 and Example 1 were relatively close, indicating that the taste of the two freeze-dried apricot samples was similar, which also indicated that the pretreatment of Example 1 had a higher retention rate of the taste of freeze-dried apricot, and could better reflect the taste of freeze-dried apricot. The distance between the pretreatment of Comparative Example 2 and Comparative Example 3 and the pretreatment of Comparative Example 1 and Example 1 was far, indicating that the taste of the former was different from that of the latter.

[0100] (9) Effect of different pretreatments on hardness

[0101] Hardness is an important parameter for measuring the quality of dried products, which affects the taste and overall acceptability of food. For freeze-dried apricot, products with moderate hardness are more popular with consumers. As shown in Figure 9 , the hardness of the samples in Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 1 was 530.63g, 612.22g, 1356.29g and 980.72g, respectively. Compared with Comparative Example 1, the hardness of the samples in Comparative Example 3 and Comparative Example 2 increased significantly (p<0.05). In summary, different pretreatment methods and conditions had different effects on the hardness of the samples. However, in this study, the pretreatment of Comparative Example 3 and Comparative Example 2 led to an increase in the hardness of the apricot slices, which was considered in combination with the SEM results. Figure 5 , the stronger damage to the cell wall caused by freezing treatment resulted in a higher hardness of the sample due to the irregular arrangement of the cell wall, while the interaction of ultrasonic and freeze-thawing appropriately reduced the hardness, not only preventing the sample from breaking during drying, but also maintaining the integrity and forming rate of the freeze-dried apricot slices, and the hardness was suitable for consumer demand.

[0102] Although the present application has been disclosed in its preferred embodiments with reference to the accompanying drawings, it is not intended to limit the present application thereto, and various modifications and alterations can be made thereto by those skilled in the art without departing from the spirit and scope of the present application, and the scope of protection of the present application should be defined by the appended claims.

Claims

1. A pretreatment method for vacuum freeze-drying and hanging-dried apricots, characterized in that, The preprocessing method is as follows: S1 Pre-treatment: The dried apricots are screened and then cut into thin slices with a thickness of 5 mm - 8 mm. S2 Freeze-Thaw Treatment: Freeze the dried apricots and then thaw them until the internal temperature of the dried apricots remains constant before sealing them. The S2 freeze-thaw process includes the S21 freezing process and the S22 thawing process; among which... S21 Freezing treatment: Freeze the dried apricots at -18°C to -22°C; S22 Thawing process: Thaw the dried apricots that have been frozen at 20°C - 25°C until the internal temperature of the dried apricots remains constant, then seal them. S3 Ultrasonic Treatment: The thawed and sealed dried apricots are ultrasonically treated. In the ultrasonic treatment, water at 20 °C-25 °C was used as the medium. The thawed and sealed dried apricots were submerged in the water, and ultrasound was applied to the water. The ultrasonic power was 180 W-220 W, the ultrasonic frequency was 38 kHz-42 kHz, and the ultrasonic treatment time was 20 min-30 min.

2. The vacuum freeze-drying method for pre-treating dried apricots according to claim 1, characterized in that, In the S1 pretreatment, the screening methods include washing, removing the stem and removing the pit.

3. The vacuum freeze-drying method for pre-treating dried apricots according to claim 1, characterized in that, The freezing time for S21 is 20-30 hours, and the internal temperature of the dried apricots remains constant after freezing.

4. The vacuum freeze-drying method for pre-treating dried apricots according to claim 1, characterized in that, The sealing method is sealing with a sealing bag.

5. A type of dried apricot food product, characterized in that, The product is prepared by pretreatment according to any one of claims 1-4, followed by vacuum freeze-drying.

6. The dried apricot food product according to claim 5, characterized in that, Vacuum freeze drying includes two processes: freezing and drying. The freezing time is 2-4 hours, the freezing temperature is -25℃ to -40℃, and the drying temperature is 20℃ to 25℃.

7. The dried apricot food product according to claim 5, characterized in that, The drying time of the pretreated and vacuum freeze-dried apricots is ≤14h, and the total energy consumption is ≤42kWh / kg; the FRAP content is ≥7.12mg Trolox / g, the TFC content is ≥1.67mgRE / g, and the TPC content is ≥7.70mg GAE / g.

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