Method for slowing down sugar flowing and sand returning of preserved strawberries through vacuum treatment

Abstract

The invention discloses a method for slowing down sugar flowing and sand returning of preserved strawberries through vacuum treatment, and belongs to the field of agricultural product processing. The preserved strawberries which are high in fruit maturity and prone to the sugar flowing phenomenon are prepared by conducting vacuum drying for 6-8 h at the temperature of 50-60 DEG C after sugaring, and the viscosity of the preserved strawberries is obviously lower than that of a conventional hot air drying sample after 30 days; for the preserved strawberries which are relatively low in maturity and easy to have a sand return phenomenon, sucrose and trehalose are compounded according to a ratio of 5: 3, and the sand return phenomenon of the preserved strawberries can be effectively slowed down by performing vacuum impregnation for 1-2 hours. According to the method, the spatial distribution of sugar in strawberry tissues and the migration characteristic between water molecules in the preserved strawberries and the environment are changed by adjusting the vacuum action link, action intensity and action time, and the sugar flowing or sand returning phenomenon of the preserved strawberries is effectively slowed down.

Description

Technical Field

[0001] This invention belongs to the field of agricultural product processing, specifically a method for slowing down the sugar exudation and crystallization of strawberry preserves through vacuum treatment. Background Technology

[0002] Strawberry preserves belong to the category of candied fruit products. They are made by sugaring and drying strawberries to achieve a moist and elastic texture. However, sugar runoff and crystallization during storage can affect their commercial value. Sugar runoff manifests as a surface covered with a sticky liquid. This phenomenon is generally believed to be related to a high proportion of invert sugar, which absorbs moisture and forms a semi-fluid, high-concentration sugar solution. Strawberry preserves with runoff stick to packaging and leave stains, affecting the product's ease of handling and acceptability. Crystallization occurs when crystalline sugar blooms on the surface of the strawberry preserves, making them harder and causing a gritty texture when chewed, thus reducing their quality.

[0003] According to reports, highly mature strawberry preserves are more prone to sugar runoff (patent CN 202510325379.1), which may be related to factors such as their low sucrose content and high proportion of water-soluble pectin in cell walls [Food Science, 2025, 46(8): 41-50]. However, strawberries with high maturity have an advantage in aroma and color when used to make strawberry preserves. Currently, the control of sugar runoff in strawberry preserves is mainly achieved through improvements in sugar soaking and storage conditions. Sugar conversion can be controlled by optimizing the sugar concentration and ratio, as well as the sugar soaking temperature and time, or by using airtight packaging to reduce the intrusion of external moisture to slow down sugar runoff. Studies on strawberry preserves made by sucrose soaking suggest that the main reason for the return of sugar crystals is due to excessive sucrose content and insufficient invert sugar content. The main solution to the return of sugar crystals in strawberry preserves is to add citric acid appropriately during sugar preparation to maintain an organic acid content of 0.3%-0.5% in the sugar solution, so that the sucrose is properly converted and the invert sugar content in the sugar solution and strawberry preserves is maintained at about 2 / 3. Under the general trend of sugar reduction, new functional sugars or sugar alcohols are increasingly used in dried fruit. These functional sugars or sugar alcohols can also cause dried fruit to crystallize, and the theory of controlling sucrose conversion can no longer support the current product quality control needs. Trehalose is a widely used sucrose substitute, but the latest research [Food Science, 2025, 46(7): 100-106] shows that when trehalose is used to replace sucrose in the production of strawberry dried fruit, crystallization is more likely to occur. This is related to the mechanical characteristics of strawberry cell walls, the osmotic dynamics of different sugars in the intercellular spaces, the binding of different sugars with macromolecules such as pectin, and the degree of water freedom [Food Research International 221(2025)117421]. Therefore, in the production of reduced-sugar strawberry dried fruit, patent CN 202510325379.1 can delay the crystallization of strawberry dried fruit by limiting the strawberry variety, maturity, and sugar ratio.

[0004] In recent years, the research and application of physical fields in food processing have become increasingly widespread. Physical fields can alter the coiling and folding configurations of macromolecules such as polysaccharides and proteins, as well as intermolecular forces such as hydrogen bonds and van der Waals forces, thereby affecting physicochemical properties such as water retention, hardness, and elasticity, and ultimately changing the pore distribution, texture, and mouthfeel of food. Vacuum treatment is a widely used method that, by creating a low-oxygen environment and influencing heat and mass transfer and cell deformation through pressure differences, can reduce oxidation, enhance color, and accelerate penetration. However, the actual effects vary depending on the materials, processes, time, and intensity of the vacuum treatment.

[0005] Strawberry preserves are typically made using atmospheric pressure hot air drying, where hot air removes surface moisture, creating a moisture gradient from the inside out to achieve the desired moisture content and suitable texture [Food Industry Technology, 2023, 44(16): 51-58]. In recent years, drying methods have diversified, with vacuum drying becoming widely used. The vacuum environment can affect the distribution and migration path of moisture and sugars inside the preserve, and may even affect the binding mode of macromolecular and small molecule sugars in the preserve, thus affecting the hygroscopic or volatile properties of the surface moisture in the strawberry preserve. In addition, vacuum impregnation can cause the fruit tissue to swell, promoting the rapid penetration of external solutes into the pore space of the food matrix. However, because strawberries are delicate and easily damaged, the production of strawberry preserves requires a long sugar-soaking time. How different types of sugar, vacuum intensity, application steps, and application times affect the characteristics of strawberry preserves still requires extensive experimentation. Summary of the Invention

[0006] The purpose of this invention is to provide a method for mitigating sugar exudation and graininess in strawberry preserves through vacuum treatment. Vacuum drying is used to reduce sugar exudation in strawberry preserves. By optimizing the synergistic control parameters of vacuum degree and drying time, a vacuum drying technology is developed to slow down sugar exudation in highly ripe strawberry preserves. This technology precisely controls the rate of moisture migration and the distribution of sugar during the drying process, effectively reducing sugar exudation. Vacuum sugaring is used to reduce graininess in strawberry preserves. By controlling vacuum conditions and optimizing the sugar solution formula, a scientifically sound vacuum sugaring scheme for strawberry preserves is developed. This scheme promotes the uniform penetration and distribution of sugar within the fruit, thereby effectively mitigating graininess.

[0007] The specific technical solution of this invention is as follows:

[0008] This invention provides a method for slowing down the sugar exudation and crystallization of strawberry preserves through vacuum treatment, comprising the following steps:

[0009] The strawberry preserves are strawberry preserves with a moisture content of 20% obtained by sugaring, dehydration, and drying.

[0010] Vacuum drying slows down the sugar exudation of highly ripe strawberry preserves;

[0011] Vacuum sugaring slows down the crystallization of underripe strawberry preserves;

[0012] Highly mature strawberries: 100% red area, single fruit weight 15-20g;

[0013] The low-ripe strawberries are characterized by 30%-50% red area and a single fruit weight of 8-12g.

[0014] The method for slowing down the sugar exudation of strawberry preserves by vacuum drying provided by the present invention includes the following steps:

[0015] Select highly mature frozen strawberries, each weighing 15-20g, add a 0.3% (w / v) food-grade sodium sulfite solution, soak and thaw at room temperature, then remove the strawberries and mix them with 40% (w / v) of sucrose, seal, and sugar-soak for 72 hours. After that, remove the strawberries and vacuum-dry them at 0.08 MPa, 50-60℃ for 6-8 hours to obtain strawberry preserves.

[0016] The method for reducing the graininess of dried strawberries through vacuum sugaring provided by this invention includes the following steps:

[0017] The strawberries were frozen strawberries at low ripeness, with a single fruit weighing 8-12g. They were soaked and thawed in a food-grade sodium sulfite solution with a mass-volume concentration of 0.3% at room temperature. Then, they were sugar-preserved using sucrose, trehalose, or a combination of the two, with the total sugar content being 40% of the strawberry mass. After mixing the strawberries and sugar, they were vacuum-sugar-preserved at a vacuum degree of 0.08Mpa for 1-2 hours, and then transferred to normal pressure for sugar preservation. The cumulative sugar preservation time was 72 hours. Finally, they were dried in hot air at 60℃ for 12 hours to obtain strawberry preserves.

[0018] Preferably, in the combined sugar infusion of sucrose and trehalose, the trehalose mass fraction is 15%;

[0019] Preferably, after 2 hours of vacuum sugaring, the mixture is then immersed at normal pressure for 70 hours.

[0020] Beneficial effects

[0021] Highly mature strawberries have large cell vacuoles, making them more prone to collapse during dehydration. They also have a high proportion of reducing sugars, making them more susceptible to sugar leakage when making strawberry preserves. This invention addresses this by using vacuum drying to dry strawberries, which increases the toughness and elasticity of the strawberry cell walls after sugaring. This promotes the uniform distribution of sugars, reduces pores formed by water evaporation, and significantly slows down the occurrence of sugar leakage.

[0022] Low-ripe strawberries are small with rigid cell walls and a low proportion of reducing sugars. Using only sucrose to make strawberry preserves easily leads to a grainy texture. This invention uses a vacuum environment to sugar-soak strawberries, promoting rapid penetration of the sugar solution into the fruit pulp, preventing surface sugar oversaturation, thus inhibiting sugar crystal formation. Furthermore, the optimized ratio of sucrose to trehalose achieves a synergistic spatial effect between sugars and polysaccharides such as pectin, improving water retention, effectively delaying graininess, and meeting the requirement for sugar reduction. Attached Figure Description

[0023] Appendix Figure 1 strawberry preserves pictures

[0024] Appendix Figure 2 Adhesion and PAS staining of hot-air dried and vacuum-dried strawberry preserves

[0025] Appendix Figure 3 The hardness of candied strawberries preserved under normal pressure and vacuum pressure, and the sensory scores of candied strawberries with added trehalose. Detailed Implementation

[0026] To make the objectives and technical solutions of this invention clearer, examples are provided below for further illustration. It should be understood that these examples are for illustrative purposes only and not for limiting the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0027] Example 1

[0028] A method for slowing down the sugar exudation and crystallization of strawberry preserves through vacuum processing includes the following steps:

[0029] 1) Select fully red Monterey strawberries weighing 15-20g each without mechanical damage, remove the stems and wash them;

[0030] 2) Weigh 200g of the strawberries from step 1) into a sample container, add 1L of food-grade sodium sulfite solution with a strawberry mass fraction of 0.3% and soak for 4 hours;

[0031] 3) After thawing, drain the liquid from the strawberries, add 80g of sucrose and mix well with the strawberries. Let them sugar-soak and dehydrate for 72 hours.

[0032] 4) Remove the dehydrated strawberries after sugaring and place them in a vacuum drying oven. Dry them for 6 hours at a vacuum of 0.08 MPa and a temperature of 60°C to obtain strawberry preserves.

[0033] Example 2

[0034] Unlike Example 1, in step 1), the weight of a single strawberry is 8-12g. In step 3), 50g of sucrose and 30g of trehalose are added. During the sugaring process, a vacuum pump is used to create a vacuum. When the vacuum degree reaches 0.08Mpa, the control valve is closed, and the vacuum degree inside the bottle is maintained at 0.08Mpa for 2 hours before being released. The sugaring continues for 70 hours without vacuum. In step 4), the strawberry preserves are placed in a hot air circulating drying oven and dried for 12 hours.

[0035] Example 3

[0036] Unlike Example 1, in step 1), the weight of a single strawberry is 8-12g. In step 3), a vacuum pump is used to create a vacuum during the candy-soaking process. When the vacuum reaches 0.08Mpa, the control valve is closed, and the vacuum inside the bottle is maintained at 0.08Mpa for 2 hours before being released. The candy-soaking continues for 70 hours without vacuum. In step 4), the strawberry preserves are placed in a hot air circulating drying oven and dried for 12 hours.

[0037] Example 4

[0038] Unlike Example 1, in step 1), the weight of a single strawberry is 8-12g. In step 3), 80g of trehalose is added. During the sugaring process, a vacuum pump is used to create a vacuum. When the vacuum reaches 0.08Mpa, the control valve is closed. The vacuum inside the bottle is maintained at 0.08Mpa for 2 hours before being released. The sugaring continues for 70 hours without vacuum. In step 4), the strawberry preserves are placed in a hot air circulating drying oven and dried for 12 hours.

[0039] Comparative Example 1

[0040] Unlike Example 1, in step 4), the sugar-soaked and dehydrated strawberries were placed in a hot air circulating drying oven and dried at 60°C for 12 hours.

[0041] Comparative Example 2

[0042] Unlike Example 2, in step 3), no vacuum is applied, and the sugar is soaked for 72 hours under normal pressure.

[0043] Comparative Example 3

[0044] Unlike Example 3, in step 3), no vacuum is applied, and the sugar is soaked for 72 hours under normal pressure.

[0045] Comparative Example 4

[0046] Unlike Example 4, in step 3), no vacuum was applied, and the sugar was soaked for 72 hours under normal pressure.

[0047] Comparative Example 5

[0048] Unlike Example 2, in step 3), the vacuum inside the bottle was maintained at 0.08 MPa for 3 hours and then the vacuum was released, and the sugaring continued for 69 hours under normal pressure.

[0049] This invention measures the adhesiveness of dried strawberries. The specific method is as follows: the dried strawberry is considered as a symmetrical structure along the longitudinal axis from the stem to the top of the fruit, with the midpoint of its symmetry axis as the center point. During the test, the center point of the dried strawberry is aligned with the center point of the apex of the cylindrical probe of the BROOKFIELDCT3 texture analyzer. The TA39 probe is used, and the longitudinal force direction is measured. The parameters are set to 1.0 mm / s, the trigger force is 7g, and the compression distance is 4mm. Five repeated tests are performed, and the adhesiveness value of the strawberry is recorded.

[0050] This invention involves PAS staining and optical microscopy observation of strawberry preserves. The strawberry preserves are considered to have a symmetrical structure along a longitudinal axis from the stem to the top of the fruit, and are cut perpendicular to this axis. The sections are 0.5 cm thick and are subsequently fixed, dehydrated, paraffin-insulated, embedded, and sectioned, then observed under an optical microscope.

[0051] This invention measures the hardness of dried strawberries. The specific method is as follows: the dried strawberry is considered as a symmetrical structure along the longitudinal axis from the stem to the top of the fruit, with the midpoint of its symmetry axis as the center point. During the test, the center point of the dried strawberry is aligned with the center point of the apex of the cylindrical probe of the BROOKFIELDCT3 texture analyzer. The TA39 probe is used, and the longitudinal force direction is measured. The parameters are set to 1.0 mm / s, the trigger force is 7g, and the compression distance is 4mm. The test is repeated 5 times, and the hardness value of the strawberry is recorded.

[0052] This invention provides a sensory evaluation of dried strawberries. The specific method involves selecting a panel of 10 sensory evaluation professionals to conduct a comprehensive assessment of the dried strawberries, evaluating aspects such as flavor (30 points), texture (30 points), mouthfeel (30 points), and color (10 points). The sensory scoring criteria for dried strawberries are shown in Table 1.

[0053] Table 1 Sensory Evaluation Criteria for Dried Strawberries

[0054]

[0055] Figure 1The results showed that after 30 days of storage, the hot-air dried strawberry preserves in Comparative Example 1 exhibited significant sugar leakage, as indicated by the black arrows in the figure, with obvious sugar syrup seepage. In contrast, the vacuum-dried strawberry preserves in Example 1 did not show this sugar leakage phenomenon, and the drying time was significantly shortened. This indicates that vacuum drying not only slows down the sugar leakage phenomenon in strawberry preserves but also reduces energy consumption and improves production efficiency. Furthermore, it avoids the damage to heat-sensitive nutrients caused by prolonged high temperatures while mitigating the sugar leakage phenomenon. Simultaneously, after 30 days of storage, the degree of crystallization in vacuum-sugared sucrose and trehalose composite strawberry preserves, sucrose strawberry preserves, and trehalose strawberry preserves (Examples 2-4) was lower than that in the un-vacuum-sugared sucrose and trehalose composite strawberry preserves, sucrose strawberry preserves, and trehalose strawberry preserves (Comparative Examples 2-4), demonstrating that vacuum sugaring effectively delays the crystallization of strawberry preserves. The degree of crystallization of vacuum-sugared sucrose and trehalose composite strawberry preserves (Example 2) was lower than that of vacuum-impregnated sucrose strawberry preserves (Example 3) and vacuum-impregnated trehalose strawberry preserves (Example 4). Meanwhile, the degree of crystallization of un-vacuum-sugared sucrose and trehalose composite strawberry preserves (Comparative Example 2) was lower than that of un-vacuum-impregnated sucrose strawberry preserves (Comparative Example 3) and un-vacuum-impregnated trehalose strawberry preserves (Comparative Example 4), indicating that the combination of sucrose and trehalose can effectively delay the crystallization of strawberry preserves.

[0056] Figure 2 The results showed that after 30 days of storage, the adhesiveness of strawberry preserves in Example 1 and Comparative Example 1 was 6.82 g and 19.92 g, respectively. The strawberry preserves with flowing sugar exhibited higher adhesiveness. Vacuum drying slowed down the sugar flow, thus reducing the adhesiveness. PAS staining and microscopic observation clearly revealed the sugar distribution structure of the strawberry preserves. The pith of the vacuum-dried strawberry preserves in Example 1 was relatively intact, with only a few small cracks, and the sugar was evenly distributed in the pith, resulting in a more uniform and symmetrical shape. In contrast, the pith of the hot-air-dried strawberry preserves in Comparative Example 1 mainly showed pores of varying sizes, with the sugar primarily distributed in the outer peripheral tissue. Vacuum drying promoted the migration of sugar from the surface to the pith and formed a tighter bond with the cell wall pectin, effectively inhibiting the surface sugar flow of the strawberry preserves.

[0057] Figure 3The results showed that after 30 days of storage, the hardness values ​​of the strawberry preserves in Examples 2-4 were 130.4g, 151.2g, and 217.8g, respectively, while the hardness values ​​of the strawberry preserves in Comparative Examples 2-4 were 240.0g, 272.0g, and 320.8g, respectively. The hardness of the strawberry preserves in the Examples was significantly lower than that in the Comparative Examples. The hardness of the strawberry preserves that had crystallized increased, and vacuum sugaring effectively delayed the crystallization process, reducing the hardness. Furthermore, the hardness of the strawberry preserves in Example 2 was lower than that in Examples 3 and 4, and the hardness of the strawberry preserves in Comparative Example 2 was lower than that in Comparative Examples 3 and 4. The combination of sucrose and trehalose reduced the hardness of the strawberry preserves. Sensory evaluation was conducted on the color, texture, taste, and flavor of strawberry preserves with trehalose content ranging from 0% to 40% of strawberry mass. When the trehalose content was greater than 15%, the sensory score began to decline, indicating that an excessively high trehalose content would affect the taste of the strawberry preserves. A trehalose content between 0% and 15% is suitable, and preferably, the trehalose content is 15%.

[0058] Furthermore, in Comparative Example 5, the strawberry preserves that were vacuum-soaked for 3 hours experienced structural collapse, affecting their chewiness and appearance. This indicates that excessively long vacuum soaking times are not conducive to the formation of a good tissue morphology in the strawberry preserves, and the preferred vacuum soaking time is 1-2 hours.

Claims

1. A method for slowing down the sugar exudation and crystallization of strawberry preserves through vacuum treatment, characterized in that, Includes the following steps: The strawberry preserves are strawberry preserves with a moisture content of 20% obtained by sugaring, dehydration, and drying. Vacuum drying slows down the sugar exudation of highly ripe strawberry preserves; Vacuum sugaring slows down the crystallization of underripe strawberry preserves; The high-maturity strawberry has 100% red area and a single fruit weight of 15-20g. The low-ripe strawberries are characterized by 30%-50% red area and a single fruit weight of 8-12g.

2. The method for slowing down the sugar exudation of strawberry preserves by vacuum drying according to claim 1, characterized in that, Includes the following steps: Select highly mature frozen strawberries, each weighing 15-20g, add a 0.3% (w / v) food-grade sodium sulfite solution, soak and thaw at room temperature, then remove the strawberries and mix them with 40% (w / v) of sucrose, seal, and sugar-soak for 72 hours. After that, remove the strawberries and vacuum-dry them at 0.08 MPa, 50-60℃ for 6-8 hours to obtain strawberry preserves.

3. The method for reducing the graininess of dried strawberries by vacuum sugaring as described in claim 1, characterized in that, Includes the following steps: The strawberries were frozen strawberries at low ripeness, with a single fruit weighing 8-12g. They were soaked and thawed in a food-grade sodium sulfite solution with a mass-volume concentration of 0.3% at room temperature. Then, they were sugar-preserved using sucrose, trehalose, or a combination of the two, with the total sugar content being 40% of the strawberry mass. After mixing the strawberries and sugar, they were vacuum-sugar-preserved at a vacuum degree of 0.08Mpa for 1-2 hours, and then transferred to normal pressure for sugar preservation. The cumulative sugar preservation time was 72 hours. Finally, they were dried in hot air at 60℃ for 12 hours to obtain strawberry preserves.

4. The method according to claim 3, characterized in that, In the combined sugar-preserving process of sucrose and trehalose, the preferred trehalose mass fraction is 15%, and the preferred vacuum sugar-preserving time is 2 hours followed by atmospheric pressure immersion for 70 hours.

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