Preparation method of preserved citrus

Abstract

The invention provides a preparation method of preserved citrus. The method comprises the following steps: cutting navel oranges or citrus peel residues into strips, and pre-cooking with a mixed solution of citric acid and a ginger extract; performing quick-freezing treatment on the precooked peel residues to form a directional micro-pore structure; performing vacuum sublimation treatment after quick freezing treatment; atomizing the composite sugar solution, and performing sugar permeation treatment on the raw materials after vacuum sublimation; performing curing treatment on the raw materials subjected to sugar permeation; and drying the cured raw materials to a target water content to obtain the preserved citrus. The method has the advantages that the residual sugar content on the surface is smaller than or equal to 0.1% and is reduced by 96% or above compared with 2.8% of a traditional process, and the sugar wiping procedure is completely omitted. According to the method, 'freezing-sublimation sugar crystal 'directional filling is adopted, internal ice crystals are formed through quick freezing, and directional micro-channels are reserved through vacuum sublimation; and the gas-phase mass transfer is realized, the sugar directly enters the pulp through the directional micro-pore channels, and'internal filling and no sugar hanging outside 'is realized, so that the liquid sugar liquid is fundamentally prevented from being attached to the surface. The sugar permeation time is shortened to 25-35 minutes from 8-12 hours in the traditional process, and the equipment turnover rate is increased.

Description

Technical Field

[0001] This invention belongs to the field of industrial production technology of citrus preserves, and specifically relates to a method for preparing citrus preserves. Background Technology

[0002] There are two major pain points in the current industrial production of citrus preserves: ① High sugar concentration and long-term soaking cause sugar to stick to the surface, resulting in high drying energy consumption; ② Insufficient sugar penetration into the pulp results in a sweet taste on the outside and bland taste on the inside, and requires 8-12 hours of vacuum sugar penetration, which is inefficient.

[0003] Researchers found the following shortcomings when using existing ultrasonic-vacuum sugar infiltration processes such as CN102326653A: 1) Sugar penetration rate is only about 55% after 4 hours of infiltration; 2) Surface residual sugar content is >3%, requiring secondary sugar wiping or cold air knife treatment, increasing the processing steps; 3) Prolonged drying at temperatures above 70℃ leads to darkening of color and fading of flavor. Furthermore, CN102326653A uses a three-stage drying technology of vacuum-hot air-dehumidification, but it does not solve the problem of uneven sugar infiltration; CN115968575A uses ultrasonic + negative pressure sugar infiltration technology, but requires 65% high-concentration sugar syrup, and the surface is prone to crusting; another method uses microwave-enzyme synergistic technology for sugar infiltration, but enzyme activity control is difficult, easily producing bitterness. Our company's original process: 70% sugar solution vacuum infiltration for 4 hours + drying at 70℃ for 6 hours, sugar penetration rate 57%, surface residual sugar 2.8%.

[0004] Existing technologies will cause the following negative effects: 1) High residual sugar on the surface, requiring additional sugar wiping, increasing labor costs and detracting from the appearance; 2) Slow deep sugar penetration, requiring more than 8 hours of sugar penetration, resulting in low equipment turnover; 3) High-temperature drying will cause browning on the product surface and loss of flavor. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing citrus preserves. The technical solution of this invention is as follows: A method for preparing citrus preserves, the method comprising the following steps: S1: After processing the strips of navel orange or citrus peel, pre-cook them in a mixed solution of citric acid and ginger extract; S2: Quick-freeze the pre-cooked skin residue to form a directional microporous structure; S3: After quick-freezing, vacuum sublimation is performed; S4: After atomizing the compound sugar solution, the raw material after vacuum sublimation is subjected to sugar infiltration treatment; S5: The raw materials after sugar infiltration are then cooked. S6: Dry the matured raw materials; You will get dried citrus fruit.

[0006] Preferably, in step S1, the navel orange peel residue contains 85% water, and the pre-cooking of the mixed solution of citric acid and ginger extract is carried out in a mixed solution with a mass concentration of 0.5-1.5% citric acid and 1-1.5% ginger extract for 25-35 minutes, and the raw material is cut into strips with a specification of 3mm×3mm×40mm.

[0007] Preferably, in step S2, quick freezing includes: a first stage: slowly lowering the ambient temperature from the pretreatment temperature to -3℃ to -5℃ at a rate of 0.5-1℃ / min, and maintaining it for 10-20 minutes; a second stage: rapidly transferring to an environment of -35℃ to -40℃ for deep freezing until the core temperature reaches below -18℃. The formation of radially dominant, branching, directional micropores lays the structural foundation for the subsequent directional and rapid penetration of sugar solution.

[0008] Preferably, in step S3, the vacuum sublimation treatment is carried out at 15-25 Pa, a temperature of -10 to -5 °C, and a time of 1-2.5 h; the ice crystals are converted into water vapor and escape directly without passing through the liquid state, and the pore walls do not collapse.

[0009] Preferably, in step S4, the compound sugar solution comprises the following components by mass fraction: 20-35% white sugar, 15-20% fructose syrup; 1-3% flavor microcapsule powder, 1.5-2.5% gum arabic; 0.4-0.6% amino acids, and 0.8-1.2% reducing sugar.

[0010] More preferably, in step S4, the compound sugar solution further includes the following components by mass fraction: sorbitol 3-5%, guar gum 0.2-0.5%.

[0011] Further preferably, the flavor microcapsule powder is prepared by spray drying citrus essential oil and maltodextrin. The specific preparation method is as follows: maltodextrin, gum arabic and β-cyclodextrin are used as composite wall materials in a mass ratio of (85-90):(8-12):(2-5), and they are dissolved in water and stirred thoroughly. The concentration of the wall material solution is 25%-35%. The core material is citrus essential oil, and the mass ratio of the core material to the wall material is 1:(4-5). The citrus essential oil is added to the wall material solution, and shear emulsification is performed to form a stable emulsion system. The emulsified emulsion is dried by a spray dryer with the following drying parameters: inlet air temperature 160°C and outlet air temperature 65°C.

[0012] Preferably, in step S4, the pH value of the compound sugar solution is adjusted to 5.0-5.5 using a buffer solution of citric acid and sodium citrate; The conditions for sugar mist infiltration are: sugar solution temperature 50-55℃; atomized particle size 10-50 micrometers; treatment time 25-35 minutes.

[0013] Preferably, in step S5, the curing process includes three stages: the first stage: temperature 65-70℃, humidity 60-70%, duration 20-30 minutes; the second stage: temperature 50-60℃, humidity 80-85%, duration 30-40 minutes; and the third stage: temperature 35-45℃, humidity 40-50%, duration 20 minutes.

[0014] Preferably, the drying in step S6 is a combination of puffing and air-energy drying, including: First stage: Extrusion and drying stage: A closed cylindrical extrusion and drying equipment is selected, with a continuous annular corrugated partition in the inner layer for the citrus preserves to pass through; superheated steam is introduced into the outer layer, with the temperature controlled at 120℃-150℃ and the pressure maintained at 0.1MPa-0.2Mpa. The sugar-soaked citrus preserves with a moisture content of 45%-50% are evenly spread on the conveyor belt. The conveyor belt speed is adjusted to 5m / min-10m / min, so that the preserves stay in the puffing and drying zone for 3min-5min. At the end of this stage, the surface of the citrus preserves dries rapidly, the volume expands slightly, the internal moisture evaporates quickly, and the moisture content drops to about 30%. The second stage: air-source heat pump drying stage: conventional air-source heat pump drying equipment is used, with the drying temperature set at 45℃-60℃, relative humidity controlled at 20%-30%, and hot air flow rate at 1m / s-2m / s; the puffed and dried citrus preserves are directly fed into the air-source heat pump drying equipment via a sealed conveyor belt, with the conveyor belt speed reduced to 2m / min-5m / min, and the drying time lasting 4h-8h; air-source heat pump drying causes the remaining moisture inside the preserves to slowly migrate to the surface and evaporate, ultimately stabilizing the product moisture content at around 20%, ensuring uniform product taste and quality.

[0015] The beneficial effects of this invention are as follows: 1. The residual sugar content on the surface is ≤0.1%, which is more than 96% lower than the 2.8% of the traditional process, and the sugar wiping process is completely omitted.

[0016] 2. This invention employs a "freezing-sublimation sugar crystal" directional filling process. Rapid freezing forms internal ice crystals, while vacuum sublimation leaves directional micropores. This enables gas-phase mass transfer, allowing sugar to directly enter the fruit pulp through these micropores, achieving "internal filling and no external sugar coating," fundamentally preventing liquid sugar syrup from adhering to the surface. The sugar infiltration time is reduced from 8-12 hours in traditional processes to 25-35 minutes, improving equipment turnover.

[0017] 3. By adopting low-temperature dynamic ripening and coupled drying technology, the browning rate is controlled below 5%. The components such as white sugar and gelatin in the compound sugar solution work synergistically to form a unique three-dimensional network structure; the product's texture and elasticity are improved, and there is no sugar analysis or crystallization.

[0018] 4. By precisely controlling the pH value of the compound sugar solution at 5.0-5.5, under this condition, some sucrose will be moderately hydrolyzed into glucose and fructose. These monosaccharides have small molecular weights and high osmotic pressure, which can significantly accelerate the sugar infiltration process. At the same time, some sucrose is still retained to maintain the pure sweetness of the product and avoid the sweetness imbalance caused by complete hydrolysis. In addition, it was unexpectedly discovered that this pH condition can further reduce the amount of residual sugar on the surface.

[0019] 5. Through two-stage precise temperature control (first stage: slow nucleation: 0.5-1℃ / min to -3~-5℃; second stage: rapid deep freezing: -35~-40℃), a radially-oriented, branching micropore structure is formed inside the pulp. Sublimation is performed for 2 hours at 20Pa and a plate temperature of -10℃, allowing ice crystals to directly vaporize and escape, avoiding pore collapse caused by the liquid water phase transition and preserving the complete three-dimensional network structure. The sugar solution is atomized into 10-50μm particles and directly penetrates into the pulp in gaseous form through the micropores at 50-55℃, preventing liquid sugar solution from adhering to the surface. Ripening is precisely controlled in three stages: 65-70℃ activates the internal Maillard reaction, generating endogenous aromas; 50-60℃ gum arabic hydrates to form a three-dimensional network structure, locking in sugars; 35-45℃ moisture redistribution prevents surface crusting. Attached Figure Description

[0020] Figure 1 Physical diagram of Embodiment 1 of the present invention; Figure 2 Comparative Example 1 of this invention: physical object drawing; Figure 3 Comparative Example 2 of this invention: physical object diagram; Figure 4 Comparative Example 3: Physical Image of the Invention; Figure 5 Comparative Example 4 of this invention: physical object diagram; Figure 6 Comparative Example 5: Physical image of the present invention; Figure 7 Comparative Example 6 of this invention: physical object diagram; Figure 8 Comparative Example 7 of this invention: physical object diagram; Figure 9 Comparative Example 8 of this invention: physical object diagram; Figure 10 Comparative Example 9 of this invention: physical object diagram; Figure 11 Comparative Example 10 shows a physical object of this invention. Detailed Implementation

[0021] The technical solution of the present invention will be further explained and described below with reference to specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention. The flavor microcapsule powder used in the examples and comparative examples was prepared by spray drying citrus essential oil and maltodextrin. The specific preparation method was as follows: maltodextrin, gum arabic and β-cyclodextrin were used as composite wall materials in a mass ratio of 85:10:3. They were dissolved in water and stirred thoroughly, and the concentration of the wall material solution was 28%. The core material is citrus essential oil, and the mass ratio of the core material to the wall material is 1:5. The citrus essential oil is added to the wall material solution, and shear emulsification is performed to form a stable emulsion system. The emulsified emulsion is then dried using a spray dryer with the following drying parameters: inlet air temperature 160°C and outlet air temperature 65°C.

[0022] Sugar permeability test: Total sugar content minus residual sugar content, the method for total sugar content test is GB / T 10782 method; Surface residual sugar: This is calculated by rinsing the product with pure water, spinning it dry, and weighing it to determine the proportion of the reduced weight in the original product.

[0023] Texture elasticity: Sensory evaluation, divided into four levels: excellent, medium, good, and poor.

[0024] Browning rate detection: the proportion of discolored particles in the total amount.

[0025] Sensory evaluation was conducted blindly by 10 evaluators and included four aspects: color, aroma, chewiness, and aftertaste.

[0026] Example 1 Raw material: 27.3 kg of navel orange peel residue (85% water content); Step 1: Pre-cook: Cut into strips: 3mm×3mm×40mm, pre-cook in a mixed solution of 0.5% citric acid and 1% ginger extract for 30 minutes; Step 2: Quick freezing: In the first stage, the temperature is lowered to -4℃ at a rate of 0.8℃ / min and maintained for 15 minutes; in the second stage, the temperature is transferred to -38℃ for deep freezing. Step 3 Vacuum sublimation: 20 Pa, plate temperature -10℃, time 2 h; Step 4: Sugar mist infiltration: The composite sugar solution consists of 30% white granulated sugar, 13% trehalose, 4% sorbitol, 2% gum arabic, 0.3% guarana gum, 2% flavor microcapsule powder, 0.5% L-proline, and 1% xylose. The pH of the composite sugar solution is adjusted to 5.2 using a buffer solution of citric acid and sodium citrate. The sugar solution temperature is 52℃, the atomization particle size is 30μm, and the treatment time is 30min. Step 5: Curing: First stage: 68℃, 65% humidity, 25min; Second stage: 55℃, 82% humidity, 35min; Third stage: 40℃, 45% humidity, 20min. Step 6 Drying: First stage: Extrusion drying stage: A closed cylindrical extrusion drying equipment is selected, with a continuous annular corrugated partition in the inner layer for the citrus preserves to pass through; superheated steam is introduced into the outer layer, with the temperature controlled at 130℃ and the pressure maintained at 0.15Mpa. The sugar-coated citrus preserves with a moisture content of 45%-50% are evenly spread on a conveyor belt, and the conveyor belt speed is adjusted to 8m / min, so that the preserves stay in the puffing and drying zone for 4 minutes. At the end of this stage, the surface of the citrus preserves dries rapidly, the volume expands slightly, the internal moisture evaporates rapidly, and the moisture content drops to about 30%. Second stage: air-source drying stage: conventional air-source drying equipment is used, the drying temperature is set at 55℃, the relative humidity is controlled at 25%, and the hot air flow rate is 1.5m / s. The puffed and dried citrus preserves are directly put into the air-source drying equipment through a sealed conveyor belt, the conveyor belt speed is reduced to 3m / min, and the drying time lasts for 5 hours.

[0027] Example 2 The quick-freezing temperature was -35 ℃, and the rest was the same as in Example 1.

[0028] Example 3 Soak in sugar for 40 minutes, and the rest is the same as in Example 1.

[0029] Example 4 Sugar solution composition: 22% white sugar, 16% fructose syrup, 13% trehalose, 5% sorbitol, 2.2% gum arabic, 0.4% guarana, 1.5% flavor microcapsule powder, 0.55% L-proline, and 1.1% xylose; pH adjusted to 5.0; the curing process was adjusted as follows: first stage: 66℃, 68% humidity, 28 min; second stage: 58℃, 83% humidity, 32 min; third stage: 42℃, 48% humidity, 22 min; the rest is the same as in Example 1.

[0030] Example 5: Sugar solution composition: 33% white granulated sugar, 19% fructose syrup, 11% trehalose, 3.2% sorbitol, 1.8% gum arabic, 0.25% guar gum, 2.8% flavor microcapsule powder, 0.45% L-proline, and 0.9% xylose; pH adjusted to 5.5; quick-freezing process adjusted as follows: first stage: temperature dropped to -3.5℃ at 0.6℃ / min and maintained for 18min; second stage: deep freezing at -36℃; the rest is the same as in Example 1.

[0031] Comparative Example 1 Cutting and pre-cooking: Same as in Example 1; Vacuum sugar infiltration: 70% sucrose solution, vacuum sugar infiltration for 8 hours; Drying: 70℃ hot air drying for 8 hours.

[0032] Comparative Example 2 Sugar solution composition: sucrose replaces all white sugar, trehalose, and sorbitol (total sugar content 62%); the rest of the process is the same as in Example 1. Comparative Example 3 Sugar solution composition: Gum arabic and guar gum are omitted; the rest of the process is the same as in Example 1.

[0033] Comparative Example 4 Sugar solution composition: sorbitol is omitted, and the amount of trehalose is increased to 17%; the rest of the process is the same as in Example 1.

[0034] Comparative Example 5 Sugar solution composition: L-proline and xylose are omitted; the rest of the process is the same as in Example 1.

[0035] Comparative Example 6 Sugar solution composition: 35% white sugar, 25% trehalose; the rest of the process is the same as in Example 1. Comparative Example 7 Sugar solution composition: 55% fructose syrup, 5% maltitol, 1% pectin; process is the same as in Example 1.

[0036] Comparative Example 8 The same compound sugar solution as in Example 1 was used, but the process was changed to soaking and sugar infiltration at normal pressure for 30 minutes. The rest of the process was the same as in Example 1.

[0037] Comparative Example 9 The sugar solution composition is the same as in Example 1, but the pH value is adjusted to 3.0 (too acidic), and the rest of the process is the same as in Example 1.

[0038] Comparative Example 10 The sugar solution composition is the same as in Example 1, but the pH value is adjusted to 7.0 (too weak acidity), and the rest of the process is the same as in Example 1.

[0039] Table 1

[0040] The sugar penetration rates of the embodiments (1-5) of this invention are all between 73.8% and 76.8%, significantly higher than the 57.5% of the conventional process in Comparative Example 1, demonstrating the significant advantage of this invention in improving sugar penetration efficiency. Comparative Example 9 showed the highest sugar penetration rate (79.2%) at pH 3.0, but with an imbalance in sweetness quality; the lowest sugar penetration rate (61.5%) was observed at pH 7.0, indicating that an acidic environment is beneficial for sugar penetration, but excessive acidity can affect product flavor. The sugar penetration rates of Comparative Example 2 (single sugar source) and Comparative Example 7 (commercially available formula) were 62.3% and 58.2%, respectively, significantly lower than those of the embodiments of this invention, demonstrating the synergistic effect of the compound sugar source. The sugar mist penetration process is the core technology for solving the problem of residual sugar on the surface, reducing the amount of residual sugar by more than 96%; the pH value is precisely controlled within the range of 5.0-5.5, achieving the optimal balance between penetration efficiency and sweetness quality; the synergistic effect of the compound components is significant, with each functional component playing an important role in improving sugar penetration rate, improving texture, and preventing crystallization. Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing citrus preserves, characterized in that: The method includes the following steps: S1: After processing the strips of navel orange or citrus peel, pre-cook them in a mixed solution of citric acid and ginger extract; S2: Quick-freeze the pre-cooked skin residue to form a directional microporous structure; S3: After quick-freezing, vacuum sublimation is performed; S4: After atomizing the compound sugar solution, the raw material after vacuum sublimation is subjected to sugar infiltration treatment; S5: The raw materials after sugar infiltration are then cooked. S6: Dry the matured raw materials; You will get dried citrus fruit.

2. The method for preparing citrus preserves according to claim 1, characterized in that: In step S1, the pre-cooking of the citric acid and ginger extract mixed solution is to pre-cook in a mixed solution with a mass concentration of 0.5-1.5% citric acid and 1-1.5% ginger extract for 25-35 minutes.

3. The method for preparing citrus preserves according to claim 1, characterized in that: In step S2, quick freezing includes: a first stage: slowly reducing the ambient temperature from the pretreatment temperature to -3℃ to -5℃ at a rate of 0.5-1℃ / min and maintaining it for 10-20 minutes; a second stage: rapidly transferring to an environment of -35℃ to -40℃ for deep freezing until the core temperature reaches below -18℃.

4. The method for preparing citrus preserves according to claim 1, characterized in that: In step S3, vacuum sublimation treatment is performed at 15-25 Pa, at a temperature of -10 to -5 °C, for a time of 1-2.5 h.

5. The method for preparing citrus preserves according to claim 1, characterized in that: In step S4, the compound sugar solution comprises the following components by mass fraction: 20-35% white sugar, 15-20% fructose syrup, 15-25% trehalose; 1-3% flavor microcapsule powder, 1.5-2.5% gum arabic; 0.4-0.6% amino acids, and 0.8-1.2% reducing sugar.

6. The method for preparing citrus preserves according to claim 5, characterized in that: In step S4, the compound sugar solution also includes the following components by mass fraction: sorbitol 3-5% and guar gum 0.2-0.5%.

7. The method for preparing citrus preserves according to claim 5, characterized in that: Flavor microcapsule powder is prepared by spray drying citrus essential oil and maltodextrin. The specific preparation method is as follows: maltodextrin, gum arabic and β-cyclodextrin are used as composite wall materials in a mass ratio of (85-90):(8-12):(2-5). They are dissolved in water and stirred thoroughly. The concentration of the wall material solution is 25%-35%. The core material is citrus essential oil, and the mass ratio of the core material to the wall material is 1:(4-5). The citrus essential oil is added to the wall material solution, sheared and emulsified, and the emulsified emulsion is dried by a spray dryer.

8. The method for preparing citrus preserves according to claim 5, characterized in that: In step S4, the pH value of the compound sugar solution is adjusted to 5.0-5.5 using a buffer solution; The conditions for sugar mist infiltration are: sugar solution temperature 50-55℃, atomized particle size 10-50 micrometers, and treatment time 25-35 minutes.

9. The method for preparing citrus preserves according to claim 1, characterized in that: In step S5, the curing process includes three stages: the first stage: temperature 65-70℃, humidity 60-70%, duration 20-30 minutes; the second stage: temperature 50-60℃, humidity 80-85%, duration 30-40 minutes; the third stage: temperature 35-45℃, humidity 40-50%, duration 20 minutes.

10. The method for preparing citrus preserves according to claim 1, characterized in that: The drying process in step S6 includes: First stage puffing drying: puffing drying equipment is selected, with a continuous annular corrugated partition in the inner layer for the citrus preserves to pass through; superheated steam is introduced into the outer layer, with the temperature controlled at 120℃-150℃ and the pressure maintained at 0.1MPa-0.2MPa; the citrus preserves with a moisture content of 45%-50% are evenly spread on the conveyor belt, and the conveyor belt speed is adjusted to 5m / min-10m / min, so that the preserves stay in the puffing drying zone for 3min-5min; The second stage is air-source drying: an air-source drying device is used, with the drying temperature set at 45℃-60℃, the relative humidity controlled at 20%-30%, and the hot air flow rate at 1m / s-2m / s; the puffed and dried citrus preserves are directly fed into the air-source drying device via a sealed conveyor belt, with the conveyor belt speed reduced to 2m / min-5m / min, and the drying time lasting 4h-8h.

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