Dry flowers, dried and preserved, the existing flowers, color, capital, rhyme, and dry flowers are not lost, and are more and more popular. It is very suitable for dark coffee shops. , hotel and home decor. Vacuum freeze-drying is a new method of drying flowers in recent years. Drying large flower plants with this drying technique can basically maintain the shape, color and aroma of dry flowers, without pollution, and has a prospect of in-depth research.
Basic principle of freeze drying
Vacuum freeze-drying is to freeze the material below the eutectic point temperature, so that the moisture in the material becomes solid ice, and then the ice is directly sublimated into water vapor under appropriate vacuum. The water vapor is condensed by a water vapor condenser (water trap) in a vacuum system to obtain a technique for drying the product.
The phase equilibrium relationship of water is the basis for studying and analyzing the principle of freeze-drying of water-containing materials. When the vapor pressure around ice is lower than 610.5Pa, the heating of ice can be directly sublimated into water vapor, which is the theoretical basis of sublimation drying, vacuum freeze-drying. The basic working principle of zui is also here.
Operation process
The freeze-drying process is divided into three processes: pre-freezing, sublimation drying, and analytical drying. Since the flowers are composed of stems, leaves, stamens and petals, the water content of each part is different. If the drying effect of each part of the plant is studied, the moisture content of each part should be determined first, and their respective eutectic points should be measured by instruments. And eutectic points.
Prefreezing process
Pre-freezing is a process in which the plants are preliminarily frozen into a certain shape by a general freezing method before the plants are freeze-dried. In vacuum freeze-drying, the high temperature of the frozen zui is an important factor affecting the quality and energy consumption of freeze-dried products. The pre-freezing temperature must be lower than the eutectic temperature, otherwise it will cause undesirable phenomena such as foaming on the surface of the product and shrinkage of the product. In general, the frozen temperature of the product is lower than the eutectic temperature of 5 to 10 ° C. However, the freezing temperature cannot be set too low. Otherwise, problems such as large investment and high energy consumption will occur.
In addition, the pre-freezing rate is also an important factor affecting the drying rate and product quality. The sublimation rate is faster at slow freezing and longer during fast freezing. A suitable cooling rate should be based on a relatively slow cooling rate while ensuring that the material quality is not affected.
2. Sublimation drying process
In order to maintain the continuous drying of sublimation, two basic conditions must be met, namely the continuous supply of heat and the continuous elimination of steam generation. The drying process is gradually dried from the surrounding to the inner center. The heat transfer driving force in the drying process is the temperature difference between the heat source and the sublimation interface, and the mass transfer driving force is the vapor partial pressure difference between the sublimation interface and the cold trap, and the temperature difference The larger the heat transfer rate, the greater the vapor partial pressure difference and the faster the mass transfer rate.
In sublimation drying, the heating temperature should not exceed the eutectic temperature of the material. If the temperature is too high, the ice crystals will melt, which will affect the quality of the product. In addition, the heating method in the sublimation drying process directly affects important process parameters such as material temperature distribution, sublimation interface temperature, sublimation interface water vapor flux and drying time.
3. Analyze the drying process
Analytical drying is the process of removing water present in the matrix of the material in the form of free or bound water after sublimation drying. The analytical drying of the material is generally accomplished by temperatures that are much higher than the sublimation temperature.
The high temperature and low pressure are beneficial to the analytical drying process, and the materials which are not dominant in the analysis drying time are dried, such as frozen tomato slices, wherein the frozen structure water and a large amount of free water can be frozen, accounting for 95%, and the analytical drying time is short. The freeze-drying cycle mainly depends on the sublimation drying time. At this time, considerations for increasing the drying rate should be based on sublimation drying.
Regardless of sublimation drying or analytical drying, the requirements for cold trap temperature are similar. The literature shows that when the cold trap temperature is lower than -50 °C, the temperature of the cold trap is lowered too much, which is of little significance for shortening the drying time. Currently, the temperature of the cold trap is generally set to -45 to 50 °C.
Process and characteristics of vacuum freeze-dried flowers
1. Characteristics of vacuum freeze-dried flowers
(1) There are a wide range of raw materials. So far, there are about 2,000 to 3,000 kinds of dry flower plants commonly used in countries around the world.
(2) The posture is natural and simple, not only has the natural charm of the plant, but also the original shape and color of the plant.
(3) It is convenient to use and manage, not only can maintain its shape and color for a long time, but also has a long storage and sales period.
(4) The rehydration performance is good. After the edible flowers are rehydrated, they can maintain their original beautiful shape and cause people's appetite.
(5) Its shrinkage rate is much smaller than that of dried flowers obtained by other methods, and it can maintain the fresh form to a large extent.
(6) The freeze-drying equipment has a large investment cost and high operating cost, resulting in high cost.
2. Practice of vacuum freeze drying flowers
In order to explore the new technology of dry flower processing, Li Baoguo et al. performed freeze-drying tests on purple Limonium, yellow marigold and red rose flowers, and determined the average moisture content of each part. The pre-freezing temperature in the test was -65 ° C, vacuum The degree is set to 10Pa. Using vacuum freeze-dried flowers, the shape of the flower is kept good, and some colors will change slightly. The stems of the rose and the grass, and the flowers of the blood grass are basically not contracted, and the petals of the marigold and rose are slightly deformed. The color of the rose flower is darker than the color of the flower, the color of the yellow marigold is lighter, and the color of the purple blood grass is basically unchanged after lyophilization. Dry air drying, when the drying temperature is selected as 50 ° C, dry flowers will produce severe shrinkage deformation. In addition, Li Baoguo also found in the test that the dried flowers are not as strong as the flowers, and the whole dried flower plants become brittle and easy to break. broken.
Xu Chenghai did a freeze-drying test of Limonium and Wenzhu, and the obtained Limonium grass maintained its original shape, and the color of the flower was natural and realistic. It can be seen that the use of suitable lyophilization equipment and optimization experiments can greatly shorten the freeze-drying time, mass production, and commercialization requirements.
Zhu Wenxue studied the lyophilization test of peony flower of Luoyanghong. It can be seen that the eutectic and eutectic temperature of the stamen and petals are higher, the eutectic and eutectic temperatures of stems and leaves are lower, and the pre-freezing temperature is selected. The temperature of the sublimation interface should be based on the eutectic temperature and eutectic temperature of the stem. Zhu Wenxue found that different drying conditions have different effects on peony flower drying. When the pre-freezing temperature is -15 ° C and the heating temperature is 20 ° C, it is dried for 12 hours, the stems and leaves are basically dry, but the petals are still damp and local fading occurs, which does not meet the drying requirements. When the pre-freezing temperature was -30 ° C and the heating temperature was 0 ° C, after drying for 45 hours, the whole plant was found to be basically dry. After drying, the petals were slightly darker than the flowers, the stems were not contracted and deformed, and the leaves and petals were slightly deformed. . Similar to the conclusion reached by Li Baoguo, the dried plants also have the phenomenon of becoming brittle, easily broken and broken. Therefore, Zhu Wen believes that although the peony that is not pretreated can be dried under certain conditions, and can basically maintain the shape, color and floral fragrance, the dried flowers can not achieve the effect of flowers, and the drying time is long, therefore, it should be studied The peony flower pretreatment process develops techniques to increase plant toughness and reduce brittleness to ensure subsequent processing and viewing.
In order to understand the characteristics of dry flowers in more depth, Zhang Zhengyong also observed the micromorphology of branches, leaves and petals of peony under scanning electron microscope, and found that there are large structural differences in various parts of peony plants, and this structural difference It will inevitably affect the heat and mass transfer process inside, resulting in different drying characteristics of each part, thus affecting the overall effect of plant drying. The surface cells of the petals of peony are densely arranged, so the water is not easily discharged and the drying speed is slow. The surface of the stem has a vertical strip-like texture structure, and the inside has a hollow mesh-like duct structure. The epidermal cells on the positive surface of the leaves are arranged neatly and densely, without a stomata, and the epidermal cells are round; the back surface has evenly distributed stomata. Since the stem and the blade have a mesh-like duct structure and pores, the water content is easily discharged and the drying speed is fast. The results of microstructure analysis are consistent with the results of lyophilization test, which makes the research on dry flower enter the microscopic world from the macroscopic world, which has certain guiding significance for the improvement of dry flower processing technology.
3. Things to be aware of in freeze-dried flowers
Vacuum freeze-drying technology Dry flower is a brand new dry flower making technology. Although there has been a lot of research and application in food and medicine drying, because the flower is different from food and medicine, the drying index is mainly the shape and color of the flower. Therefore, it is not possible to completely adopt the drying process of foods and medicines, and it is necessary to carry out research and improvement of the drying process for the different biological characteristics of flowers and the requirements of dried flowers. Referring to domestic and foreign literatures, the main problem in the current freeze-dried flowers is that the pretreatment process of flowers is immature, and the untreated flowers are vacuum-dried and dried, and the plants become brittle, easily broken and broken. Therefore, the direction of further research is to study the pretreatment process before the flower material is dried and to further optimize the vacuum freeze-drying process.
Basic principle of freeze drying
Vacuum freeze-drying is to freeze the material below the eutectic point temperature, so that the moisture in the material becomes solid ice, and then the ice is directly sublimated into water vapor under appropriate vacuum. The water vapor is condensed by a water vapor condenser (water trap) in a vacuum system to obtain a technique for drying the product.
The phase equilibrium relationship of water is the basis for studying and analyzing the principle of freeze-drying of water-containing materials. When the vapor pressure around ice is lower than 610.5Pa, the heating of ice can be directly sublimated into water vapor, which is the theoretical basis of sublimation drying, vacuum freeze-drying. The basic working principle of zui is also here.
Operation process
The freeze-drying process is divided into three processes: pre-freezing, sublimation drying, and analytical drying. Since the flowers are composed of stems, leaves, stamens and petals, the water content of each part is different. If the drying effect of each part of the plant is studied, the moisture content of each part should be determined first, and their respective eutectic points should be measured by instruments. And eutectic points.
Prefreezing process
Pre-freezing is a process in which the plants are preliminarily frozen into a certain shape by a general freezing method before the plants are freeze-dried. In vacuum freeze-drying, the high temperature of the frozen zui is an important factor affecting the quality and energy consumption of freeze-dried products. The pre-freezing temperature must be lower than the eutectic temperature, otherwise it will cause undesirable phenomena such as foaming on the surface of the product and shrinkage of the product. In general, the frozen temperature of the product is lower than the eutectic temperature of 5 to 10 ° C. However, the freezing temperature cannot be set too low. Otherwise, problems such as large investment and high energy consumption will occur.
In addition, the pre-freezing rate is also an important factor affecting the drying rate and product quality. The sublimation rate is faster at slow freezing and longer during fast freezing. A suitable cooling rate should be based on a relatively slow cooling rate while ensuring that the material quality is not affected.
2. Sublimation drying process
In order to maintain the continuous drying of sublimation, two basic conditions must be met, namely the continuous supply of heat and the continuous elimination of steam generation. The drying process is gradually dried from the surrounding to the inner center. The heat transfer driving force in the drying process is the temperature difference between the heat source and the sublimation interface, and the mass transfer driving force is the vapor partial pressure difference between the sublimation interface and the cold trap, and the temperature difference The larger the heat transfer rate, the greater the vapor partial pressure difference and the faster the mass transfer rate.
In sublimation drying, the heating temperature should not exceed the eutectic temperature of the material. If the temperature is too high, the ice crystals will melt, which will affect the quality of the product. In addition, the heating method in the sublimation drying process directly affects important process parameters such as material temperature distribution, sublimation interface temperature, sublimation interface water vapor flux and drying time.
3. Analyze the drying process
Analytical drying is the process of removing water present in the matrix of the material in the form of free or bound water after sublimation drying. The analytical drying of the material is generally accomplished by temperatures that are much higher than the sublimation temperature.
The high temperature and low pressure are beneficial to the analytical drying process, and the materials which are not dominant in the analysis drying time are dried, such as frozen tomato slices, wherein the frozen structure water and a large amount of free water can be frozen, accounting for 95%, and the analytical drying time is short. The freeze-drying cycle mainly depends on the sublimation drying time. At this time, considerations for increasing the drying rate should be based on sublimation drying.
Regardless of sublimation drying or analytical drying, the requirements for cold trap temperature are similar. The literature shows that when the cold trap temperature is lower than -50 °C, the temperature of the cold trap is lowered too much, which is of little significance for shortening the drying time. Currently, the temperature of the cold trap is generally set to -45 to 50 °C.
Process and characteristics of vacuum freeze-dried flowers
1. Characteristics of vacuum freeze-dried flowers
(1) There are a wide range of raw materials. So far, there are about 2,000 to 3,000 kinds of dry flower plants commonly used in countries around the world.
(2) The posture is natural and simple, not only has the natural charm of the plant, but also the original shape and color of the plant.
(3) It is convenient to use and manage, not only can maintain its shape and color for a long time, but also has a long storage and sales period.
(4) The rehydration performance is good. After the edible flowers are rehydrated, they can maintain their original beautiful shape and cause people's appetite.
(5) Its shrinkage rate is much smaller than that of dried flowers obtained by other methods, and it can maintain the fresh form to a large extent.
(6) The freeze-drying equipment has a large investment cost and high operating cost, resulting in high cost.
2. Practice of vacuum freeze drying flowers
In order to explore the new technology of dry flower processing, Li Baoguo et al. performed freeze-drying tests on purple Limonium, yellow marigold and red rose flowers, and determined the average moisture content of each part. The pre-freezing temperature in the test was -65 ° C, vacuum The degree is set to 10Pa. Using vacuum freeze-dried flowers, the shape of the flower is kept good, and some colors will change slightly. The stems of the rose and the grass, and the flowers of the blood grass are basically not contracted, and the petals of the marigold and rose are slightly deformed. The color of the rose flower is darker than the color of the flower, the color of the yellow marigold is lighter, and the color of the purple blood grass is basically unchanged after lyophilization. Dry air drying, when the drying temperature is selected as 50 ° C, dry flowers will produce severe shrinkage deformation. In addition, Li Baoguo also found in the test that the dried flowers are not as strong as the flowers, and the whole dried flower plants become brittle and easy to break. broken.
Xu Chenghai did a freeze-drying test of Limonium and Wenzhu, and the obtained Limonium grass maintained its original shape, and the color of the flower was natural and realistic. It can be seen that the use of suitable lyophilization equipment and optimization experiments can greatly shorten the freeze-drying time, mass production, and commercialization requirements.
Zhu Wenxue studied the lyophilization test of peony flower of Luoyanghong. It can be seen that the eutectic and eutectic temperature of the stamen and petals are higher, the eutectic and eutectic temperatures of stems and leaves are lower, and the pre-freezing temperature is selected. The temperature of the sublimation interface should be based on the eutectic temperature and eutectic temperature of the stem. Zhu Wenxue found that different drying conditions have different effects on peony flower drying. When the pre-freezing temperature is -15 ° C and the heating temperature is 20 ° C, it is dried for 12 hours, the stems and leaves are basically dry, but the petals are still damp and local fading occurs, which does not meet the drying requirements. When the pre-freezing temperature was -30 ° C and the heating temperature was 0 ° C, after drying for 45 hours, the whole plant was found to be basically dry. After drying, the petals were slightly darker than the flowers, the stems were not contracted and deformed, and the leaves and petals were slightly deformed. . Similar to the conclusion reached by Li Baoguo, the dried plants also have the phenomenon of becoming brittle, easily broken and broken. Therefore, Zhu Wen believes that although the peony that is not pretreated can be dried under certain conditions, and can basically maintain the shape, color and floral fragrance, the dried flowers can not achieve the effect of flowers, and the drying time is long, therefore, it should be studied The peony flower pretreatment process develops techniques to increase plant toughness and reduce brittleness to ensure subsequent processing and viewing.
In order to understand the characteristics of dry flowers in more depth, Zhang Zhengyong also observed the micromorphology of branches, leaves and petals of peony under scanning electron microscope, and found that there are large structural differences in various parts of peony plants, and this structural difference It will inevitably affect the heat and mass transfer process inside, resulting in different drying characteristics of each part, thus affecting the overall effect of plant drying. The surface cells of the petals of peony are densely arranged, so the water is not easily discharged and the drying speed is slow. The surface of the stem has a vertical strip-like texture structure, and the inside has a hollow mesh-like duct structure. The epidermal cells on the positive surface of the leaves are arranged neatly and densely, without a stomata, and the epidermal cells are round; the back surface has evenly distributed stomata. Since the stem and the blade have a mesh-like duct structure and pores, the water content is easily discharged and the drying speed is fast. The results of microstructure analysis are consistent with the results of lyophilization test, which makes the research on dry flower enter the microscopic world from the macroscopic world, which has certain guiding significance for the improvement of dry flower processing technology.
3. Things to be aware of in freeze-dried flowers
Vacuum freeze-drying technology Dry flower is a brand new dry flower making technology. Although there has been a lot of research and application in food and medicine drying, because the flower is different from food and medicine, the drying index is mainly the shape and color of the flower. Therefore, it is not possible to completely adopt the drying process of foods and medicines, and it is necessary to carry out research and improvement of the drying process for the different biological characteristics of flowers and the requirements of dried flowers. Referring to domestic and foreign literatures, the main problem in the current freeze-dried flowers is that the pretreatment process of flowers is immature, and the untreated flowers are vacuum-dried and dried, and the plants become brittle, easily broken and broken. Therefore, the direction of further research is to study the pretreatment process before the flower material is dried and to further optimize the vacuum freeze-drying process.
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