With the continuous development and progress of the food processing industry, the application of drying technology is becoming more and more widespread, especially in fruit and vegetable processing. Drying technology can quickly evaporate excess water in fruits and vegetables to achieve the goal of fast freshness retention and quality assurance, and make fresh fruits and vegetables into dried fruits and vegetables. Therefore, it has a relatively broad application market. This article introduces several drying techniques in fruit and vegetable processing, and analyzes their specific applications and effects in fruit and vegetable processing.
1、 Application of Heat Pump Drying Technology in Fruit and Vegetable Processing
1. The principle of heat pump drying technology.
Heat pump drying technology mainly uses heat pump operation to achieve a drying level of moisture in indoor humid air, and then reheats the dehumidified air. Drying materials through heat pumps can fully utilize the evaporation potential of water vapor to reduce energy loss. It does not require the removal of wet air or the addition of fresh air, and can also prevent material pollution caused by air changes. Therefore, it is more suitable for drying materials such as food and medicine. The temperature required for heat pump drying is relatively low, usually between 20 ℃ and 80 ℃. Compared to other drying equipment, the investment is less and the power consumption is also very low. Therefore, it is an efficient and energy-saving drying technology that has been widely used in industrial and food drying processes.
The heat pump mainly utilizes high-level energy to transfer heat from the low-level heat source to the high-level heat source device. The principle of the heat pump drying technology is shown in Figure 1. The main components of a heat pump machine include an evaporator, condenser, compressor, drying chamber, etc. During the specific operation process, the compressor increases the temperature and pressure of the gaseous heat pump working fluid. After the condenser releases heat and condenses into a liquid state, the liquid heat pump working fluid undergoes expansion treatment. After heat absorption and evaporation in the evaporator, it is transformed into a low-pressure and low-temperature gas state. After the gaseous heat pump working fluid enters the compressor, the closed circuit cycle of the heat pump working fluid is completed. In addition, after the dry air heated by the condenser enters the drying chamber, the dry material comes into contact with the humid and hot air, and then enters the dehumidification evaporator to release water and discharge it outside the machine. At this time, the cooled dry air is then processed by the condenser and enters the drying chamber, completing a closed air circulation process.
2. Characteristics of heat pump drying technology.
(1) Efficient and energy-saving. One of the outstanding characteristics of heat pump drying technology is energy conservation, high thermal efficiency, and the use of one part of electrical energy can be converted into more parts of thermal energy, with an energy-saving efficiency of nearly 50%. Moreover, the entire drying process is carried out in a closed system, where moist and hot air is discharged from the drying chamber, and then heated into dry and hot air through condenser treatment. The dried material is subjected to moisture absorption treatment, which continuously circulates until the drying goal is fully achieved. (2) Improve the qualification rate of dry materials. To ensure the high quality of dried products, it is necessary to control the temperature and humidity in the drying environment. Heat pump dryers can achieve a low temperature and low relative humidity drying environment, and are widely used in the drying process of many heat-sensitive materials such as agricultural products and medicinal herbs. Moreover, heat pump drying is a relatively mild drying method, which requires a low temperature during the drying process. The evaporation rate of surface water and the migration rate of internal water to the surface are very close, ensuring good quality, good color, and high grade of the dried items. After heat pump drying of fruits and vegetables, not only does it not seriously damage the appearance and color of the fruits and vegetables, but it can also reduce cracking and deformation during the drying process, in order to improve the quality of food processing. (3) High degree of automation and good environmental benefits. The operation of heat pump drying devices uses clean energy such as electricity, which rarely produces pollution during use. Moreover, it is easy to control and has a high degree of automation, making it widely applicable in cities. (4) Work environment hygiene. When using heat pump drying technology, the heat pump will work in a closed state. During the specific drying process, except for condensed water, there will generally be no exhaust gas or waste liquid discharged. Therefore, the processing process is very hygienic, which can ensure the safety of operators and create good environmental benefits. (5) Low processing cost. Compared to other food drying and processing technologies, the equipment investment and operating costs of heat pump drying technology are not high. The investment cost of heat pump drying equipment is only about one-fourth to one-fifth of the funds required for the production of freeze sublimation drying technology, and its operating cost is only one-third of that of other drying methods, which is relatively low. At the same time, the heat pump dryer has a long service cycle and good continuity in the production process. In the later stage, the production volume of the product can be increased by reducing the operating cost of the equipment to ensure that production meets market demand. (6) Can accurately control the temperature and humidity of the drying medium. The use of heat pump drying technology for processing fruits and vegetables can accurately control the temperature, humidity, and airflow speed of the dried fruits and vegetables, especially some fruits and vegetables that are prone to spoilage under high temperatures. It can effectively improve the quality of the dried products while ensuring their morphology and color stability. (7) Heating temperature is low. The heating temperature range of the heat pump drying process (20 ℃ -100 ℃) is relatively lower than that of general drying techniques, and the adjustment range of relative humidity is generally 15% -80%. The relatively loose application range can provide suitable drying treatment for more types of fruits and vegetables.
2、 Application of Vacuum Freeze Drying Technology in Fruit and Vegetable Processing
1. The principle of vacuum freeze-drying technology.
Vacuum freeze-drying technology is the process of freezing wet materials to a temperature below the eutectic point, so that the moisture in the material becomes solid ice. In a high vacuum environment, the ice is directly sublimated into water vapor by heating the material, and then the water vapor can be condensed through the water vapor condenser in the vacuum system, ultimately obtaining dry products. This drying processing technology places water-bearing materials in a vacuum environment, providing a certain amount of heat, allowing the water in the materials to be directly sublimated and discharged, ultimately obtaining dry products. This is currently a commonly used preservation processing technology internationally, and the products obtained through this process are also known as freeze-dried products.
From the application of vacuum freeze-drying technology, food dehydration can be achieved through heating and evaporation, or through freezing sublimation. Among them, freeze sublimation treatment can effectively remove water from fruits and vegetables. Considering that the water in fruits and vegetables is not pure water, the sublimation pressure and temperature are relatively low, so the vacuum freeze drying treatment method will have a better dehydration effect. In the application process of vacuum freeze-drying technology, it is necessary to quickly freeze fruit and vegetable products to below the melting point temperature, in order to obtain dried and dehydrated food. During the processing, the drying layer gradually thickens from the surrounding to the inside, forming a porous structure on the surface of fruits and vegetables. After completing sublimation and heat absorption, necessary energy replenishment is required in the final stage to transfer through the drying layer to the frozen part. On the sublimation surface at the boundary between drying and freezing, the water is heated and sublimated, completing the process of small molecules transferring outward. During the entire drying process, fruit and vegetable materials must be in a vacuum frozen state, and the temperature of the items needs to be controlled below the three-phase electrical temperature.
2. The characteristics of vacuum freeze-drying technology.
(1) It has a complete range of colors, shapes, and flavors. Freeze dried fruits and vegetables are dried in a vacuum environment. After rapid freezing, the water in the fruits and vegetables exists in the form of ice crystals, and soluble substances in the food are precipitated in their original positions, avoiding the use of general drying methods to promote the capillary flow of internal water to the surface. After freeze-drying treatment, fruit and vegetable products are less prone to oxidation, and the treated fruit and vegetable tissue structure is stable, not prone to spoilage and destruction. It can achieve the effect of continuously ensuring pigments and aromatic substances, making the quality and taste of dried fruit and vegetable products better. The volatile heat sensitive components in fruits and vegetables will not be lost due to high temperature, so the color, aroma, and flavor of food can be preserved to the maximum extent. After vacuum freezing and dehydration, some fresh fruits and vegetables will taste more fragrant and sweet due to their concentrated aroma and sweetness. (2) Ensure that the nutritional content of food is more comprehensive. The use of vacuum freeze-drying technology can continuously reduce the loss of some heat sensitive components and highly oxidizing substances in low temperature and low pressure environments, effectively preserve nutrients such as vitamins, carbohydrates, proteins, and maximize the preservation of substances such as chlorophyll, biological enzymes, and amino acids in food. Freeze dried food is processed in a low-temperature environment, which is not easy to breed bacteria and its nutritional components are not easily lost. Therefore, fruit and vegetable products treated with vacuum freeze drying not only have a longer shelf life, but also have relatively healthy nutritional components. (3) Good rehydration performance. Freeze dried fruits and vegetables are frozen before sublimation to fix their skeleton. After water sublimation, the solid skeleton can be kept stable and not deformed, and dried products can also maintain their own form to the maximum extent. In further drying treatment, the solid ice crystals in fruits and vegetables sublimate into water vapor, which will lead to the appearance of certain pores on the surface of the fruits and vegetables, which can maintain good rehydration performance. After dehydration treatment, vacuum frozen food has a lighter quality and can be stored and stored at room temperature. It can be used as an effective food supply in special working environments such as field operations, military operations, and navigation. Freeze dried fruits and vegetables are washed and cut in advance, so they can be consumed directly without the need to add antioxidants or preservatives during processing. Some freeze-dried fruits and vegetables can be consumed directly without rehydration, making them crispy and delicious overall. They are commonly used in the fast food industry and are also a popular family convenience food.
3、 Other drying techniques applied in fruit and vegetable processing
In the drying of fruits and vegetables, in addition to the two drying techniques mentioned above, joint drying techniques can also be used. This is a drying scheme that integrates multiple drying techniques, and at least two or more drying techniques are used to achieve complementary advantages between different drying techniques. This drying treatment method leverages the advantages of different drying techniques and utilizes effective methods in the corresponding drying stages. It has the advantages of short drying time, low energy consumption, high quality, high safety, good drying treatment efficiency, and can ensure product quality and reduce processing costs.
It should be noted that fruit and vegetable foods may undergo certain changes during the drying process. One is physical changes, which can cause shrinkage and cracking during the drying process of fruits and vegetables, gradually hardening the surface, and possibly forming porous structures. The second is chemical changes. After drying, the nutritional composition, color, and flavor of fruits and vegetables will undergo certain changes. So in the drying process, it is necessary to ensure that the nutritional components of the food are not lost as much as possible, avoid causing too much damage to its appearance, and keep the dried fruit and vegetable products bright in color and crispy in taste. This requires careful consideration when selecting specific drying techniques.
In the reduced rate drying stage of fruit and vegetable drying, it is necessary to reduce the evaporation rate of food surface moisture to ensure that it is consistent with the gradually decreasing internal moisture diffusion rate of the food, and to prevent adverse reactions caused by excessive heating of the food surface. Secondly, attention should be paid to adjusting the relative humidity of the drying medium reasonably at the end of the drying period, taking into account the expected moisture effect of the dried product, in order to minimize the damage to the appearance, color, nutritional composition, and other aspects of the fruits and vegetables during the drying process, ensure the rationality of the drying process, and thus maximize the preservation of the freshness and nutrition of the fruits and vegetables, and produce high-quality dried fruits and vegetables. Thirdly, in the mechanical processing equipment for fruit and vegetable drying, it is necessary to reasonably set drying conditions, control drying time, ensure effective connection of various drying processes, and create a good environment and conditions for the drying treatment of fruits and vegetables.
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