New technology for drying fruits and vegetables and existing problems
China's fruit and vegetable resources are abundant. With the increase in the demand for high-quality convenience foods and the rapid development of the export industry, the drying and processing of various fruits and vegetables such as grapes, apples, peppers, bamboo shoots, garlic, mushrooms and so on are becoming more and more popular. The traditional drying method has natural drying, dry and hot air drying. From the current point of view, these methods are more suitable for China's national conditions, and can also meet domestic needs. However, these methods have the disadvantages of high energy consumption, small production capacity, low efficiency, non-compliance with product process and hygiene requirements, and the quality of the products is difficult to guarantee, which is not conducive to the further development of China's fruit and vegetable drying industry.
Therefore, research on the new technology of fruit and vegetable drying has practical significance for China's fruit and vegetable drying industry. At present, the new technology of fruit and vegetable drying mainly includes microwave drying technology, far infrared drying technology, vacuum freeze drying technology and joint drying technology. These new technologies have their own advantages, but there are also some problems?
1, microwave drying technology
The microwave is a penetrating electromagnetic wave having a wavelength between 1 mm and 1 m and a frequency of 300 MHz to 300,000 MHz. In the industry, China is allowed to use microwaves with frequencies of 9158MHz and 284508MHz for heating. The penetration of microwaves is stronger than that of infrared rays, and microwave heating is not added by external heat sources, but is generated directly inside the heated object. Therefore, microwave drying has the advantages of short drying time, uniform heating, sensitive reaction and high thermal efficiency.
In addition, due to the selective heating effect of microwave on moisture, the raw material can be rapidly dried at a lower temperature, which is of great significance for improving product quality and reducing the loss of nutrient content of the product. Microwave drying technology has been widely used in foreign countries. There are also nearly 30 years of development, all industries are promoting this technology? The technical route of microwave drying is as follows: fruits and vegetables → washing → peeling and nucleation, slicing → color protection → blanching → drowning → microwave drying → softening → Finished product → packaging
The key to microwave drying technology is the good operation of microwave equipment. The domestic equipment has a certain gap between the overall operation reliability, automatic control level, mechatronics, production supporting capacity and other foreign equipment, which promotes microwave drying technology. What is the impact? Is it an urgent problem?
2, far infrared drying technology
Far-infrared drying technology is a new economical drying technology whose principle is exactly the same as that of near-infrared drying, except that the wavelength of the rays is different. The main features of far infrared drying: * is fast drying speed, high production efficiency, and its drying efficiency is generally one tenth of hot air drying. The second is that the hot air drying consumes less power. The third is that the quality of the dried product is good, and the far infrared rays are simultaneously absorbed under the surface layer and the surface layer, so the drying is uniform and the physical properties of the product are good. The fourth is the small size and low cost of the equipment?
At present, most of the far-infrared drying technology is still in the laboratory research stage. If it is necessary to further promote the equipment, process and other aspects, can we truly meet the requirements of industrial production?
3, vacuum freeze drying technology
Vacuum freeze drying technology is a new drying technology that integrates vacuum technology, refrigeration technology and drying technology. Because vacuum freeze-drying can maintain the original color, flavor, nutrients and biological activity of the food to a large extent, it can quickly restore the original appearance by rehydration, and the food is simple, convenient, light and easy to transport, so the vacuum freeze-drying technology is One of the more extensive drying techniques currently being studied?
Vacuum freeze-drying of agricultural products and foods is to first freeze the moisture in the product, and then vacuum to directly convert the ice crystals into gaseous sublimation to dehydrate the product. In the process of freeze-drying, the freezing, vacuuming and heating processes are carried out continuously, so the equipment requirements are high?
Vacuum freeze-drying is generally divided into three stages: the pre-freezing stage, the sublimation stage and the analytical stage. The lyophilization process must be developed in stages and then integrated into a whole to form a temperature, pressure and time curve, ie a freeze-drying curve. A freeze-drying curve must be developed for each new product. This also indicates that it is necessary to find suitable temperature, pressure and time conditions for different samples to be processed by different lyophilizers in order to obtain a product of good quality and low cost. The technical route of vacuum freeze-drying of fruits and vegetables is as follows: raw materials → selection → cleaning → slitting → freezing → sublimation drying → packaging → finished products
Vacuum freeze-drying is still in its infancy in China's fruit and vegetable processing industry. Most of the enterprises that produce vacuum freeze-dried products have small scale, low production capacity and low economic efficiency. Domestically produced equipment is difficult to meet production requirements, so it is necessary to import a large number of equipment, which greatly increases the production cost, which is a big problem in the promotion process of vacuum freeze-dried products. The actual production lacks the necessary, the control link is weak, and the product quality is difficult to be guaranteed. With the development of the fruit and vegetable processing industry, vacuum freeze-dried products have excellent quality, and their demand will become larger and larger. Should we conduct more in-depth research on technology and equipment in the future, and provide necessary for actual production?
4, joint drying technology
Combined drying technology is a composite drying technology that combines two or more drying modes according to the characteristics of materials, and is combined in stages, such as hot air-freezing combined drying, hot air-microwave combined drying, hot air-microwave freezing combined. Drying, etc. It is a product developed by a combination of various methods such as hot air drying, microwave drying, vacuum drying, freeze drying, etc., involving physics, fluid mechanics, cryogenic refrigeration, heat and mass transfer, automatic control, etc. Comprehensive and highly applicable technology?
The combined drying technology is characterized by the advantage of utilizing various drying techniques, using different drying methods in different drying stages, and removing most of the free water and colloidal bound water by evaporation or sublimation, respectively. The advantages of combined drying are reduced drying time, reduced energy consumption, improved quality, easy operation, environmental protection, safety and efficiency, and greater cost reduction and food quality maintenance.
The current difficulty in joint drying technology is the determination of the preferred switching point. Since the state of different fruit and vegetable tissues varies greatly, a large number of tests are needed to determine the preferred process switching points for different drying methods in combined drying. To achieve automatic control of joint drying technology, it is necessary to establish a reasonable mathematical model. Although there are a large number of model studies, the versatility of the theoretical model and the actual drying process are still very different. Many parameters need to be studied in depth. Obtained, this is also a difficult point of this technology. In addition, the joint drying technology equipment also needs continuous improvement. The equipment itself has high energy consumption, difficult to control operating conditions, and difficulties in online testing. Further research and solutions are needed.
5, summary
Fruits and vegetables are an important source of nutrients such as vitamins, minerals, dietary fiber and bioactive substances in daily life. Nowadays people are increasingly pursuing nutritious, safe and convenient processed foods. Therefore, dried fruits and vegetables are an important part of them. It is of great practical significance to develop various drying techniques for fruits and vegetables. While continuously developing various drying technologies, combined drying technology is The future development trend of fruit and vegetable drying?
Source: Food Engineering Author: Ann Yu
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