Research on the combined application of infrared heating technology and other technologies in fruit and vegetable drying
With the increasing demand for product quality, using infrared heating technology alone can no longer meet the drying requirements, which has prompted people to combine infrared heating technology with other advanced drying technologies. The so-called joint drying refers to a mixed drying technology that fully utilizes the characteristics of each drying method and combines two or more drying methods.
The drying process can be optimized to improve product quality, reduce drying time, and increase economic benefits. At present, there are already infrared hot air (IR-HA) combined drying, infrared microwave (IR-MD) combined drying, infrared vacuum (IR-VD) combined drying, and infrared heating technology combined with other advanced technologies for drying both domestically and internationally.
1. Infrared hot air combined drying
Hot air drying can timely remove the water vapor evaporated from the surface of the material, allowing the internal moisture of the material to diffuse. Infrared heating technology can simultaneously heat the internal and external moisture of the material, with temperature and humidity gradients in the same direction, thereby accelerating the drying process of the material.
Nathakaranakule et al. studied the far-infrared assisted hot air drying of longan, and the results showed that this technology can not only significantly improve the drying rate and rehydration rate, but also reduce the shrinkage and hardness of the product, and the appearance of longan dried products is dark red.
Ponkham et al. conducted a study on the combined drying of ring-shaped pineapple slices using far-infrared hot air, and found that the diffusion coefficient was mainly affected by the far-infrared intensity and hot air temperature; The fourth power model has a high degree of fitting for color changes during the drying process; The modified Midelli Kucuk model has a high degree of similarity in the shear force of dry products; The quadratic model has a high degree of fit to the shrinkage kinetics during the drying process.
Somkiat et al. also systematically studied the heat and mass transfer processes of far-infrared hot air combined drying of fruit peels.
2. Infrared microwave combined drying
Infrared radiation has strong penetration power, and is used for drying fruits and vegetables at a fast speed and with good quality. Microwaves have stronger penetration and are heated both inside and outside, making them more suitable for agricultural products that are difficult to dry and dehydrate or difficult to lose moisture during later drying.
Wang et al. studied the far-infrared microwave (FIR-MD) drying characteristics of peaches, and the results showed that with the increase of far-infrared and microwave power, the dehydration rate also increased; Far infrared power, microwave power, and conversion point moisture content have a significant impact on energy consumption and product sensory quality.
Cao Xinzhi and others studied the infrared microwave (IR-MD) combined drying characteristics of carrots, using the quality of carrots as the evaluation index. Through instrument and sensory analysis, it was found that the quality of the finished product was better and the drying rate was faster when the infrared microwave combined drying was used.
Gan Sijia first uses far-infrared to kill yellow cauliflower to a certain degree (with a moisture content of 40%), and then uses microwave drying to the specified moisture content (13%). This method can not only shorten the drying time but also ensure product quality. After testing, the protein preservation rate is 87.3%.
3. Infrared vacuum combined drying
The characteristic of infrared vacuum drying is to dry in a vacuum environment, where the internal pressure of the drying chamber is greater than the external pressure. Fruits and vegetables achieve rapid drying under the influence of pressure difference and humidity gradient.
Mongpraneet et al. studied the far-infrared vacuum drying of Welsh onions, and the results showed that the entire drying process can be divided into three stages: increasing rate, constant rate, and decreasing rate. The drying rate and chlorophyll retention rate of the product are mainly affected by radiation intensity.
Yang Zhiwei et al. optimized the processing conditions of dried mango using response surface methodology. The optimal process conditions were: drying time of 4.5 hours, absolute pressure of 42.3 kPa, and far-infrared heating temperature of 55.4 ℃.
Swasdisevi et al. conducted a study on the joint drying of banana slices using far-infrared vacuum (FIR-VD) technology. By comparing the predicted values of moisture content and temperature during the joint drying process with experimental results, it was found that the established mathematical model can fully describe the changes in moisture content and temperature during the joint drying process.
4. Other combined drying technologies
In addition, research has also reported the combination of infrared heating technology with heat pumps, gas jet impingement methods, and low-pressure superheated steam combined drying.
Xu Gang et al. conducted a study on the far-infrared heat pump (FIR-HPD) combined drying process for carrot slices, and found that the Z-optimal process parameters for the FIR-HPD combined drying process for carrots are: material hot stamping time of 120 seconds, HPD temperature of 45 ℃, FIR heat source radiation power of 2 kW, and material water content at the HPD and FIR switching point of 50%.
Zheng Xia et al. studied the infrared combined gas jet impingement method for Hami melon slices, and the results showed that the entire drying process was in the deceleration stage; This method can shorten the drying time to 2.0-3.5 hours.
Nimmol et al. studied the heat transfer process of banana slices dried by combining far-infrared and low-pressure superheated steam, and the results showed that the drying kinetics and heat transfer of banana slices were greatly affected by the temperature and pressure of the drying medium.
Infrared heating technology, as a new type of heating technology, has the advantages of high efficiency, energy conservation, and no pollution. Compared with traditional heating technology, it has a wider range of applications. However, in China, the application of infrared heating technology in combination with other technologies in fruit and vegetable drying is still in its early stages and has not yet formed a scale, especially in determining the infrared heating mechanism and its Z-optimal conversion point. There is still a significant gap compared to developed countries.
Therefore, in order to achieve automatic control of combined drying technology, it is necessary to establish a reasonable mathematical model, which is a focus and difficulty of future research. In addition, the equipment of joint drying technology also needs to be continuously improved, and automation control, safe operation, and online precise detection will become the future development direction.