Common Drying Methods for Fruits and Vegetables in Industrial Production and Their Pros and Cons

Water is the main component of fresh fruits and vegetables, typically ranging from 70% to 95% in content. Water in fruits and vegetables exists in three different states: free water, colloidal-bound water, and chemically-bound water. Free water moves through the capillaries and by osmosis within the fruits and vegetables, having a greater mobility and being easily removed during drying. Some colloidal-bound water can be removed during drying, whereas chemically-bound water generally cannot be removed by drying.

The primary reason for drying fruits and vegetables is to prevent rot and extend their shelf life. The principle behind this is that the bound water, which cannot be removed by drying, represents the equilibrium moisture content of the fruit or vegetable tissue under drying conditions. The dehydration drying process transforms fruits and vegetables from a wet state to a dry state, removing a significant amount of free water and some colloidal-bound water. As the internal moisture content decreases, the water activity also reduces, inhibiting or hindering microbial growth and enzyme activity in the fruits and vegetables, thereby extending their storage life.

The basic process involves transferring heat from a heat source to the fruits and vegetables, causing continuous migration and surface evaporation of moisture within the tissue and cells, achieving the drying effect. Dehydrated fruits and vegetables retain most of their nutrients, and despite some differences in taste and appearance compared to fresh ones, their smaller size, lighter weight, and convenience for transportation make them popular among consumers.

1. Solar Drying: This is the oldest method of drying, using solar energy to dry fruits and vegetables. It is very cost-effective, requiring only manual slicing and placement. However, due to its slow drying rate and significant changes in product color and appearance, it is not suitable for industrial mass production and is better suited for home-based or small-scale production.
2. Hot Air Drying: This technique is the most widely used due to its low cost and ease of operation. It uses hot air as the drying medium to vaporize surface moisture and gradually transfer moisture from the inside to the surface of the material. The continuous rise in surface temperature during drying creates a temperature gradient, which can impede moisture migration and slow the drying process. Although hot air drying is effective, it can lead to changes in the color of fruits and vegetables and loss of internal nutrients, alongside issues like long drying times, low energy efficiency, and reduced product quality during storage.
3. Microwave Drying: This technique involves the orientation and rapid oscillation of polar molecules like water under a microwave electromagnetic field, generating significant heat through friction-like interactions. Microwaves preferentially heat water molecules, enabling moisture to migrate from the interior to the exterior and then evaporate, rapidly achieving drying. Advantages include fast drying speeds and simultaneous internal and external drying. However, the high energy consumption per unit, substantial initial equipment investment, and potential health risks from microwave radiation are notable drawbacks. Some countries have restricted microwave heating of food due to health concerns.
4. Freeze Drying: This technique involves rapidly freezing the moisture in the material to ice, then removing the water by sublimation under high vacuum conditions at low temperatures. Freeze-dried products maintain their shape and internal structure, offering excellent rehydration quality and preserving nutrients, making it particularly suitable for drying heat-sensitive and oxidizable foods. Despite producing high-quality dried products, freeze drying is limited by its slow speed, high energy consumption per unit, and substantial equipment investment, making it less feasible for small to medium-sized enterprises.
5. Osmotic Drying: This technique involves immersing materials in solutions like sugar or salt water, which remove moisture through osmosis. Osmotic dehydration is quick and minimally impacts the structural integrity of the material, effectively preserving the original cellular structure, color, taste, and nutrients, while also inhibiting microbial growth and extending shelf life. This technique is primarily used for producing candied fruits and pickled vegetables.
6. Heat Pump Drying: This technology extracts heat from a low-temperature source and effectively uses it at a higher temperature. Recently, heat pump technology has been increasingly applied in the drying of aquatic products, medicinal materials, and agricultural by-products. Its principles are similar to those of hot air drying but with different heat sources, offering advantages such as high-quality dried products, energy saving, and no pollution.
7. Infrared Drying: Infrared radiation directly heats the water molecules in the material, causing the temperature to rise and the moisture to evaporate, achieving dehydration. The principle is based on the diffusion of moisture from the inside to the surface of the fruit or vegetable, where it evaporates into the surrounding environment. Infrared rays can penetrate to a certain depth in fruits and vegetables

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