Application of Yingtai Vacuum Freeze-Drying Technology in Fruit Drying

Application of Yingtai Vacuum Freeze-Drying Technology in Fruit Drying  

Fruit freeze-drying is an advanced food processing technology. Its core principle involves the direct sublimation of ice (solid water) into water vapor under low temperatures (typically below -30°C) and in a vacuum environment, thereby removing moisture from frozen fruits.  

Pretreatment  

Pretreatment has long been used as a technique to accelerate the drying rate and improve the quality of the final product. Traditional pretreatment methods—chemical, thermal, and mechanical—are employed to overcome the water barrier posed by the waxy skin of fruits such as cherry tomatoes, grapes, plums, blueberries, and cranberries during drying, as well as to inactivate enzymes through blanching. Although chemical and thermal pretreatments are highly effective in increasing the drying rate, they can negatively affect the sensory and physicochemical quality of the product. Blanching may lead to the leakage of soluble compounds and cause undesirable structural changes. In other cases, when rigorous mechanical pretreatment is applied, issues such as fruit integrity damage and leakage become significant. Recent technologies, including high-pressure processing, pulsed electric fields, and supercritical carbon dioxide, have been studied as pretreatments to enhance the drying rate of fruits like mangoes and sweet peppers, with promising results.  

1. Sorting and Cleaning  

   Fresh fruits are strictly sorted to remove rotten or spoiled parts and are graded by size and ripeness. They are then thoroughly cleaned using running water, ozone, or light saltwater soaking to remove surface dirt, pesticide residues, and microorganisms.  

2. Cutting and Pitting/Peeling  

   Depending on the fruit characteristics and product requirements, fruits are peeled, pitted, and stemmed (e.g., apples, strawberries), and cut into uniform pieces or slices to ensure consistent sublimation rates and improve drying efficiency.  

3. Color Protection Treatment  

   To prevent enzymatic browning (common in fruits like apples and bananas), blanching (short-term heating with hot water or steam to inactivate enzymes) or soaking in color-preserving solutions (e.g., vitamin C or citric acid solutions) is often applied to maintain the natural color of the fruits.  

4. Pre-Freezing  

   The processed fruit materials are rapidly frozen at temperatures below -30°C to ensure that the internal moisture completely forms tiny ice crystals. This step lays the foundation for subsequent vacuum sublimation drying and helps prevent damage to the cellular structure.  

Comparison of Different Drying Methods  

Common drying methods for whole berries (e.g., blueberries, cranberries, raspberries) include hot air drying, vacuum freeze-drying, and ultrasound-assisted drying. Drying is a food processing technology that extends shelf life by reducing water activity and maintaining product quality. It helps minimize spoilage and contamination, thereby ensuring quality and stability. Additionally, drying facilitates packaging, handling, and transportation.  

01. Hot Air Drying (HAD)  

    Hot air drying is a widely used drying method due to its low cost and ease of use. However, it results in prolonged drying time, exposure to oxidation, and the development of off-flavors. Prolonged heating at high temperatures can cause berries to darken in color, significantly reduce heat-sensitive nutrients such as vitamin C and anthocyanins, and result in a tough, shriveled texture with poor mouthfeel. The fruit skin may also become overly hardened ("case-hardening"), hindering internal moisture evaporation.  

02. Vacuum Freeze-Drying (FD)  

    Another common method for drying berries is vacuum freeze-drying, which offers faster drying speeds, lower temperatures, and operation in a low-oxygen environment. This method maximizes the retention of the berries' natural shape, color, flavor, and most nutritional components (especially heat-sensitive substances). The final product is crispy and porous with excellent rehydration properties.  

03. Ultrasound-Assisted Drying (UAVD)  

    Ultrasound-assisted drying technology is generally used to shorten drying time and improve drying efficiency. It enhances the dehydration rate without heating under vacuum conditions. Ultrasound improves internal moisture transfer, induces cavitation, and creates microscopic channels to facilitate water removal. Additionally, ultrasound generates cavities that help remove tightly bound internal water without causing significant heating.