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About onion preservation and preservation method
Storage Conditions
1. Temperature: -1 to 0°C.
2. Relative Humidity: 65% to 70%.
3. Gas Composition: Oxygen (Oâ‚‚) at 5%, Carbon Dioxide (COâ‚‚) at 10% to 12%.
Storage Methods
1. Traditional Outdoor Pile Storage: This method is commonly used in areas like Beijing, Tianjin, and Tangshan in China. It involves piling dried onions outdoors in a rectangular shape, typically 5–6 meters long, 1.5–2 meters wide, and 1.2–1.5 meters high. The site should be elevated, well-ventilated, and easy to drain. A layer of straw is laid first, followed by the onions. The pile is then covered with straw mats and secured with ropes or wires. To protect from rain, plastic film can be placed on top. During storage, regular temperature checks are important to detect any heat spots and address them promptly.
2. Braided Hanging Method: Sun-dried onions are braided into 1-meter-long strands, which are then hung on racks placed indoors or outdoors. The rack height is usually between 1.2 and 1.5 meters, and the length depends on available space. The braids are kept about 20 cm above the ground to prevent moisture absorption. Plastic film is used to cover the onions and protect them from rain. Depending on the weather, mats or covers can be added or removed to avoid cold damage and maintain dryness. This method is ideal for small-scale home use due to its simplicity and effectiveness.
3. Large Plastic Container Storage: High-quality red onions are selected, and any damaged ones are removed. They are then stored in sterilized baskets using a 0.5% bleach solution. These baskets are placed in shaded areas for storage until late July. In dry and well-ventilated spaces such as open-air sheds or indoor racks, the storage frames are built with materials like round wood or angle iron. Each frame has a 15–20 cm ditch around it. The bottom of the frame is wider than the shelf, and the onions are stacked inside. After stacking, the entire structure is wrapped in plastic film, with the edges rolled up and buried in the surrounding ditch. Natural oxygen control is applied to maintain an O₂ level of 5% and CO₂ of around 13%. If oxygen levels drop too low, ventilation through the openings is necessary. Every week, 4000 ml of chlorine gas is introduced to prevent microbial growth. When temperatures drop, additional straw mats can be placed around the containers for insulation.
Main Problems and Preventive Measures During Storage
During storage, onions may face several issues, including root development caused by high humidity, which leads to nutrient loss and disease susceptibility, and sprouting after the dormancy period. To prevent these problems, the following measures are recommended:
1. Maintain a relatively dry environment with a relative humidity of 65% to 70%.
2. Ensure proper drying before storage so that the leaves turn yellow and the outer layers shrink.
3. Apply Qingxinsu (0.25%) treatment to inhibit sprouting.
4. Use gamma radiation at doses between 25–80 Gy to extend shelf life and prevent spoilage.
By implementing these storage methods and preventive strategies, the quality and longevity of stored onions can be significantly improved, making them suitable for both commercial and household use.