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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‚‚): 5%, Carbon Dioxide (COâ‚‚): 10% to 12%.
Storage Methods
1. Traditional Outdoor Heap Storage
This is a traditional method used in areas like Beijing, Tianjin, and Tangshan in China. It involves piling onions outdoors in a structured way for long-term storage with good results. The process begins by selecting a high, well-ventilated, and easily drained site. A layer of soil is prepared, followed by a base of crop stalks. Dried onion bulbs are then stacked on top of the straw. The heap is usually rectangular, measuring about 5–6 meters in length, 1.5–2 meters in width, and 1.2–1.5 meters in height. After the structure is built, it is covered with curtains, and the surrounding areas and the top are wrapped with straw mats and secured with ropes or wires. To protect against rain, a plastic sheet can be placed on top. During the early storage period, regular temperature checks are essential to detect any hot spots and to cool them down promptly.
2. Braided Hanging Storage
Another effective method is to braid sun-dried onions into bundles, each about 1 meter long. These braids are then hung on racks placed either indoors or outdoors at a height of 1.2–1.5 meters. The length of the rack depends on the available space. The braided onions are hung so that the bottom is approximately 20 cm above the ground. A plastic cover is used to protect against rain, and depending on the weather, mats or other covers can be added to prevent cold or frost. Covers should be removed when conditions improve to avoid moisture buildup. This method is ideal for small-scale home use, as it is easy to manage and provides excellent results.
3. Large Plastic Container Storage
For larger quantities, red onions are selected, and those damaged by rain are removed. They are then placed in sterilized baskets using a 0.5% bleach solution and stored in shaded areas until late July. In dry, well-ventilated spaces such as open-air sheds or indoor racks, the storage structures are built according to the volume of onions. Typically, these racks can hold between 1,000 and 2,000 kg. The frames can be made from wooden poles or iron angles, and a 15–20 cm deep trench is dug around the frame. The base of the rack is extended by 20–30 cm beyond the shelf, and the onions are piled inside. Once stacked, the entire structure is covered with plastic film. The bottom and sides of the container are rolled up and buried in the trench to ensure an airtight seal.
Natural oxygen control is applied to maintain an oxygen level of about 5% and carbon dioxide at around 13%. If the oxygen level drops too low, ventilation is done through the openings. Every week, 4,000 ml of chlorine gas is introduced to inhibit bacterial 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. One common problem is the development of fibrous roots due to high humidity, which leads to nutrient loss and increased susceptibility to diseases. Another issue is germination after the dormancy period. To prevent these problems, the following measures are taken:
1. Maintain a relatively dry environment with a relative humidity of 65% to 70%.
2. Dry the onions properly before storage so that the leaves turn yellow and the outer layers become shriveled.
3. Treat the onions with Qingxinsu (0.25%) to delay sprouting.
4. Apply gamma radiation at doses ranging from 25 to 80 Gy to suppress germination and extend shelf life.
These methods help ensure that stored onions remain fresh, safe, and of high quality throughout the storage period.