For kiwifruit storage in controlled atmosphere (CAA) cold storage facilities, the rapid oxygen reduction method should be implemented under controlled low-temperature conditions to achieve optimal results. This low-temperature gas regulation not only suppresses respiration and ethylene production in fruits, delaying ripening, but also maintains a high relative humidity of over 90% within the sealed environment, ensuring the fruits remain fresh and plump.
The specific operation of low temperature controlled atmosphere storage is:
Place pre-cooled fruits into crates (10-15 kg per crate), then cover each crate with a plastic bag 0.06-0.08 mm thick. The bag is equipped with ventilation holes (secured by rubber tubes) and sealed either by tightening the bag opening or clamping the holes with water-tight clips to form airtight closure. Each crate is positioned on dedicated racks within the cold storage unit, which maintains a temperature range of 0-2°℃. All operations must be conducted strictly within the controlled environment.
When oxygen reduction is required, the rapid oxygen reduction method should be employed. First, use an air pump to evacuate air from the bag, then fill it with nitrogen. Repeat this process 2-3 times until oxygen levels reach the desired threshold. If nitrogen is unavailable, the natural oxygen reduction method can be used: seal the fruit in a bag and remove as much air as possible during sealing. Instead of filling with nitrogen, rely on the fruit's respiration to consume oxygen and increase CO₂ levels, thereby suppressing ethylene production. Both rapid and natural oxygen reduction methods require daily gas monitoring. When oxygen drops below 2% or CO₂ exceeds 5%, replenish oxygen and remove excess CO₂ to prevent damage from hypoxia or hyperoxygenia. For CO₂ removal, CO₂ scrubbers are typically used in controlled atmosphere storage facilities, while ordinary plastic bags generally employ lime for this purpose.
When large quantities are stored, use controlled atmosphere storage (CAS) with high-density gas systems. The optimal gas composition for kiwifruit preservation is 2%-3% oxygen and 3%-5% carbon dioxide. Native to southern China, kiwifruit has earned the title "King of Fruits" due to its rich vitamin C content and other nutrients. With expanded cultivation areas in recent years and improved living standards, this fruit has become accessible to ordinary households, becoming a common sight on dining tables. However, as a typical respiration-responsive berry with thin skin and high moisture content, kiwifruit is prone to spoilage. As the saying goes, "soft within seven days, rotten within ten, half spoiled within two weeks." Conventional storage methods struggle to maintain its quality for extended periods, which has hindered further development in kiwifruit production.
I. Hardware equipment and standards for low ethylene gas storage and preservation of kiwi fruit
(1) Air-tightness of Cold Storage: The air-tightness of cold storage facilities is crucial for maintaining optimal preservation of kiwifruit under low-vinylene modified atmosphere (MAD). The facility must meet the following standard: When pressurized to 30mm water column, the internal pressure should reach at least 4.4mm water column within 30 minutes; otherwise, the low-vinylene MAD environment cannot be established. Our air-tight cold storage maintains an internal pressure of 12mm water column under identical conditions. Additionally, to ensure environmental integrity and facilitate inspection access, a small door accommodating personnel entry should be excavated slightly below the main storage door.
(2) The refrigeration system equipment shall be compatible with the storage capacity.
(3) Modified atmosphere and monitoring system equipment, including carbon dioxide (CO2) remover, ethylene (C2H4) remover, nitrogen (N2) generator and storage tank, O2 and CO2 gas component monitor, ethylene analyzer.
II. Key technical indicators of low ethylene gas storage and preservation of kiwifruit
Kiwifruit, a typical respiration-threshold berry, is highly sensitive to ethylene (C₂H₄) ripening effects. Three critical factors must be addressed during storage: 1) maintaining and enhancing its vital activity; 2) reducing environmental ethylene concentration; 3) preventing water loss. Therefore, the key technical indicators for low-ethylene controlled atmosphere storage of kiwifruit should be categorized into three types:
(1) Indicators that weaken and maintain its vital activity:
1. Temperature index: -0.5--0.50℃. Through experiments, the suitable storage and preservation temperature of kiwi fruit should be-0.5--0.50℃. Higher than 0.50℃, the respiration of kiwi fruit will become active and shorten the preservation time; lower than-0.50℃, when the temperature of the storage is not uniform, the kiwi fruit near the evaporator and air duct may be frost damaged.
2. O2 concentration index: 2-3%, which is used to keep the respiration of kiwi fruit at a minimum. When O2> 3%, N2 should be injected to reduce the O2 concentration.
3. CO2 concentration index: 3-5%, which is used to inhibit the respiration of kiwi fruit. When CO2 is less than 3%, no treatment is needed under normal circumstances, and the CO2 concentration will be automatically increased through the respiration of kiwi fruit; when CO2 is more than 5%, it should be removed immediately, otherwise, kiwi fruit will suffer from CO2 poisoning.
(2) Reduce the concentration of ethylene in the environment:The concentration of ethylene should be less than 0.02ppm. In the process of the experiment, we use the automatic interval working ethylene dehydrator, and the effect of ethylene removal is very ideal, basically controlled below 0.01ppm.
(3) Humidity (RH) index: The humidity should be kept above 90%. When RH is less than 90%, kiwi fruit will suffer from dehydration and wrinkling. If the humidification equipment can not meet the requirements, water should be sprinkled on the floor to increase the humidity.
Third, choose the appropriate kiwi fruit variety
There are numerous kiwifruit varieties, each with significantly different storage durability. Comparative experiments conducted on four cultivars (Haiwode, Qinmei, Yate, and Qin Cui) currently cultivated in large areas revealed the following results: Storage durability ranking is Haiwode> Qinmei> Yate> Qin Cui. After six months of storage, Haiwode maintained normal flesh and core texture, while Qinmei developed a hard core. Yate exhibited hardened core with partial whitening of flesh and diminished flavor, whereas Qin Cui showed relatively soft flesh. Based on these findings, Haiwode and Qinmei are recommended for storage purposes.
4. Select kiwifruit with good traits for storage
(1) The kiwi fruit to be stored should not have trauma or scratch, and the villi should be well preserved.
(2) Kiwifruit should be harvested when fully ripe. The normal color of kiwifruit is yellowish-brown, with slight variations in color intensity depending on the variety. Ripe kiwifruit exhibits deeper coloration, superior texture, and better storage tolerance. In contrast, unripe fruits appear lighter with bluish tint, have inferior texture, and show poor storage stability. Experimental studies demonstrate that premature harvesting of kiwifruit from the same variety significantly reduces its shelf life.
V. Do a good job in warehousing
Kiwifruit ripens quickly at room temperature, so the following points should be paid attention to when warehousing:
(1) Before entering the warehouse, the temperature of the cold storage has been reduced to a suitable temperature and the airtightness is good.
(2) The harvesting, transportation, sorting, cooling, and storage of kiwifruit should be carried out in a seamless and timely manner. Kiwifruit must be transported immediately after picking. During transportation, they should be packed separately to prevent bruising and kept ventilated. For long-distance transport, cooling facilities should be used or nighttime transportation should be adopted. When sorting, handle the fruits quickly and gently while maintaining a low-temperature environment. After sorting, store them immediately in cold storage. The entire process from picking to storage should generally not exceed two days.
VI. Storage and stacking principles
The storage of kiwifruit should adhere to the following general principles: Prevent mutual compression among fruits, ensure good ventilation between boxes for easy access, and maintain secure stacking. These principles primarily aim to protect kiwifruits from crushing injuries, ensure proper air circulation for balanced cooling, inhibit mold growth, maintain equilibrium in O2, CO2, C2H4 concentrations and humidity levels, and prevent stack collapse that could cause personnel injuries or unnecessary losses. During storage, we used plastic turnover boxes (48cm × 32cm × 27cm in dimensions) as containers, which not only provided convenience but also effectively protected the fruits from damage, achieving excellent results. When stacking, reasonably divide each stack size based on factors like convenience, warehouse capacity, and wind direction. First, leave 20cm of space at the base of the stack, ensuring a minimum distance of 40cm between the stack and surrounding walls to facilitate air circulation and protect the warehouse's airtight layer. Then proceed with vertical stacking. To reinforce stability, add wooden boards every ten layers to connect boxes into a unified structure. After completion, cover the top layer of kiwifruits with a wet burlap sack to maintain moisture retention and reduce dripping water impact on individual fruits.
VII. Daily operation requirements
After the kiwifruit is stored in the warehouse, the storage body should be sealed quickly and adjusted according to the technical indicators of storage and preservation. During operation, it is necessary to enter the warehouse through the small door in time for necessary inspection according to the situation. However, attention should be paid to: at this time, the interior of the warehouse is severely oxygen-deficient, so it is necessary to carry oxygen supply equipment when entering.
Viii. Kiwi fruit out of the warehouse
The distribution of kiwifruit from storage facilities should adhere to centralized dispatch principles, with optimal timing for minimizing frequent environmental fluctuations during intermittent releases. Our commercial trials demonstrate that under controlled temperature and humidity conditions—maintaining 2-3% oxygen levels, 3-5% CO₂ concentration, and ethylene levels below 0.02 ppm—the storage lifespan of Qinmei kiwifruit exceeds six months, with a shelf life of at least two weeks and a 98.03% good fruit rate. The flesh hardness reaches 6.82kg/0.5cm2, while the rot rate remains at 0.77%, showing even longer storage longevity compared to Hayward varieties. Compared with kiwifruit stored in conventional cold storage facilities, those in controlled atmosphere storage exhibit 3.4 times higher flesh hardness, 1.3 times more normal fruits, a 61.9% reduction in fruit loss, and a 40% decrease in diseased fruit rates.
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