At present, air-conditioned storage is widely used for apples both domestically and internationally. However, certain apple varieties, such as Asarum and Hongyu, require higher storage temperatures than conventional refrigeration allows. To prevent excessive ripening and microbial activity, controlled atmosphere (CA) storage has emerged as an effective solution. Various modified atmosphere packaging (MAP) techniques have been applied across different regions, including varieties like Marshal, Golden Delicious, Guoguang, Qinguan, and newly developed cultivars. These methods help extend the storage period, maintain fruit quality, and reduce the occurrence of diseases such as red spot, tiger skin, and browning. Additionally, CA storage reduces microbial rot and dehydration, ensuring that apples remain fresh and firm.
One common method involves using plastic film bags. After pre-cooling and sorting on the farm, apples are placed in lined fruit boxes or baskets, sealed in plastic bags, and stored. The films used are typically made of polyethylene or non-toxic PVC, with a thickness of 0.04 to 0.06 mm. This technique creates a microenvironment that slows down respiration and ripening. For example, Shanghai Fruits Co., Ltd. successfully used this method to transport Red Star apples from Yantai to Shanghai. The apples maintained a hardness of 7.2 kg upon arrival, dropping to 5.6 kg after six months of refrigeration, and further to 4.6 kg and 3.1 kg under controlled atmosphere conditions.
Another approach is the use of plastic sheeting in refrigerated rooms, clay cellars, or ventilated warehouses. High-pressure PVC films, about 0.1 to 0.2 mm thick, are glued into rectangular covers to hold hundreds or even thousands of kilograms of apples. Once sealed, oxygen and carbon dioxide levels are adjusted using either rapid or natural oxygen reduction methods. In recent years, silicone rubber film diffusion windows have become popular, allowing gas exchange without manual intervention. These membranes are placed in specific areas of the packaging, enabling the automatic regulation of internal gas composition based on the apple variety and membrane type.
Modern air-conditioned storage systems allow precise control of temperature, humidity, and gas composition, making them ideal for long-term preservation. Apples should be stored within 24 hours of harvest and cooling. The storage temperature is usually 0.5–1°C higher than standard refrigeration, which helps reduce CO₂ damage in sensitive varieties. For apples prone to cold injury, slightly higher temperatures can minimize damage.
Reducing oxygen concentration alone improves storage results, but combining it with a controlled level of CO₂ often yields better outcomes. Most apple varieties tolerate up to 5% CO₂, while some, like the Golden Crown, can handle up to 8–10%. Recently, a short-term high-CO₂ pretreatment at the start of storage has shown promising results. For instance, applying 15–18% CO₂ for 10 days before switching to normal conditions helped maintain fruit firmness. Similar research is ongoing in China, with combined temperature and gas control methods showing improved results.
A two-way variable-air storage system, known as TDCA (Two-Directional Controlled Atmosphere), was developed by several research institutes. It involves gradually lowering the temperature and adjusting gas levels over time. This method proved more effective than low-temperature storage and comparable to standard modified atmosphere conditions. By using natural air temperatures initially, it mitigates COâ‚‚-related harm, suppresses ethylene production, and delays aging, ultimately maintaining fruit quality and firmness.
To manage ethylene accumulation, activated carbon or bromine-saturated activated carbon can be used to absorb it. For example, placing 0.05% activated carbon per fruit weight in small plastic bags helps maintain hardness in Red Star apples. Potassium permanganate (KMnOâ‚„) can also be used, either by spraying its solution or embedding it in porous materials that absorb the saturated solution. These methods help control ethylene levels and improve post-harvest quality.
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Chemical plastic products bring people all kinds of convenience, but also bring unimaginable trouble. As some waste plastics will not degrade under natural conditions, the combustion will release harmful gases, causing difficult pollution to the ecological environment. So scientists around the world are starting to develop self-destructing, or autolytic, plastics that can break down on their own to solve the problem. Some call it "green plastic". Companies in many countries have introduced their own biodegradable plastics. Biologists at the University of Mitzgan in the United States first proposed the idea of "growing" degradable plastics. Using potatoes and corn as raw materials, they inserted the plastic's genetic material so that it could be artificially grown without harmful ingredients. Imperial Chemical Forests USES bacteria to make sugars and organic acids into biodegradable plastics. The method is similar to the fermentation process used to produce ethanol, except that the bacteria, alkaloids, turn the feed into a plastic called PHBV. Bacteria accumulate the plastic as a store of energy, just as humans and animals store fat. When the bacteria have accumulated as much as 80% of their body weight in PHBV, they break down the cells with steam to collect the plastic. PHBV has properties similar to polypropylene, a material that is stable even in humid environments after being discarded, but which degrades into carbon dioxide and water in the presence of microorganisms.
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