Construction of Controlled Atmosphere Fresh-Keeping Cold Storage Facilities

　　Modified Atmosphere Fresh-Keeping Cold Storage
　　Modified atmosphere storage, also known as controlled-atmosphere storage, is currently the most advanced method for preserving and storing fruits and vegetables. Building on conventional refrigeration, this method involves adjusting the composition of the storage atmosphere by precisely controlling environmental factors such as temperature, humidity, carbon dioxide concentration, oxygen concentration, and ethylene levels. By inhibiting the respiration of fruits and vegetables and slowing down their metabolic processes, this technique effectively maintains their freshness and marketability, thereby extending both their storage life and shelf life (the period during which they remain fresh on store shelves). Typically, modified atmosphere storage can extend the storage life by 0.5 to 1 times compared to conventional refrigeration. Moreover, fruits and vegetables stored in a modified atmosphere chamber, upon being removed from storage, first "wake up" from their "dormant" state. As a result, their post-harvest shelf life—i.e., the period during which they remain fresh on store shelves—can be extended by 21 to 28 days, which is 3 to 4 times longer than that achievable with conventional refrigeration.

　　Table of Contents
　　1. Overview of Controlled Atmosphere Storage Technology
　　Basic Overview
　　Traditional methods for fruit and vegetable storage and preservation include simple storage, ventilated-storage facilities, radiation-based preservation, chemical preservation, and cold-storage facilities. Simple storage and ventilated-storage facilities are easy to set up and require low investment; however, their preservation effects are poor, the storage duration is short, and spoilage losses are significant. Radiation- and chemical-based preservation methods are somewhat suitable for certain fruits, but they pose risks of radiation and chemical residue contamination, and thus cannot be applied to all types of fruits and vegetables.
　　Modified atmosphere storage, under suitable low-temperature conditions, can maximize the creation of an optimal environment for fruit and vegetable preservation by adjusting the gas composition and relative humidity of the storage environment. Its effects are evident in the following aspects: The low-oxygen environment (typically with an O2 content of 1%–5%) and appropriate CO2 concentration created by modified atmosphere storage effectively inhibit respiration, thereby reducing the loss of nutrients in fruits and vegetables. At the same time, it suppresses the growth and reproduction of pathogenic microorganisms and controls the occurrence of certain physiological disorders. By removing ethylene from the storage atmosphere, the ripening-promoting effect of ethylene on fruits and vegetables is inhibited, thus delaying post-harvest ripening and senescence. Furthermore, increasing the relative humidity of the storage atmosphere reduces transpiration in fruits and vegetables, enabling long-term preservation and freshness.
　　Characteristics of fruits and vegetables stored under controlled-atmosphere conditions
　　(1) Preserves the original shape, color, and aroma of fruits and vegetables very well;
　　(2) The fruit has higher firmness than that stored under ordinary refrigeration;
　　(3) Storage time extended;
　　(4) Low fruit rot rate and low natural loss (water loss rate);
　　(5) Prolong shelf life. Due to the prolonged exposure of fruits and vegetables to low oxygen and high carbon dioxide levels, even after the modified atmosphere conditions are lifted, these produce still exhibit a long “lag effect” or dormancy period. (6) Suitable for long-distance transportation and export. The quality of fruits and vegetables is significantly improved, creating favorable conditions for export and distribution. (7) Many types of fruits and vegetables can achieve year-round supply through seasonal production and annual sales, yielding substantial social and economic benefits.
　　The composition of a controlled-atmosphere storage facility.
　　A controlled-atmosphere storage facility typically consists of an airtight enclosure, an atmosphere-control system, a refrigeration system, a humidification system, a pressure-balancing system, and an automated monitoring and control system for temperature, humidity, O2, CO2, and other gases. Characteristics of a controlled-atmosphere storage facility:
　　According to the requirements of controlled-atmosphere storage technology, a controlled-atmosphere storage facility not only possesses the refrigeration function of a conventional cold storage facility but also features unique structural designs and operational management practices. 1) Airtightness is the most significant structural characteristic that distinguishes a controlled-atmosphere storage facility from a conventional cold storage facility. It not only demands that the enclosure structure be thermally insulated to minimize the impact of external factors on the storage temperature, but also requires the enclosure to be highly sealed to reduce gas exchange between the inside and outside of the facility, thereby maintaining a relatively stable gas composition within the storage space.
　　2. Security
　　This is a requirement that comes naturally with airtightness. During the cooling, temperature recovery, and controlled-atmosphere processes in a modified-atmosphere storage facility, fluctuations in temperature and pressure within the storage chamber will create pressure differences across the enclosure structure’s surfaces. If these pressure differences are not promptly eliminated or kept within a certain range, they could lead to damage to the storage structure itself. Ensuring both the airtightness and the structural safety of the storage facility is yet another distinctive feature of modified-atmosphere storage systems.
　　3. High storage space in the warehouse
　　Single-story modern controlled-atmosphere warehouses are almost always single-story buildings with high interior spaces. This unique architectural form is designed with airtightness and safety as primary considerations. 4. Quick inbound and outbound operations.
　　This is another key feature of the controlled-atmosphere storage facility’s operational management: rapid entry refers to the requirement for goods to be stored as quickly as possible, so that they can enter the controlled-atmosphere storage state at the earliest opportunity. Once storage is complete, it’s best to sell off the goods in the warehouse within a short period—avoiding prolonged delays—and ensuring that the controlled-atmosphere conditions are established as soon as possible.
　　5. High pile-up filled to capacity
　　This is another key feature of management practice. Aside from reserving necessary inspection passages, goods within the warehouse should be stacked as high and fully loaded as possible, minimizing the remaining empty space inside the warehouse. This reduces the volume of gas that needs to be processed, speeds up the controlled-atmosphere conditioning process, shortens the duration of the conditioning period, and enables the controlled-atmosphere state to be established as early as possible.
　　2. Controlled-atmosphere storage structure
　　The structure of a controlled-atmosphere storage facility not only needs to have excellent thermal insulation to minimize the impact of external heat on the internal temperature, but more importantly, it must exhibit superior air-tightness to reduce or eliminate the pressure exerted by outside air on the gas composition inside the facility. This ensures rapid adjustment of the internal gas composition with minimal fluctuations, thereby improving storage quality and reducing storage costs. The structure of a controlled-atmosphere storage facility primarily consists of an air-tight layer and an insulating layer. Based on their construction, controlled-atmosphere fresh-keeping facilities can be categorized into three types: prefabricated, brick-concrete, and jacket-type. Prefabricated controlled-atmosphere storage facilities use color-coated polyurethane sandwich panels assembled together for their enclosure structure, providing thermal insulation, moisture resistance, and air-tightness. These facilities are quick to construct, aesthetically pleasing, and elegant; however, they are slightly more expensive. They are currently the most commonly used type for newly built controlled-atmosphere storage facilities both domestically and internationally. Controlled-atmosphere storage facilities are equipped with specialized controlled-atmosphere doors that must possess excellent thermal insulation and air-tightness properties. Moreover, during long-term storage after the facility has been sealed, it is generally not advisable to open the controlled-atmosphere door casually, as this could lead to gas exchange between the inside and outside of the facility, causing fluctuations in the internal gas composition. To facilitate monitoring the condition of fruits and vegetables stored inside, observation windows should be provided. After the controlled-atmosphere storage facility is completed, an air-tightness test must be conducted. The air-tightness performance should reach at least 300 Pa, with a half-pressure decay time of no less than 20–30 minutes.
　　3. Controlled Atmosphere System
　　To ensure that a controlled-atmosphere cold storage facility achieves the required gas composition and maintains relative stability, in addition to a well-sealed enclosure meeting the specified requirements, a corresponding system comprising gas-adjusting equipment, pipelines, and valves—known as the controlled-atmosphere system—is also necessary. The entire controlled-atmosphere system includes an oxygen removal unit or nitrogen generation system, a carbon dioxide removal system, an ethylene removal system, and an automatic monitoring and control system for temperature, humidity, and gas composition.

　　1. Deoxygenator
　　This is currently the most advanced oxygen-reduction equipment for controlled-atmosphere storage facilities. Its operating principle involves using a fan with a pressure lower than 24 kPa to circulate and remove oxygen, followed by activation and desorption using a vacuum pump. The motor employs variable-frequency speed control technology. This technology is often mistakenly thought of as VSA nitrogen generators. The key difference between this oxygen removal machine and a VSA nitrogen generator lies in their power sources: while a VSA nitrogen generator still relies on compressed air—albeit at relatively low pressure—the presence of oil and gas in the air source can lead to degradation of the VSA generator's feedstock. In contrast, the oxygen removal machine uses an oil-free, low-pressure fan, ensuring that the feedstock remains free from oil contamination. Moreover, its circulating air volume is more than five times greater than that of a VSA nitrogen generator. This oxygen-reduction equipment boasts an efficiency 40% higher than membrane nitrogen generators and PSA nitrogen generators, and 30% higher than VSA nitrogen generators. It also achieves energy savings of 40% compared to conventional nitrogen generators. Currently, only a handful of companies in Italy and Germany possess the technology for oxygen removal machines. In China, only Tianjin Jiesheng Company has achieved a technological breakthrough in this field. Their oxygen removal machines even slightly outperform foreign brands in terms of oxygen-reduction capacity, and their process is highly mature and has been widely adopted in numerous controlled-atmosphere storage facilities.
　　2. Nitrogen Generation System
　　Nitrogen generators have generally gone through a development process that includes catalytic combustion-based nitrogen generation, nitrogen generation via carbon molecular sieve adsorption, nitrogen generation via hollow-fiber membrane separation, and nitrogen generation via vacuum low-pressure adsorption deoxygenation (i.e., VSA). Currently, carbon molecular sieves, hollow-fiber membrane separation, and VSA-based nitrogen generation are widely adopted.
　　2. Carbon Molecular Sieve Nitrogen Generators: Carbon molecular sieve nitrogen generation employs the principle of pressure swing adsorption to produce nitrogen. Since oxygen molecules and nitrogen molecules have different kinetic diameters, oxygen molecules diffuse hundreds of times faster than nitrogen molecules. Moreover, the amount of gas adsorbed is directly proportional to the pressure. Leveraging the significant difference in adsorption capacity between oxygen and nitrogen over short periods, a programmable controller rapidly switches between two adsorption towers according to a predefined time sequence. By combining pressurized oxygen adsorption with depressurized oxygen desorption, oxygen is effectively separated from air. Carbon molecular sieve nitrogen generators boast high nitrogen purity (up to 99.9%), simple equipment design, and low cost. However, these units feature numerous valves that undergo frequent switching—each valve may be opened and closed 200,000 to 400,000 times per year—resulting in considerable noise levels. Therefore, it is crucial to ensure the quality of the valves; otherwise, the reliability of the equipment could be compromised.
　　2.2 Hollow-fiber membrane nitrogen separation systems utilize the difference in permeation rates of oxygen and nitrogen through the walls of hollow-fiber membranes to separate oxygen from air. Currently, hollow-fiber membrane nitrogen generators are the most widely used equipment for controlled-atmosphere storage. Such systems consist of a compressor, an air tank, a refrigerant dryer, filters, a heater, hollow-fiber membranes, pipes, and valves. They feature the following characteristics: (1) Simple design, compact footprint, and easy installation; (2) Only the air compressor needs to be started to produce nitrogen-enriched air; (3) The nitrogen concentration can be adjusted between 95% and 99%, offering flexible operation and enabling quick start-up and shutdown; (4) Safe and reliable—since the separator contains no moving parts, it can operate continuously and stably; (5) The separation process involves no phase change, no pressure loss, and low energy consumption; (6) Easily scalable for smaller sizes; (7) Low investment cost.
　　2. The vacuum low-pressure adsorption deoxygenation nitrogen generator utilizes the principle of CMS activated carbon adsorption and regeneration to remove O2 from the atmosphere and inject high-purity nitrogen into the storage chamber. It consists of two tanks filled with CMS activated carbon, a pump assembly, valves, piping, and a control unit. It features the following characteristics: (1) Operates at low pressure (0.8 bar), achieving energy savings of approximately 80% compared to PSA and membrane nitrogen generators with similar performance. (2) Improves oxygen reduction efficiency by more than 30%, enabling the oxygen content in the controlled-atmosphere storage chamber to be maintained below 1%, and even down to as low as 0.3%. (3) Low maintenance costs and high reliability. The key activated carbon adsorption module in the equipment has a service life of over 2–3 years. (4) Works more effectively in conjunction with the storage chamber’s leak-sealing system to prevent gas leakage from the controlled-atmosphere storage chamber. (5) Features internal gas circulation within the controlled-atmosphere storage chamber, further enhancing operational cost savings.
　　3. Carbon Dioxide Removal System
　　It is primarily used to control the carbon dioxide content in controlled-atmosphere storage facilities. By relying entirely on the carbon dioxide released during the respiration of fruits and vegetables, the system increases the CO₂ concentration within the storage facility. An appropriate level of CO₂ provides protective effects for fruits and vegetables, ensuring excellent storage and preservation results. However, excessively high CO₂ concentrations can harm these produce items; therefore, removing (or washing) excess CO₂ and carefully adjusting and controlling the CO₂ concentration are crucial for improving the quality of fruit and vegetable storage. Typically, CO₂ removal devices come in four main types: (1) slaked lime removal systems; (2) water-based removal systems; (3) activated carbon removal systems; and (4) silicone rubber membrane removal systems. Among these, the activated carbon removal system leverages the strong adsorption capacity of activated carbon to capture CO₂. Once the activated carbon reaches saturation, fresh air is introduced to desorb the CO₂, thereby restoring its adsorption performance. This type of system is currently widely adopted for CO₂ removal in controlled-atmosphere storage facilities. The operational capacity of a CO₂ removal system should be determined based on factors such as the respiration rate of the stored fruits and vegetables, the free gas volume within the controlled-atmosphere storage facility, the storage capacity of the facility, and the target CO₂ concentration required inside the storage facility.
　　4. Ethylene Removal System
　　Ethylene is a gas naturally produced and released by fruits and vegetables during ripening and post-ripening. It is a plant hormone that promotes respiration and accelerates post-ripening, thereby having a ripening-promoting effect on fruits stored after harvest. In the storage of fruits sensitive to ethylene, it is essential to remove ethylene completely. Therefore, in fruit and vegetable storage, efforts must be made both to inhibit ethylene production and to eliminate ethylene accumulation within the storage facility. Currently, two methods are widely used and relatively effective: the potassium permanganate chemical ethylene removal method and the air oxidation removal method. The chemical ethylene removal method involves filling washing equipment with an ethylene-absorbing agent. A commonly used ethylene absorber consists of a saturated potassium permanganate solution adsorbed onto porous materials such as crushed bricks, vermiculite, or zeolite molecular sieves. When ethylene comes into contact with potassium permanganate, it is oxidized and thus removed. This method is simple and extremely low-cost; however, its ethylene removal efficiency is relatively low, and potassium permanganate, being a strong oxidizing agent, can cause skin irritation. At present, the air oxidation removal method relies on the principle that ethylene reacts with oxygen under catalytic conditions and at high temperatures to produce carbon dioxide and water, thereby removing ethylene. Compared to the potassium permanganate method, this approach requires higher initial investment costs. Nevertheless, it has gained widespread acceptance due to the following distinct advantages:
　　(1) The ethylene removal efficiency is high, capable of eliminating 99% of the ethylene content in the air within the storage facility, thereby keeping the ethylene concentration inside the storage room at a level of 1–5 μL/L. (2) It reduces fruit spoilage by simultaneously removing ethylene and performing high-temperature sterilization and disinfection of the air within the storage facility. (3) This device is multi-functional: while removing ethylene, it also eliminates aromatic gases released by fruits, thereby mitigating the adverse effects of these gases that would otherwise accelerate fruit ripening.
　　Note: With the exception of ethylene-sensitive fruits (primarily subtropical and tropical fruits) such as kiwifruit and bananas, temperate fruits like apples and pears do not require ethylene removal equipment.
　　Currently, the more advanced ozone-based ethylene removal technology is gradually replacing high-temperature catalytic ethylene removal systems. The greatest advantage of this ethylene removal technology is that it operates at low temperatures without causing fluctuations in storage temperature. Moreover, its power consumption is only 500 watts—just one-tenth of that consumed by high-temperature catalytic ethylene removal systems. The primary function of the automated detection and control system in a controlled-atmosphere storage facility is to continuously monitor and display real-time data on temperature, humidity, O2, and CO2 levels within the storage chamber, ensuring that these parameters meet the specified controlled-atmosphere technical requirements and enabling automatic (or manual) adjustments to maintain optimal controlled-atmosphere conditions. In modern controlled-atmosphere storage facilities with a high degree of automation, automated detection and control equipment is typically employed. Such equipment consists of sensors for (temperature, humidity, O2, and CO2), controllers, computers, sampling tubes, valves, and other components. The entire system is centrally managed by a single computer that enables remote, real-time monitoring. This central computer can acquire O2, CO2, temperature, and humidity data from each individual sub-storage chamber, display operational curves, automatically print records, and remotely start or stop various system components. Additionally, the central computer can adjust control parameters in real time based on the specific conditions of the stored materials. The central control computer features a Windows-based user interface, allowing operators to access all relevant information conveniently and intuitively.
　　6. Refrigeration System
　　A refrigeration system is a closed-loop system composed of machinery, equipment, and piping, valves, control components, and other elements necessary for achieving mechanical refrigeration. The refrigeration system used in controlled-atmosphere storage facilities is essentially the same as that used in conventional cold storage facilities. However, the refrigeration system in controlled-atmosphere storage facilities boasts higher reliability, a greater degree of automation, and the ability to maintain the required internal temperature over extended periods during the controlled-atmosphere storage of fruits and vegetables. Typically, either an ammonia refrigeration system or a single-stage compression direct-expansion refrigeration system using fluorocarbons is employed.

　　7. Humidification System
　　Compared with conventional refrigerated storage facilities for fruits and vegetables, controlled-atmosphere storage allows for longer storage periods but also results in higher moisture loss from the produce. To inhibit moisture evaporation and reduce the vapor pressure difference between the storage environment and the stored fruits and vegetables, the controlled-atmosphere storage facility must maintain an optimal relative humidity level. This is crucial for minimizing moisture loss and preserving the crispness and freshness of the produce. Generally, it is best to keep the relative humidity inside the storage facility within the range of 90% to 95%.
　　The commonly used humidification methods for controlled-atmosphere storage facilities include the following:
　　(1) Ground-level water-based humidification;
　　(2) Fill the chiller base with water;
　　(3) Spray humidification;
　　(4) Centrifugal atomization humidification;
　　(5) Ultrasonic atomization humidification.
　　8. Controlled Atmosphere Storage Pressure Balancing System
　　In the structural design of controlled-atmosphere fresh-keeping cold storage facilities, the safety of the facility must also be taken into account. Since a controlled-atmosphere cold storage facility is a sealed-type cold storage, as the temperature inside the facility drops, the gas pressure within it also decreases, thereby creating a pressure difference between the inside and outside of the facility.
　　According to relevant data, when the temperature difference between inside and outside the storage facility reaches 1℃, the atmosphere will exert a pressure of 40 Pa on the enclosure structure. The greater the temperature difference, the larger the pressure difference will be. If this pressure difference is not promptly eliminated or kept within a certain range, it could damage the storage facility. To ensure the safety and airtightness of the controlled-atmosphere storage facility and to provide the necessary convenience for its operation and management, such facilities should be equipped with a pressure-balancing system, including safety valves and buffer gas storage bags. The safety valve is a unique safety device designed to maintain pressure equilibrium between the inside and outside of the controlled-atmosphere storage facility once it is sealed. It prevents excessive positive and negative pressures from building up inside the facility, thereby protecting the enclosure structure and its airtight layer from damage.
　　During operation, modified atmosphere storage facilities may experience slight pressure imbalances. The function of the buffer gas bag is to eliminate or mitigate these minor pressure imbalances. When the pressure inside the storage facility slightly exceeds atmospheric pressure, some of the internal gas flows into the buffer gas bag. Conversely, when the pressure inside the facility slightly falls below atmospheric pressure, the gas from the buffer gas bag automatically replenishes the modified atmosphere chamber. The gas bag converts minor fluctuations in the storage facility’s internal pressure into corresponding changes in the volume of gas contained within the bag, thereby reducing or nearly eliminating the pressure differential between the inside and outside of the facility and minimizing—or even neutralizing—the forces exerted on the building envelope due to this pressure difference. The buffer gas bag is made from a flexible material that boasts excellent air-tightness and a certain level of tensile strength.