The present invention relates to oxygen control systems and more particularly to an oxygen control system for controlling the level of oxygen in fresh food storage containers.
The advent of containerized refrigerated cargo transport has allowed fresh agricultural products to be shipped to ever more distant markets and has enabled the transfer of containers between different modes of transport (e.g., truck to ship to truck) as required without rehandling of the cargo. Insect infestation of raw agricultural products is a universal problem in produce storage, processing and shipping that must be effectively addressed to minimize health concerns and to maintain an appetizing appearance, shelf life and marketability. Cargos that cross national boundaries are also generally subject to stringent regulations for all the same reasons as well as to prevent the importation of harmful new agricultural pests. Compared to losses in the field, the losses become increasingly significant during transportation as further value is added to the product in harvesting, packaging and shipping. Disinfestation can be achieved by heating or freezing but not without some sacrifice in quality, and the use of insecticides and fungicides is becoming increasingly unacceptable.
The use of containerized cargo transport in which containers carry a dedicated refrigeration unit ("reefers") has presented the opportunity to employ controlled-atmospheres for preservation of perishable products. Controlled-atmosphere (CA) shipment of food products and other perishables for preservation has received significant attention in recent years. For example, Transfresh Corporation has developed technology to effectively "seal" reefers and to sufficiently control the oxygen and other gas composition within the container to extend the fresh shelf-life of many agricultural and perishable products. Recently, systems have been developed for active continuous removal of oxygen leaking into "sealed" reefers to very low levels (&lt;1000 ppm) to extend CA technology to the shipment of fresh meat and fish products.
The principle of hermetic storage has been known from pre-industrial times and has been widely implemented for grain, bean and oil-seed storage in underground pits; the first large scale tests were run in Paris from 1819 to 1830. This principle has been steadily improved upon and extended ever since. Existing oxygen removal systems for shipment of fresh meat and fish are on-board systems that actively maintain oxygen at the lowest level possible, compensating for any air leakage throughout the shipping period (2-3 weeks). On-board systems need to be small for the least product displacement and for conservation of power; this is achieved by pre-flushing the container, tight sealing against leaks and pressure control to reduce the initial power demand and the subsequent load on the system. This very low concentration of oxygen would generally be lethal to most insects but it is not generally compatible with produce that respires. Iceberg lettuce, for example, requires &gt;1200 ppm of oxygen to prevent spoilage, but, &lt;1800 ppm at 36.degree. F. over a three-day exposure to kill green peach aphids; this is followed by 10 days at 2% O.sub.2 in transit, the normal CA shipping environment. The requirements for a disinfestation system then would be:
1) Controlled oxygen levels rather than minimum oxygen maintenance. PA1 2) Continuous oxygen concentration measurement of sufficient resolution to set the required oxygen levels for a variety of products.
The specific technology solutions that have been used to address the above problems will now be discussed.
a. Modified-Atmosphere Disinfestation.
Hermetic storage has long been used for disinfestation of grain, beans and oil-seeds stored in underground pits. This process involves sealing the product in a pit, bin or silo and allowing the respiration of the stored product to reduce the oxygen to a level lethal to insects. Primitive versions of hermetic storage are in active use in many third world countries with mild winters, and more modern concrete storage containers are in continuous use for corn storage in Cyprus and Kenya to provide famine protection. This concept of modifying atmospheres was extended in the 1860s by adding nitrogen or "burned air" to grain storage silos. However serious interest in using the technique in a practical, routine manner was not pursued until the 1950s and 1960s. Until this time conventional fumigants and grain protectants had been successful in controlling stored-product pests. At that time, people began to realize that the chemicals, if used improperly, left objectional residues and were hazardous to apply. There was also a potential for the development of insect resistance to these chemicals. Research and development in controlled atmosphere disinfestation has been on-going since the 1960s with considerable success. It has been found that low temperatures, low oxygen and high carbon dioxide separately or in various combinations can be effective for disinfestation. For fresh produce there is the additional requirement to maintain sufficient oxygen to prevent acceleration of ripening or spoilage. The conditions of atmosphere and exposure time to achieve disinfestation without excessive ripening tends to be complex and unique to the combination of agricultural product and indigenous pest(s).
A method and apparatus for storing foodstuffs using an electrolytic cell is described in U.S. Pat. No. 4,212,891 issued to Fujita and Kudo. This apparatus included a liquid alkaline electrolyte electrolytic stack and system, but did not include integral reactor feedback sensing. Moreover, with an alkaline system, the ambient CO.sub.2 reacts with the electrolyte and frequently there is liquid electrolyte leakage into the seal and manifold area, leading to competing reactions and shunt currents. Also, at near-ambient temperatures, water and electrolyte concentration management is difficult with a liquid electrolyte system. This makes feedback sensing and control difficult, especially if the same stack would be used for oxygen depletion, monitoring and control.
b. Controlled-Atmosphere (CA) Technology.
The Tectrol.RTM. CA systems developed by Transfresh and in wide use (more than 22,000 CA-equipped reefers worldwide) are implemented in three stages: 1) sealing of specially equipped containers after loading; 2) inert-gas flushing down to 1-2% oxygen; and 3) controlled leakage of air and removal of carbon dioxide to compensate for respiration, based on regular oxygen and carbon dioxide concentration measurements during shipment. The addition of oxygen is accomplished with a unique air-exchange system; a slide valve inside the Tectrol controller, which is mounted in an access panel at the front of the reefer, allows precise amounts of pressurized air generated by the reefer fan to be pushed out through an air-exchange outlet port and outside air to be drawn into the low pressure side of the fan through an air-exchange inlet port. The removal of carbon dioxide is accomplished by periodically circulating the reefer atmosphere through a lime bed based on carbon dioxide sensor measurements. The atmosphere thus achieved lowers respiration extending the fresh life of the produce over a longer period of time but it is not typically adequate to effect disinfestation.
It is therefore a principal object of the present invention to provide a proton-exchange-membrane based electrochemical oxygen control system that can deplete, monitor and control oxygen in a closed container to levels sufficient to achieve disinfestation.
It is another object to the present invention to provide a proton-exchange-membrane based electrochemical oxygen control system that can provide feedback sensing to control oxygen levels using the same electrochemical cells stack for oxygen depletion, monitoring and control.