The Controlled Atmosphere

Controlled atmosphere is a term used until the late 2000s to refer to any of the following processes:

- Controlled atmosphere.- Modified atmosphere.- Controlled environment.- Packaging by injecting gas.- Vacuum packaging.- Vacuum packaging with adhered film.

Today there are differences between these processes, being:

1.Controlled atmosphere

The controlled atmosphere is a refrigeration technique of conservation in which it intervenes by modifying the gaseous composition of the atmosphere in a cold room, in which a control of regulation of the physical variables of the environment (temperature, humidity and air circulation) is carried out. Controlled atmosphere (CA) is understood as the preservation of fruit and vegetable products, generally in an atmosphere depleted in oxygen (O2) and enriched in carbon dioxide (CO2). In this case, the composition of the air is precisely adjusted to the requirements of the packaged product, remaining constant throughout the process. This technique associated with cold, accentuates the effect of cooling on the vital activity of tissues, avoiding certain physiological problems and reducing losses due to rots. The action of the atmosphere on the respiration of the fruit is much more important than the action of low temperatures. This controlled atmosphere slows down the biochemical reactions causing a greater slowness in respiration, delaying ripening, being the fruit in latent conditions, with the possibility of a vegetative reactivation once the fruit is placed in normal atmospheric air.

2.Modified atmosphere

The technique is based on the use of nitrogen alone or mixed with carbon dioxide, and on the reduction of the oxygen content to levels normally below 1%. The modified atmosphere is achieved by vacuuming and subsequent reinjection of the appropriate mixture of gases, in such a way that the atmosphere achieved in the container varies over time depending on the needs and response of the product. In the modified atmosphere packaging (EAM) technique, four basic components must be taken into account: the packaging used, the gas mixture, the packaging materials and the packaging equipment; all of them conditioned in turn by the nature of the product to be packaged. The normal composition of the air used in the EAM is 21% oxygen, 78% nitrogen (N2) and less than 0.1% carbon dioxide. CO2 is a highly soluble gas in water and with bacteriostatic and fungistatic properties, which slows the growth of fungi and aerobic bacteria. CO2 acts by lengthening the vegetative phase of microbial growth. Carbon dioxide is not totally inert and can influence the color, consistency and other attributes of vegetable quality.

CO2 concentrations must be between 20 and 60%, and its action at low temperatures is more effective. In modified atmosphere packaging, efforts are made to reduce the oxygen content as much as possible in order to reduce the deterioration of the products due to oxidation. Nitrogen is characterized by being an inert gas. The use of N2 prevents the collapse of the containers in those cases in which the product absorbs CO2. The factors that affect the intensity of these processes and the conditions of handling and marketing, must be taken into account to design the characteristics of the system: product-packaging-environment. Therefore, for packaging in a modified atmosphere, a polymer film with suitable permeability characteristics must be selected. The use of films of different permeability will lead to the formation of an atmosphere of different equilibria and therefore the evolution of the fruits will also be different.  The individual wrapping of the fruits with a retractable film forms a second external sheet of protection and a microatmosphere around the fruit. This barrier prevents moisture loss, protects against the spread of rot and improves hygienic conditions in handling.

3.Controlled environment

The controlled environment implies total control, not only of the gases in the atmosphere but also of the temperature, relative humidity content, etc., during the distribution phases.

4.Packaging by injecting gas

Gas packaging consists of dragging the air inside the container and replacing it with another gas, such as carbon dioxide or nitrogen. In this packaging, the air is physically displaced and the internal atmosphere may or may not be totally modified. In commerce, this packaging is generally used to remove oxygen from inside the packaging of very low humidity granulated products, such as coffee, or from the headspace of oxygen-sensitive liquid products, such as juices.

5.Vacuum packaging

Vacuum packaging consists of the total elimination of air from inside the container without it being replaced by another gas. In vacuum packaging, there is a pressure difference between the outside and inside of the container. Therefore, when the container is rigid, such as a metal or glass container, the effect of the pressure difference could lead to the entry of air or microorganisms. In the case of semi-rigid containers, the pressure difference can cause the collapse of the container and subsequent damage to the product upon contact with it, as well as the appearance of leaks. Metabolically active vacuum-packed foods, such as meats or mixed salads, continue with their respiratory activities, thus consuming the small amount of oxygen present in the tissues of the product, which increases the vacuum and produces carbon dioxide and water vapor. From a practical point of view, the vacuum packaging of a metabolically active product is therefore transformed into controlled atmosphere packaging. For almost two decades, vacuum packaging has been the method of choice for large beef and pork meat pieces and is a technique that is still used for the packaging of some meat pieces intended for retail.

6. Vacuum packed with bonded film

The chosen packaging material must be able to keep the gas mixture constant, preventing the entry of oxygen and the leakage of carbon dioxide. It is also important that it has the characteristics of anti-fog and permeability. With the quality of anti-fog we prevent water droplets from water vapor from condensing on the inner surface of the container. The welding of the containers, in addition to being resistant and waterproof, must facilitate the opening of the bag. The following will be described in summary form the different types of plastic films that are currently used in the packaging of fresh fruits and vegetables.

6.1. LAMINATED FILMS.

These films are made up of sheets of different materials joined by an adhesive, in the form of a sandwich. Laminated films offer a better engraving quality since the printed surface is incorporated between the numerous sheets that constitute them and this prevents wear during handling. The disadvantage of this type of film is that the manufacturing process is expensive, which makes this type of materials not widely used. Laminated films have excellent engraving quality by being usually printed on the back over the polypropylene and embedded in the film. They are usually used with products of low or medium respiratory activity, since the layers interfere with the mobility of oxygen to the interior of the container.

6.2. BUILT FILMS.

They are characterized by being sheets produced simultaneously that are joined without the need for adhesive. They are cheaper than laminated films, however the latter seal better, as the polyethylene melts and rebuilds more safely. Built films are etched on the surface and tend to wear out with machinery during filling and sealing. The rate of oxygen transmission into the container is higher than in laminated films.

6.3. MICRO PERFORATED FILMS.

They are used in those products that require a high oxygen transmission rate. These are films containing small holes about 40-200 microns in diameter that run through the film. The atmosphere inside the container is determined by the total area of perforations on the surface of the container. Micro-perforated films maintain high relative humidity levels and are very effective in prolonging the half-life of products that are particularly sensitive to losses due to dehydration and deterioration by microorganisms.  6.4. MICROPOROUS MEMBRANES.

The microporous membrane is used in combination with other flexible films. It is placed on a film impermeable to oxygen which has a large perforation. In this way it is achieved that all gas exchanges occur through the microporous membrane, which has pores of 0.2-3 microns in diameter. The rate of oxygen transmission can be varied by changing its thickness or by modifying the number and size of the micro pore that make up the membrane.

6.5. SMART MOVIES.

Encompassed within the so-called active containers, they are those that are formed by membranes that create a modified atmosphere within it and that ensure that the product does not consume all the oxygen inside and becomes an anaerobic atmosphere. These smart membranes or films prevent the formation of unpleasant tastes and odors, as well as the reduction of the risk of food poisoning due to the production of toxins by anaerobic microorganisms. These sheets are capable of withstanding variations in storage temperature between 3o to 10oK and increase gas permeability (oxygen transmission rate) a thousand times when the temperature increases above the established limit temperature, avoiding the appearance of anaerobiosis processes.

6.6. THE FLOW-PACK

The flow-pack is a packaging system that is applied to numerous products. The container consists of a sheet of film, usually polypropylene, which the machine forms and seals to form the container. It is characterized by a longitudinal suture in the center and sutures at the front and rear ends. In horticultural products, this type of packaging can be used with or without a tray, as is the case with strawberries and tricolor peppers respectively. After clarifying the concept of controlled atmosphere (modern), let's see what are the special constructive characteristics of the chamber and the type of equipment and accessories. Let's start from the base that the chamber is usually built with sandwich insulating panels of high density and mechanical resistance. The chamber must withstand the pressure difference of 25 millimeters water column (mm.c.w.) between inside and outside. In addition several additional equipment such as:

A. Carbon dioxide (CO2) absorbers

CO2 absorbers have been developed for the removal of CO2 from cold rooms. In addition, the absorber has the quality of removing a part of the ethylene produced (C2H4). The absorber consists of a container filled with activated carbon, a fan, an air conduction system and a command part. Operation

Air from the chamber is regularly sent through the activated carbon filter. The CO2 and C2H4 molecules adhere to the activated carbon and disappear from the chamber's atmosphere. This process is called absorption. After performing a few absorption actions, the activated carbon is saturated and can no longer eliminate the CO2 and C2H4 molecules. Activated carbon should be cleaned with outside air to facilitate the removal of absorbed gases. This process is called Regeneration. These systems work completely automatically, the regeneration is carried out continuously, without having to intervene. The absorber has a built-in panel for use, so that it can be programmed comfortably and easily by camera. If the absorber is attached to an analysis system, desired CO2 values can be programmed. If these values are exceeded, the absorber is activated. If the system detects a fault, it will be marked optically and acoustically.

B. Ethylene catalysts (C2H4).

An ethylene catalyst is used to remove ethylene from cold rooms.

Ethylene gas is produced by the products that breathe accelerating the maturation process, until it reaches rot. It also causes an accelerated aging process. The removal of this gas allows a longer shelf life of the products. It also allows to proceed to ripen the fruit at the time you want, providing ethylenic gas to the chamber.

Operation The catalyst has two columns each has a heat storage medium with platinum catalyst, two heat elements and a fan. The air of the chamber to be treated is guided by one of the columns heating it. Then the air is guided with a high temperature by the catalyst, 300° celcius, in which the ethylenic gas decomposes. Then the air flow is passed through the again, to remove the remaining ethylene molecules. The air is cooled and returned to the chamber. As it is a decomposition operation, there is no need for any regeneration. The operation allows to decompose the ethylene present up to a level of 1 ppb (parts per billion). With its heat recovery system and the mastery of a necessary optimum temperature, our catalyst consumes only the indispensable energy. The ethylene catalyst is easy to use through the use panel located on the outside of the frame. The catalyst can be used through a personal computer (PC) if it is connected to a PC analysis system. If the system detects any fault, it is acoustically and optically signaled. There is also another equipment to remove ethylene, the OXTOMCAV, this equipment, works with filters of the ionized type, which ionize the molecules that pass through the filter, thus decomposing said molecules and transforming it into secondary compounds such as oxygen (ionized oxygen) and water vapor.

When it reaches 5%, the burner stops definitively, since the same fruit is responsible for lowering the level of O2 since it needs to breathe absorbing O2 and releasing CO2. By means of a faster rate of lowering the percentage of oxygen, a system called nitrogen gasifier was created.

C. Oxygen Burner ( O2 )

The function of the oxygen burner is to absorb the air from the environment by means of a turbine and channel it to the boiler, burning the O2 at a temperature of 80° celcius with which it is reduced from 21% to 5% of O2 and increasing the CO2 from 0.03% to 13.5% in which it enters the chamber in process, modifying its atmosphere. This process takes about 4 to 5 days to lower the percentage of oxygen from 21% to 5%.

D. NITROGEN GASIFIER: For 7 years, the quality differences obtained by establishing the controlled atmosphere on a day with N2 scanning were compared, versus lowering O2 levels between 4 to 10 days with the oxygen burner, depending on the size of the chamber. It was observed that considerably the fruit is in better condition. Proof of this, we can say that this nitrogen sweeping technique is much more reliable and better than the oxygen burner, being replaced by it.  The decrease in O2 is produced by scanning with pure N2 so that the atmosphere can be established in a few hours. This system has some additional advantages, since together with sweeping the O2, the N2 displaces the excess CO2 and ethylene without injecting volatile products into the chamber.

OperationThe injection is carried out from equipment external to the refrigerator. A mobile or fixed nitrogen (thermos) reservoir that has a temperature of approximately -196° celcius (at normal atmospheric pressure) in a liquid state and takes it to a gasifier, passing through it, which takes it to a temperature appropriate for injection of approximately 5°celcius. The only installation required is a network of pipes that go inside the chambers with nitrogen, through a valve that can be remote control or integrated into the command equipment.

Advantages of this System

The sweeping of the cameras with N2 presents a series of advantages that are very attractive for the user:

(a) High speed of establishment of the atmosphere; what the generator achieves in days the sweep with N2 obtains only in hours.b) The injection into the chamber is clean, free of hydrocarbons that may affect the fruit.c) The injection of gas at low temperature does not require the cold equipment.d) The system is simple, safe and does not require maintenance.e) In case of failure of the seal of the chamber or the CO2 absorber,   this system allows you to quickly set O2 levels.

E. MEASURING DEVICES:

To achieve a guarantee of success in the conservation of AC, it is essential to be able to accurately measure and analyze the air in the chamber. Reliable measuring and analysis devices are essential tools. Sensors are the newest generation of enduring quality. Stable, precise and with a fast reaction time and minimal energy consumption.

F. GAS ANALYZER:

The gas analyzers have a ceramic sensor incorporated for each of the different gases, for a range of 0% up to 25%. Normally this type of analyzers carry 3 sensors, one of oxygen, another of carbon dioxide and another of ethylene. These analyzers can be supplied in wall-mounted, portable or integrated version in the analysis systems, or simply by control through a processor by means of PLC (programmable logic controller).

G. OVER PRESSURE SAFETY VALVE:

As a security measure of the chambers in case of injection of nitrogen and oxygen. When the chamber was completely insulated in a hermetic way, an overpressure would arise and oxygen would come out of this valve. This valve must be open during the operation of the oxygen burner or nitrogen gasifier, since through this valve the oxygen existing inside the chamber comes out that is pushed by the same nitrogen. Once 5% oxygen is reached, this valve must be closed.

H. DEPRESSION SAFETY VALVE:

As a safety measure to avoid depressions in the chambers and as a safety valve of the nitrogen reserve lung. This is necessary to prevent a structure drop because oxygen would look for the easiest side to escape, breaking the roof if necessary, that is why the depression valve exists. This is one of the most worrying conditions, when a chamber encounters an oxygen depression, that is why respirators called lungs are placed on top of the chamber.

I. U-GAUGE:

J. PRESSURE DIFFERENCE ABSORBERS:

To avoid outside air inlets into the chamber, it is necessary to place compensatory lungs and pressure balancing valves, which prevent major modifications of the interior atmosphere in the chamber for any reason. When closing the flapper, over pressure safety valve; The access valve of the respiratory lungs opens. This occurs when the percentage of oxygen reaches 5%.

The photo shows the air-compensating lungs, which are located at the top of the chamber.

K. TIGHTNESS OF THE CAMERAS:

In chambers with very low O2 atmospheres, an adequate tightness or tightness is especially necessary to limit the entry of external air into the chamber, below the levels of respiratory oxygen consumption that the fruit or vegetable itself is able to carry out. For this, various materials are used to ensure the achievement of a hermetic layer throughout the perimeter of the chamber, without forgetting, either, the structural characteristics of the walls, the pavement, the doors and all the ducts and pipes that penetrate from the outside to the interior of the enclosure. The main sealing materials used are: plastic fabrics, polyester, polyurethane and metal coatings. Each system has its advantages and disadvantages and, in general, until after the first years of operation, no problems are detected. In this sense, it is mandatory to periodically perform airtightness tests in order to diagnose and correct any cause of poor tightness.

Operating Procedures for handling in controlled atmosphere chambers:

1.0.- Before each process:1.1.- Washing and disinfecting of floor and wall.1.2.- Calibrate environment and pulp sensors.1.3.- Calibrate gas analyzer.1.4.- Inspect PVC ducts (the connections between chamber and equipment).1.5.- Perform pressure test (from 30 to 10mm of water column.) for 30 minutes, thus seeing the tightness of the chamber.2.0.- Before the closing of the chamber:2.1.- The chamber must be filled to its maximum capacity.2.2.- Verify correct stowage of bins in chamber.2.3.- Cover the upper run of bins with plastic. 2.4.- Install upper walkways.2.5.- Verify that the sample is inside the chamber, in a place of easy access, no more than 5 meters from the upper or lower hatch. 2.6.- Energize cold system with fast speed of evaporator fans. 2.7.- Verify the operation of the gas valves for the controlled atmosphere and the safety valves.2.8.- Before the total closure of the chamber, the pulp temperature must be a maximum of 3° Celsius (fruits), depending on the product this value changes.2.9.- Indicate the danger conditions due to low percentage of oxygen. 2.10.- Seal door and access hatches.3.0.- Once the chamber is closed:3.1.- Once the controlled atmosphere chamber is closed and sealed, place the evaporator fans are passed at low speed.3.2.- Program the percentages of oxygen and carbon dioxide for the automatic work of the CO2 absorber, and the nitrogen generator, that will depend on the type of product.3.3.- Program the timers for the automatic work of the ethylene catalyst, if the type of product so warrants.3.4.- Turn on O2 burner, or the nitrogen gasifier, ethylene catalyst and CO2 absorbers.3.5.- Perform the sweep with nitrogen, or with the O2 burner, (according to the requirement of the installation),   until it reaches 5% of O2 (approx).3.6.- Reaching 5% oxygen, close Flapper valve and open valve of the lungs.3.7.- Control, measure and record, every four hours the percentages of O2, CO2, C2H4, pulp and ambient temperature and relative humidity (HR%).3.8.- Check every 15 days the calibration of the gas analyzer.4.0 Opening of the chamber:4.1.- Stop, if active, the burners and absorbers of CO2 and C2H4.4.2.- Open hatches, without stopping the cold, until the oxygen concentrations of the chamber are equalized with respect to that of the outside (environment 21% of approximate oxygen).4.3.- Ventilate the chamber to evacuate the high levels of CO2 and nitrogen.4.4.- Signal the danger during the process of stabilization of the gases in the chamber.4.5.- Fulfilled the above, open the main door of the chamber for adequate natural ventilation.4.6.- Remove sensors, disassemble the walkway and remove upper plastic.4.7.- Archive a summary of all the parameters that were being controlled (temperatures, gas concentrations, etc.).

ADDITIONAL SECURITY MEASURES

Due to the lack of oxygen inside the chamber, personnel entering the chamber are exposed to great risks. To prevent any accident resulting from these environmental conditions, the personnel who enter must take into account the following safety recommendations:

a) When the operator enters a controlled atmosphere chamber in regime he must do so with autonomous breathing equipment.b) Whenever an autonomous equipment is occupied, then it must be filled with air before it is occupied again.c) A second operator must be present inside the chamber, also with autonomous equipment, for a possible risk of his partner.d) There must be visual communication with a third operator from the outside in at all times, they are even tied to the waist to be attached.e) The time of staying inside the chamber must not exceed 50% of the duration of the air tube.f) Do not enter alone and without authorization to the controlled atmosphere chambers in process.g) Do not enter the controlled atmosphere chamber with little air in the cylinders of the autonomous equipment.h) Do not enter the chamber to make repairs of any kind.

As has been observed, the controlled atmosphere technique is a complement to refrigeration systems and highly recommended for products that breathe such as fruits and vegetables (after being harvested), since the controlled atmosphere delays the process of metabolism of the fruit, with which the life time becomes much longer during conservation and after conservation, maintaining a product in optimal marketing condition; in the physical aspect, flavor, texture and maturity. Do not forget that there are two ways to remove oxygen, one by nitrogen sweep and the other by the oxygen burner. Per oxygen burner it takes about 6 days, compared to the nitrogen generator that does it in hours, and at the same time the CO2 absorber is energized, that the function of this is to remove the CO2 and that an inert gas enters. The ethylene catalyst is also energized to remove the ethylene produced by the product (in those that generate ethylene), since we know that it is harmful within the atmosphere.

The biggest disadvantage is the high cost since the technology used asks us to add equipment that controls the atmosphere inside the chamber and must be handled accurately by micro processors and if it were not so we run the risk of irreparable damage to the product, produced by a mismanagement of relative humidity, unwanted gases, oxygen levels below 1%, carbon dioxide levels above 15%, etc. Finally, companies dedicated to fruit and vegetable sectors must adopt this technique if they want to optimize the conservation of their product, to commercially own a competitive product.

Well, this work must be done as a team, between refrigerators and food technologists, since with this technology, every day that passes, new products and processes are being integrated. In addition, the continuous experimentation and evaluation of the final product, shows us the change of the values of the gases of the atmosphere of the chamber and the temperatures and times. This is seen in information from experiences on the Internet and in technical or engineering publications, for refrigeration (ASHRAE) and for food.

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