Chapter 3

Storage

3.1 The need for storage

Most fruit and vegetable production in temperate areas is seasonal. In contrast, cultivation and harvest periods are much longer in tropical and subtropical areas. Demand is year round and it is normal practice to use storage in order to ensure continuity of supply. Moreover, storage is a strategy for achieving higher returns. Produce can be held temporarily to overcome gluts thus limiting price falls or to address shortage periods when prices are high.

Storage time depends on the intrinsic characteristics and perishability of the product. Shelf life ranges from short - e.g. raspberries and other berries - to those which naturally adapt to longer storage periods - e.g. onions, potato, garlic, pumpkins, etc. Storage conditions also depend on specific product characteristics. For example, some commodities tolerate temperatures close to 0 °C such as leafy vegetables. Others, such as most tropical fruits (Table 5), cannot tolerate exposure to temperatures below 10 °C.

To optimize storage conditions, not more than one crop should be stored in the same room, unless this is for a short period of time. Sharing the same storage area can result in: differences in temperature and relative humidity conditions; chilling and ethylene sensitivity; odor contamination and other problems affecting shelf life and quality.

3.2 Requirements and general characteristics for a storage facility

Generally, storage facilities are linked or integrated to packinghouses or other areas where there is a concentration of product. However, often storage can also be undertaken on-farm, either naturally or in specifically designed facilities. Even under conditions of mechanical refrigeration, location and design have an impact on system operations and efficiency. First, climate is an important factor for the location of the storage facility. For example, altitude reduces temperature by 10 °C for every 1 000 meters of elevation. It also increases overall efficiency of the refrigeration equipment by facilitating heat exchange with ambient temperature, thereby reducing energy costs. Shading particularly of loading and unloading areas reduces thermal differences between field and storage temperatures.

Table 5: Recommended temperature and relative humidity for fruits and vegetables and the approximate storage life under these conditions.

Source: Cantwell, 1999; Sargent, et al., 2000; McGregor, 1987.

Building design is an important factor to be taken into consideration. For example, a square shaped floor perimeter is thermally more efficient than a rectangular one. The roof is the most important part of the structure. This is because it has to protect produce from rain and radiant heat. Its slope should allow easy fall off of rainwater; its dimensions should exceed the perimeter of the building to protect walls from the sun and provide a dry area around the building in rainy weather. Floors should be of concrete, isolated from soil humidity, and elevated to avoid penetration of water. Doors need to be wide enough for mechanised handling.

Storage facilities should be thoroughly cleaned before filling. This includes brushing and washing of walls and floors to eliminate dirt and organic debris that could harbor insects and diseases. Before product is placed in the storage room, inspection and presorting should be undertaken. This is in order to remove all potential sources of contamination for the remaining load. Product should be stacked in such a way that there is free circulation of air. During storage, it should also be possible to carry out quality control inspections. If the storage facility becomes full during a long harvest period, it needs to be organized around the principles of the system "first in first out".

3.3 Storage systems

As a rule, there are many ways of storing a product. The length of storage time can be longer in specifically designed structures. With refrigeration and controlled atmospheres, storage periods can be even longer. The technology utilized depends on whether the benefits (higher prices) outweigh the costs.

3.3.1 Natural or field storage

This is the most rudimentary system and is still in use for many crops. For example, roots (carrots, sweet potato, and cassava) and tubers (potato). Crops should be left in the soil until preparation for the market. This is similar to how citrus and some other fruits are left on the tree. Although storing products under natural conditions is widely practiced, it leaves them exposed to pests and diseases as well as to adverse weather conditions. This can have a detrimental effect on quality.

Another method widely used is field storage in heaps. This method ensures that produce is free from soil humidity and is protected from the weather with a tarpaulin, straw, or plastic materials (Figure 49). It is a low cost alternative for bulky crops that require large buildings. For example, potato, onions, pumpkins, sweet potato etc. Field storage in bins is a more recent variation where a pair of them (one on top of the other, the one above protected from the weather) is left in the field. It has the additional advantage of making it possible to undertake mechanical handling later.

3.3.2 Natural ventilation

Amongst the wide range of storage systems, this is the most simple. It takes advantage of the natural airflow around the product to remove heat and humidity generated by respiration. Buildings providing some form of protection from the external environment and with gaps for ventilation can be used. Produce can be placed in bulk, bags, boxes, bins, pallets etc (Figure 50). Although simple, some key concepts need to be taken into account for the efficient operation of this system.

1. Differences in internal temperature and relative humidity conditions compared to conditions externally, need to be minimal. What this means is that this system can only be used with crops that store well under natural conditions such as potato, onions, sweet potato, garlic, pumpkins, etc.
2. For adequate ventilation, openings need to be wide. This means they need to be fitted with screens to keep animals, rodents, and pests out.
3. As with any other type of fluid, air follows the path of least resistance. This means that if product is stored in a compact mass, air will circulate to remove heat and gases which have accumulated as a result of respiration. Efficient ventilation requires adequate space. However, this reduces storage capacity.
4. Hot and humid air rises within the storage facility. If no ventilation gaps exist, this leads to the build up of hot and humid areas which in turn affects the quality of stored goods. This presents the ideal conditions for the development of disease.

Within certain limits, it is possible to take advantage of natural changes in temperature and relative humidity. This can be achieved by selectively opening and closing the storage ventilation. At noon, ambient temperature and relative humidity are higher and lower, respectively. However, at night the opposite happens. To reduce temperature of stored products, buildings should be left open when external air temperatures are lower. Internal relative humidity can also be managed in a similar way.

External conditions constantly change, even during the same day. However, in comparison to air, stored mass is slower to gain and release heat. In order to handle this efficiently, internal and external electronic sensors for temperature and relative humidity are required. In addition to this, although crops suitable for this type of storage have low respiratory rates, some ventilation may be required. This is in addition to the automated opening and closing schedules.

3.3.3 Forced air ventilation

Heat and gas exchange can be improved provided air is forced to pass through the stored product. This system allows for more efficient utilization of space for bulk storage. Air conducts run under a perforated floor (Figure 51) and air is forced through the product. Again, as air follows the least resistance path, loading patterns as well as fan capacity and conduct dimensions should be carefully calculated. This is to ensure that there is uniform distribution of air throughout the product.

Removable perforated ducts can be used for storage space when there are no products in storage (Figure 52).

Figure 49: Field storage of onions in heaps covered with straw.

Fan selection is the most critical factor and specialized personnel should design the system based on volume and number of air changes per unit of time required. The latter is a function of respiratory rates of products to be stored. Static pressure or resistance to the airflow by conducts and stored mass should be considered. Ideally, sensors reacting to the internal/external ambient relationship should control the system. If closed, internal air circulation only occurs. On the other hand, if opened internal atmosphere is replaced by ventilation. A partial opening produces a mix of internal and external air to reach the desired combination of temperature and relative humidity.

Figure 50: Storing garlic in shelters with natural ventilation.

3.3.4 Refrigeration

Controlling temperature is one of the main tools for extending postharvest life: low temperatures slow product metabolism and the activity of microorganisms responsible for quality deterioration. As a result, reserves are maintained with a lower respiration rate, ripening is retarded and vapor pressure between product and ambient is minimized, reducing water loss. These factors contribute towards maintaining freshness by reducing the rate at which quality deteriorates and the nutritional value of the product is preserved.

A refrigerated room is a relatively airtight and thermally insulated building. The refrigeration equipment should should have an external escape outlet to release externally the heat generated by the product. Refrigeration capacity of the equipment should be adequate to extract the heat generated by crops with a high respiration rate. It is also important to precisely control temperature and relative humidity conditions inside the refrigerated storage environment.

Refrigerated space depends on the maximum storage volume. Other factors to be considered include walkways and aisles to handle the product mechanically and the additional space to ensure uniform distribution of cold air. It is not uncommon to find that produce occupies only 75-80% of total surface area. Chamber height depends on product and stacking pattern: three meters for hand stacking but more than six may be required if forklifts are utilized.

Figure 51: Forced air storage facilities. Product is piled up to the yellow line. Air from the floor openings is forced to pass thought the stored mass. An inspection alley is located on the upper right hand side with ladders to sample product during storage.

Figure 52: Inside of a forced air storage facility. Air conducts are removed and empty space is used to shelter farm machinery and equipment when there are no products in storage.

Refrigerated rooms can be made with concrete, metal, wood, or other materials. All external surfaces should be thermally insulated, including the floor and ceilings. Type and thickness of insulation material depends on building characteristics, produce to be stored and the difference in temperature required between external and internal conditions. Polyurethane, expanded polystyrene, cork and other such materials can be used as insulation materials. Avapor barrier should be placed on the warm side of the insulation material.

Mechanical refrigeration has two main components: the evaporator, inside the storage area and the condenser which is outside connected by tubing filled with refrigerant. Normally, both elements are finned coils made of high thermal conductivity materials and integrated to a fan. This facilitates heat exchange. An evaporator is placed in the upper part of one of the walls (Figure 53) forcing cold air to flow parallel to the ceiling. Returning air is forced past the evaporator transferring to the coil the heat extracted from the product. A refrigerant absorbs this heat as it changes to gas, cooling the air, which is forced again into the room as cold air. The refrigerant is transported as gas to the condenser (outside of building) where under the pressure provided by a compressor, it is transformed again into the liquid form. The internal heat is then released outside. With this repeated cycle, the system behaves like a pump - heat is extracted from the stored product and then released outside. Another key aspect of the mechanical refrigeration system is the expansion valve, which regulates the evaporation and flow of refrigerant. Ammonia and Freon gas are the most widely used refrigerants. However, they are now being replaced by more environmentally friendly products.

In addition to design and consideration of building materials, to gain maximum benefit from refrigeration the following conditions need to be met: refrigeration capacity needs to be adequate - this is in order to extract respiration heat from the product as well as conductive heat (through floors, walls, and ceiling); convective heat gains (door openings), and the heat produced by equipment (forklifts, lights, pumps, etc.).

Every crop has an optimal combination of temperature and relative humidity for storage. In many cases, there are differences even within varieties (Table 5). As previously mentioned, it is recommended not to store more than one crop in the same room, unless this is for a very short period (less than a week) or during transportation. Very incompatible crops should not be in the same room for more than 1 or 2 days.

3.3.4.1 Precooling

Refrigeration equipment is designed to keep product chilled. However, they are not capable of reducing field heat rapidly. Field temperature is close to the ambient one and is much higher if produce is not protected from the sun. When produce is exposed to colder ambient conditions, it loses field temperature only slowly. It may take up to 24 or 48 hours in order to reach the new ambient temperature. The rate at which temperature falls depends on a number of factors. These include: differences in temperature, individual volume of product, total mass required for precooling and capacity of the refrigeration equipment. Metabolic activity (respiration, ethylene production, biochemical, and enzymatic reactions) also decreases with temperature - when storage temperature is reached rapidly, this results in reduced losses in energy, stored reserves, and quality.

Precooling is the rapid reduction of field temperature prior to processing, storage, or refrigerated transport. Generally it is a separate operation requiring special facilities, but complementary to cold storage. As deterioration is proportional to the time produce is exposed to high temperatures, precooling is beneficial even when produce returns later to ambient conditions. It is critical in maintaining quality in fruits and vegetables and forms part of the "cold chain" to maximize postharvest life.

Product temperature loss is not linear. This is because it is rapid at the beginning but slows down as it approaches the medium refrigerating temperature. Operation costs increase for each degree reduced. In commercial operations, produce is precooled to reach 7/8^th of the difference between field and the final temperature required. The remaining 1/8^th is lost during refrigerated storage or transport (Figure 54). For example, a product precooled with a field temperature of 30 ºC followed by exposure to a refrigerating medium of 10 ºC, should be terminated when 7/8^th of the temperature difference is removed (final temperature = 12.5 ºC)

T_final = T_{initial product} - [ 7 x (T_{initial product} - T_refrigerant)/8 ]

T_final = 30 - [ 7 x (30 - 10 )/8 ] = 12.5 ºC

The rate of cooling depends on individual volume and the exposed surface of product. The difference in temperature between product and the refrigerating medium also needs to be taken into account. For example, due to large exposed surfaces, leafy vegetables cool almost 5 times faster than large fruit such as melons or watermelons. Other factors which have an influence include the type of cooling medium and the amount of circulation surrounding the product. Water has more capacity to absorb heat than air and rapid circulation increases their cooling capacity.

Each system listed below has its advantages and disadvantages.

3.3.4.1.1 Room cooling

This is probably the most widely used system and is based on the product's exposure to cold air inside a refrigerated room (Figure 53). It is simple to operate as the product is cooled and stored in the same room. However, the removal of heat slowly makes this system unsuitable for highly perishable commodities. This is because the product needs at least 24 hours to reach the required storage temperature. Almost all crops are suitable for this type of cooling but it is mainly used in potato, onions, garlic, citrus, etc. (Table 6).

3.3.4.1.2 Forced air cooling

This system includes cold air being forced to pass through produce by means of a pressure gradient across packages (Figure 55). Cooling is 4 to 10 times more rapid than room cooling and its rate depends on airflow and the individual volume of produce.

Amongst the wide range of systems available, this is probably the most versatile. This is because it can be applied to all crops (Table 7), particularly berries, ripe tomatoes, bell peppers and many other fruits. It is slow compared to hydrocooling but is a good alternative for crops requiring rapid heat removal which cannot tolerate wetting or chlorine of cooling water. However, inadequate airflow may produce dehydration. Package ventilation openings should be large enough to allow adequate air flow, particularly if products are stacked or palletized. Adequate airflow is necessary. This is because fruits in the center of packages tend to lose heat at a slower rate, compared to those on the exterior.

3.3.4.1.3 Hydrocooling

The refrigerating medium is cold water. Because of its higher capacity to absorb heat, it is faster than forced air cooling. Hydrocooling can be achieved by immersion (Figure 56) or through means of a chilled water shower. In this final system, produce must be arranged in thin layers for uniform cooling. Not all crops can be hydrocooled. This is because they need to be able to tolerate wetting, chlorine, and water infiltration. Tomato, asparagus and many other vegetables are hydrocooled commercially (Table 8). Chlorination of water (150-200 ppm) is important to prevent accumulation of pathogens.

3.3.4.1.4 Ice cooling

This is probably one of the oldest ways to reduce field temperature. The most common method of ice cooling is at the individual pack level - crushed ice is added to the top of the product before the package is closed. Ice layers may also be interspersed with produce. As it melts, cold water cools the lower layers of product. Liquid icing is another system where a mix of water and crushed ice (40% water + 60% ice + 0,1% salt) is injected into open containers so that a big ice block is formed. The main disadvantage of ice cooling is that it is limited to ice tolerant crops (Table 9). It also increases costs because of the heavier weight for transportation and the need for oversized packages. In addition to this, as water melts, storage areas, containers, and shelves become wet.

Figure 53: Inside a refrigerated storage. Evaporator is located in the upper part on one of the walls.

3.3.4.1.5 Evaporative

This is one of the most simple cooling systems. It involves forcing dry air through wet product. Heat is absorbed from product as water evaporates. This method has a low energy cost but cooling efficiency is limited by the capacity of air to absorb humidity. As a result, it is only useful in areas of very low relative humidity.

Figure 54: Temperature loss of a product exposed to a refrigerating media.

3.3.4.1.6 Vacuum cooling

Is one of the more rapid cooling systems. However, this is accomplished at very low pressures. At a normal pressure of 760 mmHg, water evaporates at 100 ºC, but it does at 1 ºC if pressure is reduced to 5 mmHg. Product is placed in sealed containers where vacuum is performed (Figure 57). Vacuum cooling produces about 1% product weight loss for each 5 ºC of temperature reduction. Modern vacuum coolers add water as a fine spray in the form of pressure drops. Similar to the evaporation method, this system is in general appropriate for leafy vegetables. This is because of their high surface-to-mass ratio (Table 10).

Table 6: Crops usually room cooled.

Source: Sargent, et al., 2000; McGregor, 1987.

3.3.4.2 Chilling injury

Refrigeration is the most widely used method for extending the postharvest life of fruits and vegetables. However, low temperatures may produce injuries to plant tissues. Freezing (prolonged exposure to temperatures lower than 0 °C); forms ice crystals inside tissues. This causes damage. Symptoms are readily apparent when thawing occurs - there is loss of turgidity and a general breakdown of plant tissues. One of the main causes for this injury is unattended or malfunctioning refrigeration equipment.

Chilling injury on crops that do not tolerate long exposure to temperatures in the range of 0 - 15 °C are less noticeable. Most chilling sensitive crops are of tropical or subtropical origin. For example, tomatoes, peppers, eggplants, pumpkins, summer squash, sweet potato, banana etc. Some temperate crops may also be sensitive. For example, asparagus, potato, some apple varieties, peaches etc. Critical temperatures for these crops range from 0-5 °C, while those of tropical origin are from 7-15 °C.

Symptoms of chilling injury depend on the type of crop and become noticeable when product is returned to ambient temperature. In banana, for example, a blackening of the skin and softening takes place while in tomato, pepper, eggplants and other fruits, sunken areas are apparent. This is usually associated with decay organisms (Figure 58) and followed by rapid and uneven ripening. In many cases internal darkening or other discolourations are present. Severity of chilling injury depends on crop, temperature and length of exposure. As a general rule, immature fruits are more susceptible to damage than mature ones.

From a physiological point of view, chilling injury is the result of a cumulative breakdown of cellular metabolism. This is reversible during the first phase. A small rise in temperature restores the product to its former condition provided injuries are of a temporary nature. Different studies show that periodic (from 6-7 to 15 days) and short interruptions (5 to 48 hours) of cold storage through increases in temperature (from 12 to 25 °C) contribute towards extending post harvest life (Fernández Trujillo, 2000). Chilling injury is cumulative. In many cases it is the result of field, storage, and/or low transport temperatures.

3.3.4.3 Ethylene and other gaseous contamination

Under relatively airtight storage conditions, metabolic gases accumulate and ethylene and other volatiles are some of the most frequent contaminants.

Table 7: Crops usually precooled by forced air.

Source: Sargent, et al., 2000; McGregor, 1987.

Table 8: Crops normally hydrocooled.

Source: Sargent, et al., 2000; McGregor, 1987.

Ethylene is a fitohormone, which regulates many growing, development and senescence processes in plant tissues. It is produced in large quantities by climacteric fruits during ripening. It is also induced by certain types of stress such as physical injuries and is also part of the healing process. Ethylene is released as a gas and accumulates to physiologically active levels when not eliminated by ventilation or chemical means.

When ethylene releasing and sensitive (Table 11) crops are placed in the same room, undesirable reactions take place.For example, these include an increase in respiratory rate, ripening and senescence, loss of green colour, yellowing, necrotic areas on plant tissues, formation of abscission layers, sprouting in potatoes, development of bitter flavour in roots, asparagus toughening, etc. Indirect effects include an increase in sensitivity to chilling, or susceptibility to pathogens and the stimulation of some decay organisms. The level of ethylene in storage areas should be less than 1 ppm to avoid problems.

Aroma, odors, and other volatiles form an integral part of the metabolism of the plant. As with ethylene, there is contamination when producing species and sensitive crops share the same storage area (Table 11).

3.3.4.4 Relative humidity

Fruits and vegetables are largely composed of water. An important factor in maintaining post harvest quality is to ensure that there is adequate relative humidity inside the storage area. Water loss or dehydration means a loss in fresh weight. This in turn affects the appearance, texture, and in some cases the flavor. Water loss also affects crispiness and firmness. Consumers tend to demand and associate these qualities with freshness, perceiving them as just harvested.

Figure 55: Inside a forced air precooling facility. Pallets are arranged to form an aisle. Tops are covered with a canvas leaving both sides exposed to the cold air. Air from the plenum tunnel is removed creating a negative pressure that forces cold air to pass through the load.

The percentage of relative humidity is the most widely used parameter to express the amount of water in the air. It is defined as the relationship between the pressure of water in the air and the temperature at saturation point. As with other gases, water vapour moves from higher to lower pressure areas. In plant tissues, water is mainly present as cellular liquids, but in equilibrium with the intercellular spaces where it exists as a vapor saturated atmosphere (100% relative humidity). Exposure to identical air conditions of relative humidity and temperature will prevent water loss from tissues.

Figure 56: Hydrocooling produce and direct loading to a truck.

Capacity of air to hold water increases with temperature. The reverse is also true. This means that refrigeration increases the relative humidity of air. However, in some cases humidifiers are needed to increase the moisture content so as to reach the ideal conditions for storage. Onion, garlic, pumpkin etc provide some exceptions. They are best stored at relative humidity in the range of 60-70%. Most fruits and vegetables are required to be kept at a relative humidity of 90-95%, while some others at values close to saturation (Table 5).

3.3.4.5 Short term storage - Refrigerated transport

Refrigeration in cold stores is not always used to maximize the postharvest life. On the contrary, it is probably used more often during the short time required for the sequence of activites in the cold chain ending at the consumption point. Refrigerated transport is probably the best example of this. However, there are many other opportunities for the use of temporary cold storage e.g. during preparation and sale of product for the market. For example, holding product until processing, packaging, or transport is carried out. Other examples include the use of refrigerated facilities at wholesale or retail. Cold storage is also used in the home to prolong the shelf-life of products.

It is hard to define what constitutes "short term and long-term storage". This is because 7 days is a long time for raspberries while for potato, onion, garlic and other products that require longer periods of storage this is considered to be relatively short. In this chapter, "short-term storage" is defined as from a couple of hours to up to 7 days approximately.

Table 9: Crops that can be ice cooled.

Source: Sargent, et al., 2000; McGregor, 1987.

Table 10: Crops that can be vacuum cooled.

Source: Sargent, et al., 2000; McGregor, 1987.

It is preferable not to store different crops together. However, this is common practice and is unavoidable in many cases, particularly at distribution or retail. This does not pose a problem provided products are not exposed to suboptimal conditions for too long and build up of ethylene is avoided. A strategy widely practiced is to set cold chambers at an average of around 5 °C and 90-95% level of relative humidity.

If possible, mixed loads should have different regimes depending on the specific combination of fruits and vegetables in store. This is assuming that ambient ethylene concentration does not exceed 1 ppm. The University of California (Thompson, et al., 1999) recommends three combinations of temperature and relative humidity: 1) 0-2 °C and 90-98% RH for leafy vegetables, crucifers, temperate fruits and berries; 2) 7-10 °C and 85-95% RH for citrus, subtropical fruits and fruit vegetables; 3) 13-18 °C and 85-95% RH for tropical fruits, melons, pumpkins and root vegetables. On the other hand, Tan (1996) recommends 5 different storage conditions: 1) 0 °C and 90-100% RH; 2) 7-10 °C and 90-100% RH 3) 13 °C and 85-90% RH; 4) 20 °C and 5) ambient conditions. Other species are divided into five groups. Group 1 - apple, apricot, figs, ripe kiwifruit, peaches, pears, leafy vegetables, grape, beet, crucifers, celery, etc. Group 2 - avocado, cantaloupes, and honey dew melons, guava, cucumber, snap beans, peppers, summer squash, eggplants and in general citrus etc. Group 3 - banana, cherimoya, papaya, potatoes, pumpkin, etc. Group 4 - pineapple and Group 5 - garlic, nuts, onions, potato and shallots.

Figure 57: Vacuum cooling. Both cooler ends are lifted to allow moving produce in and then closed to create vacuum inside.

Figure 58: Symptoms of chilling injury are usually small depressed areas on fruit surface that later are colonized by deteriorating microorganisms.

Transport is an example) of temporary refrigerated storage. Mixed loads cause incompatibility problems highlighted previously. Because packaging dimensions are different, they are usually not fully stackable and the ventilation openings of packages of different dimensions do not match to each other. This prevents ventilation and creates microambient conditions which are undesirable.

3.3.5 Combination of storage systems

Facilities for long-term storage of potato, onion, sweet potato etc, often include using a combination of forced air systems as well as heating and/or refrigeration equipment. Because these are crops that initially require a curing period, hot and humid air is introduced at the beginning. Later, the temperature is reduced with either through forced air cooling or natural ventilation. Adequate temperatures are obtained by mixing external and internal atmospheres and if required, the air is heated or refrigerated. In this way, the same building is used for both curing and storage - an important consideration in mechanised harvesting systems.

3.3.6 Controlled atmospheres

With atmosphere modification, the low metabolic rate achieved with refrigeration is extended even further. As a result, the storage period is prolonged without further losses in quality.

Composition of normal atmosphere at sea level is around 78,1% nitrogen, 21% oxygen y 0,03% carbon dioxide. A "controlled" or "modified" atmosphere is obtained when its composition varies from the norm. In controlled atmosphere, gas composition is exactly maintained. It is often used for extremely long periods of storage in purpose built facilities. Modified atmospheres, on the other hand, are obtained when produce is packed in semi permeable films and are used for short periods. The atmospheric composition inside the package changes until it is in equilibrium with the ambient one. Equilibrium atmosphere depends on product, film characteristics, and storage temperature.

Table 11: Ethylene and odor producers and sensitives.

Source: The Packer, 1996; Gast and Flores, 1992; McGregor, 1987; Cantwell, 1999.

The modification of storage atmosphere delays the biochemical and physiological changes associated with senescence. This mainly involves the respiratory rate, ethylene production, softening and compositional changes. Other effects include the reduction in sensitivity to ethylene, and in some cases chilling and the severity of pathogen attack. The atmospheric composition can also be used to control insects. The risk of using abnormal atmospheres is that they may cause fermentation, tissue asphyxia (Figure 59), and the development of off-odors or off-flavors.

From the construction point of view, controlled atmosphere facilities are similar to refrigeration facilities. However, they should be airtight to allow creation of an atmosphere different from normal. The Oxygen consumption and its replacement by carbon dioxide by respiration, create the atmosphere. When the appropriate combination has been reached, a limited intake of oxygen is required to satisfy the reduced rate of respiration. Accumulation of carbon dioxide is removed by means of different methods. Because internal atmosphere behaves differently, a pressure compensating system is required to attain equilibrium with the external or ambient atmosphere. As controlled atmosphere rooms are kept locked until the end of the storage period, inspection windows are required to control refrigeration equipment. Product should also be placed at the top of one of the walls (Figure 60). Atmospheric composition is crop specific. However, as a general rule the most common combinations are 2-5% oxygen and 3-10% carbon dioxide (Kader, 1985).

Many crops benefit from atmosphere modification. However, usage is limited. It is difficult to define products ideal for storing under controlled atmosphere. However, one of the most important factors is that investment and operating costs should be recovered. Other factors include: First, products should be seasonal and have a stable demand during a long marketing period. Second, product should have some unique qualities and not be easily substituted by similar products. In other words, it is beneficial to use controlled atmosphere technology when there are no competitor products on the market. This may go some way towards explaining why its usage is limited to specific crops, particularly apples and pears.

Figure 59: Blackening due to tissue asphyxia of an artichoke head caused by storing in an inadequate atmosphere (photograph bt A. Yommi, INTA E.E.A Balcarce).

Figure 60: Inspection window in a room with controlled atmosphere.