Long-Term Storage of Carrots

Agdex#:	732/258
Publication Date:	12/98
Order#:	98-073
Last Reviewed:	12/98
History:	Original Factsheet
Written by:	Hugh W. Fraser - Engineer (Horticultural Crop Structures & Equipment Specialist)/OMAFRA; Jim Chaput - Crop Technology Branch/OMAFRA

Table of Contents

1. Introduction
2. Growing and Harvesting Carrots
3. Storage in Side-Slatted, Hardwood Bins or Bulk Piles
4. Cooling with Refrigeration or with Cold Outside Air
5. Air Distribution
6. Maintaining a Relative Humidity (RH) Over 95%
7. Providing Exchange of Inside Air with Outside Air
8. Monitoring Air Temperature and Relative Humidity
9. Cleaning and Sanitizing Storage Surfaces
10. Storage Diseases and Disorders
11. Other Considerations

Introduction

Ontario is one of the few places in the world where topped carrots are stored for any length of time. Ontario grows about 3000 ha (7400 acres) of carrots, storing about 60% of them, or about 81,000 tonnes (89,000 tons). Carrots are stored from two to 12 months. In Ontario, carrots are mainly grown and stored in the Bradford area north of Toronto, and in Southwestern Ontario where processing carrots are grown and stored. There are also many carrot storages south of Montreal in Quebec.

In Ontario, storage sizes range from about 100 tonnes stored (110 tons) to over 1000 tonnes stored (1100 tons) in one room. These storages allow carrots to be successfully marketed throughout the entire year. Long-term storage broadens a grower’s marketing options, spreads out labour needs, and improves financial sustainability. Ontario carrots are mainly marketed in Canada and in the Eastern USA.

This Factsheet covers nine management factors:

• Growing and harvesting good quality carrots
• Storage in side-slatted, hardwood bins or bulk piles
• Cooling with refrigeration or cold outside air
• Air distribution by refrigeration evaporator coil fans or fan-pressurized plenums or ceiling ducts
• Maintaining a relative humidity over 95%
• Providing exchange of inside air with outside air
• Monitoring air temperature and relative humidity
• Cleaning and sanitizing storage surfaces
• Storage diseases and disorders

Growing and Harvesting Carrots

Healthy, good quality carrots must be grown using the principles of an integrated crop management program incorporating all aspects of carrot production. Careful monitoring of the soil nutrition, selected varieties, insect, disease and disorder problems are key steps to harvesting a quality crop that will keep in storage. Harvest carrots for storage when fully mature, since they are less sensitive to oxidative browning of their surfaces. Carrots are harvested mechanically either by pull-type or digger-type harvesters. See Figure 1. Many processing carrots are also crowned prior to digging. Harvesting equipment should be in top working order and clean of residues from other fields to avoid spreading diseases. Most carrots are not washed before storage, because carrots with some soil residue seem to maintain their quality better in storage than washed carrots. However, in bulk storages, most of the carrots are washed, but washing must be done on the day of harvest to be beneficial. The fewer diseases that go into storage, the less likelihood of quality problems during storage. Cull poor quality carrots before storage. They have little to no value, and could create problems. This includes cut, broken, misshapen and injured carrots.

Figure 1. Good storage starts with gentle, mechanical harvesting of fully mature carrots. Typical harvest rates are 17,000-20,000 kg (37,000-44,000 lbs) per hour.

Storage in Side-slatted, Hardwood Bins or Bulk Piles

Storage in slatted, hardwood bins

Most Ontario carrots are stored in side-slatted, hardwood bins that are 122 x 122 x 91 cm high, holding about 570 kg of carrots (48 x 48 x 36 in., holding 1,250 lbs). See Figure 2. Some are two-way forklift entry, while others are four-way entry, which are easier to maneuver. Weight held per bin can vary, depending on the carrots’ moisture content and size. The usable storage volume of these bins is about 1.0 m³ with side slats about 2.5 cm apart for good ventilation (35 ft³ volume, with side slats about 1 in. apart). Assuming the bins were stacked tightly against each other, and keeping an air gap around the perimeter of the storage, a 12.2 x 24.4 x 5.2 m high carrot storage would hold up to 855 bins with 9 bins wide x 19 bins long x 5 bins high and 485 tonnes, or about 1.6 t/m² (40 x 80 x 17 ft with 534 tons, or 0.17 tons/ft²). See Figure 3.

Some advantages of side-slatted hardwood bins include:

• they are readily available
• they are easy to maneuver with forklifts
• they allow good air movement through them if the air is distributed well in the storage
• they stack well, providing they are well built

Figure 2. Carrots stored in side-slatted, hardwood bins.

Some disadvantages of using side-slatted hardwood bins include:

• they cost $50.00 (Canadian) each when new, or $50/bin ÷ 570 kg/bin = 8.8¢/kg stored (4¢/lbs)
• they last about 10 years with repairs, but may not stack well after that time
• they will each absorb several kilograms of water during storage, stealing moisture from the carrots
• they may harbour diseases
  

Figure 3. Floor plan of 12.2 x 24.4 x 5.2 m high storage.

Some growers have experimented with plastic bins, which do not absorb water, are more hygienic, and stack better to greater heights than side-slatted hardwood bins. They cost about $125 (Canadian) for a 119 x 119 x 72 cm bin, holding 385 kg, or 32.5¢/kg stored (47 x 47 x 28 in., holding 850 lbs, or 14.7¢/lbs). See Figure 4. The usable storage volume of these bins is about 0.68 m³ (24 ft³).

Assuming a life expectancy of 25 years, these plastic bins have an annual cost per kg stored of:
$125/bin ÷ 385 kg/bin ÷ 25 year life = 1.3¢/kg/year

Wooden bins have an annual cost per kg stored of:
$50/bin ÷ 570 kg/bin ÷ 10 year life = 0.9¢/kg/year

(In Imperial measure, this is 0.6¢/lbs/year for plastic bins versus 0.4¢/lbs/year for wooden bins.) Even though they cost about 50% more to own, the longer life and good storage features of plastic bins can make them attractive as a long term investment.

Figure 4. Some growers are trying plastic bins.

Storage in bulk piles

Experience has shown that carrots can be stored up to 4.9 m (16 ft) deep without bruising damage. See Figure 5. The same size storage as in Figure 3 of 12.2 x 24.4 x 5.2 m would hold about 690 tonnes, or about 2.3 t/m² of floor area in bulk piles (40 x 80 x 17 ft, holding 760 tons, or 0.24 tons/ft²). See Figure 6. Even though there is at least a 1 m (3.3 ft) wide fan-pressurized air distribution plenum down one side of the storage to allow for ventilating under the pile, the bulk storage is still up to 40% more space efficient than the bin storage, because there is no space lost for the bins, particularly if the bins were slightly spaced in storage.

Some advantages of bulk pile storages include:

• they can be filled or emptied rapidly using bucket loaders, conveyor belts, or water flumes
• they result in more uniform cooling, as cooling air is delivered at even spacings under the pile and must pass around each carrot
• they are easy to humidify because water vapour can be added to the fan-pressurized air plenum
• the cooling front moves in one direction

Some disadvantages of bulk pile storages include:

• they can result in more bruising in handling
• if problems develop deep in the pile, access to get bad carrots out is difficult, if not impossible
• carrots must be free of tops and trash for good air distribution; dirt is not usually a problem, and processing carrots are seldom cleaned
• they need more expensive structural walls to withstand the carrots’ high side-wall pressures

Figure 5. Carrots being conveyed into a bulk storage.

Figure 6. Floor plan of a 12.2 m x 24.4 m x 5.2 m storage.

Cooling with Refrigeration or with Cold, Outside Air

Carrots should be cooled to 0°C (32°F) soon after harvest. Ideally, they should be 7/8 cooled in one to two days to reduce disease. This industry term means the time to cool to 7/8 of the temperature difference between the starting carrot temperature and the cooling air temperature. So, if carrots were harvested at a soil temperature of 12°C (54°F) and were cooled using 0°C (32°F) air, the 7/8 cool time would be the time to reach 1.5°C (34.7°F). Soil temperature can range from about 15°C (59°F) to 0°C (32°F) in the harvest season.

In practice, the 7/8 cooling time takes longer than 1 to 2 days because storages are filled quickly, high capacity refrigeration just during harvest is expensive, and it is difficult to use high enough airflow rates. Removing the first few degrees of heat is the most critical. The longer the filling time, and/or the later the harvest in cooler weather, the less cooling is required. With a large crop and a limited time available, harvesting proceeds quickly, regardless of outside temperature.

Using Refrigeration to Cool the Carrots

There are two types of refrigeration systems used in carrot storages. The most common, is a conventional one, using evaporator cooling coils hanging on the ceiling along one wall, or two opposite walls. Fans on these units pull the ‘warm’ air from around the carrots across the evaporator’s cooling coils, then blow the resulting ‘cold’ air across the ceiling and over the bins of carrots. See Figure 7.

Figure 7. Most carrot storages are cooled using a refrigeration system with evaporator coils.

Generally, bins are placed tightly against each other, with their forklift openings precisely lined up in the direction of the airflow from the evaporator coils. This allows air to travel through these openings and back to the fan, ensuring that cooling air must pass as close to the carrots as possible for optimum heat transfer. Spacing bins can also work, but this wastes space and can result in air short-circuiting. Two methods of bin placement are shown in Figure 8.

Some advantages of a conventional refrigeration system include:

• it is easy to install, maintain and run
• it is easy to find qualified service personnel
• it gives good control on air temperature
• it allows prompt cooling in a warm harvest season, and extends the marketing season

Some disadvantages of a conventional refrigeration system include:

• it is expensive to install, maintain and run
• it can dry out the air and shrink the carrots
• is difficult to add humidity to the air
• it can freeze carrots in top bins

Figure 8. Option 1 shows bins with all their forklift openings lined up (dotted arrows) with the airflow (solid lines). Option 2 shows the first three rows of bins running with their forklift openings opposite to the airflow direction, to help ensure a uniform pull of air through all the bins.

The second type is called a Filacell system. Storage air is passed through a refrigerated water shower to cool and humidify it. The storage is maintained at over 95% RH with this system. See Figure 9.

Some advantages of using a Filacell include:

• it has a high refrigeration capacity
• it has high airflows to distribute cold air
• it maintains a very high relative humidity
• it is easy to duct its cold air to all areas of storage

Some disadvantages of using a Filacell include:

• it is more costly than conventional refrigeration
• it requires a large adjacent room for equipment
• it keeps the storage no lower than 0.5°C (33°F)
• it is less familiar to service people

Both conventional refrigeration and Filacell systems have their advantages and disadvantages, but most growers who want to store carrots into the late spring usually must have one of these types of refrigeration systems in place.

Figure 9. Some storages are cooled with a Filacell. Warm air is pulled through a refrigerated water shower, which both cools and humidifies it.

Using Cold, Outside Air to Cool the Carrots

In the past, carrots were cooled using cold outside air. Vents and storage doors were opened to allow cold air to circulate around the carrot bins. Fans were also used to help ensure all areas received good airflow. This system worked as long as the carrots were marketed early in the winter, since quality deteriorated over time. Some small storages are still operated in this manner, but their operators know that the carrots must be marketed before a certain date. Storages managed in this manner are decreasing.

Using outside air to cool the carrots makes sense, since there is usually an unlimited amount of cooling capacity outside during the harvest season. The problem is that a cool harvest season can not be guaranteed every year. Tests on air and soil temperatures during the harvest period have demonstrated that the air temperature is lower than the soil temperature for much of the time. See Figure 10. Freshly harvested carrots are about the same temperature as the soil, so as long as the outside air temperature is lower than the carrot temperature, it can be used to cool the carrots.

Figure 10. Temperatures are usually low enough to consider doing most, if not all, of cooling with outside air.

Suppose a storage held 600 tonnes of warm bulk pile carrots at 10°C with 15,000 L/s of outside air (25 L/s/t) being pulled into the storage and through the pile (660 tons at 50°F with 31,800 ft³/min, or CFM). Table 1 shows the cooling capacity of this air depending on the temperature difference between the carrots and the outside air.

Outside air should only be circulated in the storage when it is cold enough to do some cooling. This can be done successfully using an automatically controlled set of intake and exhaust dampers that mix outside cold air with warmer air from inside the storage.

Carrots can tolerate fairly cold air blown over them for short periods without the risk of drying them out, providing the air is above freezing. It is better to remove the field heat quickly while sacrificing a small amount of shrinkage through drying.

Some advantages of using cold, outside air for cooling carrots include:

• it is less costly to install and maintain
• it hastens cooling when the outside air is cold
• it gives fresh air exchange with the outside
• it forces installing a good air distribution system

Some disadvantages of using cold, outside air for cooling carrots include:

• it is totally ineffective during warm weather
• it usually requires a backup refrigeration system
• it requires humidification of the dry outside air
• it needs expensive control equipment

Air Distribution

Regardless whether outside air, or a refrigeration system is used to cool the carrots, having lots of cooling capacity doesn’t mean much if there is a poor system of distributing the cooling air. In order to understand air movement, one must realize that:

• Air always takes the path of least resistance, and will only go into dead areas if it is forced to
• Air will short circuit along unrestricted spaces or alleys if they are parallel to the airflow
• Cold air is heavier than warm air
  

There are three methods of air distribution possible in carrot storages with variations on each being used:

• Refrigeration evaporator coil fans to distribute cooling air around and through the carrot bins
• A fan-pressurized plenum to distribute cooling air either around carrot bins, or under bulk piles
• Ceiling ducts to distribute cooling air around carrot bins

Refrigeration evaporator coil fans to distribute air

This is the most common method for air distribution in carrot storages. Evaporator coils were shown in Figure 7. Most growers stack bins tightly and force air to travel through the forklift openings and the slatted sides of the bins. See Figure 11.

Figure 11. Side view of a carrot storage using refrigeration evaporator coil fans to blow cold air (dotted arrows) to the far side of the storage, then draw it back through the forklift openings of tightly stacked bins.

Here are tips on using this air distribution system:

• Air will short-circuit back to the evaporator coils above the top of the bins if their is no place for the cold air to drop at the opposite wall
• Evaporator coil fans will blow air 10-15 m (32-50 ft)
• Obstructions such as lights, or beams, in front of the evaporator fans will interfere with the airflow
• Stack one row of bins under the refrigeration evaporator coils first, then add one row at a time, to the far wall, to help stop air short-circuiting
• Some operators turn only the first three rows of bins under the evaporator coils so their forklift openings run opposite to the airflow. Air is blown over the bins, then pulled back through the bin forklift openings, meeting resistance as it encounters the bins turned the opposite way, and creating a more uniform airflow.
• Stack bins squarely to line up forklift openings
• Leave a 0.6 m (2 ft) space along the wall under the evaporator coils and the opposite wall for air entry/exit, and for walking access
• Leave at least 0.5m (1.5 ft) between the top bins and ceiling
• Leave only enough space along the side walls, 0.3 m (1 ft) adjacent to evaporator fans to protect the walls, otherwise air might short-circuit there
• Carrots in the top bins at the ceiling might need shielding from the freezing air dropping off the evaporator coils

Fan pressurized plenum to distribute cooling air

Fan pressurized plenums must be used with bulk carrots. Most plenum systems include outside air as the cooling source, with the plenum running along one wall of the building. See Figure 6. Cold air is introduced from outside, then blown, or pushed, from the plenum either through slots that line up with the forklift openings of bins, or up through bulk piles via steel or rigid plastic piping. See Figures 12, 13, 14 and 15.

Figure 12. High capacity fans at the end of the plenum pressurize it, then blow cold air through slots in the plenum wall and through bin forklift openings, or up through a bulk pile of carrots via large diameter steel or rigid plastic pipes.

Figure 13. For bulk pile storages, large diameter steel or rigid plastic pipes carry the air from the plenum to under the pile. Air is then distributed through holes in the pipes, then up through the pile of carrots.

Bins are shown in Figure 14, although it could be a bulk pile. Note the last three rows of bins are placed with their forklift openings in the opposite direction to the airflow. Although this may seem contrary to what was shown in Figure 8 and described above, it isn’t. This is because the airflow direction is different, and air that enters the forklift openings from the air plenum meets resistance as it encounters the bins turned the opposite way at the far side of the storage, creating a more uniform airflow. Again, this helps stop air from simply short-circuiting directly through the forklift openings. The reason three rows of bins are turned is because this is how much room is needed to turn a forklift if the bins are only two-way entry. In other words, in Figure 8 the air is pulled through the bins (Option 2), while it is pushed through in Figure 14.

Figure 14. Bin layout for fan-pressurized air distribution plenum, with outside air and optional refrigeration evaporator coils. The last three bin rows are placed sideways to create more uniform airflow.

Some of the components of a fan-pressurized plenum system that require careful design include:

• the air plenum and steel pipe duct under the carrots, whose cross-sections must allow air to travel at speeds less than 5 m/s (1000 ft/min)
• the optional refrigeration evaporator coils in the return airflow to the pressurizing fan to help cool during warm autumns and in the late spring
• the minimum fan capacity of 15-30 L/s/t of carrots @ 30 mm water static pressure (30-60 CFM/ton @ 1.25 in.) Less air is needed for bulk piles than for bins, because cooling air is delivered right to the carrots in bulk piles.
• the type of humidification equipment that is essential when using outside, dry air for cooling
• the cold air intake and warm air exhaust shutters
• the electronic control equipment for mixing air streams, and controlling temperature and RH

The advantages of using a fan pressurized plenum system include:

• it helps improve airflow throughout the storage
• it cools the carrots faster and more uniformly
• it makes it easier to add humidity to the air
• it allows entry of fresh outside air
• it cools more efficiently
• it keeps the carrots at a more even temperature

Ceiling ducts to distribute cooling air

If air circulation is a problem in a refrigeration evaporator coil or Filacell system, an airbag along the ceiling can help distribute air. It can be used to introduce outside air for cooling, providing there are automatically controlled intake and exhaust dampers and added humidity.

Figure 15. Cross-section of two storages with fan-pressurized plenums. Air is blown from plenum into storage and through bins, or bulk piles. Pipes under bulk piles have round holes on each side at 4 and 6 o’clock to prevent carrot blockage.

Maintaining a Relative Humidity (RH) Over 95%

One of the worst things that can happen to carrots during storage is they dry out. This can occur because of the vapour pressure deficit (VPD) between them and the surrounding storage air. To demonstrate the VPD problem, one must remember that warm air can hold more water than cold air. For example, air at 30°C and 90% RH in the summer holds 40 times as much water as air at -20°C and 90% RH in the winter. Nature hates to have differences such as this, so water vapour will move from areas of high vapour pressure to areas of low vapour pressure.

Carrot storage should be held at 0°C (32°F) and 95% RH. For simplicity, assume a small volume of air in this storage can hold one drop of water. Now assume that a freshly harvested carrot has a temperature of 15°C (59°F). Because carrots are so filled with water (as most produce is), we can assume that the relative humidity of the internal atmosphere of the carrot is at virtually 100% RH. So, if the carrot’s internal air temperature is 15°C, the same small volume of air in the carrot can hold about three drops of water. This difference between the carrot and the air causes water vapour to move away from the carrot and into the drier storage air, drying the carrot over time. The carrot will continue to lose moisture as long as it remains warmer than the cold storage air. This demonstrates the importance of removing the field heat from the carrots as quickly as possible, since rapid cooling helps reduce the VPD and subsequent drying. All produce loses moisture similarly. Tests with long term storage of apples have shown that most of the moisture lost occurs in about the first week of storage. There is no reason to doubt that the same thing happens with carrots. So, prompt, uniform cooling of carrots to 0°C, with a high relative humidity in the cold storage is imperative.

There are three types of humidification systems currently used in carrot storages; centrifugal; high pressure water; and water/air pressure systems. It is difficult to add moisture to cold air and the key to success is to produce extremely fine droplets of water to the airflow of the storage. Simply adding water to the floor will not provide adequate humidification. See a qualified supplier of humidification equipment for details.

Providing Exchange of Inside Air with Outside Air

There is little documented evidence of the benefits of providing the exchange of inside air with outside air to remove unwanted gases such as ethylene, which can cause carrots to become bitter in taste. Using outside air to cool the storage automatically provides air exchange. However, in storages cooled exclusively with refrigeration, there is virtually no air exchange with outside air. Older storages are less tight than newer ones, so some air exchange already occurs.

Some industry people suggest one air change every 24 hours. Suppose the storage in Figure 3 used this exchange rate. The total empty volume of a 12.2 x 24.4 x 5.2 m storage is about 1500 m³ (40 x 80 x 17 ft or 53,000 ft³), but when full of carrots in bins, only about 50% of the room is air, the other 50% being carrots, and bins. So, in this storage, there would be only about 750 m³ of air. One air change every 24 hours would be a constant airflow of about:

750 m³ ÷ 24 hours ÷ 3600 seconds/hour x 1000 L/m³
= 8.7 L/s (18.4 CFM)

This is a very low airflow. It would be impossible to buy a farm-type fan that delivers this low a rate. Even a small 175 mm (7 inch) farm-type fan delivers at least 50 L/s (100 CFM). Instead, it would be better to run a small fan on a timer to pull a little bit of air out of the storage at a time. If the storage construction was not too tight, then leakage around doors would probably provide enough air intake for the fan. Otherwise, install a very small intake shutter on the opposite wall to the fan.

Monitoring Air Temperature and Relative Humidity

Good quality, electronic, remote sensing temperature sensors are a must in all carrot storages. Never try to save money by purchasing cheap equipment, since it is one of the only ways to tell what is happening with the carrots. Think of these sensors as inexpensive insurance. Temperature sensors should be placed high, low, near walls and within the bins or pile.

Measuring the relative humidity of air at low temperatures is difficult, unless sophisticated and expensive equipment is used. The cheapest method is to use a sling psychrometer, but its readings can only be used as a guideline. The air in the carrot storage should feel cold, clammy and bone chilling. This means that the air has a high relative humidity.

Cleaning and Sanitizing Storage Surfaces

Cleaning and sanitizing bins, the storage, and handling equipment may help eliminate disease-causing organisms. Cleaning is not the same as sanitizing. Cleaning includes the removal of soil, mould, juice and vegetation by brushing, scraping and high pressure washing. Cleaning compounds such as soaps lower the surface tension of water so that soils can be loosened and flushed away. Sanitizing reduces the pathogenic and spoilage micro-organisms. Cleaning must be done before sanitizing, otherwise the sanitizers will not be effective. Clean rinse water should be used after sanitizers. Although chemicals can be used to help eliminate disease-causing organisms, there really is no substitute for good general cleanliness methods, liberal amounts of hot water, and lots of elbow grease. Leaving bins outside during the off-season in the sun and rain may help, but may not eliminate the possibility of problems in the following storage period. Table 2 lists some commonly used sanitation mixtures. Always wear a suitable respirator and exercise caution when sanitizing bins, equipment or the storage as the fumes can be toxic and corrosive.

Storage Diseases and Disorders

The two most common diseases of stored carrots are Sclerotinia white mould, and bacterial soft rot. See OMAFRA Factsheet, Identification and Management of Carrot Root Diseases, Order no. 98-001 for more details. Other storage diseases may include; crater rot, cavity spot and black rot (black mould). If temperatures are too high in storage, the carrots may sprout, which greatly reduces marketability.

Other Considerations

Other considerations for carrot storages include:

• Be careful using equipment around carrots that produces ethylene, such as propane-fired forklifts, since ethylene can cause carrots to have a bitter taste.
• It is not advisable to store carrots with any other horticultural crop, even those with similar storage requirements, especially those that produce a lot of ethylene, such as fruits.
• CA storage is not recommended since carrots are too dense to be affected by a high carbon dioxide, low oxygen atmosphere.