Storage of Canola

 
 
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  Seed maturity and condition | Seed moisture temperature and time | Conditioning of canola | Heated-Air drying | Summary

J. T. Mills - Agriculture and Agri-Food Canada, Winnipeg, Canada January - 1996
Revised by M. Hartman – Alberta Agriculture and Rural Development, 2011

Factors affecting canola quality during storage include: seed maturity and condition; seed moisture, temperature, oil content and elapsed time in storage; molds, insects, and mites; dockage; cultivar type; weather during storage period; and the storage structure and handling methods used.

Seed Maturity and Condition

The original condition of a grain lot at harvest is probably the most important factor affecting its storability. Freshly harvested canola can display a high respiration rate for up to 6 wk before becoming quiescent and is often referred to as "sweating". Fully ripened seeds of low moisture content are metabolically dormant and respiration is negligible. The apparent respiration of freshly harvested and stored grain has been well researched and is predominantly due to the growth of micro-organisms such as fungi. Sweating therefore represents heat and moisture respired during final ripening of immature seeds from late branches, immature weed seeds and other dockage, and from field fungi normally found on grain. Sweating may also be caused by convection currents that develop quickly within bins with grain from different unloads of varying moisture and temperature. Dockage is known to accumulate at bin walls during unloading and can be significantly higher in moisture (Prasad et al., 1978) Some researchers (Pronyk et al., 2004; Sinha and Wallace, 1977) have monitored freshly harvested canola in steel bins and found no evidence of an initial sweating process. Regular monitoring is required because increases in heat and moisture favour the growth of storage molds. Mold growth and respiration produces additional heat and moisture and then the temperature within the seed bulk escalates quickly. Eventually the seeds may become heat-damaged. The risk of sweating is reduced by storing the crop in a dry and cool condition.

Immature seeds are distinctly green when crushed. Based on crushed seed examination, No. 1 and No. 2 Canada Canola Grades may contain a maximum of 2.0 and 6.0% distinctly green seeds, respectively (Canadian Grain Commission, 1994). Financial penalties are levied for distinctly green seeds in stocks offered for commercial sale because of added refining costs and adverse effects on the shelf-life of canola-based food products. Green seed is a problem when fields are harvested before the majority of seeds have turned color, when the crop was frozen, or when heat and drought stress occurred before maturity. During normal development, the green color disappears upon maturation but arrested development can result in chlorophyll retention. Leaving B. rapa canola in the swath for 4 days reduced the proportion of distinctly green seeds at harvest (Cenkowski et al., 1989a). Drying high moisture (>30% moisture) samples at 80C is also effective in reducing the proportion of distinctly green seeds to less than 3%, however 20C to 40C are the best drying temperatures for samples containing less moisture. Long-term storage (five months) may slightly reduce the percentage of green seeds (Table 1). Several researchers (Cenkowski and Jayas, 1993; Johnson-Flanagan et al., 1994) have reported that rehumidification at moderate temperatures (20-25C) of green seed can slightly reduce the chlorophyll contents; however this will predispose the lot to spoilage unless it is subsequently dried and cooled.

Developing canola seeds are frequently affected by localized frost in the Canadian prairies. Occasionally, an early frost will cause damage over widespread areas. In late August 1982, a frost arrested seed development over much of the canola growing area of Manitoba and Saskatchewan. Approximately 38% of the total crop was graded No. 3 Canada (3CR) or Sample Account Damaged compared to 4% of the crop in these grades in 1981. Most of the 1.16M tonnes of frost-damaged seed produced in 1982 was not readily marketable and required storage over winter. Quality changes were observed in 60 bins of farm- stored canola monitored within a 200-km radius of Winnipeg, Manitoba from November 1982 to April 1983. In November, the contents of one bin reached 102C with steam observed arising from the centre surface; corresponding temperatures for non-heating bins were 8 to 13C. Generally, the quality of seeds in official grades 2CR, 3CR and Sample Account Damaged did not decline in storage. The percentage of green seeds declined slightly in 28 bins of B. napus examined during the storage period (Table 1). Spoilage and heating problems in frost-damaged canola are most likely to occur during the first months of storage and can be prevented by use of aeration, careful bin management with frequent monitoring, and the use of small, readily accessible granaries (Mills et al., 1984).

Table1: Changes in the level of frost damaged green canola
seeds during storage.
Seed Grade
% Green Seed
Early November
Mid April
2 CR
4.3
4.4
3 CR
8.0
6.6
Sample
18.2
17.6
+Mills et al. (1984)

Occasionally, canola crops are covered by snow before they can be harvested. In October 1984, a large amount of canola was covered by snow in Northern Alberta and seed was harvested the following spring. Samples of this canola were compared to samples of fall-harvested seed from the same area. Spring-harvested seed stored more poorly than fall- harvested seed, having greater increases in free fatty acids, conductivity (electrolyte leakage when placed in water) and storage (post-harvest) molds, and larger losses in viability (indicators of quality loss) when stored hermetically at 10% or 12.5% moisture levels (Daun et al., 1986).

Seed Moisture, Temperature, and Time

The moisture level and temperature of the grain influences events that occur during storage and may lead to spoilage and self-heating. Canola and other high oil seeds are more prone to deterioration in storage than cereal grains and must be stored at a lower moisture level to prevent molding. Under the Canada Grains Act, the maximum moisture at which canola can be sold as straight grade (dry) is 10% moisture (Canadian Grain Commission, 1994). Because seed with 10% moisture can be sold without penalty, such a figure is often assumed to represent a safe level (Moysey and Norum, 1975). The upper safe relative humidity limit is 70% at moderate temperatures at which point molds begin to grow. This equates to 8.3% moisture for canola and 13.9% moisture for wheat (Triticum aestivum L.) at 25C (Mills and Sinha, 1980; Hall, 1980). At high grain temperatures (30 – 40C), mold can occur even when moisture contents are below 8% (Sathya et al., 2009) Both moisture level and relative humidity are dependent on temperature (Mills, 1989).


The oil fraction of canola seed absorbs less moisture than the starch and fibre fractions, therefore the equilibrium moisture level for canola is much lower than that of wheat (Thomas, 1984). The amount of water that must be evaporated from canola to safeguard it from molding is therefore greater than with cereal grain. Modern canola varieties have higher oil content and thus the safe moisture and temperature levels for storage needs to be lower. Australian researchers have published moisture isotherms for canola and related this to varying oil contents (Cassells et al., 2003). A practical guideline for the effect of oil content on safe storage moisture is: for every 1% higher oil content, decrease the safe moisture by 0.1%. For example, canola with 48% oil should be stored at 0.5% lower moisture % than canola with 43% oil. Although producers cannot easily measure oil content at harvest, if combined canola is larger in size than normal and cooler and wetter growing conditions during seed fill occurred, then the seed will be higher in oil content.
Temperature is important for three main reasons: i) temperature and moisture influence enzymatic and biological activities and thus the rate of spoilage; ii) temperature differences within bulk commodities favor mold development through moisture migration; and iii) the high temperature of seeds harvested and binned on a hot day is retained within un-aerated bulks for many months due to the insulating effects of the bulk seeds (Mills, 1989). Canola in the swath, combine hopper, truck and bin can be several degrees warmer than ambient air on sunny days (Prasad et al.,1978).

Temperature differences result in moisture moving from warmer to colder areas of the bin. During late fall, cold air sinks in the grain at the outside of the bulk and warm moister air, in the centre of the bulk, rises and condensation may occur when it reaches the cold seeds near the surface. This free moisture and warm temperatures near the surface can lead to rapid spoilage. In late spring and summer, it is possible to get moisture migration in the opposite direction if the outside temperature is warmer than the seeds. Warming action from the sun on the bin causes air to move up near the outside wall of the bin and down through the centre of the bulk. Moisture is reabsorbed by the cooler canola in the centre of the bin. Removing a portion of the seeds from the centre of the bin is a method of interrupting the increase in temperature and moisture in the central core. Friesen and Huminicki (1986) report that significant migration occurs in canola at moisture levels as low as 8% when placed into storage at high temperature and not cooled by aeration.

Moisture level and temperature determine the safe storage period for canola; the storage time chart shown in the following graph which predicts the keeping quality of canola over 5 months, under varying temperatures and moisture.


If the temperature or moisture level of the canola falls within the spoilage area of the chart, either the seed moisture or temperature or both need to be reduced. Moisture level can be reduced either by delaying combining to allow further drying in the swath or by artificially drying the seed. The temperature can be reduced by aerating the bin contents. If the seed is binned at above 25C, or if pockets of immature seeds or green weed seeds are present, 8.3% moisture is too high for long-term, safe storage. For storage longer than 5 months, canola should be binned at a maximum of 8% moisture and cool temperature (Mills, 1989).

To successfully store canola for periods of 6 to 24 months, particular attention must be given to conditioning and monitoring. Quality seed may be stored 2 to 3 years if its moisture and temperature are properly maintained (Thomas, 1984).

Molds, insects and mites
Seeds in storage provide a good substrate for storage molds, the most important cause of seed deterioration (Christensen and Kaufmann, 1969). Molds spores, occurring in the soil and on decaying plant material in the field, are coated onto the seeds during harvesting operations. Fungi common on freshly harvested canola (such as Alternaria and Cladosporium) tend to decline during early storage and if suitable conditions exist, spoilage fungi increase (Sinha and Wallace, 1977). Each species of storage mold flourishes at a different relative humidity level and temperature. Some species, for example Eurotium amstelodami Mangin, grow at low humidities, affect seed germination, and produce water as a consequence of metabolism during their growth. Higher moisture levels enable more damaging molds to grow. These molds include Aspergillus candidus Link and Penicillium species, all of which impair seed germination and are often associated with hot spots - areas within bulk seed that have a higher temperature than the surrounding material (Mills, 1989). Molding and heating can occur very quickly in moist canola, and where this happens, the seeds are likely to clump together. There can be a marked increase in the level of free fatty acids, probably brought about by the growth of molds (Nash, 1978). Heated seeds are brown instead of a normal yellow color and produce a distinct tobacco-like odour in the oil and meal which is difficult to remove by processing (Thomas, 1984). The result is that the quality of the stored product for processing is greatly reduced.

The speed at which molding occurs in freshly harvested canola is important because it influences drying and storage management decisions. Burrell et al. (1980) determined the spoilage time of freshly harvested rapeseed stored aerobically in tubes at five temperatures ranging from 5C to 25C and at seven moisture levels ranging from 6% to 17% at each temperature. Spoilage as expected was more rapid at higher temperatures and higher moisture levels. Seed clumping preceded the appearance of visible mold colonies and seed germination was affected much later. Seeds at 25C and 10.6% moisture clumped together after 11 days and visible mold colonies appeared after 21 days, however germination was still unaffected after 40 days. Burrell et al. (1980) suggest that seed clumping (Table 3) is the best criterion for determining the maximum period available for drying before fungal growth because the appearance of the seed will have already deteriorated by the time fungal colonies become visible. Under normal harvest conditions, seed over 10% moisture should be dried within 1 - 2 weeks to avoid spoilage.

Table 3: Maximum period (days) without visible "clumping" of rapeseeds by molds+
Initial Moisture (%)
Temperature (C)
25
20
15
10
5
17
4
4
6
11
20
15.6
4
6
6
11
28
13.7
4
6
11
20
46
12.3
8
6
18
25
109
10.6
11
18
42
42
238
8.9
23
48
116
279
>300
6.7
69
180
>300
>300
>300
+Burrell et al. (1980)

During 1974 to 1976, considerable spoilage and heating of stored canola occurred on western Canadian farms (Mills, 1976; Daun and Mills, 1979). Spoilage and heating of the 1974 and 1975 crops were reported by 19-25% of the managers of western Canada Pool elevators.

Generally, crop districts with a high elevator spoilage coincided with districts of high incidence of farm spoilage and heating in 1974. The higher the seed moisture level, the higher the probability of spoilage and heating even with turning (moving grain from bin to bin during storage) (Mills, 1976). Monetary losses resulting from bin-heating of canola were estimated to be at least $Cdn 3 million for the 1975 and 1976 crop years.
Insects occur in stored rapeseed bulks (Sinha and Wallace, 1977) but vary in their ability to survive and establish infestations. On whole seed, the merchant grain beetle (Oryzaephilus mercator (Fauvel)) was able to multiply 1.87 times in 12 wk, but the rusty grain beetle (Cryptolestes ferrugineus (Stephens)) failed to complete its life cycle. Generally, whole seeds are less vulnerable to infestation than crushed seeds and only a few insect species are adapted to the high oil content of rapeseed (Sinha, 1972). The optimum temperature for rapid growth of insects is in the range of 30C to 35C; their activity is greatly retarded by temperatures below 18C. If bulk seed is cool and dry, insects will not thrive.

However, seed may go into storage at acceptable levels of moisture and temperature and then at a later date develop pockets of high moisture and temperature suitable for insect activity (Muir, 1973). Infestation of binned seed by insects and mites will reduce the safe storage times shown in Table 2 (Mills and Sinha, 1980).

Mites carry mold spores in and on their bodies. Mites may eat the surface and interior of canola seeds affecting seed weight and quality (Hudson et al., 1991) and often feed on molds; heavy contamination by some mite species will leave a distinctly minty odor (Mills, 1989). Sinha and Wallace (1977) found that canola was more vulnerable to pest infestation than barley (Hordeum vulgare L.) when stored in farm bins in Manitoba, Canada during 1973-76. Unlike barley, canola was heavily infested by grain mites and their predators. It was suggested that the prey mites multiplied by feeding (probably selectively) on fungal species, dockage (grain dust, broken grain kernels, weed seeds, etc.). The prey mite populations were effectively checked by predatory mites favored by low temperatures during the cooler months. Turning canola in the spring reduced temperature and moisture differences between the warm centre and cooler edges, but also dispersed mold spores and mites throughout the bulk.

Armitage (1984) studied the distribution of the three most important British genera of stored-product mites in bulks of canola, wheat, and barley at a range of relative humidities over a 10 yr period. In winter, in both aerated and nonaerated bulks, grain feeding Acarus and Glycyphagus spp. were frequently present on seed surfaces dampened by moisture. In summer, the mites were usually most abundant below the surface, regardless of the relative humidity of the bulks. The difference between summer and winter distributions appeared to be related to the drying of the surface layers during the spring and summer; this may have caused the mites in those layers to reproduce less quickly. Populations of the predatory mite (Cheyletus eruditus (Schrank)) were usually more evenly distributed than those of Acarus and Glycyphagus and appeared less sensitive to seasonal moisture and temperature changes. Aeration of 9% moisture seeds, at air temperatures below 5C for 4 months, reduced the population of Acarus spp. to 4000 kg-1, compared to 36,000 kg-1 in nonaerated bins.

Molds, insects and mites occurring in grain bulks seldom act alone but interact with the grain and with each other. Fleurat-Lessard (1973) and Sinha and Wallace (1977) have stated that high populations of molds and mites often co-occur and interact together in farm-stored rapeseed.

There are several products available such as aluminum phosphide to treat insect infestations in canola bins. Do not use malathion on canola before or during storage or in the empty bins where canola will be stored. If the bin was treated previously, do not store canola in bins within six months of treatment. Turning the grain in the winter will reduce the temperature and thus deter insect growth.

Dockage
Dockage is material that must be removed from grain by the use of approved cleaning equipment in order for grain to be eligible for the highest grade for which it may qualify (Canadian Grain Commission 1994). The amount of dockage in farm deliveries of canola to elevators in western Canada over the years prior to herbicide tolerant (HT) varieties has averaged 9% (Manitoba Agriculture, 1980). Current dockage levels in HT canola are much less than with conventional herbicides (O’Donovan et al. 2006). Dockage in canola consists mainly of wild oats, other weed seeds, volunteer cereal grain, broken seeds, broken pods and soil particles. This dockage normally has a moisture level 3% to 4% higher than that of the canola seed. Large amounts of broken seeds influence the rate of respiration because they provide a substrate for the growth of molds (Thomas, 1984). During binning of canola, the chaff concentrates towards the wall of the bin. Fines, or particles smaller than canola, increase the resistance to air flow, while chaff or particles larger than canola decrease the resistance. An equal proportion of fines and chaff in a canola load increases the resistance to air flow. Spreaders have not been found to distribute dockage and chaff more uniformly than spout fills (Jayas et al., 1987).

Cultivar type
Few storability studies have compared B. napus and B. rapa cultivars, or specialty oil types (high erucic, high oleic, low linolenic). Mills et al. (1978) studied the storage quality of 106 samples of B. napus and 71 of B. rapa. The samples were obtained from primary elevators across western Canada and assessed as sound, spoiled or heated. High fat acidity and conductivity levels, low pH, weathered seed surfaces, strong off-odors, and a brown seed interior on crushing, low germination levels, and high frequency of Aspergillus spp. storage molds were correlated and indicated deteriorating quality in both species of canola. Canola samples with a sweet odor, no visible mold, less than 1% other seeds, greater than 90% germination, more than 97% yellow seeds after crushing, conductivity of less than 75mhos, and a fat acidity value of less than 30 mg KOH per 100 g of moisture-free seed were considered high quality (Mills and Sinha, 1980).

Storage and handling
Canola is very sensitive to heating in storage (Mills, 1976) and therefore requires better bin construction than that required for cereals to exclude moisture. The small size and free flowing characteristics of canola mean that high quality construction is necessary to prevent leakage. Roof and door openings, joints between structural components, and even bolt holes must be sealed to avoid losses. As heating and moisture migration problems tend to be more severe in larger storage structures, canola should be stored in the smallest bins available, without sacrificing convenience and efficient handling.

Do not use malathion on canola before storing or in the empty bins where canola will be stored. If the bin was treated previously, do not store canola in bins within six months of treatment.

Storage in wooden granaries does not facilitate control of seed leakage and also provides access for the entrance of moisture, insects and rodents. Steel granaries, on the other hand, require almost no maintenance and can be more easily sealed against pests and weather. Also, if conditioning of hot or damp grain is necessary, metal bins are best suited to the controlled movement of air through the grain mass. Regardless of the construction material used, storage structures must be as weatherproof as possible, yet still allow easy access to the bin for sampling and monitoring. The weather proofing process must include the floors of bins that are set on concrete. Concrete floors may resist the movement of water through the slab, but moisture can still enter the bin in the form of vapour. For this reason, it is important to place a vapour barrier, such as polyethylene, between the concrete and the gravel base.

A more recent type of storage being adopted is a plastic polymer membrane (“harvest bag”, “silo bag”) that if filled like a sausage in the field. Harvest bag adoption has been higher in Argentina and Australia, where farmers often do not have sufficient permanent storage capacity. These harvest bag systems are cost competitive and provide flexible choice of storage location and surge capacity in years with high yield. Recent studies in Argentina and Australia indicate that canola can be successfully stored for a year with low moisture canola and careful bagging (Ochandio et al., 2010; Darby and Caddick, 2007). The harvest bags probably are best suited for short term storage of a few months. Shortcomings noted in the Australian report include: easily torn or punctured membrane which leads to localized spoilage; overall inadequate sealing (need to achieve air-tightness) under farm situations; little protection against wildlife access; localized condensation on the inside of plastic when grain is damp; and insect disinfection method are not currently available. There may also be limitations under western Canada conditions such as below freezing temperatures during harvest or unloading that causes problems with the working properties of the plastic membrane.

The susceptibility of canola to heating justifies extra care when placing the grain into storage. Because bulk canola contains seed of varying ripeness and surface borne fungi from the field, it may undergo a sweating process before becoming dormant. Therefore the top hatch of the bin should be left open initially for several days during dry weather to allow heat and moisture to escape. The efficiency of conditioning operations will be enhanced by cleaning the seed prior to storage (Thomas, 1984).

During transportation, all cracks in trucks and other equipment must be sealed with duct tape or caulking to prevent leakage and tightly covered to prevent canola seeds being blown away. Augers should operate at full capacity to prevent the seeds from flowing back down the tube and belt conveyors should be enclosed in a trough to keep the seeds from running off-line. Kernel damage during handling is usually not a problem unless below 7% moisture (Harner, 1989).

Conditioning of Canola

The term conditioning usually refers to those processes that involve the movement of air through seed to ensure safe storage over a period of time. Conditioning systems are used to cool or dry freshly harvested hot or moist seed, to avoid spoilage in storage. These systems also help to prevent moisture migration caused by temperature gradients which can occur in moist seed. Conditioning also reduces the effects of sweating, and is used even in areas where the seed usually can be harvested in a satisfactory state. Conditioning systems can extend the harvest season since canola can be removed from the field in a tough (>10.1% moisture) or damp (>12.5% moisture) condition; thus the harvest can be started earlier and continued later. The degree to which the harvest season can be extended will depend on the level of conditioning available. Conditioning also reduces field losses, as advancing the harvest means there will be less exposure of the canola to weather conditions that can affect the yield and grade. In addition, harvesting at a higher moisture level will reduce machine shatter losses and premature shattering of pods. The ability to condition canola also helps producers to avoid harvesting and selling of overly dry grain, thus minimizing economic losses, and to take advantage of good but short-lived market conditions prior to the main harvest (Thomas, 1984).

Proper operation of conditioning systems, particularly natural-air systems, is dependent on the knowledge of the state of the seed. Monitoring of the seed condition is necessary to avoid danger of spoilage and to know when the operation is complete. A final sampling for both temperature and moisture content is advisable before long-term storage. The most critical factor in monitoring seed condition is an awareness of any changes that have occurred. Accurate monitoring, therefore, requires repeated sensing of conditions at specific locations. Permanent sensors in a bin add to the cost of a monitoring system, but ensure that measurements are always taken at the same place. Portable probes can be used effectively, but do not provide the same precision for repeated monitoring (Thomas, 1984).

Conditioning systems can be divided on the basis of both the purpose and the state of the air used in the operation. Natural-air systems use the surrounding or ambient air to condition the grain, whereas, heated-air conditioning systems use energy to heat the ambient air. Heated-air conditioning systems have a higher capacity for drying canola because of the increased drying ability of heated, low-humidity air and the higher airflow rates usually used. Conditioning systems are usually separated into aeration, natural-air drying, and heated-air drying categories; combinations of these systems, involving two or more of these operations, are also used (Thomas, 1984).

Aeration systems are used to preserve seeds by cooling and by preventing moisture migration. They are used during seed storage, between harvesting and drying operations and after heated-air drying.

Operation of aeration systems
The purpose of an aeration system is to produce the lowest practical temperature and the least temperature variation within the stored seeds. The amount of air required to change the temperature of the seed will produce very little change in moisture level. At moisture contents above 11%, aeration should not be used alone unless seed temperatures are near or below 0C. Management of aeration systems differs in fall, winter, spring, and summer seasons (Friesen and Huminicki, 1986).

The airflow rates for aeration of canola are normally 1-2 L.s-1.m-3. With an airflow rate of 1 L.s-1.m-3 about 150-200 hours of fan operation are needed to change the temperature throughout the bin; at 2 L.s-1.m-3 this time is halved (Friesen and Huminicki, 1986). Aeration fans should be started as soon as the canola covers the floor of the bin, so that immediate cooling can take place. Fans must be operated continuously until the temperature of the canola is near the average outside temperature. The operation of aeration equipment during extended periods of high relative humidities (over 80%) may promote mold growth, even in dry canola. However, continuous aeration through one or two days of high relative humidity will not damage the canola, as long as an equal time of dry weather follows. Since aeration is essentially a cooling procedure, the temperature of the air is more important than the relative humidity. When the outside temperature has dropped below the temperature of the stored canola by 5 to 10C, the canola should be cooled again. No conditioning operation is complete until the temperature and moisture level of the entire bulk have reached the desired level. After the bulk has reached the desired storage temperature, the bin should be checked periodically for evidence of heating or moisture migration.

Aeration can be accomplished by moving air upward or downward through the grain bulk (See the following graphics on air movement").



There are advantages and disadvantages to each direction, but in most situations upward air movement is preferred. Upward air movement permits the aeration progress to be easily determined by checking the canola temperature at the top of the bin. Also, with an upward air flow, the fan can be started prior to filling and air leaving the duct will help to keep the perforations clear of fines as filling progresses. The disadvantage of moving air upward is the potential for condensation to form on the underside of the roof when aerating warm seed in cold weather. Moving air downwards and exhausting it at the bottom minimizes condensation. However, in downward air movement, the canola at the bottom is the last to cool and the hardest to check to determine when aeration is complete. A further disadvantage of downward movement is that when warm seed is added to the bin, the heat from the added seed is drawn through the previously cooled seed and warms it up again. When aerating grain in summer, downward movement will draw the hot-air from under the roof down through the rest of the seed (Friesen and Huminicki, 1986), another disadvantage.

Under hot, humid harvesting conditions, such as often occur during late July and August in southwest Ontario, aeration can result in a greater potential for condensation within the bin. Moisture condensed on the inner bin roof will then drip onto the bulk surface favoring moldy crust development and insect pest populations. In this case, cross-ventilation to remove warm air rising from the bulk surface is effective before condensation occurs. Delaying aeration until air temperatures have moderated will avoid the problem (Mills, 1990).

Aeration of canola
The specific requirements of aeration systems for canola are determined by the susceptibility of the seed to spoilage and its physical characteristics. The small size of both canola seeds and the void spaces around the seeds increases the resistance of this crop to airflow. Aeration fans must operate at static pressures two to three times greater in canola than in cereals, consequently systems designed for cereals may not produce adequate airflow rates through canola; and the risk of spoilage to the seed may be substantial. In B. napus varieties of canola, aeration fans typically operate at static pressures of 200 pascals at a seed depth of 3.4 m, and 500 pascals at 8.3 m depth. The smaller seeds in the B. rapa varieties can increase these static pressures to 300 pascals and 750 pascals for the 3.4 m and 7.3 m depths, respectively (Thomas, 1984).

Many duct and perforated floor arrangements are available for use with aeration systems (Friesen and Huminicki, 1986). Given the sensitivity of canola and the difficulty in forcing air through it, a large perforated floor area is required (Thomas, 1984). Perforations must be small enough that seeds cannot enter the air passages. A 6-m deep bin requires a minimum of at least 15% perforated floor area; a 10-m deep bin should have at least 25% perforation to avoid excessive air velocities (Friesen and Huminicki, 1986). A completely perforated floor usually produces uniform airflow throughout the bulk and reduces the chance of unventilated spoilage pockets developing (Mills, 1990). Uniform air distribution is more difficult to achieve in flat (horizontal) grain storages than in cylindrical bins because flat storages usually have less uniform grain depths; to help offset the associated air-distribution problems, higher airflow rates of 2 to 3 L.s-1.m-3 are used. Some common duct arrangements used for flat storages are described in Friesen and Huminicki (1986). Foreign material in the canola bulk may reduce the overall static pressure requirements, but increase the possibility of spoilage. Furthermore, the concentration of dockage at the centre and outer edges of the bin creates uneven resistance to airflow and hinders the effectiveness of conditioning operations. The uniform distribution of fine and coarse material is advantageous when canola is being aerated (Thomas, 1984).

Natural, near-ambient, or unheated-air drying
Moisture can be removed from stored canola by passing outside air at high flow rates through the bulk with the only heat coming from the fan and motor. Grain in a ventilated bin begins to dry where the air enters the bulk, usually at the bottom of the bin. A drying front develops and moves slowly upward through the bulk. Below the drying front the grain is at the temperature of the incoming air and at a moisture level in equilibrium with the incoming air. Incoming air at 70% relative humidity, for example, will result in moisture levels of between 8 to 9% for canola seed (Table 4).

Table 4: Relationship between seed moisture level and relative humidity of ambient air for drying of oilseed rape+

Relative humidity (%)
5057657277828688
Moisture level (%)
66.67.48.210.011.212.813.9
+ Bailey (1980)

The grain above the drying front will remain at a moisture level within about 1% of its initial storage condition. The rate of movement of the drying front is mainly affected by the airflow rate per unit mass of seed. To dry all the stored crop in the least possible time requires a uniform air pattern throughout the bulk. The airflow pattern in a bin equipped with a completely perforated floor and a levelled grain surface is uniform unless a centre core of densely packed grain and dockage has formed under the filling spout (Mills, 1990). The required airflow rate for unheated-air drying depends on the type of grain, when the grain is harvested, its initial moisture level, and the outdoor air conditions (Friesen and Huminicki, 1986). Typical airflow rates for unheated-air drying are in the range of 5 to 25 L.s-1.m-3 (Thomas, 1984). For the lowest equipment and operating costs, the lowest recommended airflow is used. Minimum airflow rates for in-bin drying are chosen so that the crop dries just before it undergoes unacceptable spoilage (Mills, 1990). Theoretical minimum airflow rates for natural-air drying of canola in the area of Winnipeg, Manitoba without loss in quality are given in Table 5 (Muir and Sinha, 1986).

Table 5: Theoretical airflow rates for natural-air drying of canola+

Predicted Minimum Air Flow Rate Requirements
for Natural-air Drying in the WinnipegArea
Initial Moisture Level (%)
11
13
15
Air Flow Rate, (L/s)m3*
August 15
11
15
28
September 1
13
20
23
September 15
17
20
24
+Muir and Sinha, 1986.

*Predictions are based on the top layer dried to 10.0 % moisture within 15 - 20 days for the median of 17 years of recorded meteorological data.

A more comprehensive simulation model has been developed by Arinze et al. (1993) for in-bin drying of canola under Saskatoon, Saskatchewan weather conditions.

The drying of canola by near-ambient air is currently favored in western Canada rather than the use of hot-air. Near-ambient drying is preferred because there is a saving of energy, a smaller initial investment in equipment, and an improvement in the quality of the dried product compared to that dried with hot-air systems (Jayas and Sokhansanj, 1985).

Operation of natural-air drying systems
The fans should be started as soon as the canola covers the perforated areas of the bin floor and should be operated continuously in the fall until either the crop temperature is reduced to 0C or the crop is dry. In spring, if drying was not completed the previous fall and no spoilage has occurred, drying is continued when the air temperature rises above 0C. Even under humid or rainy conditions the fan is operated continuously to ensure that the main drying front will continue moving through the bulk despite the risk of rewetting the bottom slightly. As long as the fan operated for a few days after the humid period, the moisture will redistribute through the bulk and will not cause spoilage. Rewetting can be an economic benefit if the canola at the bottom has dried below the maximum allowed selling moisture level. Although it improves the storage quality, any drying below this regulatory value reduces the saleable mass, and thus the monetary value of the bulk (Mills, 1990).

Natural-air drying of canola
The proper design of natural-air drying systems is important because of the need to dry the bulk quickly enough to prevent spoilage. High airflow rates are used which result in high static pressures. The airflow resistance of a grain mass is directly related to the depth, so reducing the grain depth is one way of bringing static pressures into an acceptable range. Fans drying B. napus varieties of canola operate at typical static pressures of 1,000 pascals at a depth of 3.0 m, and 2,000 pascals, at a depth of 4.3 m. Using comparable fans to condition B. rapa varieties, depths are restricted to 2.6 m and 3.6 m, respectively (Thomas, 1984). Bulk density and porosity are major considerations in designing near-ambient drying and aeration systems because these physical properties affect the resistance to airflow of the stored mass (Bern and Charity, 1975). Jayas et al. (1989) examined the bulk density and porosity of B. rapa (cultivar Tobin) and B. napus (cultivar Westar) using loose and dense filling methods (dense fill simulates packing when seed falls from a significant height). The bulk density of Westar was 3.6% lower than that of Tobin canola. Dense fill resulted in bulk densities about 12% higher and porosities about 14% lower than the respective properties for loose fill. High flow rates require an effective design for the air distribution system. The ducts delivering air to the bin, together with any transitions along this network, must have sufficient area for the required airflow. Many people recommend fully perforated floors for natural-air drying systems, especially for canola (Friesen and Huminicki, 1986). Natural air drying can lower canola moisture content up to 2% over 2 months after harvest (Sinha et al., 1981).

Heated -Air Drying

Heated-air drying is used when aeration or natural-air drying fails to adequately condition canola. This may occur when ambient (outdoor) weather conditions are wet and cold or when canola is very damp following harvest. Rapid drying is essential to prevent spoilage. Other circumstances for heated-air drying are the need for an early harvest and rapid drying to meet best markets or contractual obligations, and to reduce the risks at harvest to producers having lower field-harvesting capacities. Hot-air drying differs from natural-air drying in that heated-air will absorb considerably more moisture from the grain, and the warming of canola forces moisture out much more rapidly (Thomas, 1984). Bin, batch, and continuous heated-air dryers may be used to dry canola. Multistage drying, using grain dryers and high-capacity aeration systems are also used effectively (Friesen, 1981; Thomas, 1984). An efficient system of augers, hoppers and other handling equipment is necessary when heated-air drying systems are used to ensure a continuous flow of grain from the field to the bin (Friesen, 1981).

Drying temperatures
Canola destined for seeding purposes should be dried at less than 45C; however, for oil extraction, seeds can be dried at up to 82C. Lower temperatures are used when canola is damp (over 12.5 % moisture) or when it is to be stored for over six months. Overdrying causes cracking of the seed coats; damaged seeds undergo a marked rise in the level of free fatty acids causing a reduction in oil quality. Seeds dried to moisture levels below 6% are very fragile and subject to mechanical damage during handling, whereas seed above 7% moisture will not suffer cracking (Nash, 1978). Pathak et al. (1991) observed visible cracks and blackening when seeds were dried at an elevated temperature of 250C. Canola drying temperatures are shown in Table 6.

Table 6: Safe drying temperatures for canola+

Maximum Temperature of Drying Air C
Commercial Grain
Grain Condition
(% moisture)
Seed GrainMixed During
Drying
Unmixed
Tough (>10%)
49
82
71
Damp (>12.5%)
43
71
60
+Thomas (1984)

The maximum air plenum temperature for drying canola depends on seed moisture level, seed viability temperature, expected storage period, type of dryer used, and other factors. Generally, a wetter seed requires a longer drying process at a lower drying temperature. Seed viability is adversely affected when drying temperature is too high; damage is more likely to occur when the seeds are dry or nearly dry. To prevent seed damage, it is important that maximum seed temperature does not exceed the maximum allowable temperature for the type of seed and its intended purpose. A non-recirculating batch dryer or a dryer which does not mix or circulate the seed requires a lower operating temperature as seeds next to the hot-air plenum will warm to near the hot-air temperature (Friesen, 1981; Thomas, 1984). For these dryers, temperatures 5 - 10C lower than those listed for commercial use are advisable.

Drying considerations
Drying decisions will depend upon the maximum periods available for drying before mold growth occurs (Table 3). The drying rate for canola is less than that for cereals because of the reduced airflow through the smaller, more densely packed seed. As canola offers more resistance to airflow than cereal grains, the fan on a dryer operating at the same speed used for grain will produce a higher static pressure but considerably less airflow. This causes the temperature of the hot-air plenum to rise unless the fuel flow is reduced. Another consideration in drying canola is leakage. Grain dryers are designed mainly for wheat and corn (Zea mays L.) and must be adjusted and checked for canola losses through leakage or being blown from the drying chamber by the higher static pressure. Screens and floors of dryers more than 10 years old should be checked for rust perforation to prevent canola leakage (Harner, 1989). Green weed seeds and canola stems and pods may interfere with the passage of canola through the dryer and at high drying temperatures stationary canola may catch fire (Thomas, 1984). Canola seeds may also ignite when they are passed by the burner. Fire risk when drying canola may be reduced by cleaning the seed to remove light or fine material before drying, removing accumulations of debris from the walls and other areas of the dryer, using wind deflectors to prevent drawing airborne material through the burner, avoiding overdrying the seed, and putting canola through the dryer on warm sunny days without starting the burner (Mills, 1989).

Summary

Stored canola differs from stored wheat because, unlike wheat, adverse changes can occur very rapidly. Canola grain may go through a period of active respiration after binning, and if the heat and moisture is not quickly removed, mold growth and increased respiration soon occurs (Mills, 1989). Seeds can be conditioned to avoid spoilage in storage, to extend the harvest season, and to reduce field losses. Conditioning systems using aeration, natural-air drying or heated-air drying or a combination of these can ensure safe storage (Thomas, 1984). Going from cereal grains to canola during drying operations requires temperature readjustment because reduced airflows increase drying times and the possibility of unsafe temperature buildup (Canola Council of Canada, 1981). Under western Canadian conditions, canola can be stored readily for long periods of time at moisture levels of 8 to 9% if seed temperatures are below 20C and insect and mite infestations are not present (Thomas, 1984).


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