- The Vegetable Farmer
With around 9,500 ha grown last year, onions are one of the most important stored vegetable crops in the UK, and a combination of ambient, refrigerated and controlled atmosphere storage means that domestic crops can be supplied for 52 weeks of the year. Richard Crowhurst reports.
With the storage of onions being so important, a recent course on Best Practice Onion Storage, organised by Agri-tech Register and Training for Innovation and Skills (ARTIS) and taught by Andy Richardson of Allium & Brassica Agronomy at Kirton in Lincolnshire, proved popular with a number of growers and dedicated store managers.
The day began with an overview of onion production in the UK, including the potential effects of last year's difficult season on supply and imports this year. With production across Europe down, total availability is some 6-700,000 tonnes down on normal.
With respect to stored crops, Andy explained that, "what you do at harvest can make your life in storage so much easier." The natural dormancy of onions can vary greatly between different varieties and types, and the warmer the temperature, the quicker the crop respires and the sooner dormancy is broken, although, "the thing that really breaks dormancy is fluctuating temperatures," he added.
If using the chemical sprout suppressant maleic hydrazide (MH) in the field, the timing of application is crucial and should be some 10-14 days before harvest when 10-50 per cent of the crop's foliage has fallen over. Andy added that it is important to apply MH on its own, rather than with other crop protection products, as their formulations can result in MRL exceedances and to use the recommended water volume for the particular product being used. Ethylene is also used as a sprout suppressant in some stores, although there is a feeling that onions stored with ethylene, and in CA stores may have a shorter shelf life in terms of sprouting when packed.
Store design and function
Having harvested the crop at the right time and in good condition, crop quality is determined by the store and its management, and the next part of the course looked at onion store design and function, including drying and air movement.
"It doesn't matter if the air is blown or sucked through the crop, it is the air movement which does the drying," explained Andy. "If you can stick your hand in the airflow and feel air movement, then it is probably sufficient."
While feeling for air is a quick practical check, the course went into detail on the volumes of air required, how the relative humidity (RH) and temperature of the air interact, and fan types and how to size them.
In a bulk store, the initial drying period requires 250ft3/min (cfm) of air per tonne of crop, which is between 2½ and 4 times that usually used with potatoes. This is one reason why the best results come from using dedicated rather than multi-purpose stores.
Air volumes below this slow the drying period and can increase susceptibility to diseases such as Fusarium and neck rot. While greater air volumes will dry the crop quicker, they will require larger burners to heat the air, which is less economic, and they may also dry the crop too quickly resulting in skin cracking.
Onions are either stored in bulk (usually on a slated floor) in piles up to 16 ft deep when first filled, or in box stores. These days most onion boxes can hold up to 3 tonnes of crop, although some systems use larger 17 tonne “container” sized boxes. These are based around letter box systems (which blow the air) or suction systems which move air through the pallet bases of the box and then through the crop.
"As we use the pallet bases as a plenum, you need to plug them at the end of the row to ensure air movement through the crop," explained Andy. "Box stores tend to work very well for the first four or five years or so, but after that there is a danger that the boxes become leakier and so air flow is less controlled.”
In theory, if the airflow is correct a box store will dry the crop quicker than a bulk store, but it can be more difficult to dry the crop evenly. Air turbulence next to the fans can also make drying in that region of the door difficult and so fans should be set back from the first lateral opening by 1-2 meters.
Baffles can also be installed in the tunnel to improve air flow and ensure air reaches the furthest laterals. Suction systems are better at reducing turbulence, but they have the disadvantage that they need to control the RH and temperature in the entire store, rather than just the incoming air. Box construction is particularly important in suction systems so that air cannot short circuit through box slats for example.
Another issue that both growers and store managers need to consider is that harvesting capacity has increased over the years and so the capacity of a store for stage 1 drying of the crop (which requires much more air movement than holding crops) may be insufficient.
For example, a 1,200 tonne store may only have a drying capacity of 300 tonnes, so to obtain maximum crop drying it will need to be filled in four separate batches. The fan size drives this capacity and course attendees were taught not just how to calculate how big fans need to be, but also how to check the size of their current fans and calculate the air movement that they can achieve. The size of the fan also determines the size of the duct or plenum, the burner and the ventilation louvres and the calculations for determining these were also explained.
Different strategies for drying, curing and holding crops in both ambient and refrigerated stores were compared, and Andy said that most differences in practice occur in the approach taken to Stage 2 drying.
The RH used for the first stage of drying is also determined by the crop condition at harvest, with more mature senescent crops benefiting from a higher RH in the duct. The end of Stage 1 drying occurs when the crop is “rustling dry”, typically at a temperature of 26oC and a RH around 60%. This is where management practice can differ between an 'extended Stage 1' and Stage 2: the main difference is in the RH in the duct and the use of sensors to control burners and fans according to RH in the store.
"In a way there is no right or wrong approach," added Andy. "We are trying to teach storage as a science, but it is also an art and you use your experience. The faster that you dry the crop, the fewer disease issues you will have, but you then have to consider increasing skin issues, and you will probably have to contend with different crops in the same store. This is why a good store manager is worth his weight in gold."
Once all the moisture has been dried from the neck of the crop, which normally occurs after three or four weeks, the crop is cooled by around 0.5-1oC per day until the desired holding temperature is reached. Different combinations of ambient and refrigerated air are used at different stages.
The target crop temperature should be set 0.5-1oC below the average actual crop temperature, while the duct temperature should be set 2.5-3oC below the target temperature. Andy explained that it is normally relatively easy to cool crops to 14-15oc using ambient air and that as a result, fridge units should not be switched on until the temperature is 15oC or less, and then can be used to
obtain holding temperatures of 0.5-1oC at an ideal RH of 75-80%.
When ready for use, crops which are 2oC or more below the ambient temperature should be warmed up or conditioned prior to use. While some large growers and packers have dedicated conditioning areas, other will have to manipulate the store controls appropriately.
The final part of the course looked at problems, including the identification and assessment of storage diseases, physiological issues and internal growth. In particular Andy pointed out that with the loss of thiram seed treatments, neck rot could become more of a problem.
"For control we are now down to a single active ingredient that we know some strains are already resistant to. It is a disease that we have largely forgotten about and it is often confused with bacterial rotting, although it doesn't smell and you get grey/black fluff forming between the scales of the bulb."
The incidence of Fusarium has also increased over the last ten years, although the reasons for this increase are not fully understood. "In five out of the last six years Fusarium has been the number one disease on drilled onion crops," he pointed out.
"The more times you grow onions on the same field the more susceptible they seem to be, but rotation length seems to have little effect on disease incidence. We've got to look at tackling it in different ways with integrated control, and that includes Stage 1 drying."