Onion storage revolution?

All year round marketing, the ‘holy grail' of UK grown pre-pack quality onions has long eluded British growers/packers. Whilst new varieties and modern storage techniques such as low oxygen, controlled atmosphere (CA) have undoubtedly helped to extend the season, there is still almost a two month gap in supply of supermarket quality onions from June to August.

For a number of years the UK has relied on the sprout suppressant maliec hydrazide (MH) to extend the marketing season. Whilst this chemical is very effective at reducing internal growth it does leave a detectable residue in the bulb. With minimising pesticide residues firmly on the agenda of the multiple retailers the challenge is to maintain or extend the UK season whilst producing a residue free onion! A new storage technique unveiled by David O'Connor of the Allium & Brassica Centre at the two day UK Onion and Carrot Conference & Exhibition last November promises to answer both of these difficult issues.

The current widespread use of MH in the UK to extend storage life is not without its problems:

•  MH is the only pesticide used in onion production that leaves a detectable residue, typically of between 4-6mg/kg (MRL 10mg/kg). Three supermarkets currently ban the use of MH and others restrict its use.

•  Obtaining uniform uptake of MH by an onion crop is something of a lottery and is determined by a number of factors including variety and crop maturity stage/weather at the time of application. As well as crop to crop variation, bulb to bulb variation can occur which can lead to some bulbs containing up to 3-4 times more MH than others. Sprouting is normally well controlled in onions when MH levels are in excess of 3mg/kg but breaks down where MH levels are below 2mg/kg

A New Approach

In 2000 major UK based potato packers and growers Greenvale began looking at the use of ethylene as an alternative growth suppressant to MH treatment and CIPC gassing of potato crops. Results from these initial trials showed promise and as part of more extensive trials in September 2002 it was suggested by Sandy Norman (Tesco technologist) and major Essex onion producers P G Rix (Farms) that a box of onions should be placed in an ethylene treated potato store. Despite much initial scepticism based on the different physiology of the two crops, to everyone's surprise the onions came out exceptionally well the following June.

More extensive onion trials followed in 2003/4 and 2004/5 funded initially by P G Rix (Farms) and Bedfordshire Growers, then joined by Moulton Bulb Co. and G's Marketing and more recently by Rustler Produce. Research for the group was undertaken and coordinated by the Allium & Brassica Centre and the project was supported by Greenvale's newly formed subsidiary company Restrain Co.

Over the past two seasons extensive testing of the Restrain ethylene generation system has taken place in commercial ambient, refrigerated and CA onion stores as well as by research organisations such as Institute for Vegetable Crops, Skierniewice, Poland and Applied Plant Research (APR), Lelystad, Netherlands.

Results from extensive trials undertaken to date show:

•  Use of the Restrain system consistently shows significant reductions in internal shoot growth by up to 48% when compared to untreated crops in all storage regimes (up to 70% in Dutch trials).

•  The use of the Restrain system on MH treated samples also showed positive benefits on internal growth suppression in two thirds of samples tested, particularly where varieties were susceptible to sprouting, bulb MH levels were low, or where bulb to bulb variation in MH was high.

•  Positive effects on shelf life were also noted with Restrain treated onions showing consistently lower growth levels in all samples after 14 days at 20 ° C when compared to the untreated.

•  Most importantly however no residues were found. Ethylene naturally dissipates from the onion over a 3 day period when it is removed from the treated atmosphere, after this period no detectable residues are left within the bulb.

Results overall show that use of the Restrain system gives comparable control of internal shoot growth to MH treatment. Indeed in many cases Restrain in combination with MH gives more consistent results, particularly in inherently poorer storing varieties or where MH levels are low or variable. As a gas ethylene is able to reach the parts other treatments cannot reach! distributing itself evenly around the store. As a result, all of the crop receives a uniform dose and bulb to bulb variation is eliminated.

The only area where ethylene under performs MH is with regard to shelf life. Whilst Restrain treated crops have significantly better shelf life than untreated crops, once treated bulbs are removed from store the gas easily dissipates from the onion bulb and the growth inhibiting effect is lost, unlike MH which leaves a persistent residue.

The use of ethylene gas in onion storage is currently permitted in the UK under commodity approval regulations. The Restrain Co. are currently working hard to generate the necessary data to support a full approval for use on bulb onions which should be available within the next 2 years.

The Restrain system comprises of a small machine, which safely and accurately generates and controls the production of ethylene gas within the store. The system is easily retrofitted to all existing onion stores from 12 to 2000 tonnes in size, simply by plugging in to any 13amp socket and needs little maintenance apart from a monthly top-up of Restrain fluid. This season nearly 15,000 tonnes of commercial onion crops in both refrigerated and CA stores are being treated with the Restrain system.