At the approach of the harvest season, there’s always a lot of discussion about the best ways of harvesting and storing onions. What are the risks? Precisely which method should you use to dry them? Heaters or condensation dryers? What temperature?
Evert Steenge (Onion adviser) and Huub Kasius (AgroVent BV), aim to answer these questions in the following article, which will be part 1 of a 2 part series.
Drying onions: facts and fables...
We’ve already noticed in the last few seasons that drying and storing onions is becoming more challenging. Higher temperatures combined with extreme and unpredictable precipitation mean that the first few days at the start of storage can be a headache.
Higher temperatures and a lot of precipitation mean an increase in the risk of bacteria: Erwinia carotovora and/or Pseudomonas alliicola are splashed onto the leaves and enter the plant though the stomas. As the plant gets older the bacteria descend to the neck. This also applies for the mould Botrytis aclada, also known as onion neck rot.
The spores reach the leaves from onion waste. This is why it is extremely important to harvest and dry out the onions as early -but responsibly- as possible (when the foliage is 50% green). Outside, drying air has to be used with the lowest possible humidity. Many allow their onions to continue to grow in the expectation of extra tonnage, but long term research by Applied Plant Research [praktijkonderzoek plant en omgeving or PPO] demonstrates that, while allowing further growth produces 7.1% extra gross, the net product gain is only 1.1%.
Stories that the neck will close up prematurely if onions are dried too quickly (before the onion has lost its moisture) are absolute nonsense. The moment of harvest and how fast you dry are crucial for the percentage of rot in the store. It is important that bacteria and spores have as little time as possible to enter the bulb via the neck.
Unfortunately, onions are often dried without a heater, or with heaters which warm up outside air by just a few degrees. Onions which should be dry in five or six days (so that the neck can’t roll between thumb and index finger) are only dry after 10 or 14 days. Bacteria and the spores of the neck rot mould have then had all the time they need to enter the onion. Moisture is also sucked in because the difference between the day and night time temperature can easily be more than 40 C. If the hatches are also (partially) closed then the result is zero. The risk of penicillin is great, the skin of the onion disintegrates and becomes patchy. The quality of the onion quickly deteriorates and that’s something which we don’t want, of course...
What does this mean for the drying methods?
So we need to dry onions as quickly as possible at the highest possible temperature and with the lowest possible humidity, and with enough air, of course.
Experience with condensation drying over the years has shown that drying with a low relative humidity (as low as 50%) delivers the best result, and that the risk of dehydration is much less than one might expect. Research in the past has shown that using heaters is a good method of drying onions as quickly as possible, but considerations of energy cost mean that it is done too little in practice, leaving condensation drying as the only good alternative.
In order to reach a low humidity with conventional methods, big gas heaters have to be used to increase the air temperature, an average of approx. 5-7 degrees Centigrade in our climate. We also want the shortest possible drying time, so that you actually want to heat the air to more than 30 degrees. That demands a large heating capacity. The authors' categorical advice is, in any case, not to heat continually with a low heating capacity and partially closed hatches; the risk of condensation is simply too great!
The big difference between drying with heaters and drying with condensation dryers, is that a condensation dryer dries from the very first minute!
Whatever method you choose, we just have to accept that we can’t expect to get a dry and good product every year with just ‘free’ outside air.
Next week we will publish an article on the different options growers have for drying.