Know yours. Retail onions are generically labelled now but under a new classification system, mild ones suitable for eating raw are to be labelled as mild.

Mild flavoured onions, suitable for eating raw, are to be labelled as mild under a new system for classifying onions.

Australia’s onion industry has decided to support the development of a mild onion certification system following research from the NSW Department of Primary Industries (DPI) which demonstrates that pungency can be reliably assessed.

NSW DPI postharvest researcher, Dr John Golding, says the lack of a reliable, cost-effective test has been the major barrier to date for the development of an Australian mild onion industry.

Dr Golding concluded it was possible to consistently grade onions according to taste after carrying out research with Food Science Australia in Sydney and at the Wagga Wagga Agricultural Institute.

Using a specially constructed onion press, NSW DPI researchers measured levels of the chemical that causes pungency in onions - pyruvate - in 1500 onions.

These levels were then cross checked against rankings collated from a sensory tasting panel and more than 100 consumers.

A trained panel of sensory experts and untrained consumers took part in these taste trials, and both groups were able to reliably and accurately perceive differences between higher and lower levels of pyruvate.

"As expected, onions with the lower levels of pyruvate were equally liked and those onions with the higher levels of pyruvate were equally disliked," Dr Golding said.

Dr Golding presented the highlights of his research at the Australasian Postharvest Conference, held recently in Terrigal, on the Central Coast.

He also spoke about follow-up research, analysing levels of pyruvate in mild onions purchased anonymously over several months from both NSW and South Australian retail outlets.

This study found that some onions sold as mild were in fact not.

"Because pungency increases with storage, it is recommended that mild onions be consumed no more than 30 days after harvest," Dr Golding said.

Research has shown pungency varies with differing soil types and nutrient levels, making it necessary to collect onion samples for pungency testing just before harvest.

Onions with pungencies close to the limit for the mild classification may need to be tested 30 days after the pre-harvest test, while those with lower pungency may not need re-testing if sold within 60 days of the pre-harvest test.

The project was facilitated by Horticulture Australia Limited in partnership with the Australian Onion Industry Association and was funded by the onion levy.

Related science and research pages:

• Postharvest research
• Horticulture research unit

• Contact John Golding, Gosford Horticultural Institute, (02) 4348 1926, john.golding@dpi.nsw.gov.au