Four years of work by Western Australian researchers has opened the door for exports of Australian grapefruit to one of the world's biggest markets.Agriculture and Food Minister Terry Redman said the decision by Japan's Ministry of Agriculture, Forestry and Fisheries was welcome news, potentially worth millions of dollars to Australia's growing grapefruit industry.

"Japan is the global pace maker for quarantine standards in many horticultural products," Mr Redman said.

"If our grapefruit is now acceptable to that market, it is likely to lead to other opportunities."

The main barrier to Japanese imports of Australian citrus has been the possible presence of Mediterranean and Queensland fruit flies.

In 2006, Japan granted access for oranges, lemons, mandarins and tangelos following extensive research and verification work led by Dr Francis de Lima at the Department of Agriculture and Food.

The same WA team has since worked to ensure that grapefruit can also match the exacting Japanese quarantine requirements by developing a cold disinfestation protocol for fruit flies.

This work has been supported by Rewards Group, Citrus Australia, the Federal Department of Agriculture, Fisheries and Forestry, Horticulture Australia Limited and Biosecurity Australia.

"As grapefruit comprise about half of Japan's citrus imports, this is likely to have even greater long-term value for Australian industry than the original work on other citrus varieties," the Minister said.

"Cold treatment of fresh produce is a delicate balance in killing potential insect pests without damaging the fruit.

"Under the new protocols, the temperature at the centre of fresh grapefruit must be at or below 2C for 18 days, or at or below 3C for 20 days. This can be achieved during overseas transit."

Mr Redman said Japan's decision could open sales to other northern hemisphere destinations such as the United States, Taiwan, Korea, China and Thailand.

Japanese quarantine inspectors who come to Australia annually to clear fruit have now returned home, so the new trade cannot begin until 2011.