Patience is a must for scientist Grant Bignell and field operations manager Dave Bruun who have taste-tested thousands of weird and wonderful fruits from orchard trials at Queensland's Maroochy Research Station.

Their harvest comes in a dizzying array of shapes, colours and sizes, and some completely impractical for transport.

"A lot of the stuff that comes out of the breeding work isn't at all attractive," Mr Bignell said.

"We get some really ugly shapes, you can see one down the end there just looks like a big mess of grapes, but other times we see some beautiful shapes and some beautiful colours.

"It's sometimes the luck of the draw when you're in the breeding project."

Australia leading the way
Custard apples, also known as sugar-apples and sweetsop, are native to the tropical Americas and West Indies but Australia has the world's largest breeding program and a market niche in terms of timing of production.

"It's a very tropical flavour, a rich creamy taste. It feels like it's a bit of a treat when you're eating it, because it's so sweet," Mr Bignell said.

"We're seeing some different flavours coming out of the breeding project as well, because of the species we're bringing in, so bringing in those red fruits we're getting a really tangy flavour, a lolly flavour in some of those as well."

Since the trials began a quarter of a century ago, the researchers have released just one new commercial variety — Maroochy Gold — but, although it thrived on farms in the Glass House Mountains and Atherton Tablelands, it did not perform well in northern New South Wales.

They expect it will take another two to three years to release three new green-skinned varieties and a stunning red-skinned fruit is now being tested on a small scale on farms.

"It's been a long process for the industry," Custard Apples Australia president Daniel Jackson said.

"The Maroochy Gold tested really well in a localised area, but when it got sent commercially to other growing regions, it was really hit and miss. People had trees for six or seven years, that still weren't bearing fruit.

"So that has left us very cautious to race into releasing another variety and is a perfect example of why we need to test in all the different regions."

Mr Jackson said the focus had switched to developing red-skinned fruit.

"It seems to be the search for the Holy Grail that everyone wants the perfect red one. "We have one there that we're very, very happy with, but we might just do a couple of little tweaks with the breeding and just see if we can improve it, hopefully in the next three to four years.

"People's eyes light up when they see the red custard apple, it definitely has potential for the Asian markets for sure," Mr Bignell said as he detailed the years of hard work involved in the breeding project.

From the initial crosses made by hand pollination at Nambour, it took three to four years for the plants to grow.

It then takes up to five to six years for each stage, from small-scale testing, to large-scale testing and the final decision on whether to go to commercial release.

The breeding project received funding from Horticulture Australia using the Custard Apples Australia levy, matched by the Federal Government, with support from the Queensland and New South Wales Governments.

Dave Bruun, who has worked at the Queensland Department of Agriculture and Fisheries research station for 30 years, said the custard apple breeding project had come a long way since its inception.

"Years ago, a lot of the fruit just tasted horrible and you would get the odd good one. Now we're consistently getting good tasting, good characteristics — it's a happening thing," he laughed.

Daniel Jackson said around 830,000 kilograms of custard apples were produced in Australia last year and this year's crop was expected to set a record, thanks to new farmers entering the industry.

There are 130 growers listed with Custard Apples Australia, with farms ranging in size from 100 to 2,000 trees.

Who is buying custard apples?
Mr Jackson said the Asian market was quite large, in the Western Sydney and Melbourne fruit stores.

"There definitely is increasing demand as people become aware of the health benefits and taste of custard apples," he said.

"We also ran an information project in Coles and Woolworths and we're led to believe that increased demand inside the chain stores, which was quite good because Australians aren't really accustomed to eating custard apples or paying for custard apples because there's always been a backyard tree."

Outside the research station two new varieties have been produced in Australia.

"We have had another success with the KJ Pinks, which was a budsport of an existing old variety of Pinks Mammoth custard apple, which became commercial and that's probably been the most widely uptaken variety for the last 15 years," Mr Jackson said.

He explained that a budsport is a part of leaf, shoot or flower that, due to genetic mutation, clearly differed from the rest of the plant and could be grafted to produce new plants, which retain this genetic difference as a new cultivar.

Yanalla Farms in the Glass House Mountains is also breeding a new variety called PinksBlush, which Karen Martin's father-in-law first found around 20 years ago on a single branch of an existing variety of custard apple called Pinks Mammoth, in their Glass House Mountains orchard.

The late-fruiting variety of pink-skinned custard apple has led to booming demand, but PinksBlush is currently only being cultivated on their farm.