Citrus Bitterness, Begone!
Extracting juice for lab tests, chemist Shin Hasegawa seeks to
genetically engineer citrus trees to form a natural debittering enzyme.
Troublesome compounds called limonin and nomilin can make even the freshest
citrus fruit taste bitter.
Some kinds of citrus trees thwart these natural chemicals by neatly
attaching a sugar (glucose) molecule onto them. This simple maneuver converts
limonin and nomilin to nonbitter compounds called limonoid glucosides.
ARS chemist Shin Hasegawa and colleagues were the first to discover limonoid
glucosides about 7 years ago. So far, they've uncovered about 20 of these
Now, Hasegawa and co-researcher Wan-Jean Hsu want to genetically engineer
citrus trees to boost the rate at which they convert the bitter parent
compounds into benign limonoid glucosides. Tomorrow's genetically engineered
navel orange trees, for instance, would naturally debitter their own fruit.
Currently, limonin causes navel orange juice to turn bitter and unmarketable
just hours after it's squeezed.
The bitterness reaction, says Hasegawa, can also be triggered by freezing
weather and costs navel orange growers in California about $8 million a year in
lost sales. Hasegawa and Hsu are based at the ARS Western Regional Research
Center in Albany, California.
To sweeten citrus of the future, Hasegawa wants to find the gene that cues
trees to hook a sugar molecule onto limonin or nomilin. Genes do that by
instructing the tree to form an enzyme that carries out the task.
Hasegawa has tracked down an enzyme in navel oranges that fastens a glucose
molecule to limonin.
"This enzyme, limonoid UDP-D-glucose transferase," he says,
"is the product of a gene, so it can lead us back to it." He did the
work with chemist Charles G. Suhayda and with Mitsuo Omura of the Japanese
government's Fruit Tree Research Station in Okitsu, Shimizu, Japan.
Hasegawa estimates his Okitsu collaborators may pinpoint the debittering
gene by the end of this year. Once that happens, the biotechnologists will try
to rebuild the gene, to make it more powerful. Then they will slip it into
citrus cells that are growing in laboratory petri dishes. The researchers can
nurture the cells into healthy young trees with the retooled gene working
Besides blunting bitterness of high-tech navel oranges of the future,
limonoid glucosides may be featured in an innovative beverage for Japanese
One of Japan's largest producers of fruit juices, teas, and other drinks
intends to produce an updated orange juice that offers concentrated extracts of
such limonoid glucosides as limonin 17-beta-D-glucopyranoside. The company, a
part of Wakayama Prefectural Federation of Agricultural Cooperatives, became
interested in the debittering compounds when work at Baylor University showed
that the chemicals reduced the size of cancerous oral tumors in laboratory
Hasegawa and colleagues discovered limonin 17-beta-D-glucopyranoside and
furnished vials of the tasteless, odorless white concentrate for the Baylor
experiments. Edward G. Miller, who is in the Baylor College of Dentistry,
conducted the tests.
Wakayama Corporation will use limonoid glucosides extracted from
juice-processing leftovers, including peels, seeds, pulp, and albedothe
thin white skin that encloses citrus segments.
The new product, not yet named, will boast about three times more limonoid
glucosides than a typical 8-ounce glass of orange juice. Wakayama plans to
begin test-marketing the juice-based drink this year. By Marcia
USDA-ARS Process Chemistry and Engineering Research Unit, Western Regional
Research Center, 800 Buchanan St., Albany, CA 94710; phone (510) 559-5819, fax