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close this bookCERES No. 158 March - April 1996 (FAO Ceres, 1996, 50 p.)
close this folderCerescope
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Lab results: Sweetening the bitter fruit

U.S. and Japanese scientists are cooperating in an attempt to remove the bitterness from the juice of navel oranges by genetic engineering. All citrus fruits contain a substance that turns into bitter-tasting limonin when juice is extracted or the fruit is processed into fresh segments or slices. Taste-panel studies indicate about 30 per cent of consumers object to the taste of juice with as little as two parts per million of limonin.

The bitterness is far more pronounced in navel oranges, and their juice has to be diluted to make it saleable. But the bitterness of limonin and the closely related compound nomilin can be neutralized in some kinds of citrus by an enzyme that attaches to each bitter molecule a molecule of glucose, which transforms them into non-bitter compounds called limonoid glucosides. Shin Hasegawa and his team of researchers at the U.S. Department of Agriculture's Western Regional Research Center in Albany, California, have identified the enzyme in navel oranges that links the glucose molecule on to limonin or nomilin and isolated about 20 liminoid glucosides thus produced. Hasegawa's group is cooperating with Mitsuko Omura of the Japanese government's Fruit Tree Research Station in Okitsu, Shimizu, in an attempt to work back from the enzyme to find the gene that directs the enzyme's production. “The Japanese group is a pioneer in transgenic work with citrus,” Hasegawa said. “It is doing the genetic engineering part of the research.”

The Japanese scientists will introduce the debittering gene, perhaps modified to increase its power, into single navel orange cells growing in laboratory petri dishes. From these engineered single citrus cells trees can grow and bear fruit with the modified genes at work.