New route to sweet navel orange juice.

New route to sweet navel orange juice

Some fruits -- especially navel oranges -- make for sweet eating but bitter drinking. The reason is the natural conversion of limonoids in the squeezed juice into intensely bitter chemicals such as limonin. Last year, Shin Hasegawa reported finding bacteria that could subtly transform limonin into nonbitter alternatives (SN:8/10/85,p.89). Now this biochemist, who is with the Agriculture Department's Fruit and Vegetable Chemistry Laboratory in Pasadena, Calif., reports identifying a potentially less costly treatment: spraying leaves of susceptible fruit plants with auxins--a plant hormone--before harvest.

Hasegawa's studies have shown auxins such as naphthaleneacetic acid to be a potent inhibitors of nomilin, the chemical precursor of limonin. In 30 experiments, he demonstrated that auxin treatment of developing plants can block formation of up to 90 percent of the nomilin produced by similar but untreated plants -- three times the reduction necessary to reduce limonin to nondetectable levels in navel orange juice.

Though Hasegawa's studies have not yet involved navel oranges, he says, "we are very confident that [auxins] will work in them" because the bitterness affecting their juice develops by the same nomilin-to-limonin conversion that occurs in the lemons and other fruits he studied. Still to be determined are the optimal timing, dose and frequency of auxin treatment. Hasegawa hopes to explore such factors in field tests, perhaps beginning next spring. If the process proves effective and economical, he believes it could bring California growers of susceptible fruit another $6 million to $8 million a year by allowing them to sell for juice any yields in excess of what can be marketed as fresh fruit.