Limonin Monolactone, the Nonbitter Precursor Responsible for Delayed Bitterness in Certain Citrus Juices

SUMMARY– Tissues of early-season navel oranges and grapefruit were found by paper electrophoretic procedures to contain a nonbitter precursor of limonin, but no significant amounts of limonin. Limonin is the intensely bitter triterpenoid dilactone responsible for the bitterness which develops in certain navel orange juices on standing. The nonbitter precursor was identified as limonin monolactone by comparison with the authentic compound prepared by partial hydrolysis of limonin and by acid-catalyzed conversion into limonin. Limonin monolactone is stable in the tissues of the intact fruit (which are not bitter) because it is apparently not in direct contact with the acidic juice. It is slowly converted into limonin (and the juice becomes bitter) when the fruit tissues come in contact with the juice, after the juice is expressed from the fruit. Limonin monolactone was not detected in late-season navel oranges or grapefruit. This agrees with the fact that juice made from these fruits does not contain limonin. Work is in progress to determine whether the naturally occurring limonin monolactone is the A-ring monolactone, the D-ring monolactone, or a mixture of the two.