Limonin reactions

The quality of extracted citrus juices depends on enzyme reactions that occur not only in the fruit during the development period, but also in the juice during processing. When juice is extracted from citrus fruit, enzymes are released from their normal restraint in the cell. Several of these enzymes catalyze reactions that adversely affect taste and appearance of the juice. Unless the reactions are controlled, the juice products will not meet the standards of quality set up by the USDA Food Safety and Quality Service. The two reactions of commercial importance are the hydrolysis of pectin to pectic acid, which clarifies juice, and the lactonization of limonoic acid A-ring lactone to the bitter compound, limonin. Research efforts to identify and characterize the reactions, to isolate and purify the enzymes, and to develop methods to control the reactions are described in this review. 

Although limonoic acid A-ring lactone has been shown to be a substrate of a lactonase that catalyzes the formation of limonin in citrus fruit, several other enzymes in citrus can also use the A-ring lactone as substrate, however, the products are not bitter products. These enzymes and their reactions are reviewed in the next section. 

Pectinesterase and limonin D-ring lactonase are the only enzymes known to catalyze reactions that adversely affect the quality of citrus juices. Bruemmer et al. (64) listed other enzymes that have been detected in citrus juices and described some of the reactions that can occur in the juices. None of the reactions appear to noticeably affect the quality of commercial juices. Freshly extracted citrus juices contain esterase (EC 3.1.1.1) (65, 66) and phosphatase (EC 3.1.32) (66, 67) activities. Native substrates in orange juice for peroxidase... 

The colorimetric method (Wilson and Crutchfield 1968) and the fluorometric method (Fisher 1973) involve the chemical derivation of the limonoids. However, the reactions are not specific for limonin, and these techniques depend on isolating limonin from interfering compounds. Kruger and Colter (1972) developed a derivitization procedure to allow for the non-volatile limonin in citrus juices to be analyzed by gas chromatography. 

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