Limonoic acid

Limonoic Acid A-Rinq Lactone Content 3 ppm fresh weight mg per fruit... 

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. 

Citrus leaves were shown to be the site of limonoic acid A-ring lactone biosynthesis in citrus (55). The lactone accumulated to the level of 2000 ppm in very small leaves but as the leaf grew, the lactone content declined. The lactone content of the fruit increased as the level in the leaves declined. Hasegawa and Hoagland (55) also showed that limonoic acid A-ring lactone was not synthesized in the fruit but in the leaves. The radioactive labeled lactone was isolated from a fruit adjacent to a leaf actively synthesizing it from labeled acetate, indicating that the lactone was synthesized in the leaves and transported to the fruit (55). 

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. 

Maier, V. P. Margileth, D. A. Limonoic acid A-ring lactone, a new limonin derivative in citrus. Phytochem., 1969, 8, 2A3-2A8. 

Fig. 25.13. Limonoic acid A-ring lactone and nonbitter dehydro-derivative of limonin.

No evidence of limonoid biosynthesis in fruit or seed tissues exists, despite the fact that most of the limonins are found in the seeds of mature fruits. Limonoid synthesis occurs in the leaves and limonoids are transported into the fruits (Maier, 1983). In citrus tissues, the naturally occurring precursor of limonin is a salt of limonoic acid A-ring lactone (60) (Fig. 25.13) in which the A ring is closed and the D ring is open. This tasteless compound is stable only in the salt form (Maier, 1983). In the presence of acid or the enzyme citrus limonoate D-ring hydrolase, the D-ring lacton-izes to form limonin (19). The rate of lactonization is accelerated by pasteurization of the juice. In the fruit, the precursor appears to be located in a compartment of the cell where the pH is neutral or alkaline, probably the cytoplasm (Maier, 1983). 

Hasegawa et al. (56) dj ected the enzymic conversion of 19-deoxylimonoic acid 3-methyl C ester to the 17-dehydro derivative by albedo tissue slices of Navel oranges. They isolated the product and identified it by TLC as the reaction product formed when the substrate was dehydrogenated by limonoate dehydrogenase (EC 1.1.1)... 

The intact fruits do not contain bitter limonin, but rather a nonbitter precursor, limonoate A-ring lactone (8). When juice is extracted, this nonbitter precursor is gradually converted to limonin under acidic conditions and the conversion is accelerated by the action of limonin D-ring lactone hydrolase, which has been isolated from citrus (9). The bitterness due to nomilin in juices most likely develops in a manner similar to that of limonin bitterness, but this has not yet been directly proven. However, the contribution of nomilin to juices is minor. It occurs mainly In grapefruit juices (4). 

Fmit tissues do not normally contain bitter limonin, but instead contain the nonbitter precursor of limonin, limonoate A-ring lactone (2). Limonoate A-ring lactone is the predominant limonoid aglycone present in fmit tissues of most citms species, and it is gradually converted to limonin after the juice is extracted. This conversion proceeds under acidic conditions below pH 6.5 and is accelerated by the... 

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