Limonoid Bitterness

Limonoids are a group of chemically related triterpene derivatives found in the Rutaceae and Meliaceae families. There are two groups of limonoids the aglycones and their corresponding glucosides. A total of 36 limonoid aglycones have been isolated from Citrus and its hybrids (Table 1). Five of them are bitter in taste -limonin, nomilin, nomilinic acid, obacunoic acid and ichangin (Fig. 1). In citrus juices, limonin is the major cause of delayed bitterness. 

Nomilin the general biosynthetic precursor of all limonoids, is biosynthesized via the terpenoid pathway from acetate and/or mevalonate in stems. Nomilin is then translocated to other plant tissues, including leaves, seeds, and fruit tissues, where it is converted to the other limonoids independently (Hasegawa et al. 1986 Ou et al. 1988). In most citrus species and hybrids, nomilin is converted to obacunone and then to obacunoate. Obacunaote is then converted to limonin probably via the intermediate ichangin. Nomilinic acid is the predominant acidic limonoid in citrus, but it does not seem to be directly involved in the biosynthetic pathways of major limonoids. 

Limonoid aglycones have been shown to possess certain biological activities such as the inhibition of cancerous tumour growth in laboratory animals (Lam et al. 1989 a,b Miller et al. 1989) and antifeedant activities against insects (Alford and Bentley 1986 Liu et al. 1990). Recently, the glucoside of limonin, limonin 17-P-d-glucopyranoside, was found to possess anticancer activities as well (Miller et al. 1992). 

Limonin 3. Deacetylnomilin 5. Ichangin 7. Calamin 9. Methyl deacetylnomilinate 2. Nomilin 4. Obacunone 6. Ichangensin 8. 6-Keto-7p-deacetylnomilol 

Nomilinic acid 12. Obacunoic acid 14. Isoobacunoic acid 16. Isolimonic acid 11. Deacetylnomilinic acid 13. Trans-obacunoic acid 15. Epiisoobacunoic acid 17. 19-Hydroxydeacetylnomilinic acid 

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The relationship between the Simaroubaceae and limonoid bitter principles has been confirmed by showing that glaucarubolone (102) has a triterpenoid origin. ... 

Biochemistry and Biological Removal of Limonoid Bitterness in Citrus Juices... 

Limonoids are a group of chemically related triterpene derivatives found in the plant families, Rutaceae and Meliaceae. Among 37 limonoids reported to occur in citrus and hybrids, four are known to be bitter. They are limonin, nomilin, ichangin and nomilinic acid. Limonin is the major cause of citrus juice bitterness and is widely distributed. Nomilin is also known to contribute to limonoid bitterness in grapefruit juice (4). 

The technical feasibility of an immobilized cell system was demonstrated on a laboratory scale for use in reducing limonoid bitterness of citrus juices. The treatment caused no significant adverse effect on the other juice constituents as judged by analysis of ascorbic acid, citric acid, malic acid, sucrose, glucose and fructone. 

At present there are no totally satisfactory methods of commercially removing limonoid bitterness. Several new approaches are being developed which may lead to a more complete solution to the problem. These include specific adsorbents of limonoids, a biological reactor that uses immobilized bacterial cells, and the preharvest approaches which are presented in the text. 

A species of enigmatic affinities, Harrisonia abyssinica, usually placed in the Simaroubaceae (but see the discussion above), contains both the widespread limonoid bitter principle, obacunone (47) and a second limonoid, harrisonin (48). Harrisonin has insect antifeedant activity, is cytotoxic, and has antibacterial activity. This compound inhibits feeding... 

Dreyer D L 1968 Limonoid bitter principles. Fortschr Chem Org Naturst 26 190- 244... 

Dreyer, D. L. Limonoid Bitter Principles. Fortschritte der Chemie organischer Naturstoffe 26, 190 (1968). 

Manners GD, Breksa AP III, Schoch TK, Hidalgo MB. Analysis of bitter limonoids in citrus juices by atmospheric pressure chemical ionization and electrospray ionization liquid chromatography-mass spectrometry. J Agric Food Chem 2003 51(13) 3709-3714. 

Limonoids are a group of chemically related triterpene derivatives found in the Rutaceae and Meliaceae. Limonin, a bitter member of the group, occurs widely in citrus juices. It has commercial significance because bitterness (excessive bitterness in the case of grapefruit) reduces juice quality. Dreyer (1) and Connolly et al. (2) have reviewed the chemistry and biochemistry of limonoids. 

More recently Maier et al. (3) published a comprehensive review of the limonoid constituents of Citrus and the impact of limonin bitterness on juice quality. This paper summarizes the chemical, biochemical and juice quality aspects of limonoids in Citrus (and related genera) and presents relevant advances since previous reviews. 

While most of the limonoids of Citrus have been isolated from seeds, several occur in detectable amounts in other parts of the fruit. Minor amounts of deacetylnomilin, nomilin, obacunone, de-acetylnomilinic acid and nomilinic acid were identified in extracts of navel orange peel (37). 17-Dehydrolimonoate A-ring lactone was isolated from peel and juice of navel oranges (38) and nomilin has been reported to occur in grapefruit juice and juice vesicles (39). The only Citrus limonoids known to be bitter are limonin, VI, XII, obacunoic acid and IX (3). 

Control of Juice Bitterness. A number of advances have been reported in this field since it was last reviewed (3). A commercial application of the cellulose acetate adsorption technique for the removal of limonin from citrus juices was undertaken (49). New sorbent gel forms of cellulose esters for adsorption of limonin were developed (50). Knowledge was gained that limonoids are biosynthesized in citrus leaves and translocated to the fruit (12) and that specific bioregulators can inhibit accumulation of XIV in citrus leaves (15). Additional studies were carried out on the use of neodiosmin to suppress limonin and other types of bitterness (30,51). The influence of extractor and finisher pressures on the level of limonin and naringin in grapefruit juice was reported (34). Also, further studies were conducted on the microbial sources and properties of limonoate dehydrogenase (52), the enzyme that converts XIV to XV and can be used to prevent limonin from forming in freshly expressed citrus juices (53). 

Bitterness, caused by naringin, is also removed by an enzyme, naringinase. Another possibility would be to eliminate one or more of the enzymes in the limonin biosynthetic pathway by using recombinant DNA techniques. Limonin, a triterpenoid, is probably synthesized via the mevalonate pathway, as are the monoterpenoid flavor compounds. It appears that nomilin, a precursor of limonin, is synthesized in the stems and roots of citrus and then the precursor transported to the fruit where it is converted by several enzymes to limonin and other bitter limonoids (46). 

V. P. Maier S. Hasegawa R. D. Bennet L. C. Echols, Limonin and Limonoids-Chemistry, Biochemistry and Juice Bitterness. In Citrus Nutrition and Quality, S. Nagy, J. A. Attaway, Eds. ACS Symposium Series No. 143 American Chemical Society Washington, DC, 1980 pp 63-82. 

Several limonoids are known to be bitter principles of citrus (Rutaceae). A typical example is limonin. Although fresh juice does not elicit a bitter taste, sometimes it becomes bitter after heating or storage. This is explained by the formation of bitter-tasting limonin by deglycosylation and further cyclization from limonin glucoside, which is present in citrus fruit tissue and seeds and does not exhibit bitterness.146 Recently, it was reported that limonin had antitumor activity.147 Besides limonin, nomilin and obakunone, which are considered to be... 

Limonoids are C2(, nortriterpenoids deriving from a C30 triterpene precursor. The best known limonoids are the Azadirachta indica (neem tree) antifeedant azadirachtin (C50L C40 C6 -C60(epoxide methylene cross-link) furan) and the Citrus species (Rutaceae) bitter antifeedant limonin (G50L G40 G6 G6 C50L(epoxide)-furan). Limonin gives a delayed bitter taste to Citrus fruit. The limonoids are typically bitter compounds with insect antifeedant activity... 

Azadirachtin, meliacin, azadironic acid bitter limonoids (Nortriterpenoids). Nimbibin, nimbin, salanin. Sesquiterpenes. Limonoids,... 

This review paper summarizes the biochemical and biological aspects of limonoids in citrus and presents possible preharvest approaches to reduce bitter limonoids in citrus fruits. 

Bitterness due to limonoids in a variety of citrus juices is a major problem of the citrus industry worldwide and has significant negative economic impact. Recently, Maier, et al. 

Limonin and nomilin are two bitter limonoids present in citrus juices. Systematic organoleptic tests showed that the bitterness threshold is 6 ppm (5) for limonin, and 6 ppm (6) or 3 ppm (7) for nomilin. The bitterness due to limonin develops gradually in juices after extraction from certain varieties of oranges, grapefruit, lemon, Natsudaidai, mandarin and some other minor citrus such as Iyokan and Ponkan. This phenomenon is generally referred to as "delayed bitterness". 

Citrus ichangensis possesses an unusual distribution of limonoids (20). Unlike other species of citrus, fruit tissues and seeds of . ichangensis contain very high concentrations of nonbitter ichangensin and very low concentrations of bitter limonin. It also contains relatively high concentration of nonbitter deacetylnomilin. Quantitative analysis showed that a ratio of ichangensin to limonin in the fruit tissue is approximately 50 to 1. 

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