Citrus bitterness, control

Since the determination of the structure of the citrus bitter principle limonin in 1960, the number of structurally defined members of this class of natural products (now called limonoids) has grown to several hundred. Corey s group reported the first enantioselective synthesis of a limonoid, the protolimonoid, by a route which is both short and stereo-controlled (Scheme 9.45) [17j]. Brief exposure of chiral epoxide 35 to 1.2 equiv of MeAlCl at -78°C provided two diastereomeric tricyclic ketones in a 1.5 1.0 ratio with ketone 36 as the minor component. The predominating dia-stereomer in the mixture, in which the appendage a to the carbonyl is axially oriented, could be isomerized to 36 by treatment of the crude mixture with sodium ethoxide in ethanol. Overall, the cationic cyclization and epimerization permit the rapid stereoselective establishment of three rings and five stereocenters in 43% yield. Slow addition of a solution of AIBN and BUjSnH to a heated solution of xanthate... 

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). 

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. 

When one examines the analytical methods which are presently available for these bitter compounds, it is clear why the study and control of bitterness in citrus has been so hampered and quality control virtually lacking. For the flavanone, naringin, only an approximate test is available (Davis Test) (see Chapter 5 for a discussion of this method) and for limonin there is no method available for monitoring in processing plants. 

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