Flavanone bitter

In terms of taste, the flavanone glycosides can be divided into two groups. The first group consists of compounds that are bitter. These are glycosides... 

The conversion of the bitter flavanone glycosides into those of the corresponding chalcones by alkali-catalyzed fission of the pyrone ring, and of the dihydrochalcone glycosides by hydrogenation thereof (see Scheme 1)... 

In addition to the bitter acids and essential oils, the flowers of hops offer a rich array of polyphenolic compounds, primarily chalcones and their accompanying flavanones, many of which are prenylated derivatives (Stevens et al., 1997,1999a, b). The most prominent flavonoid in all plants studied was xanthohumol [342] (3 -prenyl-6 -0-methylchalconaringenin chalconaringenin is 2, 4, 6, 4-tetrahydroxychalcone) (see Fig. 4.11 for structures 342-346). Several additional chalcones—variously adorned with 0-methyl and/or C-prenyl functions—were also encountered, along with their respective flavanones. Three new compounds were described in the Stevens et al. 

Bar-Peled M, Lewinsohn E, Fluhr R, Gressel J (1991) UDP-rhamnose flavanone-7-0-glucoside-2"-0-rhamnosyltransferase purification and characterization of an enzyme catalyzing the production of bitter compounds in Citrus. J Biol Chem 266 20953-20959... 

Neohesperidin dihydrochalcone (NeoDHC) is a phenolic compound prepared from the bitter citrus flavanones naringin and neohesperidin (Horowitz Gentili, 1985). NeoDHC is a white solid with solubility in water of 0.5 g/1, which increases with temperature, but as use level is low, sufficient for most applications. 

Another important interaction is that of limonin with the bitter flavanone glycoside naringin. Both of these bitter substances are present in grapefruit juice and Guadagni et al. (25) found that they interact at subthreshold levels in an additive way. Less than threshold amounts of limonin or naringin contribute to the bitterness of a mixture of the two compounds. The bitterness of the mixture can be predicted by adding the taste-unit contribution of each component (taste unit = concentration/ threshold). 

One of the earliest methods to measure the bitter naringin and other flavanones in grapefruit juice was developed by W. B. Davis in 1947 (12). This test is based on the reaction of dilute alkali with flavanones to form the corresponding yellow chalcones. The flavanone concentration is then determined by measuring the absorbance of the chalcones at 427 nm. Davis pointed out that the procedure was not specific for any flavanone but could be used to determine the principle flavanones in citrus juice, i.e., naringin in grapefruit juice and hesperidin in orange juice. 

He suggested that the method might also be suitable for the determination of flavones and flavonols. This method is still widely used to measure naringin in grapefruit juice albeit it is not specific for naringin, it is a simple, rapid and inexpensive method of analysis. However, since grapefruit contains both bitter and nonbitter flavanone glycosides, Davis values are only a crude approximation of bitterness. 

Chromatographic methods were developed to separate a few of the citrus flavonoids from the complex mixture of citrus flavonoids. The early paper chromatographic methods for flavanones (22, 23) were difficult to quantitate because of band broadening and uneven solvent development. Several thin layer chromatographic (TLC) methods were developed to separate the bitter from the nonbitter flavanone glycosides (24, 25, 26, 27). 

Gas-liquid chromatography (GLC) has notTeen employed for the analysis of flavanone glycosides because they are non-volatile and thermally unstable. One GLC method (28) has been developed for the analysis of the flavanone aglycones. However, the method is extremely time consuming in that the samples must be extracted, hydrolyzed and derivatized before analysis. Furthermore, the procedure cannot distinguish between bitter and nonbitter flavonoids. 

Flavonoids have no odor or mouth feel and, in general, do not contribute significantly to the color of most citrus juices. Their primary effect on citrus quality is due to the bitter taste of certain flavanone glycosides. Thus, quantitative descriptions of desirable citrus qualities are usually based on the absence or maximum concentration limits for these compounds. 

Interestingly, flavanone glycosides exist as structural isomers of which one will be intensely bitter while the other is tasteless. The flavanone portion of the bitter molecule is tasteless, while the glycoside portion is tasteless or slightly sweet (lj. Bitterness is observed only when the sugars and the... 

Figure 4. Minor structural changes to the aglycone portion of the molecule can destroy bitterness. Adding a single double bond between Carbons 2 and 3 will convert bitter flavanone neohesperidosides to tasteless flavone neohesperidosides.

Hagen et a/L (42) determined the relative amounts of all the flavanone glycosides in Texas grapefruit. Their results are shown in Table III. Naringin is the dominant flavonoid in grapefruit and is primarily responsible for the immediate bitter taste in grapefruit. The equally bitter poncirin and the much less bitter neohesperidin are present in relatively small amounts and do not contribute significantly to overall bitterness. 

Chalcones and Pihydrochalcones. Chalcones and dihydro-chalcones are Intensely sweet compounds (39) that are effective in raising the threshold at which the bitterness of naringin and limonin is perceived (46). As illustrated in Figure 5, chalcones are easily formed fromTlavanone glycosides by the addition of alkali and dihydrochalcones are formed from hydrogenated chalcones. Like the flavanone neohesperidosides, the chalcones and dihydrochalcones vary in the intensity of their taste response. 

Chalcones have been proposed (48) as a precursor that is enzymatically cyclized to form flavanone glycosides. The enzyme responsible for this stereospecific ring closure disappears or is inactivated as the fruit matures (5). However, no chalcones have been isolated from citrus. Therefore, if chalcones are a part of the metabolic pathway that leads to the formation of flavanone glycosides they probably exist as short lived, unstable intermediates. It appears unlikely that these compounds play a significant role in mitigating bitterness at their natural levels. 

Figure 5. Conversion of bitter flavanone neohesperidosides to the corresponding intensely sweet chalcones and dihydrochalcones...

Both directly and indirectly, flavonoid composition affects the quality of individual citrus cultivars and hybrids. Some cultivars, such as the sour orange, are directly affected by the presence of bitter neohesperidosides to the point they are unpalatable. The quality of other cultivars may be indirectly affected due to the presence of bitterness suppressing flavone neohesperidosides. Certain hybrids, such as the K early, also contain bitter flavanone glycosides at levels which reduce its acceptability. Therefore, the knowledge of the relative type and... 

Introduction. Naringin is a bitter flavanone glycoside found in grapefruit. It is not the only bitter compound in... 

As previously discussed by Maier and co-workers (Chapter 4), in Navel, Shamouti and certain other orange cultivars, the presence of limonin, a bitter triterpenoid, causes many economic and organoleptic problems and greatly affects the taste quality of processed fruit. Limonin is also prevalent in the grapefruit but the intrinsic quality of this fruit is further complicated by the presence of naringin, a bitter flavanone neohesperidoside (Chapter 5). 

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. 

Phenolics. The phenolics of citrus represent a varied and widely studied class of compounds. They range from the lipid-soluble methoxylated coumarins and psoralens to the water-soluble glycosides of the flavanones and flavones. They include the intensely bitter naringin and the highly insoluble hesperidin. Different classes within the group have characteristic UV spectra which have been used to detect adulterations in juices and oils. 

Flavanone glycosides hesperidin and naringin are responsible for the bitterness in oranges and grapefruit, respectively. 

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