Naringin bitterness

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. 

The alert panel supervisor will also come to know about each regular panelist s ability to detect the many off-flavors that could possibly occur in citrus products. Of primary concern, other than the common sweet and sour flavor attributes, are the panelists abilities to specifically identify (a) bitterness (naringin and/or limonin induced), (b) heated, processed or pumpout off-flavor, (c) excess peel oily, (d) excess and/or poor essency flavor, and (e) the following off-flavors cardboard, tallowy, castor oil, diacetyl or buttermilk, green or immature, overmature or stale fruit, and spoiled fruit. 

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. 

Kesterson JW, Hendrickson R (1957) Naringin, a bitter principle of grapefmit. Univ Fla Exp Stn Bull 51 lA... 

Flavones contribute to plant tissue color provided that they occur in high concentrations or are complexed with metal ions. Some flavones participate in taste for example, the highly methoxylated aglycones nobiletin, sinensetin and tangeretin are responsible for the bitter taste of citrus peel. On the other hand, some glycosylated flavones (for instance neodiosmin and rhoifolin) reduce the bitterness of some substances (limonin, naringin, caffeine, quinine) [2]. 

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. 

Fruit juices can be deacidified with a weak base anion-exchange resin. Removal of compounds which cause a bitter taste is a more popular application (26,27). It is accomplished with resins that have no ion-exchange fimctionality. In essence, they are similar to the copolymer intermediates used by resin manufacturers in the production of macroporous cation and anion exchangers. These products are called polymeric adsorbents. They are excellent for removal of limonin [1180-71-8] and naringin [1023647-2], the principal compounds responsible for bitterness in orange, lemon, and grapefruit juices. The adsorbents are regenerated with steam or alcohol. Decaffeination of coffee (qv) and tea (qv) is practiced with the same polymeric adsorbents (28). 

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). 

NHD has also been found to suppress naringin bitterness (26). It was suggested as being especially useful in upgrading the flavor of low B/A, early-season grapefruit juice (27). 

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). 

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. 

Direct ultraviolet spectrophotometric methods have been developed to measure naringin in grapefruit (19J and hesperidin in orange juice (20, 21j. While these methods are rapid, they are also nonspeciTTc for flavonoid bitterness. 

Bitterness is a generally undesirable flavor component and is usually detrimental to the quality of citrus products. Any bitterness in orange or tangerine products reduces their quality, whereas, a little bitterness is actually desirable for grapefruit products. However, it has been shown (35) that as the concentration of bitter materials increase, flavor scores and overall product acceptability decrease. Thus, excessive bitterness is usually considered objectionable and the Florida Department of Citrus has enacted regulations which limit the amount of naringin which may be present during the early weeks of the season (36). 

It is difficult to quantitatively define grapefruit qualityTn terms of bitterness because individual taste thresholds and bitterness preferences vary markedly. Guadagni et al. (37) found that 7% of a 27 member taste panel could detect as Tittle as 1.5 ppm naringin in water. Yet, another 7% of that same panel could not distinguish a 50 ppm naringin solution from water alone. 

Table I. Evaluation of naringin bitterness as a factor in preference of Florida grapefruit juice ...

Figure 3. Structural isomers of naringenin illustrating the two possible configurations of the sugars attached at the 7 position. Naringin is bitter whereas narirutin...

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. 

Enzyme Debittering. One of the more promising ways to reduce naringin bitterness is to use enzymes to convert the bitter fla-vone neohesperidoside into a less bitter glucoside or non-bitter... 

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