The extraction of monoterpene alcohols from rose petals 25 years ago opened a new area of flavor research. Scientists have conducted a plethora of experiments on glycosides and glycosidical aroma compounds. Today, flavorless glycosides that represent the accumulation form of aroma compounds are found in plant tissues and fruits. In addition, the non-volatile compounds having the ability to generate volatile compounds are called terpene diphosphates. These phosphate esters (monoterpene biosynthesis intermediates) constitute an aroma reserve (e.g., papaya fruit and marjoram). Since a wide range of precursor compounds have been identified till date, among which a vast majority are glycosidic in nature. Recent analyses of the constituents of the polar plants describe the occurrence of glycoconjugates in plants.
Glycoconjugated aroma compounds
The first monoterpene glucoside was detected in rose petals, which has improved the knowledge on the distribution and occurrence of glycosidically volatile compounds in plants. Glycoconjugated aroma compounds are found in essential-oil plants and non- essential oil-bearing plants. In most cases, the glycosidical flavors exceed the free aroma amount by the ratio 5:1. Glycosides-bearing plant species are now found in more than 60 plant families. Some examples of glycosidical volatiles include: Carum copticum, glycosides found in seeds; and Apium graveolens, glycoside is found in the rhizome.
Glycosidic aroma precursors are extracted from fruit juices, wine, and plant extracts by the selective retention on C18- phase adsorbent (reversed) or on amberlite XAD-2 resin. This is followed by the desorption process of glycosides that are retained using MeOH or EtOAc.
Two lines of investigations are pursued once the precursor concentrate is obtained. In the first approach, an aglycone fraction HRGC-MS analysis is obtained after acid and enzymatic hydrolysis. Depending on the complexity and heterogeneity of the extracted glycoside, a whole new set of chromatographic separation steps are applied to yield the purest form of intact glycoconjugate.
For preliminary separations, the technique of LC is ideally preferred. Some examples include countercurrent chromatography (CCC), preparative HPLC, and size-exclusion chromatography. The CCC technique has been found to have numerous advantages in terms of polar natural products analysis that includes glycosidic aroma precursors.
The CCC technique, also called as all-liquid chromatographic technique, does not use solid sorbents. Instead of employing solid materials, CCC technique primarily relies on inexpensive solvent mixtures. CCC instruments are commercially available and have also been successfully used in flavor precursor research. In most cases, the glycosidic sub-fractions still contain some glycosides mixture. So in order to further purify, the analytical CCC technique or the analytical HPLC technique is used.
Aglycone and Glycone Structures
The aglycone structures that are frequently reported are alkanols (medium-chain) and shikimic acid metabolites, and alkenols; also includes mevalonate-derived substances with 10 monoterpenoids; 13 C13 -norisoprenoids; and 15 sesquiterpenoids (carbon atoms). A typical plant glycoside aglycone is structurally complex and has wide diversity. The carotenoid- derived group of C13-norisoprenoid glycoconjugates was detected upto 1995.
In terms of glycone structure, the sugar part that is directly bound to the aglycone is called a β-D-glucose. This glucose may or may not be substituted by other sugar units. Till date, the components that are outlined as a second sugar unit include α-L-rhamnopyranose, α-L-arabinofuranose, β-D-apiofuranose, β-D-glucose, and β-D-xylopyranose. The important fact is that β-D-glucose is the building block of glycoconjugates and this was based on the rapid development method that determined the overall glycoside content in fruits and wines. This procedure was developed by Williams and consists of the following steps: isolating glycosidic fraction by the selective retention on C 18- phase adsorbent (reversed); acid hydrolysis to liberate glucose; and last, measuring the released glucose using enzyme assay. This method is already employed with the wine industry to develop glycoconjugates in ripening fruit.
Occurrence of Glycosidical Volatiles
Glycosidical volatiles are found in higher amounts during fruit maturation. Indeed, vegetables and several fruits like the grapes have considerably high amounts of bound aroma volatiles when compared to free volatile fraction. The glycosylated aroma compounds differ from free aglycones in two key properties:
(1) they exhibit improved water solubility; and
(2) exhibit decreased reactivity. This could be the reason why glycosylated aroma compounds accumulate in the plant kingdom when compared to free aglycones.
The glycoconjugation process allows better storage in plant vacuoles thereby protecting the plant cells from any toxicity form that is exhibited by a free aglycone.
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