Comparative Physicochemical and Biochemical Characterization of Small-Molecule Glucosides

Glycosylation of small molecules can significantly improve their physicochemical and biological properties. Only recently, decisive improvements in the biotechnological production of small-molecule glucosides (SMGs) have resulted in a large number of these compounds now being commercially available. In this study, we have analyzed a number of physical, chemical, and biological parameters of 31 SMGs, including solubility, stability, melting and pyrolysis points, partition coefficient log P, minimum inhibitory concentration against Escherichia coli (MIC), and enzymatic degradability. The properties such as water solubility, pH stability, and MICs of the glycosides were strongly dependent on the structures of the respective aglycones, which is why the SMG clustered according to their aglycones in most cases. Phenolic and furanone glucosides were readily hydrolyzed by saliva and skin microflora, whereas monoterpenol glycosides were poorer substrates for the enzymes involved. The results of this comparative analysis of SMGs provide valuable information for elucidating the biological functions of SMGs and the future technological applications of these useful natural products.

Glycosidically bound aroma precursors in fruits: A comprehensive review

Fruit aroma is mainly contributed by free and glycosidically bound aroma compounds, in which glycosidically bound form can be converted into free form during storage and processing, thereby enhancing the overall aroma property. In recent years, the bound aroma precursors have been widely used as flavor additives in the food industry to enhance, balance and recover the flavor of products. This review summarizes the fruit-derived aroma glycosides in different aspects including chemical structures, enzymatic hydrolysis, biosynthesis and occurrence. Aroma glycosides structurally involve an aroma compound (aglycone) and a sugar moiety (glycone). They can be hydrolyzed to release free volatiles by endo- and/or exo-glucosidase, while their biosynthesis refers to glycosylation process using glycosyltransferases (GTs). So far, aroma glycosides have been found and studied in multiple fruits such as grapes, mangoes, lychees and so on. Additionally, their importance in flavor perception, their utilization in food flavor enhancement and other industrial applications are also discussed. Aroma glycosides can enhance flavor perception via hydrolyzation by β-glucosidase in human saliva. Moreover, they are able to impart product flavor by controlling the liberation of active volatiles in industrial applications. This review provides fundamental information for the future investigation on the fruit-derived aroma glycosides.

Rational selection of biphasic reaction systems for geranyl glucoside production by Escherichia coli whole-cell biocatalysts

Geranyl glucoside, the glucosylated, high-value derivative of the monoterpenoid geraniol, has various applications in the flavor and fragrance industry and can be produced through whole-cell biotransformation of geraniol with Escherichia coli whole-cell biocatalysts expressing the glucosyltransferase VvGT14a. However, the low water solubility and high cytotoxicity of geraniol require the design of a proper biphasic system where the second, non-aqueous phase functions as an in-situ substrate reservoir. In this work, a rational selection strategy was applied for choosing suitable sequestering phases for geranyl glucoside production by whole-cell biotransformation of geraniol. Hansen solubility parameters and octanol/water distribution coefficients were used as first principle methods in combination with extensive database research to preselect 12 liquid and 6 solid sequestering phases. Subsequently, experimental approaches were applied to determine physicochemical characteristics and the distribution of geraniol and geranyl glucoside between the phases. Moreover, the effects of the sequestering phases on the whole-cell biocatalysts and on the produced geranyl glucoside concentration were measured during parallel biotransformations in milliliter-scale stirred-tank bioreactors. The fatty acid ester isopropyl myristate emerged as the best choice due to its low viscosity, very poor water solubility, low price and compatibility with the whole-cell biocatalyst. The biphasic system containing 20% (v/v) of this solvent boosted geranyl glucoside production (4.2-fold increase of geranyl glucoside concentration in comparison to aqueous system) and exhibits advantageous partitioning of geraniol into the organic phase (logD of 2.42±0.03) and of geranyl glucoside into the water phase (logD of -2.08±0.05). The systematic selection of a suitable biphasic system constitutes basic groundwork for the development of new bioprocesses involving geraniol. Moreover, this study can serve as a guideline for selecting sequestering phases for other whole-cell biotransformation processes.

Potential applications of glucosyltransferases in terpene glucoside production: impacts on the use of aroma and fragrance

The detection of glucoconjugated forms of monoterpene alcohols in rose petals in the late 1960s opened the new field of nonvolatile aroma precursors in flavor research. It is now well established that odorless glycosides represent a significant pool of aroma precursors in plants where they act as preformed but inactivated defense or attractive chemicals. Technical improvements in the separation and identification of plant secondary metabolites have provided a multitude of chemical structures, but functional characterization of glycosyltransferases that catalyze their formation lags behind. As technical efforts and costs for DNA sequencing dramatically dropped during the last decade, the number of plant genome sequences increased significantly, thus providing opportunities to functionally characterize the glycosyltransferase gene families in plants. These studies yielded the first glycosyltransferase genes that encode efficient biocatalysts for the production of monoterpene glucosides. They have applications in the food, feed, chemical, cosmetic, and pharmaceutical industries as slow release aroma chemicals.

Controlled fragrance release from galactose-based pro-fragrances

Conjugating α,β-unsaturated ketonesviaa thioether linkage to carbohydrates is an efficient way to modulate the polarity of the precursors and thus to influence the release of damascones in functional perfumery.

Controlled Release of Volatiles under Mild Reaction Conditions: From Nature to Everyday Products

Volatile organic compounds serve in nature as semiochemicals for communication between species, and are often used as flavors and fragrances in our everyday life. The quite limited longevity of olfactive perception has led to the development of pro-perfumes or pro-fragrances--ideally nonvolatile and odorless fragrance precursors which release the active volatiles by bond cleavage. Only a limited amount of reaction conditions, such as hydrolysis, temperature changes, as well as the action of light, oxygen, enzymes, or microorganisms, can be used to liberate the many different chemical functionalities. This Review describes the controlled chemical release of fragrances and discusses additional challenges such as precursor stability during product storage as well as some aspects concerning toxicity and biodegradability. As the same systems can be applied in different areas of research, the scope of this Review covers fragrance delivery as well as the controlled release of volatiles in general.

Photochemical Release of Aldehydes from α-Acetoxy Nitroveratryl Ethers

Photolabile aldehyde-releasing precursors (alpha-acetoxy ethers) were prepared by reduction of the corresponding esters with DIBAL and quenching the intermediate aluminum hemiacetal with acetic anhydride. These species smoothly released aldehydes upon irradiation with UV light at 350 nm. Using this method, not only simple model aliphatic aldehydes were liberated but also specimens relevant for the flavor and fragrance industry (methional, phenylacetaldehyde, and (R)-citronellal). [reaction: see text]