C13-Norisoprenoid glycosides in plant tissues: An overview on their occurrence, composition and role as flavour precursors

Impact of meteorological conditions, canopy shading and leaf removal on yield, must quality, and norisoprenoid compounds content in Franciacorta sparkling wine

Climate change is a major concern in agriculture; in grapevine production, climate change can affect yield and wine quality as they depend on the complex interactions between weather, plant material, and viticultural techniques. Wine characteristics are strongly influenced by microclimate of the canopy affecting primary and secondary metabolites of the grapevine. Air temperature and water availability can influence sugar and acid concentration in grapes and relative wines, and their content of volatile compounds such as norisoprenoids. This becomes relevant in sparkling wine production where grapes are generally harvested at a relatively low pH, high acidity, and low sugar content and where the norisoprenoids significantly contributes to the final aroma of the wine. The effect of climate change on grapevine and wine, therefore, calls for the implementation of on-field adaptation strategies. Among them canopy management through leaf removal and shading have been largely investigated in the wine growing sector. The present study, conducted over 4 years (2010-2013) aims at investigating how leaf removal and artificial shading strategies affect grape maturation, must quality and the production of norisoprenoids, analyzed using an untargeted approach, in sparkling wine. Specifically, this paper investigates the effect of meteorological conditions (i.e., water availability and temperatures) and the effect of leaf removal and shading on Vitis vinifera L. cv. Chardonnay and Pinot noir, which are suitable to produce sparkling wine in the DOCG Franciacorta wine growing area (Lombardy, Italy). The effect of leaf removal and shading practices on norisoprenoids has been the focus of the study. No defoliation and artificial shading treatments play an important role in the preservation of the acidity in warm seasons and this suggests calibrating defoliation activities in relation to the meteorological trend without standardized procedures. This is particularly relevant in the case of sparkling wine, where the acidity is essential to determine wine quality. The enhanced norisoprenoid aromas obtained with a total defoliation represent a further element to direct defoliation and shading strategies. The obtained results increase knowledge about the effect of different defoliation and artificial shading applications in relation to meteorological condition supporting the management decision-making in the Franciacorta wine growing area.

Enantioselective Total Synthesis of (R,R)-Blumenol B and d9-(R,R)-Blumenol B

C₁₃-norisoprenoids are of particular importance to grapes and wines, as these molecules influence wine aroma and have been shown to significantly contribute to the distinct character of various wine varieties. Blumenol B is a putative precursor to a number of important wine aroma compounds, including the well-known compounds theaspirone and vitispirane. The enantioselective synthesis of (R,R)-blumenol B from commercially available 4-oxoisophorone was achieved using a short and easily scaleable route, which was then successfully applied to the synthesis of poly-deuterated d9-blumenol B.

Physicochemical composition, nitrogen fraction and volatile profiles of goat cheese made with artisanal liquid coagulant

In this study, physicochemical composition, nitrogen fractions and volatile compounds of goat cheeses manufactured using artisanal liquid coagulant by four different manufacturers in a mountainous area were investigated. Fresh goat cheeses were characterized by their high fat and high levels of total free amino acids. Volatile profiles were isolated by a solid-phase microextraction technique (SPME/GC-MS) and analyzed by gas chromography-mass spectrometry (GC-MS). In the cheeses, 50 aroma components were identified as ten acids, nine terpens, nine alcohols, eight esters, six aldehydes, five ketones, two volatile phenols and one norisoprenoid with 13 carbon atoms. The major aroma compounds found were nonanoic acid, 2-hexanol and acetoin.

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.

Grape terpenoids: flavor importance, genetic regulation, and future potential

Terpenes significantly affect the flavor and quality of grapes and wine. This review summarizes recent research on terpenoids with regard to grape wine. Although, the grapevine terpene synthase gene family is the largest identified, genetic modifications involving terpenes to improve wine flavor have received little attention. Key enzyme modulation alters metabolite production. Over the last decade, the heterologous manipulation of grape glycosidase has been used to alter terpenoids, and cytochrome P450s may affect terpene synthesis. Metabolic and genetic engineering can further modify terpenoid metabolism, while using transgenic grapevines (trait transfer to the plant) could yield more flavorful wine. We also discuss traits involved in wine aroma quality, and the strategies that can be used to improve grapevine breeding technology.

Research Advances on Biosynthesis, Regulation, and Biological Activities of Apocarotenoid Aroma in Horticultural Plants

Apocarotenoids, which play important roles in the growth and development of horticultural plants, are produced by the action of carotenoid cleavage oxygenase (CCO) family members or nonenzymatic cleavage actions. Apocarotenoids are commonly found in leaves, flowers, and fruits of many horticultural plants and participate in the formation of pigments, flavors, hormones, and signaling compounds. Some of them are recognized as important aroma components of fruit and flower with aromatic odor, such as βß-ionone, β-damascenone, and 6-methyl-5-hepten-2-one in tomato fruit, and have low odor thresholds with β-ionone having odor threshold of only 0.007 ppb. In this review, the main apocarotenoid aroma components in horticultural plants were listed, and factors influencing their production were discussed at first. Then, the biosynthetic pathway of apocarotenoid aromas was briefly introduced, and the CCDs gene family was highlighted, and the nonenzymatic production of apocarotenoid aromas was also mentioned. Next, chemical and molecular regulations of apocarotenoid aromas and their biological activities were summarized. Finally, further exploration aspects needed were suggested. We anticipate that this review can afford some crucial information for comprehensive application of apocarotenoid volatile compounds in horticultural plants.

Impact of agronomic practices on grape aroma composition: a review

Aroma compounds are secondary metabolites that play a key role in grape quality for enological purposes. Terpenes, C₁₃ -norisoprenoids, phenols, and non-terpenic alcohols are the most important aroma compounds in grapes and they can be found as free volatiles or glycoconjugated (bound) molecules. The non-volatile glycosylated group is the largest, and it is present in all varieties of Vitis vinifera (L.), the most widely used species for wine production. These aroma precursors represent the reserve of aroma molecules that can be released during winemaking. Their relative and absolute concentrations at fruit ripening determine the organoleptic value of the final product. A large range of biotic and abiotic factors can influence their biosynthesis in several ways. Agronomic practices such as irrigation, training systems, leaf removal, and bunch thinning can have an effect at plant level. The spraying of stimulatory compounds on fruit at different developmental stages has also been shown to modify metabolic pathways at fruit level with some impact on the aroma composition of the grapevine fruit. Viticulturists could act to promote aroma precursors to improve the aromatic profile of grapes and the wine ultimately produced. However, agronomic practices do not always have uniform results. The metabolic and physiological changes resulting from agronomic practices are unknown because there has not been sufficient research to date. This review presents the state of the art regarding the influences of vineyard agronomic management on the biosynthesis of grape aroma compounds. Although literature regarding the topic is abundant there are still many unknown biological mechanisms involved and/or that have been insufficiently studied. The aim of this work is therefore to find the gaps in scientific literature so that future investigations can focus on them. © 2018 Society of Chemical Industry.

Evolution of Volatile Compounds during the Distillation of Cognac Spirit

Cognac wine spirit has a complex composition in volatile compounds which contributes to its organoleptic profile. This work focused on the batch distillation process and, in particular, on volatile compounds specifically produced by chemical reactions during the distillation of Cognac wine spirit, traditionally conducted in two steps with charentais pot stills. The aim of this study was to characterize these volatile compounds formed during distillation. Sampling has been performed on the distillates and inside the boiler during a typical Cognac distillation. The analysis of these samples allowed us to perform a mass balance and to point out several types of volatile compounds whose quantities strongly increased during the distillation process. These compounds were distinguished by their chemical family. It has been found that the first distillation step was decisive for the formation of volatile compounds. Moreover, 2 esters, 3 aldehydes, 12 norisoprenoids, and 3 terpenes were shown to be generated during the process. These results suggest that some volatile compounds found in Cognac spirit are formed during distillation due to chemical reactions induced by high temperature. These findings give important indications to professional distillers in order to enhance the product's quality.

Regional features of northern Italian sparkling wines, identified using solid-phase micro extraction and comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry

We carried out comprehensive mapping of volatile compounds in 70 wines, from 48 wineries and 6 vintages, representative of the two main production areas for Italian sparkling wines, by HS-SPME-GCxGC-TOF-MS and multivariate analysis. The final scope was to describe the metabolomics space of these wines, and to verify whether the grape cultivar signature, the pedoclimatic influence of the production area, and the complex technology were measurable in the final product. The wine chromatograms provided a wealth of information, with 1695 compounds being found. A large number of putative markers influenced by the cultivation area was observed. A subset of 196 biomarkers fully discriminated between the two types of sparkling wines investigated. Among the new compounds, safranal and α-isophorone were observed. We showed how correlation-based network analysis could be used as a tool to detect the differences in compound behaviour based on external/environmental influences.

Identification and quantitation of glycosidically bound aroma compounds in three tobacco types by gas chromatography-mass spectrometry

Glycosidically bound aroma compounds in three different types of tobacco were investigated. After isolation of extracts obtained by Amberlite XAD-2 adsorption and ethyl acetate elution, glycosides were analyzed after enzymatic hydrolysis by gas chromatography-mass spectrometry (GC-MS) or directly after trifluoroacetylated (TFA) derivatization by GC-MS in electron ionization (EI) and negative chemical ionization (NCI) mode. In total 21 bound aglycones were identified by β-glucosidase hydrolysis. These aglycones mainly consisted of C13-norisoprenoids, aromatic components and sesquiterpenoids. Additionally, with the aid of enzymatic hydrolysis, 15 β-d-glucopyranosides and 1 β-d-rutinoside were tentatively identified by TFA derivatization. TFA method was validated by repeatability and successfully employed to analyze different types of tobacco. Principal component analysis (PCA) was carried out on identified glycoside variables to visualize the difference between the tobacco types and the relationship between the glycoside variables and the tobacco types was established.

Genome-wide identification and transcription analysis of soybean carotenoid oxygenase genes during abiotic stress treatments

Carotenoid oxygenase is a key enzyme in carotenoid metabolism leading to the synthesis of two phytohormones, abscisic acid (ABA) and strigolactone, as well as norisoprenoids. Few studies have analyzed inter-relationship of the metabolic networks of these three substances. In this present paper, soybean carotenoid oxygenase genes were identified to reveal their phylogenetic relationships, and the transcriptional response of these genes to four abiotic stresses (NaCl, PEG, high and low temperature) and ABA treatment were investigated to characterize their potential roles in plant resistance. Positive selection was found in the branches of carotenoid cleavage dioxygenase (CCD1), CCD8 and NCED (9-cis-epoxycarotenoid oxygenase), indicating an adaptive evolution in these clades. In soybean eight carotenoid oxygenase genes were identified. The transcriptional responses of almost all of them under stress and ABA conditions were significantly altered when assessed by quantitative polymerase chain reaction. Notably, CCD1 and CCD4, previously known as the key genes in norisoprenoids metabolism, showed especially strong responses to the abiotic stresses and ABA treatment. Furthermore, transcription levels of CCD7 and CCD8, key genes for the strigolactone pathway, highly increased during ABA treatment providing further evidence that ABA is involved in regulating strigolactone metabolism. All of the carotenoid oxygenase genes in soybean are involved in plant abiotic stress physiology, and ABA is presumed to be a core regulatory substance. These findings provide some insights into the mechanisms that underlie the regulation of tolerance response to abiotic stresses in soybean.

A new norisoprenoid and other compounds from Fuzhuan brick tea

Fuzhuan brick tea, a kind of dark tea consumed mainly in the border regions of Southwestern and Northwestern China since the 1860s, is produced from the leaves of Camellia sinensis var. sinensis by microbial fermentation. From this special fermented tea, a new norisoprenoid, 3R,9R-oxido-5-megastigmene, was isolated, together with α-linolenic acid, strictin, isovitexin, astragalin, (+)-catechin, (−)-epicatechin, (−)-epicatechin gallate, (+)-gallocatechin, (−)-epigallocatechin, (−)-epigallocatechin gallate and gallic acid. The structures of the compounds were identified by spectroscopic means. The new compound didn’t show any inhibition activity against the tested enteric pathogenic microorganisms at a concentration of 800 μg/mL by the hole plate diffusion method.

Evaluation of differences in the aroma composition of free-run and pressed neutral grape juices obtained from Emir (Vitis vinifera L.)

In this study, the differences in the aroma compounds released after the free-run and pressed juices of cv. Emir grape (Vitis vinifera L.) were evaluated. Aroma compounds were obtained by liquid-liquid extraction with CH(2) Cl(2) , and then analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). According to the results, pressing uniformly increased the levels of the aromatic constituents, but this treatment lowered the grape juice quality for winemaking by increasing the total phenolic compounds, browning index, and C(6) -alcohol levels (green-herbaceaous odor) as compared to the free-run juice. From all the aroma compounds identified in both juices, hexan-1-ol, (E)-hex-2-en-1-ol, isobutanol, isoamyl alcohol, and 2-phenylethanol were the most abundant volatile compounds.

Occurrence, sensory impact, formation, and fate of damascenone in grapes, wines, and other foods and beverages

Among plant-derived odorants, damascenone is one of the most ubiquitous, sometimes occurring as an apparent natural product but more commonly occurring in processed foodstuffs and beverages. It has been widely reported as a component of alcoholic beverages, particularly of wines made from the grape Vitis vinifera . Although damascenone has one of the lowest ortho- and retronasal detection thresholds of any odorant, its contribution to the sensory properties of most products remains poorly understood. Damascenone can be formed by acid-catalyzed hydrolyses of plant-derived apocarotenoids, in both aglycon and glycoconjugated forms. These reactions can account for the formation of damascenone in some, but not all, products. In wine, damascenone can also be subject to degradation processes, particularly by reaction with sulfur dioxide.

Glycosidic conjugates of C13 norisoprenoids, monoterpenoids, and cucurbates in Boronia megastigma (Nees)

Analysis of a methanolic extract of marc from Boronia megastigma (Nees) using LC-MS (APCI, nominal mass) provided strong evidence for the presence of both glycosides and malonyl glycosides of methyl cucurbates, C13 norisoprenoids including megastigmanes, and monoterpene alcohols. Subsequent fractionation of an extract from the marc using XAD-2 and LH 20 chromatography followed by LC-UV/MS-SPE-NMR and accurate mass LC-MS resulted in the isolation and identification of (1R,4R,5R)-3,3,5-trimethyl-4-[(1E)-3-oxobut-1-en-1-yl]cyclohexyl β-D-glucopyranoside (3-hydroxy-5,6-dihydro-β-ionone-β-D-glucopyranoside); 3,7-dimethylocta-1,5-diene-3,7-diol-3-O-β-D-glucopyranoside; and a methyl {(1R)-3-(β-D-glucopyranosyloxy)-2-[(2Z)-pent-2-en-1-yl]cyclopentyl}acetate stereoisomer (a methyl cucurbate-β-D-glucopyranoside); and provided evidence for 3,7-dimethylocta-1,5-diene-3,7-diol-3-O-(6'-O-malonyl)-β-D-glucopyranoside in boronia flowers.

The formation of wine lactone from grape-derived secondary metabolites

Wine lactone (i.e., 3a,4,5,7a-tetrahydro-3,6-dimethylbenzofuran-2(3H)-one, 1a/1b) was formed hydrolytically at wine pH from both racemic (E)-2,6-dimethyl-6-hydroxyocta-2,7-dienoic acid (3) and the corresponding glucose ester 2a at 45 °C but at room temperature was only formed from the acid 3. The glucose ester does not appear to be a significant precursor for the formation of wine lactone in wine. The slow formation of wine lactone from the free acid 3 indicates that the acid is not likely to be an important precursor to wine lactone in young wines unless present in high concentration (≫ 1 mg/L), but could be a significant precursor to wine lactone in wine that is several years old. The wine lactone formed in hydrolysates of the (6R)-enantiomer of 3 was partially enriched in the (3S,3aS,7aR)-enantiomer 1a when the hydrolysis was conducted at pH 3.2 and 100 °C in a closed vessel or under simultaneous distillation-extraction (SDE) conditions, and the enantiomeric excess (ee) varied from 5 to 22%. Hydrolysis of (6R)-3 in sealed ampules at 45 °C and at pH 3.0, 3.2, or 3.4 gave near-racemic wine lactone, but when the hydrolyses were conducted at room temperature, the product was enriched in the (3S,3aS,7aR)-enantiomer 1a and the ee was greater at higher pH (up to 60% at pH 3.4).

Study of 'Redhaven' peach and its white-fleshed mutant suggests a key role of CCD4 carotenoid dioxygenase in carotenoid and norisoprenoid volatile metabolism

BACKGROUND

Carotenoids are plant metabolites which are not only essential in photosynthesis but also important quality factors in determining the pigmentation and aroma of flowers and fruits. To investigate the regulation of carotenoid metabolism, as related to norisoprenoids and other volatile compounds in peach (Prunus persica L. Batsch.), and the role of carotenoid dioxygenases in determining differences in flesh color phenotype and volatile composition, the expression patterns of relevant carotenoid genes and metabolites were studied during fruit development along with volatile compound content. Two contrasted cultivars, the yellow-fleshed 'Redhaven' (RH) and its white-fleshed mutant 'Redhaven Bianca' (RHB) were examined.

RESULTS

The two genotypes displayed marked differences in the accumulation of carotenoid pigments in mesocarp tissues. Lower carotenoid levels and higher levels of norisoprenoid volatiles were observed in RHB, which might be explained by differential activity of carotenoid cleavage dioxygenase (CCD) enzymes. In fact, the ccd4 transcript levels were dramatically higher at late ripening stages in RHB with respect to RH. The two genotypes also showed differences in the expression patterns of several carotenoid and isoprenoid transcripts, compatible with a feed-back regulation of these transcripts. Abamine SG - an inhibitor of CCD enzymes - decreased the levels of both isoprenoid and non-isoprenoid volatiles in RHB fruits, indicating a complex regulation of volatile production.

CONCLUSIONS

Differential expression of ccd4 is likely to be the major determinant in the accumulation of carotenoids and carotenoid-derived volatiles in peach fruit flesh. More in general, dioxygenases appear to be key factors controlling volatile composition in peach fruit, since abamine SG-treated 'Redhaven Bianca' fruits had strongly reduced levels of norisoprenoids and other volatile classes. Comparative functional studies of peach carotenoid cleavage enzymes are required to fully elucidate their role in peach fruit pigmentation and aroma.

Timing of cluster light environment manipulation during grape development affects C13 norisoprenoid and carotenoid concentrations in Riesling

Sunlight exposure of winegrape clusters is frequently reported to increase C(13)-norisoprenoids in resulting wines, but the timing and mechanism of this influence is not well understood. Fruit zone leaf removal was applied to Vitis vinifera cv. Riesling at three timings: 2, 33 and 68 days past berry set (PBS), and compared to an untreated control. Free and total 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), vitispirane and beta-damascenone were measured in juice and wines, and carotenoid profiles were determined in grapes at midseason and maturity. Significantly higher total TDN was observed in grapes from the 33-day PBS treatment compared to the control and other treatments (195 microg/L vs 54-87 microg/L). Total vitispirane in juice was also significantly increased in the 33-day PBS treatment, while total beta-damascenone was reduced in the 68-day PBS treatment compared to the control. Existing HPLC protocols were modified to allow for quantification of zeaxanthin in V. vinifera berries, and zeaxanthin was determined to be significantly higher in the 33-day PBS treatment than the control or other treatments (p < 0.05). Total TDN in juice correlated with free TDN in wine, with 11.0% +/- 2.5% of total juice TDN converted to free TDN in wine. In contrast, total vitispirane increased significantly during fermentation, and was not correlated with vitispirane in juice. In summary, leaf removal at 33 days PBS significantly increased zeaxanthin in Riesling grapes midseason, total TDN and vitispirane in the juice of mature Riesling grapes, and free and total TDN in finished wine, while earlier or later leaf removal had no effect.

Bound volatile precursors in genotypes in the pedigree of 'Marion' blackberry (Rubus sp.)

Glycosidically bound volatiles and precursors in genotypes representing the pedigree for 'Marion' blackberry were investigated over two growing seasons. The volatile precursors were isolated using a C18 solid-phase extraction column. After enzymatic hydrolysis, the released volatiles were analyzed using stir bar sorptive extraction gas chromatography-mass spectrometry (GC-MS) and direct microvial insert thermal desorption GC-MS. The most abundant volatile precursors in the genotypes were alcohols, followed by shikimic acid derivatives. High amounts of furanone glycosides were also detected, while norisoprenoids only existed in a small amount in blackberries. The volatile precursor composition in the genotypes in the 'Marion' pedigree was very similar to their free volatile distribution. 'Logan' and 'Olallie' predominantly had bound norisoprenoids. Wild 'Himalaya' predominated with terpene alcohol and furaneol glycosides, whereas 'Santiam' and 'Chehalem' contained a high level of terpene alcohol glycosides. A similar inheritance pattern was also observed for some volatile precursors in the genotypes in the 'Marion' pedigree. A high content of linalool, hydroxylinalool, and alpha-ionol glycosides in 'Olallie' and a low content in 'Chehalem' resulted in a moderate level in their offspring 'Marion', while a low content of (E)-linalool oxide precursor in 'Olallie' and a high content in 'Chehalem' also resulted in a moderate level in 'Marion'. However, the concentration of furaneol glycosides in 'Marion' exceeded that of its two parents.

Volatile and Glycosidically Bound Composition of Loureiro and Alvarinho Wines

Composition of Loureiro and Alvarinho wines from the Vinhos Verdes region, regarding free volatile compounds as well as glycosidically bound aroma precursors, was exhaustively determined by gas chromatography—mass spectrometry after adsorption on XAD-2 resin. On the whole, were identified and quantified 120 volatile compounds in the free fraction and 77 glycosidically bound compounds, belonging to C6-compounds, alcohols, fatty acids ethyl esters, esters of organic acids, acetates, monoterpenic alcohols, monoterpenic oxides and diols, C13-norisoprenoids, volatile phenols, volatile fatty acids, and carbonyl compounds. Globally, the wines of the two cultivars present similar composition on volatiles. However, regarding varietal compounds, Loureiro wines were richer than Alvarinho ones with respect to C6-compounds and monoterpenic compounds, occurring the opposite for volatile phenols. It was also demonstrated that wines of both varieties might benefit the aroma reserve, present as glycoconjugates, as it is susceptible of being technologically explored. Linalool, Ho-trienol, (α-terpineol, contributing with fruity and floral notes, and (β-damascenone mostly for Alvarinho, confering tropical fruit notes, are the varietal compounds which may particularly influence the aroma of these wines. Respecting fermentative compounds, Alvarinho is also particularly rich in fatty acids ethyl esters related to lipid metabolism and acetates of fusel alcohols, which can provide it a fruity character; Loureiro contains higher levels of esters of organic acids and 2-phenylethanol, conferring fruity and floral notes. Sensory analysis agreed with chemical analyses showing a pronounced tree and tropical fruit character for Alvarinho wines while Loureiro wines present more intense citrus fruit notes.

Biosynthesis of plant-derived flavor compounds

Plants have the capacity to synthesize, accumulate and emit volatiles that may act as aroma and flavor molecules due to interactions with human receptors. These low-molecular-weight substances derived from the fatty acid, amino acid and carbohydrate pools constitute a heterogenous group of molecules with saturated and unsaturated, straight-chain, branched-chain and cyclic structures bearing various functional groups (e.g. alcohols, aldehydes, ketones, esters and ethers) and also nitrogen and sulfur. They are commercially important for the food, pharmaceutical, agricultural and chemical industries as flavorants, drugs, pesticides and industrial feedstocks. Due to the low abundance of the volatiles in their plant sources, many of the natural products had been replaced by their synthetic analogues by the end of the last century. However, the foreseeable shortage of the crude oil that is the source for many of the artificial flavors and fragrances has prompted recent interest in understanding the formation of these compounds and engineering their biosynthesis. Although many of the volatile constituents of flavors and aromas have been identified, many of the enzymes and genes involved in their biosynthesis are still not known. However, modification of flavor by genetic engineering is dependent on the knowledge and availability of genes that encode enzymes of key reactions that influence or divert the biosynthetic pathways of plant-derived volatiles. Major progress has resulted from the use of molecular and biochemical techniques, and a large number of genes encoding enzymes of volatile biosynthesis have recently been reported.

Isolation of some glycosides as aroma precursors in young leaves of Japanese pepper (Xanthoxylum piperitum DC.)

To clarify the formation mechanism for the major alcoholic aroma compounds in young leaves of Japanese pepper, the glycosides were isolated as aroma precursors. The presence of glycosides of the main alcoholic aroma constituents was indirectly determined by enzymatic hydrolysis and trifluoroacetylation (TFA) of the glycoside-containing fraction. After Amberlite XAD-2 column chromatography, ODS flash chromatography, and high-performance liquid chromatography (HPLC), two new compounds, namely, (3S,6S)-cis-linalool-3,7-oxide beta-D-glucopyranoside and 2-methylpropanyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside, were isolated. In addition, (3S,6R)-cis-linalool-3,6-oxide beta-D-glucopyranoside, which absolute configuration was the first determined, and six known glycosides, citronellyl beta-D-glucopyranoside, linalyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside, (Z)-3-hexenyl beta-D-glucopyranoside, benzyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside, dendranthemoside A, and 3,6-dihydroxy-5,6-dihydro-beta-ionol 9-beta-D-glucopyranoside, were isolated. All of these glycosides were isolated for the first time from the leaves of Japanese pepper. Their structures were established on the basis of spectral data and chemical evidence. The ratios of stereoisomers of the aglycon moieties of citronellyl beta-D-glucopyranoside and linalyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside were investigated by a chiral GC analysis and compared with those of free citronellol and linalool in the aroma concentrate.

Volatile compounds released by enzymatic hydrolysis of glycoconjugates of leaves and grape berries from Vitis vinifera Muscat of Alexandria and Shiraz cultivars

Glycoconjugates from Muscat of Alexandria and Shiraz leaves and grape berries were isolated by adsorption on Amberlite XAD-2 resin, and enzymatically released aglycons were analyzed by GC-FID and GC-MS. About 120 aglycons were fully or tentatively identified. Compositional differences were observed between leaves and berries of the two varieties in five aglycon chemical groups: C6 alcohols, aliphatic alcohols, monoterpenes, shikimates, and C(13)-norisoprenoids, which were much more abundant in the leaves than in the berries. The differences observed for C(13)-norisoprenoids were in agreement with their hypothetical independent biosynthesis in leaves and berries. Thus, 3-hydroxy-beta-damascone, an important norisoprenoid aglycon of grape berries, was not detected in leaves, whereas its oxidized derivative, 3-oxo-alpha-damascone, was absent in berries. Compositional differences were also observed between Muscat and Shiraz leaves. 3-Oxo-alpha-ionol was not detected in Shiraz leaves, and its retro derivatives were less abundant than in Muscat of Alexandria leaves. Conversely, in Shiraz leaves the levels of 7,8-dihydroionone derivatives, such as megastigman-3,9-diol and 3-oxo-7,8-dihydro-alpha-ionol, were higher than in Muscat of Alexandria leaves.

The (3R,9R)-3-hydroxy-7,8-dihydro-beta-ionol disaccharide glycoside is an aroma precursor in tea leaves

The disaccharide glycoside, (3R,9R)-3-hydroxy-7,8-dihydro-beta-ionyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside was isolated as an aroma precursor from the leaves of Camellia sinensis var. sinensis cv. Yabukita. Its stereochemistry was elucidated on the basis of spectral data and chemical synthesis.

C(13)-Norisoprenoid glucoconjugates from lulo (Solanum quitoense L.) leaves

With the aid of multilayer coil countercurrent chromatography, subsequent acetylation, and liquid chromatographic purification of a glycosidic mixture obtained from lulo (Solanum quitoense L.) leaves, three C(13)-norisoprenoid glucoconjugates were isolated in pure form. Their structures were elucidated by NMR, MS, and CD analyses to be the novel (6R,9R)-13-hydroxy-3-oxo-alpha-ionol 9-O-beta-D-glucopyranoside (4a), the uncommon (3S,5R,8R)-3, 5-dihydroxy-6,7-megastigmadien-9-one 5-O-beta-D-glucopyranoside (citroside A) (5a), and the known (6S,9R)-vomifoliol 9-O-beta-D-glucopyranoside (6a). Enzymatic treatment of compound 5a showed the formation of 3-hydroxy-7,8-didehydro-beta-ionone (7), an important lulo peeling volatile, which in its turn after chemical reduction and heated acid catalyzed rearrangement generates beta-damascenone (9) and 3-hydroxy-beta-damascone (10).

Total synthesis and biological activities of (+)- and (-)-boscialin and their 1'-epimers

Natural (-)-boscialin [(-)-1] has recently been described as one of the constituents of various medicinal plants. To obtain more material for investigations of its biological activities, we carried out the synthesis of (-)-1 and its isomers. Starting from the chiral building block 2, the key steps of the synthesis involved a regioselective reduction and a nucleophilic addition. The enantiomer of the natural product, (+)-boscialin [(+)-1], could be obtained via acid-catalyzed epimerization of hydroxyketone 4 to (+)-3. Starting the synthesis with (-)-3 led to (-)-boscialin [(-)-1] with the natural absolute configuration. In addition to (+)- and (-)-boscialin, the corresponding 1'-epimers (+)- and (-)-epiboscialin were also obtained. In vitro assays with (-)-boscialin [(-)-1] and its three stereoisomers were carried out to test for activity against microbes, parasites, and human fibroblasts. The investigations revealed activity against various microbes and against Trypanosoma brucei rhodesiense and also revealed cytotoxicity against human cancer cells.

Glycoconjugated aroma compounds: occurrence, role and biotechnological transformation

The present paper reviews the occurrence of glycosidically bound aroma compounds in the plant kingdom and discusses different hypotheses concerning their role in plants. Emphasis is on biotechnological methods for flavor release and flavor enhancement through enzymatic hydrolysis of glycoconjugated aroma substances.