Identification and quantification of geosmin, an earthy odorant contaminating wines

Geosmin, a Food- and Water-Deteriorating Sesquiterpenoid and Ambivalent Semiochemical, Activates Evolutionary Conserved Receptor OR11A1

Geosmin, a ubiquitous volatile sesquiterpenoid of microbiological origin, is causative for deteriorating the quality of many foods, beverages, and drinking water, by eliciting an undesirable "earthy/musty" off-flavor. Moreover, and across species from worm to human, geosmin is a volatile, chemosensory trigger of both avoidance and attraction behaviors, suggesting its role as semiochemical. Volatiles typically are detected by chemosensory receptors of the nose, which have evolved to best detect ecologically relevant food-related odorants and semiochemicals. An insect receptor for geosmin was recently identified in flies. A human geosmin-selective receptor, however, has been elusive. Here, we report on the identification and characterization of a human odorant receptor for geosmin, with its function being conserved in orthologs across six mammalian species. Notably, the receptor from the desert-dwelling kangaroo rat showed a more than 100-fold higher sensitivity compared to its human ortholog and detected geosmin at low nmol/L concentrations in extracts from geosmin-producing actinomycetes.

Volatile sensation: The chemical ecology of the earthy odorant geosmin

Geosmin may be the most familiar volatile compound, as it lends the earthy smell to soil. The compound is a member of the largest family of natural products, the terpenoids. The broad distribution of geosmin among bacteria in both terrestrial and aquatic environments suggests that this compound has an important ecological function, for example, as a signal (attractant or repellent) or as a protective specialized metabolite against biotic and abiotic stresses. While geosmin is part of our everyday life, scientists still do not understand the exact biological function of this omnipresent natural product. This minireview summarizes the current general observations regarding geosmin in prokaryotes and introduces new insights into its biosynthesis and regulation, as well as its biological roles in terrestrial and aquatic environments.

Online Solid-Phase Extraction–Gas Chromatography–Flame Ionization Detection System for Monitoring Contaminants at Parts-Per-Trillion Concentrations in Process Waters

Online monitoring of odour and taste components that occur at parts-per-trillion (ppt) levels in industrial process waters requires specialized analytical hardware that is generally not compatible with the harsh environmental conditions in these typical industrial settings. An alternative instrumental method is proposed that uses dynamic extraction in combination with gas chromatography (GC) equipped with a simple flame ionization detector (FID) to achieve these extremely low detection limits. The extraction process was fully automated by means of online solid-phase extraction (SPE). The combination of online SPE and GC–FID was used to monitor the quality of process water contaminated with 2-methylisoborneol and geosmin, which are two notorious odour and taste components, in volumes up to 1 L.

A Preliminary Study of Yeast Strain Influence on Chemical and Sensory Characteristics of Apple Cider

During the fermentation of apple juice, yeast metabolism creates complex biosynthetic pathways which produce a range of compounds responsible for the organoleptic qualities of cider. In this study, basic cider quality parameters were measured to investigate the influence of six yeast strains on cider made from three apple varieties (‘Pink Lady’, ‘Sturmer’, and ‘Bulmer’s Norman’). Measurement of pH, titratable acidity, and total phenolic content revealed that yeast can influence cider attributes, albeit variety and season dependent. Descriptive sensory analysis using a trained sensory panel was conducted on cider made from ‘Pink Lady’ apples and the same six yeast strains. The sensory panel significantly differentiated the yeast strains on the attributes of ‘fresh apple’, ‘earthy’ and ‘pear’. Identifying the variety specific influence of individual yeast strains on chemical and sensory characteristics of apple cider will provide cider makers with an enhanced understanding when choosing yeast strains.

Reduction in Off-Flavors in Wine Using Special Filter Layers with Integrated Zeolites and the Effect on the Volatile Profile of Austrian Wines

In the course of the present study, filter layers with embedded zeolites (patented layer of the company Filtrox (Zwingen, Switzerland) with the brand name Fibrafix® TX) were tested for the elimination of the wine defects, cork taint (2,4,6-trichloroanisole), mouldy aroma (geosmin), and strong “animal” phenolic aromas (4-ethylphenol, 4-ethylguaiacol, 4-ethylcatechol). The test design allowed a comparison with conventional filter layers (sterile filter layers) in a small-scale trial (25 L) as well as in a large-scale trial (125 L). By means of gas chromatography-mass spectroscopy, not only the impact compounds of the wine faults were analysed, but also the loss of volatile substances such as 15 free monoterpenes, 34 ester compounds, and 24 wood flavours. Sensory analyses were carried out by means of expert panels. The Fibrafix® TX layers were satisfactory with regard to the reduction in 2,4,6-trichloroanisole (the lead substance of cork flavour; reduction > 90%) and geosmin (the lead substance of mould flavour; reduction > 75%), but not with regard to the reduction in 4-ethylphenol, 4-ethylguaicol, and 4-ethylcatechol. However, the reduction in these off-flavours was generally not specific enough to prevent a loss of volatile compounds, especially various ester compounds and free monoterpenes. The wines obtained after filtration (in case of contamination with geosmin or TCA) were sensory faultless, but for some of the tasters, the overall quality of these wines did not correspond to the quality wine clasification.

Modifications of Grapevine Berry Composition Induced by Main Viral and Fungal Pathogens in a Climate Change Scenario

The grapevine is subject to high number of fungal and viral diseases, which are responsible for important economic losses in the global wine sector every year. These pathogens deteriorate grapevine berry quality either directly via the modulation of fruit metabolic pathways and the production of endogenous compounds associated with bad taste and/or flavor, or indirectly via their impact on vine physiology. The most common and devastating fungal diseases in viticulture are gray mold, downy mildew (DM), and powdery mildew (PM), caused, respectively by Botrytis cinerea, Plasmopara viticola, and Erysiphe necator. Whereas B. cinerea mainly infects and deteriorates the ripening fruit directly, deteriorations by DM and PM are mostly indirect via a reduction of photosynthetic leaf area. Nevertheless, mildews can also infect berries at certain developmental stages and directly alter fruit quality via the biosynthesis of unpleasant flavor compounds that impair ultimate wine quality. The grapevine is furthermore host of a wide range of viruses that reduce vine longevity, productivity and berry quality in different ways. The most widespread virus-related diseases, that are known nowadays, are Grapevine Leafroll Disease (GLRD), Grapevine Fanleaf Disease (GFLD), and the more recently characterized grapevine red blotch disease (GRBD). Future climatic conditions are creating a more favorable environment for the proliferation of most virus-insect vectors, so the spread of virus-related diseases is expected to increase in most wine-growing regions. However, the impact of climate change on the evolution of fungal disease pressure will be variable and depending on region and pathogen, with mildews remaining certainly the major phytosanitary threat in most regions because their development rate is to a large extent temperature-driven. This paper aims to provide a review of published literature on most important grapevine fungal and viral pathogens and their impact on grape berry physiology and quality. Our overview of the published literature highlights gaps in our understanding of plant-pathogen interactions, which are valuable for conceiving future research programs dealing with the different pathogens and their impacts on grapevine berry quality and metabolism.

Beta vulgaris ssp. vulgaris chromosome 8 shows significant association with geosmin concentration in table beet

Geosmin, a degraded sesquiterpene molecule with earthy and musty odor, imbues table beet with its characteristic aroma. Geosmin is heritable and endogenously produced in table beet; its earthy aroma is sought by some consumers but deters others. Geosmin biosynthesis is catalyzed by a bifunctional geosmin synthase enzyme in diverse bacteria and fungi, but a mechanism for geosmin biosynthesis in plants has not been reported. This work employed association analysis and selective genotyping of a segregating F2:3 mapping population to seek QTL associated with geosmin concentration in table beet. GBS reads were aligned to sugar beet reference genome EL10.2, and association analysis revealed two QTL for geosmin concentration on Beta vulgaris ssp. vulgaris chromosome 8. QTL at EL10.2 positions 28,017,624 and 38,488,687 each show effect size 8.7 μg·kg-1 geosmin and explain 8.5% and 6.4% of total variation in geosmin concentration, respectively. Resolution was low due to large recombination bin size and imperfect alignment between the reference genome and mapping population, but population size and selection proportion were sufficient to detect moderate to large effect QTL. This study, the first molecular genetic mapping experiment in table beet, succeeded in finding QTL for geosmin concentration in table beet, and it provides the basis for fine mapping or candidate gene investigation of functional loci for this distinctive sensory trait.

Application of natural and synthetic zeolites in the oenological field

Zeolites are crystalline hydrated aluminosilicates, of natural or synthetic origin, characterized by a microporous structure and high adsorption properties. They are employed as soil amendments and fertilizer carriers in agriculture, as catalysts, detergents, adsorbents and molecular sieves in many chemical processes, as well as in water and soil decontamination, and in food processing. They have been also tested in the oenological field for several potential applications; yet an overview on such topic is not still available. The present review summarizes the recent and innovative applications of zeolites in winemaking and supplies a critical discussion about their potential to prevent protein haze, tartrate instability or the appearance of certain defects, like light-struck off-flavour and earthy off-flavours. Further applications of these minerals in the management of winery wastes and in the analytical field are also reviewed. The outcomes of this work evidenced the need of further research on the use of zeolites in oenology for better exploiting their peculiar sorption and exchange properties, selecting the most efficient natural types and improving the performances of the synthetic ones, without disregarding the potential secondary effects of these treatments on wine quality.

Characterization of Key Aroma Compounds in Tartary Buckwheat (Fagopyrum tataricum Gaertn.) by Means of Sensory-Directed Flavor Analysis

The key odorants of tartary buckwheat (TB) were researched by a sensory-directed flavor analysis approach for the first time. After the volatiles of TB were isolated by solvent-assisted flavor evaporation (SAFE), 49 aroma-active components with flavor dilution (FD) factors in the range of 1-2187 were identified using gas chromatography-olfactometry-mass spectrometry (GC-O-MS) combined with aroma extract dilution analysis (AEDA). Geranylacetone, phenethyl alcohol, and β-damascone showed the highest FD factors of 2187. All 49 odorants were further quantitated by the internal standard curve method, and their odor activity values (OAVs) were obtained. The overall aroma of TB was successfully simulated (similarity > 98.16%) by mixing 16 odorants (OAV ≥ 1) with their natural concentrations. The omission tests revealed that geosmin, α-isomethylionone, α-methylionone, β-ionone, linalool, β-damascone, geranylacetone, guaiacol, ethyl hexanoate, geraniol, vanillin, tetrahydrolinalool, and 2,5-dimethyl-4-hydroxy-3-(2H)-furanone were the key odorants of TB. Chiral analysis showed that tetrahydrolinalool and linalool existed as racemics in the commercial TB. The relative content of R-enantiomers of α-isomethylionone and α-methylionone was slightly higher than that of their S-enantiomers. The odor thresholds of R- and S-enantiomer of tetrahydrolinalool were first detected as 0.029 and 3.8 μg/L in air, respectively.

Strategies for the identification and sensory evaluation of volatile constituents in wine

Wine aroma, which stems from complex perceptual and cognitive processes, is initially driven by a multitude of naturally occurring volatile constituents. Its interpretation depends on the characterization of relevant volatile constituents. With large numbers of volatile constituents already identified, the search for unknown volatiles in wine has become increasingly challenging. However, the opportunities to discover unknown volatile compounds contributing to the wine volatilome are still of great interest, as demonstrated by the recent identification of highly odorous trace (µg/L) to ultra-trace (ng/L) volatile compounds in wine. This review provides an overview of both existing strategies and future directions on identifying unknown volatile constituents in wine. Chemical identification, including sample extraction, fractionation, gas chromatography, olfactometry, and mass spectrometry, is comprehensively covered. In addition, this review also focuses on aspects related to sensory-guided wine selection, authentic reference standards, artifacts and interferences, and the evaluation of the sensory significance of discovered wine volatiles. Powerful key volatile odorants present at ultra-trace levels, for which these analytical approaches have been successfully applied, are discussed. Research areas where novel wine volatiles are likely to be identified are pointed out. The importance of perceptual interaction phenomena is emphasized. Finally, future avenues for the exploration of yet unknown wine volatiles by coupling analytical approaches and sensory evaluation are suggested.

Molecular Background of a Moldy-Musty Off-Flavor in Cocoa

The parallel application of aroma extract dilution analysis to the volatiles isolated from a sample of fermented cocoa seeds with a pronounced moldy-musty off-flavor and to the volatiles isolated from a flawless reference sample revealed (-)-geosmin, 4-methoxy-2,5-dimethylfuran-3(2H)-one, 1H-indole, and 3-methyl-1H-indole as potential off-flavor compounds on the basis of their odor quality and higher flavor dilution factors in off-flavor cocoa than in the reference sample. Quantitation of the four compounds in nine off-flavor cocoa samples and calculation of odor activity values (ratio of the concentrations to the odor threshold values) suggested the crucial roles of (-)-geosmin and 3-methyl-1H-indole for the off-flavor. In the chocolate industry, their quantitation can be used to objectively assess the off-flavor at the level of incoming goods inspection. Because both compounds are inhomogeneously distributed between the testa and the embryo, separate quantitation in the two parts of the seeds is required.

Role of gas chromatography and olfactometry to understand the wine aroma: Achievements denoted by multidimensional analysis

The human nose has been used as a detector in gas chromatography analysis to evaluate odoriferous compounds related to aroma and quality of wine. Several olfactometric techniques are available to access the description, intensity, and/or duration of the odor of each compound. Olfactometry can be associated with one-dimensional gas chromatography or multidimensional gas chromatography, including heart-cut gas chromatography and comprehensive two-dimensional gas chromatography. Multidimensional gas chromatography may help to resolve coeluted compounds and detect important trace components for the aroma. The identification of odor-active compounds may help to differentiate wines according to terroir, grapes cultivars used in winemaking or types of aging, understand the role of fungal infection of grapes for wine quality, find the best management practices in vineyard and vinification to obtain the greatest quality. In addition, when the instrumental techniques are combined with sensory analysis, even more accurate information may be obtained regarding the overall wine aroma. This review discloses the state of the art of olfactometric methods and the analytical techniques used to investigate odor-active compounds such as one-dimensional gas chromatography, multidimensional gas chromatography, and comprehensive two-dimensional gas chromatography. The advances in knowledge of wine aroma achieved with the use of these techniques in the target and profiling approaches were also discussed.

Enantioselective synthesis of tri-deuterated (-)-geosmin to be used as internal standard in quantitation assays

For the accurate and sensitive quantitation of the off-flavor compound geosmin, particularly in complex matrices, a stable isotopologue as internal standard is highly advantageous. In this work, we present a versatile synthetic strategy leading from (4aR)-1,4a-dimethyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one to tri-deuterated (-)-geosmin ((4S,4aS,8aR)-4,8a-dimethyl(3,3,4-2 H3 )octahydronaphthalen-4a(2H)-ol). The starting material was readily accessible from inexpensive 2-methylcyclohexan-1-one using previously published procedures.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Geosmin as a source of the earthy-musty smell in fruits, vegetables and water: Origins, impact on foods and water, and review of the removing techniques

The earthy-musty smell produced by Streptomyces sp. is assigned to geosmin and is responsible for the major organoleptic defects found in drinking water, fruits and vegetables such as grapes, mushrooms, carrots, and beet. Geosmin is also found in juices and musts before fermentation and its presence has been associated with partial presence of Botrytis cinerea. It has a variable detection threshold depending on the matrix and the detection level ranges from 5 to 50 ng/L. On the sensory level, very few individuals are immune to geosmin and although the intensity of the defect caused by this molecule decreases rapidly in the nose, a bad taste is very persistent in the mouth. As the origin of geosmin is fungal, conventional control techniques used for geosmin prevention are limited to ventilation, improving the integrity of plants and use of storage temperatures around 1 °C in a humidity-controlled environment. However, it has been demonstrated that only the combination of different prophylactic and preventive measures provide a relatively sufficient efficacy. Therefore, prevention of factors favoring the formation of geosmin is still topical. Some chemical treatments showed relatively good results against Botrytis cinerea. However, there is a requirement that must be met, namely that only one chemical per family per year must be used. Moreover, a multi-year alternation of chemical families is a strong agronomic recommendation. Regarding Penicillium, no active material is 100% approved and it negative effects plants such as beet and grapes. Consequently, the importance of finding effective ways to fight against geosmin formation is still relevant. From analytical point of view, measurement of geosmin is mainly based on gas chromatography.

Mycobiota of Slovak wine grapes with emphasis on aspergillus and penicillium species in the south Slovak wine region

The Southern Slovak wine growing region is warmest part of Slovakia and is suitable for cultivating the grapes for production of wines at high quality. From the eight vineyards were collected 8 samples of wine grapes (white 7, blue 1) during harvesting 2011, 2012 and 2013. The aim of this work was to gain more knowledge about mycobiota on grapes originating from Slovakia, to identify Aspergillus and Penicillium species according to their morphopogy and evaluate the presence of secondary metabolites (also including intracellular and extracellular mycotoxins) produced in in vitro conditions by thin layer chromatography method from fresh grape berries. Fifty wine grapes per bunch (approximately 7 - 8 berries per plate) that showed no symptoms were randomly selected on Dichloran Rose Bengal Chloramphenicol agar medium. The plates were then incubated aerobically at 25 ±1 °C for 5 to 7 days in the dark. Of these samples were identified 17 genera. One hundred percent of samples were colonies by the genus Penicillium and 75% by the genus Aspergillus. During the survey, 135 isolates belonging to 9 Penicillium species (P. aurantiogriseum, P. canescens, P. citrinum, P. crustosum, P. decumbens, P. expansum, P. funiculosum, P. chrysogenum and P. purpurogenum) and 26 isolates belonging to 3 Aspergillus species (A. clavatus, A. flavus and A. section Nigri) were isolated and identified from exogenous contamination. The main occurring penicillium species of the samples were P. expansum (37.5% Fr), followed P. citrinum, P. chrysogenum and P. crustosum (25% Fr). The main occurring aspergillus species of the samples were A. section Nigri (62.5%). Eight potentially toxigenic species were tested for their toxigenic ability. It was confirmed the production of various mycotoxins such as aflatoxin B1, citrinin, patulin, cyclopiazonic acid, penitrem A and roquefortin C. Out of 34 strains, 56% produced at least one mycotoxin.

Biocontrol of geosmin-producing Streptomyces spp. by two Bacillus strains from Chinese liquor

Streptomyces spp. producing geosmin have been regarded as the most frequent and serious microbial contamination causing earthy off-flavor in Chinese liquor. It is therefore necessary to control the Streptomyces community during liquor fermentation. Biological control, using the native microbiota present in liquor making, appears to be a better solution than chemical methods. The objective of this study was to isolate native microbiota antagonistic toward Streptomyces spp. and then to evaluate the possible action mode of the antagonists. Fourteen Bacillus strains isolated from different Daqu (the fermentation starter) showed antagonistic activity against Streptomyces sampsonii, which is one of the dominant geosmin producers. Bacillus subtilis 2-16 and Bacillus amyloliquefaciens 1-45 from Maotai Daqu significantly inhibited the growth of S. sampsonii by 57.8% and 84.3% respectively, and effectively prevented the geosmin production in the simulated fermentation experiments (inoculation ratio 1:1). To probe the biocontrol mode, the ability of strain 2-16 and 1-45 to produce antimicrobial metabolites and to reduce geosmin in the fermentation system was investigated. Antimicrobial substances were identified as lipopeptides by ultra-performance liquid chromatography tandem electrospray ionization/quadrupole-time-of-flight mass spectrometry (UPLC-ESI/Q-TOF MS) and in vitro antibiotic assay. In addition, strains 2-16 and 1-45 were able to remove 45% and 15% of the geosmin respectively in the simulated solid-state fermentation. This study highlighted the potential of biocontrol, and how the use of native Bacillus species in Daqu could provide an eco-friendly method to prevent growth of Streptomyces spp. and geosmin contamination in Chinese liquor fermentation.

Structural Studies of Geosmin Synthase, a Bifunctional Sesquiterpene Synthase with αα Domain Architecture That Catalyzes a Unique Cyclization-Fragmentation Reaction Sequence

Geosmin synthase from Streptomyces coelicolor (ScGS) catalyzes an unusual, metal-dependent terpenoid cyclization and fragmentation reaction sequence. Two distinct active sites are required for catalysis: the N-terminal domain catalyzes the ionization and cyclization of farnesyl diphosphate to form germacradienol and inorganic pyrophosphate (PPi), and the C-terminal domain catalyzes the protonation, cyclization, and fragmentation of germacradienol to form geosmin and acetone through a retro-Prins reaction. A unique αα domain architecture is predicted for ScGS based on amino acid sequence: each domain contains the metal-binding motifs typical of a class I terpenoid cyclase, and each domain requires Mg(2+) for catalysis. Here, we report the X-ray crystal structure of the unliganded N-terminal domain of ScGS and the structure of its complex with three Mg(2+) ions and alendronate. These structures highlight conformational changes required for active site closure and catalysis. Although neither full-length ScGS nor constructs of the C-terminal domain could be crystallized, homology models of the C-terminal domain were constructed on the basis of ∼36% sequence identity with the N-terminal domain. Small-angle X-ray scattering experiments yield low-resolution molecular envelopes into which the N-terminal domain crystal structure and the C-terminal domain homology model were fit, suggesting possible αα domain architectures as frameworks for bifunctional catalysis.

Development of a novel quantitative PCR assay as a measurement for the presence of geosmin-producing fungi

AIMS

To provide an efficient technique for monitoring the off-flavoured fungal compound geosmin.

METHODS AND RESULTS

Geosmin-associated gpe1 gene of Penicillium expansum displayed ≥99% similarity to cytochrome P450 gene of geosmin-producing P. restrictum, but ≤40% similarities to geosmin biosynthesis, non-cytochromic gene of Streptomyces avermitilis and cytochrome P450 genes of non-geosmin-producing Neotyphodium lolii, Phoma betae and P. paxilli. Serial 10-fold dilutions of P. expansum's DNA was subjected to a previously reported qPCR assay (Atoui et al. 2007), utilizing gpe1 specific primer pair 'SNgpe1F/SNgpe1R'. A linear relationship between DNA quantity and Cycle Threshold (Ct ), with strong correlative coefficient, was observed. Using the available physico-chemical method, geosmin was quantified in 188 grape samples. Penicillium spp's DNA was quantified in these samples, utilizing the developed qPCR assay. A strong positive correlation (R(2)  = 0·97) between Penicillium's DNA and geosmin concentration was observed. Furthermore, <50 ng μl(-1) Penicillium's DNA corresponds to geosmin level below the permitted intensity limit i.e. 4, for 'Flavour Profile Analysis'.

CONCLUSIONS

Penicillium spp., genomic DNA level can provide an efficient way to quantify geosmin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This particular qPCR technique can be utilized in numerous food industries, for the timely detection and monitoring of geosmin contamination.

Geosmin (2β,6α-dimethylbicyclo[4.4.0]decan-1β-ol) production associated with Beta vulgaris ssp. vulgaris is cultivar specific

The characteristic earthy flavor and aroma of table beet [Beta vulgaris ssp. vulgaris (garden beet group)] is due to the presence of geosmin, C₁₂H₂₂O, a volatile terpenoid compound commonly produced by many soil microorganisms. This study screened beet and related subspecies cultivars grown in three different environments (field, greenhouse in nonautoclaved soil, greenhouse in autoclaved soil) to evaluate the effect of cultivar and environment on geosmin level in table beet. There was no significant difference between years or between cultivars grown in autoclaved and nonautoclaved soil, indicating geosmin content may not be primarily attributable to microbial associations. A significant interaction between cultivar and environment was found, but generalizations could be made for high- or low-producing cultivars, demonstrating that geosmin levels were cultivar specific. 'Bull's Blood', 'Chioggia', and sugar beet exhibited the highest geosmin levels. Cultivars grown in the field had the smallest range of geosmin production, from 4.84 to 20.82 μg geosmin (kg root tissue)⁻¹. The high degree of consistency in cultivar performance across years and in ranking for geosmin levels across environments as well as the lack of a significant difference between plants grown in autoclaved and nonautoclaved soil suggests characteristic levels of geosmin may be present in and produced endogenously by cultivars of table beet. It may be possible to establish breeding populations with defined geosmin levels and to identify variety-specific aroma and flavor intensities that would be durable across environments.

Aroma characterization of chinese rice wine by gas chromatography-olfactometry, chemical quantitative analysis, and aroma reconstitution

The aroma profile of Chinese rice wine was investigated in this study. The volatile compounds in a traditional Chinese rice wine were extracted using Lichrolut EN and further separated by silica gel normal phase chromatography. Seventy-three aroma-active compounds were identified by gas chromatography-olfactometry (GC-O) and gas chromatography-mass spectrometry (GC-MS). In addition to acids, esters, and alcohols, benzaldehyde, vanillin, geosmin, and γ-nonalactone were identified to be potentially important to Chinse rice wine. The concentration of these aroma-active compounds in the Chinese rice wine was further quantitated by combination of four different methods, including headsapce-gas chromatography, solid phase microextraction-gas chromatography (SPME)-GC-MS, solid-phase extraction-GC-MS, and SPME-GC-pulsed flame photometric detection (PFPD). Quantitative results showed that 34 aroma compounds were at concentrations higher than their corresponding odor thresholds. On the basis of the odor activity values (OAVs), vanillin, dimethyl trisulfide, β-phenylethyl alcohol, guaiacol, geosmin, and benzaldehyde could be responsible for the unique aroma of Chinese rice wine. An aroma reconstitution model prepared by mixing 34 aroma compounds with OAVs > 1 in an odorless Chinese rice wine matrix showed a good similarity to the aroma of the original Chinese rice wine.

Description of the mechanisms underlying geosmin production in Penicillium expansum using proteomics

A 2D-DIGE proteomics experiment was performed to describe the mechanism underlying the production of geosmin, an earthy-smelling sesquiterpene which spoils wine, produced by Penicillium expansum. The strains were identified by sequencing of the ITS and beta-tubulin regions. This study was based on a selection of four strains showing different levels of geosmin production, assessed by GC-MS/MS. The proteomics study revealed the differential abundance of 107 spots between the different strains; these were picked and submitted to MALDI-TOF-TOF MS analysis for identification. They belonged to the functional categories of protein metabolism, redox homeostasis, metabolic processes (glycolysis, ATP production), cell cycle and cell signalling pathways. From these data, an implication of oxidative stress in geosmin production may be hypothesized. Moreover, the differential abundance of some glycolytic enzymes may explain the different patterns of geosmin biosynthesis. This study provides data for the characterisation of the mechanism and the regulation of the production of this off-flavour, which are so far not described in filamentous fungi.

BIOLOGICAL SIGNIFICANCE

Green mould on grapes, caused by P. expansum may be at the origin of off-flavours in wine. These are characterized by earthy-mouldy smells and are due to the presence of the compound geosmin. This work aims at describing how geosmin is produced by P. expansum. This knowledge is of use for the research community on grapes for understanding why these off-flavours occasionally occur in vintages.

Non-Botrytis grape-rotting fungi responsible for earthy and moldy off-flavors and mycotoxins

The grape microflora is complex and includes filamentous fungi, yeasts and bacteria with different physiological characteristics and effects on wine production. Most studies have focused on the wine microbiota, but a few studies have reported the ecology of grape microorganisms. Some of these organisms - such as non-Botrytis bunch rotting fungi, which greatly influence the safety or sensory quality of wine, due to the production of mycotoxins and off-flavors, respectively - are considered to be spoilage agents. We review here the diversity of filamentous fungi on grapes and the factors influencing their development, such as grape ripening stage, environmental factors (climate, rain and cultivation practices), grape variety and grape health status. We also discuss the pathways by which mycotoxins and off-flavors are produced, the control of the population, the metabolites responsible for wine spoilage and the methods for detecting and characterizing the microorganisms involved.

Chemical and sensory effects of the freezing process on the aroma profile of black truffles (Tuber melanosporum)

The aim of this work was to evaluate the effect of freezing black truffles (Tuber melanosporum) on their aroma both in sensory and chemical terms. The truffles were frozen at temperatures of -20 to -80°C. Descriptive and discriminative sensory and chemical analyses, based on headspace solid phase microextraction followed by gas chromatography-mass spectrometry analysis (HS-SPME-GC-MS), were carried out after 1, 20, 40 and 60 days. Fifteen compounds with high aromatic potential in truffles were determined. Their selective ion peak areas were calculated, summed and expressed as percentage of active odour compound, in order to monitor changes in odour profile. The aroma of frozen truffles differed significantly from the aroma of fresh truffles. Volatile composition data revealed that T. melanosporum aromatic profile is deeply modified as a consequence of a freezing process. These aromatic changes could explain the loss of freshness observed in all frozen truffles. Methional and some phenols were suggested as markers of freezing time. Interestingly, 1-octen-3-one appeared as a general marker of freezing process.

Determination of the microbial origin of geosmin in Chinese liquor

Geosmin is the major cause of the common earthy off-flavor in light-aroma type Chinese liquor and, thus, highly detrimental to the aromatic quality. To find out its origin, the evolving process of geosmin in light-aroma type liquor making was monitored, and microbial analysis of Daqu containing geosmin was carried out. The results showed that geosmin appeared in all the fermented sorghums at different fermentation periods. About 57% geosmin in the fermented sorghums was distilled into liquor. During the distillation process, the peak of geosmin concentration appeared when alcohol content was 50-60% vol. More importantly, high geosmin content was observed during the Daqu-making process. Furthermore, five Streptomyces strains were isolated from different types of Daqu used for the fermentation of light-aroma type liquor. All of them produced only geosmin as the main volatile metabolite but no 2-methylisoborneol (2-MIB). It appears that microorganisms developing in Daqu are responsible for the presence of geosmin in liquor. Because of the relatively low detection threshold estimated at 110 ng/L in 46 vol % hydroalcoholic solution, the presence of geosmin in Daqu may pose a risk for Chinese liquor producers.

Characterization of geosmin as source of earthy odor in different aroma type Chinese liquors

Earthy odor is one of the most frequent and serious causes for the aroma deterioration in Chinese liquor, which causes a dirty and dusty impression. The odor in Chinese liquor is similar to that of rice husk, one kind of auxiliary material widely used as a filler in the distillation process. So it is experientially hypothesized that such odor may derive from rice husk. In this paper, the gas chromatography-olfactometry (GC-O) technique and gas chromatography-mass spectrometry (GC-MS) were used to discover and identify the characteristic odoriferous zone of Chinese liquor marked by earthy odor. Geosmin was found to be responsible for this odor. The levels of the compound in ten bottled liquors and thirty liquors aging for different years belonging to four different aroma types were determined by the optimized headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) method. Quantitative analysis of bottled liquor revealed the presence of geosmin in all aroma type liquors with concentrations ranging from 1.10 μg/L to 9.90 μg/L, except for strong-aroma type liquor. Meanwhile in the aged liquors belonging to the same aroma type, geosmin was detected with significant concentrations and high odor activity values (OAVs) during different years of aging. However, geosmin was not detected in steamed rice husk nor in nonsteamed rice husk, which suggests that rice husk is not the origin of earthy odor in Chinese liquor, and there may be another origin of it during the brewing process.

PCR ITS-RFLP: A useful method for identifying filamentous fungi isolates on grapes

Restriction digestion analysis of the ITS products was tested as an easy method to identify isolates of filamentous fungi on grapes. Endonucleases SduI, HinfI, MseI, HaeIII were used. Endonucleases BfmI, Cfr9I, Hpy188I, MaeII or PspGI were used as necessary to complete discrimination. The 43 species studied generated 42 different composite profiles. Only the species P. thomii and P. glabrum gave the same composite profile. 96.3% strains tested could be differentiated to the species level with only four enzymes. Hundred ninety nine strains of filamentous fungi were isolated from various vineyards in Burgundy and identified by this method. Penicillium (58.5%) was the genus the most frequently isolated and no strains of the genus Aspergillus was isolated. P. spinolusum was the most isolated species of Penicillium (22.70%). The species C. cladiosporioides, B. cinerea, E. nigrum, A. alternata, T. koningiopsis, P. diplodiella, C. herbarum, A. alternatum, T. cucumeris and F. oxysporum were also isolated. This technique is a rapid and reliable method appropriate for routine identification of filamentous fungi. This can be used to screen large numbers of isolates from various environments in a short time. This is the first exhaustive study of fungal diversity at species level in vineyard.

Belowground volatiles facilitate interactions between plant roots and soil organisms

Many interactions between organisms are based on the emission and perception of volatiles. The principle of using volatile metabolites as communication signals for chemo-attractant or repellent for species-specific interactions or mediators for cell-to-cell recognition does not stop at an apparently unsuitable or inappropriate environment. These infochemicals do not only diffuse through the atmosphere to process their actions aboveground, but belowground volatile interactions are similarly complex. This review summarizes various eucaryotes (e.g., plant (roots), invertebrates, fungi) and procaryotes (e.g., rhizobacteria) which are involved in these volatile-mediated interactions. The soil volatiles cannot be neglected anymore, but have to be considered in the future as valuable infochemicals to understand the entire integrity of the ecosystems.

Modeling quality of premium spanish red wines from gas chromatography-olfactometry data

The aroma compositions of 25 premium Spanish red wines have been screened by quantitative gas chromatography-olfactometry and have been related to the quality scores of the wines. The study has shown that up to 65 odorants can be present in the aroma profiles of those wines, 32 of which have been detected in less than half of the samples. One new odorant is reported for the first time in wine [(Z)-2-nonenal], and only 11 odorants, most of them weak and infrequent, remain unknown. Quality was not positively correlated with any single compound or with any olfactometric vector built by the summation of odorants with similar odors. However, an olfactometric vector built by the summation of the olfactometric scores of defective odorants, such as 2-methoxy-3,5-dimethylpyrazine, 4-ethylphenol, 3-ethylphenol, 2,4,6-trichloroanisole, and o-cresol was significant and negatively related to quality. Quality could be satisfactorily explained by a simple partial least-squares model (79% explained variance in cross-validation) with just three X-variables: the aforementioned defective vector, a second vector grouping 9 other compounds with negative aroma nuances, and the fruity vector, grouping 15 compounds with fruit-sweet descriptors. This result shows that the quality of these red wines is primarily related to the presence of defective or negative odorants, and secondarily to the presence of a relatively large number of fruit-sweet odorants. Remarkably, only in a few low-quality samples could defective aroma nuances be detected, which suggests that defective and negative odorants exert a strong aroma suppression effect on fruity aroma.

Multi-residue off-flavour profiling in wine using stir bar sorptive extraction-thermal desorption-gas chromatography-mass spectrometry

A multi-residue method (MRM) for the detection and quantification of eight compounds responsible for off-flavours in wine using stir bar sorptive extraction (SBSE) followed by thermal desorption (TD) and gas chromatography-mass spectrometry (GC-MS) analysis is presented. The extraction and desorption conditions were optimised in order to get the best compromise for the simultaneous analysis of the eight target solutes, belonging to different chemical classes. The analytical conditions enable the quantification of the solutes below their respective organoleptic perception thresholds in wine. The method displayed good linearity over the concentration ranges explored in wine as well as excellent repeatability (RSD below 6%) and good reproducibility (RSD below 24%). The developed methodology was applied to the analysis of several wines and showed good agreement with the results collected with headspace solid-phase microextraction (HS-SPME) or liquid-liquid extraction (LLE) followed by GC-MS or electron capture detection (ECD). Good correlation was also found between the analytical and sensory results.

Geosmin biosynthesis. Streptomyces coelicolor germacradienol/germacrene D synthase converts farnesyl diphosphate to geosmin

Geosmin is responsible for the characteristic odor of moist soil. Incubation of recombinant germacradienol synthase, encoded by the SCO6073 (SC9B1.20) gene of the Gram-positive soil bacterium Streptomyces coelicolor, with farnesyl diphosphate (2, FPP) in the presence of Mg2+ gave a mixture of (4S,7R)-germacra-1(10)E,5E-diene-11-ol (3) (74%), (-)-(7S)-germacrene D (4) (10%), geosmin (1) (13%), and a hydrocarbon, tentatively assigned the structure of octalin 5 (3%). Individual incubations of recombinant germacradienol synthase with [1,1-2H2]FPP (2a), (1R)-[1-2H]-FPP (2b), and (1S)-[1-2H]-FPP (2c), as well as with FPP (2) in D2O, and GC-MS analysis of the resulting deuterated products supported a mechanism of geosmin formation involving proton-initiated cyclization and retro-Prins fragmentation of the initially formed germacradienol to give intermediate 5, followed by protonation of 5, 1,2-hydride shift, and capture of water.

Impact of the Botrytis cinerea strain and metabolism on (−)-geosmin production by Penicillium expansum in grape juice

Geosmin, an off-flavour of some rotten grapes, has been implicated in wine defects. Botrytis cinerea and Penicillium expansum were the most common among the numerous microorganisms isolated from rotten grapes. P. expansum produces geosmin on model media but not healthy grape juice. However, geosmin synthesis by P. expansum was demonstrated in grape juice and on crushed grapes that had been pre-cultured with certain B. cinerea strains. 34 out of 156 B. cinerea strains ([bot +] phenotype) isolated from the centre of grape bunches were able to induce high geosmin production, up to 494 ng/l, by P. expansum in grape juice. A study of the impact of grape juice composition on geosmin synthesis by P. expansum revealed the importance of nitrogen composition, particularly amino-acid deficiency. Metabolism of amino acids by B. cinerea was shown to be favourable to geosmin synthesis by P. expansum. However, the amino-acid and ammonium concentrations in grape juices pre-cultured with B. cinerea [bot -] and [bot +] strains were very similar implying that other factors are involved as well. Indeed, an ethanol-precipitable fraction, probably a polysaccharide, synthesized by B. cinerea [bot -], but not [bot +] strains, inhibited geosmin production by P. expansum.

Rapid headspace solid-phase microextraction/gas chromatographic/mass spectrometric assay for the quantitative determination of some of the main odorants causing off-flavours in wine

In this study we present a rapid and simultaneous assay method using headspace (HS) solid-phase microextraction (SPME)/gas chromatography (GC)/electron impact (EI) mass spectrometry (MS) (selected ion monitoring) for contaminants causing the principal organoleptic defects of wine (2,4,6-trichloroanisole, 2,3,4,6-tetrachloroanisole, pentachloroanisole, 2,4,6-tribromoanisole, 1-octen-3-ol, geosmin, 2-methylisoborneol, 3-isopropyl-2-methoxypyrazine, fenchol, fenchone, 2-methoxy-3,5-dimethylpyrazine, 4-ethylphenol, 4-ethylguaiacol, 4-vinylphenol, 4-vinylguaiacol, 3-isobutyl-2-methoxypyrazine, guaiacol and ethyl acetate). The method was validated according to protocols NF ISO 5725-1, 2 and NF V03-110. Its characteristics (limit of detection (LOD), limit of quantification (LOQ), uncertainties) were determined after having optimised the SPME parameters. The target contaminants were quantified in the wines below their threshold of perception with a satisfactory relative standard deviation for all the analytes except ethyl acetate (RSD=36%); for that, the assay method permits clear differentiation of the wines that are at risk of presenting an acescent character, i.e. containing more than 120mgL(-1) ethyl acetate. The target volatile and odorous substances were determined at concentrations significantly below their threshold of perception in a hydroalcoholic context and their threshold of recovery in wines.