Quantification of cork off-flavor compounds in natural cork stoppers and wine by multidimensional gas chromatography mass spectrometry

The microbiota of cork and yellow stain as a model for a new route for the synthesis of chlorophenols and chloroanisoles from the microbial degradation of suberin and/or lignin

BACKGROUND

The main application of cork is the production of stoppers for wine bottles. Cork sometimes contains 2,4,6-trichloroanisole, a compound that, at a concentration of ng/L, produces an unpleasant musty odor that destroys the organoleptic properties of wine and results in enormous economic losses for wineries and cork industries. Cork can exhibit a defect known as yellow stain, which is associated with high levels of 2,4,6-trichloroanisole. We describe how the microbiota of cork and yellow stain define a novel mechanism that explains the formation of chlorophenols and chloroanisoles (including 2,4,6-trichloroanisole) from p-hydroxybenzoate produced during lignin and/or suberin breakdown.

RESULTS

Electron microscopy revealed that cork affected by yellow stain exhibited significant structural degradation. This deterioration was attributed to the presence of higher microbial populations compared to those found in standard cork. Cork microbiota is rich in filamentous fungi able to metabolize lignin. A metataxonomic analysis confirmed that yellow stain contained significantly greater populations of fungal species belonging to Absidia, Geomyces, Mortierella, Mucor, Penicillium, Pseudogymnoascus, Talaromyces, and Umbelopsis. It also contained significantly greater amounts of bacteria belonging to Enterobacterales, Streptosporangiales, Tepidisphaerales, Pseudomonas, and several members of Burkholderiaceae, particularly species of the Burkholderia-Caballeronia-Paraburkholderia group. The extraction of aromatic compounds from cork samples allowed the identification of several compounds typically observed following lignin depolymerization. Notably, p-hydroxybenzoic acid and phenol were detected. Two strains of the genus Streptomyces isolated from yellow stain were able to biotransform p-hydroxybenzoate into phenol in resting cell assays. Phenol could be efficiently chlorinated in vitro to produce 2,4,6-trichlorophenol by a fungal chloroperoxidase, an enzymatic activity commonly found in filamentous fungi isolated from cork. Finally, as has been widely demonstrated before, 2,4,6-trichlorophenol can be efficiently O-methylated to 2,4,6-trichloroanisole by many of fungi that inhabit cork.

CONCLUSIONS

Chlorophenols and chloroanisoles can be produced de novo in cork from p-hydroxybenzoate generated by the microbial biodegradation of lignin and/or suberin through the participation of different types of microorganisms present in cork. The natural origin of these compounds, which are of great interest for the chlorine cycle and represent a new source of environmental contamination that differs from that caused by human activity, is described. Video Abstract.

3-Alkyl-2-Methoxypyrazines: Overview of Their Occurrence, Biosynthesis and Distribution in Edible Plants

Pyrazines are ubiquitous in nature - biosynthesized by microorganisms, insects, and plants. Due to their great structural diversity, they own manifold biological functions. Alkyl- and alkoxypyrazines for instance play a key role as semiochemicals, but also as important aroma compounds in foods. Especially 3-alkyl-2-methoxypyrazines (MPs) have been of great research interest. MPs are associated with green and earthy attributes. They are responsible for the distinctive aroma properties of numerous vegetables. Moreover, they have a strong influence on the aroma of wines, in which they are primarily grape-derived. Over the years various methods have been developed and implemented to analyse the distribution of MPs in plants. In addition, the biosynthetic pathway of MPs has always been of particular interest. Different pathways and precursors have been proposed and controversially discussed in the literature. While the identification of genes encoding O-methyltransferases gave important insights into the last step of MP-biosynthesis, earlier biosynthetic steps and precursors remained unknown. It was not until 2022 that in vivo feeding experiments with stable isotope labeled compounds revealed l-leucine and l-serine as important precursors for IBMP. This discovery gave evidence for a metabolic interface between the MP-biosynthesis and photorespiration.

A Review on Wine Flavour Profiles Altered by Bottle Aging

The wine flavour profile directly determines the overall quality of wine and changes significantly during bottle aging. Understanding the mechanism of flavour evolution during wine bottle aging is important for controlling wine quality through cellar management. This literature review summarises the changes in volatile compounds and non-volatile compounds that occur during wine bottle aging, discusses chemical reaction mechanisms, and outlines the factors that may affect this evolution. This review aims to provide a deeper understanding of bottle aging management and to identify the current literature gaps for future research.

The Impact of Different Closures on the Flavor Composition of Wines during Bottle Aging

Wine flavor undergoes major changes during bottle aging and can be influenced by the type of closure. The interaction between wine, the type of closure and the external environment has the potential to significantly influence the overall quality of bottled wines, especially when the storage period is relatively long (more than five years). Therefore, the choice of closure (cork, synthetic or screw cap) deserves special attention in order to establish the ideal sealing conditions for optimizing wine flavor attributes. The contribution of different closures to the quality of bottled wine is through mass transfer phenomena, including permeation, sorption (scalping) or desorption of chemicals between closure materials and wines. Thus, this article aims to review the impact of different closures on the flavor composition of wines during post-bottling conditions. The implications of closures on wine sensory properties are also discussed.

Corky off-flavor compounds in cork planks at different storage times before processing. Influence on the quality of the final stoppers

BACKGROUND

Cork companies store cork planks before processing them for a minimum of 6 months to dry up and to stabilize their texture and chemical composition, although many companies extend this storage period up to 12 months. However, there is no information about the influence of this seasoning period on their 'corky' off flavors. For this reason, the main compounds responsible for the 'cork taint' of planks stored before processing from 6 to 12 months were investigated.

RESULTS

Four haloanisoles and three halophenols were identified: 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole (TeCA), pentachloroanisole (PCA), 2,4,6-tribromoanisole (TBA), 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP), and 2,4,6-tribromophenol (TBP). All of the planks presented some haloanisole or halophenol after 6 and 9 months of storage, which practically disappeared after a year of storage. These compounds were only detected in the cork stoppers made from planks with 6 and 9 months of storage. Of the alkylmethoxypyrazines, 2-methoxy-3,5-dimethylpyrazine (MDMP), 3-isopropyl-2-methoxypyrazine (IPMP), and 3-isobutyl-2-methoxypyrazine (IBMP) were identified. The MDMP was detected in a larger number of planks with 6 months of storage and at higher concentrations than IPMP and IBMP. However, MDMP was not detected in the cork stoppers made from planks at 6, 9, and 12 months of storage.

CONCLUSION

A storage time of 6 months before processing of raw cork planks would be sufficient to obtain cork stoppers with low concentrations of corky off-flavor compounds. An increase in storage up to 9 or 12 months would result in practically 'cork taint'-free natural stoppers. © 2021 Society of Chemical Industry.

Significance and Transformation of 3-Alkyl-2-Methoxypyrazines Through Grapes to Wine: Olfactory Properties, Metabolism, Biochemical Regulation, and the HP-MP Cycle

3-Alkyl-2-methoxypyrazines (MPs) contribute to the herbaceous flavor characteristics of wine and are generally considered associated with poor-quality wine. To control the MPs in grapes and wine, an accurate understanding of MP metabolism is needed. This review covers factors affecting people in the perception of MPs. Also, the history of O-methyltransferases is revisited, and the present review discusses the MP biosynthesis, degradation, and biochemical regulation. We propose the existence of a cycle between MPs and 3-alkyl-2-hydropyrazines (HPs), which proceeds via O-(de)methylation steps. This cycle governs the MP contents of wines, which make the cycle the key participant in MP regulation by genes, environmental stimuli, and microbes. In conclusion, a comprehensive metabolic pathway on which the HP-MP cycle is centered is proposed after gaining insight into their metabolism and regulation. Some directions for future studies on MPs are also proposed in this paper.

Characterization of Key Aroma Compounds in a Commercial Rum and an Australian Red Wine by Means of a New Sensomics-Based Expert System (SEBES)-An Approach To Use Artificial Intelligence in Determining Food Odor Codes

Although to date more than 10 000 volatile compounds have been characterized in foods, a literature survey has previously shown that only 226 aroma compounds, assigned as key food odorants (KFOs), have been identified to actively contribute to the overall aromas of about 200 foods, such as beverages, meat products, cheeses, or baked goods. Currently, a multistep analytical procedure involving the human olfactory system, assigned as Sensomics, represents a reference approach to identify and quantitate key odorants, as well as to define their sensory impact in the overall food aroma profile by so-called aroma recombinates. Despite its proven effectiveness, the Sensomics approach is time-consuming because repeated sensory analyses, for example, by GC/olfactometry, are essential to assess the odor quality and potency of each single constituent in a given food distillate. Therefore, the aim of the present study was to develop a fast, but Sensomics-based expert system (SEBES) that is able to reliably predict the key aroma compounds of a given food in a limited number of runs without using the human olfactory system. First, a successful method for the quantitation of nearly 100 (out of the 226 known KFOs) components was developed in combination with a software allowing the direct use of the identification and quantitation data for the calculation of odor activity values (OAV; ratio of concentration to odor threshold). Using a rum and a wine as examples, the quantitative results obtained by the new SEBES method were compared to data obtained by applying an aroma extract dilution analysis and stable isotope dilution assays required in the classical Sensomics approach. A good agreement of the results was found with differences below 20% for most of the compounds considered. By implementing the GC × GC data analysis software with the in-house odor threshold database, odor activity values (ratio of concentration to odor threshold) were directly displayed in the software pane. The OAVs calculated by the software were in very good agreement with data manually calculated on the basis of the data obtained by SIDA. Thus, it was successfully shown that it is possible to characterize key food odorants with one single analytical platform and without using the human olfactory system, that is, by "artificial intelligence smelling".

The predominant phytoplankton of Pseudoanabaena holding specific biosynthesis gene-derived occurrence of 2-MIB in a drinking water reservoir

With the increasing occurrence of water eutrophication and blooms, earthy-musty odor problems caused by cyanobacteria have been more and more apparent. These problems have a serious impact on aquatic ecosystems and drinking water safety and become one of the priorities of the water environment researches and managements. In the present study, genes associated with 2-MIB synthesis in cyanobacteria were studied by systematic investigation on molecular characterization and quantity of 2-MIB-producing cyanobacteria in China. Our results founded that Pseudoanabaena is an important 2-MIB-producing phytoplankton and predominant in summer in Qingcaosha reservoir, and the copy number of mic and 2-MIB concentration have strongly positive correlation. We also demonstrated that abundance of mic in water was significant correlation with that in sediment. These results will help us to understand the main taxa of the odoriferous cyanobacteria in the water bodies in China, understand the genetic basis of the odor substances in the cyanobacteria. Rapid, reliable monitoring and ecological research methods for the production of odor cyanobacteria in water bodies can be established based on these results.

Flavor and flavor chemistry differences among milks processed by high-temperature, short-time pasteurization or ultra-pasteurization

Typical high-temperature, short-time (HTST) pasteurization encompasses a lower heat treatment and shorter refrigerated shelf life compared with ultra-pasteurization (UP) achieved by direct steam injection (DSI-UP) or indirect heat (IND-UP). A greater understanding of the effect of different heat treatments on flavor and flavor chemistry of milk is required to characterize, understand, and identify the sources of flavors. The objective of this study was to determine the differences in the flavor and volatile compound profiles of milk subjected to HTST, DSI-UP, or IND-UP using sensory and instrumental techniques. Raw skim and raw standardized 2% fat milks (50 L each) were processed in triplicate and pasteurized at 78°C for 15 s (HTST) or 140°C for 2.3 s by DSI-UP or IND-UP. Milks were cooled and stored at 4°C, then analyzed at d 0, 3, 7, and 14. Sensory attributes were determined using a trained panel, and aroma active compounds were evaluated by solid-phase micro-extraction or stir bar sorptive extraction followed by gas chromatography-mass spectrometry, gas chromatography-olfactometry, and gas chromatography-triple quad mass spectrometry. The UP milks had distinct cooked and sulfur flavors compared with HTST milks. The HTST milks had less diversity in aroma active compounds compared with UP milks. Flavor intensity of all milks decreased by d 14 of storage. Aroma active compound profiles were affected by heat treatment and storage time in both skim and 2% milk. High-impact aroma active compounds were hydrogen sulfide, dimethyl trisulfide, and methional in DSI-UP and 2 and 3-methylbutanal, furfural, 2-heptanone, 2-acetyl-1-pyrroline, 2-aminoacetophenone, benzaldehyde, and dimethyl sulfide in IND-UP. These results provide a foundation knowledge of the effect of heat treatments on flavor development and differences in sensory quality of UP milks.