Wine is a highly complex mixture of compounds which largely define its appearance, aroma, flavour and mouth-feel properties. The compounds responsible for those attributes have been derived in turn from three major sources, viz. grapes, microbes and, when used, wood (most commonly, oak). The grape-derived compounds provide varietal distinction in addition to giving wine its basic structure. Thus, the floral monoterpenes largely define Muscat-related wines and the fruity volatile thiols define Sauvignon-related wines; the grape acids and tannins, together with alcohol, contribute the palate and mouth-feel properties. Yeast fermentation of sugars not only produces ethanol and carbon dioxide but a range of minor but sensorially important volatile metabolites which gives wine its vinous character. These volatile metabolites, which comprise esters, higher alcohols, carbonyls, volatile fatty acids and sulfur compounds, are derived from sugar and amino acid metabolism. The malolactic fermentation, when needed, not only provides deacidification, but can enhance the flavour profile.

The aroma and flavour profile of wine is the result of an almost infinite number of variations in production, whether in the vineyard or the winery. In addition to the obvious, such as the grapes selected, the winemaker employs a variety of techniques and tools to produce wines with specific flavour profiles. One of these tools is the choice of microorganism to conduct fermentation. During alcoholic fermentation, the wine yeast Saccharomyces cerevisiae brings forth the major changes between grape must and wine: modifying aroma, flavour, mouth-feel, colour and chemical complexity. The wine bacterium Oenococcus oeni adds its contribution to wines that undergo malolactic fermentation. Thus flavour-active yeasts and bacterial strains can produce desirable sensory results by helping to extract compounds from the solids in grape must, by modifying grape-derived molecules and by producing flavour-active metabolites. This article reviews some of the most important flavour compounds found in wine, and their microbiological origin.

Yeast and bacterial modulation of wine aroma and flavour

During hydrolysis of lignocellulosic biomass, monomeric sugars and a broad range of inhibitory compounds are formed and released. These inhibitors, which can be organized around three main groups, furans, weak acids and phenolics, reduce ethanol yield and productivity by affecting the microorganism performance during the fermentation step. Among the microorganisms that have been evaluated for lignocellulosic hydrolysate ethanol fermentation, the yeast Saccharomyces cerevisiae appears to be the least sensitive. In order to overcome the effect of inhibitors, strategies that include improvement of natural tolerance of microorganism and use of fermentation control strategies have been developed. An overview of the origin, effects and mechanisms of action of known inhibitors on S. cerevisiae is given. Fermentation control strategies as well as metabolic, genetic and evolutionary engineering strategies to obtain S. cerevisiae strains with improved tolerance are discussed.

Increased tolerance and conversion of inhibitors in lignocellulosic hydrolysates by Saccharomyces cerevisiae

High-Definition Macromolecular Composition of Yeast RNA-Processing Complexes

Although Escherichia coli has long been recognized as the best-understood living organism, little was known about its abilities to use aromatic compounds as sole carbon and energy sources. This review gives an extensive overview of the current knowledge of the catabolism of aromatic compounds by E. coli. After giving a general overview of the aromatic compounds that E. coli strains encounter and mineralize in the different habitats that they colonize, we provide an up-to-date status report on the genes and proteins involved in the catabolism of such compounds, namely, several aromatic acids (phenylacetic acid, 3- and 4-hydroxyphenylacetic acid, phenylpropionic acid, 3-hydroxyphenylpropionic acid, and 3-hydroxycinnamic acid) and amines (phenylethylamine, tyramine, and dopamine). Other enzymatic activities acting on aromatic compounds in E. coli are also reviewed and evaluated. The review also reflects the present impact of genomic research and how the analysis of the whole E. coli genome reveals novel aromatic catabolic functions. Moreover, evolutionary considerations derived from sequence comparisons between the aromatic catabolic clusters of E. coli and homologous clusters from an increasing number of bacteria are also discussed. The recent progress in the understanding of the fundamentals that govern the degradation of aromatic compounds in E. coli makes this bacterium a very useful model system to decipher biochemical, genetic, evolutionary, and ecological aspects of the catabolism of such compounds. In the last part of the review, we discuss strategies and concepts to metabolically engineer E. coli to suit specific needs for biodegradation and biotransformation of aromatics and we provide several examples based on selected studies. Finally, conclusions derived from this review may serve as a lead for future research and applications.

http://envismadrasuniv.org/Bioremediation/pdf/Biodegradation%20of%20aromatic.pdf

Biodegradation of Aromatic Compounds by Escherichia coli

Publisher Summary This chapter reviews the scientific knowledge of the role of microorganisms, especially yeast, in the development of wine flavor. Specific attention is given to the contribution of esters, higher alcohols, volatile thiols, volatile phenols, and monoterpenoids to the flavor profile. It is well known that grapes of different cultivars display characteristic aromas that are distinctive of the wines. However, it can be shown that although some volatile aroma substances arise from components of the grapes, many of these compounds are changed and a substantial portion of wine flavor substances are formed during yeast fermentation. Therefore, wine has more flavor than the grape juice it is fermented from, and the importance of yeast and other wine-related microorganisms is central to the development of wine flavor. Furthermore, different biosynthetic pathways are interactive during the formation of the aroma of alcoholic beverages, and different factors play their part in the formation of the total aroma.

Yeast modulation of wine flavor.

PAD1 encodes phenylacrylic acid decarboxylase which confers resistance to cinnamic acid in Saccharomyces cerevisiae.

In accordance with the present invention, there are provided methods to modulate plant lignification. For example, the level of the enzyme phenylalanine ammonialyase (PAL), a key enzyme in the production of lignin precursors, can be increased or decreased, as desired, to increase or decrease lignification levels. Alternatively, the level of lignin precursors in the plant can be modulated either by enhancing the plant's ability to produce such precursors, or by causing the compounds which are required for the production of lignin precursors to undergo alternative chemical conversions (which prevent the incorporation of such compounds into lignin by diverting the lignin precursors to other chemical pathways). In addition, there is provided a method for producing plants having modified lignin composition and content, by converting a portion of the cinnamic acid in said plant to styrene. In another embodiment, there is provided a method for producing styrene and/or styrene derivatives, by contacting cinnamic acid or a cinnamic acid derivative with PAD under decarboxylation conditions.

Monika Clausen

Papers

PAD1 encodes phenylacrylic acid decarboxylase which confers resistance to cinnamic acid in Saccharomyces cerevisiae.

Methods for the modulation of lignification levels

Hormone-Inducible Expression and Metal Affinity Chromatography of Recombinant Proteins in Saccharomyces cerevisiae