ReviewMicrobial modulation of aromatic esters in wine: Current knowledge and future prospects
Introduction
Wine is a complex mixture of hundreds of compounds, many of which contribute substantially to the colour, mouthfeel or aromatic properties of this beverage. The aroma of wine has received much research attention over recent decades, with numerous components being identified as playing a role in specific sensory notes. The distinctive flavour and aroma of wine is determined by many variables, including grape variety, viticultural and winemaking practices and wine maturation and storage conditions. A range of microorganisms come into contact with wine during its production, and thus their metabolic activities and synthetic and degradative enzymes, may influence wine aroma. In this review we examine the types and concentrations of one important group of aroma-active compounds, the esters, and consider their sensory contribution, the impact of wine-associated yeast and bacteria on wine ester makeup and explore the potential for modulation of this profile by selective use of microbes or their derivatives. Where wine-related information is not available, this review will draw on findings from other food-related organisms involved in similar processes.
Section snippets
Esters in wine
Esters are flavour compounds that occur widely in a variety of food products (Gatfield, 1992). In fermented beverages such as wine and beer, they are frequently in trace amounts, such that individually they are often below aroma threshold concentrations, and collectively they generally do not exceed concentrations of 100 mg/l. Nonetheless, as a group these compounds are the next major constituents in wine (Fig. 1) after water, ethanol and fusel alcohols (Etiévant, 1991), and are the primary
Enzymatic synthesis and hydrolysis of esters
The enzymatic accumulation of esters in wine during fermentation is known to be the result of the balance of the enzymatic synthesis and hydrolysis reactions involving esterases (EC 3.1.1.1) and synthesis reactions involving alcohol acetyltransferases (EC 2.3.1.84) (Lilly et al., 2000, Mason and Dufour, 2000, Matthews et al., 2007, Verstrepen et al., 2003c). Wine microflora possess these enzymes and enzymatic activity in microbial strains used during the production of wine is of great
Microbes and aroma
As stated, the initial conversion of grape must into wine occurs via an alcoholic fermentation largely carried out by one or more strains of yeast, typically S. cerevisiae. Ethanol produced during the primary fermentation progressively limits the growth of other microbes that are potentially undesirable, thereby helping to protect the wine from spoilage (Renouf, Claisse, & Lonvaud-Funel, 2007). Nevertheless, without their deliberate elimination, bacteria and other yeasts, including non-
Impact of ester synthesis and hydrolysis on wine
Esterolytic activity during wine production has the potential to either increase or decrease the amount of esters present in wine and therefore its perceived quality. The ester-associated aroma profile does depend on the esters involved, but also the compounds liberated by the esterases (for example fatty acids and higher alcohols). There is a large variation in ester production between yeast strains used for the primary fermentation and investigations into structural changes in the expressed
Methods of measuring esters and esterase activity
The general method in the literature for measuring substrate activities of esterases uses a series of p-nitrophenyl esters of C2–C16 fatty acid compounds, and substituted ethyl and acetic acid ester compounds (Bendicho et al., 2001, Fenster et al., 2003a, Fenster et al., 2003b, Matthews et al., 2006, Matthews et al., 2007). The p-nitrophenyl-linked substrates and rates of p-nitrophenyl release are easily quantified by measurement of absorbance at 410 nm. Assays with ethyl esters of C2–C6 fatty
Potential applications of current knowledge
There are several ways in which the findings reported to date could be utilised in order to achieve outcomes for fermented beverage production. These include:
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Strain selection, including non-Saccharomyces yeast, to positively enhance the ester profile of wine. Strategies for extending the survival of non-Saccharomyces with favourable effects on ester production, might also be considered.
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Preparation of esterase enzyme extracts that are better able to function under the harsh and changing
Conclusions
A considerable amount of research has been directed at the accumulation of esters during fermentation and their role in the fruity aroma of wine. From this research it is clear that esters are extremely important for the flavour profile of wine. Much of this work has focused on formation during the primary fermentation and a large amount of the research focus has been on the determination of the genes involved in ester synthesis and hydrolysis in Saccharomyces sp. and the environmental
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