Esters are the aromatic, fruity compounds in wine,
formed during fermentation, malolactic fermentation (MLF) and ageing. There are
many factors that influence the types and quantities of esters present and
although all of them are not positive contributors, as a group, they are a
major constituent of wine.
Research has identified 11 different esters in grape
juice, but wine can have up to 83; this affects the wine’s flavour and
complexity in proportion to their presence. Esters are formed when an acid
reacts with alcohol, eliminating a water molecule in the process.
With the many different acids and alcohols found in
wine, there is considerable potential for the formation of a wide range of
esters. The alcohol in the reaction can be ethanol, or any other alcohol
produced by yeast cells, especially from the degradation of amino acids.
In wine, esters can be classified into two groups:
those formed enzymatically and those formed during wine ageing, through a
chemical esterification between alcohol and acids at a low pH.
Esters can furthermore be grouped into two classes,
namely ethyl esters and acetate esters. The ethyl esterscomprise a short-chain alcohol group (ethanol) and a
longer-chain acid group (such as medium- to long-chain fatty acids). Ethyl
esters have a strong influence on a wine’s aroma and examples of ethyl esters
are ethyl hexanoate, with reported aroma characteristics that include fruity,
strawberry, green apple and anise; ethyl octanoate, with sweet, fruity, ripe
fruit, burned and beer characteristics; and ethyl decanoate, which imparts an
oily, fruity and floral character.
The acetate
esterscomprise an acid group (short-chain acetic acid) and a
longer-chain alcohol group (fusel alcohols), mostly complex alcohols derived
from amino acid metabolism. This group includes esters such as isoamyl acetate,
with aromas of banana and fruits; and isobutyl acetate, with fruity and apple
flavours.
Ethyl acetate is quantitatively the most prominent
ester in wine, due to its spontaneous or enzymatic formation from ethanol and
acetic acid – it is therefore both an acetate and/or ethyl ester.
Ethanol and acetate are present in wine in relatively
large concentrations; ethyl acetate is therefore often an important contributor
to wine aroma. At low concentrations it may give desirable and fruity character
to the wine; however, at higher concentrations it can impart a solvent or nail
varnish aroma, and contribute to the perception of volatile acidity (VA).
Ester formation
Esters are typically synthesised in grapes, but rarely in amounts that are
of sensory significance. Isoamyl acetate, the commonly detected banana and
fruity character in Pinotage, is one exception, while the synthesis of ethyl 9-hydroxynonanoate
by Botrytis cinerea may contribute to the distinctive aroma of botrytised wines.
The formation of esters during fermentation is a dynamic process, with numerous variables
interacting, affecting ester production and concentration. These variables
include the quantity of esters or their precursors originally present in the
grape; the temperature of fermentation; the yeast strain that predominates; and
the nutrients present, especially nitrogen compounds and must solids. Grape
cultivars also differ considerably in terms of ester content.
Malolactic fermentation
(MLF) can furthermore
impact the ester profile of a wine. The aroma compound that is mostly produced
during MLF is ethyl lactate (ethyl 2-hydroxypropanoate), which is
characteristically milky, soapy, buttery and fruity. The production of this
ester is coupled to lactic acid formation and its synthesis can be correlated
with the percentage of degradation of malic acid.
During fermentation, significant modifications in
composition takes place, this is then followed by ageing when chemical constituents generally react slowly in order
to move to their equilibrium position, resulting in gradual changes in flavour.
During bottle ageing, hydrogen ions – which are found
in higher concentration in low pH wines – serve as a catalyst in the formation
of esters. However, these hydrogen ions also encourage esters to split back
into acids and alcohols. These two counterbalancing acts gradually move a wine
closer to equilibrium, where there are equal parts of alcohol, acids, esters
and water.
During this period, the ester-influenced bouquet of
the wine constantly changes due to the concentration, formulation and splitting
of different esters. This is partly why a wine will have one set of aromas when
it is young and other aromas later in its life.
The importance of yeast strain
Along with other sensory important compounds, esters
can be produced as secondary metabolites by many yeast species during
fermentation.It is clear that yeast and bacteria have the ability to alter a
wine’s volatile composition – specifically the profile of esters – which
emphasises the importance of strain choice inwinemaking on the ester profile of
the finished product.
The precise yeast strain used during fermentation and
the fermentation temperature are two of the most important indicators of the
types of esters that will develop. While this helps to partially explain why
Chardonnay grown in the same vineyard, but made by two different producers,
could have different aromatics, indigenous yeasts can impart distinct regional
characteristics to wine which are integral to their authenticity.
Effects of esters
Esterolytic activity during wine production has the
potential to either increase or decrease the amount of esters present and
therefore its perceived quality.
The ester-associated aroma profile not only depends on
the esters involved, but also on the compounds freed by the esterases, for
example fatty acids and higher alcohols.
When esters are formed, a water molecule is released
in the process, which binds to hydrogen ions. This effectively raises the
wine’s pH, softens and mellows it (maturation), gives off fruity aromas and, at
a later stage, imparts more developed bouquets.
In fermented beverages such as wine and beer, esters
are frequently found in trace amounts. Individually, they are often below the
aroma threshold concentrations, while collectively they generally do not exceed
concentrations of 100 mg/l.
In low concentrations, these compounds are perceived
as generically fruity or floral and can boost the awareness of these
characteristics of the grape cultivar. However, if they are present in too high
a concentration, they can mask the other varietal aromas and impart a synthetic
fruit or fruit-candy aroma to the wine, decreasing its complexity.
Although ethyl acetate provides the familiar fruity
character to young wine, it is a product of ethanol reacting with volatile
acetic acid – at higher concentrations, this produces the pickled, vinegary
aroma associated with volatile acid spoilage. Wines that generally contain more
than 90 mg/L of ethyl acetate or 200 mg/L of total esters are considered to be
faulty.
Esters are the most commonly encountered functional
groups in wine and these complicated compounds have an important impact on the
flavour and complexity of a wine. They are influenced by each other, other
compounds and external factors, but although they are considered the perfume of
wine, they aren’t always perceived positively.
SA LAW
Flavourants of vegetable origin or extracts thereof,
as well as flavourants that are nature-identical may not be added to wine.
This, as stipulated by the Liquor Products Act 60 of
1989.
Sources:
• Cape Wine Academy
• Wikipedia
• http://enologyaccess.org: Diagnostic Key Classes
of Off-Character Compounds Esters
• www.elsevier.com/locate/foodchem: Microbial
modulation of aromatic esters in wine:Current knowledge and future prospects
• Wine Science: Principles and applications (third
edition); by Ronald S. Jackson.