Agriculture Reference
In-Depth Information
of course 'No, Iain: it's the other way round - cooking
is
chemistry' (O'Hare and
Grigor 2004).
10.5.1 Natural esters
While these are perhaps of limited interest chemically, they are probably the
largest volume of all the 'synthesised' natural aroma chemicals. Natural alcohols
and natural acids are readily available from fermentation, especially ethanol itself
and isoamyl alcohol derived from fusel oils (see below), i.e. the higher boiling
residues from distillation of 'liquors' such as whisky and brandy.
The simplest approach to esters is direct reaction between an alcohol and an
acid, with the water produced being removed by azeotropic distillation. With the
more volatile esters, the ester itself can be used to form an azeotrope; with esters
of water-soluble alcohols such as ethyl esters, the presence of too much alcohol in
the distillate can stop the water from separating out and a volatile hydrocarbon
such as toluene may be added to help overcome this. From
n
-butyl esters onwards
this is not a problem as the alcohol is immiscible with water and separation is not
an issue.
All of this applies to naturals as much as synthetics. However, the reaction is
slow and the 'chemical' approach of adding a strong acid catalyst is not possible.
For esters of higher boiling alcohols such as isoamyl alcohol [10.24] and
cis
-3-
hexenol [10.13], this is not much of a problem as the reaction mix can be heated
to a suffi ciently high temperature to achieve a reaction in a matter of a few hours,
for example the formation of the familiar isoamylacetate [10.25]. With the
formation of ethyl esters, however, the low boiling point of ethanol (78°C) means
the reaction is very slow, leading to reaction times running into many days, with
each passing day increasing the temptation to add a catalyst! This can be done
naturally, of course, by using esterases (lipases) to accelerate the reaction,
especially in transesterifi cation reactions.
[10.24] [10.25]
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