Abstract:
The branched chain 2-methylbutyl, 3-methylbutyl and 2-methylpropionate ester constituents of apple aroma are thought to arise by oxidative degradation of the amino acids isoleucine, leucine and valine respectively.
We were interested to define the relative importance of enzymic activity and specificity of substrate concentrations in determining the proportions of these esters in apple aroma.
Skin slices from the apple cultivars ‘Fuji’, ‘Red Delicious’, ‘Royal Gala’, ‘Braeburn’, and ‘Granny Smith’ were incubated with an equimolar mixture of d10-isoleucine, d3-leucine and d8-valine and the aroma volatiles were collected and analysed for deuterium incorporation using GC and GCMS.
Addition of amino acid precursors increased total aroma volatile production by the apple tissue slices.
The 2- (and 3)-methylbutyl and methylbutanoate esters comprised 82 – 99% of the major labelled volatiles, with different ratios of 2-methyl/3-methyl isomers (from 0.3 to 10) occurring for each cultivar (Table 1). Individually d9-2-methylbutyl acetate, ethyl d9-2-methylbutanoate and 3-d3-methylbutyl acetate were the major volatiles produced.
Metabolism to d3-acetate esters (largely butyl d3-acetate) contributed 0.4 to 2.6 % of labelled volatiles from four of the cultivars, but 17.6% (16.1% butyl d3-acetate) of the labelled volatiles produced by ‘Royal Gala’ apples (Table 1).
The distribution of deuterated metabolites in the aroma volatiles generally reflected the quantitative distribution normally found for each apple cultivar.
In some cases, the synthesis of labelled volatiles was accompanied by reduced biosynthesis of the unlabelled analogues, suggesting that there may be competition between substrates, and that enzymic activity as well as substrate concentrations may limit aroma volatile production.
| Class of labelled |
Precursor |
‘Fuji’ |
‘Red |
‘Royal |
‘Braeburn’ |
‘Granny
|
| aroma volatile |
|
|
Delicious’ |
Gala’ |
|
Smith’
|
| 2-methylpropionate |
valine |
0.5 |
0.0 |
trace |
1.4 |
5.6
|
| 2-methylbutyl and 2- |
isoleucine |
25.2 |
48.7 |
56.3 |
73.4 |
82.3
|
| methylbutanoates |
|
|
|
|
|
|
| 3-Methylbutyl and 3- |
leucine |
73.6 |
49.0 |
26.0 |
23.6 |
8.0
|
| methylbutanoates |
|
|
|
|
|
|
| d3-acetates |
isoleucine |
0.4 |
1.3 |
17.6 |
1.3 |
2.6
|
| Ratio of 2-Me/3-Me |
|
0.34 |
0.99 |
2.2 |
3.1 |
10.3
|
| butyl compounds |
|
|
|
|
|
|
|
