The 'secondary volatiles' are compounds formed from non-volatile precursors,
such as linoleic acid, during disruption of fruit tissue by the action of enzymes and, to a
smaller extent, by heating during processing (Schreier, 1986) . Berger ( 199 1 ) stated that
the flavour profile of apples and apple juice can change rapidly after disintegration of
cells due to several reactions: (a) lipoxygenase-induced degradation of unsaturated CI8
fatty acids, leading to intensely aromatic C6 or C9 cleavage products within minutes; (b)
enzymatic hydrolysis of carboxylic acid esters, leading to a loss of fruity odour
components; (c) modification of monoterpenes and their precursors, leading to
rearranged and new products; and (d) formation of aromatic acid esters from activated
precursors. Drawert
et ai.( 1 986) identified cis-3-hexenal, trans-3-hexenal, cis-3-hexenol
and phenyl ethyl alcohol in apple homogenates. These C6 volatile compounds are found
in low levels in headspace of intact apples, but are abundant in apple juice and essence
(Paillard, 1990). They are formed by the enzymatic oxidation of linoleic and linolenic
acids after the fruit is crushed and exposed to oxygen (Hatanaka, 1 993) with maximum
production during the first 30 minutes after maceration of 'Cox's Orange Pippin' apple
peel (Drawert
et aI. ,1 986). Cold storage of such apple peel for 24 hours at 6°C
favoured formation of these secondary aldehydes, in contrast with those stored at
ambient temperature (Drawert et aI. , 1 9 86). Feys et al. ( 1980b) reported that reducing the time between homogenisation at room temperature and distillation of apples at low temperature to 1 0 minutes could not prevent formation of significant amounts of hexanal and trans-2-hexenal. Addition of 0. 1 % linoleic acid and 0. 1 % linolenic acid to apples during homogenisation resulted in a 3-fold and 4-fold increase in n-hexanal and trans-2- hexenal, respectively, over control (Feys et aI. , 1 980b). Production of secondary volatiles from fatty acids is common in damaged plant tissues, such as leaves and fruits (Galliard and Matthew, 1 977a; Hatanaka, 1 993; 1 996), and they play an important role in the overall flavour in a range of plant products, including tea (Hatanaka, 1 996), apple juice and essence (Dtirr and Schobinger, 1 98 1 ; Flath et aI. , 1 967).
The formation of secondary volatiles, such as hexanal and trans-2-hexenal follows hydroperoxidation of linoleic and linolenic acids by the lipoxygenase enzyme (Galliard et at. , 1 977; Gatfield, 1 988). However, enzymatic oxidative degradation of lipids is preceded by the action of acyl hydrolase, which liberates free fatty acids (Galliard, 1 980). The lipoxygenase [linoleate : oxygen oxidoreductase] of apple has been partially purified; it is membrane-bound, has a pH optimum at 6.0 and converts linoleic and linolenic acids into hydroperoxides (Kim and Grosch, 1 979). These hydroperoxides are relatively unstable being cleaved rapidly by a hydroperoxide lyase enzyme
(Fig. 1.7)
to form volatile carbonyl compounds (Eriksson, 1 979; Galliard and Matthew, 1977a; 1 977b) which are cytotoxic to proteins and membrane structures of cells (Schreier, 1984). In leaves, the lipoxygenase and hydroperoxide lyase enzyme systems are bound to the thylakoid membrane of chloroplasts (Hatanaka, 1 993 ; 1 996) . Lipoxygenases from different sources (ie. species of plants) differ considerable in their pH optima, substrate specificities and especially in the isomeric structure of their reaction products (Gatfield, 1 988; Schrodter, 1 990) . Thus, depending on the type of lipoxygenase and plant tissue, the reaction produces either 9- or 1 3-hydroperoxides or a mixture of both (Schreier, 1 984). The enzymatic cleavage of the 1 3-hydroperoxides leads to formation of hexanal and cis-3-hexenal, where the latter may undergo isomerisation by isomerase enzyme to give trans-2-hexenal (Gatfield, 1 988). The volatile products from the 9-hydroperoxides are cis-3-nonenal trans-2-nonadienal and cis-6-nonadienal, respectively (Eriksson,
9-Hydroperoxy [C-1 8:2] cis-3-Nonenal (ADH) Lipid
�
polyuc Acyl Hydrolase)
1 3-hydroperoxy [C-1 8:2] (Lipoxygenase) 13-hydroperoxy [C-1 8:3] 9-hydroperoxy [C-1 8:3]
"'" '
Ccis-trans-isomerases/
double-bond reducing enzymes trans-2,cis-6-N onadienal
�" /
trans-2-Hexenal . 1 (ADH)I .
trans-2-hexen- l -olFig. 1.7
Possible biochemical pathways for the fonnation of C6-aldehydes and alcohols from fruit lipid (after Eriksson, 1979).Lipoxygenase activity has been found to increase just before the respiration climacteric and ethylene production during ripening of apples (Meigh et al., 1 967). The increased lipoxygenase activity is found in the peel of apples and this has been suggested to be responsible for the decrease of linoleic and linolenic fatty acids during ripening, both on and off the tree (Meigh and Hulme, 1965).
The double bond position of the C6 aldehyde and alcohol secondary volatiles has been found to have some impact on their olfactory characteristics. The fruity and sweet character is found with aldehydes and alcohols having a double bond at the C-2
position, a high green and fresh note is found with cis-3-hexenol, and a strong spicy and grassy green odour is associated with cis-3-hexenal. Compounds having a double bond at C-4, such as 4-hexenal give an impression of spicy and vegetable-like green smell, while those with a double bond at C-5, such as 5-hexenol and 5-hexenal, have strong oily-fatty, insect-like green and herbal odours (Hatanaka, 1 993).
Investigations on the biogenesis of apple volatiles have shown that some of the volatile compounds are 'primary' , which are considered to be direct metabolites produced by intracellular biogenetic pathways and these volatiles are emitted by the whole, intact fruit. These compounds comprise the odour from the fruit and the fIrst impression given by the smell. Their quality and quantity depend on genetic factors and are influenced by ripening and storage, which will be discussed later. On the other hand, some of the volatile compounds, such as n-hexanal and trans-2-hexenal, are 'secondary' products, which formed very quickly during disruption of cell structure due to enzymatic reactions in the present of oxygen. These compounds are produced