Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 74

of aroma-characteristic lactones was the
lowest at CI-inducing temperatures, whilst
low-temperature conditioning alleviated
the development of CI and maintained
higher levels of lactones. Furthermore,
changes in the lactones of peach fruit
during cold storage are suggested to be a
consequence of the altered expression of
PpACX1 and long-chain enzyme activity
(Xi et al. , 2012). The addition of exogenous
linoleic acid to chilled tomato fruit resulted
in an increase in the corresponding volatile
hexanal (Boukobza and Taylor, 2002). In a
recent study in tomato fruit, increased
linoleic and linolenic acid content caused
by the overexpression of fatty acid de-
saturase (FAD) resulted in a signifi cant
accumulation of C6 compounds (Dom-
ínguez et al. , 2010).

short-term storage, the inhibition of LOX
activity in CA-stored 'Mondial Gala' apple
fruit resulted in decreased emission of
esters despite substantial ester-forming
capacity that allowed for the recovery of
ester production during shelf-life, whilst in
the case of long-term storage (6 months),
strong inhibition of AAT activity in
combination with LOX caused un-
recoverable diminution of the generation of
esters (Lara et al. , 2007). A recent study
suggested that the deduced respiration rate
might infl uence the modulation of the
different energy-carrying compounds and
the overall activity of the enzymes such as
LOX, ADH, PDC and AAT (Ortiz et al. ,
2010), resulting in lower fruit aroma
quality and consumer acceptability after
CA storage.

5.4.4 Atmosphere 5.4.5 Metabolic engineering

An enormous volume of research has been
reported on the controlled atmosphere
(CA) storage of fruit to retain quality for a
longer period. The results from 'Rich Lady'
peach fruit showed that CA storage (3%
O 2 + 10% CO 2 at 2°C) was characterized by
higher production of characteristic
G -decalactone and J -dodecalactone com-
pared with air-stored fruit, resulting in
improved fl avour perception and thus
consumer acceptability after CA storage
(Ortiz et al. , 2009). However, emission of
straight-chain esters in 'Tardibelle' peach
fruit was signifi cantly reduced after 7 days
of shelf-life after CA storage (Ortiz et al. ,
2010). The blockage of ester-formation
capacity was more severe and even
unrecoverable in apple (Lara et al. , 2007)
and pear fruit (Lara et al. , 2003) after
extended storage under CA, leading to
decreased overall aroma quality and fruit
acceptability.
The observed differences in AAT
activity alone were not enough to explain
the deduced straight-chain esters caused by
CA storage, indicating that an altered
supply of alcohol and acyl-CoA precursors
has an important role in the modulation of
volatile esters (Lara et al. , 2003, 2006). For

Notable success has been reported in
enhancing and improving fruit aroma
quality using transgenic techniques. The
fi rst attempt to add a new volatile com-
pound to fruit was performed in tomato by
introducing a Clarkia breweri linalool
synthase gene under the control of the
fruit-specifi c E8 promoter, leading to sig-
nifi cant accumulation of ( S )-linalool and
8-hydroxylinalool (Lewinsohn et al. , 2001).
The fi rst successful enrichment of aroma
and fl avour in fruit through metabolic
engineering was achieved by expressing
lemon basil ( Ocimum basilicum ) geraniol
synthase in tomato (see Plate 5). The
genetically engineered tomato fruit had an
increased content of monoterpenes such as
nerol, citronellol, citronellal, citronellic
acid, citronellyl acetate and rose oxide
(Davidovich-Rikanati et al. , 2007), giving
the fruit lemon and rose aroma properties.
A panel of 82 people tested the genetically
modifi ed tomato fruit, which was preferred
by 49 members of the panel, whilst four
expressed no preference. The effects of
modifi ed expression of fruit TPS, CCD,
ADH, LOX and FAD on volatile profi les
have been reviewed by Dudareva and
Pichersky (2008).