Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 64

biotechnological programmes and is of
keen public interest. Metabolic engineering
of volatile compounds using transgenic
techniques has been widely applied in
plants (Dudareva and Pichersky, 2008) and
depends on the discovery of genes
involved in the biosynthesis of volatiles.
Advances in gene cloning associated with
the formation of aroma volatiles during
fruit ripening have been reviewed recently
(Defi lippi et al. , 2009a), and more reviews
in terms of gene evaluation and functions
involved in the biosynthesis of volatile
compounds are also available (Chen et al. ,
2011; Strommer, 2011).
A decline in fruit fl avour quality has
occurred over the last two decades (Klee,
2010). Recently, a growing number of
consumers are willing to pay a premium
for better aroma quality, and the number of
scientifi c papers investigating the char-
acterization and regulation of aroma
volatiles has increased greatly. In addition,
with the development and application of
state-of-the-art techniques, certain meta-
bolic pathways involved in the bio-
synthesis of aroma volatiles have been
established recently. Even for defi ned
metabolic pathways, the regulatory mech-
anisms during fruit ripening are not fully
understood. The formation and regulation
of aroma volatiles is becoming an
important research topic. To further our
understanding of the mechanism of aroma
quality formation in ripening fruit, the
present chapter should serve as a guide to
review the latest progress in the
identifi cation of characteristic aroma vola-
tiles of ripening fruit, summarizing the
regulation of the biosynthesis of volatiles
that are clearly associated with ripening.

compounds within the complex mixtures.
Over the past few decades, a variety of
technologies have been developed to
capture and identify the aroma volatiles of
ripening fruit, including solid-phase micro-
extraction (SPME), gas chromatography
(GC), mass spectrometry (MS) and data
analysis.
Distillation, employed since the middle
ages, is the early separation process to
isolate volatile compounds from fruit. In
1925, over 2000 l of Valencia orange juice
was used as starting material to concentrate
volatiles to levels at which they could be
measured (Rouseff et al. , 2009). With the
application of GC and MS, the initial
required volume of grapefruit juice was
reduced to 100 l to identify sulfur volatiles.
The volume was further reduced to 10 ml
with improvements in instrumentation
such as headspace SPME, which is a rapid
and simple analytical technique. The
principle of SPME is the partitioning
process of the analyte between the fi bre
coating and the sample. The development
and application of SPME in aroma volatiles
have been reviewed (Augusto et al. , 2000;
Wardencki et al. , 2004; Qualley and
Dudareva, 2009).
GC is a common chemical analysis
technique for separating and analysing
compounds in a complex sample. A
modern GC instrument was invented in
1952 by James and Martin and was applied
to the separation of volatile compounds
from citrus juices in the 1950s and 1960s
(Rouseff et al. , 2009). However, the
identifi cation of volatiles was insuffi cient
until GC was coupled to MS in the 1960s.
Although more and more forms of MS have
been developed, the fragmentation pattern
based on electron-impact quadrupole MS
is probably the most-used MS form.
Currently, the available NIST Mass
Spectral library ( http://www.sisweb.com/
software/ms/nist.htm) is used to guide the
identifi cation of compounds, which has
greatly facilitated separating and identify-
ing fruit aroma volatiles.
SPME-GC-MS has been widely used to
profi le volatiles released from fruit. Con-
ventional statistical analysis is not

5.2 Identifi cation of Aroma Volatiles

5.2.1 Technologies used for analysis of
aroma volatiles

A key prerequisite to understanding the
function and biosynthesis of aroma vola-
tiles released from ripening fruit is the
separation and identifi cation of volatile