Potato Flavor - The Potato: Botany, Production and Uses - page 354

19.5 Flavor and Tuber Storage

pathways that lead to flavor compound biosyn-
thesis is required. As reviewed in this chapter,
knowledge of the key potato tuber flavor con-
stituents is still in its infancy; however, some
progress has been made, and candidate flavor
compounds are emerging. In other fruit and
vegetables, there is more detailed knowledge of
flavor metabolites, and it is of relevance to con-
sider approaches to isolating the biosynthetic
genes in these species. In tomato fruit, for ex-
ample, several approaches have been taken to
identify flavor genes. Using a genetics approach,
it has been shown that the number of genes that
impact on tomato flavor is likely to be quite large.
In tomato, c .100 quantitative trait loci (QTLs) af-
fecting volatiles and their precursors have been
identified (Causse et al ., 2001; Saliba-Colombani
et al ., 2001; Schauer et al ., 2006; Tieman et al .,
2006; Mathieu et al ., 2008). Similar approaches
in potato have been undertaken, and the results
should be published in the near future (G.J. Bryan,
UK, 2011, personal communication). The rela-
tively large number of QTLs that affect tomato
flavor emphasizes the complex nature of flavor
development, and indicates that multiple path-
ways and control mechanisms conspire to develop
flavor attributes.
With the knowledge of the key metabolites
and the genes involved in their biosynthesis and
turnover, the opportunity arises to use trans-
genic approaches to manipulate the expression
level of key genes and determine the effects on
the level of the target metabolite, and ultimately
organoleptic quality. For example, in tomato fruit,
transgenic expression of a geraniol synthase
gene from basil altered the profile of terpenoid
volatiles significantly, and the transgenic fruit
could be distinguished by taste panellists, 60%
of whom preferred them to the non-transgenic
control.
In potato, several studies have led to the
production of transgenic lines that contain al-
tered levels of potential flavor compounds. For
example, overexpression of an Arabidopsis cys-
tathionine g-synthase gene in potato tubers led
to a sixfold increase in tuber methionine content
(Di et al ., 2003). The increased methionine con-
tent resulted in an increase in methional content
(up to 4.4-fold) in baked tubers; however, sen-
sory evaluation of these transgenic lines has not
been reported. Other transgenic experiments
have demonstrated that tuber fatty acid levels

A key feature of the potato crop is its storability,
enabling the product to be supplied to both the
fresh and processing industries for several months
postharvest. Surprisingly, few studies have ad-
dressed changes in flavor quality during storage,
presumably because it has been difficult to deter-
mine the key metabolites that contribute to this
trait. It is known that there are significant changes
in amino acid content including the major umami
amino acids, glutamate and aspartate (Brierley
et al ., 1997). The significant effects of storage time
on the volatile flavor components of baked pota-
toes have also been investigated (Duckham et al .,
2002). As for amino acids, other flavor precursors,
including fatty acids and sugars, change in levels
during storage. Although potato lipids account for
only 0.8-1.3 mg g - 1 dry weight (Galliard, 1973),
70- 75% of lipids are the relatively reactive poly-
unsaturated linoleic and linolenic acids, pre-
cursors of a wide range of volatile compounds
(Galliard, 1973). Some studies report an increase
in total fatty acid levels on storage (Cotrufo and
Lunsetter, 1964; Cherif and Ben Abdelkader,
1970), whereas others report genotypic vari-
ations in fatty acid levels, depending on the dur-
ation of storage (Dobson et al ., 2004). Changes in
tuber reducing sugar during storage are well char-
acterized (for example, Finglas and Faulks, 1984;
Brown et al ., 1990) and generally increase during
storage, although the extent of the increase is cul-
tivar dependent (Blenkinsop et al ., 2002).
Quantitative descriptive analysis of tuber
samples from Tuberosum and Phureja ger-
mplasm stored for 3 months revealed significant
changes in flavor. Principal component analysis
enabled the identification of the main metabolites
driving these changes. Among the volatiles that
were found at enhanced levels following storage
were several aldehydes; propanol, 5- methylhex-
anal, and 2- hexenal, all known to be derived from
fatty acids via enzymatic and non-enzymatic pro-
cesses (Dobson et al ., 2004).

19.6 Genes Involved in Potato Flavor

Compound Biosynthesis

In order to breed potatoes with enhanced flavor,
knowledge of the genes involved in the metabolic