Potato Flavor - The Potato: Botany, Production and Uses - page 348

which are intimately interrelated. Recently, these
complex pathways were analysed by state-of-the-
art metabolomic approaches. Thus, Roessner-
Tunali et al . demonstrated an inverse correlation
between sucrose content and free amino acid
level in the tuber (Roessner-Tunali et al ., 2004).
Later, the same group published methods, based
on [U- 13 C] glucose labeling of potato tuber discs
and gas chromatography-mass spectrometry
(GC-MS), to investigate metabolic fluxes to dif-
ferent primary metabolites including amino
acids (Roessner-Tunali et al ., 2004). Despite
these advances, there are still important ques-
tions to be addressed concerning potato tuber
amino acid content. For example, it is still un-
clear what proportions of amino acids are syn-
thesised de novo in the tuber compared with
those imported from the phloem (Fischer et al .,
1998), but clearly amino acid transporters
could have a key role (Koch et al ., 2003). It is
well known that glutamate and aspartate and
their respective amides are found in the phloem
sap of many species; however, the relative con-
tribution to final tuber contents remains to be
elucidated (Koch et al ., 2003). Recent work
has also identified protein kinases involved in
the regulation of carbon and amino acid me-
tabolism (Halford et al ., 2004); however, the
role of these regulators in potato tubers remains
unknown.

resource for the identification of key metabolites,
and ultimately genes that underpin these traits.
The study of Morris et al . has addressed one as-
pect of this issue, by measuring the levels of
umami compounds in tuber samples that had
been subjected to detailed sensory analysis by a
trained panel (Morris et al ., 2007). The free
levels of the major umami amino acids, gluta-
mate and aspartate, and the 5¢- nucleotides, 5¢-
GMP, 5¢- AMP, were measured in potato samples
during the cooking process. Tubers were sam-
pled several times during the growing season.
The levels of both glutamate and 5¢- nucleotides
were significantly higher in mature tubers of
two S. tuberosum group Phureja clones com-
pared with two S. tuberosum group Tuberosum
cultivars. The equivalent umami concentration
was calculated for five clones/cultivars, and
there was a strong positive correlation with the
acceptability scores derived from a trained evalu-
ation panel, suggesting that umami was an im-
portant component of potato flavor ( Fig. 19.1 ).
There was a large effect of the developmen-
tal stage on the level of cooked 5¢- nucleotides.
Interestingly, as tuber development progressed,
the effect on 5¢- nucleotide levels was different
for the Phureja and Tuberosum cultivars. In
small developing tubers of both cultivars,
5¢- nucleotide levels were generally lower, and
levels in the Tuberosum cultivars were not sig-
nificantly different to those in the Phureja
clones (Stage H1 in Fig. 19.2 ) . Conversely, at
tuber maturity, whereas low levels of 5¢- GMP
could be detected in the Tuberosum tubers, sig-
nificantly higher levels were present in cooked
Phureja tubers (Stage H2 in Fig. 19.2 ). In add-
ition, levels of 5¢- GMP remained higher in the
Phureja tubers, compared with Tuberosum,
after 6 weeks storage at 4°C (Stage H3 in Fig. 19.2 ).
The implications of this finding, in view of the
demonstrated association of umami level and
taste, is that these compounds contribute to taste
differences of tubers at different developmental
stages (Fig. 19.2 ).
Previously, it has been suggested that
5¢- nucleotides accumulate due to the action of
nucleases during cooking processes, particu-
larly due to RNA degradation (Buri and Solms,
1971). Ribonucleases are active under the pH
and temperature conditions that occur during
heating, being particularly active at around
50°C (Singh et al ., 1991). As the temperature

Levels of umami compounds
in different potato genotypes

In order to extend studies of potato flavor, a use-
ful approach has been to compare potential fla-
vor metabolite levels in potato germplasm with
very different sensory properties. A valuable re-
source in this regard is Solanum tuberosum group
Phureja, differentiated from S. tuberosum group
Tuberosum on the basis of a number of import-
ant tuber quality traits such as flavor, texture,
color, and reduced tuber dormancy (De Maine
et al ., 1993, 1998; Dobson et al ., 2004; Morris
et al ., 2004; Ghislain et al ., 2006). Despite being
able to differentiate the Phureja group of land-
races based on geographical origin, this group is
very similar genetically to the Tuberosum group
(Spooner et al ., 2007). Whatever the final taxo-
nomic outcome, because many of the Phureja
group can be clearly differentiated based on tuber
quality trait parameters, they form a useful