Potato Flavor - The Potato: Botany, Production and Uses - page 347

umami-tasting glutamate glycoconjugates, pro-
duced by a Maillard reaction (Beksan et al ., 2003),
the potential indirect effects of sugars on potato
flavor should not be overlooked

5¢- AMP = 0.18), and 1218 is a synergistic con-
stant. All concentrations must be in g 100 g - 1 .
Glutamate is the most potent umami amino
acid, with aspartate showing only 7% of the taste
activity of glutamate. 5¢- GMP is the most potent
common 5¢- nucleotide, having a 2.3-fold greater
taste activity than 5¢- IMP (Yamaguchi et al .,
1971). Common chemical structural motifs ap-
pear to be necessary for a taste-enhancing effect;
in particular, two negative charges at a distance
of 3-9, or preferably 4-6, carbon atoms are es-
sential (Ney, 1971). In addition to glutamate and
aspartate, the C4-dicarboxylic acids, succinic
acid and tartaric acid, fulfill this criterion and
also exhibit some umami taste, probably as a re-
sult of the negative charge distance (Ney, 1971;
Velisek et al ., 1978). More controversial is the
question of whether small peptides can also give
rise to an umami taste. However, several gluta-
mate glycoconjugates formed during processing
by the Maillard reaction have umami-like proper-
ties, and these compounds may contribute to
overall flavor (Beksan et al ., 2003).

5 ¢ - nucleotides in potato tubers

Early work on umami in potato addressed the
question of the origin of the 5¢- nucleotides (Buri
and Solms, 1971). Barely detectable levels of
5¢- nucleotides could be measured in raw tuber
samples, whereas appreciable levels (for example
64.4 µmoles kg - 1 fresh weight 5¢- guanosine
monophosphate) accumulate in steamed sam-
ples. It was concluded that the origin of the
5¢- nucleotides was RNA that had been degraded
enzymatically by the action of RNases or
phophodiesterases. More detailed analysis of
5¢- nucleotide formation during cooking of potato
tubers followed (Dumelin and Solms, 1976).
There were no clear-cut conclusions from this
work, apart from the recognition that potato
tubers contained a complex enzyme system that
was able to attack ribonucleic acid and to form
different intermediates and end products of deg-
radation. The interactions of phosphodiesterase I,
phosphodiesterase II, and ribonucleases are likely
to be important, but details of the interactions
could not be elucidated from this study. However,
temperature and pH conditions are of great im-
portance to these reactions.

Umami in potato

Umami was first considered as a component of
potato flavor in a paper published in 1970 (Buri
et al ., 1970). This report detailed levels of gluta-
mate and 5¢- nucleotides in tuber samples from
two potato cultivars, Bintje and Ostara. Using a
taste panel, it was reported that the higher levels
of glutamate and 5¢- nucleotides in the Bintje
sample corresponded to an “enhanced” taste.
Following on from this, it was suggested that the
flavor of boiled potato was due largely to the
natural mixture of glutamic acid and other
amino acids in combination with guanosine- 5¢-
monophosphate (GMP) and other 5¢- ribonucleo-
tides produced on cooking (Solms, 1971). Thus,
chemicals that impart an umami flavor are likely
to be an important component of potato flavor.
Indeed, some authors claim that there is only a
small contribution from volatile (olfactory) com-
ponents, and that chemicals representing the
so-called sweet, sour, salty, and bitter tastes do
not make a major contribution to potato flavor
(Solms, 1971; Solms and Wyler, 1979). Thus,
these authors suggest that the presence of salt,
sugars, or glycoalkaloids do not enhance potato
flavor, although their presence at high levels
may decrease palatability. With the discovery of

Glutamate in potato tubers

As well as 5¢- nucleotides, the levels of amino
acids, most importantly glutamate but also to a
lesser degree aspartate, are contributors to the
umami taste. The amino acid biosynthetic net-
works are complex and heavily regulated, al-
though most of the biosynthetic genes have
been identified (Halford et al ., 2004). The levels
of amino acids in potato tubers have been the
subject of several studies—for example, Brierley
et al . surveyed tuber amino acid content in sev-
eral cultivars at harvest and during storage
(Brierley et al ., 1997). Asparagine and glutam-
ine were the major amino acids, although the
levels of glutamic and aspartic acids were signifi-
cant. During storage, the level of glutamate in-
creased (at 10°C the increase was c .fourfold).
Amino acid metabolism is a central part
of primary metabolism, the various pathways of