Umami Compounds Are a Determinant of the Flavor
of Potato (Solanum tuberosum L.)
WAYNE L. MORRIS, HEATHER A. ROSS, LAURENCE J. M. DUCREUX,
JOHN E. BRADSHAW, GLENN J. BRYAN, AND MARK A. TAYLOR*
Quality, Health and Nutrition, Scottish Crop Research Institute, Invergowrie,
Dundee DD2 5DA, United Kingdom
Vegetable flavor is an important factor in consumer choice but a trait that is difficult to assess
quantitatively. The purpose of this study was to assess the levels of the major umami compounds in
boiled potato tubers, in cultivars previously assessed for sensory quality. The free levels of the major
umami amino acids, glutamate and aspartate, and the 5′-nucleotides, GMP and AMP, were measured
in potato samples during the cooking process. Tubers were sampled at several time points during
the growing season. The levels of both glutamate and 5′-nucleotides were significantly higher in mature
tubers of two Solanum phureja cultivars compared with two Solanum tuberosum cultivars. The
equivalent umami concentration was calculated for five cultivars, and there were strong positive
correlations with flavor attributes and acceptability scores from a trained evaluation panel, suggesting
that umami is an important component of potato flavor.
KEYWORDS: Aspartate; glutamate; flavor; 5′-nucleotide; potato; Solanum tuberosum; Solanum phureja;
Increasingly, potato tuber quality traits are assuming a greater
importance in breeding programs, as consumers demand greater
variety and retailers wish to market cultivars that have distinctive
commercial advantages. However, as with many food crops,
potato flavor is difficult to assess in breeding programs.
Assessments are highly subjective and require trained sensory
panels. These have a low sample throughput and are conse-
quently expensive. As a result, flavor is generally assessed only
in the later stages of a breeding program after selection for more
easily quantifiable traits. In fact, most of the potential flavor
improvements are likely to be discarded, and to a large extent
the marketplace determines whether a new cultivar is acceptable
to consumers (1).
The volatiles produced by raw and cooked potatoes have been
studied extensively (2–4), and over 250 compounds have been
identified in potato volatile fractions. Attempts have been made
to discriminate which of these components are important for
potato flavor, which are specific to the method of cooking,
cultivar differences, the effects of agronomic conditions, and
the effects of storage (5–9). Overall, there is no clear-cut
identification of which volatiles (if any) are the key contributors
to cooked potato flavor and taste.
In addition to the volatile compounds produced on cooking
of potato tubers, soluble cellular constituents are likely to be
important in flavor also (10). Interactions between tastants and
aroma compounds probably give rise to the overall sensory
quality of the cooked tuber. The soluble, matrix-associated
compounds define the basic taste parameters, sweet, sour, salty,
or bitter and umami (a Japanese word meaning delicious).
Compounds including monosodium glutamate (MSG), several
process-derived glutamate glycoconjugates, adenosine 5′-mono-
phosphate (5′-AMP), inosine 5′-monophosphate (5′-IMP), and
guanosine 5′-monophosphate (5′-GMP), are well-known to show
umami-like sensory characteristics (11, 12). Umami compounds
generally enhance flavor and mouthfeel, giving the impression
of creaminess and viscosity to savory dishes (10). The umami
taste intensity increases exponentially when glutamate interacts
with 5′-ribonucleotides (13). The synergistic effect between
certain free amino acids and 5′-nucleotides can be measured
using the equivalent umami calculation [see Materials and
Methods (13)]. Glutamate is the most potent umami amino acid,
with aspartate showing only 7% of the taste activity of
glutamate. The taste activity of 5′-GMP is the most potent
common 5′-nucleotide, having a 2.3-fold greater effect than 5′-
IMP (13). The intensity of the umami taste may be enhanced
by a range of salts including sodium, potassium, and magnesium
(14) as well as certain organic acids such as succinate (15).
As early as 1971 it was suggested that boiled potato taste
was largely due to the natural mixture of glutamic acid and
other amino acids in combination with the 5′-GMP and other
5′-nucleotides produced on cooking (16). Indeed, some authors
claim that there is only a small contribution from volatile
(olfactory) components and that chemicals representing the so-
called sweet, sour, salty, and bitter tastes do not provide a
* Corresponding author (e-mail email@example.com; telephone
+44 1382 562731; fax +44 1382 562426).
J. Agric. Food Chem. 2007, 55, 9627–9633
10.1021/jf0717900 CCC: $37.00
2007 American Chemical Society
Published on Web 10/19/2007
cooked potato taste (16–18). Thus, the presence of salt, sugars,
or alkaloids does not enhance potato flavor, although their
presence at high levels may decrease palatability.
Limited data on the levels of several umami compounds in
potato tubers have been published (17). Although raw potatoes
contain only very small amounts of 5′-nucleotides, cooked
potatoes contain appreciable levels, higher than most other plant-
derived foods examined. Several studies have addressed the
levels of free amino acids (including glutamic acid) in potato
tubers and also examined changes in amino acid levels during
storage (19, 20). It is clear that the soluble protein and amino
acid contents of potato tubers are modified substantially during
storage, with larger effects observed after 3 months of storage
at 10 °C than at 4 °C (20). At harvest, the major free amino
acids are asparagine, glutamine, glutamic acid, arginine, and
aspartic acid, with some cultivar-dependent variations in the
contribution of these amino acids (20, 21). Additionally, the
soil nitrogen fertilization regimen affects amino acid content,
with a high level of nitrogen fertilization associated with
increased glutamine content (22). Potassium fertilization can
enhance tuber potassium content (23), and as potassium may
enhance umami taste intensity (14), this could also be a factor
in potato flavor.
Only sparse taste panel data are available to support the
importance of umami compounds in defining potato flavor. The
effects of supplementing boiled potato with glutamic acid and
glutamic acid plus nucleotides both resulted in a “stronger”
potato taste, as assessed by a trained panel consisting of 18
judges. Additionally, an aqueous mixture of amino acids and
nucleotides that reproduces the levels found in boiled potatoes
“had practically no odor, but an agreeable basic potato-like taste”
(16). Furthermore, a mixture with components replicating the
amino acid and nucleotide concentrations of a preferred boiled
potato was judged to taste better than that replicating a less
preferred boiled potato (24). More recently, it was shown that
supplementation of mashed potato with different amounts of
monosodium glutamate and disodium inosate and guanylate can
enhance the sensory evaluation scores for mashed potato,
although no information on endogenous levels was included in
this analysis (25). In general, correlation of sensory evaluation
scores for cooked potato flavor from different cultivars with
umami compound measurements is lacking. Recent work has
demonstrated that boiled tubers from Solanum phureja score
better in sensory evaluations than those from Solanum tuberosum
(26). The S. phureja tubers have a distinctive (preferred)
mouthfeel and a higher intensity of flavor attributes than S.
tuberosum. The aim of the current paper is to compare the levels
of the amino acids and ribonucleotides with umami taste in S.
phureja and S. tuberosum tubers at a range of tuber develop-
mental stages and during several processing methodologies. By
comparing the levels of the these umami compounds with
sensory evaluation data, the aim was to determine whether or
not these compounds are likely to be important components of
MATERIALS AND METHODS
Plant Material. Potato-breeding clones and cultivars examined in
this study were grown in field trials during 2006 at Gourdie Farm
(Dundee, U.K., 56°, 28′, 27′′ north; 3°, 4′, 11′′ west) using normal
agronomic practices. Tubers were planted on April 26, 2006, and were
analyzed at three harvest periods: H1, harvested early in development
(harvested on July 12, 2006) when average tuber weight was 20–40 g;
H2, harvested at maturity when average tuber weight was 150–200 g
(harvested on October 9, 2006, following acid burndown of foliage);
H3, mature tubers that had been stored at 4 °C for 6 weeks.
Approximately 1 kg of average-sized mature tubers was selected and
pooled, and each tuber was manually cut into eighths. These pools were
subdivided into nine with each constituting a single representative
sample. Triplicate samples were used for three cooking time points
(either steaming or boiling as indicated in the text). All raw and cooked
tuber samples were immediately frozen in liquid nitrogen and freeze-
dried. The freeze-dried samples were ground in a laboratory mill fitted
with a 0.5 mm sieve and stored at -20 °C prior to analysis. The clones
used in this study were S. phureja 333-16, Mayan Gold, Inca Sun, and
DB257-28 and S. tuberosum cultivars Montrose, Pentland Dell, Maris
Piper, and Record.
Analysis of 5′-Nucleotides. Triplicate freeze-dried potato samples
(approximately 100 mg) were extracted into 5 mL of 5% perchloric
acid (VWR Ltd., Lutterworth, U.K.) by vortexing for 5 s, followed by
end over end mixing on a blood rotator for 60 min at 4 °C. Following
centrifugation at 4 °C at 3000g for 5 min, 2.5 mL of supernatant was
transferred to a fresh 15 mL centrifuge tube containing 50 µL of BDH
4080 indicator dye (VWR Ltd.). The pH range of the sample was
adjusted to approximately 6.5 by the dropwise addition of a solution
of 5 mol/L K2CO3(VWR Ltd.). After removal of potentially explosive
insoluble KClO3 salts, by centrifugation at 3000g for 10 min, the
supernatant was applied to a strong anion exchange–solid phase
extraction (SAX SPE) column (100 mg, acetate counter ion, Alltech,
Carnforth, U.K.). The column was washed with 4 mL of deionized
H2O prior to elution of 5′-nucleotides with 4 mL of 2 mol/L formic
acid (VWR Ltd.). Extracts were lyophilized and resuspended in 200
µL of deionized water prior to analysis by high-performance anion-
exchange chromatography (HPAEC), using a CarboPac PA-1 column
[Dionex (UK) Ltd., Camberley, U.K.], as detailed in ref 27. Nucleoside
monophosphate reference standards 5′-AMP and 5′-GMP (Sigma-
Aldrich, Gillingham, U.K.) were used for peak identification and
quantification using standard curves. 5′-Nucleotide recovery percentages
were consistently 55–65% as determined by comparing spiked (with
standards) and unspiked samples that had undergone the extraction
procedure. The results presented are not corrected for recovery
Analysis of Amino Acids. Triplicate freeze-dried potato samples
(approximately 100 mg) were extracted into 5 mL of buffer consisting
of methanol/water/acetic acid [49:49:2 v/v/v (28)]. Samples were
vortexed for 5 s followed by end over end mixing on a blood rotator
for 60 min at 4 °C. The extracts were centrifuged at 3000g for 5 min,
and the supernatant was passed through a 0.45 µm filter (VWR Ltd.)
prior to analysis. Amino acids were derivatized with o-phthaldialdehyde
prior to separation by high-performance liquid chromatography (HPLC)
(29). Separation was performed using a Zorbax Eclipse AAA column
(4.6 × 150 mm, 5 µm) on an Agilent 1100 HPLC equipped with a
G1313A autosampler, a G1312A binary pump, and a G1315A
fluorescence detector (Agilent Technologies, Wokingham, U.K.). A
binary solvent gradient of 0–14% B (0–6 min), 14% B (6–11 min),
14–50% B (11–16 min), 50% B (16–20 min), 50–100% B (20–30 min),
100% B (30–32 min), and 100–0% B (32–36 min) at a flow rate of 0.8
mL min-1was used [solvent A, 83 mmol/L sodium acetate/methanol
(4:1) with tetrahydrofuran added at 1% v/v; solvent B, 83 mmol/L
sodium acetate/methanol (1:4)], and the column temperature was kept
at 20 °C. Fluorescence detection was set at excitation ) 360 nm,
emission ) 455 nm, and PMT gain ) 10. Amino acids were quantified
by comparison with the AA-S-18 (Sigma-Aldrich, Gillingham, U.K.)
reference amino acid mixture supplemented with asparagine, glutamine,
tryptophan, and γ-aminobutyric acid. Amino acid recovery percentages
were consistently 80–90% as determined by comparing spiked (with
standards) and unspiked samples that had undergone the extraction
procedure. Data shown are not corrected for recovery percentage.
Equivalent Umami Calculation. The equivalent umami concentra-
tion (EUC, grams of MSG per 100 g) is the concentration of MSG
equivalent to the umami intensity given by a mixture of MSG and the
5′-nucleotide and is represented by the addition equation (13)
where Y is the EUC of the mixture, ai is the concentration of each
umami amino acid (Glu or Asp), ajis the concentration of each umami
J. Agric. Food Chem., Vol. 55, No. 23, 2007 Morris et al.
5′-nucleotide (5′-GMP or 5′-AMP), biis the relative umami concentra-
tion (RUC) for each umami amino acid to MSG (Glu ) 1, Asp )
0.077), bj is the RUC for each umami 5′-nucleotide to 5′-IMP (5′-
GMP ) 2.3, 5′-AMP ) 0.18), and 1218 is a synergistic constant. All
concentrations must be in grams per 100 g.
Sensory Evaluation. Potatoes were stored in a cold store at 2 °C
from harvest until being assessed for sensory characters. Potatoes were
peeled and cut into cubes of approximately 30 g weight. Samples (500
g) were boiled with 1000 mL of boiling distilled water and 1% cooking
salt for a predetermined length of time predicted by initial penetrometer
tests (between 6 and 10 min). Samples were drained of excess water,
squeezed through a potato ricer, then transferred to prewarmed bowls,
covered with foil, and kept warm in an oven at 70 °C prior to serving
to a trained sensory panel for assessment. The panel members were
trained following standard guidelines (30) ensuring the use of a
standardized vocabulary, and all panel members demonstrated good
sensory acuity. The sensory assessors rated the potatoes according to
a series of sensory attributes, including a “catch all” trait, acceptability,
which measures an assessor’s subjective opinion of the degree to which
he or she likes or dislikes the flavor of the cooked potatoes. The flavor
attributes intensity, creaminess, and sweetness were also assessed. All
samples were coded and presented in a defined order to allow
assessment of sample, assessor, order of tasting, carry-over, and session
effects. All attributes were scored on a scale from 0 (poor rating for
the character) to 100 (good rating for the character). Five clones per
session were tested, and each clone was evaluated in three separate
sessions. For each session there were between 8 and 12 assessors. Data
were collected using a computer-assisted interface. All assessments were
carried out in triplicate in isolated, purpose-built booths with controlled
airflow and lighting. Assessors were invited to rinse their palates
between samples. The experimental results were collated and analyzed
using a proprietary package (KwikSense, Hannah InterActions Ltd.,
Ayr, U.K.) as well as Genstat version 9 (Lawes Agricultural Trust).
The mean values for each attribute (three replicates) were computed.
Data for five genotypes for the flavor attributes, flavor intensity, flavor
creaminess, flavor sweetness, and the overall acceptability attribute are
In-Gel RNase Assay. Substrate-based sodium dodecyl sulfate–poly-
acrylamide gel electrophoresis of potato protein extracts was performed
as described in ref 31. Lyophilized tuber powder (0.2 g) was extracted
in 2 mL of citrate protein extraction buffer [150 mmol/L citric
acid–Na2HPO4, pH 3, 0.1 mmol/L phenylmethanesulfonyl fluoride
(PMSF)]. In addition, proteins were extracted using an acetate buffer
[0.1 mol/L sodium acetate, pH 5.2; (32)] with the addition of PMSF at
a final concentration of 0.1 mmol/L. All protein extractions were
performed at 4 °C. Samples were clarified by centrifugation (10 000g
for 10 min), and the protein was quantified using the DC Protein Assay
(Bio-Rad, Hertfordshire, U.K.). Proteins (40 µg) were separated by
electrophoresis prior to visualization of RNase activity as described
RNase Activity Assay. RNase enzyme activity assay was performed
spectrophotometrically as described previously (32) with the addition
of PMSF at a final concentration of 0.1 mmol/L. Absorbances at 260
and 280 nm were recorded, and one enzyme unit is defined as a change
of 1 OD at 260 nm per milligram of protein per hour.
Phosphohydrolytic Enzyme Activity Assays. Freeze-dried powder
(0.5 g) was extracted into 5 mL of sodium citrate extraction buffer
[0.05 mol/L, pH 6.0 containing 2 mmol/L cysteine and 0.1 mmol/L
PMSF; (33)] by grinding in a mortar and pestle for 10 s. Samples were
clarified by centrifugation (10000g for 5 min); 2.5 mL of extract was
desalted using a Sephadex G25 gel filtration column (NAP10 column,
GE Healthcare UK Ltd., Buckinghamshire, U.K.) pre-equilibrated with
sodium citrate extraction buffer. Specific phosphohydrolytic enzyme
activities were determined (34) using a variety of synthetic substrates:
p-nitrophenyl phosphate for the phosphomonoesterase activities, bis-
p-nitrophenyl phosphate as substrate for nonspecific phosphodiesterases,
and thymidine 5′-monophospho-p-nitrophenyl ester substrate for phos-
phodiesterases that can specifically hydrolyze the 5′-phosphodiester
bonds. Each substrate (Sigma-Aldrich, Gillingham, U.K.) was used for
assays performed under alkaline (33 mmol/L Tris-HCl buffer, pH 8.7
(35)) and acidic (33 mmol/L ammonium acetate buffer, pH 5.7)
conditions. All assays were performed at 50 °C; the liberated p-
nitrophenol was monitored at 405 nm, and all analyses were performed
Statistical Analysis. Student’s t test method was used (paired, two-
tailed distribution) to test the statistical relationship between cultivars
for amino acid and 5′-nucleotide levels and nuclease activities.
Formation of 5′-Nucleotides during Cooking. To investigate
the formation of 5′-nucleotides during cooking, tubers (cultivars
S. phureja Mayan Gold and S. tuberosum Pentland Dell) were
analyzed at time points during the boiling or steaming process.
Nucleotides were analyzed using HPLC and quantified by
comparison to authentic reference standards. In accordance with
published literature (16) 5′-nucleotide levels were very low in
raw tubers. However, 5′-GMP and 5′-AMP accumulated in
tubers during cooking (Figure 1). 5′-GMP and 5′-AMP levels
rose sharply during the first 5 min of cooking time before
reaching a plateau. The levels of 5′-nucleotides were slightly
higher (although not significant using Student’s t test) in steamed
tubers compared with boiled tubers, possibly indicating that
steaming is the better cooking method. Interestingly, levels of
the most potent umami 5′-nucleotide, 5′-GMP, were ca. 2–3-
fold higher in the Mayan Gold compared with Pentland Dell
tubers for both cooking methods at the plateau level (Figure
1A,B). Additionally, the same trend was observed for 5′-AMP,
where the levels were ca. 2- and 4-fold higher in Mayan Gold
than in Pentland Dell for boiling and steaming, respectively
Once it was established that 5–10 min of cooking time was
optimal for the release of 5′-nucleotides, levels were compared
in S. tuberosum (cultivars Montrose and Pentland Dell) and S.
phureja tubers (cultivars Inca Sun and Mayan Gold) throughout
development and storage. Levels of 5′-nucleotides were deter-
mined in raw and steamed samples at the three different harvests,
defined under Materials and Methods (Figure 2). The formation
of 5′-nucleotides upon cooking is again evident for all three
harvest time points. At harvest stage 1 the levels of 5′-GMP
and 5′-AMP are not significantly different (at P ) 0.05 level)
between the S. phureja and S. tuberosum cultivars (Figure 2).
However, at harvest stage 2 the 5′-GMP levels were significantly
higher (P e 0.015) in S. phureja compared with S. tuberosum
(e.g., Inca Sun 2.6-fold higher than Montrose). The levels of
5′-GMP were maintained in the S. phureja Inca Sun tubers at
harvest stage 3, whereas the levels in S. tuberosum cultivars
and Mayan Gold declined slightly. A similar pattern was
observed for levels of 5′-AMP during development and storage
(Figure 2B). For example, Inca Sun 5′-AMP levels were 2.0-
and 2.6-fold higher than those in Montrose at harvest stages 2
and 3, respectively.
Tuber Free Amino Acid Content. Amino acids were
analyzed using HPLC and quantified by comparison to authentic
reference standards. The levels of amino acids determined were
in the same range as previously published values (20, 21).
Tubers were analyzed at the same three harvest points as used
for the 5′-nucleotides. In both S. phureja and S. tuberosum
cultivars the amides asparagine and glutamine predominated and
comprised between 20 and 50% (w/w) and between 15 and 45%
(w/w), respectively, of the total free amino acid pool (see
Supporting Information Table 1). The profiles of individual
amino acids showed variation between cultivars, and the total
amino acid levels were generally around 2-fold lower in tubers
harvested early in development (H1) compared with harvest
stages 2 and 3. Of particular interest were the levels of the
umami amino acids glutamate and aspartate (Figure 3). The
Umami Compounds in PotatoJ. Agric. Food Chem., Vol. 55, No. 23, 2007
most striking difference observed was the higher levels of
glutamate in the S. phureja cultivars compared with S. tubero-
sum cultivars. The levels of glutamate at tuber maturity were
significantly (P e 0.08) higher in Mayan Gold and Inca Sun
compared with Montrose and Pentland Dell.
Equivalent Umami Concentration. The intensity of umami
flavor depends on the synergistic interaction between 5′-
nucleotides and certain flavor amino acids. Equivalent umami
concentrations (EUC) were calculated as described above using
the levels of aspartate, glutamate, 5′-GMP, and 5′- AMP and
the equation given in ref 13 (Figure 4). The EUC values
calculated for S. phureja cultivars Mayan Gold and Inca Sun
were not significantly different (at P ) 0.1 level) from those of
S. tuberosum cultivars Montrose and Pentland Dell at the earliest
harvest stage (H1). However, at later harvest stages the EUC
values were significantly higher in S. phureja compared with
S. tuberosum with differences ranging from between 2.6-fold
for Mayan Gold versus Pentland Dell harvest stage 3 (P )
0.046) and 6.5-fold for Inca Sun versus Montrose harvest stage
3 (P ) 0.0062).
5′-Nucleotide-Liberating Enzyme Assays. Previously it has
been suggested that the presence of 5′-nucleotides in potato
tubers may be due to the enzymatic breakdown of RNA during
cooking (17). To investigate this hypothesis, nuclease enzyme
activities (RNase and phosphomono- and diesterases) were
compared in S. tuberosum (Maris Piper and Pentland Dell) and
S. phureja (Mayan Gold and 257-28) mature tubers. Substrate-
based in-gel RNase assays showed no differences in RNase
Figure 1. Comparison of the kinetics of 5′-nucleotide formation during
cooking for potato cultivars Pentland Dell (PD) and Mayan Gold (MG):
(A) GMPboiled; (B) GMPsteamed; (C) AMPboiled; (D) AMPsteamed.
n ) 3.
Figure 2. Effect of tuber developmental stageon5′-nucleotidelevels in
rawandcooked(Ckd) potatocultivars MayanGold(MG), IncaSun(IS),
Pentland Dell (PD), and Montrose (MON). H1, harvest 1; H2, harvest
2; H3, harvest 3 (see text for details). Error bars represent the SEM,
n ) 3.
Figure3. Effectoftuberdevelopmental stageonflavoraminoacidlevels
in cooked potato cultivars Mayan Gold (MG), Inca Sun (IS), Pentland
Dell (PD), and Montrose (MON). Glu, glutamic acid; Asp, aspartic acid;
H1, harvest 1; H2, harvest 2; H3, harvest 3 (see text for details). Error
bars represent the SEM, n ) 3.
J. Agric. Food Chem., Vol. 55, No. 23, 2007 Morris et al.
banding patterns between the S. tuberosum and S. phureja
cultivars (Supporting Information Figure 1). Additionally, no
significant differences were observed between the S. tuberosum
and S. phureja cultivars for total RNase activity assays (Sup-
porting Information Table 2). As the differences in the levels
of 5′-nucleotides could not be attributable to variation in RNase
activity, the activities of phosphohydrolytic enzymes were
investigated. Both phosphomonoesterase and phosphodiesterases
(specific and nonspecific) were examined in S. tuberosum and
S. phureja cultivars (Supporting Information Table 2). No
significant differences were observed in phosphohydrolytic
Sensory Evaluation and Umami Level Correlation. Sen-
sory evaluations of boiled mature tubers from both S. tuberosum
(cultivars Maris Piper and Record) and S. phureja (clones
DB333-16, DB257-28 and the variety Mayan Gold) were carried
out by a trained panel. Overall acceptability and the flavor
attributes flavor intensity, flavor creaminess, and flavor sweet-
ness scores were plotted against calculated EUC values to
determine the level of correlation (Figure 5). Clear and positive
correlations were observed between all sensory scores and EUC
values (for flavor intensity, R2) 0.83; flavor creaminess, R2)
0.85; flavor sweetness, R2) 0.86; and acceptability, R2) 0.79).
Previously it has been hypothesized that potato flavor is
largely due to the levels of umami compounds that develop in
cooked tubers (16, 17). Evidence in support of this hypothesis
is presented in this paper. It is clearly demonstrated that there
are significantly higher levels of glutamate and 5′-GMP in
cooked mature tubers of S. phureja cultivars than in those of S.
tuberosum cultivars. Glutamate is the most potent commonly
occurring amino acid in terms of its umami flavor, and 5′-GMP
is the most potent 5′-nucleotide. Calculation of the equivalent
umami concentration, taking into account the levels of the major
umami amino acids (glutamate and aspartate) and 5′-nucleotides
(5′-GMP and 5′-AMP), shows levels that are up to 2.3-fold
higher in S. phureja tubers compared with the highest levels
measured in mature S. tuberosum cultivars. Sensory evaluations
of boiled tubers from five cultivars were carried out using a
trained panel. There are good correlations between the overall
acceptability score and flavor attributes and the EUC value. The
three phurejas that were analyzed are related in that 333-16
and Mayan Gold share a common parent with 257-28. Mayan
Gold and 257-28 were both considered to have commercially
acceptable flavors, and the former is now available in super-
markets. In contrast, 333-16 was rejected for commercialization
because of unacceptable taste and texture. The level of detection
of glutamate in solution varies between individuals, but a mean
value of 1.5 mmol/L or ca. 250 mg/L has been published (36).
It may be significant that the EUC for S. tuberosum cultivars is
close to this threshold, whereas for S. phureja, the level will
clearly exceed the threshold and perhaps accentuate the differ-
ence in taste. Interestingly, both glutamate and 5′-nucleotide
levels were found to be elevated in the S. phureja tubers; the
main factor affecting the increased EUC was the elevated level
The formation of 5′-nucleotides was examined during the time
course of cooking using several different boiling or steaming
methods. In raw tubers the levels of 5′-nucleotides were very
low, indicating that these compounds are formed during
processing. More consistent and higher levels (ca. 10%) of 5′-
nucleotides were measured on steaming than by boiling in water.
5′-GMP levels were maximal after 5–10 min of steaming for
both S. phureja and S. tuberosum cultivars. Previously it has
been suggested that 5′-nucleotides accumulate due to the action
of nucleases during cooking processes, particularly due to RNA
degradation (37). Ribonucleases are active under the pH and
temperature conditions that occur during heating, particularly
at around 50 °C (38). As the temperature of potato tissues
increases slowly from 40 to 60 °C during some cooking
processes (for example, boiling), nuclease activity may be
significant (17). Despite the proposed involvement of RNases,
no differences in RNase, or phosphohydrolytic enzyme, activities
could be detected in extracts from S. tuberosum and S. phureja
under our assay conditions. It may be that more subtle
differences in these activities, which escape detection in our
extraction and assay methods, account for the difference. For
example, the RNase activity maxima may be different in S.
phureja and S. tuberosum.
There was a large effect of developmental stage on the level
of cooked 5′-nucleotides. Interestingly, as tuber development
progressed, the effect on 5′-nucleotide levels was different for
the S. phureja and S. tuberosum cultivars. In small developing
tubers of both cultivars 5′-nucleotide levels were generally lower
and levels in the S. tuberosum cultivars were not significantly
different from those in the S. phureja cultivars. Conversely, at
tuber maturity, whereas low levels of 5′-GMP could be detected
Figure 4. Effect of tuber developmental stage on equivalent umami
concentrations (EUC) incookedpotatocultivars MayanGold(MG), Inca
Sun (IS), Pentland Dell (PD), and Montrose (MON). H1, harvest 1; H2,
harvest 2; H3, harvest 3 (see text for details). Error bars represent the
SEM, n ) 3.
concentration of potato: (squares) flavor intensity; (circles) acceptability;
Maris Piper (MP) and Record were compared with S. phureja clones
DB333-16 and DB257-28 and cultivar Mayan Gold (MG).
Umami Compounds in PotatoJ. Agric. Food Chem., Vol. 55, No. 23, 2007
in the S. tuberosum tubers, significantly higher levels are present
in cooked S. phureja tubers. In addition, levels of 5′-GMP
remained higher in the S. phureja tubers, compared with S.
tuberosum, after 6 weeks of storage at 4 °C. The implication 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. The
changes that occur during tuber development that account for
the differences in 5′-nucleotide levels are not yet clear. It is
possible that changes in starch structure or content and/or
textural differences may underpin the effect. It is known, for
example, that S. phureja tubers cook significantly more quickly
than S. tuberosum tubers (26). This may affect the accessibility
of RNases with their substrate, assuming this is the mode of
A consistently higher glutamate level was measured in the
mature S. phureja tubers than in those from S. tuberosum. The
amino acid biosynthetic networks are complex and heavily
regulated, although most of the biosynthetic genes have been
cloned. Additionally, the contribution of amino acids synthesized
in the tuber and amino acids imported from leaves remains to
be fully resolved (39), but clearly amino acid transporters could
have a key role (40). In view of the importance of amino acids
in quality traits, understanding the mechanisms that control
storage organ levels should be a priority.
As there are multiple factors that may be affecting the levels
of the key umami metabolites, it is a challenging problem to
dissect the molecular mechanisms involved. A factor not
considered in this study is the potential for inorganic ions such
a potassium and magnesium to modulate the umami taste. The
levels of these ions should be included in further work. It is of
significance that in crosses of S. phureja and S. tuberosum there
is considerable variation in the levels of these compounds (data
not shown). This may imply that a genetic approach might be
applicable, for example, comparing quantitative trait loci (QTLs)
for sensory traits with the map location of candidate genes. Other
recently developed tools include a nearly whole transcriptome
microarray for potato, which could be used to identify gene
expression differences correlated with the trait of interest.
AMP, adenosine 5′-monophosphate; Asp, aspartic acid; EUC,
equivalent umami concentration; Glu, glutamic acid; GMP,
guanosine 5′-monophosphate; HPAEC, high-performance anion
exchange chromatography; HPLC, high-performance liquid chro-
matography; IMP, inosine 5′-monophosphate; IS, Inca Sun; MG,
Mayan Gold; MON, Montrose; MSG, monosodium gluta-
mate; OD, optical density; PD, Pentland Dell; PDE, phosphodi-
esterase; PME, phosphomonoesterase; PMSF, phenylmethane-
sulfonyl fluoride; PMT, photomultiplier tube; QTL, quanti-
tative trait loci; SAX-SPE, strong anion exchange–solid phase
Supporting Information Available: Free amino acid contents
of potato cultivars, 5′-nucleotide-liberating enzyme activity
assays, and in-gel RNase assay. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Received for review June 18, 2007. Revised manuscript received
September 6, 2007. Accepted September 9, 2007. This work was
supported by the Scottish Executive Environment and Rural Affairs
Umami Compounds in Potato J. Agric. Food Chem., Vol. 55, No. 23, 2007