Maltol and ethyl maltol: from the larch tree to successful food additive. Synthesis of a natural compound led to improved flavor- and aroma-enhancing capabilities


LeBlanc, D.T.; Akers, H.A.

Food Technology 43(4): 78-84

1989


The history, production and application of the food additives maltol and ethyl maltol are described.

MALTOL
AND
ETHYL
MALTOL•
From
the
Larch
Tree
to
Successful
Food
Additive
Synthesis
of
a
natural
compound
led
to
improved
flavor-
and
aroma-enhancing
capabilities
Darren
T.
LeBlanc
and
Hugh
A.
Akers
IT
SEEMS
UNUSUAL
that
a compound
first
isolated
from
the
bark
of
the
larch
tree
over
a
century
ago
could
have
such
a
powerful
impact
on
the
food
flavoring
industry
and
lead
to
the
synthe-
sis
of
an
even
more
powerful
homolog.
But
such
was
the
case
with
maltol.
The
compound
known
as
maltol
was
found
to
enhance
or
modify
the
flavor
and
aroma
of
foods
and
beverages
for
heightened
consumer
appeal.
Its
demand
eventually
surpassed
the
capacity
to
iso-
late
it
from
natural
sources,
and
so
now
maltol
is
produced
synthetically.
A
related
compound,
ethyl
maltol,
was
introduced
to
the
food
industry
several
years
after
the
shift
to
chemically
produce
maltol.
When
compared
to
maltol,
it
has
qualitatively
similar
yet
much
improved
flavor-
and
aroma-
enhancing
capabilities.
This
article
will
review
the
history,
production,
and
application
of
these
two
successful
food
addi-
tives.
History
In
1861,
maltol
was
first
isolated
from
the
bark
of
the
larch
tree
(Pinus
larix,
Linn.)
by
the
British
chemist
J.
Stenhouse
(1861,
1862)
during
his
inves-
tigations
on
the
chemistry
of
tanning.
The
larch
was
considered
at
that
time
to
be
a
pine
and
its
scientific
name
provided
Stenhouse
the
basis
for
naming
this
substance
larixinic
acid.
Stenhouse
obtained
larixinic
acid
from
larch
bark
by
water
extraction,
evaporation,
and
crystallization,
fol-
lowed
by
sublimation
which
purified
the
sub-
stance.
Care
had
to
be
taken
to
prevent
exposure
of
larixinic
acid
to
iron
or
iron
salts
and
avoid
the
formation
of
a
deep-purple-colored
liquid,
from
which
it
could
not
be
recovered.
Physical
proper-
ties
such
as
melting
point,
sublimation
tempera-
ture,
and
crystalline
geometry
provided
Stenhouse
with
evidence
to
distinguish
larixinic
acid
from
substances
that
had
been
previously
isolated
from
plants.
Stenhouse
concluded
that
larixinic
acid
existed
in
the
bark
and
was
not
a
product
of
the
isolation
procedure.
An
interesting
feature
in
Sten-
house's
original
publication
was
an
error
in
the
Author
Akers
is
a
Professor,
Department
of
Chemistry,
Lamar
University,
Beaumont,
TX
7710.
Author
LeBlanc,
at
the
time
of
the
project,
was
an
undergraduate
at
the
university.
Send
reprint
requests
to
author
Akers.
empirical
formula
of
larixinic
acid
due
to
confusion
concerning
the
composition
of
water
(which
was
thought
to
be
HO)
and
relative
atomic
masses.
When
Stenhouse's
combustion
data
is
used
with
the
correct
water
formula
and
relative
atomic
mass
values,
the
empirical
formula
of
larixinic
acid
(maltol)
results.
Stenhouse
was
the
first
to
com-
ment
on
its
pleasant
odor
(at
room
temperature)
and
its
slightly
bitter
and
astringent
taste
in
pure
form.
A
description
based
on
taste
during
Sten-
house's
period
was
considered
part
of
the
proce-
dure
for
characterizing
new
substances.
In
the
late
1800s,
the
brewing
industry
intro-
duced
a
caramel-colored
malt,
which
contained
a
higher
sugar
content
than
ordinary
malts.
The
beer
prepared
from
this
malt
gave
a
violet
color
with
ferric
chloride.
The
causitive
agent,
originally
thought
to
be
salicylic
acid,
was
determined
by
Brand
(1894)
to
be
a
substance
other
than
salicylic
acid.
This
crystalline
substance
that
reacted
with
ferric
chloride
to
produce
the
violet
color
was
recovered
from
malt
by
sublimation.
In
spite
of
a
negative
reaction
with
Millon's
reagent
(a
test
for
phenols
such
as
salicylic
acid),
its
possession
of
a
positive
ferric
chloride
test
for
phenols
and
its
isolation
from
malt
provided
Brand
with
the
basis
for
naming
this
substance
maltol.
Brand
showed
that
maltol
was
produced
during
the
roasting
process
in
malt
production.
The
correct
molecular
formula
of
C
6
H
6
0
3
for
maltol
was
determined
by
Brand
who
suspected
that
this
formula
resulted
from
the
loss
of
three
molecules
of
water
from
glucose
(or
any
other
hexose).
Brand
suggested
that
maltol
was
3,6-dihydroxy-7-oxabicyclo
[2.2.1]
hepta-2,5-diene.
Shortly
after
Brand's
discovery
of
maltol,
Kiliani
and
Bazlen
(1894)
oxidized
maltol
with
potassium
permanganate
and
observed
water,
carbon
dioxide,
and
acetic
acid
as
products.
They
observed
that
only
one
acidic
hydrogen
and
just
the
monoacyl
derivative
of
maltol
could
be
formed.
These
obser-
vations
required
the
existence
of
only
one
hydroxyl
group
and
a
methyl
group
in
the
structure
of
maltol,
thereby
excluding
the
structure
proposed
by
Brand.
By
comparisons
made
with
the
chemis-
try
of
pyromeconic
acid,
Kiliani
and
Bazlen
deduced
that
maltol
must
be
a
methylpyromeconic
acid.
Several
years
later,
Feuerstein
(1901)
isolated
a
78
FOOD
TECHNOLOGY—APRIL
1989
Matto!
and
Ethyl
Maltol
(continued
substance
from
the
needles
of
the
silver
fir
(Abies
alba,
Mill.)
and
found
it
to
be
identical
with
maltol.
This
latter
observation
soon
led
to
the
conclusion
by
Peratoner
and
Tamburello
(1903)
that
maltol
was
larixinic
acid.
Peratoner
and
Tamburello
(1905)
later
went
on
to
hydrolyze
the
methyl
derivative
of
maltol
with
barium
hydroxide
and
analyze
the
products
recovered.
In
addition,
deriv-
atives
of
maltol
were
compared
with
those
of
pyromeconic
acid
and
it
was
concluded
that
the
2-methyl
derivative
of
pyromeconic
acid
was
mal-
tol
(3-hydroxy-2-methyl-4-pyrone).
Formation
Maltol
occurs
naturally
in
certain
conifers,
but
it
also
forms
when
certain
disaccharides
are
heated
(pyrolysis).
Maltol
has
been
identified
in
a
wide
variety
of
heated
materials
such
as
bread
crusts,
coffee
and
cocoa
beans,
cereals,
dried
whey,
soy
sauce,
and
chicory
(Ensminger
et
al.,
1983).
Maltol
has
also
been
observed
to
be
a
product
of
the
alkaline
decomposition
of
the
antibiotic
strepto-
mycin
B
(Schenck
and
Spielman,
1945).
Maltose
when
heated
to
191°C
decomposes,
in
part,
to
maltol
(Ensminger
et
al.,
1983).
This
latter
obser-
vation
explains
the
formation
of
maltol
during
the
production
of
malt
or
baked
goods.
Reducing
sug-
ars,
when
heated
with
amino
acids,
will
also
dehy-
drate
into
maltol
by
a
nonenzymatic
browning
reaction
of
the
Maillard
type
(Patton,
1950).
Mal-
tol
is
formed
in
less
extreme
pH
and
tempera-
ture
conditions
when
amino
acids
(catalyst)
are
present.
Because
maltol
sublimes
at
room
temperature,
it
contributes
to
the
odor
of
baked
goods,
caramel,
and
cotton
candy.
This
realization
and
the
thought
of
replacing
maltol
"lost"
from
food
stuffs
by
sublimation
led
to
its
use
as
a
food
additive.
Production
Dow
Chemical
Co.,
Midland,
Mich.,
was
the
first
corporation
to
commercially
exploit
the
use
of
maltol
as
an
agent
for
improving
the
flavor
and
aroma
of
foods.
Maltol,
under
the
trade
name
Palatone,
was
introduced
in
1942
as
a
flavor
enhancer
for
fruit
flavors
(Sanders,
1966).
Dow's
maltol
was
recovered
from
tars
obtained
as
a
byproduct
of
the
destructive
distillation
of
wood
at
its
Cliffs
Dow
facility
in
Marquette,
Mich.
(Goos
and
Reiter,
1946).
In
spite
of
the
tendency
for
maltol
produced
in
this
manner
to
contain
impuri-
ties
which
adversely
affected
its
use
as
a
food
additive,
during
the
1940s,
Dow
sold
5
to
6
tons
of
Palatone
annually
for
about
$11
a
pound,
mainly
to
the
General
Foods
Corp.
(Anonymous,
1962).
Because
the
demand
for
this
product
increased
and
the
production
was
seriously
limited
by
the
amount
of
tars
available,
attempts
were
made
to
develop
a
synthetic
method
of
production.
The
first
successful
synthesis
method
published
started
with
pyromeconic
acid,
which
itself
was
an
expensive
material,
and
gave
maltol
in
low
yields
(Spielman
and
Freifelder,
1947).
During
the
1960s,
a
series
of
U.S.
Patents
were
granted
to
the
Chas.
Pfizer
and
Co.,
New
York,
N.Y.,
for
the
synthesis
of
maltol
from
kojic
acid
(Pfizer,
1977).
Kojic
acid
could
be
obtained
cheaply
as
a
fermentation
prod-
uct
of
an
aspergillus
fungus
(Beelik,
1956)
and
allowed
the
yield
of
maltol
to
be
increased
substan-
tially
to
a
commercially
feasible
level.
The
synthet-
ic
routes
involved
the
oxidation
of
kojic
acid
to
comenic
acid
and
the
addition
of
formaldehyde
to
the
2-position
and
its
subsequent
reduction
to
a
methyl
group.
Decarboxylation
could
occur
before
or
after
the
addition
of
formaldehyde.
By
substi-
tuting
acetaldehyde
for
formaldephyde,
an
analog
of
maltol
called
ethyl
maltol
(on
the
model
of
ethyl
vanillin
as
noted
below),
was
produced
(Rennhard,
1971).
To
date,
ethyl
maltol
has
not
been
observed
as
a
naturally
occurring
substance
(Freydberg
and
Gortner,
1982).
Pfizer
produces
and
markets
mal-
tol
and
ethyl
maltol
under
the
trade
names
of
Veltol
and
Veltol-Plus,
respectively.
The
most
significant
foreign
producers
include
Firmenich
Inc.
of
Geneva,
Switzerland,
who
several
years
ago
marketed
the
maltols
under
the
name
Corps
Pra-
line,
and
Otsuka
Chemical
Co.,
Osaka,
Japan,
who
currently
markets
these
compounds
under
the
trade
name
Piromaltol
and
Ethyl
Pyromaltol
(Stettler,
1988).
The
increase
in
flavor
enhancement
properties
(see
below)
in
the
maltol
to
ethyl
maltol
transition
is
analogous
to
the
increase
in
vanilla-like
flavor
of
ethyl
vanillin
over
vanillin
(Hodge,
1967).
Vanillin,
the
naturally
occurring
substance
responsible
for
the
flavor/odor
of
vanilla,
contains
a
methoxy
group;
ethyl
vanillin,
a
component
of
most
synthet-
ic
vanilla
preparations,
contains
the
ethyoxy
group.
A
compound
isomeric
with
maltol,
thus
called
isomaltol,
has
also
been
found
to
contribute
to
the
odor/flavor
of
baked
goods
(Hodge
and
Moser,
1961).
Many
aspects
of
the
history
of
this
com-
pound
are
similar
to
that
of
maltol,
including
a
publication
reporting
an
incorrect
structure
(Backe,
1910).
The
correct
structure,
3-hydroxy-
2-furyl
methyl
ketone,
has
appeared
in
the
litera-
ture
(Fischer
and
Hodge,
1964).
Properties
Maltol
and
ethyl
maltol
are
both
slightly
acidic
substances
that
form
salts
with
bases.
Both
com-
pounds
are
white,
crystallize
readily,
have
low
melting
points,
and
sublime
at
room
temperature.
They
possess
a
cotton
candy,
caramel-like
aroma.
The
sublimation
of
these
compounds
is
significant
because
it
is
responsible,
in
part,
for
their
aroma
and
odor-altering
properties.
Better
retention
of
maltols
is
provided
if
packaging
and
storage
is
in
tight,
inert
containers.
Ethyl
maltol
sublimes
more
readily
and
has
a
greater
solubility
in
water
than
maltol,
a
factor
which
might
account
for
its
greater
flavor-enhancing
activity.
Because
both
substances
are
chelators
and
readily
form
complexes
(many
are
colored)
with
metals,
care
should
be
taken
to
insure
that
the
maltols
or
products
composed
of
them
are
not
packaged
in
certain
containers
made
of
metals
or
some
grades
of
stainless
steel;
glass
or
plastic
containers
are
more
suitable
(Pfizer,
1977).
Judging
from
nuclear
magnetic
resonance
data,
80
FOOD
TECHNOLOGY-APRIL
1989
Maltol
and
Ethyl
Maltol
(continued
maltol
exhibits
some
aromatic
character
(Lassack
and
Pinhey,
1968).
Consumption
and
Toxicity
Maltol
and
ethyl
maltol
are
added
to
foods
in
minute
amounts.
In
1970,
the
average
daily
intake
per
person
in
the
U.S.
of
maltol
and
ethyl
maltol
was
estimated
at
0.4
and
0.3
mg,
respectively.
Combined,
this
corresponds
to
about
0.01
mg/
kilogram
of
body
weight
for
a
person
who
weighs
60
kg
(Freydberg
and
Gortner,
1982).
Studies
indicated
that
ethyl
maltol
was
slightly
more
toxic
than
maltol
when
administered
as
a
single
dose
to
laboratory
animals.
In
repeated
doses,
however,
the
opposite
was
true.
When
mal-
tol
was
fed
to
rats
(dogs)
at
the
rate
of
1,000
(500)
mg/kg/day,
there
was
significant
body
weight
depression,
kidney
damage,
and
death
among
the
different
individual
test
animals.
Ethyl
maltol
at
the
same
dosage
caused
no
significant
effects
to
either
animal
type
except
for
a
mild
weight
loss.
Ethyl
maltol
was
fed
daily
to
rats
and
dogs
up
to
and
including
200
mg/kg/day
for
as
long
as
two
years
without
any
adverse
toxic
effects.
The
ani-
mals
mated
and
no
effects
on
the
fertility
or
offspring
development
were
noted.
Neither
com-
pound
produced
any
allergic
reaction
or
sensitiza-
tion
(Gralla
et
al.,
1969).
The
metabolic
fate
of
maltol
and
ethyl
maltol
has
been
investigated
in
the
dog.
Orally
dosed
animals
readily
absorbed
the
materials
from
the
gastrointestinal
tract;
neither
substance
was
detected
in
the
feces
after
oral
dosing.
Within
24
hours
of
an
orally
or
intravenously
administered
dose,
both
substances
were
rapidly
metabolized
and
excreted
in
the
urine
as
glucuronide
and
sulfate
derivatives.
These
metabolic
fates
are
both
common
to
exogenous
phenols
and
related
com-
pounds
(Rennhard,
1971).
The
U.N.
Joint
FAO/WHO
Expert
Committee
on
Food
Additives
concluded
that
up
to
2
mg/
kg/day
(120
mg/day
for
a
60
kg
person)
was
an
acceptable
level
of
consumption
of
ethyl
maltol
for
humans.
This
value
is
many
times
greater
than
the
current
average
consumption
levels
for
both
com-
pounds
(Freydberg
and
Gortner,
1982).
Both
compounds
are
included
in
a
list
proposed
by
the
Flavor
and
Extract
Manufacturers'
Associa-
tion
(FEMA)
as
substances
generally
recognized
as
safe
(GRAS)
for
use
in
foods.
Maltol
and
ethyl
maltol
have
been
given
the
following
respective
FEMA
(or
GRAS)
reference
numbers:
2656
and
3487.
The
Food
and
Drug
Administration
(FDA)
includes
both
compounds
in
its
list
of
"synthetic
flavoring
substances
and
adjuvants"
that
are
safe
for
use
in
foods.
The
Bureau
of
Alcohol,
Tobacco,
and
Firearms
claims
that
both
compounds
can
be
safely
added
to
alcoholic
beverages
if
amounts
do
not
exceed
100
ppm
for
use
as
a
stabilizing
agent
or
250
ppm
for
use
as
a
smoothing
agent.
Typically,
maltol
is
added
to
foods
at
levels
ranging
from
50
to
200
ppm
while
ethyl
maltol
is
added
at
amounts
corresponding
to
the
range
of
1
to
50
ppm
(Pfizer,
1977).
Applications
At
the
recommended
concentrations,
the
maltols
do
not
contribute
a
flavor
of
their
own
but
modify
or
enhance
the
inherent
flavors
of
the
foods
to
which
they
are
added.
Their
diversity
of
action
in
modifying
flavors
offers
a
variety
of
routes
to
the
production
of
new
food
products
with
unusual
flavor
sensations
(Pfizer,
1977).
The
actual
mecha-
nism
for
the
flavor-modifying
effects
of
the
maltols
is
unknown
(Lindsay,
1985)
so
food
flavor
research
must
proceed
in
the
future
in
an
empirical
man-
ner.
Pfizer
states
that
maltol
creates
a
"velvet
mouth"
sensation,
especially
in
sweet
foods
(Anon-
ymous,
1962).
The
commercial
trade
name
"Vel-
tol"
might
have
originated
as
a
contraction
of
velvet-maltol,
which
is
reminiscent
of
Brand's
orig-
inal
malt-phenol
combination
suspected
of
produc-
ing
maltol's
name.
Maltol,
a
potent
enhancer,
replaced
flavor
and
aroma
enhancers
which
were
not
as
effective
in
low
concentrations.
Coumarin,
an
enhancer
widely
used
in
the
past,
had
to
be
cautiously
added
so
that
its
powerful
natural
aroma,
resembling
that
of
vanilla
beans,
would
not
overpower
the
desired
enhancing
qualities.
Maltol
could
easily
replace
four
times
its
weight
of
coumarin
(Stephens
and
Allingham,
1968).
Since
1954,
coumarin
has
been
classed
by
the
FDA
as
a
toxic
substance
and
subsequently
was
banned
as
a
food
additive.
Because
of
its
effectiveness
and
lack
of
toxicity,
maltol
filled
the
void
left
by
the
demise
of
couma-
rin.
Tests
conducted
by
Pfizer
(1977)
and
indepen-
dent
groups
have
shown
that
ethyl
maltol
is
two
to
six
times
more
effective
as
a
flavor
enhancer
than
maltol.
In
addition,
effects
not
possible
to
achieve
with
maltol
are
observed
with
ethyl
maltol
because
of
its
effectiveness
at
such
low
concentrations
(Pfizer,
1977).
Upon
realization
that
the
relative
costs
per
weight
of
maltol
and
ethyl
maltol
are
comparable,
it
becomes
apparent
that
ethyl
maltol
is
a
more
economical
and
convenient
flavor
enhancer
than
maltol
(Stephens
and
Allingham,
1968).
The
maltols
may
be
added
to
the
food
or
per-
fume
directly
in
a
dry
form
or
as
a
solution.
Care
must
be
taken
to
evenly
distribute
the
additive
in
the
final
product
because
minute
quantities
have
such
a
powerful
effect
(Stephens
and
Allingham,
1968).
The
greatest
application
of
maltol,
and
later
ethyl
maltol,
it
was
discovered,
was
in
synthetic
berry
and
citrus
fruit
flavorings.
These
additives
intensify
and
produce
the
characteristic
fruity
flavors
of
strawberry
and
raspberry
in
fruit-
flavored
drinks
and
enhance
the
flavors
of
orange,
pineapple,
and
black
cherry
as
well.
They
have
been
used
along
with
sodium
citrate
and
sodium
gluconate
to
minimize
the
bitter
aftertaste
of
the
artificial
sweetener
saccharin,
used
in
dietetic
products.
The
maltols
are
added
to
other
beverages
which
improve
in
overall
flavor
and
aroma
such
as
grape
and
apple
juice,
fortified
wines
(port,
tokay,
muscatel,
and
sherry),
liqueurs
(cordial
and
bran-
82
FOOD
TECHNOLOGY-APRIL
1989
Maltol
and
Ethyl
Maltol
(continued
dy),
ale,
and
even
tomato
beverages,
of
which
acidity
and
aftertastes
are
also
muted
(Dow,
1959;
Pfizer,
1977).
Flavor
in
addition
to
textural
qualities
such
as
richness
and
creaminess
are
improved
upon
adding
the
maltols
to
ice
creams,
puddings,
frozen
cus-
tards,
and
gelatin
desserts
(Pfizer,
1977).
The
maltols
will
lessen
the
yeasty
taste
of
baked
foods.
Although
the
maltols
have
been
shown
to
survive
baking
(Dow,
1959;
Lindsay,
1985),
they
(especially
ethyl
maltol)
should
be
added
as
late
as
possible
and
at
the
lowest
temperature
feasible
to
minimize
losses
due
to
volatilization.
They
impart
a
fresh
baked
or
browned
odor
to
breads
and
enhance
the
flavor
character
of
cakes,
cookies,
pie
fillings,
tarts,
and
pastries,
especially
those
which
have
fruit
flavors
(Dow,
1959;
Pfizer,
1977).
The
maltols
increase
the
richness
and
smooth-
ness
of
chocolates
in
addition
to
improving
aroma
and
mellowing
of
harsh
flavor
qualities.
Miniscule
amounts
of
the
maltols
can
make
a
"chocolate
candy
taste
more
chocolatey"
with
a
decrease
in
essential
flavor
components
(Anonymous,
1962;
Pfizer,
1977).
The
maltols
have
been
used
in
products
other
than
foods
where
they
often
mute
undesirable
flavors
and
odors.
Many
tobacco
products,
per-
fumes,
colognes,
baby
talcs,
shampoos,
cough
syr-
ups,
multivitamin
tablets,
scented
candles,
and
after
shave
lotions
contain
these
modifiers
(Pfizer,
1985).
When
maltol
is
added,
the
content
of
sugar
used
in
some
foods
can
be
reduced
by
5
to
15%
with
no
apparent
loss
in
sweetness.
Maltol
itself
does
not
have
a
sweet
taste.
For
materials
composed
mainly
of
sugar
(candies,
carbonated
beverages,
and
fruit
drinks),
any
reduction
in
sugar
can
represent
a
substantial
savings
for
the
manufacturer
(Bou-
chard
et
al.,
1968).
A
Call
to
Consumers
The
maltols
have
outlasted
other
food
additives,
because,
in
addition
to
being
powerfully
effective
at
low
concentrations,
they
have
unique
flavor-
and
aroma-enhancing
properties.
As
long
as
consumers
search
for
food
products
possessing
an
attractive
flavor
and
aroma,
there
will
be
a
demand
for
these
two
successful
food
additives,
and
as
food
produc-
ers
are
aware,
any
food
product
having
a
wider
consumer
acceptance
will
show
an
increase
in
sales.
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The
authors
would
like
to
thank
Dr.
H.H.
Rennhard
and
B.O.G.
Schueler
for
their
interest
and
help
with
this literature
investigation.
-Edited
by
Donald
E.
Pszczola,
Assistant
Editor
84
FOOD
TECHNOLOGY
-APRIL
1989