On the human consumption of the red seaweed dulse (Palmaria palmata (L.) Weber & Mohr)

Abstract

The red seaweed dulse (Palmaria palmata) is one of the more popular seaweed species for human consumption in the Western world. With a documented historical use up to present days in Ireland, Brittany (France), Iceland, Maine (USA), and Nova Scotia (Canada), it has remained a snack, a food supplement, and an ingredient in various dishes. The trend towards more healthy and basic foodstuffs, together with an increasing interest among chefs for the seaweed cuisine, has posed the need for more quantitative knowledge about the chemical composition of dulse of relevance for human consumption. Here, we report on data for amino acid composition, fatty acid profile, vitamin K, iodine, kainic acid, inorganic arsenic, as well as for various heavy metals in samples from Denmark, Iceland, and Maine.

Full text

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Journal of Applied Phycology
ISSN 0921-8971
Volume 25
Number 6
J Appl Phycol (2013) 25:1777-1791
DOI 10.1007/s10811-013-0014-7
On the human consumption of the red
seaweed dulse (Palmaria palmata (L.)
Weber & Mohr)
Ole G.Mouritsen, Christine Dawczynski,
Lars Duelund, Gerhard Jahreis, Walter
Vetter & Markus Schröder

1 23
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On the human consumption of the red seaweed dulse
(Palmaria palmata (L.) Weber & Mohr)
Ole G. Mouritsen &Christine Dawczynski &
Lars Duelund &Gerhard Jahreis &Walter Vetter &
Markus Schröder
Received: 13 November 2012 /Revised and accepted: 26 February 2013 /Published online: 27 March 2013
#Springer Science+Business Media Dordrecht 2013
Abstract The red seaweed dulse (Palmaria palmata) is one
of the more popular seaweed species for human consumption
in the Western world. With a documented historical use up to
present days in Ireland, Brittany (France), Iceland, Maine
(USA), and Nova Scotia (Canada), it has remained a snack,
a food supplement, and an ingredient in various dishes. The
trend towards more healthy and basic foodstuffs, together with
an increasing interest among chefs for the seaweed cuisine,
has posed the need for more quantitative knowledge about the
chemical composition of dulse of relevance for human con-
sumption. Here, we report on data for amino acid composi-
tion, fatty acid profile, vitamin K, iodine, kainic acid,
inorganic arsenic, as well as for various heavy metals in
samples from Denmark, Iceland, and Maine.
Keywords Dulse .Palmaria palmata .Foodstuff .Amino
acids .Fatty acids .Vitamin K .Iodine .Arsenic .Kainic
acid .Heavy metals
Introduction
Whereas the human consumption of whole and processed
seaweeds from a wide range of genera enjoys a continuous
record over millennia in Asia (Arasaki and Arasaki 1983;
Basanti and Gualtieri 2006;Mouritsen2013; Rhatigan
2009; Mouritsen 2012a;Pereira2012;Pomin2012;
Fleurence et al. 2012), the main uses of seaweeds in the
Western world are mostly restricted to seaweed extracts for
producing hydrogels (Bixel and Porse 2011). Although
there is a living tradition for eating whole seaweeds in a
few scattered coastal areas, e.g., in Ireland, Brittany, Iceland,
Maine, and Nova Scotia, this tradition appears to have been
discontinued most other places for reasons still not known,
e.g., in Scotland and Norway. However, in recent decades
consumers have started asking for seaweeds both in restau-
rants and food stores, a trend that first seemed to be fueled
by the holistic health-food movement and now apparently is
carried by a demand for new, interesting, and healthy (“nat-
ural”) foodstuffs that can be produced in a sustainable
fashion. The bad connotations previously associated with
O. G. Mouritsen (*):L. Duelund
MEMPHYS-Center for Biomembrane Physics,
Department of Physics, Chemistry, and Pharmacy,
University of Southern Denmark,
55 Campusvej, 5230 Odense, Denmark
e-mail: ogm@memphys.sdu.dk
L. Duelund
e-mail: lad@memphys.sdu.dk
O. G. Mouritsen
Nordic Food Lab,
93 Strandgade, 1401 Copenhagen, Denmark
C. Dawczynski :G. Jahreis
Institute of Nutrition, Friedrich Schiller University Jena,
Dornburger Strasse 24, 07743 Jena, Germany
C. Dawczynski
e-mail: christine.dawczynski@uni-jena.de
G. Jahreis
e-mail: gerhard.jahreis@uni-jena.de
W. Vetter :M. Schröder
Institute of Food Chemistry (170b),
University of Hohenheim,
Garbenstr. 28, 70599 Stuttgart, Germany
W. Vetter
e-mail: walter.vetter@uni-hohenheim.de
M. Schröder
e-mail: markus.schroeder@uni-hohenheim.de
J Appl Phycol (2013) 25:1777–1791
DOI 10.1007/s10811-013-0014-7
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the word “seaweed”have increasingly vanished and it is
becoming common knowledge that seaweeds are not only
edible but also tasty and healthy (Mouritsen 2013; Edwards
et al. 2011; Holdt and Kraan 2011; Stegel et al. 2011).
The red seaweed dulse (Palmaria palmata) is considered
as one of the more delectable for human consumption with a
large and fairly unexplored potential for use in the cuisine
(Mouritsen 2012a), both in the home kitchen (Rhatigan
2009) and in gastronomy (Mouritsen et al. 2012). Dulse is
a common seaweed in the cold Atlantic waters. Since it can
both be harvested in the wild and cultured in the sea and on
land in pools, it holds a fine potential for commercialization
both as a whole food as well as an ingredient. Interestingly,
as we shall review below, dulse is one of the few seaweed
species that can be documented to have been used for
human consumption in Europe over centuries, possible
millennia.
A wider use of dulse for human consumption, it be as whole
food, as functional and novel food, or as ingredients, requires
access to reliable data for its chemical composition. Some data
for dulse can be found scattered in the literature, but for
several nutritionally important components, such as fatty
acids, as well as potential contents of harmful compounds,
only limited data is available. One motivation for publishing
the present paper hence stems from the needs expressed by
consumers and seaweed harvesters/farmers/producers for
quantitative information on seaweed composition inparticular
with regard to nutritional value, gastronomic potential and
flavor, as well as contents of potentially harmful compounds.
The paper is organized as follows. After a brief description
of the species dulse (P.palmata), a short review is provided of
the historical and present uses of dulse for human consump-
tion. Data is then presented for amino acid composition, fatty
acid profile, vitamin K, iodine, arsenic, kainic acid, as well as
various heavy metals. Some of this data has not been pub-
lished before, whereas other data is compiled from the scien-
tific literature. The contents data is discussed in relation to
implications for the use of dulse as human foodstuff, and in
some instances a comparison is made with other commonly
used seaweeds for human consumption.
Dulse, Palmaria palmata (Linnaeus) Weber & Mohr
Dulse is a relatively small, intertidal or shallow subtidal, red
seaweed common in the North- and North-East Atlantic
Ocean. Its fronds grow to a length of 50 cm and width of
3–8 cm with a soft, leathery and skin-like texture (Braune
and Guiry 2011). In the wild, it is found in cold and turbu-
lent waters on substrates of rocks or other large seaweeds,
e.g., kelps like Laminaría hyperborea, and it has a small
and rather fragile, disk-like holdfast. The colors of the wet
fronds are purple, crimson, or brownish–red, which turns to
a pinky–red after drying and bleaching in the sun.
In the wild, the thallus of dulse is upright, elongated, and
wedge-shaped, often ending in forks as illustrated in Fig. 1,
exposing a directional structure with the shape of a hand or
palm (as mirrored in its Latin name palmata). In aquacul-
ture, it can assume an almost isotropic shape when subject to
the conditions of being grown freely in a pool with con-
stantly moving and swirling water, as illustrated by the
image of the specimen shown in Fig. 2.
Dulse harvested in the wild, dried, and packed in closed
plastic bags is sold commercially by a number of small
companies, e.g., in Ireland, Brittany, Spain, Iceland, Maine,
Nova Scotia, and California. There is a small production of
cultivated dulse in Ireland, Brittany, Spain, Hawaii, and
most recently Denmark (Le Gal et al. 2004;Pangand
Lüning 2004; Martínez et al. 2006; Pereira 2012; Hafting
et al. 2012; Mouritsen et al. 2012). In Europe, the most
successful attempt to cultivate dulse takes place in the open
sea along the coastline of northern Spain, an endeavor that is
now responsible for the largest commercial production of
red algae outside of Asia (Martínez et al. 2006).
In the present work, we use a variety of different supplies
of dulse. Some are harvested in the wild in Iceland and
Maine at low tide and are available commercially, and others
have been farmed on land in tanks in Denmark.
A brief history of dulse as human foodstuff
A very rich cultural history is associated with dulse that has
been eaten for centuries by humans living along coastlines
Fig. 1 Schematic illustration of the red seaweed species dulse,
P. palmata (drawing by Jonas Drotner Mouritsen)
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around the world, especially those of the Northern Atlantic
and Pacific Oceans (Mouritsen 2013).
In her book on the traditional Irish seaweed kitchen, Dr.
Prannie Rhatigan writes that dulse (dillisk or duileach in Gaelic)
is likely to have been used in the diet of the costal populations
of Ireland more than five millennia back. The first written
records stem back to the fifth century Ireland where dulse was
used as a condiment with bread, butter, and milk. The Irish and
Scottish immigrants are likely later to have carried this tradition
to North America. Stories are told of Irish monks of the twelfth
century gathering dulse for distribution to the poor, who prob-
ably ate it for lack of anything better (Rhatigan 2009).
Ireland maintains a rich tradition of using algae in soups,
both as thickeners (for example, carrageen Chondrus crispus)
and as ingredients. In earlier times, fresh dulse was gathered at
ebb tide and spread out to dry in the sun on a tin roof or on
roofing stones. The dried dulse could be eaten raw between
two pieces of buttered bread or used in a stew or soup.
In Brittany, the use of seaweeds as human food goes back
at least as far as it does in Ireland. The vernacular names for
dulse, dillisk in Ireland and tellesk in Brittany, are indicative
of a common origin. The Centuries old Breton practice of
using dulse for animal fodder continues to the present. Uses
of dulse for feeding livestock are also mirrored in terms like
“cow weed”in England and “horse seaweed”in Norway.
It is possible that the idea of eating dulse came to Iceland
and the Faroe Islands from Ireland, England, and Scotland and
then migrated from Iceland to Norway, where the consump-
tion of dulse, in particular, was widespread in coastal areas. In
all of these regions, it was easy to gather dulse along the low-
lying shores and dry it for later use. Dulse was used as a
trading commodity on Iceland since the 700s and was one of
the goods exchanged between coastal and inland dwellers. It
was effectively a hard currency; the cost of renting a farm was
often expressed in terms of a quantity of seaweeds. The
seaweeds were collected at the end of June by gatherers who
lived in tents pitched along the beach for the duration of the
harvest. People came to the seashore to barter for seaweeds—
for a kilogramof dried fish, one could buy a kilogram of dulse.
The harvested dulse was rinsed inwater and then spread out to
dry. In the course of the desiccation process, salts and amino
acids sometimes seeped out to form a layer on the surface of
the seaweed. The greater the deposit of this salty–sweet pow-
der, the more attractive the seaweed was considered to be
(Kristjánsson 1980).
Written sources such as law codes and sagas, in particular
the famous Egil Skallagrimsson’s saga, record the use of
seaweeds as human food on Iceland as far back as the tenth
century. A number of different types of seaweeds were
harvested,in particular red algae. Icelanders, and possibly also
Norwegians, ate fresh dulse baked in bread and dried and
salted dulse as a sort of snack. For the preparation of a meal,
the seaweed was mixed with butter or lard and served with
dried fresh or cooked potatoes and turnips. Another way of
preparing dulse was to cook it with milk or put it in porridge.
Finally, dulse has also been added to bread dough in order to
make the flour stretch farther. The Norwegian Vikings prob-
ably brought dried seaweeds with them as provisions for their
long expeditions maybe having realized that it protected the
seafarers against scurvy. In the early 1900s, toasted dulse was
served as a snack in Irish and Scottish pubs to stimulate beer
consumption. On the Westman Islands and other parts of
Iceland, dried dulse is still an ordinary snack food. Dulse is
nowadays used in many places in Europe and North America
as a dietary supplement (Erhart and Cerier 2001; Maderia
2007;Cooksley2007;Mouritsen2013; Rhatigan 2009).
It should be mentioned that seaweeds have been used
throughout times for medicinal purposes (Arasaki and
Arasaki 1983;Smit2005;Pomin2012). Their medical
potential is likely to be connected to their rich content of a
variety of bioactive compounds (Holdt and Kraan 2011).
The Icelanders knew how to apply poultices containing
dulse to fight infection, while seaweed extracts were thought
to counter sea sickness and hangovers. Dulse has success-
fully been used as an insecticide as well as a vermifuge and
antihelminthic treatment both for animals and humans in
Ireland, Wales, in the Mediterranean region, as well as in the
Orient (Michanek 1979; Arasaki and Arasaki 1983; Guiry
1978; Rhatigan 2009). The antihelminthic property of dulse
may derive from a content of kainic acid, which we shall
address later since this compound is also a neurotoxin.
Present-day uses of dulse in the cuisine
Although dulse appears to be one of the seaweeds with the
more interesting potential for gastronomical applications, it
is surprisingly little used in the modern cuisine (Rhatigan
Fig. 2 Black-and-white photographic image of a specimen of dulse
(P. palmata) farmed in a pool with swirling seawater in Horsens,
Denmark (courtesy of Rasmus Bjerregaard, Blue Food; photograph
by Jonas Drotner Mouritsen)
J Appl Phycol (2013) 25:1777–1791 1779
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2009; Mouritsen 2013). In modern household cuisine, dulse
could be incorporated into bread, fish and vegetable soups,
and fish dishes, or simply eaten toasted as a snack that goes
well with a dark beer or an aperitif. It can be fried to a crisp
in a little oil or butter and used as an agreeable substitute for
fried bacon, e.g., in an omelet. Because it has a somewhat
sweet taste, dulse is a good complement for root vegetables
and corn. It can also be crushed and sprinkled on salads and
vegetables. When toasted, dulse loses some of its red color
and turns brownish (Mouritsen 2013).
It appears that freshly harvested and wet dulse is by most
considered less palatable than dried dulse by most consumers.
During drying, and possible subsequently during partial rehy-
dration or roasting, the dulse develops more interesting flavors
(Rhatigan 2009; Mouritsen 2013). Since dulse contains simpler
polysaccharides than those found in, e.g., the brown seaweeds
(Holdt and Kraan 2011), dulse presents itself with a more
delicate flavor and softer texture. When dried, dulse develops
notes of licorice and smoke, and when toasted it has a nutty taste.
In some cases, the fresh dulse is preserved in salt, but it is
usually dried and sometimes chopped up into pieces or ground
into coarse granules. As mentioned, in the course of the drying
process, salts and amino acids may seep out onto the surface
and show up as white spots on the reddish–purple blades.
When fresh, very young dulse can be eaten as a salad,
possibly after being soaked in fresh water, which causes its
cells to burst. Freshly harvested dulse is often too tough to
eat raw, but drying makes it easier to chew and brings out its
pleasant salty and nut-like taste. Dulse should not be exten-
sively cooked or boiled as this causes it to break up.
In most places, dulse is usually picked by hand by local
harvesters and dried on site, after which it is brought to small
factories where it is sorted by hand, and epiphytes, shells, as
well as small crustaceans and bivalves are picked off. The
bone-dry dulse is placed in a sealed room to absorb moisture
for a day and a night and then let to ripen for a couple of
weeks. In the course of the ripening process, the seaweeds’
own enzymes tenderize the blades so that they become both
softer and more flavorful. In tightly sealed packages, these
chewy, but still dry blades have a shelf life of about a year. If
they are kept longer than that, the enzymes break the dulse up
into pieces and it becomes unsuitable for eating. Some pro-
ducers make a dulse product that is lightly smoked, e.g., over
apple wood (Mouritsen 2013).
Dulse has found its way into the so-called New Nordic
Cuisine (Mouritsen et al. 2012) where aqueous extracts of
dulsehavebeenusedfordashi and dishes infused with dashi,
e.g., fresh cheese, bread, and aqua vitae. For these applica-
tions, the subtle floral flavor of dulse is brought out in full.
Chemical composition of dulse
Amino acids and proteins
Data for the amino acid contents of dulse are compiled in
Table 1with the purpose of putting emphasis on the amino
Table 1 Contents of amino
acids (AA) in dulse (P. palmata)
a
Mai et al. (1994)
b
Galland-Irmouli et al. (1999)
c
Mouritsen et al. (2012)
Amino acid (AA) % of total AA mg/100 g AA in aqueous extract
Essential AA
Cystine (Cys) 1.3
a
Isoleucine (Ile) 3.7
a
, 5.3
b
2.8 (Denmark)
c
Leucine (Leu) 7.1
a
, 7.8
b
12 (Denmark)
c
Lysine (Lys) 3.3
a
, 8.2
b
0.7 (Denmark)
c
Methionine (Met) 2.7
a
, 1.9
b
0 (Denmark)
c
Phenylalanine (Phe) 5.1
a
, 5.2
b
0.1 (Denmark)
c
Tyrosine (Tyr) 3.4
a
, 4.5
b
0 (Denmark)
c
Threonine (Thr) 3.6
a
, 5.5
b
1.6 (Denmark)
c
Valine (Val) 6.9
a
, 7.3
b
2.6 (Denmark)
c
Tryptophan (Trp) –
Non-essential AA
Alanine (Ala) 6.7
a
, 7.5
b
25± 6 (Denmark)
c
, 12± 2 (Iceland)
c
Arginine (Arg) 5.1
a
, 6.2
b
0 (Denmark)
c
Aspartic acid (Asp) 18.5
a
, 9.3
b
27± 8 (Denmark)
c
, 11± 2 (Iceland)
c
Glutamic acid (Glu) 9.9
a
,13
b
40± 10 (Denmark)
c
, 10± 5 (Iceland)
c
Glycine (Gly) 13.3
a
, 7.2
b
3.0 (Denmark)
c
Histidine (His) 0.5
a
, 2.1
b
0.3 (Denmark)
c
Hydroxyproline (hPro) 2.3
a
Proline (Pro) 1.8
a
, 4.4
b
23 (Denmark)
c
Serine (Ser) 6.3
a
, 4.6
b
24 (Denmark)
c
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acids that in free form are of interest for the flavor of the
seaweed, in particular when used for forming aqueous ex-
tracts for soup broths (dashi) with umami flavor (Mouritsen
et al. 2012). Values of free amino acids in such extracts are
also listed in Table 1.
The total protein content in dulse has been reported to be
in the range of 8–35 % (Morgan et al. 1980; Misra et al.
1993; Galland-Irmouli et al. 1999) and it is subject to
substantial geographical and seasonal variations (Rødde et
al. 2004). Most typical values are around 20 %.
The most abundant amino acids are alanine, aspartic
acid, glutamic acid, and glycine. Early work has also
found by alcohol extraction a pool of free amino acids
dominated by alanine, aspartic acid, and glutamic acid
(Channing and Young 1953) as well as serine and proline
(Coulson 1953; Laycock et al. 1979). The percentage
concentrations of the free amino acids extracted by warm
water extraction for soup broth purposes (dashi) listed in
Table 1(Mouritsen et al. 2012) show correspondingly
large amounts of alanine, aspartic acid, glutamic acid,
proline, and serine. In addition, the aqueous extract con-
tains measurable amounts of leucine.
Fatty acids
We have analyzed the lipid content and the corresponding
fatty acid profiles of dry samples of dulse from three loca-
tions: Maine (USA), Denmark, and Iceland. For each loca-
tion, the analysis has involved freshly harvested and dried
samples (May) as well as samples from the previous harvest
(October the year before), saved in the dry state in a sealed
bag under dark conditions. The resulting data are shown in
Table 2together with previously published data for Spanish
dulse (Sanchez-Machado et al. 2004).
Since most of the data only refer to measurements on a
single or a couple of samples, we are unable to perform a
statistical analysis. The total lipid content varied from about
0.4–1.8 %, depending on location and the age of the sample.
Table 2 Total fatty acid content (percentage of total dry weight) and fatty
acid composition (percentage of total fatty acid) in dulse
(P. p a l m a t a ), in the present work based on measurement of percentage
FAME. New and old refer to samples that are respectively harvested in
the spring (May) one year and autumn (October) the previous year. Data
for the samples from Maine, Denmark, and Iceland refer to single samples
Fatty acid (FA) Maine
a
(new) Maine
a
(old) Denmark
a
(new) Denmark
a
(old) Iceland
a
(new) Iceland
a
(old) Spain
b
Total fat % 0.35 0.54 1.84 0.47 1.53 0.63 0.7–1.8
C14:0 22.51 21.80 8.50 15.66 5.98 11.02 16.76±0.61
C16:0 47.63 53.35 31.89 50.10 20.83 33.74 45.44± 1.84
C16:1 ω-7 5.26± 0.63
C16:2 ω-4 –
C16:3 ω-4 1.20± 0.16
C18:0 1.84 2.30 0.81 2.62 0.40 0.95 1.28± 0.12
C18:1 ω-9 9.04 8.14 4.80 8.05 3.30 6.76 3.13± 0.47
C18:1 ω-7 2.08± 0.33
C18:2 ω-6 0.36 0.36 1.28 2.60 0.34 0.47 0.69± 0.13
C18:3 ω-3 0.11 0.06 0.93 1.01 0.19 0.12 0.59± 0.26
C18:3 ω-6 0.05 0.05 0.21 0.25 0.09 0.08
C18:4 ω-3 0.17 0.05 1.07 0.94 0.66 0.24 0.74± 0.47
C20:1 ω-9 0.20± 0.10
C20:4 ω-6 (ARA) 0.16 0.16 1.11 1.13 0.32 0.59 1.45± 0.31
C20:4 ω-3 0.08 0.02 0.17 0.05 0.09 0.10 0.14± 0.03
C20:5 ω-3 (EPA) 1.76 2.65 40.56 4.55 59.69 38.84 24.05± 2.59
C22:6 ω-3 (DHA) 0.02 0.01 0.07 0.08 0.03 0.01
ΣSaturated FA 51.59 57.82 33.98 56.28 22.13 35.98 60.48± 2.58
ΣMonounsaturated FA 16.19 13.24 7.94 12.76 6.05 9.47 10.67±1.55
ΣPUFA 3.08 3.62 46.11 11.08 61.77 40.72 28.86 ± 3.94
Σω-6 0.70 0.65 2.86 4.22 0.95 1.34 2.14±0.45
Σω-3 2.18 2.80 42.95 6.71 60.78 39.35 25.52±3.34
Ratio ω-3/ω-6 3.11 4.31 15.01 1.59 63.97 29.37 11.93
a
This work
b
Sanchez-Machado et al. (2004)
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Furthermore, a significant variation is observed in the fatty
acid profiles and the contents of polyunsaturated fatty acids
(PUFA), in particular eicosapentaenoic acid (EPA).
Iodine
Iodine in dulse can exhibit a rather wide range of values as
shown in Table 3, typically in the range of 10–100 μgg
−1
depending on location and time of harvest.
Arsenic
The contents of total arsenic and inorganic arsenic of dulse
from various sources are given in Table 3. Arsenic content
varies widely with location and age of the specimen. For
example, dulse from Maine (Maine Coast Sea Vegetables;
Shep Erhart, personal communication) has been tested neg-
atively (<0.02 μgg
−1
) for inorganic arsenic in the case of
younger, whole broad-leaf material, whereas a granular
product produced from older plants was found to contain
0.3 μgg
−1
inorganic arsenic. Similarly farmed Danish dulse
contains about the same content for older leaves (0.3 μgg
−1
),
whereas young leaves have three times less (0.1 μgg
−1
).
Heavy metals
Table 3lists some of the values for heavy metal contents from
our work and from the literature. The levels of cadmium in
dulse from different sources are generally found to be below
our detection limit (1 μgg
−1
). The present work has not
applied sufficiently accurate methods to make a reliable as-
sessment of mercury and the level is found to be below the
detection limit (1 μgg
−1
). More accurate approaches (Almela
et al. 2002) have found levels of around 0.01 μgg
−1
Hg in
Spanish dulse, or below the detection limit (0.04 μgg
−1
)in
dulse from Maine (Shep Erhart, personal communication,
2012). Lead contents are less or around 1 μgg
−1
.
Vitamin K
Samples of fresh, newly dried fresh, as well as stored dry
dulse were analyzed for their contents of vitamin K
1
(phylloquinone) and each analysis was performed twice.
The results indicate, as shown in Table 3, that the contents
are fairly low, in the range of 2–7μgg
−1
. The fresh and
freshly dried dulse contains the larger amount of vitamin K
1
.
There is only little data published about the vitamin K
contents of seaweeds. Interestingly, in a classic paper on
vitamin K in plants (Dam and Glavind 1938), co-authored
by Henrik Dam, the Noble Laureate 1943 for the discovery
of vitamin K, the vitamin K content is reported for four
different seaweed species, sea lettuce (Ulva lactuca), gut
weed (Ulva clathrata, formerly Enteromorpha clathrata),
dulse (P. palmata,formerlyRhodymenia palmata), and
knotted wrack (Ascophyllum nodosum). According to this
classic paper, the vitamin K content in dulse amounted to
17 μgg
−1
.
The USDA National Nutrient Database for Standard Ref-
erence (USDA 2013) lists vitamin K
1
contents for Irish
moss (Chondrus cripus), konbu (Saccharina japonica), nori
(Porphyra spp.), and wakame (Undaria pinnatifada)tobe
0.3, 3.3, 0.2, and 0.3 μgg
−1
, respectively, where the tabu-
lated contents have been multiplied by a factor of 5, ac-
counting for the fact that the dry matter constitutes about
20 % of the wet matter. We have measured vitamin K
1
in
Danish farmed sugar kelp (Saccharina latissima)bythe
same techniques as for dulse and found 1–6μgg
−1
.
Kainic acid
The kainic acid contents are determined of dry dulse from
three different locations: harvested wild dulse from Iceland
and Maine and farmed dulse from Denmark. The results for
the contents of kainic acid are reported in Table 3. General-
ly, the levels measured were rather low. We have only found
one earlier published study of kainic acid in wild dulse
(Ramsey et al. 1994) and one unpublished report on both
Tab l e 3 Contents of potentially harmful compounds in dulse (P.
palmata). Values are in units of μgg
-1
on a dry-weight basis
Compound Content (dry weight basis)
I <5 (Iceland)
a
;5–7 (Denmark)
a
; <5 (Maine)
a
;
10–100
b
;15–55
c
;80
d
; 10.2
e
Cd <1
a
,ND
h
(Maine); <1 (Denmark)
a
; <1 (Iceland)
a
; 0.7
f
Hg ND (Iceland)
a
; ND (Maine)
a,h
; <1 (Denmark)
a
; 0.0105
f
Pb <1 (Iceland)
a
;<1
a
,ND
h
, 1.7
h
(Maine);
<1 (Denmark)
a
; 1.1
f
As (total) 1 (Iceland)
a
, 7.9, 10 (Maine)
h
; 7.56
f
; 12.8
c
;10–13
g
As (inorg) 0.056 (Maine)
a
; ND, 0.333 (Maine)
h
; < 0.03 (Iceland)
a
,
0.1, 0.3 (Denmark)
a
; 0.44
f
Vitamin K 2–7 (Denmark)
a
,17
i
Kainic acid 0.22 (Maine)
a
; 1.0 (Denmark)
a
; 21 (Iceland)
a
2.5
j
,12
j
, 130
j
ND non-detectable (for lower detection limits, see text)
a
This work
b
Mabeau and Fleurence (1993)
c
Holdt and Kraan (2011)
d
Morgan et al. (1980)
e
MacArtain et al. (2007)
f
Almela et al. (2002)
g
Morgan et al. (1980)
h
Shep Erhard (private communication)
i
Dam and Glavind (1938)
j
Lüning (2008)
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wild and farmed dulse presented at a conference (Lüning
2008). As we shall discuss below, the study by Ramsey et al.
detected substantially higher levels of kainic acid than we
did in our samples as well as in those reported on by Lüning.
Discussion
There are no daily recommendations for the intake of sea-
weeds. Since seaweeds are part of a whole kingdom, such a
recommendation may not even make sense. However, sea-
weeds constitute a substantial part of the diet in many coun-
tries, in particular in East Asia, where seaweeds are a valued
source for tasty and nutritious food. A safe recommendation
based on, e.g., the traditional consumption of seaweeds in
Japan would amount to a daily intake of about 5 g dry weight
(Mouritsen 2013), corresponding to 30–50 g wet weight
depending on the species. Since different seaweeds have
different chemical contents and hence different nutritional
value, another safe recommendation would be to compose
these 5 g of dry seaweed, at least averaged over the week,
by different species. Dulse could be one of them. Based on the
data presented in this paper, we are now in a position to
discuss whether dulse consumption ofthe amounts mentioned
above could present any potential risks for human health.
In most countries, there are no special regulations
enforced for the use of seaweeds and algae as human food
except that they have to conform to the general safety
regulations for food and their contents, specified by a pro-
visional tolerable weekly intake (PTWI) recommended by
the World Health Organization (WHO) referring to an aver-
age adult body weight of 68 kg. France, as an exception, has
authorized a specific list of seaweeds for human consump-
tion (Mabeau and Fleurence 1993; Burtin 2003). This list
includes dulse. Moreover, the French list specifies upper
limits for the contents of inorganic arsenic, lead, cadmium,
tin, mercury, and iodine in seaweed. The legislation regard-
ing seaweeds has recently been reviewed by Holdt and
Kraan (2011).
The Tolerable Upper Intake Level (DRI Report 2001)is
defined as the highest level of daily intake that is likely to pose
no adverse health effects in most human individuals. The
European Commission has issued upper levels for the amount
of particular environmental compounds, such as lead, mercu-
ry, and cadmium in foodstuffs and as additives (EU 2008). We
shall refer to these limit values in the discussion below.
The assessments made below of potential risks involved
with the ingestion of dulse take a daily consumption of 5 g
dry weight or 30–50 g wet weight as a basis. Even regular
consumers of seaweed products would not reach these
amounts. It should be noted that the actual composition of
the dulse used for human consumption in a prepared meal
can be different from that quoted in Tables 1,2, and 3above.
Depending on methods of preparation, such a soaking,
boiling, heating, marinating, roasting, etc., the amounts of
the various compounds in the seaweed material and in the
medium in which it is prepared can vary substantially. For
example, iodine can seep out during soaking, and fatty acids
can oxidize during heat treatment.
Amino acids and proteins
Dulse has a relatively high protein content, typically around
20 %, which is greater than that of foods such as chicken or
almonds. Dulse contains more protein than other seaweeds
popular for human consumption, such as konbu (S. japonica,
7 %), wakame (Undaria pinnitifada, 13 %), and winged kelp
(Alaria esculenta, 18 %) but much less than nori (Porphyra
spp., 45 %) (Mouritsen 2013;HoldtandKraan2011). The
distribution of total content of amino acids shown in Table 1
indicates that dulse contains most essential amino acids. How-
ever, it lacks tryptophan and contains possibly only marginal
amounts of cystine (Mai et al. 1994).
The large content of free glutamic acid and aspartic acid
imparts dulse and extracts of dulse witha strong umami flavor
(Mouritsen et al. 2012) making dulse an attractive component
for flavoring food. The umami capacity is comparable to the
celebrated Japanese konbu. Hence dulse in food may not only
have a gastronomical interest but also be valuable as a means
to regulate food intake and improve nutrition and health
(Mouritsen 2012b). In addition to the umami flavoring amino
acids, dulse contains substantial amounts of the sweet amino
acids proline, serine, and alanine, and bitter amino acids like
isoleucine, leucine, and valine. Altogether, the amino acid
profile in dulse imparts a more complex taste to dashi pre-
pared from dulse compared to the classical Japanese dashi
prepared from konbu (S. japonica;Mouritsen2012a,b).
Fatty acids
The total fatty acid content of dulse is typically around 1.8–
2.0 % (Morgan et al. 1980; Mishra et al. 1993) but can also
be as low as 0.2 %. We have found values ranging from 0.4
to 1.8 % (cf. Table 2). The fatty acid contents in both
samples from Maine are in the low end of this interval,
whereas all samples from Denmark and Iceland are in the
high end. It appears that in these last samples, the fatty acid
contents are significantly lower at the end of the growth
season than in samples drawn from the early harvest.
The fatty acid profiles for dulse in Table 2, showing the
percentage of the different fatty acids out of the total lipid
content, highlight a distinct feature of dulse: it is generally very
low in ω-6 and high in ω-3 fatty acids. Again, the samples from
Maine differ from those from Denmark and Iceland. EPA is the
most prominent species of the ω-3 fatty acids and, although the
total fatty acid content is low, the relative amount of EPA is
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very high in Icelandic dulse, where the ratio of ω-3/ω-6 fatty
acids is extremely large. For both the Danish and Icelandic
samples, it appears that the relative contents of EPA are lower
in samples from the late harvest, possibly due to a longer
storage period that may have procured a degradation of the
PUFA. Analysis of a Danish sample harvested in October and
analyzed before storage showed a total lipid content of about
0.7 % and an abundance of EPA in the range of about 20 %
(data not shown). This suggests that both seasonal differences
as well as sensitivity to degradation during storage affect the
contents of this valuable PUFA in dulse. We have insufficient
data to explain the differences found between the dulse from
Maine and the dulse from the two other geographical locations.
One has to go to green seaweeds, like sea lettuce
(U. lactuca), to find larger ω-3/ω-6 ratios (Holdt and Kraan
2011) compared to dulse. Dulse hence displays an extremely
favorable ratio of ω-3/ω-6 that is much higher than for most
other seaweeds. Only microalgae like Spirulina have a higher
ω-3/ω-6 ratio (Tokuşoglu and Ünal 2003), which partly is due
to high levels of DHA. Similar to other macroalgae (Holdt and
Kraan 2011), dulse has very low if any amounts of DHA but
large amounts of EPA.
Iodine
Iodine is an essential element for synthesis of two thyroid
hormones, triiodothyronine and thyroxine, that control hu-
man metabolism, growth, and brain development
(Braverman 1994). In order to maintain proper thyroid func-
tion, the human requirement is about 2 μg per day per kg
body weight, i.e., about 140 μg daily for an adult. In some
countries, iodine is added to table salt in order to assure that
the population gets sufficient daily iodine supply. Adverse
effects such as endemic goiter or cretinism are well known
in the case of iodine deficiency resulting in an underactive
gland (hypothyroidism). Less common are effects due to an
overactive thyroid caused by excessive amounts of iodine
(hyperthyroidism). Since it is the thyroid that sets the re-
quirements and limits to dietary iodine, it is the so-called
thyroid stimulating hormone level that is used to set the
upper limit of tolerance for iodine. Some has estimated this
level to be 1,100 μgday
−1
(DRI Report 2001)whereas
others state that safety limits for adults go up to 15–20 times
the recommended daily intake (FAO Report 2001).
Seaweeds as well as other seafood are a rich source in
dietary iodine. The variability of the iodine content in sea-
weeds is however enormous (van Netten et al. 2000; Teas et
al. 2004;HoldtandKraan2011) ranging over 3 orders of
magnitude from about 5 μgg
−1
in the red seaweed nori
(Porphyra spp.) to 5,000 μgg
−1
in the brown seaweed konbu
(S. japonica). In general, brown seaweeds are those species
with the highest contents of iodine. Konbu is likely to be the
main source of the extremely high intake of iodine among
Japanese, mounting to 2–3 mg day
−1
(FAO Report 2001;
Zava and Zava 2011). It is possible that the Japanese as well
as other populations exposed to a large iodine pressure in the
diet have developed an ability to secrete the excess iodine.
AsshowninTable2, we have found iodine levels in all dulse
samples investigated to be around 5 μgg
−1
or less. Hence, an
adult needs to consume about 30 g of dry dulse to fulfill the
daily requirements for iodine. Hence, dulse is not a way to
obtain enough dietary iodine. On the other hand, a typical daily
consumption of dulse, in contrast to some brown seaweeds,
does not pose any risk with regard to iodine overload.
Arsenic
The possible nutritional importance of arsenic and the details
of its metabolic function are unknown (Feldman et al. 2000).
It is important to distinguish between inorganic and organic
arsenic. Whereas authorities not always distinguish between
these forms and refer to total arsenic when issuing recommen-
dations, it is important to stress that it is arsenic in the inor-
ganic forms (trivalent arsenite and pentavalent arsenate) that is
associated with the largest potential health risk (Lewis 2007).
Upon ingestion, inorganic arsenic is quickly methylated in the
liver and most of it is excreted with the urine. Inorganic
arsenic is a well-known poison to which humans and animals
have been exposed from time to time throughout history. Its
most serious effects include encephalopathy and gastrointes-
tinal adverse conditions, possibly leading to death. Other
adverse effects include peripheral neuropathy, risk of various
cancers, and skin diseases. In contrast to inorganic arsenic,
there is no known adverse effect of organic arsenic, e.g.,
arsenoribosides and arsenobetaine (DRI Report 2001).
The average person’s total intake of arsenic is about 10–
50 μgday
−1
. Values about 1,000 μg are however not unusual
following consumption of fish or mushrooms (DRI Report
2001). Authorities have set no value for arsenic from food and
recently canceled an earlier PTWI value of 15 μgkg
−1
(WHO
2011a). Since a major daily source of arsenic in some areas
can be drinking water, some agencies have set a maximum
upper limit of arsenic in water to be 50 μgL
−1
and in some
cases even lower than 10 μgL
−1
(WHO 2011c).
ThedatareportedfordulseinTable3show that the amount
of total arsenic is 1–10 μgg
−1
. A portion of 5 g dulse will
therefore maximally contain 5–10 μg arsenic, which is below
even the strictest limit for 1 L of drinking water (WHO 2011c).
Table 3shows that the levels of inorganic arsenic in dulse
are typically 1–2 orders of magnitude less than the total
content of arsenic. The inorganic arsenic content depends on
the age of the seaweed. For example, in the case of dulse from
Maine, levels are below the detection limit (0.02 μgg
−1
)for
young dulse, whereas older specimens can contain up to
0.3 μgg
−1
. Since we have analyzed only few samples, it is
obvious that other factors than age can be important.
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Whereas arsenic is less of a concern in red algae like dulse,
it can be a major problem in some brown seaweeds. In
particular hijiki (Sargassum fusiforme), which is a staple
seaweed in the Japanese cuisine, can contain very large
amounts of arsenic, e.g., 140 μgg
−1
As of which 85 μgg
−1
is inorganic As. Although a large part of this can be extracted
in water during preparation, hijiki is forbidden to be sold as
food in a number of countries (Almela et al. 2002).
Heavy metals
The contents of heavy metals are of major concern in all food
from the ocean, including seaweeds. Seaweeds absorb rapidly
and very effectively accumulate cadmium, lead, copper, nick-
el, and mercury (Prasher et al. 2004; Dawczynski et al. 2007a;
Schäfer et al. 2009). In particular, mercury in the form of
methyl mercury is of major concern since it exhibits adverse
effects on the central nervous systems, the endocrine system,
as well as organs like the kidneys (Clarkson and Magos 2006).
Mercury is not an essential element for the biological function
of seaweeds and the contents vary significantly depending on
the pollution level of the ambient seawater. The PTWI
value for mercury is 4 μgkg
−1
(WHO 2011b), i.e., about
40 μgday
−1
for an adult. EU regulations have no entry for
seaweeds, whereas the French list stipulates <0.1 μgg
−1
.EU
regulations for food put 1.0 μgg
−1
for a variety of seafood and
0.1 μgg
−1
for food additives. According to Table 3,mercury
levels in dulse is certainly below these limits, and a standard
daily consumption of 5 g dry dulse will amount to an intake of
mercury below limits set by PTWI.
Turning to cadmium, a PTWI of 7 μgkg
−1
corresponds to
approximately 70 μgday
−1
for an adult (WHO 2011c). Again,
EU regulations have no entry for cadmium in seaweeds as
such, whereas the French list stipulates <0.5 μgg
−1
.EU
regulations for food set at 0.05–1.0 μgg
−1
for a variety of
seafood, 1.0 μgg
−1
for food additives, and 3.0 μgg
−1
for
additives consisting of dried seaweeds or products derived
from seaweeds. The levels of cadmium found for dulse in
Table 3are well within all these limits.
Finally, the French list puts <5.0 μgg
−1
for lead, whereas
EU regulations for food has 3.0 μgg
−1
for food additives.
The PTWI value for lead was earlier 25 μgkg
−1
corre-
sponding to about 25 mg day
−1
for an adult, but this value
has recently been withdrawn and no health protective value
was put forward (WHO 2011c). The levels of lead found for
dulse in Table 3are well below the old limit.
Hence, we can conclude that even in the case where an
individual takes his full 5 g dry seaweed portion a day in the
form of dulse there is no conflict with the safety limits and
regulations issued by the authorities. Of course, the caveat to
this statement is that other special food items consumed
along with the dulse may contribute to the load and the
intake of seaweeds should then be reconsidered.
Vitamin K
Vitamin K is a group of fat-soluble compounds that come in
two natural variants, vitamin K
1
and vitamin K
2
. Vitamin K
1
is synthesized in green plants and algae and K
2
in bacteria,
e.g., in the human colon. None of the two variants are
known to be toxic as such to humans even in large oral
doses of 20 mg or more (FAO Report 2001) and no adverse
effects have been reported from human consumption of
vitamin K in food or supplementation. However, since vita-
min K is a coenzyme required for the formation of blood
coagulation factors, excessive and uncontrolled intake of
vitamin K, e.g., from seaweeds in the diet, may interfere
with medication in individuals who are subject to blood-
thinning treatment, e.g., involving warfarin. Clinical studies
have shown that intermittent and smaller changes in vitamin
K intake do however not require permanent changes in the
dosing of warfarin (Bartie et al. 2001).
The adequate daily intake of vitamin K for adults is 90–
120 μg and recommended daily intakes are 200–500 μg(DRI
Report 2001). Large variations in dietary vitamin K
1
are
known to be caused by intake of fresh parsley
(1,640 μg(100g)
−1
), spinach (480 μg(100g)
−1
), water cress
(250 μg(100g)
−1
), and broccoli (103 μg(100g)
−1
)(USDA
2013). The results in Table 3from our own measurements on
dulse show that dried dulse may contain 200–700 μg
(100 g)
−1
vitamin K
1,
which corresponds to 40–140 μg
(100 g)
−1
on the basis of wet, fresh seaweed. Since a typical
daily intakeof dry dulse is maximally about 5 g,it appears that
even individuals subject to a blood-thinning medication need
not worry about a diet containing moderate amounts of dulse.
Kainic acid
Kainic acid is an amino acid (cf. Fig. 3) that belongs to a
group of compounds known as kainoids. These compounds
are neuroactive, and the most potent kanoid is the amino
acid domoic acid that can cause amnesic shellfish poisoning
(Clark et al. 1999). Whereas domoic acid has been found in
some red macroalgae (Holdt and Kraan 2011), it is predom-
inantly produced in diatoms that can accumulate in shellfish
and other marine organisms. Kainic acid has been found in a
few different macroalgae, e.g., Digenea simplex (Murakami
et al. 1953) and in some strains of P.palmata (Laycock et al.
Fig. 3 Chemical structure of kainic acid
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1989;Ramseyetal.1994;Lüning2008), where it is a
secondary metabolite. The work by Ramsey et al. (1994)
showed that some strains of P.palmata are producers of
kainic acid and 1′-hydroxykainic acid, whereas others are
nonproducers. These authors also found, somewhat surpris-
ingly, that the producing strains produced kainic acid in
concentrations that were not very sensitive to growth con-
ditions, such as temperature and nitrogen availability.
Ramsey et al. (1994) report the absence of kainic acid in
broad-leaf, wild-type fronds of dulse from the Atlantic coast
of Nova Scotia, whereas certain strains from the upper Bay
of Fundy and samples of P. palmata var. sarniensis from
Finavarre (Ireland) had large amounts, up to 4,000 μgg
-1
dry
weight. Even larger amounts (>10,000 μgg
−1
dry weight)
were detected in certain dwarf mutants of dulse from New
Brunswick.
The results of our analyses of kainic acid in dulse from
three different locations and sources (cf. Table 3)showin
accordance with the data of Ramsey et al. (1994)thatdulse
from Maine/Nova Scotia contained only sub-μgg
−1
amounts
of kainic acid. In contrast, the levels of kainic acid detected in
the wild samples from Iceland and the farmed samples from
Denmark were found to be finite but rather small, ranging
from 1 to 21 μgg
−1
(Table 3). Similar low values have been
reported by Lüning (2008) for wild-harvested dulse from
Ireland (12 μgg
−1
) and France (130 μgg
−1
) and even lower
for cultivated dulse (2.5 μgg
−1
). The very significant differ-
ence between on the one side the data reported by Ramsey et
al. and on the other side the data reported in the present paper
and in the unpublished work by Lüning suggests that theolder
work by Ramsey et al. may be effected by the method of
analysis used or the samples were contaminated by epiphytes
that produce kainic acid. It is of interest to note that the content
of kainic acid in the farmed dulse appears to be lower than in
the wild dulse of European origin.
The neurotoxic effect of kainic acid is caused by a bind-
ing of kainic acid to certain non-NMDA ionotropic kainic
receptors that are involved in excitatory neurotransmission
and synaptic plasticity (Swanson and Sakai 2009). Due to its
structural similarity to the neurotransmitter glutamate,
kainic acid is a potent glutamate agonist (Nadler 1979;
Laycock et al. 1989). Kainic acid in large doses is known
to exhibit neurotoxic effects possibly leading to brain dam-
age (Lothman and Collins 1981; Nadler et al. 1981; Coyle
1983; Strain and Tasker 1991). The neurotoxic action seems
to be related to the ability of the kainic acid ligand to
depolarize the neurons and provoke intensive neuronal fir-
ing. Animal studies with rats and mice have shown that
injection of kainic acid into the brain or intraperitoneally
can lead to seizures, hippocampal damage, and behavioral
changes. The doses used are however extremely large, typ-
ically from 4 to 32 mg kg
−1
body weight. Doses of those
orders would correspond to up to more than 2 g pure kainic
acid for an adult human being. There appears to be no
published data regarding human safety values, neither are
there any published studies relating oral intake of foodstuffs
containing kainic acid in humans to neuronal activity. In
order to reach the hazardous levels of kainic acid dosed in
the mice and rat experiments, a total amount of about 30 kg
dry dulse of the variety with the highest concentration of
kainic acid reported in Table 3need to be applied. Since it is
unlikely that a human being would consume such a large
amount of dulse in one meal and since the consumed dulse
furthermore has to pass through the gastrointestinal system
before possibly making it into the bloodstream and from
there across the blood–brain barrier, we find it unlikely that
human consumption of dulse of the varieties studied in the
present paper will present any serious danger to human
health caused by kainic acid.
Conclusion
Seaweeds have been eaten by people around the world for
centuries possibly millennia. However, in contrast to the re-
mains of bones and scales of animals and fish, seaweeds
usually leave no traces of having been used as foodstuff. Still,
there is every reason to assume that people in all coastal areas
going back to Paleolithic times have eaten whatever they could
find in the sea. The combination of fish, shellfish, and sea-
weeds has provided our ancestors with all the essential nutri-
ents they needed for sustaining life and for evolving from the
early hominids to Homo whose main characteristics is a large
brain-to-body weight ratio and a brain packed with PUFA
(Cunanne and Stewart 2010). It is interesting to note that those
nutrients believed to be most critical for the development of the
brain are ω-3 fatty acids (EPA and DHA) and five minerals:
iodine, iron, copper, zinc, and selenium. Without a sufficient
supply of these, the genetic potential for the evolution of a
large and complex brain could not be maximized (Cunnane
2005). Algae hence contain most of the required elements for
human brain evolution and development.
The oldest documented use of seaweeds for human con-
sumption derives back to 12,000 BCE from a hearth exca-
vated at Monte Verde in southern Chile (Dillehay et al.
2008). Researchers have used the finds of about 20 different
marine macroalgae, including the genera Porphyra,
Gracilaria,Sargassum,Macrocystis, and Durvillaea, to ar-
gue that this part of the South American continent was
settled by people who had arrived from the north by follow-
ing the shoreline rather than by moving inland.
It is important to keep the evolutionary background of
Homo in mind when we discuss what is the “natural”diet
for the modern human being. Genes evolve very slowly, but
the expression of the genes can change quickly in response
to environmental changes, such as the diet. There is ample
1786 J Appl Phycol (2013) 25:1777–1791
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reason to assume that the many diet-related lifestyle dis-
eases, such as heart and coronary diseases, cancer, diabetes,
obesity, and certain mental diseases that make up the burden
of illness reflect on an imbalance in our diet with respect to
the ratio, ω-3/ω-6, between ω-3 and ω-6 fatty acids
(Simopoulos 2002; Mouritsen and Crawford 2007). The
characteristic feature of fats in seafood, including seaweeds,
is that ω-3/ω-6 is close to or larger than 1. This is in strong
contrast to the modern European diet that has typically
ω-3/ω-6 values in the range of 0.1–0.2, and North American
diets with the ratio being close to 0.05 (Simopoulos 2002).
In this light, seaweeds are in some ways optimal for human
nutrition (MacArtain et al. 2007; Mouritsen 2013; Holdt and
Kraan 2011; Pereira 2012) because they not only contain
high levels of important macro and micro elements, vita-
mins, proteins, iodine, soluble, and insoluble dietary fibers,
but in particular the PUFA as listed in Table 2in the case of
dulse. Although the total lipid content of seaweeds is gen-
erally low, about 2–3 % on a dry weight basis and for dulse
typically a little less 2 %, it should be remarked that gener-
ally more than half of the lipid is made up of unsaturated
fatty acids, of which the majority are essential ω-6 and ω-3
PUFA with very long and super-unsaturated fatty acids,
specifically ARA in the ω-6 family and EPA and DHA in
the ω-3 family. Whereas some microalgae contain both EPA
and DHA, macroalgae such as dulse (Table 2) contain very
little, if any DHA. We have found no DHA of any signifi-
cance in our samples of dulse but high levels of other PUFA
and in particular EPA, the ratio ω-3/ω-6 being higher than in
most other macroalgae (Khotimchenko et al. 2002; Colom-
bo et al. 2006; Dawczynski et al. 2007b; Mouritsen 2013;
Holdt and Kraan 2011).
Dulse (P.palmata) is one of the most popular, if not the
most popular seaweed species for human consumption in
those coastal regions in the Western world that have a
tradition for consuming seaweeds, e.g., Iceland, Ireland,
Maine, Nova Scotia, and Brittany. The trend towards more
healthy and basic foodstuffs, together with an increasing
interest among chefs for the seaweed cuisine, has put
renewed focus on seaweed species that have been consumed
by humans for centuries. In this context, dulse is gaining
renewed interest, which is exposing a need for more quan-
titative knowledge about the chemical composition of this
species of relevance for human consumption.
In the present paper, we have presented data for the
composition of the red edible seaweed dulse (P.palmata).
Due to its attractive taste, dulse is one of the more interest-
ing seaweed species for human consumption as a whole
food. We have therefore focused attention on those compo-
sitional aspects that pertain to taste, nutritional value, as well
as compounds that may be undesirable. There is some data
for the chemical composition of dulse scattered in the sci-
entific literature and we have attempted to compile this data
along with the new data we have obtained. The composition
of dulse as well as other seaweeds can be very sensitive to
which strain is pertains to, where it is grown, at which
season it is harvested, and not least the composition of the
water, it has grown in, with respect to nutrients and contents
of toxic metals. The data presented in Tables 1,2, and 3are
therefore invariably subject to uncertainties and variations.
Dulse is not necessarily the best choice of seaweed in
terms of mineral and vitamin contents but, on the other
hand, it is far richer in potassium salts than in sodium salts
compared to all other seaweeds. In fact, one of the more
interesting properties of dulse from the point of view of
nutrition is its contents of sodium and potassium with a
K/Na ratio of around 4–5 (Morgan et al. 1980; Rødde et
al. 2004; MacArtain et al. 2007). In young Danish dulse, we
have found an even more favorable K/Na ratio of about 7.
In all, we have reported on data for amino acid composition,
fatty acid profile, vitamin K, iodine, kainic acid, inorganic
arsenic, as well as various heavy metals in dulse (P. palmata)
from different sources. We have discussed the contents of
potentially harmful substances and concluded that the contents
found in the specimens of dulse we have analyzed are not
likely to inflict any danger on human health.
Materials and methods
Seaweed supplies
Cultivated dulse was grown in open tanks (pools) at Blue
Food (Horsens, Denmark). The growing seaweeds were fed
with seawater and by using air turbulence to move the
seaweeds, to provide nutrition, and to facilitate photosyn-
thesis. The controlled cultivation in pools enables not only a
fouling-free quality, but also facilitates a highly red pigmen-
tation and a large protein content. The cultivated dulse was
dried in open air in sun light immediately after harvest.
Commercially available, dried dulse was supplied by
Icelandic Blue Mussel & Seaweed (Stykkishólmur, Iceland)
and by Maine Coast Sea Vegetables (Franklin, Maine, USA).
The dulse is hand-harvested from wild resources at the west-
ern coasts of Iceland and along the Atlantic coast of the Fundy
Bay (Maine and Nova Scotia), respectively. The fresh dulse
was dried immediately after harvesting.
For each geographical location, two sets of dried dulse
were studied: dulse from the early, new harvest 1 year (May)
and dulse stored under dry and dark conditions from the late
harvest of the year before (October).
Protein and amino acid analysis
The analysis procedure has been described earlier (Mouritsen
2012b). Free amino acids were extracted in water. Two types
J Appl Phycol (2013) 25:1777–1791 1787
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of water were used, ordinary tap water (water hardness=
20°dH) and filtered, demineralized soft water. All extractions
are based on 10 g dry seaweed in 500 mL water placed in a
plastic bag sealed under a vacuum (sous-vide) of 98.5 kPa in a
Komet Plus Vac 20 (Plochingen, Germany) and immersed
over a period of 45 min in a water bath at the prescribed,
constant extraction temperature. All used chemicals were
from Sigma Aldrich (Denmark) and of HPLC quality or
better. Amino acid analysis was performed on Biochrome
31 amino acid analyzer (Biochrome, UK). Prior to analysis,
proteins were precipitated by addition of trichloroacetic
acid, and lipids were extracted with hexane. The amino
acids were identified and quantified by comparison with
pure amino acid standards with major focus on glutamic
acid, aspartic acid, and alanine in the de-protonated
state.
Fatty acid analysis
Seaweeds were dried at 30 °C for 24 h and pulverized/
homogenized with a lab mill (Retsch ultra centrifugal mill, ZM
100, Haan, Germany). Dry weight refers solely to this method.
Lipids for FA analysis were extracted using a methanol/
chloroform/water mixture of 1:2:1 (according to Folch et al.
1957). For the preparation of fatty acid methyl esters (FAME),
sodium methylate (NaOCH
3
; Acros: 419605000) was used.
The FAME was purified by means of thin layer chromatogra-
phy (using hexane/diethyl ether/acetic acid (85:15:0.2, v/v/v)).
The quantification of the fatty acids was realized using a gas
chromatograph (GC-17 V3; Shimadzu, Japan) equipped with
an auto sampler, automatic injection system (AOC-5000), and
flame ionization detector. Analyses were performed using a
fused silica capillary column (DB-225 MS from Agilent
Technologies, USA, 60 m×0.25 mm internal diameter,
0.25 μm film thickness) and H
2
as carrier gas. This
column was suitable to receive a successful separation
of FAME ranging from C4 to C22 (including straight
and branched structures) in a time-saving manner. FA
concentrations were expressed as percentage of the total
area of all FA peaks (percentage of total FAME).
Iodine and heavy elemental analyses
The analysis for I, Cd, Hg, and total As was performed at a
certified laboratory (Kolbe, Mülheim a. d. Ruhr, Germany)
based on samples of dried dulse.
Inorganic arsenic analysis
Inorganic arsenic was assessed by the protocol GMUAS01.01
in the case of dulse from Iceland, Denmark, and Maine. The
measurements were performed at an authorized laboratory at
the Danish Veterinary and Food Administration, Aarhus,
Denmark. The detection limit of this protocol is 0.03 μgg
−1
.
Added to the sample was 0.07 M HCl/3 % H
2
O
2
. During this
process, As(III) is oxidized to As(V) and hence all inorganic
arsenic in the sample is now to be found as As(V). The total
inorganic arsenic is extracted in a microwave oven. The
quantitative amount of inorganic arsenic is determined by
chromatographic separation from other arsenic species by
HPLC followed by detection using ICP-MS.
The data for the inorganic arsenic in dulse from Maine
was also assessed by the U.S. Environmental Protection
Agency EPA 1632 testing protocol that has a detection limit
of 0.024 μgg
−1
(Shep Erhart, private communication).
Vitamin K analysis
The analysis for vitamin K (K
1
, phylloquinone) was
performed at a certified laboratory (DB Lab, Odense,
Denmark). Dried or fresh wet dulse was finely chopped,
freeze-dried, and extracted to n-hexane. Hexane was evap-
orated and the solid residue was dissolved in a mixture of
tetrahydrofuran and ethanol. Vitamin K was separated from
this mixture by HPLC on a reverse-phase C18 column and
detected at 248 nm.
Kainic acid analysis
Kainic acid standard Solid kainic acid monohydrate
(2.2722 mg) was diluted in 0.5 mL methanol. An aliquot
of 10 μL of the homogenous solution (0.045 mg) was
diluted in 1.0 mL methanol and 100 μLwereusedfor
silylation as described below.
Seaweed extraction The extraction of the dried dulse samples
was performed according to Pleasance et al. (1990). About
2.5 g of dried seaweed (Denmark, 2.54 g; Iceland, 2.67;
Maine/USA, 2.63 g) was cut in pieces. About 30 mL methanol
was added and the solution was heated and refluxed for 5 min.
After cooling, the supernatant was transferred to a 25-mL
flask. The residue was refluxed with ∼30 mL water. After
cooling, the water extract was also transferred to the 25-mL
flask. Methanol extracts were greenish (indicative of chloro-
phyll) while aqueous extracts were brownish and foaming up
(indicative of polysaccharides).
Cartridge clean up The C
18
cartridges (Strata C18-E, 55 μm,
70 Å, 500 mg/3 mL, Phenomenex) were preconditioned with
5 mL methanol followed by 5 mL water. Then, 1 mL
methanolic extract was placed on the column and eluted with
5 mL methanol/water (1:9, v/v). The eluate was collected and
evaporated to dryness at 90 °C. The residue was dissolved
with 1 mL methanol, ultrasonicated, heated to ∼40 °C and
finally membrane filtered. One hundred microliter of the
pellucid solution was used for silylation.
1788 J Appl Phycol (2013) 25:1777–1791
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Silylation The aliquot (100 μL) was evaporated to dryness,
50 μLN,O-bis(trimethylsilyl) trifluoroacetamide:
trimethylchlorosilane 99:1 and 50 μL pyridine were added.
The derivatization reaction was performed for 30 min at
70 °C. After cooling, 400 μLn-hexane and 5 μL(0.025mg)
5α-cholestane (syringe standard) were added, the vial was
shaken and used for GC/MS measurements.
GC/EI-MS determination of kainic acid: A 5890 series II
gas chromatograph connected to a 5972 mass selective
detector (Hewlett-Packard/Agilent, Germany) was
used for sample analysis. A fused-silica capillary column
coated with DB-5MS (phenyl arylene polymer virtually
equivalent to a (5 %-phenyl)-methylpolysiloxane)
(30 m×0.25 mm internal diameter, 0.25 μm film thickness,
Hewlett-Packard/Agilent) was installed in the GC oven.
Injector and transfer line temperature was set at 250 and
300 °C, respectively. Helium (99.9990 %, Westfalengas
Münster/Germany) was used as the carrier gas in constant flow
mode (1 mL min
−1
). The oven program started at 50 °C. After
1 min hold time, the oven was heated at 20 °C/min to 180 °C,
3 °C/min to 210 °C, and finally at 20 °C/min to 300 °C
(10 min). The measurements started after a solvent delay of
5min.Quantificationwasperformedintheselectedionmon-
itoring mode. From 5.0 to 18.0 min, we recorded m/z73, m/z
122, m/z312, m/z313, m/z386, and m/z429. From 18.0 min
until the end of the run, we recorded m/z73, m/z217, m/z218,
m/z357, m/z372, and m/z373. Determination of kainic acid
was based on m/z312 (quantification ion) and m/z313 (veri-
fication ion). The syringe standard 5α-cholestane was deter-
mined with m/z217 (quantification ion) and m/z218
(verification ion). The limit of detection of kainic acid was
about 60 pg.
Acknowledgments Mariela Johansen is gratefully acknowledged
for translation of OGM’s Danish book on seaweeds into English.
Rasmus Bjerregaard (Blue Food) is thanked for supplying speci-
mens of farmed dulse. Shep Erhard (Maine Coast Sea Vegetables)
has generously made information available regarding chemical
composition of dulse from Maine and provided samples for anal-
ysis. Símon Sturluson (Icelandic Blue Mussel & Seaweed) is
thanked for supplying samples of wild Icelandic dulse. Lars
Williams (Nordic Food Lab and Restaurant Noma) performed
some of the aqueous extracts of dulse. Eyjólfur Friðgeirsson
(Íslensk hollusta ehf) is acknowledged for correspondence regard-
ing dulse (søl) in Iceland. Susan Løvstad Holdt (The Danish
Seaweed Network) is thanked for useful references. Poul Erik
Nielsen (Gourmettang) is acknowledged for information on the
composition of French seaweed products. Inge Rokkjær (Danish
Veterinary and Food Administration, Aarhus, Denmark) is thanked
for performing the analyses for inorganic arsenic. Helpful corre-
spondence with Dr. Dorthe Dideriksen (Odense University Hospi-
tal) on pharmacological effects of kainic acids is gratefully
acknowledged. Mette Rindom Nørrelykke is thanked for giving
us access to some unpublished data for fatty acid contents of
Danish dulse. MEMPHYS Center for Biomembrane Physics is
supported by the Danish National Research Foundation. This
work was supported by grants from the Danish Food Industry
Agency (J.nr. 3414-09-02518) and from Lundbeckfonden.
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