Journal of Ethnopharmacology 145 (2013) 378–380
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Journal of Ethnopharmacology
journal homepage: www.elsevier.com/locate/jep
Ethnopharmacological communication
Edible bird’s nests—How do the red ones get red?
Paul Pui-Hay But a,n, Ren-Wang Jiang a,b, Pang-Chui Shaw a,c
a
b
c
School of Life Sciences, The Chinese University of Hong Kong, Tai Po Road, Shatin, N.T., Hong Kong, PR China
Guangdong Province Key Laboratory of Pharmacodynamic Constituents of Traditional Chinese Medicine and New Drugs Research, Jinan University, Guangzhou, PR China
Institute of Chinese Medicine, The Chinese University of Hong Kong, Tai Po Road, Shatin, N.T., Hong Kong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2012
Received in revised form
12 October 2012
Accepted 25 October 2012
Available online 7 November 2012
Ethnopharmacological relevance: Red edible bird’s nests are regarded as of higher beneficial value for
health and hence fetch a higher price than the white ones. Their red colour remains a myth.
Aim of the study: To determine if white edible bird’s nests can turn red by vapours generated from
sodium nitrite in acidic conditions and by vapours from ‘bird soil’.
Materials and methods: White edible bird’s nests were exposed to vapours from sodium nitrite
dissolved in 2% HCl or from ‘bird soil’ in hot and humid conditions.
Conclusions: Vapours from sodium nitrite dissolved in 2% HCl or from ‘bird soil’ containing guano
droppings from swiftlet houses were able to turn white edible bird’s nests red. The reddening agent in
‘bird soil’ was water-soluble and heat-stable. The red colour of edible bird’s nests is likely caused by the
environmental factors in cave interiors and swiftlet houses.
& 2012 Elsevier Ireland Ltd. All rights reserved.
Keywords:
Edible bird’s nest
Cubilose
Colour
Nitrite
Swiftlet
1. Introduction
Edible bird’s nests (EBNs) or cubiloses are treasured as a health
tonic and gastronomic delicacy in Chinese cuisine. Recent research
suggests that EBNs and EBN extracts have interesting bioactive
properties (Ng et al., 1986; Kong et al., 1987; Guo et al., 2006;
Yagi et al., 2008; Aswir and Wan Nazaimoon, 2011; Abidin et al.,
2011; Matsukawa et al., 2011; Vimala et al., 2011; Roh et al., 2012;
Zhang et al., 2012).
EBNs are produced by certain swiftlets of the genera Aerodramus,
Apus and Collocalia (Marshall and Folley, 1956; Kong et al., 1989; Lim
and Cranbrook, 2002). They secrete a ‘nest cement’ from salivary
glands for the construction of their nests with or without binding
together with other materials. The nest cement is soft and sticky
when fresh, but gradually dries and hardens when exposed to air (Lim
and Cranbrook, 2002). The major component in nest cement is sialic
acid-rich glycoproteins (Kathan and Weeks, 1969).
Typically, EBNs are dull white or dirty white to somewhat dull
yellowish. Occasionally, in caves and swiftlet houses, there are
some samples that are partially or completely dull orange red to
brownish red (Lim and Cranbrook, 2002). These red EBNs are
called Xueyan or Xueyanwo [literally blood swiftlet or blood
swiftlet nest] in Chinese or red ‘blood’ nests in the West
(Marcone, 2005). Red EBNs are traditionally promoted as having
higher beneficial values to health. In the market, red EBNs fetch a
n
Corresponding author. Tel.: þ852 9366 0328; fax: þ852 2603 5646.
E-mail address: [email protected] (P.P.-H. But).
0378-8741/$ - see front matter & 2012 Elsevier Ireland Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jep.2012.10.050
higher price than the white ones. Marcone (2005) described them
as ‘‘premium red ‘blood’ nests’’ as compared to the white ones
which are regarded as of ‘lower’ grade.
The origin of red colour in EBNs has been a mystery. A century-old
legend claimed that the red colour came from the blood in the saliva
of exhausted swiftlets hurrying to finish their nests before laying
eggs. Other fabrications associate the red colour with oxidation of iron
seeping along with cave drippings into the nests or with the seaweeds
and mollusks taken by the swiftlets, which actually are insectivorous.
A fourth interpretation goes to artificial dyes. A visit to Indonesia by
the first author, however, discovered that large quantities of red EBNs
are the product of exposing white EBNs to vapours from ‘bird soil’
(swiftlet guano droppings gathered from swiftlet caves or houses).
There, EBN traders collect white EBNs from caves and swiftlet houses,
moisten and place them at room temperature on trays kept in closed
containers partly filled with moist ‘bird soil’ and an interior temperature of 40–50 1C. Under such hot and humid conditions, the EBNs
would turn red in 1–4 weeks in the presence of vapours from ‘bird
soil’. To follow up on this lead, we check if sodium nitrite and swiftlet
guano droppings play any role in turning white EBNs red.
2. Materials and methods
2.1. Samples and chemicals
Cleaned white EBNs (3.5–4 g each) from Indonesian swiftlet
houses were obtained from a local chain-store of edible bird’s
nests. ‘Bird soil’ was obtained from a supplier in Jakarta, Indonesia.
P.P.-H. But et al. / Journal of Ethnopharmacology 145 (2013) 378–380
Sodium nitrite and hydrochloric acid were from Sigma-Aldrich and
Riedel-de Haen, respectively. Wide-mouth glass jars (9 9 12 cm3)
were from a local departmental store.
2.2. Effect of sodium nitrite
Sodium nitrite (0.5 g) and a piece of support (5 5 3 cm3)
were placed on the bottom inside of a jar. Distilled water or 2% HCl
was added to a depth of 1 cm in the jar. A piece of white EBN was
moistened with distilled water and then placed on the support. A
similar set up was made with 2% HCl but without sodium nitrite.
The jars were properly sealed and placed in dark at room
temperature (22–25 1C). Colour changes were monitored for 48 h.
379
The jars with 2% HCl and sodium nitrite released air bubbles
shortly after mixing them together. The EBNs in these jars also
displayed colour changes, turning strong yellow in 6 h and orange
brown in 48 h (Fig. 1). EBNs fumigated by 2% HCl alone and by
sodium nitrite dissolved in water remained whitish.
It is known that sodium nitrite would liberate nitric oxide,
which would combine with myoglobin, the pigment responsible
for the natural red colour of uncured meat. They form nitric oxide
myoglobin, which is deep red. EBNs, however, are not known to
contain myoglobin. This finding demonstrates that nitric oxide
could turn glycoproteins red. The actual mechanism deserves
further research and may offer new approaches for the food
industry.
2.3. Effect of ‘bird soil’ and sodium nitrite
3.2. Effect of ‘bird soil’ and sodium nitrite
3 kg of ‘bird soil’ were thoroughly mixed and cleaned of feathers,
rocks and vegetative materials. One-third of the ‘bird soil’ was soaked
in 10-folds of distilled water by volume for 4 h twice. The water was
drained off, and the washed ‘bird soil’ was kept in 4 1C until use.
Another one-third of the ‘bird soil’ was placed in an oven at 120 1C for
5 h and then kept in an air-tight box. These two batches were labelled
as ‘washed bird soil’ and ‘heated bird soil’, respectively. The remaining
one-third of the ‘bird soil’ was labelled as ‘untreated bird soil’. Each
batch of ‘bird soil’ was used to fill three glass jars to about 4 cm tall
each. Distilled water was added to keep the ‘bird soil’ thoroughly wet
but not soggy. A piece of support (5 5 3 cm3) was placed on the
soil. A piece of white EBN, wetted with distilled water, was placed on
the support. The jars were properly sealed. Some washed ‘bird soil’
was mixed with sodium nitrite, and this ‘nitrite-enriched bird soil’
filled a fourth set of three jars to the same standard as the other three
sets. The jars were placed in a foam box, filled with water to twothirds the height of glass jars so as to keep the temperature inside the
box uniform at around 50 1C. The box was covered to keep the jars in
darkness and placed in a cast iron chest bearing a light bulb of 200 W
on the lower side of the lid. Temperature inside the foam box was
monitored by a digital thermometer.
‘Bird soil’ is used in Indonesia for fumigating EBNs and turning
them red under dark, enclosed, warm and humid conditions.
These conditions may possibly imitate the conditions deep in cave
interiors where swiftlets rely on echolocation to enable them to
3. Results
3.1. Effect of sodium nitrite
In this experimental design, only vapours from water, sodium
nitrite and HCl could reach the EBNs sitting on the support.
Fig. 2. Photo showing edible bird’s nests after exposure to vapours from ‘bird soil’
for 16 days except specified: clockwise from left: (a) original EBN (0 days),
(b) ‘heated bird soil’, (c) ‘washed bird soil’, (d) ‘nitrite-enriched bird soil’ (7 days),
and (e) ‘untreated bird soil’.
Fig. 1. Photo showing glass jars and EBNs after 48 h of fumigation by (from left to right): (a) sodium nitrite in distilled water, (b) sodium nitrite in 2% HCl, and (c) 2% HCl.
380
P.P.-H. But et al. / Journal of Ethnopharmacology 145 (2013) 378–380
penetrate, orientate and navigate in total darkness (Lim and
Cranbrook, 2002). Our experiments aimed to determine if (a)
‘bird soil’ could turn EBNs red, (b) reddening agents in ‘bird soil’
was water-soluble, (c) reddening agent in ‘bird soil’ was heatlabile, and (d) sodium nitrite added to ‘washed bird soil’ would
recover or enhance the reddening effect.
Our results revealed that the EBNs in both ‘untreated bird soil’
and ‘heated bird soil’ jars turned bright yellow to orange on day 7 of
fumigation and brownish red on day 16. The EBNs in the jars of
‘nitrite-enriched bird soil’, however, had already turned brownish
red on day 7. On the other hand, the EBNs exposed to vapours from
‘washed bird soil’ turned weakly yellow on day 16 (Fig. 2). These
results indicate that ‘bird soil’ could definitely induce a colour
change in EBNs. The reddening agent in ‘bird soil’ is water-soluble
but not ‘heat-labile’. Addition of sodium nitrite in washed ‘bird soil’
could bring back the reddening property. Apparently, the reddening
agent in ‘bird soil’ bears resemblance to nitrites, which may also
account for the high contents of nitrites in red EBNs.
4. Discussion
EBNs are regarded as ‘‘Caviar of the East’’ (Marcone, 2005) and
‘‘undoubtedly one of the most expensive foods per unit weight in
the world’’ (Lim and Cranbrook, 2002). It is not surprising to find
various adulterants including karaya gum, red seaweed, Tremella
fungus, pork rind and egg white as imitations of EBNs or for
adding weight to EBNs (Marcone 2005; Lin et al., 2009; Wu et al.,
2007, 2010). However, little attention has been paid to red EBNs.
Some recent queries consider red EBNs artifacts, while some
others consider them imitations of nature. Marcone (2005), on
the other hand, regarded them as ‘‘premium ‘red blood’ nests’’ in
contrast to the ‘lower’ grade white nests. Here, red EBNs can be
produced by vapours generated from ‘bird soil’, or more specifically, water-soluble but heat-stable components liberated from
‘bird soil’. Moreover, sodium nitrite is able to turn white EBNs red
in weakly acidic conditions, but the mechanism of this colourchange awaits further investigation. The results also suggest that
red EBNs in cave interiors or swiftlet houses are very likely the
results of environmental conditions therein.
Another reflection from this study is that ethnopharmacological studies have convincingly helped document the claimed
values and identify the bioactive components of indigenous
materials developed through empirical knowledge gained down
the ages. The contributions of scientific studies to clarify misconceptions, refute unreliable claims and reject faked products
are equally important. The myth of the red colour of EBNs has
perpetuated for over a century in Chinese communities and many
consumers believe in claims of their supposedly higher values to
health and thus pay a higher price for them. Simple experiments
can equally offer sound findings for refining understandings of
ethnopharmacological products.
With our findings about the origin of the red colour in red EBNs,
further research is expected on the chemical nature and mechanisms of the colour changes, especially when the colour modification
in glycoproteins may have good applications in the food industry.
Pharmacological and clinical studies would need to document if red
EBNs have stronger health benefits. A simple starting point is to
compare the amounts of sialic acids and proliferative strengths
between white and red EBNs. Management of the environmental
conditions in swiftlet houses, with an objective of either enhancing
or preventing colour changes in EBNs, can also take benefits from
such studies.
Acknowledgements
Partial support was received with gratitude from Hing Kee Java
Edible Bird’s Nest Co. Ltd. Advice from Dr. Lim Chan Koon is
deeply appreciated. Encouragement from Kent Hau and Wilson
Ng is deeply appreciated.
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