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,ca School of Life Sciences, The Chinese University of Hong Kong, Tai Po Road, Shatin, N.T., Hong Kong, PR China
b Guangdong Province Key Laboratory of Pharmacodynamic Constituents of Traditional Chinese Medicine and New Drugs Research, Jinan University, Guangzhou, PR China
c Institute of Chinese Medicine, The Chinese University of Hong Kong, Tai Po Road, Shatin, N.T., Hong Kong, PR China
a b s t r a c t
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 of edible bird’s nests is likely caused by the
environmental factors in cave interiors and swiftlet houses.
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 hardenswhen 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 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 amystery. 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 colourwith oxidation of iron
seeping alongwith cave drippings into the nests orwith 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 fromcaves 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. 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.
2.3. 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 200W on the lower side of the lid. Temperature inside the foam box was monitored by a digital thermometer.
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. 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.
3.2. Effect of ‘bird soil’ and sodium nitrite
‘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 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.
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’. 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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