A Little Spit Goes a Long Way: Exploring The Chemistry Behind Saliva’s Cleaning Abilities

Have you ever used your own saliva to clean things, even if it felt a bit yucky? Well, this article hopes to relieve you from the disgust, and show you how your spit could actually outclean pre-existing soap. Now if you thought science was all about being nerdy, you should think twice. 

The Ig Nobel Prize - What exactly is it? 

The word ‘Ig Nobel’ is a wordplay mixing up ‘ignoble’, meaning dishonourable or unworthy, and the infamous Nobel Prize. Ignoble prize’s history begins all the way back in 1991, in the scientific humour magazine AIR (Annals of Improbable Research) which aims to “honour achievements that first makes people laugh, and then think”. It was initially awarded in a lecture hall at the Massachusetts Institute of Technology, but moved to Harvard university during 1994, and finally returned to MIT after covid. Besides a silly handmade trophy, the winners are also awarded a banknote of 10 whole trillion dollars… but in Zimbabwean currency, which is around US$0.40. 

“The Stinker”, official mascot of the Ig Nobel Prizes (Improbable Research)

Background

Now, the spotlight of this article is on Paula Romão, Adília Alarcão and the late César Viana, of Portugal, who has been awarded the 2018 Ig Nobel Chemistry award. Their research question measured and spoke of the degree to which human saliva is a good cleaning agent for dirty surfaces. This query has originated from conservators - people who repair and preserve art - using saliva from centuries ago to clean delicate surfaces including gold leaves, ceramic and paintings to avoid damage. Their three test criteria for evaluation were no penetration into lower layers of paint, no partial dissolving of the pigments and good cleaning efficiency.

Method

Romão and his team have collected data through wiping different surfaces with cotton swabs wetted with multiple substances including: saliva, ethanol, xylol (used in paints/varnish), 2-methylheptane (used in fuel application) and aqueous ammonia (used in cleaning). 

Results

Key: 

+: Criteria completely met

o: Criteria partially met

-: Criteria not met

	Colour	Pigment	Saliva	2-Methylheptane	Xylol	Ethanol
Oil paint	Black	Charcoal	+	o	o	o
Flesh tones	White lead/ochre	+	-	-	-
Green	Malachite	+	-	-	-
Red	Vermillion	+	o	-	+
Tempera paint	Blue	Azurite	o	o	+	+
Brown	Ochre	+	o	o	o
Red	Vermillion	o	+	o	o
White	White lead	+	-	o	+

The Magazine of Chemistry Europe

Results - Cleaning gold leaf surfaces

Gustav Klimt, ‘Adele Bloch-Bauer I'

(1907) (Photo: Neue Galerie New York via Wikimedia Commons)

Key:

2-MH: 2-Methylheptane

NH3: Ammonia

H2O: Water

Saliva	2-MH	Xylol	Ethanol	2-MH/Xylol 2:1	2-MH/Xylol 3:1	NH3/H2O 1:1	NH3/H2O 1:3	NH3/H2O 1:5
+	o	o	+	+	o	-	-	o

The Magazine of Chemistry Europe

Conclusion

Surprisingly, their results showed that saliva was actually the best cleaning agent, passing all three criteria most of the time. 

Science behind it

Now we have proved that human saliva was the most effective cleaning solvent. So what’s the explanation for this? 

First, we can evaluate the physical properties of saliva compared to water and other substances. Saliva has a lower surface tension of 45mN/m compared to water (70mN/m), therefore being able to spread and wet tiny cracks and pores. Also, saliva is viscous and thicker than water, so it does not soak too deeply into the paint layers, reducing the risks of rapid drying, swelling and lifting of the paint layers. (A. V. Nieuw Amerongen et al., Glycobiology 1995, 5, 733.)

Then, we can use various scientific methods to analyze human saliva’s chemical properties. Romão and his team first heated fresh saliva up to 80°C and found out that it lost its cleaning abilities, suggesting protein denaturation (IGCSE Biology unit 5 - ask your teachers!). They used chromatography to analyze the different substances, especially proteins that exist in human saliva to identify which chemicals were actively cleaning. Then, they have found out that only a fraction of solution containing α‐amylases acted as a cleaner, further proving that cleaning with saliva is an effective chemical process of enzymatic degradation of the dirt layer with a side of mechanical degradation using cotton swabs. (N. Ramasubbu, V. Paloth, Y. Luo, G. D. Brayer, M. J. Levine, Structure of human salivary alpha-amylase at 1.6 A resolution: implications for its role in the oral cavity, Acta Cryst. 1996, 52 Sect. D, 435.)