Chemistry with Spit and Polish

Research projects that win the IgNobel Prize are meant to first make us laugh, and then think. This principle is definitely demonstrated by the winners of the 2018 prize for chemistry: Paula Romão, Adilia Alarcão, and César Viana observed that for generations, conservators have preferred to clean the surfaces of artworks with saliva, so they set out to find scientific support for this process. The thought of such a research project probably first makes you smile; but if saliva really does have cleaning power, we should figure out what the chemical reason for this might be.

1 The IgNobel Prize

The IgNobel Prize is awarded every year in the Sanders Theater at Harvard University in Cambridge, Massachusetts, USA. The name “IgNobel” is a play on the homophone ignoble, in which the prefix “ig” converts the adjective “noble” to its opposite.

Winners in ten different disciplines are selected by the journal, Annals of Improbable Research. The American award ceremony is not as stiff and elaborate as its Swedish counterpart, and time flies as paper airplane tossing, musical performances, and diverse short presentations raise the spirits of the 1500 audience members.

In contrast to the colourful proceedings, the prize itself is modest, consisting merely of a certificate and a hand-made trophy that is newly designed every year and made from cheap materials. The prize value of 100 trillion dollars is attention-grabbing at first glance, but close examination reveals that it is not in US dollars, but in Zimbabwe dollars, with a total value of barely 3 Euros [2]. As of October 1, 2015, the Zimbabwe dollar is no longer legal currency, having been replaced by dollar bond notes.

2 Research by the Winner of the 2018 IgNobel Prize in Chemistry

The award-winning research project of 2018 started with the observation that conservators have been preferentially using their own saliva to clean gold-leaf works and oil paintings for generations. The chemists tested this process on five gilded and painted sculptures of the 18th century. The surfaces were carefully wiped with cotton swabs wetted with either saliva or a different solvent. The solvents tested on the oil and tempera paint surfaces were saliva, ethanol, xylol, and 2-methylheptane (iso-octane), the gold-leaf surfaces were additionally swabbed with aqueous ammonia (see Tab. 1) [3].

Table 1. Surface cleaning of art works with saliva and other solvents. (“+” = Criteria met: no penetration into deeper paint layers, no dissolving of pigment, good cleaning power. “o” = Criteria partially met. “-“ = Criteria not met.)

	Color	Pigment	Formula	Saliva	2-Methylheptane	Xylene	Ethanol
Oil paint	black	charcoal	C	+	o	o	o
	flesh tones	white lead /ochre	(PbCO₃)₂•Pb(OH)₂ + Fe₂O₃	+	–	–	–
	green	malachite	Cu₂CO₃(OH)₂	+	–	–	–
	red	vermillion	HgS	+	o	–	+

Tempera paint	blue	azurite	(CuCO₃)₂• Cu(OH)₂	o	o	+	+
	brown	ochre	Fe₂O₃	+	o	o	o
	red	vermillion	HgS	o	+	o	o
	white	white lead	(PbCO₃)₂•Pb(OH)₂	+	–	o	+

Cleaning of Gold Leaf Surfaces

Saliva	2-Methylheptane	Xylene	Ethanol	2-Methyl-heptane/Xylene 2:1	2-Melhyl-heptane/Xylene 3:1	NH₃/H₂0 1:1	NH₃/H₂O 1:3	NH₃ /H₂O 1:5
+	o	o	+	+	o	–	–	o

Experienced conservators use three criteria to evaluate the various solvents:

No penetration into lower layers of paint
No partial dissolving of the pigments
Good cleaning efficiency

The findings of conservators were confirmed. In this comparison, saliva is the best cleaning agent. Only vermillion (cinnabar, mercury sulphide), and azurite blue (basic copper carbonate) are partially dissolved in matte tempera paint layers. In tempera paints, the pigments are suspended in a water/oil emulsion stabilized with egg or a different binder.

If saliva really is so good at cleaning, one thing is clear: There is chemistry behind it. Let’s get to the bottom of it.

3 Human Saliva as a Solvent

Our saliva glands produce 0.5 to 1.5 L of saliva daily. Without our noticing, the saliva carries out many tasks in our body. These include moistening our mouth and throat, buffering the oral pH value, making bites of food easier to swallow, protecting our teeth from demineralization [4], and fending off infections through antibacterial activity. In addition, saliva significantly contributes to the sensation of flavour by dissolving the flavourful components out of our food and delivering them to our taste buds as we chew [5]. This broad spectrum of tasks is reflected in the complex chemical composition of saliva [6–11] (see Tab. 2).

Table 2. Composition of human saliva [6,7].

Inorganic Components	Saliva
Water	94 %
pH	5.75–7.05
Hydrogen carbonate	15–80 mmol/L
Phosphate	4 mmol/L
Sodium	20–80 mmol/L
Chloride	30–100 mmol/L
Potassium	20 mmol/L
Calcium	3–4 mmol/L

Organic Components	Saliva
Urea	120–200 mg/L
Glucose	10 mg/L
Lactate	0.3–1.8 mmol/L
Lipids	10–100 mg/L
Fatty acids
Triglycerides
Glycolipids
Cholesteryl esters
Phospholipids
(Glyco)proteins	0.5–3.0 g/L
Mucins
Immunoglobulins
Proline-rich glycoproteins
Lysozyme
Lactoferrin
α-Amylase
Histatin

Some of the superior cleaning power of saliva relative to water is due to its lower surface tension of 45 mN/m (water’s is 70 mN/m). Saliva’s high viscosity is also an advantage, preventing rapid drying and deeper penetration into the paint layer. This is why conservators see saliva as less “aggressive” than water. The high viscosity results from glycoproteins known as mucins (lat. Mucus = mucous) [12,13].

Hydrogen carbonate and phosphate have been excluded from the list of compounds in saliva that have cleaning powers, as their removal by dialysis does not reduce the cleaning activity of the saliva.

However, if fresh saliva is briefly heated to 80 °C, it immediately loses its cleaning effect, which suggests protein denaturation. After chromatographic separation of the proteins in saliva, only the fraction containing α‐amylases acted as a cleaner. This result was confirmed by the fact that aqueous extracts of other α‐amylase sources, including baker’s yeast, potatoes (it is said that oil painting can be cleaned by careful rubbing with freshly sliced potatoes), and Bacillus subtilis have cleaning powers comparable to human saliva.

Let’s take a closer look at the α‐amylases found in human saliva.

3.1 Our α‐Amylases

Human saliva α‐amylase consists of 496 amino acids [14]. α‐Amylases catalyse the stepwise hydrolytic splitting of starch to form maltoriose and maltose (see Fig. 1). This reaction is exergonic and occurs spontaneously in the presence of these enzymes.

Figure 1. Degradation of starch with saliva α‐amylase. [Image: fvasconsellos1SMD (PDB) wikimedia commons: public domain].

The catalytic activity of the α‐amylases in our saliva can be confirmed in a simple self-experiment. A bite of soft white wheat bread (made of flour, salt, yeast, and water) is chewed in the mouth at least 30 times, allowing it to be mixed with saliva. The relatively high concentration of about 50 mg of α‐amylase in 100 mL of saliva is enough to allow us to detect the sweet degradation products by their taste.

In the stomach, low pH values immediately denature the α‐amylases, however, the enzyme remains active for a few minutes on the interior of larger, rapidly swallowed bites, continuing this breakdown of starch, which is known as predigestion. Starch is only fully broken down into glucose once it reaches the small intestine. The α‐amylases required are secreted there by the pancreas.

3.2 Other Saliva Components with Cleaning Powers

Analysis of the dirt removed from art works indicates that there are other active cleaning agents in saliva: Free amino acids are obtained from proteins by proteases, free long-chain fatty acids are released from fats and phospholipids by lipases. Cleaning with saliva is thus a combination of mechanical (polish) and enzymatic degradation of the dirt layer with a significant use of cotton swabs, and a lot of fresh saliva (spit).