Allowing you to eat is the most obvious but far from the only favor granted by saliva. Silletti removes a bottle of wine vinegar from a tote bag. With a dropper, she squirts some on my tongue. “Do you feel it? Saliva is coming in the mouth to dilute the acid.” It’s as though I’d taken a sip of tepid water. “The communication between the brain and mouth,” says Silletti with infectious wonder. “It’s so fast!”
Vinegar, cola, citrus juice, wine, all are in the acid range of the pH scale: from around pH 2 to 3. Anything under pH 4 will dissolve calcium phosphate, a key component of tooth enamel. The process is called demineralization. Take a drink of anything acid, and if you are paying attention, you will notice a sudden warm slosh: parotid saliva arriving like the cavalry to bring the pH back up to the safe zone. Earlier, Silletti paged through a Dutch-language textbook on saliva (speeksel) to show me close-up photographs of teeth belonging to dry-mouthed people—those with Sj?gren’s syndrome or whose salivary glands have been damaged by radiation treatments. “It’s really shocking,” she said, and it was: gaping brown lesions all along the gum line. “Their teeth are so soft that they cannot even eat properly.”
Sugar contributes to tooth decay only indirectly. Like humans, bacteria are fond of it. “Bacteria get all crazy—party, party—they metabolize the sugar, break it down, and they release their metabolites, and these are acid” (though not as acid as cola or wine). In other words, sugar itself doesn’t cause cavities; it’s the acidic metabolites of the bacteria that feed on the sugar. As with acidic foods, saliva dilutes the acid and brings the mouth back to a neutral pH.
You may be wondering, though probably not, why newborns—who have no teeth to protect—produce excessive volumes of drool. Silletti has answers. One is simple mechanics. “They lack teeth to physically keep it in there.” Your lower incisors are a seawall holding back the salivary tide. The other reason is the newborn’s high-fat, 100 percent whole-milk diet. Baby saliva—so cute!—contains extra lipase, an enzyme that breaks down fats. (Adults have lipase mainly in their intestines.) More saliva means more lipase. As babies move on to a more varied diet, the salivary lipase tapers off.
The main digestive enzyme in stimulated saliva—everyone’s, regardless of age—is amylase. In Silletti’s dancing Italian accent it sounds like a liqueur or a European ingénue. Amylase breaks starches down into simple sugars that the body can use. You can taste this happening when you chew bread. A sweet taste materializes as your saliva mixes with the starch. Add a drop of saliva to a spoonful of custard, and within seconds it pours like water.
This suggests that saliva—or better yet, infant drool—could be used to pretreat food stains. Laundry detergents boast about the enzymes they contain. Are these literally digestive enzymes? I sent an e-mail to the American Cleaning Institute, which sounds like a cutting-edge research facility but is really just a trade group formerly and less spiffily known as the Soap and Detergent Association.
With no detectable appreciation for the irony of what he had written, press person Brian Sansoni referred me to a chemist named Luis Spitz. And when Dr. Spitz replied, “Sorry, I only know soap-related subjects,” Sansoni—still without a trace of glee—gave me the phone number of a detergent industry consultant named Keith Grime.
When I’d composed myself sufficiently, I put in a call to Grime. The answer is yes. Higher-end detergents contain at least three digestive enzymes: amylase to break down starchy stains, protease for proteins, and lipase for greasy stains (not just edible fats but body oils like sebum). Laundry detergent is essentially a digestive tract in a box. Ditto dishwashing detergent: protease and lipase eat the food your dinner guests didn’t.
Credit for the idea of using digestive enzymes for cleaning goes to chemist and Plexiglas inventor Otto R?hm. In 1913, R?hm extracted enzymes from livestock pancreases and used them to presoak dirty fabric, perhaps the clothes of the slaughterhouse staff in exchange for the pancreases; history has forgotten the details. Extracting enzymes from animal digestive tracts is costly and labor-intensive. For the first commercially produced laundry enzyme, scientists turned to a protease created by bacteria. Commercial lipase followed soon after. Here the gene was transferred to a fungus. Fungi are bigger and thus easier to deal with. You don’t need a microscope to see your herd, or crop, or whatever collective noun applies to fungi.
Grime told me about an enzyme found on the forest floor that breaks down the cellulose in dead, fallen trees. When he worked at Procter & Gamble, he tried it out as a fabric softener. (That’s how softeners work. They ever so mildly digest the fibers.) That didn’t work out, but the enzyme did something even better. It digested the cotton fibrils that tangle up and form pills on your sweater. (Crushingly, the anti-pilling enzyme doesn’t work on wool.) We had traveled a long distance from saliva, and I had not asked the question I’d called to ask. It was time to come in from the forest.