by Mary Roach
* * *
* Using the tongue is less peculiar than it seems. Before doctors could ship patients’ bodily fluids off to labs for analysis, they sometimes relied on tongue and nose for diagnostic clues. Intensely sweet urine, for instance, indicates diabetes. Pus can be distinguished from mucus, wrote Dr. Samuel Cooper in his 1823 Dictionary of Practical Surgery, by its “sweetish mawkish” taste and a “smell peculiar to itself.” To the doctor who is still struggling with the distinction, perhaps because he has endeavored to learn surgery from a dictionary, Cooper offers this: “Pus sinks in water; mucus floats.”
* The shipping of bodily fluids was a trying business in the 1800s. One shipment to Europe took four months. Bottles would arrived “spilt” or “spoilt” or both. One correspondent, taking no chances, directed Beaumont to ship the secretions “in a Lynch & Clark’s pint Congress water bottle, carefully marked, sealed and capped with strong leather and twine, cased in tin, with the lid soldered on.”
6
Spit Gets a Polish
SOMEONE OUGHT TO BOTTLE THE STUFF
ERIKA SILLETTI STUDIES saliva in a sunny top-floor lab in the Dutch town of Wageningen. A Gaudi poster hangs on one wall, and the windows look recently washed. The day I arrive, she wears a tailored wool skirt, short but not overly so, black leather boots, and a dove-gray cashmere sweater. If you saw a picture of Silletti in a magazine, you might make yourself feel better by assuming that the creamy skin tone and flawless symmetry of her features had been photoshopped. Only one thing fits my imagined notion of what saliva science looks like: a two-foot-tall, free-standing steel paper-towel holder with the fattest roll of paper towels I’ve ever seen.
I came upon Erika Silletti while roaming the abstracts of a dental conference. She later told me the presentation she gave there was met with blank looks. “They think of it as lubricating, and that’s it!” She went back to her hotel room and called her boyfriend in tears.
It is safe to say that no one in this world understands and appreciates saliva like Erika Silletti.*
HUMANS SECRETE TWO kinds of saliva, stimulated and unstimulated, no more alike than most siblings. The prettier child is stimulated saliva. It comes from the parotid glands, between cheek and ear. When a plate of Erika Silletti’s spaghetti carbonara makes your mouth water, that’s stimulated saliva. It makes up 70 to 90 percent of the two to three pints of saliva each of us generates daily.
We’re going to gather some now. Silletti pulls on a pair of blue latex gloves that so pleasingly complement the gray of her sweater that they look like part of the ensemble. She picks up two stoppered plastic vials. Inside each is a second, smaller vial, which contains a tightly compressed, cylindrical cotton wad. This is the Salivette saliva collection system. Silletti uncaps a Sharpie and marks an M, for Mary, on one, and an E on the other.
The Salivette instructions are printed in six languages. Erika Silletti, born in Italy, fluent in English, living in the Netherlands, can read three. “Kauw dan 1 minuut lichtjes op de wattenrol.” “Masticate delicatamente il tampone per un minuto.” “Gently chew the tampon for one minute.” This is the simplest way to collect stimulated saliva without also collecting the food that stimulated it: you chew the collection device. This is “mechanical stimulation” (as opposed to gustatory or olfactory stimulation, which we’ll come to). Il tampone will wick our flow, and then Silletti will place each back in its vial and put them in a centrifuge. The liquid will be spun from the cotton and flow down through an opening at the bottom of the inner vial, ending up in the outer vial.
The Salivette makes an unmistakable point: your parotid glands don’t care what you chew. There is nothing remotely foodlike about superabsorbent cotton, yet the parotids gamely set to work. They are your faithful servants. Whatever you decide to eat, boss, I will help you get it down.
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 Sjögren’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 Röhm. In 1913, Röhm 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.
“If you dribble something on your shirt while you’re eating,” I asked Grime, “does it make sense to dab it with saliva? As a kind of natural laundry presoak?”
“That’s an interesting thought.”
Dr. Grime carries a Tide stain pen. He does not use his own spit.
Art conservators do. “We make cotton swabs on bamboo sticks and moisten the swab in our mouths,” says Andrea Chevalier, senior paintings conservator with the Intermuseum Conservation Association. Saliva is especially helpful for fragile surfaces that solvents or water would dissolve. In 1990, a team of Portuguese conservators pitted saliva against four commonly used nonanatomical cleaning solutions. Based on its ability to clean but not damage water-gilded gold leaf and low-fired painted clay surfaces, saliva “was judged the ‘best’ cleaner.” Denatured saliva, stripped of its enzymatic powers, was also tested and proved inferior to straight spit.
For more typical cleaning jobs, painting conservators, like laundry formulators, turn to commercially produced digestive enzymes. Protease, the protein digester, is used to dissolve washes made from egg white or hide glues. (Less enlightened conservators of yore used to spread glue made from rabbit hides onto canvases to consolidate flaking paint.) Lipase, the fat digester, is used to eat through the layers of linseed oil that eighteenth- and nineteenth-century painters applied to improve light refraction and “feed the surface” of their artworks.
Chevalier volunteered that some conservators’ saliva cleans noticeably better than others’, and that this occasionally prompted speculation about how many martinis these individuals were having at lunch. In reality, there are naturally large individual differences in the chemical makeup of people’s saliva.
And in people’s flow rates. Silletti and I, for instance, chewed our cotton wads for the same amount of time. I produced .78 milliliters of stimulated saliva; she produced 1.4. She tried to reassure me. “It doesn’t say anything about how good you are or how good I am with saliva.”
“Erika, I’m a dried-up husk.”
“Don’t say that, Mary.”
Silletti excuses herself. “I want to go get some ice. The reason is that even after one minute, this will start to smell very bad.”*
While she is out, I will take this opportunity to share with you the extremely surprising findings on the topic of olfactory stimulation of saliva. The notion that food smells make your mouth water is, science says, erroneous. Science has said this over and over, most recently in 1991, at King’s College London. Ten subjects donned plastic odor-delivering face masks and nickel-sized Lashley cups. (The Lashley cup, a sort of glandular beret, fits on top of the parotid and collects its secretions.) Food odors wafted into the volunteers’ noses: vanilla, chocolate, peppermint, tomato, and beef. Only one smell, and in only one subject, caused a significant increase in salivation. Oddly, this subject was a vegetarian, and she was smelling beef. Upon questioning, the woman revealed that the smell had nauseated her. The salivation was the kind that precedes throwing up.
It is easy to criticize that study. Sitting in a lab with a plastic mask on your face and sniffing chemically synthesized odorants does not approximate the typical mealtime mouth-watering scenario. This does, though. In 1960, a bright-eyed, full-lipped young physiologist named Alexander Kerr fried up bacon and eggs in his lab at Harvard. He did so in front of three hungry volunteers, whose parotid flow was measured via a type II outflow recorder*—the Lashley cup having not yet been invented. Even here, no one salivated any more than he had before the cooking began. The subject identified as A.G. didn’t buy it. A.G. was positive he could feel his mouth “watering profusely” in the moments before he began eating. Kerr insisted that wasn’t so. He told A.G. that the feeling was an artifact caused by shifting his attention to the inside of his mouth and suddenly becoming “conscious that his mouth contains saliva.” I have seen the data, but I too find it hard to believe Dr. Kerr.
IT’S BEEN SNOWING all morning. Wet clumps of flakes flock the trunks and branches of the trees outside the lab. Silletti joins me at the window. She holds the small glass beakers that contain our fresh-from-the-centrifuge stimulated samples.
“It’s beautiful,” I am saying. Silletti agrees, but I notice she isn’t looking out the window. Is it possible she thinks I am referring to the contents of the beakers? I’d say that, yes, it is possible. You’ve never seen such clear, clean-looking spit. Stimulated saliva looks, tastes, and flows like water—it is, in fact, 99 percent water. Water with some proteins and minerals. Like water from different springs, each person’s saliva contains minerals in unique proportions. (People whose saliva naturally contains a lot of salt are slightly oblivious to it in their food.)
“So somebody,” I observe, “could do a taste test with various salivas.”
“If somebody would like to do that, yes.”
Somebody—really everybody—wouldn’t. I point to the beaker labeled E. “What about just your own? Would you ever—”
“No, I wouldn’t. Even me. Although actually, you are drinking it all the time.”
“Right, so—”
“No.”
An intriguing double standard applies to your own saliva. As long as it stays in your mouth, it’s benign, welcomed even, no more offensive than the water it tastes like. Outside your mouth, it’s almost as vile and contemptible as a stranger’s. As part of a study, our friend from the University of Pennsylvania psychology department, Paul Rozin, asked subjects to imagine a bowl of their favorite soup and to rate their liking of it. He then asked them to rate that same bowl of soup after they’d imagined spitting into it. Forty-nine out of fifty subjects lowered their rating. Among certain castes in India, writes Edward Harper in “Ritual Pollution as an Integrator of Caste and Religion,” spitting on someone puts even the spitter “in a state of severe impurity,” because it is assumed that some of his saliva has “rebounded onto him.”
The saliva taboo can make life burdensome for researchers. Silletti’s colleague René de Wijk did a study years ago that looked at how the salivary breakdown of starch mobilizes fats and enhances flavor. (Fat is the main carrier of flavor.) To do this study, he needed his subjects to rate the taste of custard samples with and without a drop of their saliva added. You can’t just have them spit in it, he explained, because then they won’t go near it. He had to collect samples of their saliva without telling them why, and then add it behind their backs, like a spiteful waitress.
The same double standard applies to all “body products,” as Rozin calls them, managing to make snot and saliva sound like spa purchases. We are large, mobile vessels of the very substances we find most repulsive. Provided they stay within the boundaries of the self, we feel no disgust. They’re part of the whole, the thing we cherish most.
Paul Rozin has given a lot of thought to what he calls the psychological microanatomy of the mouth: Where, precisely, is the boundary between self and nonself? If you stick your tongue out of your mouth while eating and then withdraw it, does the ensalivated food now disgust you? It does not. The border of the self extends the distance of the tongue’s reach. The lips too are considered an extension of the mouth’s interior, and thus are part of the self. Though culture shifts the boundaries. Among religious Brahmin I
ndians, writes Edward Harper, even the saliva on one’s own lips is considered “extremely defiling,”* to the extent that if one “inadvertently touches his fingers to his lips, he should bathe or at the least change his clothes.”
The boundaries of the self are routinely extended to include the bodily substances of those we love. I’m going to let Rozin say this: “Saliva and vaginal secretions or semen can achieve positive value among lovers, and some parents do not find their young children’s body products disgusting.”
I recall being told, in grade school, that Eskimos kiss by rubbing noses. Is this an example of a culture reluctant to accept the saliva of a loved one? Gabriel Nirlungayuk, my go-to man for all things Eskimo/Inuit, confirmed that the kunik, or nose rub, has been and remains the traditional alternative to the kiss. “Even to this day, now that my children are adults, I kunik them when I have been away for a long time.” But never girlfriends. By the time Nirlungayuk was a teenager, kissing “whiteman’s way” had caught on. Nobody seemed to have a problem with extending those boundaries. If anything, the Inuit are leaders in the field. “Sometimes when my ingutaq—granddaughter—is full of snot, my wife or myself will wipe it clean with our mouths and then spit it out. But we would never consider this with other kids.”
A similar psychology applies to breast milk. It’s considered natural for a child to consume a mother’s milk, or even for a lover to do so, but not a stranger. (Hence the 2010 hullaballoo over the New York City restaurateur who invited diners to try cheese made from his wife’s breast milk.) So reliable is breast-milk consumption as a delimiter of intimate family that Islam recognizes a category called “breast milk son,” which confers an exemption to the rules on segregation of the sexes. A man can be alone with a woman if she’s immediate family or if she breast-fed him as a child.* (Sisters sometimes breast-feed each other’s infants, thereby creating breast-milk relatives.) Milk is thicker than blood, or about the same consistency.