Salt Talks

David Pacchioli
March 01, 1994

"A civilized life is impossible without salt."

—Pliny the Elder

How do we begin to capture a substance as basic to our lives and legends, as omnipresent, as simple and complex, as sustaining and lethal, prosaic, exalted, easy to overlook, and irreplaceable, as cheap, plain, powerful, savory, sharp, gritty, and pure as sodium chloride?

Sneak up behind it, maybe, and put some salt on its tail?

Let's be straight. It isn't considered the best of form, today, to admit to an affinity for sodium chloride. Salt is either bad for us or boring. Above all else, it sometimes seems, we desire to be "salt-free:" liberated from the tang we have tasted in sweat and seawater, the stuff we have slung against both the Devil and Mr. Freeze. The grit which built the earliest roads of trade, and now eats away at those of concrete. The mineral that made McDonald's.

Pepper, salt's punchy calypso partner, has had a far better press in recent years. Waiters don't come around asking whether we'd like a little fresh ground sodium chloride. But try sprinkling pepper on your popcorn some cinematic evening. Or gargling with it in warm water the next time your throat is sore.

No. Salt, let it be affirmed, is the profounder stuff. Flavor itself. The genuine article. Nonpareil. A substance we ought definitely to be true to, and take things with a grain of. Nor, as it turns out, is that even the half of it.

"The history of salt," says Dennis Kostick of the U.S. Bureau of Mines, "is synonymous with the history of mankind."

Salt Shaker

Salt Shaker

Kostick was a plenary speaker at the Seventh Symposium on Salt, held in Kyoto, Japan, in April of 1992. Six hundred and fifty people from 35 countries attended this latest gathering, among them earth scientists, engineers, physiologists, economists and doctors, from universities, government agencies, and the salt industry. H. Reginald Hardy, Penn State professor of mining engineering, was there. Hardy also served as co-editor of the meeting's published proceedings, an enterprise that ran to two volumes and 1,340 pages.

The topics covered ranged from the densification behavior of wet granular salt to the origins of borates in the saline lakes of China, from solar evaporation to salt's role in the development of capitalism.

There was stuff on the physiology of salt balance, mine safety, and environmental impact.

Papers were presented on the use of brine shrimp in the biological management of solar saltworks, the effects of salt on the softening of cooked Japanese radish roots, and the salt tolerance of mangroves.

Some bit of attention was focused, in other words, on everything you might ever want to know about salt.

It was no accident that Salt '92 was held in Japan. Because the environment for salt production in his country is so poor, explained Masayuki Ohno of the Japanese government's Salt Technology department, "the Japanese have had to commit themselves to salt-making with their full ingenuity and passion."

It's true, the islands of Japan are surrounded by salt water. As explained by Ohno, the Japanese word for salt, shio, is in fact the same word used for the ebb and flow of the tide. But the traditional method used by coastal peoples for extracting salt from the sea—solar evaporation—requires two things not readily available in Japan: lots of flat land, for the great shallow evaporation fields, and reliable sun, without too much rain.

As a result, the Japanese, from at least the 6th century, developed their own two-step technology for concentration, or saikan, and crystallization, or sengo. Seaweed, soaked in seawater, was gathered and allowed to dry. The salt that precipitated on the seaweed was rinsed off into more seawater, producing a concentrated brine. This brine was then heated in clay pots until the salt turned to crystals.

Legend says that the god Shio-Tsuchi-Oji taught this method to the ancestors of present-day Japanese. Shrines remain scattered around Japan, and salt-making is demonstrated in annual rituals by Shinto priests.

Meanwhile, here in the 20th century, the Japanese have been leaders in advancing salt-getting technology. The state-of-the-art for "concentrating" salt water, ion-exchange membrane electrodialysis, was pioneered by Asahi Glass, now Corning Asahi in the 1950s. (The same technology was researched in the United States, at the same time, for desalination.) The Japanese have been improving electrodialysis ever since, working to make it more efficient and less expensive.

The appetite for salt is a basic biological motivation—as basic as hunger, thirst, or the sex drive. You can see it, says Penn State neuroscientist Ralph Norgren, in the way a porcupine that gets into a backwoods toolshed will eat the cork handle off your favorite fishing pole.

"He doesn't want the cork," says Norgren. "He's after the salt from your hands."

Norgren, who was not at the international symposium, has done studies in which rats, deprived of sodium in their diet, will avidly drink the saline equivalent of seawater in order to make up the deficit their brains perceive. "That's not a learned behavior," he says. It's a hard-wired survival instinct. The same motivation exists in humans—but that's not what keeps Uncle Larry reaching for the Doritos.

That, says Norgren, is salt preference. There's a big difference.

Animals, particularly herbivores, have to struggle to get enough salt to keep their metabolisms balanced. Deer visit salt licks. Range cattle chew on bones. Once, humans were in the same boat, but that time has long since passed. Today, our foraging takes place at the 7-Eleven, and we get so much salt that our real biological need hardly ever surfaces. "A person who does hard physical work all day in a very hot sun, and drinks only water—there you might get into salt need," Norgren says. "But for the rest of us, it just doesn't happen."

For Norgren, salt appetite makes an ideal model for understanding the biological basis for motivation: the neural mechanisms that control basic behaviors. "It's a nice system to study," he says, "because the object of motivation, the sodium ion, is so specific, and is recognized by as single sensory system—taste." In one series of experiments, Norgren and his team are recording electrical signals from single rat brain cells to see how they change with the taste bud's response to salt.

The craving we humans actually notice is at least partly a learned behavior. Until they are about four months old, notes Leann Birch, Penn State professor of human development, babies are indifferent to salt. Yet, she adds, young children have very strong preferences for salt, stronger than experience alone would seem to warrant: "If you take the broth from Campbell's chicken noodle soup, and you give it to kids to taste test—well, you know how salty that is already, but you have to keep adding more and more salt before they'll say that's the way they like it. You can't get too much salt, as far as the kids are concerned."

Birch has done other studies showing that preschool children learn via experience, including parental example, which foods are appropriate for salting and which are not. "Kids learn to prefer a food the way they have had it."

Ultimately, she says, it's hard to untangle what's inherent from what's learned. "There's definitely a built-in preference for salt, but then salt is so pervasive in our cuisine that it's what people are used to."

Whatever the reasons, we tend to eat many times more salt than our bodies actually need. But is all this excess necessarily bad for us?

It's a very old question. The earliest known medical writing, the Nei Ching, quotes the Yellow Emperor: "If too much salt is in the food, the pulse hardens, the complexion changes, and tears make their appearance." Five thousand years later, the effect of salt in the body has become a battleground. In the last two decades, a case has been built for the "salt hypothesis," which links salt intake with high blood pressure. Salt has been labeled a killer in the popular press. Broad public health initiatives seek to restrict salt intake by the general population.

Now some experts are expressing a different view. J. Swales, a British physiologist and another plenary speaker in Kyoto, acknowledges that the body's essential sodium balance can be maintained on a salt dose one-tenth the daily average. But, Swales adds, "to say this means sodium intake is grossly excessive is misleading." Optimal intake, he argues, is a difficult question.

Although salt restriction has been shown to lower blood pressure in some patients, Swales says, in others the connection is not that clear. There seems to be a genetic component involved: Some people are more sensitive to salt than others. What's more, there is no evidence to suggest that lowering salt intake can prevent the onset of high blood pressure in otherwise healthy people. A policy of universal sodium restriction, he warns, is unwarranted, and may court other problems.

Friedrich Luft of the University of Erlangen Nurnberg in Germany, reviewing recent research on the issue, accedes that dietary salt reduction is no panacea. Nevertheless, decreased salt intake is at least "a valuable adjunct" in certain patients, and until reliable means of identifying salt sensitivity are available, is a reasonable approach to the non-drug management of hypertension. Such management, Luft concludes, must also consider weight reduction, lowered alcohol consumption, and exercise, among other factors.

Shiriki Kumanyika of Penn State's Hershey Medical Center would agree. Kumanyika is an epidemiologist who specializes in minority health. African-Americans and other minorities, she notes, have far higher rates of high blood pressure than the general population. A number of factors come into play, and among them one of the strongest is salt intake. Well-targeted educational programs, she argues, are the only way to fight the problem. Programs that show that healthy eating includes regulation of salt.

It's not easy to live with less salt, when we're so used to having it. In an effort to ease the pain, food manufacturers have experimented with salt substitutes, including monosodium glutamate, potassium chloride, and citric acid, among others, but with little success. As for reducing salt content in their products outright, Birch says, "I think the industry is terrified to do it—afraid they'll lose money. And they're probably right."

Reginald Hardy has his own, admittedly anecdotal, observations on the dietary consumption of salt. "I happen to collect antique china," he says. "If you look at the salt shakers from 50 years ago, you'll see that the holes in them are huge. You can't turn them over without getting a mound of salt on your plate. People were eating a lot more salt then than they are now."

Fact is, worldwide use of salt has increased, and dramatically, over the last 50 years—but none of that increase is related to salt's consumption at the table.

Since 1950, Dennis Kostick reports, annual worldwide production of salt has quadrupled, from 48 million tons to 196 million tons. A big chunk of that increase is tied to salt's use in the production of glass, plastics, and household chemicals. Even more, the jump is attributable to the rather unglamorous business of winter highway maintenance.

In the winter of 1940, a mere 150,000 tons of rock salt were scattered on the highways of North America. After World War II, with sprawling infrastructure, massive production of automobiles, and the growth of interstate trucking, that number began to swell: by 1960, to 2.1 million tons, and by 1972 to a whopping 10.2 million tons. It has since leveled off at about 9 million tons.

Europe has similarly upped its seasonal self-seasoning, albeit starting a bit later: Widespread salting of French highways, for instance, began only with the Grenoble winter Olympics in 1968.

This great increase has not been without its negative effects. The chemical aggressiveness of the chloride ion—its propensity for eating steel and concrete—has been well and sadly observed by car owners and pothole dodgers. Heavy salt applications blight roadside vegetation, and recently there has been concern too about run-off causing increased sodium levels in drinking water.

As a result there has been considerable research into alternative melting agents. People have looked for more benign substances to put out. The best thing they've come up with so far, something called CMA, is environmentally neutral, but it costs anywhere from 10 to 25 times what rock salt does, depending on whose statistics you look at. (Some say that if you factor in all the damage saved, the costs begin to even out.)

Other approaches include working on improved protective coatings for road surfaces, bridges, and auto bodies, and using additives that might neutralize the offending ion. Perhaps the easiest solution is simply to use less salt. The slogans and titles that have sprung up in the German Salz im Winterdienst ("Salt in Winter Maintenance") and corresponding publications elsewhere have a distinctly dietary flavor:

"Just enough salt and not too much."

"Maximum safety with a minimum of salt."

"Sensible Salting Program."

When Hardy looks at salt, he doesn't see a condiment or a de-icer. He sees a building material.

Not a traditional, discrete, above-ground building material, like wood or brick—although legend has it that a Saharan city called Taghaza was built with bricks of salt. Rather, Hardy sees his palaces underground: caverns hollowed from salt and filled with supplies of crude oil, natural gas, or liquid chemicals.

Most big cities in the southern U.S., Hardy says, are served by a number of such repositories: slender caverns, maybe 50 feet across, a couple of thousand feet long, tucked thousands of feet below the surface. The gas industry uses these facilities to siphon off extra gas during times of low customer usage and then boost the flow during peak consumption hours, a practice known as "peak shaving."

Salt makes a good "host" or "medium" for these hollows because the sheer size of the deposits available—often miles of solid salt—make a massive, uniform area to burrow into. A second plus, Hardy says, is the mineral's forgiving nature. "If it fractures, it tends to heal back up again. It grows back together like a wound. Most other rocks stay fractured."

But a salt cavern has to be carefully designed. Whether carved out by traditional hard-mining techniques like those used for digging coal, or shaped via solution mining, in which fresh water is pumped into a cavity and salt water pumped out, a cavern must allow for salt's special characteristics and the effects of surrounding underground forces. This is no easy task.

Mechanically, "salt is actually one of the most complicated solids there is," Hardy explains. "It behaves like an elastic material at very low stresses, and like a plastic material at higher ones."

The same weakness that makes salt "heal," for instance, contributes to its tendency not to stay in one place. Simply put, when you dig a hole in it, salt tends to fill the hole back up. At a mine Hardy once visited in Austria, a location that has been worked for a thousand years, underground entries that were once six feet high and five feet wide are now two by two. "Salt moves in," he says.

Sometimes it moves in quicker than other times. In the early days of salt-cavern storage during the 1970s, loss of volume was a serious problem: One facility in Mississippi lost 40 to 60 percent of its available space in the first five years of operation. Faced with these constrictions, the gas industry approached researchers like Hardy. Over the course of a decade he and many others worked at understanding salt's physical properties, then developed mathematical models to predict closure. At last, Hardy presented the American Gas Association with a 600-page monograph on cavern design. ("Storage," he says in precis, "is a balance between too much inventory and too little [outward] pressure.")

But we still don't completely understand salt's mechanics.

How, for instance, will a salt repository hold up over the long term—hundreds, even thousands, of years? This was the big concern when salt caverns in the southern U.S. were being considered as possible nuclear waste sites in the early 1980s. "Mathematical equations have been developed to predict long-term behavior," Hardy says, "but anything we say is based on extrapolation, and that can be dangerous, of course." The presence of impurities can change the whole crystal structure, significantly altering behavior. And how will salt react under complicated states of loading?

In the end, Hardy says, salt caverns were rejected as industrial nuclear waste containers—at least in the United States—because of one very basic flaw: the fact that salt is water soluble. He is a bit rueful about this development: Careful design, he argues, can allow for a certain amount of dissolution. But the nuclear industry has already moved on to consider other possibilities, materials like tuff (a volcanic residue), granite, and basalt.

This change of focus means most of the support for salt mechanics research in the United States has disappeared, Hardy says. Germany and France are now the leaders in the field. But Hardy and a handful of graduate students press on in a couple of specialized areas.

In Kyoto, Hardy presented a paper based on a series of recent experiments with acoustic emission, a sophisticated monitoring technique that allows an investigator to "hear" low-level seismic events emitted by solids under stress. (Penn State's Rock Mechanics Laboratory has been involved in acoustic emissions research since 1964.)

A chief problem in the designing of any underground structure, he explains, is trying to determine the stresses at work, forces which are complex, countervailing, and hard to see from above ground. Methods have been developed to measure these stresses in situ, but, says Hardy, they're expensive, and they depend on a material being elastic, which salt is not.

Recently, however, he found a novel way around the problem, via acoustic emission and the so-called Kaiser effect.

"During the study of the mechanical behavior of metals," he writes, "J. Kaiser at the University of Munich in 1950 discovered that the acoustic emission response of metals showed a pronounced 'memory' of the maximum previously applied stress." That is, Kaiser applied different levels of pressure to certain metals wired for micro-noises, and did this in cycles, say heavy, light, light, heavy for a given piece of metal. What he found was that once a heavy load had been placed on a given piece of metal, there was very little acoustic emission activity in that piece until the same heavy load was applied again. The material "remembered"—and made noise about—the heaviest load it had ever felt.

In the ensuing years, the Kaiser effect has been researched and observed in many other materials. Eventually, Hardy began to wonder whether it might not also hold true for salt. If so, a salt sample could recall in the lab the stresses it had felt underground.

After a series of trials, he found out that the Kaiser effect is indeed present in salt. "You can predict stresses quite well." What this result means, Hardy adds, winking only slightly, is that along with all of its other properties—its gustatory, medicinal, structural, mystical, and miscellaneous powers—there's at least one further thing that can be said of the stuff he has spent a lifetime studying:

Salt talks.

H. Reginald Hardy, Jr., Ph.D., is professor of mining engineering and director of the Pennsylvania Mining and Mineral Resources Research Institute in the College of Earth and Mineral Sciences, 110 Mineral Sciences Building, University Park, PA 16802; 814-863-3098. Hardy attended the Seventh International Symposium on Salt, held in Kyoto, Japan, April 6-9, 1992, and served as one of four co-editors of the symposium's proceedings, published by Elsevier of Amsterdam in 1993.

Ralph Norgren, Ph.D., is professor of behavioral science in the College of Medicine, The Milton S. Hershey Medical Center, Pennsylvania State University, 500 University Drive, Hershey, PA 17033; 717-531-8521.

Leann L. Birch, Ph.D., is professor and head of the department of human development and family studies in the College of Health and Human Development, 105 South Henderson Building, University Park; 814-863-0435.

Last Updated March 01, 1994