Cracking Up

A friend and I disagree on how walnuts are removed from their shells commercially. He has an idea that the shells are drilled and something is injected to make them explode apart. I thought there must be something to do with heat and/or gases, but nothing explosive.

How, exactly, are walnuts removed so cleanly from their shells?

Not with explosions, as far as I can determine. (I tried; read further on.) Among many attempts to improve upon vises, hammers, high-heeled shoes and other spur-of-the-moment tactics, 257 nut-cracking patents have been granted by the U.S. Patent and Trademark Office since it awarded the first patent in 1790.

Walnuts have been consumed since prehistoric times, and we have been frustrated with extracting the kernels ever since. The walnut is literally a tough nut to crack, at least without scattering fragments of kernel and shell. But if you scorn the venerable-but-cheap, hand-squeezed gadget that doubles as a lobster cracker, the Indiana Nut Growers Association (www.nutgrowers.org/QA/nutcrackers.htm) recommends several innovative nutcrackers from $25 to more than $100.

One of the most ingenious INGA-recommended devices is the $30 Texas Native Inertia Nutcracker (www.inertianutcracker.com), which is powered by rubber bands with the assistance of Isaac Newton's First Law: "A nut at rest will remain at rest until whacked by something heavy" (not his exact words). This clever device hits the shell hard enough to crack it, but stops just short of the kernel.

THE SHELL GAME

Turning from one-nut-at-a-time methods to industrial mass nutcracking, I replied to the reader who submitted the question above by admitting I didn't know how they do it commercially, and suggested that he contact Crain Walnut Shelling Inc. in Los Molinos, Calif., to see if the company would divulge its methods. He did so, and we can all be gratified to learn that commercial walnut processors don't have a much better batting average than we do for obtaining nut halves rather than broken pieces. Here is the reply he received from Chuck Crain, president of the company: " . . . half walnut kernels are extracted with a rotary sheller that compresses the shell gradually until it cracks. Halves are the most desirable size of kernel, measured by sales value. The compression process only allows us to obtain between 10 percent and 60 percent halves. The difference in percentage of yield (in halves) is based on the variety and therefore the characteristics of the nut being cracked. The most favorable variety measured by half yield is the Chandler variety, and it can produce up to 60 percent halves by mechanical cracking. There are approximately 35 commercial varieties grown in California. Any kernels not remaining intact are sold as pieces for various baking applications."

If it need be pointed out, walnuts are no small potatoes. According to the California Agricultural Statistics Service, some 325,000 tons of English walnuts were harvested in California in 2004. Many of them are sold in their shells in sizes named jumbo, large, medium or baby, and exported for other countries to grapple with.

The shelled kernels are packaged and sold as halves, chopped, finely diced, sliced, chips or ground -- all labeled as if they had been deliberately created, rather than having been sifted from the rubble at the cracking factory. Halves are, of course, the most expensive, but the others are good for baking or for nutty food manufacturers -- that is, manufacturers of nutty foods.

There are black walnuts and English walnuts. The black walnut tree (Juglans nigra) is native to the United States and grown in the eastern part of the country, but mostly for its wood. The common English walnuts we see in the markets (J. regia) originated in Persia but came to be known as English because the British merchant fleet carried them around the world in trade. They are grown mostly in California.

When I was 12 in Poughkeepsie, N.Y., we used to play in a nearby grove of black walnut trees. The ripe nuts we picked off the ground were embedded in soft green husks, moist with a liquid that stained our hands brown for several days -- even in the unlikely event that we washed them. When we persisted and extracted the nuts from the hulls, we found them devilishly hard to crack. So mostly, we just threw the things at one another. Our mothers did not appreciate what they did to our clothes.

Alan Davidson, in his "Oxford Companion to Food," relates that the Romans used to throw walnuts at weddings. In rice, there is much to be preferred. Both black and English walnuts are noted for their relatively high oil content, ranging from 60 to 70 percent of the kernels' weight. English walnut oil is particularly rich (10 to 12 percent) in linolenic acid, some of which is converted by body enzymes into a heart-healthful omega-3 fatty acid. Hazelnuts, in comparison, contain about 61 percent oil and virtually no linolenic acid.

BACK TO THE DRAWING BOARD

I couldn't let the explosion theory go by without a test. Walnuts contain 4 percent water, so I figured that if I put them in the microwave oven and zapped them until their water turned to steam, the steam pressure would . . . well, I decided to find out what would happen.

I put a dozen whole walnuts in a covered plastic container in the microwave oven, donned my safety goggles, turned the oven on high and waited for the explosions.

Nothing happened. After three minutes of cringing, I removed the container and opened it. There was lots of steam inside, some of which had condensed, but the walnuts were intact. Every one of them.

CONCLUSION

Walnut shells are not airtight; they can leak water vapor through the seams. So much for the Big Bang theory of nutcracking.

But wait! Newton's Second Law says (again, I paraphrase) that the walnut's acceleration is inversely proportional to its mass. In other words, the heavier the walnut, the less its cracked components will fly away. So what if I were able to grow huge, heavy walnuts? Wouldn't they just fall apart gently when cracked, without scattering their fragments?

I'm working on it.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. His latest book, "What Einstein Told His Cook-2: Further Adventures in Kitchen Science," will be published by W.W. Norton in April. He can be reached at wolke@pitt.edu.