Weekly Column_120 / August 16, 2012

Food Science Wonders and Mysteries Solved

By Charles Platkin, PhD

Why is it that when you and your partner both eat garlic, you don’t smell it on the other person? Also, why is it so difficult to get rid of garlic breath?
According to Luke LaBorde, Ph.D., a professor of food science at Penn State University, “You probably don’t notice the smell because your olfactory system is saturated and your brain no longer receives ‘garlic signals.’ It’s the same as if you worked in a horse barn: After a while you don’t notice the smell.” It makes sense. Clearly, we can be “desensitized” to our own bad breath. “So, it would make sense that if we had bad garlic breath, and we became desensitized to it, we would similarly become desensitized to our partner’s simultaneous garlic breath,” says Stephen J. Pintauro, a professor at the University of Vermont.

One of the reasons that garlic breath is so strong is that it’s “full of sulfurous compounds that ‘feed’ the bacteria in the mouth, and bad breath (halitosis) is caused as a result,” says Gerard E. Mullin, M.D., M.H.S., the director of gastroenterology at Johns Hopkins Hospital.

However, the more relentless bad breath associated with garlic originates in the intestinal tract. “Sulfur-containing gases from the garlic are produced in the intestines and are only slowly eliminated by metabolism,” says Pintauro.

LaBorde also notes that the chemical responsible for garlic breath enters the bloodstream and circulates. And, “The volatile garlic compounds diffuse from the blood to the air deep within the lungs, and we breathe them out.”

Why does parsley counteract bad breath?
The experts say it’s the chlorophyll — the green pigment in plants responsible for photosynthesis — that gets rid of bad breath. Chlorophyll tablets have been recommended for reducing odors, but their effectiveness has not been entirely proved. The exact mechanism is not known, says LaBorde.

Why do you tear up from an onion? And how can it be stopped?
According to Pintauro, when you slice an onion, you disrupt cells in the vegetable that contain an enzyme (lachrymatory-factor synthase) that catalyzes the production of volatile irritants. The synthase enzyme converts the sulfoxides (amino acids) of the onion into sulfenic acid. The unstable sulfenic acid then rearranges itself into syn-propanethial-S-oxide, which gets into the air and comes in contact with your eyes. “Once it hits the eye, it turns into a mild form of sulfuric acid, which causes burning,” adds Don Schaffner, a food science professor at Rutgers University. The lachrymal glands become irritated, and the eyes tear.

Pintauro recommends chilling the onion before chopping. “Enzyme activity is greatly reduced at lower temperatures. So, you can minimize the production of the volatile irritants by chopping cold onions. Also, I have heard that simply wearing swimming goggles may help.” Swanson has a few other suggestions, including: “Wear contacts; peel and slice under running cold water or close to running cold water to aspirate the volatiles away with the water; light a candle before chopping and chop onions in proximity to the candle so the tear-causing volatiles will be volatilized in the rising heat from the candle and away from your eyes.”

What in asparagus causes a person’s urine to smell?
“The smell is likely due to the normal metabolism of certain amino acids that are high in asparagus,” says Pintauro. There is some debate as to exactly which sulfur compounds and which amino acids are involved, but it clearly is a sulfur-related smell. “One of the compounds identified is methyl mercaptan. This chemical smells like rotten cabbage. One of its industrial uses is as an additive to natural gas, so that you will smell it if there is a gas leak or if you’ve accidentally forgotten to turn off a gas burner,” adds Pintauro. According to Johns Hopkins’ Mullin, only half of the population will notice this odor after eating asparagus. Why? Scientists are not sure. It could be because half of the population has the “gene enabling us to break down the sulfurous amino acids in asparagus into their smellier components, or that everyone digests asparagus the same way, but only about half of us have a gene that enables us to smell the specific compounds formed in the digestion of asparagus.”

If you have fish smell on your hands, does rubbing them on stainless steel eliminate that odor?
“Stainless steel contains iron, chromium, nickel and other metals. The chromium at the surface reacts with oxygen to form an extremely thin chromium oxide film that protects the metal underneath from oxidizing (rusting). It is theoretically possible that a small amount of these metals either directly combines with the compounds responsible for fish odor or catalyzes their oxidation by air to form less volatile and therefore less odoriferous compounds,” says LaBorde. However, Dr. Mullin recommends rubbing your hands with a wedge of fresh lemon and then washing them with vinegar and water for the best results.

Why does spinach make your teeth feel funny?
“Spinach, like many other plants but more so, contains very small crystals of oxalic acid as a waste product of the plant’s metabolism. They are not very soluble in water, and one can sense them as a very fine grit on the teeth and tongue,” says LaBorde. And although oxalic acid can be harmful, the amount in spinach is too small to do any damage.

Does salt make water boil quicker?
No. “Actually, the effect of any dissolved solute, including table salt, is to increase the boiling point of water. So, water containing salt should take longer to boil than water with no added salt,” says LaBorde. And, according to Barry Swanson, Ph.D., professor of food science at Washington State University, this phenomenon, known as the colligative properties of solutions, is the same reason salt is spread on icy roads during winter — to lower the ice’s melting point and keep the ice off the roads.

However, there is one reason that salt might appear to increase the speed with which water boils: “Salt decreases the surface tension of water so that smaller, more foamy — and thus noticeable — bubbles can be formed. Without salt, fewer and larger bubbles are formed. Perhaps this observation is confused with faster boiling time,” Swanson adds.






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