Erie, Pa. --- A Penn State Erie chemist has identified a previously unnoticed component of what makes limes smell, well, like limes.
Dr. Mary Chisholm, associate professor of chemistry at Penn State's Behrend College, has identified a balsamic, woodsy scented material, 7-methoxycoumarin, as an unexpected part of the lime fragrance.
It's a surprise, Chisholm says, because it doesn't come from the same class of chemicals, the terpenes, from which many lime odorants do and is less volatile.
While limes are not a major part of the human diet, they are important to the beverage industry. Lime or lemon flavor or fragrance is used in many of the most popular softdrinks including Coca Cola, 7-Up, Sprite, and Dr. Pepper. Other applications include perfumes and the confectionery industries. By identifying all of the constituents in the lime aroma, Chisholm hopes to eventually to be able to duplicate it in the laboratory.
Chisholm presented her findings in a paper, "Differences in the Aroma Composition of Cold Pressed and Distilled Essential Oils of Key and Persian Limes," at the national meeting of the American Chemical Society in Boston in August. Her co-authors were three of her former undergraduate students, Matthew A. Wilson, Nicole R. Taylor and Aisha B. Mitchell.
The Penn State researcher says, "There isn't a single compound in lime oil that is responsible for the distinctive lime aroma. Rather, a number of different compounds, interacting together produce that pure, simple smell."
Foods, in general, tend to have aromas that result from complex interactions, Chisholm adds. A food odor can often become significantly different if just one constituent is changed or absent.
Chisholm notes that 7-methoxycoumarin had been reported in lime oil previously but only as one of several solid components of the peel, not as an odorant. Three major constituents -- linalool, neral and geranial -- produce 80 percent of the odor activity. However, without the newly identified component, limes just don't smell like limes, she says.
While Chisholm uses the latest in chemical instrumentation and techniques to study lime oil's components, she and her students use an old- fashioned device -- the human nose, to identify odorants and to detect changes in the aroma.
"The human nose can detect odors at far lower levels than instruments," she says. "The human nose is the most sensitive detector on the market to measure aroma intensities.
"We take very simple combinations of only two or three compounds found in lime oil and have the students sniff them and describe what they smell," she explains. "We then dilute the mixtures to find out whether the aroma changes."
Chisholm, who received the 1998 Behrend Council of Fellows Faculty Research Award, adds that the project is an excellent tool for educating and training students who learn to apply their analytical skills as well as to operate equipment they may encounter in the work world after graduation. The students also learn to be better "sniffers." Although the stereotype holds that females have the better sense of smell, Chisholm has found that males are often her best "noses."
In addition to the new lime aroma component, the research has also turned up information about the ways in which distilled versus un-distilled or "cold-pressed" lime oils smell. Chisholm says the beverage industry primarily uses the distilled oils which tend to have higher aroma intensities. Linalool is a major odorant in both distilled and cold-pressed lime oils. Neral and geranial and their variants contribute more to the undistilled lime oil fragrance.
Persian limes, the larger green kind usually found in U.S. supermarkets, and Key limes, the more seasonally available small ones that turn yellow on ripening, have similar aroma intensities, she says. However, beverage companies most often prefer the Key limes.
Chisholm says, "There is five times as much 7-methoxycoumarin in Persian lime as Key lime oil, which means that it has five times the impact in Persian lime. In Key lime it is not a major odorant, but rather one of many secondary odorants."
There is very little information in the chemical literature on citrus aroma intensities, notes the Penn State scientist. She intends to continue the studies on lime oils, including examining aroma interactions that may lead to making synthetic lime oil in the laboratory. She thinks there's a lot more she and her students can "sniff out" about limes.
Editors: Dr. Chisholm is at (814) 898-6412 or at mac8@psu.edu by email.