Infrared Spectroscopy Checks on Diet Drinks

Hank Hogan

Sugar is sweet but aspartame is sweeter, permitting it to be used as a low-calorie sugar substitute in diet soft drinks and other foods. However, some people believe that aspartame’s metabolic by-products strike a sour note and can cause health problems if too much is ingested. Hence, there is a need for a rapid, simple and reproducible method to determine its concentration in foodstuffs.

Recently, a group of researchers from the University of Hawaii in Honolulu and from Hankyong National University in Ansung, South Korea, demonstrated that multibounce attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) could do the trick. Using this method, they found aspartame concentrations in soft drinks to be in the 0.4 to 0.5 mg/ml range. These results were within a few percent of those determined by high-performance liquid chromatography.

Fourier transform infrared spectra of an aspartame standard added to various drinks are shown in the range of 1600 to 2000 cm^–1. Because of the absorption of functional groups within the aspartame molecules, researchers could use this spectral region to determine the aspartame concentration. Reprinted with permission from the Journal of Agricultural and Food Chemistry.

Researcher Soojin Jun, an assistant professor at the University of Hawaii, said that the technique could satisfy those who want answers about how much of the artificial sweetener they are taking in. “They want a single-step end-point measurement of aspartame in drinks. Here, FTIR can be what they want.”

That is possible because multibounce ATR-FTIR requires very little, if any, sample preparation and does not require skilled personnel. It does, however, demand the appropriate equipment, which, in this case, involved a spectrometer from Thermo Electron Corp. (now Thermo Fisher Scientific) of Madison, Wis., equipped with a horizontal multibounce ATR accessory kit from the same company. Producing 12 reflections with an infrared beam at a penetration depth of 2 μm, the accessory had a zinc selenide crystal upon which the researchers placed the soft drink samples.

They prepared their samples from four different commercial aspartame-free soft drinks, to which they added aspartame to create blended samples with artificial sweetener concentrations ranging from 0 to 50 mg per 100 ml. They used some of the samples to calibrate their readings and the rest to validate their results.

They degassed the soft drinks before collecting spectral data from 4000 to 400 cm^–1. After analyzing these readings, they decided that the spectral region from 1600 to 1900 cm^–1 was the best for determining the concentration of aspartame. There are a number of functional groups in the molecule that exhibit strong infrared absorption in this wave number region.

They compared the concentration predicted by their infrared technique to that determined by high-performance liquid chromatography. The two methods agreed to within 2.4 to 5.7 percent, depending on the soft drink. According to Jun, the differences were not statistically significant, especially given the simplicity and rapidity of the FTIR measurements.

As for the future, he said that they will look into using similar techniques to resolve questions about other foods. One outcome might be a scanner that analyzes food properties immediately for the curious consumer.

He also noted that infrared methods could be enhanced by the addition of Fourier transform Raman spectroscopy.

The combined use of infrared and Raman spectroscopy can extract comprehensive information.

Journal of Agricultural and Food Chemistry, Feb. 13, 2008, pp. 778-783.