Diffuse Reflectance FTIR Spectroscopy Helps Find Sulfite in Beverages

Lynn M. Savage

The inorganic compound sulfite (SO₃^2–) is used to ferment or preserve various foods, including beverages such as beer and wine. Unfortunately, some people are allergic to the chemical, and it can cause breathing difficulties in individuals who have asthma because it gives off sulfur dioxide when swallowed.

According to Manas K. Deb of Pandit Ravishankar Shukla University in Raipur, India, 5 percent to 10 percent of asthma patients react adversely to sulfites, experiencing symptoms ranging from mild to life-threatening.

Investigators used diffuse reflectance Fourier-transform infrared spectroscopy to develop a technique for detecting sulfite content in beverages such as beer and wine. Shown here are the qualitative peaks at maxima of 973, 633 and 495 cm^–1 at various concentrations of sulfite (70 to 160 ng) mixed with 0.1 g of potassium bromide. Reprinted with permission of the American Chemical Society.

There are several techniques for detecting sulfites in food and drink when the content is under suspicion, including chemiluminescence and spectrofluorimetry. However, some such methods lack sensitivity or selectivity, and they all require large amounts of reagents as well as large sample volumes.

Now Deb and his colleague, Santosh K. Verma, have used diffuse reflectance Fourier-transform infrared (FTIR) spectroscopy to develop a technique that rapidly and precisely determines the sulfite content in single drops of beverage samples. According to Deb, the technique may be of importance to various food and beverage quality control authorities for routine analysis.

The researchers used FTIR spectroscopy because, although the technique has had limited use in detecting inorganic compounds, it is known to be extremely rapid. In previous investigations, they found that sulfite — a multiatomic inorganic ionic species — had several important vibrational bands. “The amplitude of the bands varied quantitatively with the variation in even the least quantities of sulfite,” Deb said.

They prepared samples of beer, red and white wine, and orange and lemon juice by placing 10-μl drops of each onto a substrate of 0.1 g of potassium bromide. They dried the combination, then ground it in a mortar and placed it into the diffuse reflectance accessory of an FTIR spectrometer made by Shimadzu Corp. of Kyoto, Japan.

Across all sample types, the researchers found that sulfites were detectable to 0.8 μg/ml. This limit of detection is comparable with other standard methods but has the advantage of requiring neither a reagent nor a large quantity of the sample. According to Deb, the technique should be widely applicable to other multiatomic ionic chemical species, such as borates, phosphates, chromates and tungstates.

Journal of Agricultural and Food Chemistry, Oct. 17, 2007, pp. 8319-8324.