Fluorescence Shows How Cheeses Change Properties After Exposure to Light
Lynn M. Savage
Have you left the cheese out on the kitchen counter again? You may wish that you hadn’t — cheeses become discolored, change flavor and lose nutrients when exposed to light, mostly because of photoinduced lipid oxidation. Some cheese makers wrap their products in packaging film designed to block the most damaging wavelengths, but most cheeses remain exposed semicontinuously from the factory to the grocer’s refrigerator case, where they sit, exposed even more, until they are purchased.
Cheeses contain chlorophyll, porphyrin and riboflavin — chemicals that, when exposed to light, start a series of reactions that lead to oxidation and the release of volatile chemical compounds. Now investigators at the University of Copenhagen in Denmark have studied how three wavelengths activate these processes in one type of cheese.
Researchers used spectrophotometry with a front-face accessory to determine the effect of several wavelengths of light on the chemical properties of a semihard cheese. Courtesy of Arla Foods amba.
Led by Charlotte M. Andersen, the researchers used three types of semihard cheese from the same producer: one with 16 percent fat content, one with 25 percent fat and one, labeled “organic,” also with 25 percent fat. They split each sample in two, exposing both pieces for four or 24 hours to 366-, 436- or 546-nm light from a mercury lamp from Osram of Munich, Germany. They chose these wavelengths because they matched the absorption of the target molecules; for example, riboflavin exhibits maximum absorption at 366 and 436 nm, but none at 546 nm.
After exposure, one portion from each sample underwent fluorescence measurement; the other half underwent gas chromatography/mass spectrometry analysis. The scientists used a PerkinElmer spectrophotometer with a front-face accessory to record changes in the cheeses’ surface fluorescence after light exposure, capturing fluorescence landscapes with excitation from 360 to 460 nm and emission from 500 to 560 nm. Alternatively, they obtained emission spectra with excitation at 410 nm and emissions captured between 580 and 770 nm.
They found that both chlorophyll and porphyrin were degraded by 436- and 546-nm light but not by 366-nm light. Riboflavin, on the other hand, degraded noticeably at 436 nm. They also found that there were only slight differences in the amount of degradation of the three compounds between four and 24 hours, suggesting that most, if not all, of the degradative process occurs in four hours or less.
In addition, they detected as many as 47 volatile compounds resulting from oxidation or other processes occurring within the cheese. Comparison of the fluorescence and chromatography data suggested a possible, if tenuous, correlation between the spectral data and the distribution of the volatile compounds.
The volatile compounds do not exhibit fluorescence, but the investigators believe that the technique can have good predictive power when it provides information about the photosensitizers that enable the degradation leading to the volatile compounds.
Journal of Agricultural and Food Chemistry, ASAP Edition, Feb. 15, 2008, doi: 10.1021/jf072672f
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