Fluorescence Spectroscopy Analyzes Cheese

Hank Hogan

Exposed to too much light and oxygen, cheese can form off-flavors, undergo changes in color and lose nutritional value. Cheese makers and distributors, therefore, search for a combination of product characteristics, storage conditions and packaging materials that minimize light-induced oxidation. To assist them, food chemists are exploring ways to evaluate storage conditions and to spot the onset of oxidation. A promising approach is fluorescence spectroscopy.

Fluorescence spectroscopy may have the potential to be used as a quality monitor in the manufacture of cheese and as a research tool for studying the photooxidation process that can cause cheese to change color and lose nutritional value.

Charlotte M. Andersen, an assistant professor in food science at The Royal Veterinary and Agricultural University in Copenhagen, Denmark, noted that spectroscopic methods generally have the advantages of being fast and of not requiring sample preparation. Fluorescence methods have the added benefit of involving only a few specific molecules, making data interpretation easier than infrared and near-infrared spectroscopic approaches. This is particularly true when fluorescence measurements are done in an excitation-emission matrix, in which the excitation source is stepped across a range and the emission spectrum is measured for every excitation step.

Given these advantages, and because fluorescence spectroscopy has been used to measure oxidation in other dairy and food products, Andersen and her colleagues at the university examined its potential for the evaluation of cheese. For their investigation, reported in the Dec. 28 issue of Journal of Agricultural and Food Chemistry, they worked with samples of Danbo cheese that were packaged with two films of different transmission coefficients and ultraviolet cutoffs and in a modified atmosphere in which the oxygen was largely replaced by carbon dioxide. They stored the cheese for various times from 0 to 84 days under fluorescent lighting.

Periodically pulling cheese samples from storage, they measured two emission-excitation matrices using an LS50 B spectrofluorometer from PerkinElmer Life and Analytical Sciences Inc. of Wellesley, Mass. In one, they measured emission spectra from 280 to 600 nm with excitation every 10 nm from 260 to 360 nm and a slit width of 7 nm for both excitation and emission. In the second, they measured emission spectra from 390 to 650 nm with excitation every 10 nm from 360 to 460 nm and a slit width of 5 nm. Because the spectrofluorometer uses monochromators rather than filters for wavelength selection, Andersen said, additional equipment was not necessary.

In analyzing the results, the researchers uncovered three components in the first matrix. Based on the excitation and emission peaks, one probably arises from vitamin A in the cheese, although Andersen cautioned that this has not yet been verified. If it is indeed vitamin A, it could serve as a monitor because the vitamin is degraded by exposure to oxidation. The other two were related to protein structure.

In the second matrix spectrum, they found two components — one from riboflavin and the other from oxidation products.

This assignment was based in part on changes to the spectral components with storage time and conditions.

Andersen noted that several parameters should be investigated further. She also said that fluorescence could be used as a quality monitor for cheese manufacture and as a research tool to better understand the photooxidation process.