Fluorescence Spectroscopy Detects Adulterated Honey

As a premium-priced, all natural product, honey is sometimes adulterated with substances such as sugar syrup, molasses, starch, and water to fool the consumer with a finished product that is less than pure. Detecting adulterated honey is a challenge due to natural variations in composition arising from nectar differences. Additional variability is added via processing and storage conditions.

As research has demonstrated, fluorescence measurements are ideal for a rapid screening of honey samples because fluorescence is nondestructive and simple to perform. Little to no sample preparation is necessary and testing does not require complicated instrumentation or highly trained personnel.

Experimental Conditions

We used a QE Pro-FL spectrometer and a 365 nm LED as the fluorescence excitation source. Fluorescence spectra were measured for several different types of honey, including clover honey, golden blossom honey, orange blossom honey, and organic honey.

Undiluted honey was pipetted into a disposable cuvette and placed in a 4-way cuvette holder with the excitation and emission fibers arranged at 90 degrees. The fluorescence spectra measured for the honey samples (measurement time was kept constant for all the samples) are shown in Figure 1. Differences in fluorescence intensity and subtle differences in spectral shape are observed for all of the samples.

Figure 1: Fluorescence of pure honey with 365 nm excitation. The spectra are dominated by fluorescence response due to the flavonoids present in the honey.

The broad fluorescence peak observed between 400–700 nm in each spectrum results from the presence of flavonoids (antioxidant compounds) in the sample. Variations in the shape of these fluorescence spectra are attributed to differences in the flavonoid composition of the nectar used in the honey. The small peak at 365 nm is not fluorescence from the honey but excitation energy that is scattered into the spectrometer by the undiluted, optically dense honey samples.

The flavonoids that dominate the fluorescence spectra for honey shown in Figure 1 are polyphenols. These plant metabolites determine the color, aroma, and flavor of the honey, and provide antioxidant and other health benefits. The fluorescence spectrum for each honey sample illustrates the sensitivity of fluorescence spectroscopy for characterizing honey.

Results

The measurements we conducted focused on the fluorescence of a small set of pure honey samples using a single excitation wavelength. Additional measurements could be performed by mixing and matching our modular spectroscopy components. For example, we could use a range of LEDs for fluorescence excitation to find the optimal excitation wavelength for the detection of honey adulterants.

Conclusions

Regardless of our approach, through the use of modular spectroscopy components, measurements can be taken out of the laboratory setting to test honey quality during bottling or at the point of sale to authenticate that the honey is pure.

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