*3.1. Fluorescence Spectra Characteristics of Honey and Syrup*

To first determine if the FDP is a common phenomenon among fluorescence spectrophotometers from different manufacturers, and to test for variation between instruments, the fluorescence spectra for each honey type and sugar syrup were measured by the three brands of fluorescence spectrophotometers (Figure 1). By removing the specific optical filter during the acquisition process, the results show that the FDP is a common phenomenon among the different manufacturers. Furthermore, the application of fluorescence spectroscopy in combination with measurements of the FDP intensity can be used as a suitable indicator for the adulteration of honeys with a variety of sugar syrups.

**Figure 1.** Comparison of honey and syrup fluorescence spectra generated by fluorescence spectrophotometers from three different manufacturers ((**a**) pure honey analyzed by a Hitachi F-4500; (**b**) pure honey analyzed by a Lingguang F97; (**c**) pure honey analyzed by a Shimadzu RF-5301PC; (**d**) syrup analyzed by a Hitachi F-4500; (**e**) syrup analyzed by a Lingguang F97; (**f**) syrup analyzed by a Shimadzu RF-5301PC).

Although we found differences in the absolute, or quantitative, measurements between instruments from the different manufacturers, the apex and the area ratio of fluorescence emission spectra and the area ratio of the FDP between honey and syrup were uniform across all three instruments (Table S2). Data in our study were collected using a Hitachi F-4500 (Tokyo, Japan).
