**4. Conclusions**

We reported the development of a novel fluorescence screening method combining the FDP intensity with the apex wavelength of fluorescence emission spectra in order to identify syrup adulterants in honey. We demonstrated that the FDP intensity decreases significantly and the apex wavelength undergoes a distinct, dose-dependent blueshift with the addition of increasing amounts of syrup to pure honey. Using this method, 10% syrup and higher can be detected in adulterated honey. To validate this method, we analyzed 20 syrup-adulterated honey samples and successfully detected 11 out of 15 with HFCS, 1 out of 2 with rice syrup, 2 out of 2 with beet syrup and 1 (out of 1) with maltose syrup, thus demonstrating 75% accuracy. In comparison with other more sophisticated, labor-intensive and expensive spectroscopy methods, the analysis time was less than 1 min, and the instrument can be operated with minimal training and no subsequent statistical analyses to interpret the data. For most consumers, honey is purchased for its nutritional and medicinal benefits, and especially for its purity. This is also the first report, to our knowledge, on the application of the FDP to research on food adulteration. These experiments provide an exciting route toward wide adoption of this technique by honey producers and regulatory agencies.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods11152316/s1, Figure S1: The relationship of amount of syrup added and area of FDP. Table S1: Fluorescence Spectra Information of acacia honey Samples. Table S2: The comparison of fluorescence spectra information from 3 different manufacturers.

**Author Contributions:** Methodology, investigation, writing—original draft, funding acquisition: S.Y.; methodology, investigation, formal analysis: M.S.; methodology, investigation: X.W.; investigation, writing—review and editing, supervision: J.S.; conceptualization, writing—review and editing, supervision, funding acquisition: X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Agricultural Science and Technology Innovation Program under Grant, the project of the Bee Industry for Quality Improvement and the Shanxi Agricultural University Science and Technology Innovation Fund (grant number 2020BQ83).

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** We are grateful to senior engineer Xinying Liu from the Shandong Bee Industry and Bee Products Quality Monitoring for providing the positive samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
