*2.5. Hydroxymethylfurfural Analysis*

The HMF content in the analyzed honey samples was determined chromatographically with the application of a Knauer HPLC system (Knauer GmbH, Berlin, Germany) equipped with an UV K-2501 detector and with a reversed-phase C-18 Vertex Plus Eurospher column (BGB Analytik Vertrieb GmbH, Lörrach, Germany), as described previously [41]. The internal diameter of column was 4 mm, length 250 mm, and particle size 5 μm. The column and detector were placed on a thermostat at 30 ◦C. As a mobile phase, a mixture of methanol and deionized water (10:90, *<sup>v</sup>*/*v*) was used, and the flow rate was 1 mL·min−1. For the quantitative analysis of HMF, the external standard method was applied. The determinations were carried out in triplicate. Honey samples were prepared based on the European Honey Commission procedure [40].
