5 hmf

Cocoa shells, a by-product of the Kandit chocolate factory (Osijek, Croatia) were obtained in 2017. Before the extraction, the plant material was ground using a standard laboratory mill. DPPH and ethyl acetate were purchased from Sigma-Aldrich Chemie (Steiheim, Germany). All other solvents were of analytical grade and purchased from J.T. Baker (J.T. Baker, Phillipsburg, NJ, USA). Folin-Ciocalteau’s phenol reagent was purchased from Sigma-Aldrich Chemie (Steiheim, Germany). Caffeine (purity 99%), theobromine (purity 98.5%), catechin (purity 99%), epicatechin (purity 97%) gallic acid (purity 99%) and chlorogenic acid (purity 98%) were purchased from Sigma-Aldrich. Theophylline (purity 99%), furfural (purity 99%) and 5-HMF (purity 98%) were purchased from Acros Organics (Geel, Belgium). Sugars (glucose, rhamnose, arabinose, mannose, xylose and fucose) and carboxylic acids (levulinic acid, lactic acid and formic acid) with high-purity (>99%) for HPLC analysis were obtained from Sigma-Aldrich.

The mandarin peels (Citrus unshiu Marc.) of the variety “Kuno” were obtained in November 2019 from a small family farm Dalibor Ujević (Opuzen, Croatia). Before the extraction, the mandarin peels were washed several times in distilled water, freeze-dried (Alpha LSCplus, Christ, Germany) and milled using a IKA M 20 universal laboratory mill (IKA-Werke GmbH, Staufen, Germany). Carbon dioxide (CO₂) used in SC-CO₂ extraction was 99.97% (w/w) pure (Messer, Osijek, Croatia). The HPLC standard hesperidin (purity 89.5%) was obtained from Dr. Ehrenstorfer GmBH (Augsburg, Germany), while narirutin (purity ≥ 98%), naringin (purity ≥ 95%), and chlorogenic acid (purity ≥ 98%) were purchased from Sigma-Aldrich (Steiheim, Germany). Standards 5-HMF (purity 98%) and rutin (purity 97%) were purchased from Acros Organics (Geel, Belgium). Chemicals 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Folin-Ciocalteu’s phenol reagent were purchased from Sigma-Aldrich Chemie (Steiheim, Germany). All other solvents were of analytical grade (J.T. Baker, Phillipsburg, NJ, USA). Milli-Q pure water was used during experiments and analytical measurements obtained by Milli-Q Millipore system (conductivity ≤ 0.054 μS/cm).