Detection of adulterated honey produced by honeybee (Apis mellifera L.) colonies fed with different levels of commercial industrial sugar (C3 and C4 plants) syrups by the carbon isotope ratio analysis

•Fructose ratio of industrial sugars is important for detection of adulteration.•Sugar sources (C3 or C4) used for feeding colonies affect on success of analysis.•The official methods and ISCIRA could not efficiently detect the indirect adulteration with the syrups from C3 plants.•High relationships were found between the δ13C values of honey and C4%. In the present study, one hundred pure and adulterated honey samples obtained from feeding honeybee colonies with different levels (5, 20 and 100L/colony) of various commercial sugar syrups including High Fructose Corn Syrup 85 (HFCS-85), High Fructose Corn Syrup 55 (HFCS-55), Bee Feeding Syrup (BFS), Glucose Monohydrate Sugar (GMS) and Sucrose Sugar (SS) were evaluated in terms of the δ13C value of honey and its protein, difference between the δ13C value of protein and honey (Δδ13C), and C4% sugar ratio. Sugar type, sugar level and the sugar type*sugar level interaction were found to be significant (P