Determination of the 2-hydroxymethyl-5-furfuraldehyde (hydroxymethylfurfural or HMF) content in honey. HMF is hardly or not at all present in honey only just harvested. 

It is always formed afterwards from the sugars in honey under the influence of an acid environment (always present in honey; pH = ± 4) and heat. This reaction is irreversible and thus constitutes a criterion for the manner in which honey is treated. At room temperature the increase in HMF is limited; at high temperatures the HMF increases quickly.

The HMF content also increases due to the age of the honey.

HMF is determined spectrophotometrically.  The UV absorbance of a cleared solution of honey in water is measured by means of a reference solution in which the HMF is reduced by bisulphite.

Legally the HMF content may not exceed 40 mg/kg. A higher content is allowed for honey processed into other products (baker’s honey) and honey of tropical origin.