HMF (5-hydroxymethylfurfural) is an organic compound formed in honey after extraction from the comb. Freshly extracted honey has a very low level of HMF. Soon after extraction of honey the HMF level increases. The speed at which HMF level is raised depends on Processing and Storage temperature. Though HMF is not harmful to health, it is an important indicator of fresh honey.
HMF is basically a decay product of levulose. The levulose or fructose present in honey undergoes Millard’s reaction producing HMF. This reaction takes place because of the acidic environment in honey. Raise in HMF also darkens the honey. (One thing we should keep in mind that dark honey is not necessarily old honey. Many floral varieties of honey are dark in colour by nature. It is often found that darker honey is richer in nutrient as compared to light colour honey.)
There are 3 factors which primary effect the speed by which HMF is raised.
- Age of honey
- The heat used during processing
- The storage temperature of honey
Age of Honey
Freshly extracted honey has less than 10 mg/kg of HMF. After extraction from the comb, HMF is bound to increase even if it is unprocessed. Old honey may have HMF well above 100 mg/kg. Honey is suitable for consumption even after thousands of year. The HMF level is not the criteria for edibility. Some researchers even found HMF to be beneficial to health.
The heat used during processing
High heat during honey processing speeds up this reaction, producing a large amount of HMF at once. Therefore the elevated level of HMF also indicates the use of high heat during processing of honey. Ideally, honey should not be heated beyond 45 °C, But most of the commercial honey is heated above 70 °C to make it more flowable before ultrafiltration.
The storage temperature of honey
Storage of honey for a longer period of time especially at higher atmospheric temperature produces a good amount of HMF. Therefore honey should be stored at a lower temperature if you are planning to store it for a longer period of time. But one should keep in mind that if honey is stored at a lower temperature it will get crystallised.
Sometimes it is impossible to stop the raise of HMF in a hot atmosphere. HMF may continue to raise even after packing the honey (Honey may have HMF below 80mg/kg before packing, but it may increase well above 80mg/kg on the shelves because of the atmospheric temperature) This phenomenon raises an important question on the HMF parameter.
Though the HMF parameter may indicate the freshness of honey. But it is not the parameter to assess the purity of honey. Many adulterants may have nill HMF.