5-HMF (5-hydroxymethylfurfural) is a thermal decomposition product of 
fructose - one of the two primary sugars in honey.  The older the honey, 
the higher temperatures the honey has seen, and the amount of time at 
elevated temperatures are all factors in its level.  It is not deemed a 
factor for humans, and sometimes even quite to the contrary, but it has 
been implicated as "not being good" for bees.  Honey is naturally high 
in acid due to the gluconic acid from the glucose oxidase enzyme 
activity.  This also produces hydrogen peroxide that is in a 
proportional amount.  The result for a high temperature and long time 
for thermal decomposition would be 5-HMF.  In the EU (European Union) 
honey is routinely tested for HMF as an indicator for age and/or 
quality.  There are even guidelines for the sale of honey based upon the 
5-HMF levels.  These EU consumers there are expecting filtered "raw" 
honey. 

Erik

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