Hydroxymethylfurfuraldehyde.  
 
It is one of the breakdown products of sugar and naturally occurs through  
age, much accelerated through the application of heat. It's the stuff that turns 
 your strawberry jam brown.  In the UK and probably throughout the EU it is  
illegal to attempt to sell table honey with (if I have got the figure right in 
 my head) more than 40 ppm of HMF.  Above that level it has to be labelled  
as 'Baker's Honey - suitable only for cooking.'
 
Chris

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