Any treatment of honey that involves heating is liable to caromelize it:
it turns a deep red colour indicating the formation of hydroxy-methyl
fufural (HMF).  Such honey is rightly regarded as "spoiled" in some
countries (e.g. in Germany, where such honey may not be sold as "honey").
The maximum recommended temperature to which honey may be heated during
processing is 43 degrees celcius.  However, the caromelisation process is
a function of both temperature and time: a long time at even say 40 C will
cause darkening (several days), while rapid heating to a higher temperature
for short times (in a pasteuriser or a microwave oven - for at most a few
minutes) will not cause darkening.
        Crushing the combs and straining is effective but very messy - and
of course you cannot return the frame/comb to the hive for re-use.
Heather honey can only be extracted in this way (crushing the combs) because
it is thioxtropic (viscosity varies with stirring) - it cannot be extracted
in a centrifuge.
                   Geoffrey Hunter, York University, Toronto  FS300022@YUSOL