Re: microwave, RF heating

["Bernard G. Cleyet & Nancy Ann Seese" <georgeann@REDSHIFT.COM>]

I don't think conductivity has anything to do with microwave absorption*.  Water is a
rather good insulator -- Try heating a pair of glasses, one with brine, the other
deionized water.  I'll be surprised if the temps are more different than expected due to
variations in the microwave field with position -- you have a "carousel" oven?  Both water
and fats, fatty acids, glycerides, etc. have O-H bonds whose stretching frequency is close
to that of the oven's magnetron.  Amino acids, and, therefore, proteins, also have OH
bonds (in the carboxylic group), so they heat up too.  I would expect the heating to be
some monotonically increasing function of the density of OH bonds.  I'll ask my
nutritionist friend about the comparative densities in egg yolk and white.

bc

* Well, yes, surround the food with several layers of al. foil and it won't get hot!

John Denker wrote:

> In a previous note I said:
>
> > So, we know about
> >  -- the obvious process involving the gross conductivity of water, and
> >  -- spin resonance (in abnormal applied fields)
> > ... but other than that, does anybody have any experimental evidence for a
> > mechanism for RF absorption in ordinary biological tissue under normal
> > conditions?  If so, I'd love to see the evidence.
>
> Here's a small mystery;  perhaps somebody can help me with it.
>
> a) In the first paragraph of
>    http://www.sharp.net.au/product_sales/mwo/microfaq.htm
> it says:
> > One example is an egg.
> > Energy penetrates to the centre, where the fatty yolk becomes hotter than
> > the white and cooks first.
>
> b) I've seen other reports that fat absorbs microwave power at least as
> well as water.
> c) I've seen some behavior consistent with this during ordinary cooking of
> fatty foods.
>
> On the other hand....
>
> 1) One normally thinks of fats and oils as having essentially zero
> conductivity.
> 2) I did the following experiment:  25cc of water in one beaker, 25cc of
> salad oil in another beaker, put both in microwave.  The water heated up
> much faster than the oil.
>
> Does anybody understand what is going on here?