Microwave or oven?

[tim]

What is the difference between defrosting in a m/w or in 'the bottom oven' at 220F??

[tim]

[supersprout]

My eye tim, you do ask 'em :o

Heating in the oven uses convection - the transfer of heat from a gas (or liquid) to its surroundings. The specific heat of air in the oven is much less than that of water. If you put your hand into water at 200 degrees C, you would scald it, but you could put your hand into the oven at that temperature without much harm, because the amount of heat transferred to your hand depends on the specific heat of the air, and is not enough to heat your skin too much. That's why cooking e.g. potatoes takes longer in an oven than by boiling, even though the oven may be at a higher temperature.

Microwaves operate on a similar principle to radiant heating (like a grill). However, the radiation is of a longer wavelength than that used in conventional grills (which use infra-red) and can penetrate further into the food. The radiation's frequency is exactly tuned to the frequency at which water molecules can vibrate. So when a microwave is absorbed by a water molecule it transfers all its energy to the molecule, and heats it. The 'hot' molecule can then transfer its energy to its neighbours, so that heat is transferred by conduction again. Microwaves can penetrate inside water to a depth of about 1cm, so not only the surface is heated, but a region of up to about 1 cm thickness. Microwave ovens don't provide a uniform level of microwave energy, and there are 'hot' and 'cold' spots inside every microwave. If you put a potato in the microwave and cook on full power for about 40 seconds, then cut the potato open, you will find finger-like regions extending from the outside where the potato has been heated. This is in contrast to the uniform heating of 'conventional' cooking methods.

I choose the defrosting method based on the shape, constituents, and density of food to defrost. In order for the microwave to defrost satisfactorily, the food has to contain a good proportion of water. If the food contains a high proportion of fat or oil, you would have to watch for burning in the microwave, so the low oven might be a better option. If you thaw meat in the microwave you may have noticed you always get some 'cooked' meat where the uneven radiation has heated the fat in the meat. I never thaw meat in the microwave for this reason, just overnight in the fridge. For e.g. a wide, shallow lasagne, I would definitely put it in the low oven (covered to avoid evaporation). If it's a brick-shaped piece of lasagne in a bag, I would defrost it in the microwave because it would be quicker. And if it's a flat bag of pre-cooked pulses, I defrost in a bowl of hot water. We all put frozen veg straight into boiling water - instinctively we know convective heating is very efficient.

I always freeze my food flat (in self seal freezer bags) to maximise surface area for later defrosting (and can stuff more in the freezer that way)

[tim]

Well - it gives everyone the opportunity to read such erudite stuff as you produce!!

Reason - Wife always says 'why use electricity when you have the AGA'?

'Defrosting overnight'? Yes - if you decided yesterday. But, even then, a large chicken or roast can take 2+ days. Difficult to forecast. And, although cold water is a safe way, it still takes for ever.
Good point about 'flat freezing' - freezes quicker too! But so often you get something like 30 chicken wings in a block from the butcher - not easy to re-arrange!!

Recently, I've been taking a different line - slow-slow. Put it in the bottom oven & let it be for 5 hours or so. Until the thermo bell rings!

PS Have you read 'Understanding Cooking' by Lundberg et al?

[supersprout]

Oh tim, how I miss my Aga :'(. I remember putting the turkey in the bottom oven on Xmas eve night, all stuffed and nestled in its tinfoil jacket, and waking up on Christmas morning to the festive aroma wafting upstairs. The turkey was always perfectly done and tender, and no last minute paddies. And doing toast in the tennis racquet. And no microwave - like you say, no need for one ;)

Haven't read the Booke you mentioned, just off to amazon now!

[tim]

Forgot to say - I mentioned cold water, for a large thing, so that it never exceeds the critical 40F.

PS The book - I see that Amazon have 4 on offer! All different in order of attribution. Ours is the UK Edition - paperback - ISBN 0 7131 15505 1970 r/p 1990. Don't know what the others are. And, on our cover, Ceserani comes first! 376pp.

[supersprout]

Grr, no product details - will >>>>>>>>> to the library and see if it's worth £4 plus postage! Looks interesting though as the authors come from catering industry, rather than being scientists interesting in cooking. Thank you tim, this was great fun for an otherwise immobile Sunday morning :D

[tim]

oooooops - 376 PAGES (pp - not P&P)!!! Rather fewer than Harold McGee's mag op?

[Curryandchips]

Thank you both for a thoroughly enjoyable and educational thread ! Banter like this makes this forum just perfect ! I may be interested in that book too, I will check at my local library first.

[Trixiebelle]

Just leave the freezer door open accidentally ... always works for me  ;D

I've got a Rayburn but I've never cooked anything in/on it after I tried to fry and egg and it took 35 minutes  ::)

And a funny story about 'Hodder and Stoughton': When I was a teenager I had a lovely boyfriend who had a twin brother. We used to go out on summer evenings to different local pubs with him and his girlfriend. One Friday night we got to his house and the brother was nowhere to be seen ... there was just a hand-written note on the kitchen table that said 'Hodder and Stoughton'. So we set off around the wilds of Derbyshire trying to find a pub called 'The Hodder and Stoughton'  ;D

I supopose you had to be there at the time  ::) As you were!

Trixie X

[supersprout]

LOL trixie ;D.
Harold McGee twas who first got me started on the science bit tim! That would be ... ooo ... 25 years ago? :-[  8)

[tim]

Good for the mind - if not the result!

[grawrc]

Having spent a large part of today degunging my 2 ovens I would say Microwave every time!! ;)

[supersprout]

Well Tim, the Booke arrived today, and what a treasure it is! Love the format - illustrations of food and molecules, and true/false q&as all the way through. It's like a souped-up  ;) version of the Food Trivial Pursuit my son and I used to play when he was small. Thank you so much for the recommendation, this is a great £4-worth, 2nd hand from the Unilever Research Library :o!
;D ;D ;D

[tim]

Glad!