[chris (OP)]

Naked Scientists Podcast listener Ryan needs some help with this energy / food-risk related frozen ready meal question!

Am I saving energy, safely, by leaving a frozen food lunch out for 3-4 hours prior to heating it up and eating it?

I ask because I sometimes succumb to one of the sadder pastimes in office life: frozen lunches eaten at the desk.

Usually I bring it to work, pull it out of my bag, and put it in the freezer. Then around lunchtime I bring it out and microwave it for the requisite time and eat it.

Lately, however, perhaps inspired by a forgetful morning, I left the meal, still packaged, at my desk until lunchtime. It had somewhat thawed. So I microwaved it for about half the time it would have needed and it came out nice and hot.

I'm curious if the power I'm not requiring of the microwave means hazardous things for my health.

What does everyone think?

[alancalverd]

The operating frequency of a microwave cooker is tuned to the absorption spectrum of liquid water, which is slightly different from ice, so thawed food always cooks more quickly even allowing for the latent heat of melting the ice.

If the packet was unopened and the food was precooked, just requiring heating, it is unlikely that any harm will arise from a few hours thawing.

Definitely worth checking the instructions on the packet if you don't cook it the same day. If the pack says "cook from frozen" it's probably OK to refreeze - you will lose some texture but the eventual rapid cooking will kill most of the bugs.Problems arise if you re-freeze stuff that must be thawed before cooking because if the first thawing allowed n bugs to thrive and they weren't killed by refreezing, the second thawing  will generate n² bugs, which could be a serious number.