Firstly, thank you Maki for a wonderful blog, it has been my encyclopedia of Japanese food and culture for the past year since moving to Japan!
I saw a Chinese Medicine doctor a few years ago for digestive problems and she recommended that I do not eat frozen foods. She said that my digestion needs "warming foods" (this is not true for everyone) and frozen foods even when thawed/warmed still retain a certain "coldness".
My dilemma is that keeping this in mind, it is virtually impossible to keep a bento stash.
So, my question is, how bad are pre-prepared frozen foods?