Food doesn't always have to be hot to be nourishing
I was re-reading novel that used to be a favorite guilty pleasure* the other day. One minor scene in the book recounting the childhood of one of the main protagonists describes her lunch routine when she was a little girl. She would always share a lunch with her mother at her workplace. The emphasis is mine:
There was always something hot, nourishing, and delicious in [her mother's] covered basket for them to share. Many of the other workers also brought their lunches from home...
I'm sure I didn't even notice that passage when I read the book originally, but now that I am hyper-attuned to anything related to bringing lunch from home, I found the "hot, nourishing and delicious" part intriguing. First of all, presumably the mother brought the lunch from home in the morning. The time period was the 1950s, so there were no microwave ovens. So how did the mother manage to make the lunch brought in a covered basket hot? Actually I'm sure that the author did not even think this through when she wrote it; she was great at detail in most of her books, but I think she was a lot more interested in things like fashion and sex than food. She probably threw in the phrase "hot, nourishing and delicious" to convey a feeling of a mother's love for her little daughter.
In many cultures, hot food is associated with those terms 'nourishing and delicious', even if the food we actually put in our mouths is not piping hot most of the time. And this insistence on hot food is a psychological barrier for quite a few people regarding bento meals and the fact that they are mainly meant to be eaten cold/at room temperature, judging from the comments and emails I've gotten over the years. Quite a few people cannot even conceive of eating a 'cold' lunch; to them giving a 'cold' lunch to kids seems positively cruel. The comments to my Quora reply about heating up bento boxes are pretty typical.
To my mind, only a few foods have to be eaten piping hot: soups and stews and curries, hot cooked porridges, hot tea or coffee, bao or steamed buns. Many foods are a lot better when they are hot or at least warm, such as the spring rolls (harumaki) I posted a recipe for last week - on JustHungry, not JustBento, as you may have noticed. But there are plenty of foods that are delicious, even better, at room temperature - and that's what most homemade bentos are composed of. (Some, though not all, cheap storebought bentos in Japan are meant to be heated up a bit in the microwave before eating, although they can usually be eaten without heating up too.)
Some Japanese people do like to reheat their bentos - the most frequent complaint is against cold rice - so if they have access to a microwave, they'll heat up their food there. Some kindergartens have bento heating facilities, where metal bento boxes and be put in en masse and steam-heated. And of course there are thermal lunch jars and bento boxes. But as I've stated on these pages before, most bentos are eaten without reheating - and no, it's not weird at all. I think the prejudice against 'cold' food is a cultural and psychological one in the main. Is hot food always more nourishing for you than cold? Are hot Tater Tots** better for a child than a 'cold' pasta salad brought from home with lots of vegetables and protein? Obviously not if you think it through, but it's hard to shake those deeply ingrained beliefs.
So if you've been resisting the idea of packing a bento lunch because of the heat issue, try this: make a list of at least 5 foods that you love, that don't have to be piping hot or even warm to enjoy. Now imagine packing those foods in a portable container. You''ve taken the first steps towards incorporating bentos in your routine! Then, try the recipes on this site, 99% of are meant to be tasty at room temperature. (For anything that needs to be refrigerator-cold or hot, I say so clearly and suggest appropriate measures, such as using a thermal lunch jar.)
[* In case you're interested, that old guilty-pleasure book is Scruples by Judith Krantz. I found it hidden in the drawer of the bedroom was assigned to in the house my parents rented in Port Washington, Long Island, for a month while they were house hunting, after we moved back to the U.S. from Japan. I think the bedroom belonged to a teenaged girl, and the book was dog-eared at all the naughty bits. For 16-year old me, it was quite a revelation. I must have re-read that book 10 times in that month...and it may even have helped to brush up my rusty English. ^^]
[** I love Tater Tots, in moderation.]