# Thermal Mass Thermos

A ther­mos does a great job with liquids: the in­su­la­tion means you can pour in hot soup in the morn­ing, and en­joy hot soup at lunch. But it doesn’t do nearly as good a job with solids: fill one with rice and it quickly falls into the “dan­ger zone”.

One recom­men­da­tion you’ll see is that you should fill the ther­mos with hot wa­ter, let it sit, and then swap the wa­ter for the food. This does help, but be­cause the ther­mos doesn’t have that much ther­mal mass it only helps some. What if we put a rock in the ther­mos to hold heat, and pre­heat it along with the rest of the ther­mos? How much does this help? Does this help enough?

The gen­eral rule is that hot food shouldn’t be be­low 140F for more than two hours, be­cause sub­stan­tial bac­te­ria can grow, and the closer it is to 100F the worse it is. My goal here is to be able to send hot dry food with Lily’s lunch, and I pack it four hours be­fore she gets to eat it, so the ques­tion is: what’s the tem­per­a­ture two hours in, which is two hours be­fore lunch time? If that’s above 140F we’re ok, if not we should figure some­thing else out.

I ran an ex­per­i­ment on one cup of cooked rice, the kind of dry food you’re not nor­mally sup­posed to put in a ther­mos. I heated it in the microwave un­til it was steam­ing hot (it mea­sured ~188F with an in­frared ther­mome­ter):

I tested this in three con­figu­ra­tions:

• T1: a ther­mos that has been pre-heated with boiling wa­ter.

• T2: same as T1, but with the ad­di­tion of a 4.3oz (122g) rock with vol­ume 1.5oz (45ml).

• T3: same as T2, but the ther­mos has been pre­heated with two rounds of boiling wa­ter.

After two hours I mea­sured the tem­per­a­ture of the rice:
• T1: 108F

• T2: 136F (143F in the mid­dle)

• T3: 142F (148F in the mid­dle)

The rock is clearly helping a lot, though it looks I should do the full T3 treat­ment with two rounds of boiling wa­ter in this par­tic­u­lar ther­mos.

This works, but it is kind of hacky: rocks are odd shaped, hard to clean, rat­tle around, hide food, etc. How else could we solve this? Ideas:

• Use wa­ter in­stead of a rock, be­cause wa­ter has ex­cel­lent heat ca­pac­ity by vol­ume and very good heat ca­pac­ity by weight. Have two sec­tions: an outer one you pour boiling wa­ter into, and an in­ner one for the dry food. Keep heat from leav­ing the over­all en­clo­sure, but al­low heat to move freely be­tween the two sec­tions.

• Some­thing microwave-safe which uses a solid for the ther­mal mass. tick it in the microwave for a few min­utes be­fore putting the food in, or even with the food in­side. Water has a max­i­mum tem­per­a­ture of ~212F, but with a de­sign like this you could bring your ther­mal mass up to, say, 300F (wrapped in an in­su­lat­ing layer so no one got burnt).

• Use a phase-change ma­te­rial? Maybe like this travel mug but with a higher tar­get tem­per­a­ture than 136F.

• Bat­tery pow­ered cross be­tween a ther­mos and a crock pot. I do see some things kind of like this aimed at peo­ple who want to keep coffee warm in their car (no bat­tery; takes power from the car) and some ex­pen­sive ones (ex) that have good in­ter­nal bat­ter­ies. But noth­ing I can find de­signed to eat warm dry food out of?

Why isn’t there a product de­signed to keep dry food warm? Am I just not find­ing it?
• I was dis­ap­pointed that Jeff didn’t con­clude, based on the de­tailed ev­i­dence he found, that it’s prob­a­bly fine to just pack the rice in a reg­u­lar un-mod­ified ther­mos.

Or am I way off in sum­ma­riz­ing his ev­i­dence this way?