A large stainless-steel vacuum flask is the best haybox you can salvage, because a vacuum stops conduction and convection that loose insulation can only slow. Boil the food hard in a separate pot, pour it into a pre-warmed flask, seal it, and treat the hold limit and the 60-degree rule exactly as for any other box. Never put raw or under-boiled beans in a flask with just hot water.
Part of Retained Heat Cooking
A vacuum flask has two walls with the air removed between them, which leaves only radiation and the neck as routes for heat to escape.
What The Overview Claims
The overview calls a large stainless-steel vacuum thermos or insulated food jar the ultimate haybox, says vacuum insulation is 10 times more effective than any loose insulation, and says food stays above 70 degrees for 8 to 12 hours. These are the overview’s figures and they are not independently sourced here; flasks vary widely with size, age and lid, and a small flask holds little heat. Test your own with plain water and a thermometer.
Method
Step 1: Pre-warm the flask. Fill it with boiling water, close it and leave it while the food cooks, then pour that water out.
Step 2: Cook the food in a separate pot. Boil it hard for the full recipe time first (see Grains & Legumes).
Step 3: Fill the flask with the boiling food, leaving a little space at the top, and seal at once.
Step 4: Open carefully. Point it away from you; the contents may be under pressure.
The overview suggests it for overnight porridge or taking hot food on long journeys; the 8-hour limit and reheating rules still hold. Keep the food above 60 degrees, as for any other box.
Common Mistakes
- Putting raw beans or grain in with boiling water to “cook in the flask”. The contents stay at 75 to 85 degrees for hours, which is hazardous for beans.
- Using a flask with a cracked or dented inner wall. The vacuum has failed.
- Filling to the brim and screwing the cap tight. Steam pressure builds.
Safety
Pressure and scalds
A sealed flask of boiling food holds steam and can eject its stopper or spray hot contents when opened. Open it slowly, face and hands clear. See Minimum Safe Temperatures for the temperature rule that applies to what is inside.
What’s Next
- Learn next: Multi-Pot Haybox — several pots in one box
- Combine with: Porridge & Oatmeal — a classic flask food