There’s a particular kind of winter disappointment that comes from opening a flask at lunchtime and finding lukewarm soup. If you’re batch cooking soup this winter (and if you’ve got a soup maker, you probably are), a good food flask is the bit of kit that gets it to lunchtime still steaming.

My pick is the Thermos Stainless King Food Flask (470ml). It’s not new or exciting. It’s just the one that keeps doing what it says.

Why this one?

Thermos rates it at 9 hours hot and 14 hours cold. Fill it at 7.30am and you’re well within that window at 1pm, even if lunch slips to mid-afternoon. It’s double-walled vacuum insulated stainless steel inside and out, so there’s no plastic liner to stain orange from a tomato soup or hang on to the smell of last week’s curry.

A few other details that make it nice to live with:

  • The wide mouth makes it easy to pour soup in without a funnel, eat straight from it, and get a sponge right to the bottom when washing up.
  • The insulated lid doubles as a bowl, so you don’t need to pack one.
  • The outside stays cool to the touch with hot food inside, so it’s comfortable to hold and won’t scorch the inside of your bag.

Is 470ml enough?

For soup as a full lunch, 470ml is about a generous bowl, which suits most people with a bit of bread on the side. If you’re feeding a teenager or you’re out on a long, cold walk, look at the larger flasks in the same range. For chilli, stew or pasta, 470ml goes further than you’d expect because it’s denser.

The trick most people miss

Preheat it. Fill the flask with boiling water from the kettle, put the lid on, and leave it for five minutes while you heat the soup. Tip the water out, then add the soup piping hot. Skip this step and the cold steel pulls the heat straight out of your lunch, and that’s usually the real reason people think their flask “doesn’t work”.

Two other small things:

  • Fill it close to the top. Air in the flask means heat loss.
  • Don’t put fizzy drinks or anything still fermenting in it. Sealed vacuum flasks and pressure don’t mix.

Who might want something different?

If you only ever eat at your desk with a microwave ten steps away, a basic lidded tub is cheaper and does the job. But for anyone working on site, driving between jobs, at school, or just sick of the microwave queue, a proper food flask makes a winter of homemade soup actually work. This is the one I’d buy.