There are a couple of different ways a vacuum flask can keep the contents warm and it starts with the double-layered base.

The Hydro Flask insulated food jar was one of the simplest designs we tried and the 28oz (828ml) version offered some serious capacity. You can also get a smaller 20oz (591ml) version, though.

How does a vacuum flask work?

These flasks are designed to prevent any temperature drops influenced from outside factors. So, even if you’re trekking through a snowy wilderness, the tough material and vacuum insulation will ensure that your food stays nice and toasty. For kids, there are smaller capacity ones of around 300ml. These don’t weigh as much, will fit into lunch boxes and, as kids generally eat less, there’s nothing going to waste. This food flask is made from stainless steel which has a range of benefits. For starters, it’s really lightweight so it won’t weigh you down, but it’s also incredibly durable. On top of this, stainless steel is very resistant to rust so you won’t need to worry about the life of your flask. We've tested the best food flasks that will keep your food warm - or cold - for hours, without you having to worry about spillages. What to look for in an insulated food flask When we tested this flask, it kept the contents piping hot until we were ready to enjoy it and the slimline frame meant it was easy to store in our lunch bags or take on the go.

For maximum heat retention, fill your SHO Food Flask to near capacity with food or liquids. Excess empty space will result in less efficient heat retention. To remove this, simply add a couple of tablespoons of white vinegar to the flask and top it up with water. Leave this for a couple of hours or overnight to give it time to work. If you’re buying a soup flask then it’s because you want to be able to keep your soup hot and tasty until you’re ready to eat it. However, some flasks have inferior insulation while others will keep the soup hot all day long. Think about this and compare it to your needs; nobody wants to get to lunch time to find that their food is tepid at best. Is The Flask Leakproof? France, Germany, Italy, Spain, Ireland, Netherlands, Austria, Denmark, Poland, Sweden, Czech Republic Sandwiched between the inner and outer layers is a void - this vacuum stops the heat from the inner layer reaching the outer layer as heat can only be conducted through particles, whether it’s in the air, through liquids or solids.Because this is such a durable product, we think it’s ideal for outdoorsy people; perhaps if you’re going out climbing or cycling and want something that’s going to resist these kinds of environments. Moreover, it won’t leak, sweat or burn your hands, so it’s got everything going for it. On a budget – The 720°DGREE wunderJar Food Flask makes for surprisingly good value for those not wanting to overspend on a food flask. Even with its environmentally-friendly credentials, which often come with a little extra on the price tag, this particular flask is very reasonably-priced for a flask with a solid amount of volume for single portions and exceptional build quality and style.