A solid performer in testing, SHO’s food flask kept water chilled for the advertised 20 hours and even managed to keep a portion of ice cream very cold, albeit a little melted, for the same period of time. Similarly, it kept hot food such as pasta piping hot over a period of 10 hours. The flask’s design also proved practical as well as stylish, with neat touches including a handy foldable spork, which stores in its lid, and a rubber carry handle. While the 530ml version is suitable for single portions, those of you looking to share may want to opt for the more roomy 800ml option.

Capacity: Bigger capacity means more contents to enjoy. It also means more weight to carry, but only if you fill it to the brim. For best insulation, buy a flask that’s slightly bigger (but not heavier) than you need, and leave a couple of centimetres of air at the top. We’ve included capacity and weight info for all our flasks below. Button-operated lids are more efficient to use than twist-and-pour stoppers – we found the button-operated lids to be more secure than the twist-and-pour stoppers.Angle tough-to-open flasks away from your face Also, do it in an open space as this will protect you and others. Avoid abrasive detergents and materials when cleaning your flask – bleach will stain and damage stainless steel, and scouring pads will scratch the outside of your flask The space between the two flasks creates a vacuum, as labelled above, preventing heat transfer. The wall of the inner flask is silvered to prevent heat radiation deflecting heat away from the flask. Don't fully submerge the flask in water – doing this might allow water to get inside the insulation layers, affecting the quality of insulation None of the flasks leaked from the main body – however, some flasks can't be filled to their stated capacity without liquid coming out when you screw on the lid.

We've reviewed the best Thermos flasks, along with vacuum flasks from Chilly's, Stanley and Hydro Flask, and cheaper own-brand products from Wilko, Asda and Argos We hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. There have been reports of vacuum flasks exploding during use. Although rare, if you're not following proper guidelines, it is possible. Since the American manufacturer, Thermos, came to fame, the term has become slang for vacuum-insulated flasks. Although the terms are largely interchangeable, the original scientific phrasing is 'vacuum flask'.

The two glass-lined flasks we tested were the only flasks to fail our drop test: one shattered on the first drop, the other fell apart on the second. Flasks are more 'smell prone' than 'stain prone' – none of the flasks we tested had any staining after cleaning, but some retained a faint coffee smell. It’s 100% leak proof – no matter how hard we tried, we couldn’t get any liquid to escape and the automatic opening trigger means you can drink from it one-handed with ease. Finally, there’s an additional loop handle on top so it can easily be attached to bags or simply carried this way. All in all, this is brilliantly stylish and robust Invest in a sturdy bottle brush for stainless steel flasks – it will help you get to hard-to-reach areas inside the flask but could damage glass-lined flasks

Multi-wall insulation: Double-wall insulation creates a vacuumed layer, which is vital for keeping hot drinks hot and cold drinks cold. Some flasks include more layers for even better insulation. The 100% stainless steel construction of today’s flasks means there’s no risk of inner walls shattering when dropped. Don't leave food and drink in a flask for a long period of time Bacteria can breed and this produces gas. Pressure then builds up which can force the lid off the flask. We dropped each flask five times from 90cm to mimic a dropped flask. Those with large dents that inhibited function or ability were rated low. Whether it’s a chunky “food flask”, sleek “vacuum bottle” or something in between, the job of a flask is to keep food and drink hot or cold without leaking, all day long. Some of the winners on our list can insulate for more than 24 hours, which is long enough to survive the toughest hike or most gruelling commute. A Thermos King will last years but it’s not officially dishwasher-safe, and you’re not even meant to submerge it in water (if you do, you kill the manufacturer’s 50-year guarantee). Inquisitive users report that it does survive the dishwasher intact, albeit with peeling colour.Insulated cup: Good flasks also pay attention to the insulation of the detachable cup, if they have one. This is so your drink stays warm while you’re drinking it. Meanwhile, the outside stays cool, so you don’t risk burnt hands. Thermocafe vacuum insulated food flask keeps your food fresh for longer. Handy metal folding spoon included Need to know: Not dishwasher safe, button-operated lid, comes with one cup, also available in 350ml and 500ml sizes We examined each flask and its pouring mechanism and attachments for faults. We penalised flasks that had faults affecting their usability.