Away from the resources of the kitchen there is often a need to control the temperature of food both for safety reasons and for practical considerations.
Scottish born James Dewar (1842-1923) whose most important work was in the field of extremely low temperatures, developed the double-walled flask with a vacuum between the walls. He found that heat transference from either direction, from the contents of the flask outwards or from the outside world into the flask, only occurred very slowly. Thus, from equipment in the laboratory the Dewar flask was developed. Today, it is more commonly known as the vacuum flask with Thermos being perhaps the best known company associated with this product.
Originally comprising a metal outer casing and a glass inner flask, they were designed with simple uniform shapes and limited colour ranges. The advent of plastics enabled vacuum flasks to be produced in many shapes and unlimited colours, thus appealing to a wide consumer market (1 - 3).
Adaptations of the conventional glass lined vacuum flask came in the form of 'unbreakable' flasks and mugs. The Roughneck (4) and the insulated travel mugs (5 - 6) abandon the glass inner and replace it with a more robust plastic form.
Hot or cold liquids are readily accommodated in the narrow bodied flasks or jugs with pouring spouts, but more solid foods require an insulated container with a wider body and neck. The Insulex picnic jar, with carrying handle (7) exemplifies a simple solution to an everyday problem of maintaining the temperature of food on the move.
The development of the larger still, double-walled container combined with the use of packs of frozen, non-toxic, gel, saw the end of the familiar warm, curled sandwich of the traditional picnic basket. The cool-box (8) enabled a picnic meal to be made in the morning and safely consumed hours later. The slowly defrosting ice-packs (9 - 10) impart their low temperature to food within the lidded container which excludes the ambient temperature.
Ez-freeze products (11 - 12) rely on the same 'frozen' core of compartmentalized frozen gel system to maintain food in a safe condition and the freezeable double-walled liquid lined beer mug provides instant cooling for drinks on hot days (13).