We humans are unique in so many ways, but perhaps our mastery of the systems used in getting food into our mouths is the most remarkable. From our humble hunter-gatherer beginnings, in which we behaved much as any other animals, we have discovered agriculture, domesticated animals, learned to harvest milk and eggs and are nowadays even capable of growing a steak from just a few cells (we’ll temporarily gloss over the cost and taste of the finished product). However, arguably just as important as these advancements has been our ability to store food, allowing us to survive the harshest of winters and conditions in numbers few other animals could hope to match.
Our methods of food storage have varied widely over the years; beyond the simple tactic of ‘keep your food somewhere basically dry and clean’, in the last few decades we’ve moved on from our old favourites to explore as wide a variety of solutions as chemical preservatives and freeze drying. However, today I wish to explore an older, yet arguably far more interesting, method that remains our current favourite method of home food preservation: that of refrigeration.
Refrigeration, or the highly technical art of ‘making food colder so bad things can’t survive’, is an ancient idea; ice houses have been found in Iran dating from 1700BC, and were in use in both China and the Roman Empire throughout both culture’s long histories. Since making their own ice was impossible using the technology of the time, these ancient civilisations simply moved existing ice to a designated place where it with useful and came up with ingenious ways to make sure it stayed cold throughout the long summers; these great buildings would have immensely thick walls and were then packed with straw or sawdust to prevent the air circulating, thus helping to maintain their temperature. Thanks to their thick walls, ice houses were necessarily vast structures, acting rather like communal refrigerators for a local lord and his community and capable of holding up to thirty thousand tons of food.
In other countries, where snow and ice was harder to reliably come by (even in winter), refrigeration didn’t really catch on and people stuck with salting their food. However, because this a) made a lot of food taste disgusting and b) meant you still had to drink warm beer, by the seventeenth century it became relatively common for the rich across Europe to import ice (at vast expense) to their own personal ice houses, allowing them to serve fancy drinks at parties and the like and enjoy an unsalted pork roast in February. Ice was a symbol of luxury and status, which is presumably one of the reasons why ice sculptures are even today considered the pinnacle of class and fine living (that and the fact that they’re really, really cool). During the Georgian and Victorian eras, it was common practice for families going out for a day’s jolly (particularly in the colonies) to take an ice box of food with them, and there were even ice shops where the rich would go to buy high-quality, exceptionally clear ice for whatever party they happened to be hosting- but, by the end of the century that business would be long bust.
Y’see, in 1805 a man named Oliver Evans, who would later become known as ‘the father of refrigeration’, invented a device called the vapour-compression refrigeration machine. This is, basically, a series of tubes containing a stable coolant; the coolant is first compressed, then condenses (causing it to lose the heat it’s picked up- this is the vapour-compression bit), before going back inside and evaporating again thanks to a mixture of a pressure change and temperature change, thus allowing it to pick up heat. This rather convoluted evaporation/condensation procedure (first investigated by Benjamin Franklin and a chemistry professor called John Hadley half a century earlier) wasn’t actually the preferred solution for a few decades, since the earliest devices built were ‘compression-compression’ systems that used air as a coolant and were thus only able to change its pressure rather than get it to liquefy. Regardless, it was soon realised the vapour-compression system allows a device to more efficiently control the transfer of heat from in to out rather than vice versa, and is now pretty much universally used in modern day ‘heat pumps’ of all sorts.. Incidentally, heat pumps are among the most efficient systems ever devised for heating/cooling a space, and nowadays they are increasingly used (in the opposite direction, of course), to heat houses, as they use far less energy than conventional methods of heating.
But anyway; back to fridges. Evans’ design never actually built a prototype of his design, but it was picked up on and revised several times over the next seventy-odd years until the design was sufficiently advanced to be used in commercial ice makers, putting the old ice ‘manufacturers’ (who simply got their ice out of a convenient mountain lake or glacier) out of business, and by the early 20th century the devices got so good that they were able to liquefy air.
Surprisingly, it wasn’t until after this point that the modern science of refrigeration began to make it into our homes. It took until 1913 for a patent to be issued for a domestic refrigerator, and even that was just a way of keeping an existing ice box cool; it didn’t actually cool the interior of the fridge down. However, the following year the USA got the awesomely-named Kelvinator refrigerator, the first properly practical domestic fridge that held some 80% of the market by 1923. During the economic boom of the 1920s, fridges were among the many devices whose popularity exploded, and they gradually became bigger, sleeker, more practical and more efficient in the process. By the 1930s they’d even managed to find a coolant that wasn’t highly corrosive or toxic, which all seemed terribly fantastic in the days before most people knew what ‘CFCs’ and ‘the ozone layer’ were. By 1940 the idea of attaching a freezer (at a sub-zero temperature) to one’s fridge (which usually operates at about 3ºC) became commonplace, and since then most of the advancements in the field of domestic refrigeration have been limited to making fridges bigger, easier to clean (particularly with the introduction of injection-moulded plastic components), more energy-efficient and more of a middle-class fashion statement.
However, this does not mean that the science of refrigeration is slowing down: recently, a British company called Reaction Engines Ltd. demonstrated their prototype air-breathing rocket engine, whose key feature was a revolutionary new type of heat exchanger. Despite a design utilising pretty much exactly the same science you’d find at the back of your fridge at home, this heat exchange is capable of dropping the temperature of air from several hundred degrees to -150ºC; in a hundredth of a second. That change in heat energy represents roughly the power output of a medium sized power station from a device that weighs significantly less than a hatchback. I would love to explain all the mechanics of this technology to you, but right now I wish for little more than to sit back and marvel.