i.e. keeping foods at a low temperature, preserves them for a limited time. All the processes of life, indeed all chemical reactions, take place more slowly at low temperatures.
Heat is a form of energy which manifests itself as a vibration of the molecules of which things are made. In a hot body the molecules move fast; in a cold one they move more slowly. That is why chemical reactions occur more slowly in cold conditions.
There is no such thing as ‘cold’; a cold body simply contains less heat energy than a hot one. When two objects at different temperatures come together, heat flows out of the warmer body into the colder one until eventually they are at the same temperature.
When a substance changes state—that is, when a solid melts to a liquid or a liquid turns to a gas, or vice versa—there is a considerable transfer of heat energy. For example, when ice melts, a lot of heat has to pass into it to allow it to change to water. This heat is drawn in from the surroundings, so anything in contact with the melting ice loses heat and becomes colder. All the available heat is going into the melting process, so the ice and its meltwater remain at freezing point until all the ice has melted, and only then do things start to warm up.
Similarly, when any liquid evaporates to a vapour the process demands heat. The liquid can be made to evaporate by heating it. But it can also be made to evaporate without heating, by reducing the ambient pressure. In this case all the heat that is required has to come from the surroundings, which therefore become cooler. When water vapour condenses to a liquid it gives up the heat it has taken in, warming the surroundings.
The preservative effects of cold were observed early, even if the reason was not understood. The Romans used to chill perishable foods by packing them in snow brought from the Alps, using straw to insulate the snow and keep it from melting both on the journey and in use. The icehouse is another ancient invention, developed in the Middle East, taken up in Renaissance Europe, and familiar in N. America until recently. Ice is collected from lakes in winter, or from mountains, and stored in a heavily insulated building, usually sunk into the ground. With proper management there will still be some ice left by the time winter comes again.
Both these methods are strictly for the rich, who could afford to have ice transported and to build an icehouse. One other traditional method was available to all: the evaporation of water through unglazed earthenware. If water is stored in an unglazed pot, a little of it seeps through the clay and evaporates on the outside, cooling the pot and the water still inside it. Ideally the pot should be set in a shady spot exposed to any wind, since the movement of air speeds evaporation. This method is still widely used to cool drinking water in India and elsewhere. The same principle is used in the modern earthenware cooler for milk bottles.
In Italy in the early 16th century it was discovered that if ice is mixed with salt the temperature of the mixture will fall as low as –18 °C (0 °F; in fact the zero point of the Fahrenheit scale was determined in this way). This phenomenon may seem surprising, but it depends on simple facts of physics. A solution of any substance in water always has a lower freezing point than that of pure water; in the case of the strongest possible salt solution this is that stated above. When ice melts, the temperature of the ice and the meltwater around it is always exactly that of the freezing point of the water, even if this is below the normal melting point of the ice. So if ice is floating in a salt solution, it will melt at the freezing point of the solution.
In the 1830s some attempts were made to build mechanical refrigerators that used the cooling effect of evaporation, but these were unsuccessful until the British physicist William Thomson (later Lord Kelvin) worked out the principle of the heat pump in 1851. A liquid is made to evaporate by lowering the ambient pressure and without heating it, so that it becomes extremely cold. The cold vapour is piped through a closed container, and the heat of whatever is in the container passes into the vapour, so the contents are cooled. After this the vapour is led out of the container and compressed to reliquefy it; this releases the heat that has gone into the vapour. The liquid is then vaporized, and thus cooled, again. The fluid travels in an endless cycle in a closed circuit of pipework, constantly taking heat out of the container and releasing it into the open air. All that is needed is some means of driving the fluid around the circuit—in most modern refrigerators this is an electric pump.
The first workable refrigerating machinery was built in 1857 by the Frenchman Ferdinand Carré. It used a variant of the principle just described. The refrigerant fluid is ammonia dissolved in water. The ammonia is made to boil out of the water by heating it—ammonia boils at a lower temperature than water. This creates enough pressure to force the ammonia vapour into another vessel, where it condenses into a liquid. Still driven by this pressure, the liquid flows through an expansion valve which greatly reduces its pressure, causing it to evaporate and become very cold, after which it passes through the food compartment and cools the food. From here it flows to another vessel containing water in which it dissolves again, and thence back to the start of the cycle. The system could reach a temperature as low as –30 °C (–22 °F), and was soon in use for the freezing of meat.
Carré's refrigerator could equally well be used to keep foods cool without freezing them; but it was too big for home use. The domestic refrigerator had to wait until electric motors became small and reliable. The first practical model was the Kelvinator, launched in the USA in 1918. Since then the refrigerator has gradually become ubiquitous and indispensable.
An improvement came in 1931 with the invention of chlorofluorocarbons (CFCs), a group of inert gases which make excellent refrigerants. They do not need to be dissolved in water, and the cycle is the simple one described first. One other difference between the cycles is that the simple one needs a mechanical pump to drive fluid around the system; while (at least in a small machine without long pipe runs) the ammonia cycle can be powered entirely by the water heater and there need be no moving parts. This cycle is still used in small gas-powered refrigerators for caravans and boats.
In the 1970s it was established that CFCs were eroding the earth's ozone layer, and since then they have gradually been replaced by less harmful chemicals such as hydrochlorofluorocarbons (HCFCs) and butane. With this beneficial change, refrigerators regain in full their reputation as one of the great boons of the 20th century.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
