Anyone who has ever filled a gas lighter knows how cooling works: when you remove the gas cartridge's filling spout from the lighter, it hisses briefly; a tiny bit of liquid gas escapes and evaporates. That is all it takes to cause the surrounding metal and sometimes even your fingertips to drop to icy temperatures in a fraction of a second. Our refrigerators and cold-storage rooms in the food industry works according to the same principle – extraction of heat through evaporation. In refrigeration machinery, a gaseous coolant is first transformed into its liquid state in a compressor using overpressure. Under normal atmospheric pressure it evaporates again and pulls heat from the refrigerator compartment. Anything placed inside then takes on the cold temperature of the refrigerator over time. This initially happens on the surface of the chilled good; the cold only slowly makes its way deeper inside.
Cooling process directly within chilled goods
Although vacuum cooling is also based on evaporation of a liquid, this process does not take place in the cooling unit of a machine; instead it takes place directly inside the chilled good. The liquid is simply water, which is contained in most foodstuffs. As is generally known, its boiling point depends on the ambient pressure. At sea level at a "normal pressure" of 1013 mbar, it boils at 100 degrees Celsius. At 30 mbar, the boiling point is at about 25 degrees; at 10 mbar, it already evaporates at about 6 degrees. Therefore, if foodstuffs with a certain proportion of water are placed in a vacuum chamber, the water they contain can be brought to a boil without heating it. Since the vacuum also acts on the inside of the foodstuff, the evaporation taking place removes a significant amount of heat from the chilled good in one go.
More and more bakeries are putting this effect to use to accelerate the cooling of their oven-fresh baked goods. What would otherwise take up to one and a half hours is achieved in a matter of minutes in a vacuum chamber. The baked goods are immediately transported to the vacuum chamber after the baking process. Once the door is closed, the pressure there is reduced to 30 to 50 mbar. The bread and pieces of cake usually reach the desired temperature of approximately 30 degrees in under three minutes. Afterwards they can be removed and almost immediately eaten or processed further. Vacuum also helps when preparing frozen goods: compared with shock frosting, only a fraction of the energy is required.
Savings and quality gains
The energy and time savings not only mean a tremendous gain in efficiency for bakery work processes. During the cooling process using a vacuum, the swelling of starch and the desired conversion of protein in flour can progress further. This effect allows shorter baking time. When the excess water evaporates in the vacuum, crumbs and crusts obtain the perfect texture. The baked good has more volume and stays fresh and crisp for longer. Crisp and firm crusts are considered an outstanding quality characteristic in many countries. However, even if soft crusts are preferred, the same advantages apply. Precise control of the vacuum level and the cooling time make it possible to ensure that only the desired amount of water evaporates, thus eliminating dryness.
This is why vacuum cooling can also be used for lettuce, other vegetables, and flowers. The speed of the cooling process is the decisive factor here, as well. Large vacuum chambers can hold an entire truckload of freshly harvested greens and cool them from the temperature of the field to approximately four degrees within 25 minutes. And when compared to conventional cooling, there are additional advantages here, too: significantly lower energy consumption, immediate cooling through to the core and a significantly longer shelf life.
The dry COBRA screw vacuum pumps from Busch have really proven their worth for vacuum cooling baked goods, vegetables and fruit.
Under normal conditions, we are familiar with the three states or "phases" of matter: solid, liquid and gas. The last two are particularly interesting when it comes to cooling. This is due to the amount of heat required for the transition from the liquid to the gas phase. For example, if we want to increase the temperature of a litre of water by one degree, we need around four kilojoules (kJ). To bring the same litre of water to a boil and thus transform it into the gas phase, we would need 2088 kJ, even despite the fact that the water was already 100 degrees.
In a vacuum, water boils at lower temperatures, but the amount of energy needed for the phase change remains the same. So if water already evaporates at 25 degrees at 30 mbar, the exact same amount of heat is extracted from the baked good or head of lettuce as when water boils at normal pressure. Because, in physics, "cold" simply means "less warm", this heat extraction leads to more intense immediate cooling.