Cooling, freezing and icing for foodstuffs
Cooling
People have known how to cool since they started building houses. The ancient Egyptians used damp clay jugs as "refrigerators". The clay jars were filled with water or wine and were waved by slaves with palm fronds to make use of the evaporative cooling. This removed the heat from the medium in the jars and cooled them.
A simple principle that anyone can understand if they wet their finger and blow on it. Paul Carl Gottlieb technically implemented this principle in 1876 and thus invented the refrigerator.
Nowadays, almost every household, at least in the industrialised countries, has a refrigerator, since slaves with palm fronds are scarce and politically undesirable in this country.
But how does the food industry manage to keep the goods fresh before shopping?
Fruit and vegetable storage
Apples in particular can be stored well. They taste best straight from the tree, but they can still be fresh and delicious many months after harvesting. However, this requires storage in a cold store. There, the air is only one to four degrees cold, very humid and contains little oxygen. Sometimes the apples are additionally encased in a freshness gas.
With vegetables, the possible storage times are sometimes much shorter. The temperatures are somewhat higher, around 7 °C.
In pure cold storage halls, it must be avoided at all costs that the temperature drops below 0 °C, otherwise the food will freeze.
Freezing, deep-freezing and freezing
Food can be kept longer if it is frozen. The loss of vitamins and nutrients is relatively low, even after several months.
The terms "deep-freezing" or "deep-freezing" are used for industrial processes, while the term "freezing" is more commonly used for cold preservation in the domestic freezer. By removing heat through rapid shock freezing and thus stopping the growth of microorganisms, frozen food can be stored for several weeks, some even for over a year.
The low temperature of -18 °C and below causes the water in the food to freeze and form crystals. Microorganisms can no longer multiply at these temperatures. This extends the shelf life. However, the frozen food spoils due to the atmospheric oxygen, so that storage is not possible at will.
Industrial shock freezing (-25 °C and below) allows fewer large ice crystals to form, which can break open the cell structures of the food.
When freezing slowly at home, on the other hand, large ice crystals often form, which affect the cell structure of the frozen food. For example, when meat is thawed, a lot of juice escapes, which means that micro-organisms can multiply much more quickly than with snap-frozen products. When stored in the freezer, food must be hermetically sealed to prevent freezer burn. This occurs when air gets to the food, but also when there are temperature fluctuations in the food. Freezer burn is not dangerous or unhealthy, but it does change the consistency and taste of the frozen food. Meat can become very tough when it is prepared later. It also doesn't look very appetising.
On an industrial scale, of course, it is imperative to avoid freezer burn, otherwise the food is simply unsaleable.
Ice formation on the frozen food
For ice to form on the frozen food, the temperature of the food must already be below the freezing point. Only then can ice deposits occur on the surface when it comes into contact with ambient air. In this case, the water vapour contained in the air changes directly into ice (resublimation -> transition from the gaseous to the solid phase). The warmer and the more humid the contact air is, the more water vapour it contains and the more ice can form on the surface.
In an iced-up freezer, defrosting is the order of the day. Industrially, in deep freezers, one tries to avoid the formation of ice on the food by taking appropriate measures and not to interrupt the cold chain during packaging and transport to the supermarket.
How can simulation help the industry?
When air-conditioning cold storage halls, the temperature in a hall should be as uniform as possible without draughts when people walk in. In logistics centres, which include cold storage halls, freezing halls, cold storage and packaging zones, areas with different temperatures must be separated from each other as much as possible. Nevertheless, there are thermal bridges such as airlocks and gates that need to be considered.
These are classic tasks of fluid mechanics CFD, which we have been carrying out for large food companies for several years.
The complexity of the buildings, halls, ducts, heat exchangers and fans as well as the level of detail are virtually unlimited in CFD simulation. We consider entire logistics centres in our simulations, including conveyor belts, airlocks etc.
What you can build, we can calculate.
In many cases it is already sufficient to consider the "cold" flow (i.e. without temperature). The essential statements can already be obtained from an appropriate CFD model with the help of flow calculation, in which the flow distribution and velocity are evaluated. Areas with no flow, so-called dead water areas, but also areas with very high flow velocities (draught) can be identified in this way. Subsequently, optimisation can take place so that comfort and energy costs are in harmony with optimum cooling performance and temperature control.
Take advantage of our many years of experience to design cold storage halls, deep-freeze rooms and logistics centres optimally, i.e. efficiently and economically. We look forward to hearing from you.
Yours Stefan Merkle
PS: We can also simulate the melting of ice and tell you how much faster it is if you use the hair dryer on your frozen freezer at home.
PPS: Unfortunately, this is not very cheap, as it requires a lot of computing power. A new freezer is usually cheaper.