2 years, 6 months ago
How is food freeze dried?
Explain the process of freeze drying food
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M$1 Answer
Freeze-drying has been around for centuries. The Incas of Peru used mountain peaks along the Andes as natural food preservers. The extremely cold temperatures and low pressure at those high altitudes prevented food from spoiling similar to a modern freeze-drying machine or a freezer. They had storehouses for foods all over the Empire, storing potato or meat and setting them out in dry days and cold nights freeze-drying very fast.
Freeze-Dried Potatoes in the Andes.
http://cbertel.files.wordpress.com/2008/09/potatoes-freeze-drying-in-the-andes.jpg
Removing the water keeps food from spoiling for long periods. Food spoils when microorganisms, such as bacteria, feed on the matter and decompose it. To dry food is simple. Just put it out in a hot, dry area, and the water inside will evaporate: The heat gives the water molecules enough energy to "break free" of the liquid and become gas particles. Then you seal it in a container, and it stays dry.
This approach has various side effects. It is difficult to remove the water completely using evaporation because it’s not directly exposed to the air, dehydrating the food like this only removes 90% of the water, slowing down the bacteria but it will not stop completely. The heat of the evaporation process changes the shape, texture and composition of the food, much like the heat in an oven changes the food. Heat energy facilitates the chemical reactions in the food changing its form, the taste, smell or appearance. This is the basics of cooking. These changes are good for making the food taste better but if you're drying it with the intention to revitalize it later, the process will alter the quality.
According to "recipes.howstuffworks.com" the whole process of How Freeze-Drying Works, (a little bit long explanation but necessary):
---Quote--
The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form. There are two major factors that determine what phase (solid, liquid or gas) a substance will take: heat and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a certain range. Without these conditions, that phase of the substance can't exist.
Chart with the necessary pressure and temperature values of different phases of water.
http://static.howstuffworks.com/gif/freeze-drying-graph.gif
From the chart, water can take a liquid form at sea level (where pressure is equal to 1 atm.) if the temperature is in between the sea level freezing point (32°F or 0°C) and the sea level boiling point (212°F or 100°C). But if you increase the temperature above 32°F while keeping the atmospheric pressure below .06 atmospheres (ATM), the water is warm enough to thaw, but there isn't enough pressure for a liquid to form. It becomes a gas.
This is exactly what a freeze-drying machine does. It consists of a freeze-drying chamber with several shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum pump.
http://static.howstuffworks.com/gif/freeze-drying-machine.gif
With most machines, you place the material to be preserved onto the shelves when it is still unfrozen. When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber. The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.
Next, the machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure below .06 ATM. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day.
This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure. If everything works correctly, the material will go through the entire process almost completely unscathed.
---/Quote---
Freeze-Dried Potatoes in the Andes.
Removing the water keeps food from spoiling for long periods. Food spoils when microorganisms, such as bacteria, feed on the matter and decompose it. To dry food is simple. Just put it out in a hot, dry area, and the water inside will evaporate: The heat gives the water molecules enough energy to "break free" of the liquid and become gas particles. Then you seal it in a container, and it stays dry.
This approach has various side effects. It is difficult to remove the water completely using evaporation because it’s not directly exposed to the air, dehydrating the food like this only removes 90% of the water, slowing down the bacteria but it will not stop completely. The heat of the evaporation process changes the shape, texture and composition of the food, much like the heat in an oven changes the food. Heat energy facilitates the chemical reactions in the food changing its form, the taste, smell or appearance. This is the basics of cooking. These changes are good for making the food taste better but if you're drying it with the intention to revitalize it later, the process will alter the quality.
According to "recipes.howstuffworks.com" the whole process of How Freeze-Drying Works, (a little bit long explanation but necessary):
---Quote--
The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form. There are two major factors that determine what phase (solid, liquid or gas) a substance will take: heat and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a certain range. Without these conditions, that phase of the substance can't exist.
Chart with the necessary pressure and temperature values of different phases of water.
From the chart, water can take a liquid form at sea level (where pressure is equal to 1 atm.) if the temperature is in between the sea level freezing point (32°F or 0°C) and the sea level boiling point (212°F or 100°C). But if you increase the temperature above 32°F while keeping the atmospheric pressure below .06 atmospheres (ATM), the water is warm enough to thaw, but there isn't enough pressure for a liquid to form. It becomes a gas.
This is exactly what a freeze-drying machine does. It consists of a freeze-drying chamber with several shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum pump.
With most machines, you place the material to be preserved onto the shelves when it is still unfrozen. When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber. The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.
Next, the machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure below .06 ATM. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day.
This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure. If everything works correctly, the material will go through the entire process almost completely unscathed.
---/Quote---
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M$
...and low pressure (that comes with high altitude). The altitude is the setting that facilitates all this: low pressure and cold temperatures.
High altitude and low temperatures are important for freeze drying.