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Freeze Drying Seminar Report
Lyophilization is a process commonly used in industrial field to prevent the deterioration of foods and drugs that are sensitive with respect to the heat. This paper deals with problems that have to be faced when performing mass and temperature measurements of substances subjected to freeze-drying processes. A brief description of a lyophilization process is initially presented and a deep investigation is performed in order to identify the main uncertainty contributions that affect mass temperature measurements. A measurement system is then described that has been specifically conceived to work inside a freeze-dryer. Experimental results are reported that refer to the metrological characterization of the proposed measurement system and to its use for the monitoring of real freeze-drying processes. Experimental tests are also described that have been conceived to estimate the uncertainty contributions strictly related to this specific condition.
Freeze-drying, or lyophilisation, is like "suspended animation" for food. You
can store a freeze-dried meal for years and years, and then, when you're finally ready
to eat it, you can completely revitalize it with a little hot water. Even after all those
years, the taste and texture will be pretty much the same. That's some trick!
The basic idea of freeze-drying is to completely remove water from some
material, such as food, while leaving the basic structure and composition of the
material intact. There are two reasons someone might want to do this with food:
Removing water keeps food from spoiling for a long period of time. Food
spoils when microorganisms, such as bacteria, feed on the matter and decompose it.
Bacteria may release chemicals that cause disease, or they may just release chemicals
that make food taste bad. Additionally, naturally occurring enzymes in food can react
with oxygen to cause spoiling and ripening. Like people, microorganisms need water
to survive, so if you remove water from food, it won't spoil. Enzymes also need water
to react with food, so dehydrating food will also stop ripening.
Freeze-drying significantly reduces the total weight of the food. Most food is largely made up of water (many fruits are more than 80 to 90 percent water, in fact). Removing this water makes the food a lot lighter, which means it's easier to transport. The military and camping supply companies freeze-dry foods to make them easier for one person to carry. NASA has also freeze-dried foods for the cramped quarters onboard spacecraft. It's pretty simple to dry food, drugs and just about any other biological material. Set it out in a hot, arid area, and the liquid water inside will evaporate: The heat gives the watermolecules enough energy to "break free" of the liquid and become gas particles. Then you seal it in a container, and it stays dry. This is how manufacturers make dehydrated meals like powdered soup and baking mixes. There are two big problems with this approach. First, it's difficult to remove water completely using evaporation because most of the water isn't directly exposed to air. Generally, dehydrating food in this way only removes 90 to 95 percent of the water, which will certainly slow down bacteria and enzyme activity, but won't stop it completely. Secondly, the heat involved in the evaporation process significantly changes the shape, texture and composition of the material, in the same way that heat in an oven changes food. Heat energy facilitates chemical reactions in the food that change its overall form, taste, smell or appearance. This is the fundamental purpose of cooking. These changes can be good, if they make the food taste better (or taste good in a different way), but if you're drying something so you can revitalize it later, the process compromises quality somewhat.
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