Freeze-drying is not sterilization
We come across one misconception about freeze-drying quite often. Because water is removed under deep vacuum during freeze-drying, some people imagine the whole process as a kind of “boiling in the cold”. From there, it is only a short step to assuming that if the food is “boiling” during the process, it must also be heat-treated and microorganisms must be destroyed.
But that is not how it works.
Freeze-drying is primarily a drying method. Its purpose is to remove water from a product at low temperatures, not to cook, pasteurize or sterilize it. That is an important distinction not only in terms of how the technology works, but above all in terms of the safety of the resulting food.
Boiling at -30 °C is not cooking
At normal atmospheric pressure, we are used to water boiling at approximately 100 °C. But when pressure is reduced significantly, the conditions under which water can pass into the gaseous state also change.
Freeze-drying makes use of exactly this principle. The product is first deeply frozen and a vacuum is then created inside the chamber. Under suitable conditions, the ice contained in the product can pass directly into water vapour without first becoming liquid water. This process is called sublimation.
The fact that water is removed from the product under conditions that may resemble “boiling” does not mean that the food itself is being cooked.
The simplest example is a piece of raw chicken. If we put it into a freeze dryer raw, after a correctly completed process we get dry, freeze-dried chicken. From the point of view of heat treatment, however, it is still raw.
So if the meat needed to be cooked before freeze-drying because of the possible presence of pathogens such as Salmonella, freeze-drying itself does not remove that requirement. We cannot assume that removing the water also reliably destroyed pathogenic microorganisms.
That does not mean that every microorganism passes through freeze-drying completely unharmed. Freezing, drying and subsequent storage all place considerable stress on microorganisms and their numbers may decrease. Freeze-drying, however, is not a process designed or validated as sterilization. Its ability to eliminate dangerous microorganisms therefore cannot be relied upon.
Removing water also makes it much more difficult for microorganisms to continue multiplying. This is one of the reasons properly dried and packaged foods can have a long shelf life. But stopping or greatly slowing microbial growth is one thing; reliably killing microorganisms is another.
After rehydration, water returns to the product and conditions suitable for the activity of microorganisms that survived the process may be created again. Freeze-dried raw meat should therefore be handled with the same caution as raw meat before freeze-drying.
Sometimes preserving life is actually the goal
Perhaps the most interesting proof that freeze-drying and sterilization are two completely different things can be found in the use of the same principle outside the food industry.
LYOTRADE was built on the experience of people from its sister company BIOTRADE, which has long worked with laboratory, pilot and production freeze dryers for biotechnology applications, among other technologies.
In laboratories, freeze-drying is used precisely because it can stabilize biological material that would be damaged or destroyed by high temperatures.
Microbial cultures are a typical example. Under suitably controlled conditions, bacteria can be freeze-dried so that a significant proportion of the cells survive the process and become viable again after later rehydration. Entire microbial culture collections use freeze-drying as a method of long-term preservation.
The same technology is also used for other sensitive biological and pharmaceutical materials where the aim is to preserve their properties during storage. What is a major advantage of freeze-drying in one field is also a reason for caution when processing food.
The low process temperature helps preserve properties of the product that conventional heat treatment could alter. But it does not distinguish between what we want to preserve in a food and what we would rather eliminate.
It is therefore useful to remember one simple rule: freeze-drying dries food, but by itself it does not turn it into heat-treated or sterile food.
If you put cooked chicken into a freeze dryer, the result will be freeze-dried cooked chicken.
If you put raw chicken into it, the result will be freeze-dried raw chicken.
The water is gone. The need to handle the food correctly is not.
