Food preservation has been one of the most important nutritional issues in human life for centuries. Throughout history, mankind has invented various ways to store food without losing its value. Burying meat beneath the ground was a primal method among prehistoric people. For many centuries, the most popular options for food preservation were drying, salting, pickling, smoking, and fermentation. Today, however, the method that works best is lyophilization, which allows people to preserve the nutritional value of products and properly store their food.
Lyophilization – what is it?
Lyophilization, also known as freeze-drying, is a technologically advanced process applied for food preservation. The term defines removing water from a previously frozen product through ice sublimation. The ice turns directly into water vapor, bypassing the liquid state. It is obtainable thanks to the method of freezing the product inside the freeze dryer chamber and reducing the pressure in this area simultaneously. The main aim of the process is therefore to freeze water quickly and properly in order to preserve food in its best state. This activity therefore makes it possible to maintain the product of high quality and long shelf life.
This process is widely known in the pharmacy and food industry as a safe method of processing and preserving the nutritional value of the product. There is no interference with its ingredients. This way, the product can be stored for a very long time and there is no need to use any additives, pasteurization, or preservatives.
What is more, it enables the destabilization of various kinds of chemical components. Many compounds are unstable in liquid form, which makes them also temperature-sensitive. That leads to chemical reactions with others, which is very undesirable regarding food preservation. This combination of components results in a short shelf life and requires special storage of a given product. Lyophilization is a great way to avoid these problems and properly store food.
The stages of the process
Although the process may differ a little among so many companies all over the world, the main parts can be clearly specified. Lyophilization includes the following stages:
1. Pretreatment
This phase is an introductory stage to freeze-drying. It involves any technique of treating the product before freezing. The methods that can be used include product concentration, increase of the surface area, decrease of a high-vapor-pressure solvent, or formulation revision. The latter may be implemented for instance by adding components in order to increase stability. Some pharmaceutical products are required to be sterilized, and in that case, a multi-step filtration is carried out. The pretreatment phase is therefore implemented to prepare the product to be frozen.
2. Freezing
This phase includes any method used to freeze the product. The material intended for lyophilization has to be properly frozen. It is of high significance as it has a huge impact on the whole process and the final effect.
Usually, the product is frozen at a temperature of about -40 degrees Celsius in a special freezer or using liquid nitrogen. This is undertaken at normal pressure. Although large ice crystals obtained through annealing or slow freezing are the easiest way to complete the process of lyophilization, it is better to achieve it rapidly. The main reason for that is the ability of large crystals to break the cell walls, which in turn is a cause of improper freeze-drying effects. Rapid freezing offers a way to avoid such problems.
If materials possess features that lead to precipitation, annealing can be implemented. It consists of fast freezing and raising the product temperature in order to grow the crystals.
3. Sublimation (The primary drying phase)
The end of the freezing stage leads to the next phase, which is sublimation. The pressure in the freeze dryer then decreases. When it drops below the vapor pressure of ice, the proper sublimation process of the product is possible. At this point, the ice is removed from the outer layer, which is frozen. Immediately afterwards it transforms into water vapor. The latter then settles on the condenser plates. They are characterized by a lower temperature. At this point, the water can get to a solid state again.
This phase leads to the removal of about 95% of the water content in the frozen product. The sublimation stage may be a slow process. It is necessary to remember that the heat should not be too high as it can alter or even destroy the structure of the product.
4. Adsorption (The secondary drying phase)
The adsorption, which is the final phase of the whole process, leads to the removal of ionically-bound water molecules. The bonds between them and the material are broken through the temperature increase. It is higher than during the first drying stage. The structure of a freeze-dried material is porous and the product can be vacuum packed, which enables people to store it for quite a while. The final amount of water content is ultra-low (about 1-4%).
What can be a subject to lyophilization?
Freeze-drying is an operation mainly used in the food industry. One of the most popular areas where the process is implemented is fruit lyophilization. This technique is applied, among others, for cranberries, blueberries, currants, pineapples, apples, plums, mangoes, strawberries, and raspberries. When it comes to vegetables, freeze-drying is used to preserve mainly carrots, celery, parsley, leeks, beans, peas, and peppers. The process is also commonly carried out to freeze fish, meat, cold cuts, mushrooms, and coffee. This technique is a very popular practice to preserve pasta, soups, stews, and dinner sets.
What is more, the technique of freeze-drying is implemented also in the pharmaceutical area. It is so far the best way to obtain a product without water, which simultaneously has the same quality as when it is fresh. Dried algae can serve as a perfect example here as they retain their original properties and full nutritional along with biological value (vitamins, amino acids, minerals) thanks to the sublimation technique.
Advantages of lyophilization
By far, freeze-drying is the best food preservation method. Thanks to it, products retain their original condition and quality. Their nutritional value remains unchanged. They are still full of vitamins and biological substances such as proteins, carbohydrates, fats and minerals. Sublimated food therefore has a very long shelf life, which is a considerable advantage of freeze-drying. Moreover, there is no need to use any other preservatives. It is also worth mentioning that freeze-drying saves from developing various types of microflora. Freeze-drying inhibits moisture, so fungi and bacteria do not have conditions favorable to their growth. This is therefore additional protection for frozen products. An extra advantage of freeze-dried dishes is the fact that one can just pour water over them and wait a while to restore them to their original form and enjoy them.