As a consequence, the time of the second drying stage lengthens, the rehydration capacity of the product deteriorates, the product has a higher final water content. If the glass transition temperature is exceeded, the structure may collapse (porosity reduction), which is highly disadvantageous due to the reduction of the specific surface of the product. Its value must not exceed the value of the cryoscopic temperature for a given material or the glass transition temperature for a given water content. One way to assess whether the amount of heat supplied is too high is to monitor the temperature of the lyophilized material. Hence, it is extremely important to maintain a balance between the amount of heat supplied and used. If the supplied heat flux is too high, it will cause an accumulation of heat in the material and an increase in its temperature, consequently leading to the possibility of the appearance of liquid water. If too little heat is supplied, the process will be slow, which will increase its costs. This is one of the most difficult problems in freeze-drying.įreeze-drying is a mass exchange process that requires heat transport. The main feature of freeze-drying, the only one that distinguishes it from vacuum drying, is the need to keep free water frozen. The removal of a layer of ice from the surface of the capacitor.īoth the kinetics of the process and the properties of the obtained product depend on the parameters in which these phenomena occur. Therefore, the aim of this review was to characterize all stages of the freeze-drying process, discuss the phenomena taking place during those stages, present their impact on the course of the process, and explain the effect of the process conditions on the selected physical properties of different food products. However, improperly selected process parameters may cause unfavorable changes in the material, such as shrinkage, color change, collapsed structure. It is widely believed that freeze-drying is the best method of drying. The effect of freeze-drying conditions on the nutritional properties, antioxidant activities, and glass transition characteristics of different food materials can be found in the literature. Therefore, the dependence of these food properties on the parameters of freeze-drying is extremely important. The color and structure–texture properties are crucial in the quality evaluation of food by consumers. It may also reduce the period of ensuring high quality during storage. The presence of liquid water during the freeze-drying of food products may result in many changes in the composition, morphology, and physical properties of foods (e.g., shrinkage). Liquid water is the reaction medium and changes the rheological properties of the product. The conditions of the freeze-drying process should be selected in a way that does not melt the water. For example, increasing shelf temperature can lead to the defrosting of the product and the collapse of the structure or to the thermal degradation of heat-sensitive food ingredients. Setting parameters to speed up the process can lead to the deterioration of the product’s properties. Therefore, process parameters and other conditions of its course are often set so that its time is as short as possible. The cost of a product mainly depends on the freeze-drying time. The effect of freeze-drying should be considered from the economic aspect and the quality of the freeze-dried material. Secondary drying-desorption drying-drying the product to the required final humidity. Therefore, lyophilization is a highly complex and multi-step process that consists of : In the freeze-drying process, all ice water and some bound water must be removed. Free water freezes, but bound water does not freeze. The water in the products can be free water or water bound to the matrix by various forces. The removal of water by sublimation results in the creation of highly porous structure of the freeze-dried products, and the rehydration of lyophilisates occurs immediately. Raw food materials contain a lot of water, ranging from 80% to 95%. Therefore, the taste, smell, and content of various nutrients do not change. This is due to the fact that freezing water in the material prior to lyophilization inhibits chemical, biochemical, and microbiological processes. In the process, the quality of the dried product (biological, nutritional, and organoleptic properties) is retained. It is widely used for the stabilization of high-quality food, biological materials, and pharmaceuticals, such as proteins, vaccines, bacteria, and mammal cells. Freeze-drying is a process in which water is sublimated by the direct transition of water from solid (ice) to vapor, thus omitting the liquid state, and then desorbing water from the “dry” layer.
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