Freeze-drying, a method extensively employed for stabilizing high-quality food, biological substances, and pharmaceuticals, entails the sublimation of water directly from solid (ice) to vapor, thereby bypassing the liquid phase. This process preserves the integrity of the dried product, encompassing its biological, nutritional, and organoleptic properties. By freezing water within the material before lyophilization, chemical, biochemical, and microbiological processes are inhibited, thus ensuring minimal alteration in taste, smell, and nutrient content.

Raw food materials typically possess a high water content, ranging from 80% to 95%. Freeze-drying involves the removal of water through sublimation, resulting in the formation of a highly porous structure within the products. Rehydration of lyophilisates occurs rapidly. Water within the products can exist in two forms: free water and bound water, the latter being held by various forces within the matrix. While free water freezes, bound water does not. Consequently, in the freeze-drying process, all ice water and a portion of bound water must be eliminated.

Lyophilization comprises several distinct stages:

1. Freezing of the product is typically conducted under atmospheric pressure.
2. Primary drying, wherein ice sublimation occurs, often at reduced pressure.
3. Secondary drying involves the removal of residual moisture to attain the desired final humidity level.

Consideration must be given to both economic factors and product quality throughout the freeze-drying process. The duration of freeze-drying significantly impacts the cost of the final product. Consequently, process parameters are often optimized to minimize drying time. However, hastening the process may compromise product properties. For instance, elevating shelf temperature can lead to structural collapse or thermal degradation of heat-sensitive ingredients.

Careful selection of freeze-drying conditions is imperative to prevent the formation of liquid water, which can alter the rheological properties of the product and induce undesirable changes such as shrinkage. Furthermore, the color and texture of freeze-dried foods play a crucial role in consumer perception and acceptance. Thus, understanding the influence of process parameters on these properties is paramount.

While freeze-drying is widely regarded as the preferred drying method, improper selection of process parameters can result in unfavorable outcomes, including shrinkage, color alteration, and collapsed structures. Therefore, this review aims to comprehensively characterize each stage of the freeze-drying process, elucidate the underlying phenomena, evaluate their impact on process dynamics, and elucidate the effects of process conditions on various physical properties of food products.

During the freeze-drying process, encompassing stages of sublimation, primary drying, and secondary drying, several key physical phenomena significantly influence the efficacy, quality, and cost-effectiveness of the process:

Phase Transition of Water into Ice: Water contained within the product undergoes a phase transition into ice during freezing. This step is pivotal as it immobilizes ingredients, curbing foaming during pressure reduction, and restricts chemical, biochemical, and microbiological changes.

Ice to Vapor Phase Transition: Sublimation occurs as ice transitions directly into vapor, bypassing the liquid phase. This process necessitates efficient heat transport to facilitate sublimation while preventing overheating and the formation of liquid water.

Desorption of Water Molecules: Water molecules desorb from material structures during drying, necessitating the attainment of a sufficiently low pressure to facilitate their removal.

Achieving Low Pressure: Maintaining low pressure within the chamber is essential for facilitating sublimation and the removal of water vapor from the product.

Re-sublimation on Condenser Surface: Water vapor removed from the material re-sublimates on the condenser surface, aiding in its removal from the system.

Removal of Ice Layer from Condenser: A layer of ice may accumulate on the condenser surface during the process, necessitating its removal to maintain efficient operation.

The success of freeze-drying hinges on striking a delicate balance between heat input and usage. Insufficient heat may prolong the process and escalate costs, while excessive heat can lead to overheating and structural collapse of the material. Monitoring the temperature of the lyophilized material is crucial to prevent such occurrences, ensuring that it does not surpass critical thresholds such as the cryoscopic temperature or glass transition temperature.

Furthermore, the freezing stage, often overlooked in the past, plays a pivotal role in freeze-drying. Freezing water within the product offers numerous advantages, including the immobilization of ingredients, the limitation of chemical changes, and the creation of a specific ice crystal structure that influences subsequent mass movement during drying.

The freezing rate significantly impacts the physical state and morphology of the frozen material, thereby affecting the properties of the final freeze-dried product. Optimal freezing conditions depend on the material type, with considerations for achieving the desired ice crystal size to balance primary and secondary drying requirements.

Advanced techniques such as spray-freezing into liquid (SFL) technology offer enhanced control over freezing rates, facilitating the creation of porous structures within frozen materials. While this method offers advantages in terms of rapid freezing and high sublimation rates, it also presents challenges such as limited heat conduction due to small contact surfaces between frozen product spheres.

A comprehensive understanding of the physical phenomena at play during freeze-drying, particularly in the freezing stage, is essential for optimizing process parameters and ensuring the quality and efficiency of the final product.

The freezing rate in freeze-drying operations is directly influenced by the temperature differential between the cryoscopic temperature of the material and the temperature of the freezing medium and is inversely affected by the heat transfer resistance. Adjusting the temperature differential, particularly feasible in outdoor units, provides a means to regulate the freezing rate effectively.

Heat transport resistance, crucial for freeze-drying efficiency, is contingent upon the thickness of the material layer. Reduced thickness equates to lower resistance and faster processing, albeit at the expense of processing volume. To reconcile this trade-off, spin freezing is employed. This technique involves freezing the product in unit packages before freeze-drying. Swirling the package during freezing results in the formation of a thinner layer of frozen material on the package walls, augmenting the evaporation surface area. Spin freezing thus enables a wider range of freezing rate modulation.

Spin freezing holds particular relevance for the freeze-drying of oxidizable liquid or semi-liquid food products. Employing unit packages akin to vials, this method markedly reduces drying time. Additionally, vacuum packaging within the freeze-dryer chamber safeguards the product against oxidation, enhancing overall product quality and shelf life.

A way to speed up freezing, especially in the case of large-sized materials where the conduction resistance prevents rapid freezing, is to use supercooling. In the absence of crystal seeds, the material must be supercooled below the cryoscopic temperature for nucleation to form. The greater the degree of subcooling, the faster the freezing process takes place in the entire volume of the material. A higher degree of supercooling increases the rate of ice nucleation and the effective rate of freezing, yielding a high number of small ice crystals. In a study by Searles et al., it was found that the primary drying rate is about 4% lower for each degree of additional supercooling.

The supercooling state can be achieved by various methods, e.g., by adding cryoprotectants, thanks to the external magnetic field that prevents the movement of water molecules to the crystal surface. Another way is freezing at elevated pressure. By taking advantage of the pressure dependence of the freezing temperature, the product can be subjected to super atmospheric pressure and then reduced in pressure, causing it to freeze.

If it is possible to evaporate from the free surface, the self-freezing effect can be used. If the water begins to intensively evaporate, it takes away heat from the product from which it evaporates, causing it to quickly freeze in its entire volume without the need for separate unit operation.

The freezing process of coffee extract in comparison with air-freezing and contact; vacuum freezing led to significantly smaller freezing times. At the same time, some of the solvents were evaporated, thanks to which additional compaction of 26–43%, contrasting with 1–2% losses for air freezing and contact freezing, was obtained.

This fact, along with the extremely porous structure formed during vacuum freezing, makes this method particularly interesting for soluble coffee production by freeze-drying. Such a phenomenon may occur in the initial stage of drying—during the reduction of pressure in the freeze-dryer chamber. However, there are restrictions.

In the case of a non-cellular material, there is intensive evaporation during pressure reduction, and since the viscosity of the liquid at low temperature is high, it causes the material to splash. In the case of a cellular material that provides a high mass transport resistance, there are two cases for the use of this phenomenon.

Evaporation can be facilitated when the material is naturally thin (e.g., vegetables or other deciduous plants) or in the case of a thin scrap of tissue with damaged cell membranes by cutting. Another way to reduce mass transport resistance is by destroying cell membranes.

Such an effect can be obtained by acting on the structure with a pulsed electric field (PEF). PEF pretreatment provokes damage to cell membranes and accelerates mass and heat transfer processes without undesirable changes in food tissues.

The reduction of the temperature of an apple slice treated with PEF depends on the degree of structure destruction. With the degree of cell wall disintegration equal to 0.96, while reducing the pressure in the freeze-dryer to a value of 1000 Pa, the temperature of the apple decreased from 25 to −10 °C due to evaporation; meanwhile, for undamaged tissue, the temperature only decreased to approximately −5 °C.

Primary drying refers to the crucial phase of freeze-drying characterized by the sublimation of ice. When designing the lyophilization process for a specific material, several process parameters must be carefully determined. These include the pressure within the freeze-dryer chamber and the intensity of heat supplied. The heat supply flux is contingent upon the chosen heating method, whether it involves contact heating, radiation heating, or microwave heating.

In contact heating, the appropriate shelf temperature needs to be set, whereas for radiation heating, factors such as the distance from the material, range of infrared radiation, and intensity of radiation must be considered. Similarly, in microwave heating, the intensity of microwaves and their duration of operation are critical parameters. Additionally, the temperature of the condenser surface, which typically ranges from approximately −60 to −80 °C, is a key parameter determined by the design of the cooling cycle in the freeze-dryer.

During the sublimation period, it is essential to ensure that the amount of heat supplied aligns with the heat required for ice sublimation. This heat can be supplied through heat conduction, radiation, or microwave heating methods. To facilitate the sublimation process effectively, two fundamental conditions must be met: continuous removal of sublime steam from the sublimation area and the sustained provision of heat necessary for sublimation to maintain the vapor pressure differential, enabling the removal of water vapor from the chamber.

Failure to meet these conditions may result in adverse phenomena such as softening, thawing, bulging, or collapse of the material's structure. Hence, meticulous attention to these process parameters is imperative to ensure the success of the primary drying phase in freeze-drying processes.

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