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Freeze-Drying: Basic Principles and Processes

Roji Balaji Waghmare1, Manoj Kumar2, and Parmjit Singh Panesar3

1 Department of Food Engineering and Technology, Institute of Chemical Technology, Mumbai, India

2 Chemical and Biochemical Processing Division, ICAR-Central Institute for Research on Cotton Technology, Mumbai, India

3 Department of Food Engineering & Technology, Sant Longowal Institute of Engineering & Technology, Longowal, Punjab, India

1.1 Introduction

It is possible that drying food is the earliest means of preserving it for an extended period of time, making it a crucial unit operation. Drying is most commonly used to turn a wet or liquid product into a powder, flake, or solid form without altering the product's physicochemical qualities (Maisnam et al. 2015; Assegehegn et al. 2019). Foods that have been dried have a longer shelf life because the drying process stops enzyme and microbial activity (Prosapio and Norton 2018). Traditional drying, on the other hand, degrades the physical and chemical properties of food, which can make it unpalatable to customers. As a result, enhanced drying methods are crucial for food and agro-products to ensure the production of high-quality goods with reduced drying times, increased capacities, enhanced process controls, reduced operational costs, and enhanced safety. Conventional drying processes can degrade nutrients, decreasing the overall quality of dried foods. In light of this, developing and researching innovative drying tools and methods are crucial.

"Altmann used freeze-drying as early as 1890, his technique went unnoticed for over 40?years. .

. the optimism may have somewhat dimmed, the promise remains, and economic changes in the future may well stimulate another surge in development."

Meryman (1976)

The above words are extracted from the abstract of the article "Historical recollections of freeze-drying," where author was optimistic about the future of freeze-drying (FD) or lyophilization of food.

The age-old Chinese and Peruvian Incas had some knowledge of FD. These people used to leave meat outdoor in the cold season to prolong storage and improve flavor. Prior to drying, the meat was first frozen. On the mountain tops above Machu Picchu, the ancient Incas kept potatoes and other harvests. The meal in the high altitudes' low pressure slowly sublimated the water inside due to the cold mountain temperatures that caused it to freeze. Food that has been freeze-dried is lighter and stays longer than other dried samples (Hua et al. 2010).

In the nineteenth century, FD was widely used and employed in research and technology. Nevertheless, the uses of first industrial FD were not seen until after the Second World War. Initially, the medical business was the only one to use the FD procedure to produce medicines, cells, and other related items. The food industry did not begin to use FD until the middle of the twentieth century (Garcia-Amezquita et al. 2015). This technique has quickly risen to prominence as a crucial method for the long-term storage of food items, even among the many other types of drying methods. FD is frequently used in the food business since it results in superior quality dried food. FD relies on the fact that a product's solvent will eventually evaporate. In this process, the solvent - which can be water or an organic solvent - crystallizes at low temperatures before rapidly changing phase from solid to gas. Because FD is carried out at lower temperatures, the qualitative attributes of the food are preserved, and the damage to thermolabile food components is limited (Martínez-Navarrete et al. 2019). Because of this, FD serves primarily to ship a product with a longer shelf life, and the quality of food is maintained after being reconstituted with water.

FD offers numerous advantages compared to conventional drying technology. The main advantages of FD are maintenance of morphological, biochemical, and immunological characteristics; high recovery of volatiles; maintenance of structure, surface area, and stoichiometric proportions; long time frame of realistic usability; and reduced weight for storage, shipping, and handling (Ciurzynska and Lenart 2011; Isleroglu et al. 2018). Since FD is performed at low temperature, they present a lower risk for products labile to heat degradation. Therefore, FD can be applicable for valuable materials that are heat sensitive or samples sensitive to heat that cannot be treated using other processes involving high temperatures (Morais et al. 2016).

1.2 Principle of FD Process

To remove the solvent from a liquid formulation is the fundamental idea of FD, which is a method of preservation. As can be seen in Figure 1.1, there are typically three stages to the FD process: freezing, primary drying, and secondary drying. However, there are five crucial processes that must occur in order to fully comprehend the mechanics at play here: freezing, sublimation, desorption, vacuum pumping, and vapor condensation (Liu et al. 2008). At first, the liquid formulations are cooled to a low temperature, which freezes all of the water existing in the substance. In the sublimation drying process, the frozen solvent is first heated to a point where it leaves its solid state and enters the vapor phase (primary drying). After the initial freezing process, the unfrozen liquid is removed via desorption (secondary drying). Consequently, FD involves two equally important processes: freezing and drying (Tang and Pikal 2004).

Figure 1.1 Freeze-drying process, presenting freezing, primary drying, and secondary drying stages.

Source: Morais et al. (2016)/Reproduced with permission from Elsevier.

The time and energy needed at each stage is highly dependent on the nature of the product, the dryer's design and configuration, and other process variables. In addition, moisture content standards of final product vary by type. While the necessity of proper storage of freeze-dried products may not immediately jump out, it has a profound impact on the durability and safety of the dried goods. Most FD is intended for long-term storage, which means that it must be packaged and stored in such a way as to prevent the deteriorating biochemical and microbiological reactions that would otherwise occur. Storage stability has been estimated using appropriate shelf-life tests, which have included approaches incorporating accelerated shelf-life testing.

1.2.1 Freezing of Raw Material

Freezing is the first phase and one of the essential steps during FD. All the food products such as fresh produce, beverages, and coffee should be subjected to freeze before primary drying (Nowak and Jakubczyk 2020). During the preliminary phase of freezing, the sample is reduced to a temperature below its eutectic point or glass transition temperature. It is the lowest point of the combination of composition and temperature at which the sample freezes. In this step, the primary focus is on freezing any liquid water that may be present in the sample. In this step, it is advised to cool the sample to a temperature lower than the glass transition temperature for each humidity concentration. Over this point, sample is in unstable rubbery form, whereas, under this point, sample changes into glassy or amorphous phase (Garcia-Amezquita et al. 2015). This step transforms 70-90% of free liquid water present in food sample to solid frozen state. However, the remaining water is present in bound state (Bhushani and Anandharamakrishnan 2017). During FD, the suitable freezing rate relies on the type of food sample which could be solution, suspension, or biomaterials. Freezing rate depends on the temperature of the sample, temperature of the freezing medium, and also on the heat transfer resistance (Nowak 2017). To achieve the less primary drying time, it is necessary to maintain the big ice crystal size which results in less resistance to mass transfer in the final dried sample. However, development of several small ice crystals results in high resistance (Assegehegn et al. 2019). The temperature reducing the rate of cooling and freezing also has significant impact. Faster freezing results into the creation of limited glassy phase and inhibits extreme dehydration of food sample during freezing, whereas extremely fast freezing leads to the destructive changes, for example rupturing the cell wall and its other cellular components (Hua et al. 2010).

1.2.2 Primary Drying - Sublimation

Following the freezing stage and during primary drying, ice is removed by sublimation. Sublimation begins at the ice surface and proceeds across the frozen-dry matter interface, mainly dependent on the temperature-pressure combinations maintained. The drying chamber's pressure and the drying process's heat intensity are crucial factors in the FD process (Rambhatla et al. 2006). If the pressure in the compartment or the partial pressure of vapor in the compartment is lower than the vapor pressure of ice, then the frozen solvent can be evacuated from the solid state and entered straight into the vapor phase. This has been suggested that the compartment pressure should be maintained in between one fourth to one half of vapor...