#food #food_processing #foodproduction Freeze-drying, also known as lyophilization, is a food preservation method that involves removing the water content from food by freezing it and then drying it in a vacuum. The process involves three main stages: freezing, primary drying, and secondary drying. In the first stage, the food is frozen to a very low temperature, typically around -40°C to -50°C. This is done to create a solid structure that can support the food during the subsequent drying process. In the second stage, the frozen food is placed in a vacuum chamber, and the pressure is reduced to a very low level. This causes the ice in the food to sublimate, or turn directly from a solid to a gas, without passing through the liquid phase. This process can take several days, depending on the size and moisture content of the food. In the final stage, called secondary drying, the remaining water molecules are removed from the food at a higher temperature and lower pressure. This helps to ensure that the food is completely dry and stable, and can be stored for a long time without spoiling. Freeze-drying is commonly used to preserve foods that are delicate, such as fruits, vegetables, and meats, as well as pharmaceuticals and other products that need to be stored for long periods of time without degradation. The resulting products are lightweight, easy to transport, and have a long shelf life, making them useful for military rations, space missions, and other applications where fresh food is not readily available. Freeze-drying has been widely used in microbiology for many decades to stabilize and store cultures. Our understanding of freeze-drying and the factors that affect a microorganism’s ability to survive freeze-drying has improved significantly in recent years. Freeze-drying of microorganisms is now a complex process that involves culturing organisms under conditions that enhance freeze-drying tolerance, harvesting the organisms at the correct time point, and optimizing the lyoprotectant and the freezing, primary drying, and secondary drying conditions. Microorganisms vary greatly in their tolerance to freeze-drying and it is now understood that this tolerance can be enhanced by triggering various survival responses. In general, Gram-positive bacteria show a far higher tolerance to freeze-drying than Gram-positive bacteria. Similarly, spores are more resistant to freeze-drying than vegetative cells and fastidious microorganisms show the least resistant resistance to freeze-drying. There are, however, considerable differences in the freeze-drying tolerance of closely related microorganisms. Different strains of the same species can behave completely differently through the freeze-drying process. This means that the freeze-drying process must be optimized for individual strains if maximum viability and stability are to be achieved. Freeze drying is the best method of drying products/bioactive compounds, which are sensitive to heat and degrade at high temperatures (Zhang et al., 2007). Recently, MW freeze-drying (MWFD) has been considered better than freeze-drying (FD) as MWFD reduces the energy consumption by 42% and drying time by approximately 43%. In addition, in barley grass dried with MWFD, the chlorophyll and flavonoid level was more significant than in FD (Cao et al., 2018b). Another study on the drying of raspberry puree indicated that samples dried using MWFD had a longer shelf life (12 wk at 37 °C) with better chemical stability (ascorbic acid and anthocyanin) compared to FD (Ozcelik et al., 2020). Spray freeze drying combines spray drying and freeze drying. In the case of spray drying a solution is sprayed into a hot air chamber, whereas in spray freeze drying the solution is sprayed into cryogenic medium. The remaining steps are same as freeze drying (ie, freezing, primary drying, and secondary drying). Spray freeze drying stabilizes biopharmaceuticals from thermal stresses. Patil et al. explored spray freeze drying to produce dry powder formulations of whole inactivated viral influenza vaccine. These formulations contain inulin as a cryoprotectant and included adjuvants. After pulmonary inhalation these formulations produced both systemic and mucosal immune responses.65