This is a working overview of secondary drying, written for readers who want more than a one-paragraph summary but less than a textbook.
Reviewed 2025-09-24. Anything still debated is marked as such rather than presented as settled.
The process usually has three stages: freezing, primary drying, and secondary drying. Freezing sets the ice crystal structure and can determine pore size in the final cake. Primary drying removes free ice by sublimation at low pressure and low shelf temperature. Secondary drying removes bound water by raising the shelf temperature, although some water may remain as part of the solid. Cycle parameters depend on formulation, fill volume, vial type, and equipment performance.
The physical chemistry of freezing influences whether a formulation forms an amorphous glass or a crystalline solid. Amorphous systems can collapse if product temperature rises above the glass transition temperature of the freeze concentrate. Crystalline systems may show eutectic melting, where ice and solute melt together at a fixed temperature. Formulators add bulking agents, lyoprotectants, and buffers to preserve structure and biological activity. The optimum cycle keeps product temperature below critical thresholds during primary drying while allowing efficient sublimation.
Lyophilization is a dehydration process that removes water from a frozen material by sublimation under low pressure. The material is first frozen to convert liquid water into ice. Next, the pressure is reduced below the triple point of water so that ice changes directly into vapor without passing through a liquid phase. This step is called primary drying. The result is a porous solid or cake that retains the original shape of the frozen solution.
After primary drying, secondary drying removes water that remains bound to the material. This stage raises the shelf temperature while maintaining low pressure, which encourages desorption of unfrozen water. Residual moisture can be reduced to a low percentage, improving stability for many products. The process parameters, including freezing rate, shelf temperature, and chamber pressure, influence the final pore structure and reconstitution behavior. Control of these variables helps prevent collapse or meltback during drying.
A formulation often contains excipients that protect the active ingredient during freezing and drying. Bulking agents provide structure, while lyoprotectants stabilize sensitive molecules. The freezing step can produce ice crystals whose size and distribution affect the drying rate, and cycle design includes freezing, annealing, and drying phases. If the product temperature rises above a critical value, the cake may collapse or lose its porous structure. Successful lyophilization therefore depends on the interaction between formulation, equipment, and cycle design.
| Property | Value | Notes |
|---|---|---|
| Common synonyms | Freeze-drying, lyophilisation | Lyophilisation is the British spelling; the process is not simple evaporation. |
| Primary drying pressure | 0.05–0.3 mbar | Pressure must remain below the vapor pressure of ice at the product temperature. |
| Sublimation temperature | Below 0 °C | Ice changes directly to vapor while the product remains frozen. |
| Typical shelf temperature | −40 to −10 °C | Exact setting depends on formulation critical temperature and equipment. |
| Cycle duration | 12–72 hours | Time varies with fill volume, formulation, and dryer performance. |
Lyophilization removes water by freezing a material and then lowering pressure so ice changes directly to vapor. The process relies on sublimation, the phase transition from solid to gas without an intermediate liquid state. Because the material remains frozen during primary drying, the structure often stays porous. This porous matrix can rehydrate quickly when water is added back. The low pressure also allows vapor to leave the solid matrix without boiling.
A typical cycle begins with freezing, which fixes the material into a solid and determines ice crystal size. Primary drying then raises heat under vacuum so ice sublimes, often near or below the collapse temperature of the formulation. Secondary drying removes bound water that remains after ice is gone, usually by gently warming the product. Each stage balances heat input against pressure to avoid melting or structural damage. Temperature probes and pressure sensors guide the transition between stages.
In practice, lyophilization is slower and more energy intensive than simple drying. Cycle times can range from hours to several days depending on load, container, and formulation. Amorphous materials may require excipients that help preserve structure during freezing and drying. The method is widely used for biological materials, pharmaceuticals, and foods where heat drying would cause unacceptable change. Open questions remain about scaling cycles between laboratory and production equipment, and this gap affects technology transfer.
The physics of lyophilization depends on phase boundaries and heat and mass transfer. During primary drying, heat supplied to the product must equal the latent heat of sublimation, while water vapor moves through the drying cake to the condenser. If shelf temperature or pressure is set too high, the ice front can exceed the collapse temperature, causing meltback or pore collapse. If conditions are too cold, drying slows and costs rise. Formulation excipients, vial geometry, and freezing rate alter these limits.
Equipment for lyophilization includes a vacuum chamber, temperature-controlled shelves, a condenser, and a vacuum pump. A refrigeration system cools the shelves and condenser below the product's freezing point. Process monitoring often uses Pirani and capacitance manometers, thermocouples, and resistance sensors. Cycle development balances product quality with time and energy use. Some products are annealed during freezing to improve crystallization of bulking agents. Open questions remain about scaling cycles between laboratory, pilot, and production freeze-dryers.
Ross (1971), historian and professor at University of Southern California, 2018 Pulitzer Prize for History finalist Roy Rosenzweig (1971), historian and director of the Center for History and New Media at George Mason University Scott Atran (1972), anthropologist; director at Centre National de la Recherche Scientifique and presidential scholar at John Jay College of Criminal Justice Joel Black (1972), literature and film scholar Michael Gerrard (1972), professor at Columbia Law School Jerome Groopman (1972), Harvard Medical School professor and medical writer for The New Yorker Robert Hymes (1972), professor of Chinese history at Columbia University, winner of two Joseph Levenson Book Prizes George Klosko (1972), professor of philosophy at the University of Virginia Mark J. Roe (1972), professor at Harvard Law School John Servos (1972), professor and historian of science; president of the History of Science Society David Stern (1972), professor of Hebrew literature at Harvard University Tom R. Tyler (1972), professor of psychology at Yale Law School Harold Aram Veeser (1972), professor at City College of New York, known for contribution to new historicism Sean Wilentz (1972), historian and winner of the Bancroft Prize; chair of American Studies at Princeton University Angelo Falcón (1973), political scientist, president and founder of the National Institute for Latino Policy Steven Messner (1973), sociologist, professor of the University at Albany, SUNY, former president of the American Society of Criminology William C.
=== Switzerland === In Switzerland, the equivalent profession to a clinical biologist is the FAMH Specialist in Laboratory Medicine. This title is awarded by the Foederatio Analyticorum Medicinalium Helveticorum (FAMH), the Swiss organization responsible for the postgraduate training and certification of specialists in medical laboratory diagnostics. To obtain this title, professionals must complete a four-year postgraduate training program, which is carried out alongside employment in an accredited medical laboratory. The program is open to individuals holding a medical degree (MD), a pharmacy degree, or a master’s/PhD in biomedical sciences, biology, or related disciplines. The training covers several specialties, including hematology, clinical chemistry, medical microbiology, immunology, and genetics. The FAMH Specialist in Laboratory Medicine plays a key role in medical diagnostics. They are responsible for the supervision and interpretation of laboratory analyses, validation of results, and communication with clinicians. They also ensure quality management and compliance with ISO 15189 standards, while contributing to the training and supervision of laboratory technical staff. The FAMH Specialist title is federally recognized and is a mandatory requirement to lead and validate medical diagnostic laboratories in Switzerland.
Chemical biology is a scientific discipline between the fields of chemistry and biology. The discipline involves the application of chemical techniques, analysis, and often small molecules produced through synthetic chemistry, to the study and manipulation of biological systems. Although often confused with biochemistry, which studies the chemistry of biomolecules and regulation of biochemical pathways within and between cells, chemical biology remains distinct by focusing on the application of chemical tools to address biological questions.
Sources: en.wikipedia.org
The solute carrier (SLC) group of membrane transport proteins include over 400 members organized into 66 families. Most members of the SLC group are located in the cell membrane. The SLC gene nomenclature system was originally proposed by the HUGO Gene Nomenclature Committee (HGNC) and is the basis for the official HGNC names of the genes that encode these transporters. A more general transmembrane transporter classification can be found in TCDB database. Solutes that are transported by the various SLC group members are extremely diverse and include both charged and uncharged organic molecules as well as inorganic ions and the gas ammonia. As is typical of integral membrane proteins, SLCs contain a number of hydrophobic transmembrane alpha helices connected to each other by hydrophilic intra- and extra-cellular loops. Depending on the SLC, these transporters are functional as either monomers or obligate homo- or hetero-oligomers. Many SLC families are members of the major facilitator superfamily.
At the insistence of the Soviet Union, on 10 October the withdrawal of Bulgarian troops from the parts of Macedonia and Thrace that had been in Greece and Yugoslavia until the war began. On 28 October 1944, a delegation led by Foreign Minister Petko Staynov and including Ministers Nikola Petkov, Dobri Terpeshev and Petko Stoyanov signed an armistice with the Allies. Bulgaria was forced to accept harsh conditions - maintenance of the Soviet troops stationed in the country, placing the government under the control of the Allied Control Commission, and involvement in the hostilities against Germany. On 3 December, at the suggestion of Damyan Velchev, the Council of Ministers passed a decree enabling the military officers charged under the People's Court Act to go to the front and, if they showed bravery, be discharged. The next day the Communists declared the decree "counter-revolutionary" and organised demonstrations against it, and on 6 December, at the insistence of the head of the Union Control Commission, Sergei Biryuzov, the decree was revoked. In the following days, Communists and Soviet officers headed the General Staff and its Intelligence Department, and held two deputy ministerial posts in the War Ministry. On 26 January 1945, the Council of Ministers approved the "Ordinance-Law for the Protection of People's Power", which contained 18 articles - 5 of them providing for the death penalty and 4 for life imprisonment.
Another GT31 Clade 10 member, named GALT31A, encodes a β-1,6-GalT when heterologously expressed in E. coli and Nicotiana benthamiana and elongated β-1,6-galactan side chains of AGP glycans. GALT29A, a member of GT29 family was identified as being co-expressed with GALT31A and act co-operatively and form complexes. Three members of GT14 named GlcAT14A, GlcAT14B, and GlcAT14C were reported to add GlcA to both β-1,6- and β-1,3-Gal chains in an in vitro enzyme assay following heterologous expression in Pichia pastoris. Two α-fucosyltransferase genes, FUT4 and FUT6, both belonging to GT37 family, encode enzymes which add α-1,2-fucose residues to AGPs. They appear to be partially redundant as they display somewhat different AGP substrate specificities. A GT77 family member, REDUCED ARABINOSE YARIV (RAY1), was found to be a β-arabinosyltransferase that adds a β-Araf to methyl β-Gal of a Yariv-precipitable wall polymer. More research is expected to functionally identify other genes involved in AGP glycosylation and their interactions with other plant cell wall components.
Sources: en.wikipedia.org
The main change is sublimation, in which ice becomes water vapor without melting into liquid water. This occurs when the chamber pressure is held below the vapor pressure of ice while mild heat is supplied. The result is a dry, porous solid that retains much of its original shape.
Freezing determines ice crystal size, solute distribution, and the pore network left after drying. A slow or fast freezing rate can produce different cake structures and affect reconstitution. It also sets whether the formulation follows an amorphous or crystalline drying path.
It removes most free water during primary drying and part of the bound water during secondary drying. A small residual moisture content often remains and is specified for each product. Complete removal is generally neither practical nor desirable for stability.
Lyophilization removes water by sublimation from a frozen material, while evaporation changes liquid water into vapor. The low-pressure freezing step avoids the liquid phase and can preserve heat-sensitive structures.