This is a working overview of secondary drying, written for readers who want more than a one-paragraph summary but less than a textbook.
This page was last updated on 2026-07-16 and is reviewed periodically as new material appears.
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.
Quality control for lyophilized materials includes visual inspection, residual moisture measurement, and reconstitution testing. Cake appearance can reveal process problems such as collapse, shrinkage, or meltback, although appearance alone does not prove potency. Residual moisture is commonly measured by Karl Fischer titration or by loss on drying. Reconstitution time is checked because a slow or incomplete dissolve can indicate a change in pore structure. Stability studies track these attributes over time under defined temperature and humidity conditions.
Analytical methods for lyophilized solids must account for the low moisture content and the fragile cake. Karl Fischer titration is widely used for water content, while near-infrared spectroscopy can measure moisture non-destructively in sealed containers. X-ray diffraction and modulated differential scanning calorimetry help identify crystalline or amorphous phases. Residual solvent analysis may be needed if organic solvents were used during formulation. The combination of these methods supports batch release and long-term stability assessment.
Lyophilized products are typically stored as sealed solids in vials or syringes. Moisture ingress is a major concern because many dried cakes are hygroscopic and can lose stability when exposed to humid air. Storage temperature depends on the formulation; some products are kept refrigerated, while others are stable at room temperature. Container closure integrity and headspace moisture are often monitored. Light protection may also be required for some photosensitive materials.
| Property | Value | Notes |
|---|---|---|
| Common synonym | Freeze-drying | Same dehydration operation |
| Typical vacuum | 10-100 Pa | Pressure during primary drying |
| Primary drying temperature | -40 to -10 °C | Below collapse temperature for many formulations |
| Cycle duration | 12-72 hours | Varies with load, container, and formulation |
| Key phase change | Sublimation | Solid ice to water vapor |
Freeze-drying is distinct from simple evaporation and from spray drying. Evaporation removes water at temperatures above freezing, while spray drying rapidly dries droplets in a heated gas stream. Lyophilization avoids high temperatures, which can be useful for heat-sensitive materials such as proteins, vaccines, and some foods. The porous cake produced by sublimation dissolves or rehydrates more quickly than a dense dried mass. Not all materials tolerate freezing or the pH shifts that can occur as solutes concentrate during ice formation.
Lyophilization removes water from a frozen material by sublimation under reduced pressure. The process begins with freezing, which converts liquid water into ice and fixes the structure of the sample. After freezing, primary drying lowers pressure so ice changes directly to vapor without passing through a liquid phase. Secondary drying then removes bound water that remains after ice sublimation. The result is a dry, porous solid that often retains its original shape.
The low pressure used during drying allows water vapor to move from the ice surface to a cold condenser. Energy supplied as heat drives sublimation but must stay below the collapse temperature of the frozen matrix. If the product becomes too warm, the frozen structure may soften or melt, reducing pore formation and slowing drying. Formulations often include bulking agents, stabilizers, or buffers to support a rigid cake. The final moisture content depends on formulation, freezing rate, and the length of secondary drying.
Storage stability depends on water content, oxygen exposure, and temperature. Lyophilized solids are hygroscopic and can absorb water if exposed to humid air. Vials are usually sealed under vacuum or inert gas with rubber stoppers and aluminum crimps. Storage temperatures range from room temperature to refrigerated or frozen conditions depending on the material. Stability studies track potency, moisture, and physical form over time. Accelerated conditions can reveal sensitivity but may not predict long-term behavior.
Analytical methods for lyophilized materials include X-ray diffraction for crystallinity, differential scanning calorimetry for thermal transitions, and scanning electron microscopy for pore morphology. Moisture sorption analysis shows how the cake responds to humidity. These methods help distinguish amorphous from crystalline states and detect phase changes. Open questions remain about how pore structure changes during long-term storage and how best to predict collapse under varied conditions. Comparisons across studies are complicated by differences in formulation, cycle, and storage history.
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.
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.
Quality control for lyophilized materials examines appearance, reconstitution time, residual moisture, and mechanical integrity. An acceptable cake is usually uniform and porous, though appearance alone does not prove stability. Karl Fischer titration is a common method for water content, while differential scanning calorimetry can reveal glass transition events. Stability studies track potency, aggregation, and moisture over time under defined temperature and humidity conditions. Specifications are product-specific and may include sterility and endotoxin tests for sterile preparations.
Misconceptions about lyophilization include the idea that dried products are indefinitely stable or that vacuum sealing eliminates all degradation. Chemical reactions can continue in the solid state, and some proteins lose activity even at low moisture. Another misconception is that any freeze-dryer cycle can be scaled by time alone; heat and mass transfer differ with equipment and load. Open questions remain about predicting long-term stability from short accelerated studies, particularly for amorphous formulations. Real-time stability data remain the standard for shelf-life assignment.
== Appearance and use == The 917 has two trays for racks, plus a stat rack. Racks that hold five test tubes slide in on the left side of the machine. There are two reagents carousels on the right side of the 917. In the centre, towards the back, are the reaction vessels, where the chemical reactions take place.
=== Vocalizations === The American kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter". The "klee" is usually delivered as a rapid series – klee, klee, klee, klee when the kestrel is upset or excited. This call is used in a wide variety of situations and is heard from both sexes, but the larger females typically have lower-pitched voices than the males. The "whine" call is primarily associated with feeding but is also uttered during copulation. The "chitter" is used in activities that involve interaction between male and female birds, including courtship feeding, copulation, and the feeding of nestlings. Nestlings can produce calls similar to those of adults at 16 days old.
Non-professional antigen presenting cells include all nucleated cell types in the body. They use an MHC class I molecule coupled to beta-2 microglobulin to display endogenous peptides on the cell membrane. These peptides originate within the cell itself, in contrast to the exogenous antigen displayed by professional APCs using MHC class II molecules. Cytotoxic T cells are able to interact with endogenous antigen presented using an MHC class I molecule. Non-professional APCs do not typically express MHC class II molecules. However, it has been observed that antigen presentation to CD4+ cells via MHC class II is not restricted to the classically professional APCs. Other leukocytes, including granulocytes such as mast cells and neutrophils, can be induced to do so, as can endothelial and epithelial cells under certain circumstances. Even so, there is little evidence that these atypical APCs are able to activate naive CD4+ T cells.
Profilins are proteins of molecular weights of roughly 14–19 kDa. They are present as single genes in yeast, insects, and worms, and as multiple genes in many other organisms including plants. In mammalian cells, four profilin isoforms have been discovered; profilin-I is expressed in most tissues while profilin-II is predominant in brain and kidney. Asgard archaea use profilins. Multiple eukaryotic diatom species lack profilins. Profilin is essential to host cell invasion by Toxoplasma gondii. Toxoplasma profilin is the specific pathogen-associated molecular pattern (PAMP) of TLRs 5, 11, and 12. Profilin enhances actin growth in two ways:
Sources: en.wikipedia.org
Isotope masses from: Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001 Isotopic compositions and standard atomic masses from: de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683. Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005. Half-life, spin, and isomer data selected from the following sources. Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001 National Nuclear Data Center. "NuDat 3.0 database". Brookhaven National Laboratory. Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.
== Psychoactive properties == Laussmann & Meier-Giebing (2010) reported the presence of psilocybin at ~2.5% and psilocin at ~1.194% from 25 samples seized by the German government, which makes modern commercially cultivated strains of this fungus the most potent hallucinogenic mushrooms ever described in published academic research. Other researchers have documented a significant presence of serotonin and urea in this species as well as the possibly psychoplastogenic indole alkaloid baeocystin.
=== Since 1994 === A new interim constitution, introduced in 1994 after four years of negotiation, finally introduced all-race democracy and enfranchised men and women of all races on equal terms, the minimum age remaining 18 years. Parliament was reconstituted to consist of a Senate and a National Assembly.
SCl2 + Cl2 → SCl4 It melts with simultaneous decomposition above −20 °C. Its solid structure is uncertain. It is probably the salt SCl3+Cl−, since related salts are known with noncoordinating anions. In contrast to this tetrachloride, SF4 is a neutral molecule.
Bill Pittendreigh, DuPont, and other individuals and corporations worked diligently during the first few months of World War II to find a way to replace Asian silk and hemp with nylon in parachutes. It was also used to make tires, tents, ropes, ponchos, and other military supplies. It was even used in the production of a high-grade paper for U.S. currency. At the outset of the war, cotton accounted for more than 80% of all fibers used and manufactured, and wool fibers accounted for nearly all of the rest. By August 1945, manufactured fibers had taken a market share of 25%, at the expense of cotton. After the war, because of shortages of both silk and nylon, nylon parachute material was sometimes repurposed to make dresses. Nylon 6 and 66 fibers are used in carpet manufacture. Nylon is one kind of fiber used in tire cord. Herman E. Schroeder pioneered application of nylon in tires.
Sources: en.wikipedia.org
MSP using qPCR can also be performed to obtain quantitative rather than qualitative information about methylation. Miniprimer PCR: uses a thermostable polymerase (S-Tbr) that can extend from short primers ("smalligos") as short as 9 or 10 nucleotides. This method permits PCR targeting to smaller primer binding regions, and is used to amplify conserved DNA sequences, such as the 16S (or eukaryotic 18S) rRNA gene. Multiplex ligation-dependent probe amplification (MLPA): permits amplifying multiple targets with a single primer pair, thus avoiding the resolution limitations of multiplex PCR (see below). Multiplex-PCR: consists of multiple primer sets within a single PCR mixture to produce amplicons of varying sizes that are specific to different DNA sequences. By targeting multiple genes at once, additional information may be gained from a single test-run that otherwise would require several times the reagents and more time to perform. Annealing temperatures for each of the primer sets must be optimized to work correctly within a single reaction, and amplicon sizes. That is, their base pair length should be different enough to form distinct bands when visualized by gel electrophoresis. Nanoparticle-assisted PCR (nanoPCR): some nanoparticles (NPs) can enhance the efficiency of PCR (thus being called nanoPCR), and some can even outperform the original PCR enhancers. It was reported that quantum dots (QDs) can improve PCR specificity and efficiency.
The Soviet Union adopted a command economy, whereby production and distribution of goods were centralized and directed by the government. For the overwhelming majority of its existence, the USSR did not use GDP or GNP to measure its economy, instead relying on the Material Product System. The first Bolshevik experience with a command economy was the policy of war communism, which involved the nationalization of industry, centralized distribution of output, coercive or forced requisition of agricultural production, and attempts to eliminate money circulation, private enterprises and free trade. The barrier troops were also used to enforce Bolshevik control over food supplies in areas controlled by the Red Army, a role which soon earned them the hatred of the Russian civilian population. After the severe economic collapse, Lenin replaced war communism by the New Economic Policy (NEP) in 1921, legalizing free trade and private ownership of small businesses. The economy steadily recovered as a result. After a long debate among the members of the Politburo about the course of economic development, by 1928–1929, upon gaining control of the country, Stalin abandoned the NEP and pushed for full central planning, starting forced collectivization of agriculture and enacting draconian labour legislation. Resources were mobilized for rapid industrialization, which significantly expanded Soviet capacity in heavy industry and capital goods during the 1930s. The primary motivation for industrialization was preparation for war, mostly due to distrust of the outside capitalist world.
Some females stay and defend the nest until the chicks start hatching, but most leave the nesting area completely to nest again; in a good season, a female emu may nest three times. If the parents stay together during the incubation period, they will take turns standing guard over the eggs while the other drinks and feeds within earshot. If it perceives a threat during this period, it will lie down on top of the nest and try to blend in with the similar-looking surrounds, and suddenly stand up to confront and scare the other party if it comes close. Incubation takes 56 days, and the male stops incubating the eggs shortly before they hatch. The temperature of the nest rises slightly during the eight-week period. Although the eggs are laid sequentially, they tend to hatch within two days of one another, as the eggs that were laid later experienced higher temperatures and developed more rapidly. During the process, the precocial emu chicks need to develop a capacity for thermoregulation. During incubation, the embryos are kept at a constant temperature but the chicks will need to be able to cope with varying external temperatures by the time they hatch. Newly hatched chicks are active and can leave the nest within a few days of hatching. They stand about 12 cm (5 in) tall at first, weigh 0.5 kg (17.6 oz), and have distinctive brown and cream stripes for camouflage, which fade after three months or so. The male guards the growing chicks for up to seven months, teaching them how to find food.
== Processing == At room temperatures the resin reacts very slowly and if frozen will remain stable for years. Thus, prepregs can only be cured at high temperatures. They can be processed with the hot pressing technique or the autoclave technique. Through pressure the fiber volume fraction is increased in both techniques. The best qualities can be produced with the autoclave technique. The combination of pressure and vacuum results in components with very low air inclusions. The curing can be followed by a tempering process, which serves for complete crosslinking.
Sources: en.wikipedia.org
Primary drying removes ice by sublimation under vacuum. Secondary drying removes water that is bound to the material, often by warming the product after most ice has left. Both stages occur below temperatures that would cause unwanted melting.
Sublimation requires the solvent to remain solid so vapor leaves without passing through a liquid phase. If the product melts, the porous structure can collapse and drying becomes uneven. Maintaining frozen conditions preserves the intended physical form.
No, freeze-drying is a dehydration method, not a sterilization step. It can reduce water activity and limit microbial growth during storage, but it does not reliably kill microbes or remove endotoxins. Sterility must come from separate validated processes.
Most lyophilized products are stored in sealed containers at controlled temperature and humidity. Some require refrigeration, while others are stable at room temperature. Protection from light and moisture is often necessary.