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 2025-11-08 and is reviewed periodically as new material appears.
Lyophilization is a dehydration process that removes water from a frozen material by sublimation under low pressure. The material is first frozen so that water becomes ice; then the surrounding pressure is lowered below the vapor pressure of ice. Heat is applied gently so ice changes directly to vapor without passing through a bulk liquid phase. The vapor is collected on a cold condenser, leaving a dry porous matrix. This process differs from simple evaporation because the material remains frozen during the main drying stage.
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.
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. |
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.
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.
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.
Lyophilization, also called freeze-drying, removes water from a frozen material by sublimation under reduced pressure. The process begins with freezing, which converts free water into ice and may also produce a glassy phase. Primary drying then lowers chamber pressure so ice sublimes directly to vapor without passing through a liquid stage. Secondary drying raises the temperature modestly to remove bound water. The result is a porous, dry solid that usually reconstitutes quickly. Each stage influences pore structure, residual moisture, and stability.
The physics of lyophilization couples heat transfer, mass transfer, and phase behavior. Sublimation requires a vapor pressure difference between the ice front and the chamber, and the dried layer adds resistance to vapor flow. Amorphous formulations are characterized by a glass transition temperature of the maximally freeze-concentrated solute, often denoted Tg'. Crystalline bulking agents can provide structure, while amorphous excipients stabilize labile components. Open questions remain about spatial heterogeneity, edge effects, and how laboratory cycles scale to production.
Lyophilization is a drying process in which a solvent, usually water, is removed from a frozen material by sublimation under reduced pressure. The material is first solidified, then placed under vacuum so that ice transitions directly to vapor without a bulk liquid phase. This approach suits heat-sensitive substances that would degrade during conventional evaporation. Primary drying removes unbound ice, while secondary drying reduces water that remains adsorbed to the solid matrix. The result is a porous, lightweight solid that can be reconstituted later.
A typical cycle begins with freezing, sometimes including an annealing step to control ice crystal size. Freezing conditions influence the pore network that later allows vapor escape. During primary drying, shelf temperature and chamber pressure are set so heat enters the product while its temperature stays below the collapse or eutectic point. Secondary drying then raises the shelf temperature to desorb bound water and lower residual moisture. Cycle design depends on formulation, fill volume, container type, and equipment capability.
Native gels are run in non-denaturing conditions so that the analyte's natural structure is maintained. This allows the physical size of the folded or assembled complex to affect the mobility, allowing for analysis of all four levels of the biomolecular structure. For biological samples, detergents are used only to the extent that they are necessary to lyse lipid membranes in the cell. Complexes remain — for the most part — associated and folded as they would be in the cell. One downside, however, is that complexes may not separate cleanly or predictably, as it is difficult to predict how the molecule's shape and size will affect its mobility. These effects have been successfully addressed by preparative native PAGE. Unlike denaturing methods, native gel electrophoresis does not use a charged denaturing agent. The molecules being separated (usually proteins, peptides, or nucleic acids), therefore differ not only in molecular mass and intrinsic charge, but also the cross-sectional area, and thus, experience different electrophoretic forces dependent on the shape of the overall structure. For proteins, since they remain in the native state, they may be visualized not only by general protein staining reagents but also by specific enzyme-linked staining. A specific experiment example of an application of native gel electrophoresis is to check for enzymatic activity to verify the presence of the enzyme in the sample during protein purification.
==== Temperature ==== A controlled temperature is needed to ensure accurate measurements, as temperature affects many properties of a solution. If the temperature changes between measurements, this variance will be reflected in the measured refractive index.
Phytaspase is a member of the plant subtilisin-like protease family, and is commonly distinguished from the other members by its unusual and extremely high specificity towards its substrates, which resembles that of the animal caspases. Similarly to the animal caspases, the phytaspase is a cell death promoting protease.
Environmental noise arises from the surroundings of the analytical instrument. Sources of electromagnetic noise are power lines, radio and television stations, wireless devices, compact fluorescent lamps and electric motors. Many of these noise sources are narrow bandwidth and, therefore, can be avoided. Temperature and vibration isolation may be required for some instruments.
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=== Radical anion preparation === In the original ETD experiments anthracene (C14H10) was used to generate reactive radical anions through negative chemical ionization. Several polycyclic aromatic hydrocarbon molecules have been used in subsequent experiments, with fluoranthene currently the preferred reagent. Fluoranthene has only about 40% efficiency in electron transfer, however, so other molecules with low electron affinity are being sought.
=== Todd Barber === Todd Barber (Branden Cook) is Harper's ex-boyfriend from New York. The two remain sexually involved after their breakup, and he helps her forge her university transcripts when she is hired to Pierpoint. Todd later visits Harper in London; the two have a fight after Harper discovers he stole an expensive jacket from a patron at the nightclub they went to, where Todd calls Harper a coward and a hypocrite. Harper cuts ties with him for good.
In 2023, a pub in Worcester claimed to offer the cheapest pint of Guinness in the UK, at £2. The third-largest Guinness drinking nation is Nigeria, followed by the USA; the United States consumed more than 950,000 hectolitres (21,000,000 imp gal; 25,000,000 US gal) of Guinness in 2010.
Calhoun acquires a newly charged power cell from the lab's sub-basement and delivers it to Rosenberg and the others. Dr. Rosenberg then initiates the system and brings it online. They all narrowly avoid the military's invasion of the prototype labs, teleporting to the safety of an unnoticed access tunnel. They get into an SUV and leave Black Mesa. Rosenberg's fate remains unknown.
Hyperthyroidism and hypothyroidism are the two major dysfunctions associated to behavioral and brain chemistry changes due to the imbalances in the thyroid hormones, triiodothyronine (T3) and thyroxine (T4).
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== Economy == Bophuthatswana was the richest of the TBVC-states as it had platinum mines, which accounted for two-thirds of the total platinum production in the Western world. It was also rich in asbestos, granite, vanadium, chromium and manganese. Additional revenues came from the Sun City casino, which was a day trip from Johannesburg and Pretoria, where gambling was illegal under the National Party government, as it was throughout all of South Africa. Bophuthatswana had a relatively capitalist free market economy, as seen by the government's drive for infrastructure development, foreign direct investments and virtually no regulatory barriers to starting and running a business. Initially, its economy was driven by agriculture then later by mining, hospitality and banking. Bophuthatswana also issued bearer development bonds. The so-called "Bop Bonds" were redeemed by the government of the North West province from 1995 to 1997, and are now worthless as financial instruments. However, bonds in excellent condition are considered collectible. Bonds issued in 1988 and 1989, in R10 and R20 denominations, currently trade at 10–25% of original face value.
The basic components of a mature mammary gland are the alveoli (hollow cavities, a few millimeters large), which are lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join to form groups known as lobules. Each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells contract under the stimulation of oxytocin, excreting the milk secreted by alveolar units into the lobule lumen toward the nipple. As the infant begins to suck, the oxytocin-mediated "let down reflex" ensues, and the mother's milk is secreted—not sucked—from the gland into the infant's mouth. All the milk-secreting tissue leading to a single lactiferous duct is collectively called a "simple mammary gland"; in a "complex mammary gland", all the simple mammary glands serve one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands. The opening of each simple gland on the surface of the nipple is called a "pore." The presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia. Maintaining the correct polarized morphology of the lactiferous duct tree requires another essential component – mammary epithelial cells extracellular matrix (ECM) which, together with adipocytes, fibroblast, inflammatory cells, and others, constitute mammary stroma. Mammary epithelial ECM mainly contains myoepithelial basement membrane and the connective tissue.
=== Metabolism and excretion === Docetaxel is mainly metabolised in the liver by the cytochrome P450 CYP3A4 and CYP3A5 subfamilies of isoenzymes. Metabolism is principally oxidative and at the tert-butylpropionate side chain, resulting first in an alcohol docetaxel (M2), which is then cyclised to three further metabolites (M1, M3 and M4). M1 and M3 are two diastereomeric hydroxyoxazolidinones and M4 is an oxazolidinedione. Phase II trials of 577 patients showed docetaxel clearance is related to body surface area and to hepatic enzyme and alpha1 acid glycoprotein plasma levels. The following model represents docetaxel clearance in humans:
== External links == Media related to Corticotropin releasing hormone at Wikimedia Commons Overview of all the structural information available in the PDB for UniProt: P06850 (Corticoliberin) at the PDBe-KB.
I reject only the concept that black liberation can be achieved through the leadership of white liberals." He added that "the [white] liberal is no enemy, he's a friend – but for the moment he holds us back, offering a formula too gentle, too inadequate for our struggle". Biko's approach to activism focused on psychological empowerment, and both he and the BCM saw their main purpose as combating the feeling of inferiority that most black South Africans experienced. Biko expressed dismay at how "the black man has become a shell, a shadow of man ... bearing the yoke of oppression with sheepish timidity", and stated that "the most potent weapon in the hands of the oppressor is the mind of the oppressed". He believed that blacks needed to affirm their own humanity by overcoming their fears and believing themselves worthy of freedom and its attendant responsibilities. He defined Black Consciousness as "an inward-looking process" that would "infuse people with pride and dignity". To promote this, the BCM adopted the slogan "Black is Beautiful". One of the ways that Biko and the BCM sought to achieve psychological empowerment was through community development. Community projects were seen not only as a way to alleviate poverty in black communities but also as a means of transforming society psychologically, culturally, and economically. They would also help students to learn about the "daily struggles" of ordinary black people and to spread Black Consciousness ideas among the population.
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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.