A practical reference on secondary drying: what it is, how it behaves, what the literature reports, and where the honest uncertainties sit.
This page was last updated on 2026-05-27 and is reviewed periodically as new material appears.
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.
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.
After lyophilization, the product is usually a porous cake or powder with a large internal surface area. This structure can absorb moisture quickly if exposed to humid air, so vials are sealed under vacuum or an inert gas. Moisture uptake may lower the glass transition temperature of the dried matrix and accelerate chemical or physical degradation. Storage conditions therefore depend on the formulation, container, and intended shelf life. Some products remain stable at room temperature, while others require refrigeration or freezing.
Quality control for lyophilized products includes appearance, cake structure, reconstitution time, pH, residual moisture, and potency. Residual moisture is a key attribute because excess water can reduce stability, while excessively low moisture may cause structural changes or aggregation in some systems. Stability studies compare real-time and accelerated conditions to estimate shelf life. Analytical methods must be validated for the specific matrix, container, and moisture range. Sterility and container integrity are also monitored for sterile products.
Handling practices aim to prevent moisture ingress and mechanical damage. Vials should remain stoppered and crimped until use, and reconstitution should follow the labeled diluent and volume. Shipping may involve temperature-controlled containers and desiccants, but direct contact between desiccant and product is avoided. Regulatory guidance expects documented storage conditions, excursion assessments, and stability commitments. Open questions remain about how best to predict long-term stability from short accelerated studies for every formulation class.
| Property | Value | Notes |
|---|---|---|
| Primary phase change | Sublimation | Ice changes directly to vapor under reduced pressure |
| Typical chamber pressure | 0.01–0.5 mbar (1–50 Pa) | Below the triple point of water; product-specific |
| Typical product temperature during primary drying | −40 °C to −10 °C | Kept below collapse temperature |
| Typical residual moisture | 0.5–3% w/w | Target range varies by formulation and use |
| Common synonyms | Freeze-drying; lyophilisation | Lyophilization is the US spelling |
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.
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.
After lyophilization, a product's quality depends on residual moisture, cake appearance, and reconstitution time. Residual moisture is often measured by Karl Fischer titration or thermogravimetric analysis. A low moisture content can slow chemical degradation, but overly dry cakes may be brittle or slow to dissolve. Stability studies track these attributes over months under defined temperature and humidity conditions. Batch records link these measurements to specific process runs and help identify trends before a product fails specification.
Storage conditions for dried products usually aim to exclude moisture and oxygen. Vials are sealed under vacuum or with an inert gas, and stoppers must maintain a barrier during transport. Temperature recommendations vary; some materials remain stable at room temperature, while others need refrigeration or frozen storage. Humidity control is critical because dried cakes can absorb water rapidly once a container is opened. Desiccant packs and moisture-barrier bags add further protection during shipping.
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.
These data confirm that the enthalpy changes are approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term, which is much less unfavorable. In general it is difficult to account precisely for thermodynamic values in terms of changes in solution at the molecular level, but it is clear that the chelate effect is predominantly an effect of entropy. Other explanations, including that of Schwarzenbach, are discussed in Greenwood and Earnshaw (loc.cit).
This resulted in a surge of both Croatian nationalism and Slovene nationalism in response and the collapse of the League of Communists of Yugoslavia in 1990, the victory of nationalists in multi-party elections in most of Yugoslavia's constituent republics and eventually civil war between the various nationalities beginning in 1991. Yugoslavia was dissolved in 1992. The Soviet Union itself collapsed between 1990 and 1991, with a rise of secessionist nationalism and a political power dispute between Gorbachev and Boris Yeltsin, the new leader of the Russian Federation. With the Soviet Union collapsing, Gorbachev prepared the country to become a loose federation of independent states called the Commonwealth of Independent States. Hardline Marxist–Leninist leaders in the military reacted to Gorbachev's policies with the August Coup of 1991 in which hardline Marxist–Leninist military leaders overthrew Gorbachev and seized control of the government. This regime only lasted briefly as widespread popular opposition erupted in street protests and refused to submit. Gorbachev was restored to power, but the various Soviet republics were now set for independence. On 25 December 1991, Gorbachev officially announced the dissolution of the Soviet Union, ending the existence of the world's first Marxist–Leninist-led state.
Rita Levi-Montalcini and Stanley Cohen discovered NGF in the 1950s while faculty members at Washington University in St. Louis, for which they were awarded a Nobel Prize in Physiology or Medicine in 1986. The critical preliminary discovery was done by Levi-Montalcini and Hertha Meyer at the Carlos Chagas Filho Biophysics Institute of the Federal University of Rio de Janeiro in 1952. Their publication in 1954 became the definitive proof for the existence of the protein. Levi-Montalcini later remarked:The tumor had given a first hint of its existence in St. Louis but it was in Rio de Janeiro that it revealed itself, and it did so in a theatrical and grand way, as if spurred by the bright atmosphere of that explosive and exhuberant manifestation of life that is the Carnival in Rio.However, its discovery, along with the discovery of other neurotrophins, was not widely recognized until 1986, when it won the Nobel Prize in Physiology or Medicine. Studies in 1971 determined the primary structure of NGF. This eventually led to the discovery of the NGF gene. NGF is abundant in seminal plasma. Recent studies have found that it induces ovulation in some mammals. Nerve Growth Factors (NGF) were initially discovered due to their actions during development, but NGF are now known to be involved in the function throughout the life of the animal.
Sources: en.wikipedia.org
A referendum for the preservation of the USSR was held on 17 March 1991 in nine republics (the remainder having boycotted the vote), with the majority of the population in those republics voting for preservation of the Union in the form of a new federation. The referendum gave Gorbachev a minor boost. In the summer of 1991, the New Union Treaty, which would have turned the country into a much looser Union, was agreed upon by eight republics. The signing of the treaty, however, was interrupted by the August Coup—an attempted coup d'état by hardline members of the government and the KGB who sought to reverse Gorbachev's reforms and reassert the central government's control over the republics. After the coup collapsed, Russian president Yeltsin was seen as a hero for his decisive actions, while Gorbachev's power was effectively ended. The balance of power tipped significantly towards the republics. In August 1991, Latvia and Estonia immediately declared the restoration of their full independence (following Lithuania's 1990 example). Gorbachev resigned as general secretary in late August, and soon afterwards, the party's activities were indefinitely suspended—effectively ending its rule. By the fall, Gorbachev could no longer influence events outside Moscow, and he was being challenged even there by Yeltsin, who had been elected President of Russia in July 1991.
=== Improved mine safety === An ultrasonic bolt elongation monitor developed by a NASA scientist for testing tension and high-pressure loads on bolts and fasteners has continued to evolve over the past three decades. Today, the same scientist and Luna Innovations are using a digital adaptation of this same device for non-destructive evaluation (NDE) of railroad ties, groundwater analysis, radiation, and as a medical testing device to assess levels of internal swelling and pressure for patients with intracranial pressure and compartment syndrome, a painful condition that results when pressure within muscles builds to dangerous levels.
An immigration stamp is an inked impression in a passport or other travel document, typically made by a rubber stamp upon entering or exiting a territory. Depending on the jurisdiction, a stamp can serve different purposes. For example, in the United Kingdom, an immigration stamp in a passport includes the formal leave to enter granted to a person subject to entry control. In other countries, a stamp activates or acknowledges the continuing leave conferred in the passport bearer's entry clearance. Under the Schengen system, a foreign passport is stamped with a date stamp which does not indicate any duration of stay. This means the person is deemed to have permission to remain for either 3 months or the period shown on their visa, if specified otherwise. Member states of the European Union are not permitted to place a stamp in the passport of a person who is not subject to immigration control. Stamping is prohibited because it imposes a control to which the person is not subject. Passport stamps may occasionally take the form of sticker stamps, such as entry stamps from Japan. Depending on nationality, a visitor may not receive a stamp at all (unless specifically requested), such as an EU or EFTA citizen travelling to an EU or EFTA country, Albania, or North Macedonia. Most countries issue exit stamps in addition to entry stamps. A few countries issue only entry stamps, including Canada, El Salvador, Ireland, Mexico, New Zealand, Singapore, United Kingdom and the United States of America.
Sources: en.wikipedia.org
Steroidogenesis is the biological process by which steroids are generated from cholesterol and changed into other steroids. The pathways of steroidogenesis differ among species. The major classes of steroid hormones, as noted above (with their prominent members and functions), are the progestogens, corticosteroids (corticoids), androgens, and estrogens. Human steroidogenesis of these classes occurs in a number of locations:
Tennessee marble, extracted from quarries on the city's periphery, was used in the construction of numerous monumental buildings across the country, earning Knoxville the nickname, "The Marble City." Knoxville's economy slowed in the early 1900s. Political factioning hampered revitalization efforts throughout much of the 20th century, though the creation of federal entities such as the Tennessee Valley Authority in the 1930s and the ten-fold expansion of the University of Tennessee helped keep the economy stable. Beginning in the late 1960s, a city council more open to change, along with economic diversification, urban renewal, and the hosting of the 1982 World's Fair, helped the city revitalize to some extent.
=== Dry skin === Dry skin does not have enough moisture. It will show signs like flaking, tightness, roughness, redness, or cracks. If the skin is very dry, it might also look swollen or extra soft. Common areas that get dry include the face, scalp, and back—especially when exposed to air or clothing. However, places where the skin folds, like under the arms, between the toes, and around the groin, usually do not get as dry. Washing dry skin should be done daily using lukewarm water (less than 37 °C (99 °F)). Avoid long or frequent baths and showers. Try to keep bath time under five minutes per day. If the skin is extremely dry, avoid using regular soap and water, as they can make dryness worse. Instead, use gentle, alkali-free cleansers with a pH of 4 to 5, especially ones with ingredients like urea, lactic acid, or glycerin, which help keep moisture in. Moisturizing dry skin at least twice a day is important. If the skin is very dry, moisturizing more often may help. Thicker, oil-based creams work best for very dry skin. Aqueous cream should be avoided. It also helps to apply a skin protectant. Using a moisturizer with niacinamide and glycerin twice a day has been shown to strengthen the skin and reduce dryness, versus moisturizers without niacinamide.
Sources: en.wikipedia.org
Lyophilization relies on sublimation, so water moves from solid ice to vapor without becoming liquid. The material is frozen, pressure is reduced, and controlled heat is supplied. Vapor is captured on a cold condenser, leaving a dry porous solid.
The process has three main stages: freezing, primary drying, and secondary drying. Freezing sets the ice structure, primary drying removes free ice, and secondary drying removes bound water. Each stage uses specific temperature, pressure, and time settings.
No, it is a drying method rather than a sterilization method. Removing water can limit microbial growth, but it does not reliably kill microorganisms. Sterility must come from separate steps such as filtration, heat treatment, or aseptic processing.
Collapse occurs when the product temperature rises above its collapse or eutectic temperature during drying. The frozen matrix loses structure, producing a shrunken or melted appearance. This can slow reconstitution and may affect stability.