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 2026-01-13. Anything still debated is marked as such rather than presented as settled.
After drying, a lyophilized product is usually sealed under vacuum or an inert gas to limit moisture uptake. Residual water content is measured because small changes can alter chemical stability and cake appearance. Storage temperature depends on the material; many biological products are kept at 2–8 °C, while some require −20 °C or colder. Exposure to ambient humidity during handling can cause the porous solid to absorb water and collapse. Container closures and stoppers are therefore selected for low moisture transmission and compatibility.
Quality control for lyophilized materials includes visual inspection of the cake, measurement of residual moisture, and tests for reconstitution time. An acceptable cake is typically uniform and may be slightly porous; shrinkage, meltback, or cracks can indicate process deviations. Analytical methods such as Karl Fischer titration, thermogravimetric analysis, and near-infrared spectroscopy quantify water content. Reconstitution time is recorded because a very slow or incomplete dissolution can signal collapse or aggregation. Stability studies compare samples stored under defined temperature and humidity conditions over months or years.
Freezing determines ice crystal structure and pore size, which affect drying speed and product uniformity. Rapid freezing creates small crystals, while slow freezing creates larger crystals and often faster sublimation. During primary drying, chamber pressure is held below the vapor pressure of ice, and shelf temperature supplies heat for sublimation. The ice front recedes, leaving a porous matrix. Thermal limits such as collapse and eutectic temperatures set safe boundaries for formulation. These limits vary with solute composition and concentration.
Secondary drying removes bound water that remains after ice sublimation. Shelf temperature is raised gradually while pressure remains low, reducing water content to a target range. Over-drying can cause brittleness or electrostatic issues, while under-drying affects stability. The endpoint is often judged by pressure rise tests, temperature measurements, or water content analysis. Scale-up depends on matching heat and mass transfer across equipment sizes. Small changes in shelf temperature or pressure can alter cycle length substantially.
| Property | Value | Notes |
|---|---|---|
| Typical storage temperature | 2–8 °C | Common for biological materials; some require −20 °C or colder |
| Residual moisture specification | 0.5–3.0% w/w | Product-specific; measured after drying |
| Common moisture method | Karl Fischer titration | Coulometric or volumetric; detects water content |
| Cake appearance | Uniform and porous | Collapse, meltback, or cracks are deviations |
| Reconstitution time | Seconds to several minutes | Depends on formulation, cake structure, and diluent |
Freeze-dried materials are hygroscopic to varying degrees and can take up moisture after drying. Storage therefore often uses sealed glass vials, rubber stoppers, and crimp seals to limit contact with ambient humidity. A desiccant may be included for moisture-sensitive products, although it is not universal. Controlled room temperature is sufficient for many lyophilizates, while others require refrigeration or freezing. Moisture ingress remains a primary cause of cake collapse, chemical degradation, and loss of reconstitution performance.
Quality assessment of a lyophilized product includes cake appearance, residual moisture, reconstitution time, and container closure integrity. A uniform, porous cake suggests that freezing and drying stayed within the formulation's design space. Cracks, shrinkage, meltback, or a glassy film can indicate thermal abuse or a formulation problem. Analysts also test for subvisible particles and sterility when the product requires those specifications. Visual inspection alone cannot confirm biological activity or chemical stability, so it is combined with analytical methods.
Stability programs monitor lyophilized products under defined temperature and humidity conditions over time. Real-time studies at recommended storage conditions are the reference, while accelerated studies provide early signals of degradation pathways. Because a dry cake can still undergo oxidation, hydrolysis, or aggregation, stability depends on residual moisture, excipients, and container headspace. Open questions include how best to predict long-term stability from short accelerated runs and how vial-to-vial variability affects shelf life. Current guidance treats these predictions as product-specific rather than universally generalizable.
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.
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.
=== Mechanism of liver injury === The cause of the mild, transient serum enzyme elevations during imipenem-cilastatin therapy is not known. The cholestatic hepatitis attributed to imipenem-cilastatin and the carbapenems is probably immunoallergic and resembles the rare, clinically apparent liver injury that has been linked to penicillins and cephalosporins.
Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate, which is lactic acid in solution. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells. It is also used extensively to preserve food and create novel flavours. Despite the name, milk is not required by this process. If oxygen is present in the cell, many organisms will bypass fermentation and undergo cellular respiration; however, facultative anaerobic organisms will both ferment and undergo respiration in the presence of oxygen. Sometimes even when oxygen is present and aerobic metabolism is happening in the mitochondria, if pyruvate is building up faster than it can be metabolized, the fermentation will happen anyway. Lactate dehydrogenase catalyzes the interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD+. In homolactic fermentation, one molecule of glucose is ultimately converted to two molecules of lactic acid. Heterolactic fermentation, by contrast, yields carbon dioxide and ethanol in addition to lactic acid, in a process called the phosphoketolase pathway.
There are 25 known isotopes of potassium, three of which occur naturally: 39K (93.3%), 40K (0.0117%), and 41K (6.7%) (by mole fraction). Naturally occurring 40K has a half-life of 1.250×109 years. It decays to stable 40Ar by electron capture or positron emission (11.2%) or to stable 40Ca by beta decay (88.8%). This decay results in a relatively higher concentration of Argon in the atmosphere. The decay of 40K to 40Ar is the basis of a common method for dating rocks. The conventional potassium–argon dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (40Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated. The minerals best suited for dating include biotite, muscovite, metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. Apart from dating, potassium isotopes have been used as tracers in studies of weathering and for nutrient cycling studies because potassium is a macronutrient required for life on Earth. 40K occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. 40K is the radioisotope with the largest abundance in the human body. In healthy animals and people, 40K represents the largest source of radioactivity, greater even than 14C.
== Awards == 2009: BBSRC David Phillips Fellowship 2012: ERC Starting Grant 2014: Talented Young Italians Award (Research and Innovation) 2015: EFMC Prize for Young Medicinal Chemist in Academia 2015: ICBS Young Chemical Biologist Award 2016: RSC Capps Green Zomaya Award 2016: MedChemComm Emerging Investigator Lectureship 2016: Fellow of the Royal Society of Chemistry (FRSC) 2022: Prous Institute-Overton and Meyer Award for New Technologies in Drug Discovery 2023: Fellow of the Royal Society of Edinburgh (FRSE) 2025: Fellow of the Royal Society 2026: Fellow of the Academy of Medical Sciences (FMedSci)
Sources: en.wikipedia.org
From propionyl-CoA: Methylmalonyl-CoA results from the metabolism of fatty acid with an odd number of carbons, of amino acids valine, isoleucine, methionine, threonine or of cholesterol side-chains, forming Propionyl-CoA. The latter is also formed from propionic acid, which bacteria produce in the intestine. Propionyl-CoA and bicarbonate are converted to Methylmalonyl-CoA by the enzyme propionyl-CoA Carboxylase. It then is converted into succinyl-CoA by methylmalonyl-CoA mutase (MUT). This reaction is a reversible isomerization. In this way, the compound enters the citric acid cycle. The following diagram demonstrates the aforementioned reaction: Propionyl CoA + Bicarbonate → Methylmalonyl CoA → Succinyl CoA
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 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.
A ghretropin is a substance, usually a peptide, that increases the secretion of the "hunger hormone" ghrelin when consumed. Ghretropins that work on mice have been reported in the enzymatic digests of wheat, rice, and soy bean proteins. When a mouse consumes a purified version of the peptide, its plasma concentration of ghrelin and food intake increase. If they also work on humans, they may represent a way to boost the appetite of elderly and anorexic people.
Sources: en.wikipedia.org
=== Sulfhydryl-containing agents === Alacepril Captopril (trade name Capoten), the first ACE inhibitor. Zofenopril These agents appear to show antioxidative properties but may be involved in adverse events such as skin eruptions.
==== Abolition of the state ==== Socialist Studies accuses the SPGB of coming under the influence of anarchist ideas and not sufficiently emphasising the parliamentary aspect of the socialist revolution. Socialist Studies claims that the SPGB's position is that the state would be abolished immediately upon the overthrow of class society whereas Socialist Studies's position is that the state would "gradually wither away" instead. Referring to the SPGB's 1984 conference resolution (since rescinded) that "Socialism will entail the immediate abolition of and not the gradual decline of the State", Socialist Studies writes: If you are in favour of the immediate abolition of the State when Socialism is established then you are in favour of its immediate abolition here and now, which is of course the anarchist position. [...] Unless Socialists use the power of the machinery of government to dispossess the capitalist class they will be unable to establish Socialism. We find it inconceivable that intelligent Socialists of a future generation would even consider such an act of monumental stupidity.
=== Timeline of the struggle for professional equality in France === 1900: Law allowing women full access to the bar, including the right to plead. 1907: A law authorizes married women to freely dispose of their salary. Previously, all earnings went to their husbands. They are also now allowed to sit on labor tribunals. 1908: Law grants eight weeks of maternity leave, unpaid. 1920: Women can join a union without their husband's authorization. 1924: Women are allowed to take the baccalauréat exam (some had already done so in the past, notably Julie-Victoire Daubié, but from now on, secondary education is the same for both sexes, allowing more girls to aim for this diploma). 1928: Two-month fully paid maternity leave is granted in the civil service. 1938: Women's legal incapacity is lifted. They can attend university, pursue studies, and obtain an identity card or passport without their husband's permission (Renoult law). However, the husband remains the head of the household and can forbid his wife from working. 1942: Widowed women are allowed to work, even if their husbands were employed, provided this does not cause layoffs. 1944: Women are granted the right to vote. 1945: The concept of "women's wages" is abolished. The principle "equal pay for equal work" is written into the legislation. 1946: Women can become judges. 1965: Reform of matrimonial regimes: women can work and open a bank account without their husband's authorization and benefit from social advantages. 1967: Medical contraception is legalized by the Neuwirth law.
== Development == In humans, white adipose tissue starts to develop during early to mid-gestation period. White adipose tissue consists of white adipocytes, which are the lipid storage cells. They are differentiated from undifferentiated preadipocytes through transcriptional cascade. This process is regulated by the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ), a protein regulating gene involved in regulation of fatty acid storage and glucose metabolism and members of the CCAAT/enhancer-binding protein family, type of transcription factors that promotes gene expression. PPARγ is required for both the adipogenesis and maintenance of the adipocytes. White adipose tissue exists in various depots that may have different types of adipocytes. That is, different depots in different locations have different intrinsic properties. This led to various theories to find the adipogenic lineage of the white adipose tissue depots. A hypothesis is that the precursors for the different types of adipocytes are mesenchymal stem cells which differentiates by the influence of specific gene expression into specialized white preadipocytes. Such genes are Shox2, En1, Tbx15, HoxC9, HoxC8, and HoxA5. The study of the gene expression is important as they can be indicative of various health issues such as obesity related risk factors including diabetes and metabolic conditions.
Sources: en.wikipedia.org
Most are held in sealed containers at controlled temperatures, often 2–8 °C, while some require frozen storage. Protection from moisture and light helps preserve the dry matrix. Exact conditions are set by the manufacturer or study protocol.
Cake collapse suggests the material exceeded its collapse temperature during drying or later absorbed moisture. It can lead to slower reconstitution, uneven moisture, and reduced stability. Appearance alone may not reveal the cause, so process records and moisture tests are used together.
Some residual moisture is common and may be acceptable within a defined range. Very low moisture can alter stability or increase brittleness, while high moisture promotes hydrolysis and microbial risk. Specifications are based on product-specific stability data.
Conventional drying uses heat to evaporate liquid water, often at temperatures that can degrade sensitive materials. Lyophilization freezes the material first and then removes water by sublimation under vacuum. This avoids prolonged exposure to liquid water and high heat.