Phase changes: What conditions influence lyophilisation?
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Phase changes: What conditions influence lyophilisation?
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[post_date] => 2025-01-14 05:26:12
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Lyophilisation, or freeze-drying, is a technique of dehydration which utilizes low pressure, low temperature environments to induce the sublimation of water content from a material. Sublimation is the process converting a substance from a solid directly to a gas, as shown in Figure 1.
Figure 1. Generalized diagram indicating the phase change of sublimation.
Perhaps the most recognizable form of freeze-drying is freeze-dried ice cream, sometimes also called ‘astronaut ice cream’ due to its development and use in the Apollo space missions; however, lyophilisation also has many applications in the pharmaceutical agricultural industry. Freeze-drying is ideal for preservation because low water content prevents enzymes and microorganisms from spoiling or degrading the substance. Additionally a freeze-dried substance can be rehydrated easily because of the microscopic pores left behind by the ice crystals when they sublimate away. This makes lyophilisation ideal for the long term storage and relatively fast reconstitution of substances, e.g. pharmaceuticals from an inert to an active form.
There is a three stage process to lyophilisation. First the substance must be frozen, usually at very low temperatures between -50°C and -80°C. Once frozen, the substance is placed in a vacuum on plates, and a small amount of heat is added to help sublime the water directly from solid ice to water vapor. This is known as the primary drying stage, which removes 95% of a material’s water content. It can last up to several days, and requires rigorously maintained conditions due to the sensitivity of the sublimating structure. Finally, there is a secondary drying stage, which involves the temperature being raised again slightly and pressure being lowered further to sublime as much of the last 5% of moisture as possible.
Researchers have found in their own astronaut ice cream making experiments that varying the temperature conditions can influence the rate of sublimation, as well as the consistency of the dehydrated ‘cake’ product. Table 1 shows how various temperatures affect the overall structure of freeze-dried ice cream during the primary drying stage by increasing temperature.
Table 1. Results of lyophilisation of ice cream under varying temperature conditions.
Citation: Tables Adapted from: Xiang, Jun, Hey, Jeffery M., Liedtke, Volker, D.Q. Wang, Investigation of freeze–drying sublimation rates using a freeze–drying microbalance technique. International Journal of Pharmaceutics Volume 279, Issues 1–2, 26 July 2004, Pages 95–105
Nail SL1, Jiang S, Chongprasert S, Knopp SA., Fundamentals of freeze-drying. Pharm Biotechnol. 2002;14:281-360.
[post_title] => Phase changes: What conditions influence lyophilisation?
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[question] => Following the secondary drying stage, researchers found that the water vapor would consistently rehydrate the freeze-dried ice-cream, ruining their cake structure. Which of the following would best prevent the rehydration?
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[description] => Reason for the Correct Answer:
Promoting sublimation simply sublimates ice into water vapor quickly, but it does not address the problem of rehydration.
The water vapor would not dissipate over time because it is within in a sealed vacuum.
Forming less ice crystals initially in the process would result in less microscopic pores to be formed, which would discourage rehydration from occurring.
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[answers] => Array
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[each_answer] => A.Decreasing the pressure during the drying stages to promote rapid sublimation
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[each_answer] => B.Increasing the temperature at which the ice cream is frozen to form less ice crystals
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[each_answer] => C.Increasing the pressure during the drying stages to slow down the rate of sublimation
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[each_answer] => D.Utilizing a very slow sublimation rate in order to allow the water vapor to dissipate
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => If the researchers were looking to maximize the speed of making ice cream while maintaining a solid consistency of their product, which condition should they provide for freeze-drying?
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[answer] => 4
[description] => Reason for the Correct Answer:
Condition 7 involves the fastest rate but its consistency is not solid.
Condition 3 provides a solid consistency but it is not the fastest rate.
Condition 14 provides the fastest rate of sublimation while maintaining a solid consistency.
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[each_answer] => A.Condition 3
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[each_answer] => B.Condition 7
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[each_answer] => C.Condition 12
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[each_answer] => D.Condition 14
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[quiz_unique_key] => 83407773
[question] => During the sublimation of water from a solid to a gas, how is the temperature of water being affected?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
While the temperature of a phase change is dependent on pressure, it will not vary during the phase change itself.
Heat is required during a phase change, but the added heat will not increase the temperature.
Temperature does not increase during a phase change due to the enthalpy of a phase transition.
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[answers] => Array
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[0] => Array
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[each_answer] => A.The temperature varies during a phase change because it is dependent upon pressure
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[each_answer] => B.Temperature does not change during a phase change
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[each_answer] => C.Temperature is decreasing because substances release heat during phase changes
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[each_answer] => D.Temperature is increasing because it requires heat to induce a phase change
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[question] => Which of the following means of heat transfer would be least effective in warming a substance during the drying stages of lyophilisation?
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[answer] => 3
[description] => Reason for the Correct Answer:
Conduction is the primary form of heating through the drying plates the substance rests upon.
Radiation would also be relatively effective in heating the substance.
Convection would not be effective because the substance is placed within a vacuum.
)
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[each_answer] => A.Conduction
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[each_answer] => B.Radiation
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[each_answer] => C.Convection
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[each_answer] => D.All three means of transfer would be equally useful
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[question] => Often, scientists will place a super-cooled (−50°C) condenser plate in the freeze-drying vacuum to prevent water vapor from rehydrating their freeze-dried product. How would a condenser plate help to prevent rehydration of the product?
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[answer] => 1
[description] => Reason for the Correct Answer:
The ice crystals are at no real risk of melting, the temperatures during freeze drying are never above the melting point of water.
Although the name is misleading, the condenser plate is actually too cold to condense water into a liquid.
A condenser plate could be used as a surface onto which the water vapour could deposit from water vapor directly to ice. This would effectively sequester any extra water vapor from rehydrating a freeze-dried product.
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[each_answer] => A.The condenser plate could act as a surface for water vapor to undergo deposition
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[each_answer] => B.The condenser plate could absorb heat radiated during the secondary drying stage to prevent over-heating of the product
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[each_answer] => C.The condenser plate could condense water into a liquid and have it drained away
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[each_answer] => D.The condenser plate could keep the environment cooler during the primary drying stage so that none of the initial ice crystals melt into the product
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