Concentrating and preserving biological samples with freeze drying
There are a variety of evaporation methods that one can choose to reduce sample volume, heat evaporation being the most common. But it is crucial to choose the correct sample evaporation method based on how much heat your sample can withstand.
Vacuum concentration, rotary evaporation, freeze drying (lyophilisers) and nitrogen blowdown are alternative methods that process your samples under different conditions and affect your samples differently. So, it is important to identify the most appropriate sample reduction method for your application to ensure sample integrity.
Lyophilisation is one method that can be used in several applications, using a freeze dryer as the equipment for this method. Freeze dryers are commonly used in the food and pharmaceutical sectors. There is also a large variety of laboratory applications that use freeze dryers, like heat-sensitive sample preparation, plant material research, the stabilisation of living materials such as microbial cultures, long-term sample storage, preservation of whole specimens and the concentration/recovery of reaction products.
Freeze dryers use deep vacuum and heat to remove moisture from a frozen sample by a process called lyophilisation, where the frozen liquid goes directly to the gas state without going through a liquid phase. Bypassing the liquid phase allows for the biological viability of many samples to be preserved. This makes lyophilisation a unique type of evaporation method, resulting in the preservation of your biological samples.
For freeze drying to be successful, the sample to be freeze dried must first be adequately pre-frozen as the final temperature of the frozen product can affect the ability to freeze dry the sample effectively. If the amount of solvents present in the sample inhibits its freezing point, consider removing them via other evaporation techniques first, like vacuum concentration, nitrogen blowdown or rotary evaporation, so that the sample can then be frozen sufficiently during pre-freezing.
To keep a dry and structurally intact final sample, temperature and pressure play critical roles. Since vapour pressure is related to temperature, the temperature of the sample should be 15 to 20°C warmer than the freeze dryer collector temperature. Moreover, once all the ice has sublimed from the sample in the freeze-drying process, it is still possible to have bound moisture present in the product as high as 7–8% of residual sample moisture content. Therefore, it may be necessary to consider some further drying at a warmer temperature to reduce the residual moisture in your sample to optimum values.
Once your sample is dry, two factors can affect the stability of freeze-dried material — moisture and oxygen. Freeze-dried material tends to absorb moisture when exposed to air, and oxygen can destabilise it, making exposure to both detrimental to the stability of most freeze-dried material. Therefore, it is essential to choose impermeable packaging for your freeze-dried sample, which will lock out atmospheric moisture and air. It would be best to store your freeze-dried sample in low-humidity environments to reduce exposure to moisture and decrease the risk of degradation. For long-term storage of your freeze-dried samples, consider storing them in refrigerator temperatures of approximately 4–8°C as higher temperatures result in faster sample degradation.
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