Ultra-low temperature storage is a staple of laboratory life. Nearly every scientist leans on safe and protected long-term sample storage afforded by lab freezers, typically set to either -20°C, -80°C, or -196°C. With such a wide variety of sample types, from DNA and RNA samples to protein samples, cell lysates, and unicellular isolates, no one has truly determined optimal conditions for low temperature storage. Instead, many use -80°C as the standard as it has been demonstrated to be an effective storage temperature. However, some have recently begun questioning whether -70°C sample storage may work as well, with the possibility of reducing energy consumption.
Benefits of using -70°C storage
The first question when deciding to set low temperature freezers to -70°C instead of -80°C is whether it is safe for protein, DNA, and RNA samples. Scientists often rely on doing what others have done previously, not necessarily because other methods have been eliminated but because the current method is a known commodity that works. Testing new methods costs money, so if it isn’t broken, why fix it?
With climate change at the tip of everyone’s tongue lately, it makes sense to minimize energy usage wherever possible, including at the lab. Lowering the temperature of freezers has two potential benefits towards reduced energy consumption: it drives down overall energy consumption by 30%-40% and also prolongs the life of the freezer.1
Sample viability at -70°C
When answering the primary question of whether lowered freezer temperatures affect sample viability, the scientific literature provides several examples showing that it can be safe and effective:
- A study in 2004 by Epinel-Ingroff et al showed that preservation of fungal isolates for up to 8 years at -70°C resulted in a loss of only 0.7% of yeasts and 3.8% of molds. Unfortunately, this study was not conducted in parallel with similar isolates stored at -80°C as a control.2
- A study in 2012 by Beekhof et al. confirmed that storing enzyme isolates of paraoxonase-1 (PON1) for one year at -70°C could retain enzymatic activity when thawed. Again, no -80°C control was used in this study, as isolates were stored at either -20°C, -70°C, or -195°C.3
- A paper from 1972 studied mycobacteria stored at -20°C or -70°C using a new technique (at the time) to seal the containers with a vaccine-stoppered serum bottle sealed with an aluminum crimp cap. Storage at -70°C maintained both viability and virulence.4
Overall, though no studies have been performed comparing storage at -70°C to -80°C, low levels of sample degradation have been observed in nearly every study that has used -70°C as a storage temperature. Accordingly, even Qiagen recommends storing DNA samples at -70°C. Thus, increasing the temperature of -80°C by 10 degrees should be feasible to save energy in the short and long term.
Tracking samples at -70°C
Sample tracking is primarily tested at temperatures similar to what is conventionally used: -20°C, -80°C, and -196°C. Note that cryogenic labels are made to resist temperatures as low as -196°C; thus, the transition to -70°C, should these labels be applied, should not change much. Additionally, storage at -70°C is compatible with CryoSTUCK® labels, which can be applied to frozen surfaces as low as -80°C.
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References:
- Energy Transition Coordinating Council. Ultra-Low Temperature Freezers: Opening the Door to Energy Savings in Laboratories. 2016.
- Espinel-Ingroff A, et al. Long-Term Preservation of Fungal Isolates in Commercially Prepared Cryogenic Microbank Vials. J Clin Microbiol. 2004;42(3):1257-1259.
- Beekhof PK, et al. Long term stability of paraoxonase-1 and high-density lipoprotein in human serum. Lipids Health Dis. 2012;11:53.
- Kim TH, Kubica GP. Long-term preservation and storage of mycobacteria. Appl Microbiol. 1972;24(3):311-317.
