Serum for Cell and Tissue Culture: Frequently Asked Questions (FAQs)

Biological, quality and regulatory information about serum products

Why do cell and tissue cultures need serum?

Serum is added as a supplement to culture media at a concentration of 2-10% to provide a mixture of nutritional, hormonal, growth and attachment factors. Serum also acts as a buffer to the cell culture system against a variety of disruptions to cell growth and toxic effects such as pH change, proteoloytic activity, or the presence of heavy metals or endotoxin.


How is serum processed?

Serum delivered frozen to a processing facility is thawed and tested for the presence of bacteria, virus, endotoxin and for hemoglobin content. The best quality serum is then filtered through a sequence of membrane filters, terminating with a 0.1 micron pore filtration under aseptic conditions. After filtration, aseptic conditions are maintained as serum is dispensed into sterile bottles, quickly frozen to -20°C and quarantined until all lots have met or exceeded quality control standards. A certificate of analysis for each lot is available upon request.


What is the recommended storage temperature for serum?

The recommended storage temperature for serum is between -5°C and -20°C, as it is most stable when stored frozen until needed. Storing serum in a frost-free freezer is not recommended, because temperature cycling may cause bottles to crack, contributing to contamination or deterioration of the product.


How should serum be thawed?

Serum is thawed overnight at 2-8°C to prevent degradation; or, it may be thawed at room temperature with periodic agitation to resuspend the contents. Thawed serum should be mixed thoroughly before being added to cell culture medium.

As repeated freeze-thaw cycles may significantly impact serum quality, we recommend aliquoting the thawed serum for single use, and to store aliquots at -20°C. If serum is stored between 2-8°C, it should be used within 2-4 weeks.

Note: Temperatures above 37°C may degrade nutrient components and compromise serum performance.  


If serum arrives partially thawed, can I still use it?

Serum is packaged in dry ice and it should arrive frozen. However, if serum is partially thawed due to delay, it can still be used. Thaw the serum completely and aliquot into single-use aliquots for storage and further use.


What is the shelf life of serum products?

The shelf life of serum is five years from the date of manufacture.


Is serum light sensitive?

Normal handling of serum under typical room/laboratory lighting will have little effect. However, serum does contain certain constituents that are light sensitive. Exposing serum to high-intensity or short-wavelength lights is therefore not recommended.


Why is the serum cloudy?

Precipitates appear in serum for a variety of reasons:

  • Repeated freeze-thaw cycles denature serum lipoproteins and cause turbidity. This can be minimized by preparing single-use aliquots.
  • Serum is processed rapidly at cold temperatures to retain growth properties, which may leave some fibrinogen in serum, which may convert to fibrin on thawing. The excess amount of fibrin in serum is visible as flocculent material, which can be removed by centrifugation at 400xg.  Filtering serum to remove flocculent is not recommended, as filters may become clogged, or unintentional retention of desirable proteins on the filter may occur.


How long can I store media once I have added serum and all the supplements?

If aseptic handling is carefully observed, serum-supplemented media may be stored up to six weeks at 2-8°C. Media should be discarded using appropriate institutional biosafety protocols if it becomes cloudy, regardless of storage duration.


Does bovine serum product testing include tests for endotoxin, bacteria, fungi, molds, mycoplasma and bacteriophage?

Like all of our serum products, FBS is meticulously tested for the presence of these relevant microbial contaminants.  A Certificate of Analysis (CoA, CofA) for each lot is available upon request.


Are serum products free of Bovine Spongiform Encephalopathy (BSE)?

Yes. All Sigma-Aldrich sera have been sourced from countries that are free of or at low risk for BSE. The intergovernmental World Organisation for Animal Health (OIE) fights animal disease at the global level by monitoring country compliance with OIE guidelines and standards.


What is gamma-irradiated (fetal bovine) serum?

Gamma-irradiated serum has been exposed to a Cobalt-60 source at a 25-35 kGy dose to inactive viruses and other adventitious microbial agents (such as mycoplasmas). Gamma irradiation does not compromise the physiochemical properties or the cell culture performance of serum.


Why (and how) is some serum heat-inactivated?

Complement proteins naturally occurring in mammalian sera mediate cytolytic events, contract smooth muscles, and activate lymphocytic and myeloid cells.  Heat-inactivation destroys the activity of complement in serum.  This method is therefore recommended for immunological applications and for the culture of embryonic stem cells (ESCs), insect cells and smooth muscle cells.

The heat inactivation process must be carefully controlled to prevent denaturation of desirable serum proteins that are key constituents of sera necessary for cell and tissue propagation.

Heat inactivation occurs when serum is heated to 56°C for a period of 30 minutes.


Why do we see lot-to-lot variability in serum?

As serum is a solution extracted from biological sources, its composition may vary from lot to lot. To minimize application variability, we perform rigorous quality testing of each lot, and can provide lot samples to our customers for testing in their labs. We reserve material from the test batch during the customers’ lot qualification process.


The color of the FBS is not the same as my previous lot. Does this affect serum performance?

The color of the serum depends on hemoglobin concentration, and variation in this parameter will not affect serum performance.


Why is the origin of serum important?

The origin of serum (also sometimes referred to as the ‘source’) is the country where the unrefined blood was collected. This is not to be confused with the country where the serum was separated, filtered, or otherwise processed, which may differ. The certificate of origin documents from the serum supplier should clearly state the serum source and processing information.


What is the difference between USDA-origin and U.S.-origin serum?

U.S.-Origin Serum USDA-Origin Serum
U.S.-origin serum is collected from facilities within the United States of America that undergo continuous inspection by the United States Department of Agriculture (USDA). USDA-origin serum is collected from facilities outside the U.S. in countries that have been approved for import to the U.S. by the USDA.
These include Australia, Canada, New Zealand, Chile, and Central America.


How do traceability and origin impact the cost of serum
?

While “origin” refers to the country in which serum was collected, the term “traceability” refers to the documents that trace the serum from origin through manufacturing and distribution.

Serum is a commodity product for which price varies among countries of origin. For example, the cost of serum originating from Australia and New Zealand may be higher than that from other countries, due to lower supply and/or higher costs associated with collection.  

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