Will thermal inactivation of fetal bovine serum at 56 ℃ for 30 minutes produce flocculent precipitates?

Fine flocculent precipitate appearing after 30 minutes at 56°C is not uncommon in many batches and usually results from aggregation of fibrinogen, lipoproteins, or complement components; after centrifugation, the supernatant can still be used for most primary cultures. However, if the precipitate is large, the supernatant becomes turbid, or the odor is abnormal, a problematic batch or repeated freeze-thaw cycles during transport should be suspected. It is recommended to take a small amount, filter it, and perform a parallel comparison of primary cell attachment and proliferation using an old batch or commercial control serum, observing morphology for 48–72 hours. When a new lot enters the laboratory, it is best to retain a sample and record the lot number; if abnormalities occur, contact the supplier promptly for replacement, and do not use it in large quantities first.

After inactivation, aliquoting is recommended based on single-use volumes, for example, 10-20 mL per tube for primary culture, to avoid repeated freeze-thaw cycles. A single freeze-thaw cycle of serum typically does not completely abolish activity, but it will generate a small amount of protein aggregates again and may reduce the activity of labile factors; if it has already undergone one freeze-thaw cycle, it is advisable to prioritize it for routine maintenance culture rather than for critical induced differentiation or monoclonal screening. Before aliquoting, low-speed centrifugation at 4°C or standing to collect the supernatant can be performed; during aliquoting, avoid aspirating the pellet at the bottom. Label the date and number of freeze-thaw cycles, and use within 2-4 weeks after opening whenever possible.

Before inactivation, I prefer to thaw overnight at 4°C or slowly at room temperature, rather than rapidly at 37°C, as local overheating can easily produce more flocculent material. Stabilize the water bath at 56°C in advance; after placing the serum bottle in, the water bath level should ideally be 2–3 cm above the serum level, but do not submerge the bottle completely, to prevent water from entering the bottle opening. During inactivation, gently swirl every 5–10 minutes; the 30-minute timing starts when the serum temperature reaches 56°C, not when the bottle is placed in. Immediately cool in an ice-water bath after inactivation, then store temporarily at 4°C or aliquot. A small amount of fine particles is generally not a problem after centrifugation before use.