Can fetal bovine serum still have ice residue after being thawed overnight at 4 ℃?

If you are in a hurry to prepare complete medium, directly taking the supernatant when a small amount of ice crystals remains can cause uneven components, so it is not recommended. A more reliable emergency approach is a room-temperature water bath or cold water bath: wipe the bottle clean, place it in a clean bag, do not let water contact the bottle opening, and gently swirl until completely thawed; do not use 37°C just because you are short on time. Short-term room temperature generally will not have a noticeable effect on culture; the key is to avoid local overheating and contamination. If the cells subsequently show fluctuations in condition, first check the medium preparation and serum lot number rather than attributing it solely to this thawing.

More critically, aliquoting. If 500 mL is drawn multiple times, repeated freeze-thaw cycles will affect serum quality more than this short-term rewarming. It is recommended that after the first complete thaw at 4°C, aliquot into 50 mL/tube in a biosafety cabinet, keep out the amount for current use, and store the rest at -20°C. South American-origin serum may have slightly higher protein and lipid levels; if slight turbidity or a small amount of precipitate appears after thawing, do not shake vigorously—gently invert to mix. If HepG2 and 293T cultures are sensitive to lot, record the serum lot number, thaw time, and aliquot date, and perform a parallel culture comparison when changing lots.

If there are ice crystals, you can continue to place it at 4°C until it is completely thawed. It is not recommended to directly place it in a 37°C water bath. A 37°C water bath can easily cause localized overheating on the bottle wall. When ice cores remain inside the bottle, this may lead to local protein denaturation. Complement and some growth factors are relatively sensitive to temperature. If you are in a hurry, you can place the serum bottle in a room-temperature water bath or 20-25°C water, and gently swirl it every 10 minutes or so. Usually, a small amount of ice crystals will disappear relatively quickly. After it is completely thawed, be sure to mix gently before preparing culture medium. Although HepG2 and 293T have relatively good tolerance, fixed thawing conditions can reduce batch-to-batch variation.