Can a small amount of white flocculent sediment still be used after the melting of fetal bovine serum?

If a new batch shows precipitate, take photos first, record the lot number, and contact the supplier to ask whether other users of the same lot have also observed this. Normally, lipoprotein precipitation can usually be redissolved by placing it in a water bath and shaking gently; if the precipitate does not decrease after shaking and the supernatant becomes cloudy, there is a high probability of deterioration. For storage, do not place it against the wall at -20°C, to avoid local concentration and temperature differences. At the time of purchase, you can ask the supplier to provide appearance photos or videos and specify that "returns or exchanges are allowed if precipitate exceeds the standard." It is best to open and inspect within 7 days of arrival; once overdue, disputes become very disadvantageous.

To expand tumor cells and MSCs, we usually conduct a 72 hour small culture test: inoculate a 6-well plate to observe for abnormal background particles, vacuolization, and slow proliferation. Continuing to use a small amount of sediment is not a big problem, but repeated freeze-thaw cycles will worsen it, so it is recommended to pack the goods into a single dose upon arrival. It is best to perform aseptic testing: resuspend the sediment and transfer it into a blood plate or liquid culture medium. If there are no problems, only the sediment can be centrifuged to obtain the supernatant; If the background particles increase significantly, discard the entire bottle and do not take any risks.

It is not recommended to remove all precipitates directly by filtration, as filtration will result in the loss of some proteins and cytokines. I prefer centrifugation at 400-500g for 5-10 minutes and then collecting the supernatant. If there are still concerns, follow with a 0.22μm filtration. For MSCs that adhere slowly, it may be more appropriate to seed a batch directly after centrifugation without filtration to observe their condition. 'Black gum bugs' are generally not caused by serum precipitates themselves, but are more often related to mycoplasma or exogenous particle contamination. It is recommended to resuspend a small amount of the precipitate and inoculate it into bacterial/fungal culture media, while also performing PCR for mycoplasma.

A small amount of white flocculent substance appears after overnight melting at 4 ℃, mostly consisting of lipoprotein or fibrin precipitation, especially at the bottom of the bottle. Pollution and deterioration often result in turbidity, grayish yellow color, significantly acidic pH, and peculiar odor. You can first centrifuge at low speed to obtain the supernatant. I have encountered a small amount of precipitation of MSCs and tumor cells, which has little effect on the state, but a batch of low-density cells need to be made to observe the adhesion and background particles. If the precipitation does not decrease and the supernatant becomes turbid after a 37 ℃ water bath, the probability of deterioration is high, and it is not recommended to continue using it.