Can fetal bovine serum be stored longer at -20 or -80? Does repeated freezing and thawing after packaging have a significant impact?

For long-term storage, I recommend following the instructions first. Gibco South American fetal bovine serum is conventionally stored at -20°C protected from light, and generally there is no problem storing it until the indicated expiration date. In our lab, storing it at -20°C until the indicated expiration date has shown no obvious problems, but the prerequisite is not to repeatedly freeze and thaw the entire bottle. -80°C is not necessary; if you put it in -80°C, you actually need to pay more attention to ice expansion and precipitate formation during thawing. Since your lot number has just arrived and you have a -20°C freezer, there is no need to change equipment for this. Aliquotting into 50 ml centrifuge tubes is standard practice; be careful to leave 15%-20% headspace in each tube, and do not use water that is too hot when thawing.

For aliquoting, I recommend preparing amounts for 1–2 weeks of use. Do not use only the 50 mL large tube; if you have 5–10 mL sterile cryovials, that would be better, to avoid being unable to finish the thawed serum and yet reluctant to discard it. Two freeze-thaw cycles do not necessarily mean it is immediately unusable, but this is not recommended as routine practice, especially for relatively fragile adherent cells such as primary cells, MSCs, and some tumor lines, which may show slow recovery and decreased clonogenic capacity. It is recommended to set up a parallel control: take one serum aliquot that has been freeze-thawed only once and fresh serum that has not been freeze-thawed for a small-scale comparison. Since it has already arrived and the lead time is tight, as long as the lot information is kept properly and it is stored under standard -20 conditions, running a side-branch test first is the more prudent approach.

In actual use, storage at -80°C may cause slight flocculent or flaky precipitates, especially when thawing is slow. However, most of these precipitates are lipoprotein/fibrin-type and can be largely removed by low-speed centrifugation; they do not necessarily indicate poor quality. Storage at -20°C will also produce a small amount of precipitate. The key is the thawing method: place at 4°C one day in advance, or thaw in a room-temperature water bath with shaking; do not subject to repeated freeze-thaw cycles. After aliquoting, if each tube is thawed only once, the effect on cell attachment is generally minimal; if the same tube is refrozen and thawed two more times, proliferation or attachment may be affected, especially for primary cells and stem cells, which are sensitive.