What it is
Crème fraîche is heavy cream cultured with mesophilic lactic-acid bacteria until it thickens and turns gently sour. Unlike yogurt, it sets at room temperature rather than blood heat, and unlike American sour cream it is typically higher in fat (~30%+) and more stable when heated in sauces. A spoon of finished crème fraîche or cultured buttermilk is enough to seed the next batch.
The science
The classic cultures are mesophilic lactococci — especially Lactococcus lactis subsp. lactis and subsp. cremoris, often with aroma strains such as L. lactis subsp. lactis biovar diacetylactis and Leuconostoc species that produce diacetyl, the buttery note of cultured cream (Smid & Kleerebezem, 2014). As they ferment lactose to lactic acid, the pH falls and cream proteins thicken into a spoonable gel without rennet or heat-setting.
Ultra-pasteurised cream sometimes cultures poorly because the severe heat treatment damages the proteins that help the set; ordinary pasteurised cream is the more reliable base.
Safety
Start from a live commercial culture (crème fraîche, buttermilk, or a freeze-dried mesophilic packet). Use clean jars and discard if the cream smells putrid, yeasty-bitter, or shows mould. Once thick and tangy, refrigerate promptly — cold slows the bacteria and keeps the texture stable.
Signs it worked / troubleshooting
- ✅ Good: thickens in 12–24 h, clean tang, faintly buttery aroma.
- ⚠️ Still thin = cream ultra-pasteurised, room too cool, or weak starter → use pasteurised cream, hold nearer 22 °C, seed with fresher culture.
- 🚫 Yeasty, bitter, or mouldy = contamination → discard and re-start.
How to store
Refrigerate; it keeps 1–2 weeks. Re-culture from your own jar for a few generations, then refresh from a commercial starter when flavour or set drifts.
References
- Smid EJ, Kleerebezem M (2014). Production of aroma compounds in lactic fermentations. Annual Review of Food Science and Technology 5:313–326. doi:10.1146/annurev-food-030713-092524
- Wouters JTM, Ayad EHE, Hugenholtz J, Smit G (2002). Microbes from raw milk for fermented dairy products. International Dairy Journal 12(2–3):91–109. doi:10.1016/S0958-6946(01)00151-0
- FAO/WHO Codex Alimentarius (2003, rev. 2018). Standard for Fermented Milks (CXS 243-2003) — cultured cream products sit in the broader fermented-milk family.