Asked July 01, 2016, 11:14 AM EDT
District of Columbia County District of Columbia
When you say you "zapped" the cream, I take that to mean that you heated it in the microwave. Then you say, "and it seemed rather warm." In doing this you may have heated some or all of the cream too hot. Heating in the microwave does not produce an even heat so there may have been hot spots in the cream. That extra heat can have a number of effects that could adversely affect the outcome of your process:
1. The proteins in the cream may have been denatured. This may have decreased their ability to set together rather than separate into curds and whey.
2. Some of the bacteria and enzymes that are needed for positive fermentation may have been destroyed at the higher temperature.
In general, overheating can produce off-flavors.
The National Center for Home Food Preservation at University of Georgia Cooperative Extension has a good fact sheet for making yogurt at home at http://nchfp.uga.edu/publications/nchfp/factsheets/yogurt.html. It also describes some trouble-shooting options.
These resources all refer to yogurt, not crème fraiche, but the processes are very similar.
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