How to make Mascarpone cheese

The most typical cause of failure when making mascarpone is using ultra-pasteurized cream. When making mascarpone cheese (which I do often), I do exactly what you've described there. When I'm careful to make sure that the cream is not ultra-pasteurized, I have no problem. It does take quite a long time, however, for it to strain.

A couple other points here: I never let the cream boil - 180 is ideal. I heat to 180 degrees Fahrenheit, hold it at a simmer at 180 for three minutes, add the lemon juice and then cook again for 3 minutes, stirring gently, all at 180 degrees. Then I let it sit for about 45 minutes off the heat, and only then do I start the straining process.

How much lemon juice are using (to how much cream)? ;o)