I am a former barista who has encountered this phenomena many times, but could never consistently re-produce the effect.
1 Answer
Heavy cream should be lightly whipped, still in a somewhat pourable state. If it is over-whipped, it will have more of a tendency to sink.
The beverage should be at least slightly sweetened with white sugar or brown sugar. The dissolved sugar in the beverage will help to keep the cream afloat.
So, the problem could be over-whipped cream, no sugar, or both. Inconsistencies in either of those areas may be why your results varied.
Lastly, I have always been told that unwhipped or whipped heavy cream should be poured gently over the back of a warm spoon. That is supposed to ensure a gentle addition of the cream around the sides of the cup/glass that will make it float. (It's a useful technique for adding the cream.) I have seen many people do this, but I have always been successful without using a spoon.
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1Why would overwhipped cream sink? Surely it should be less dense with more air incorporated– Chris HAug 14, 2017 at 15:36
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@ChrisH I don't know the science behind it. This is what I was taught many years ago by several different bartenders when I was learning how to properly make Irish coffee. Researching online in more recent years, the information I've found agrees with this.– CindyAug 14, 2017 at 15:44