Can I make lemon curd more sour/tart after it's cooked?
I have used both citric acid (food grade, sold for canning and jam making, not the descaler) and ascorbic acid (vitamin C) in similar cases when only the acidity of a dish was insufficient. Sometimes lemons are just not sour enough.
The powder comes in very fine texture and can even made finer with a mortar and pestle. A very small amount will go a long way and that’s why it won’t affect the ratios of a dish and a can be stirred into even thick preparations like lemon curd. You can gently warm up your curd so that the butter in it softens, if necessary.