Sometimes I don't have time to cool the mixture before adding to my pastry but I can't see a major difference in outcome – the pastry does seem to be a bit more soggy when the mixture is not cooled first.
I'd love to know the specific reasoning behind it.
Best Answer
You may already know that pastry making requires everything to be cold - the fat, the water, your hands, the board, everything. This is to prevent the fat from melting into the flour prematurely, which results in cardboard-like pastry. When the fat remains cool until cooking, it melts into layers in the pastry as it cooks, making it nice and flaky.
If you put hot filling into your pastry case, you will melt some of the fat prematurely and so the pastry won't be as good as would be if you let it cool.