Boiling quinces before making jam is not always necessary
Whether to blanch the quinces depends on their firmness and the desired texture, and with the correct measurements you can make a creamy jam without doing so.
Some recipes cook the quinces directly with sugar, while others briefly boil them to soften them. Blanching them first makes cooking easier, especially when the quinces are firm and green.
Boiling them beforehand is not mandatory. Once chopped, they can be cooked in a pot with the sugar and the appropriate amount of water. Over low heat, they soften, absorb the syrup, and retain their shape better; this method is especially practical with ripe, not-too-hard quinces.
If the quinces are very hard, it's best to blanch them briefly. Cut them into pieces and submerge them in boiling water for a few minutes, without letting them soften completely; then drain them and cook them with sugar and water. Blanching them too long will cause them to fall apart during cooking.
Direct cooking enhances the color and consistency: the quince cooked with the sugar releases its juices into the syrup, and its aroma infuses better. At low or medium heat, the pieces don't fall apart, and the initial foam can be skimmed off with a spoon if necessary.
If you blanch the quince, don't wait until it's completely soft. A brief blanching, thorough draining, and immediate transfer to the pot yield better results. Some of the blanching water can also be used in the jam to preserve the quince's aroma.