Add lemon to sugar syrup at the end, just before removing it from the heat. Its purpose is to prevent the sugar from recrystallizing and to balance the sweetness; it does not need a long boil. Adding lemon too early can break down too much of the sugar, thin the syrup, and leave a sharp note. The amount should be measured: a few drops to one 5 ml measure is enough for most recipes.
Lemon’s role in sugar syrup is not to add flavor. Its acid breaks down some sucrose molecules into glucose and fructose. Because these different molecules cannot form a regular crystal structure together, the syrup does not become grainy as it cools. This breakdown happens quickly, and a few minutes in hot syrup is enough. Lemon does not need to boil with the syrup for a long time; it does its job in a short time. The same acid is also used in jams and confections for exactly this purpose.
Adding lemon too early causes 2 problems. First, the breakdown proceeds further than needed; the syrup stays thinner than expected and runs off the dessert instead of coating it. Second, the flavor of the lemon juice changes during a long boil and leaves a sharp, slightly bitter note in the syrup. That is why lemon is added just before the heat is turned off; boil the mixture for another 1–2 minutes, then remove it from the heat. If the syrup has become too thin, it can be reduced again, but this time without adding any lemon. Once the syrup reaches the right consistency, lemon should not be used to adjust it.
The amount matters too. Too little lemon will not prevent crystallization; too much will make the syrup sour and throw off the dessert’s balance. For most home recipes, a few drops to one 5 ml measure is enough. Always strain the lemon juice before adding it, because seeds and pulp can leave cloudiness and bitterness in the syrup. Freshly squeezed lemon gives a cleaner result than juice that has been left open for a long time. Bottled lemon juice also works; since its acidity can vary more than that of fresh lemon, start with a small amount.
The peel is a separate matter and is not added at the same time. It is used for flavor and aroma, not acidity, so add it to the syrup at the start of the boil and remove it by straining. Use only the yellow part of the peel. The white layer underneath gives a noticeable bitterness during a long boil, and that bitterness becomes more pronounced after cooling, so peel it thinly. If grating the peel, move the grater only over the surface and do not reach the white layer.
If you do not have a lemon, a few alternatives can be used. White vinegar does the same job but leaves its own smell. Citric acid crystals are stronger, so only a very small pinch is needed. Orange juice is a weaker acid and does not reliably prevent crystallization. Whichever one you use, the timing stays the same: add the acid at the very end, and do not stir the syrup after taking it off the heat. Stirring can bring sugar crystals from the bottom to the surface and start crystallization as the syrup cools.