Turkish delight is made by combining two mixtures: sugar syrup cooked to a certain temperature and a starch slurry cooked in water. After the two mixtures are combined, the mixture cooks over low heat for hours with constant stirring. This long cooking fully gelatinizes the starch and gives Turkish delight its elastic texture. Citric acid prevents the syrup from crystallizing again.
The first half is the syrup. Boil sugar and water in a saucepan and cook it to the hard-ball stage, about 121–130 °C / 250–265°F. At this point, much of the water in the syrup has evaporated. Monitor the temperature with a thermometer because this stage cannot be identified reliably by sight; the syrup is still clear and colorless. Once it reaches the target temperature, remove the pan from the heat and prepare the second mixture. Use a heavy-bottomed, deep-sided pan because the mixture rises as it cooks.
The second half is the starch. Dissolve cornstarch or wheat starch in cold water until smooth, then cook it in a separate pan while stirring; it will turn into a thick, sticky, translucent slurry. Bring the slurry to a boil, because undercooked starch can leave Turkish delight with a raw-flour taste and a gritty texture. To combine the two mixtures, slowly pour the syrup over the slurry while stirring constantly. Use cold, not lukewarm, water when making the slurry; starch begins to clump as soon as it is added to lukewarm water.
The main work begins after the mixtures are combined. Cook the mixture over low heat for hours, stirring continuously the whole time. During the long cooking, the starch chains fully open, the syrup settles between them, and the mixture turns translucent and golden. Turkish delight that is undercooked remains inconsistent and doughy. A good sign that it is ready is when it clings to the spoon and flows off slowly. Add flavorings, nuts, and food coloring near the end, once the mixture has reached the right consistency.
Citric acid or lemon juice is the hidden ingredient in this recipe. The acid breaks down some of the sugar and prevents the mixture from crystallizing again as it cools. Without it, Turkish delight can become grainy with sugar crystals within a few days. Use only a small amount: too much acid adds a sour note and makes the mixture too soft. The acid is usually added midway through cooking. The mixture then needs to cook even longer, so adding the acid early extends the total cooking time.
The final step is resting. Pour the hot mixture into a tray dusted with starch on the bottom and sides, smooth the surface, and leave it at room temperature overnight. The texture sets during this rest. Do not chill it in the refrigerator, because the cold surface hardens and becomes damp. Cut it with a knife coated in starch, then toss the pieces in a mixture of powdered sugar and starch. Sift the coated pieces to remove excess powdered sugar; otherwise, the Turkish delight will become damp in a short time.