Some kebab restaurants have special equipment that keep moving the ayran, so it comes with a nice foam layer. Store-bought ayran usually has added milk protein, which also may help foam. Unfortunately, it's difficult to get so much foam with homemade ayran, but using a blender, battery operated milk frother or shaking it in a closed jar helps. Some also say that switching some of the water with sparkling water makes the ayran foamier, but I haven't tried this yet.