Inhabitants of the North Central Province, which is known as the Cultural Triangle to tourists, consume mostly Nadu Haal variants. An estimated 70% of the paddy land of the North Central and Eastern province, the classical rice production areas of Sri Lanka, grow Nadu rice (which in the harvesting stage is still the same as Kekulu Rice, see below).
Nadu rice is preboiled. In other words: It’s a dried boiled rice. Preboiling is a traditional method of boiling the grain within the husk before drying it in the sun and afterwards milling it. The process has been common in Sri Lanka already in historical times. Parboiling, for comparison, is a modern method of preboiling. Parboiling is now an industrialized procedure using steam-pressure and dry-heat. The Sinhala term for ‘parboiled’ is ‘thambapu’. The parboiling process reduces the loss of nutrients during the subsequent milling process, as the hydrothermal treatment of pre-cooking the rice within the husk prior to the polishing conduces to moving micro nutrients such as water soluble vitamins from the bran into the starch of the grain, which absorbs the water and gets gelatinized. In the subsequent drying stage the parboiled rice undergoes partial retrogradation resulting in harder paddy grains. Parboiled rice has a Glycaemic Index (GI) even lower than some varieties of Basmati.
Though preboiled, Nadu Haal has a high digestible carbohydrate content of more than 80%, dry basis. However, the GI value (glycemic index), which is relevant in preventing diabetes, is lower than that of non-preboiled rice. In general, the GI of parboiled rice is much lower than that of non-parboiled rice. For examble, the GI value of Basmati is about 73, whereas that of parboiled Nadu Rice is about 40.