Chapter 7. Physico-chemical properties of konjac glucomannan
Résumé
The konjac flour is obtained from the tuber of the Amorphophallus konjac, a plant of the Araceae family. This plant is found in the wild and cultivated as a vegetable in Thailand, China, Vietnam, Korea, and Japan since the nineteenth century AD (Chua et al. 2012; Li et al. 2005). The major compound of konjac flours is glucomannan, the reserve polysaccharide of konjac. The glucomannans are also present under the same structural type in various plant species: orchid bulbs, lily, and Aloe vera seeds (Dorthe 2005). The konjac flour is obtained by crunching the tubers more or less finely. These are harvested after 2 or 3 years of plant development. The tubers are cut, dried, crushed, and sieved. The resulting flour contains between 51% and 72% dry weight of the konjac glucomannan (Fang and Wu 2004). In order to obtain pure flour containing more than 95% of the konjac glucomannan in dry mass, a purification step by hydroalcoholic washes is essential. Depending on the species, the dried crude konjac flour contains about 10%–30% starch, 2%–5% fibre, 5%–14% protein, 3%–5% reducing sugars, and 3.4%–5.3% ash, it is low in vitamins and fat. The konjac flour production accounts for 25,000 tonnes per year, with China and Japan as leading producers and consumers of the konjac flour. China has become the largest producer of the konjac ahead of Japan, exporting more than half of its production. Japan retains its konjac production for domestic consumption and has specialised in the production of pharmaceutical grade flour for export, which is more limited in volume.