Starch is a plant-based carbohydrate with a chain structure, composed of many glucose units. The main components of starch are amylose and amylopectin. Amylose has an unbranched structure made of glucose residues linked by oxygen atoms via α-1,4-glycosidic bonds. Amylose does not dissolve in cold water, but it dissolves in hot water, probably with partial degradation. Amylopectin has a branched structure with additional α-1,6-glycosidic bonds, the amount of which is estimated at approx. 4%. Amylopectin dissolves in cold water (which, however, requires prior achievement of strong starch dispersion, e.g. by treatment with dimethyl sulphoxide).
However, it should be noted that such treatment must be considered a chemical modification of starch. It may be interesting that waxy maize starch, despite containing amylopectin only, is not soluble in water. Starch in its pure form is a white, tasteless and odourless substance. When devoid of proteins, germs and seed coats, it is referred to as native starch. Native starch is insoluble in cold water and lacks binding properties. Only when water is heated does it begin to exhibit binding capabilities; the granules swell and, through water absorption, lose their specific structure. This results in a colloidal solution characterised by high viscosity. Reaching the maximum viscosity level is defined as the gelatinisation point.
Starch added to the mass improves inter-fibre bonding, reduces opacity and reduces the pick resistance of the paper surface layer.