Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2002) ww part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 910 STARCH Green fruits especially bananas often contain much starch but the ripening process changes the starch to sugars. In general starch is a white amorphous powder having a specific gravity from to . It is insoluble in cold water but can be converted to soluble starch by treating with a dilute acid. When cooked in water starch produces an adhesive paste. Starch is easily distinguished from dextrins as it gives a blue color with iodine while dextrins give violet and red. The starch molecule is often described as a chain of glucose units with the adhesive waxy starches as those with coiled chains. But starch is a complex member of the great group of natural plant compounds consisting of starches sugars and cellulose and originally named carbohydrates because the molecular formula could be written as Cn H2O x but not all now-known carbohydrates can be classified in this form and many now-known acids and aldehydes can be indicated by this formula. Starch can be fractionated into two polymers of high molecular weight. Amylose is a straight-chain fraction having high adhesive properties for coatings and sizings and amylopectin is a branched-chain fraction best known as a suspending agent for foodstuffs. Amylose is chemically identical with cellulose but the chain units of the molecule have an alpha linkage and are coiled while the cellulose molecule is rigid. It has a molecular weight of 150 000 while amylopectin has a molecular weight above 1 million. The 1-4 alpha linkage of amylopectin with random branches at the 6-carbon position makes the material easily dispersible in cold water but resistant to gelling. Amylopectin is thus best suited for thickening but because it can be combined and cross-linked with synthetic resins and is highly resistant to deterioration it is used with resins for water-resistant coatings for paper and textiles. Tapioca is the starch from the root of the large tuber Manihot utilissima now grown in most tropical countries. It .
