CEREAL STARCHES

• Physical structure of starch

  • Found in the form of granule in cereal grains
  • Granules are formed inside plastids
  • In wheat, rye, barley, and sorghum, there is a single starch granule inside a plastid
  • In rice and oats, there are several or compounds starch granules
  • Starch granules grow in layers
• Starch


  A polymer of a six-carbon sugar D-glucose
• Structure of D-glucose


  Can be shown in ring form or open chain
• Starch composition

  • D-glucopyranose polymers linked together by alpha 1,4 and alpha 1,6 glycosidic bonds
  • Amylose: smaller linear polymer, alpha 1,4 bonds
  • Amylopectin: larger branched polymer, branching at alpha 1,6 bonds
• Degree of polymerization (DP)

  • Amylose: 1,500 - 6,000
  • Amylopectin: 300,000 - 3, 000,000
• Amylose and amylopectin composition

  • Dent corn: 23-25% amylose, 75-77% amylopectin
  • Waxy corn: less than 1% amylose, more than 99% amylopectin
  • High-amylose corn: 55-70% amylose, 3-45% amylopectin
• Amylose molecules


  Form complexes with chemical compounds, e.g. iodine - amylose molecules give blue color
• Wheat starch is a major storage carbohydrate and contains about 60~75% grain and 70~80% flour
• Starch granules located in starchy endosperm cells are composed of two polymers called amylose and amylopectin
• Crystallinity of starch

  • Packing of amylose and amylopectin within starch granule is not random but very organized
  • A, B, and C X-ray patterns
  • Most cereal starches give the A pattern
• Birefringence of starch


  • Under polarized light, starch granules show birefringence in form of a "maltese cross"
• Differences between amylose and amylopectin

  • Amylose: Straight chain polymer made-up of D-glucose subunits, linked by α-1,4 glycosidic linkages, water-insoluble, does not exhibit gelling, gives blue color with iodine
  • Amylopectin: Branched-chain polymer made up of D-glucose subunits, connected by α-1,6 glycosidic linkages, water-soluble, exhibit gelling, does not give blue color with iodine
• Starch Gelatinization


  Cooking of starch in aqueous suspension such that irreversible granule swelling occurs resulting in a viscous solution
• Microscopy provides platform to identify undercooked and overcooked starch granules