In-Depth Notes on Starch Cookery

Learning Outcomes

• After studying this topic, you should be able to:

  • Explain the basic physical and chemical principles affecting the behavior of starch-based food.

  • Describe the different starch sources.

The Chemistry of Starch

• Starch is composed of polymers of glucose.

• Glucose units are linked together by glycosidic linkages.

• Starch is hydrolyzed by amylase enzymes.

• It is a primary source of carbohydrates in the human diet, synthesized by plants through photosynthesis.

Structure of Starch

• Polymers of Glucose:

  • Formed through glycosidic linkages.

• Hydrolyzed by Amylase:

  • Enzymatic breakdown occurs, making starch digestible.

• Starch synthesis occurs as follows:

  • Photosynthesis Reaction:

  • 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + O₂ + H₂O

Starch Granule

• Storage in Plants:

  • Starch molecules stored in organized units called granules.

• Birefringence:

  • Starch granules exhibit birefringence (Maltese cross) under polarized light due to their organized structures.

  • All starch granules are microscopic, varying in size and shape depending on their source.

Starch Fractions

• Starch consists of two main fractions:

  • Amylose:

  • Straight-chain polymer.

  • Responsible for gelling properties in cooked and cooled starch mixtures.

  • Amylopectin:

  • Branched-chain polymer.

  • Responsible for thickening properties due to its complex structure.

Amylose vs Amylopectin

• Amylose:

  • Contains 250-350 glucose molecules.

  • Linear structure (alpha 1,4 linkages).

  • Molecular weight: 40,000-300,000.

  • Very soluble in water.

• Amylopectin:

  • Contains 500-2000 glucose molecules.

  • Branched structure (alpha 1,4 & 1,6 linkages).

  • Molecular weight: 4-6 million.

  • Less soluble in water.

Starch Sources and Behavior

• Relative proportions of amylose and amylopectin affect starch's behavior in food products.

• Common starch sources:

  • Waxy Rice (Malagkit): 0% amylose, 100% amylopectin

  • Ordinary Rice: 16% amylose, 84% amylopectin

  • Wheat: 22% amylose, 78% amylopectin

• Waxy varieties are non-gelling due to the absence of amylose, making them suitable for freezing and thawing applications.

Testing for Starch

• Iodine Test:

  • Starch presence is indicated by a blue-black color resulting from the formation of a complex between amylose and iodine.

  • Purplish-red indicates amylopectin presence.

• Color Reaction:

  • Dark blue indicates amylose; red suggests very little to no amylose (e.g., waxy starch).

Starch Functions in Food Products

• Culinary Uses:

  • Thickening Agent: thickens soups, sauces, etc.

  • Gelling Agent: forms gels in puddings and jellies.

  • Protective Coating: forms edible films in products like chewing gums.

  • Other uses include binding, filling, coating, and coloring (dextrinization).

Key Points

• Starch is a carbohydrate made of amylose and amylopectin fractions.

• The proportions of these fractions determine starch behavior in food.

• Starch can be obtained from natural plants or genetically modified sources, or tailored for specific applications.