Lecture Notes: Carbohydrate Series - Gelatinization and Retrogradation

Introduction

• Continuation of the carbohydrate series.
• Focus on gelatinization and retrogradation.
• Background topics on carbohydrates covered in previous videos.

Starch Structure

• Major component of food plants: wheat, barley, rice, corn, potato, sweet potato, cassava.
• Source of carbohydrates and energy in diet.
• Consists of:
  • Amylose (15-25%): Linear alpha glucose units, 1-4 glycosidic bonds.
  • Amylopectin (75-85%): Branched structure, 1-6 glycosidic bonds.
• Starch granule structure: Alternation of amorphous (less crystalline) and crystalline regions.
• Water absorption occurs in amorphous regions.

Gelatinization

• Definition: Breakdown of intermolecular bonds between starch molecules (amylose and amylopectin) allowing hydrogen bonds to hold more water.
• Process:
  1. Starch Granule Swelling: Water absorbed in amorphous space, granule swells.
  2. Melting: Increased heat leads to bond breakage, especially O6-O2 bonds.
  3. Amylose Leaching: Amylose leaches out as granules dissolve and become plasticized.
• Factors affecting gelatinization:
  • Plant source, water content, pH, salt concentration, and presence of sugar, protein, or fat.
  • Example: Pasta sauce with sugar can slow down gelatinization.
• Visual Process:
  • Ambient temperature: Starch granules do not dissolve.
  • Heating: Leads to swelling and disintegration above 60°C, amylose leaches out, thickening the mixture.

Retrogradation

• Definition: Chemical reaction where amylose and amylopectin realign upon cooling.
• Process:
  • After gelatinization, cooling leads to realignment of amylose and amylopectin into a crystalline structure.
  • Forms hydrogen bonds, expels water (syneresis), related to staling of bread.
• Factors:
  • Retrogradation leads to less digestible starch.
  • Modified by additives like fat, glucose, sodium nitrate.
• Examples:
  • Bread and rice retrogradation upon cooling.

Differences Between Gelatinization and Retrogradation

• Gelatinization:
  • Process: Heat and water lead to breakdown of starch bonds, less crystalline structure.
  • Outcome: Water absorbed, starch becomes viscous.
• Retrogradation:
  • Process: Cooling leads to realignment, forming more crystalline structure.
  • Outcome: Water expelled, structure solidifies.

Conclusion

• Gelatinization involves the breakdown and water absorption.
• Retrogradation involves reformation and water expulsion.
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