Starch and Starch Products - In Depth Notes

What is Starch?

  • Definition: Starch, or amylum, is a complex carbohydrate consisting of a large number of glucose units joined by glycoside bonds.
  • Origin: The term "starch" originates from the Middle Eastern word "sterchen," meaning "to stiffen". The Latin term for starch is "amylum".
  • Significance:
    • Second Most Abundant Substance: Starch is the second most abundant organic substance on Earth.
    • Photosynthesis Product: It is an end product of photosynthesis.

Importance of Starch

  • Function in Plants: Serves as a storage form of energy.
  • Human Consumption: Most consumed dietary carbohydrate.
  • Food Industry: Used to achieve desired viscosity.
  • Non-Food Applications: Utilized for binding and stiffening in various products.

Food Sources of Starch

  • Starch is typically found in plants. Examples include:
    • Corn starch
    • Potato starch
    • Rice starch
    • Wheat starch
    • Cassava (Tapioca) starch
    • Arrowroot starch

Classification of Starch

  • Natural Starch: Extracted in its original form without alterations.
  • Modified Starch: Chemically or physically altered to improve properties (e.g., thickening, heat resistance).

Structure of Starches

  • Amylose:

    • Composed of glucose units linked from C1 to C4 in a linear chain.
    • Oxygen is consistently on one side.

  • Amylopectin:

    • Glucose units linked from C1 to C4 and C1 to C6, resulting in a highly branched and dense molecule.
    • Can contain up to 2 million glucose units, with 5% forming branches.

Microscopy and Granule Structure

  • Starch granules of amylose and amylopectin are arranged radially, containing both crystalline and non-crystalline regions.
    • Structure can be likened to onion layers.

Shapes of Starch Granules

  • Variations in shapes based on source:
    • Cassava Starch: Irregular shapes with concave granules.
    • Wheat Starch: Spherical and lenticular granules.
    • Rice Starch: Polygonal granules.
    • Corn Starch: Round and larger polygonal granules.
    • Potato Starch: Smooth ovoid granules.

Size of Starch Granules

  • Smallest: Rice granules (3-10 micrometers)
  • Largest: Potato granules (up to 100 micrometers)
  • Root crop starches are typically larger than grain starches.

Amylose vs. Amylopectin

  • Amylose: Typically consists of about 17,000 glucose units in a linear form.
  • Amylopectin: Major component in starches, comprising approximately 70-85% in non-waxy starches and 100% in waxy varieties.
  • Properties such as gelatinization and pasting are influenced by the content of amylose and amylopectin.

Gelatinization and Pasting

  • Occurs when starch is heated to critical temperature, resulting in irreversible changes:

    • Process:
    1. Starch granules swell in water but remain unsolved.
    2. Critical temperature causes the starch to absorb water, swell, and exert pressure.
    3. Amylose leaches out, increasing viscosity and forming a paste.

  • Complete Gelatinization is characterized by:

    • Loss of birefringence
    • Increase in viscosity and clarity.

Starch Viscosity

  • Defined as resistance to flow under shear forces; important in many cooking applications.
  • Types of Starches:
    • Root and waxy starches show single-stage viscosity.
    • Non-waxy grain starches exhibit two-stage viscosity: swelling followed by granule collapse.

Factors Affecting Viscosity and Gel Strength

  1. Stress (Stirring): Over-stirring can break apart gelatinized starch.
  2. Starch Type and Amount: More amylopectin increases viscosity; higher amylose makes firmer gels.
  3. Heating Rate: Fast heating can result in thicker pastes.
  4. Cooling Conditions: Fast cooling may prevent adequate structuring while slow cooling can lead to weeping.
  5. Dextrinization and Ingredients: Enzymes, acids, and fats affect viscosity and gel strength depending on their interactions with starch.

Retrogradation

  • Defined as reassociation of amylose chains by hydrogen bonding, leading to increased gel rigidity as it cools.
  • Staling: Loss of freshness in baked goods largely due to starch retrogradation.

Syneresis

  • Characterized by moisture loss from a gel, resulting in liquid expulsion known as "weeping".

Dextrinization

  • Partial hydrolysis of starch through dry heat, leading to dextrins.
    • Dextrins are sweet-tasting, water-soluble, with lower viscosity than starch gels.

Hydrolysis

  • Involves breaking glycosidic links between glucose units catalyzed by acids or enzymes like amylase, occurring during cooking or food storage.

Modified Starches

  • Altered physically/chemically to change properties for diverse applications (food, pharmaceuticals, textiles).
  • Examples:
    • Carboxymethyl Starch: More hydrophilic, useful in medications.
    • Starches Acetate: Used in films and biodegradable packing.

Functional Properties of Starches

  • Starch's roles in food include:
    • Thicker for gravies and sauces
    • Moisture retention in baked goods
    • Structuring in bread and pastries

Nutritional Importance of Starches

  • Major energy resource: provides 4 kcal/gram.
  • Integral part of a balanced diet with recommended servings from various food groups.

Resistant Starch (RS)

  • Refers to starch that is not digested in the small intestine, categorized as:
    • RS1: Physically inaccessible starch.
    • RS2: Raw starch.
    • RS3: Retrograded starch (e.g., leftover rice).
    • RS4: Chemically modified starch inclusions.

Starch Cookery

  • Functions:
    • Thickening sauces and soups
    • Gelling puddings and kakanin
    • Emulsifying and stabilizing products
  • Common Problems:
    • Lumping, thinning, weak gels, skin formation, scorching, raw flavor can all occur during cooking, with various methods of prevention related to temperature control and processing techniques.

Market Availability of Starches

  • Native starches like corn and rice flour available in stores, while modified starches primarily serve industrial users.