Starch

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Starch

• Megan Erickson
• Central Washington University

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What is it?

• Complex carbohydrate made up of two components
• Components
• Amylose
• Amylopectin
• Properties depend on amounts of the components

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Where is it found?

• Roots/Tubers
• Potato
• Arrowroot
• Tapioca
• Cereal
• Corn
• Waxy corn
• Wheat
• Rice
• Waxy rice

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Amylose

• Linear component of starch
• Contains 1,4-alpha-glucosidic bonds
• Molecular weight less than 0.5 million
• Can form coils which will trap iodine and turn
  blue

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Amylopectin

• Branched component of starch
• Contains 1,4-alpha-glucosidic as well as
  1,6-alpha-glucosidic bonds
• Molecular weight 50-500 million
• Limited coiling causes purplish-red color when
  iodine added

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Amylose vs. Amylopectin

• Starches usually contain more amylopectin than
  amylose
• Generally roots/tubers contain more amylopectin
  than cereals
• Roots/Tubers 80 amylopectin
• Cereals 75 amylopectin
• Waxy corn and rice contain virtually all
  amylopectin

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Starch Composition
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Starch Granule

• Made in the cytoplasm of plant cells
• Amylopectin forms in concentric circles with
  amyose dispersed in between
• Held together by hydrogen bonds
• The granule swells when heated in water

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Starch Granule
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Functions

• Gelatinization
• Structure in baked products
• Thickener in sauces, soups, and dressings
• Dextrinization
• Gelation
• Pie filling

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Gelatinization

• When starch is heated in water
• Hydrogen bonds break, allowing water to enter the
  granule and the granule swells
• Amylose migrates out of the granule
• H-bonding between water and amylopectin increases
• Reduced free water changes the viscosity of the
  starch mixture, thickening it

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Gelatinization and Temperature

• Gradually thicken with temperature
• Can be heated to 100oC without much granule
  rupture
• If held at 95oC will implode and lose viscosity

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Gelatinization and Type of Starch

• Best thickening ability potato starch
• Worst thickening ability wheat starch
• More amylopectinmore translucentmore stringy

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Viscosity and Type of Starch
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Gelatinization and Sugar

• Used together in pie fillings and puddings
• Sugar competes with the starch for water so less
  water available for gelatinization
• Delays gelatinization and decreases viscosity
• Increases gelatinization temperature
• The more sugar added, the longer the delay
• Disaccharides have a stronger effect than
  monosaccharides

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Gelatinization and Acid

• Used together in fruit pie fillings, specifically
  lemon fillings
• Acid breaks down starch molecules so the paste is
  thinner
• Decreases viscosity
• Acid effect can be minimized by adding after
  gelatinization or heating rapidly

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Gelation

• As a starch paste cools, a gel is formed
• Free amylose molecules lose energy as the
  temperature decreases and form hydrogen bonds
• The bonds create a network that holds the swelled
  granules in place

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Gelation and Starch Source

• The more amylopectin (less amylose), the softer
  the gel
• Potato starchhigh amylopectingood thickening
  agentsoft gel
• Corn starchless amylopectinless effective
  thickening agentstrong gel

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Gelation and Other Effects

• Heating
• Moderate temperature and rate of heating
• Enough amylose needs to be released from the
  granule without the granule bursting
• Agitation
• Agitation during cooling disrupts amylose network
• Should mix flavorings immediately after removing
  from heat

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Gelation and Other Effects

• Sugar
• Decreases gelatinization and amylose release
• Softer gel
• Acid
• Decreases gelatinization by hydrolysis of
  granules
• Softer gel

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Aging Gels

• Syneresis
• Loss of water from a gel
• Amylose molecules pull together, squeezing water
  out
• Retrogradation
• Realignment of amylose molecules
• Hydrogen bonds break and reform into more orderly
  crystals
• Can by reversed by gently heating
• Examples refrigerated pudding, stale bread

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Dextrinization

• When starch is heated without water
• A higher temperature is reached than with water
• Bonds break throughout the starch forming dextrins

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Genetically-modified Starches

• Waxy starch
• High in amylopectin
• Used in fruit pies because thickens well, but
  does not gel well
• Have good freeze-thaw stability
• High amylose starch
• Amylose creates strong bonds to form strong gels
• Used in edible films to coat food

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How to compare starches?

• Line spread test
• Measures thickening power
• Poor heated starch into cylinder, lift cylinder
  and measure spread after specified time using
  concentric circles
• Universal Texture Analyzer
• Measures gel strength
• Percent sag
• Measures gel strength
• Measure molded gel height and compare to unmolded
  gel height
• Stronger gelsmall sag, weaker gellarge sag

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Modified Starches

• Physically or chemically modifying native
  starches
• Are used for specific applications in the food
  industry, Why?
• Native starches have undesirable qualities
• Poor processing tolerance to heat, shear and acid
• Poor textures
• Do not store, hold, and freeze/thaw well

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Pre-gelatinized Starches

• Use
• Instant pudding
• Dehydrated gelatinized starch
• Heated so granule swells and then dehydrated
• Swells when water added, no heat necessary
• Decreases preparation time
• Physical change

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Thin-Boiling Starches

• Use
• Pass freely through pipes
• Acid-hydrolyzed starch
• Hydrolyzes 1,6-alpha-glucosidic bonds
• Amylopectin in smaller pieces
• Decreases thickening power, but makes a strong
  gel because hydrogen bonds form more readily

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Cross-linked Starch

• Use
• Increases storage time because of reduced
  retrogradation
• More stable at high temperature, with agitation,
  and with acid addition
• Salad dressings, baby foods, pie fillings
• Cross-linked starch molecules
• Alter hydroxyl ends under alkaline conditions by
  acetic anhydride, succinic anhydride, or ethylene
  oxide

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Viscosity and Cross-linked Starches
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Resistant Starch

• Small intestine is unable to digest, limited
  digestion in large intestine
• Classifications
• RS1 trapped in cells (seeds/legumes)
• RS2 native starch (raw potatoes, bananas, waxy
  maize)
• RS3 crystalline, non-granular starch (cooked
  potatoes)
• RS4 chemically modified
• Can contribute fiber to food without the fat that
  bran has
• Takes up less water than other fiber, making
  dough less sticky
• Smooth even texture
• Less than 3 cal/g