Polysaccharides 11

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Polysaccharides 11
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Pectin

• Pectic substances
• Middle lamellae of plant cell walls
• Functions to move H2O and cement materials for
  the cellulose network
• Get PECTIN when you heat pectic substances
  (citrus peel apple pomace) in acid
• Not a very well defined material
• Pectins from different sources may differ in
  chemical and functional details

• 85 galacturonic acid
• Some are esterified with methyl alcohol
• DE degree of esterification
• 10-15 galactopyranose, arabinofuranose rhamnose

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Pectin

• Most pectins have a DE of 50-80
• Young unripened plants/fruits have very high DE ?
  hard texture
• Old ripened plants/fruits have lower DE ? softer
  texture
• Food use
• A. Thickener - some use, but less common than
  gums
• B. Pectin gels - jelly and jams

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Pectin

• Pectin gels (Jelly)
• 1. Regular sugar/acid gel
• Pectin 0.2 - 1.5
• Low pH from 2.8 - 3.2 (suppresses ionization) -
  get less repulsion
• Sugar (65 -70) - causes a dehydration of pectin
  by competing for water through H-bonding
• Get gel by charge, hydration effect

Undissociated at low pH ? No repulsion
RAPID SET - 70 ESTERIFIED SLOW SET - 50 - 70
ESTERIFIED
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Pectin

• Pectin gels (Jelly)
• 2. Low methoxyl pectin gel
• lt 50 esterified
• Get gel due to Ca2 ion bridging
• Avoid need for sucrose (diet foods)
• Get gels over wide pH range
• Gels tend to be more brittle less elastic than
  sugar/acid gels

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Pectin

• Pectin and quality problems
• Example Cloud in citrus juices
• Normal juice - colloidal pectin - cloud
• Pectin esterase - demethoxylates pectin get loss
  of cloud - precipitation - due to H-bonding of
  COOH and Ca2 bridging
• Must heat juice to inactivate enzyme - causes
  dramatic flavor changes

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Cellulose

• Most abundant organic compound on the planet
• Plant cell wall component
• Gives tensile strength to cell wall
• Very high molecular weight insoluble polymer of
  glucose
• ?-1-4 glycosidic bonds
• These bonds give cellulose a very rigid straight
  parallel chain that has extensive H-bonds

v.s.
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Cellulose
Hydrogen Bond
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Cellulose

• Properties
• Crystalline regions have very tight H-bonding
• Insoluble in water
• No effect on viscosity (why?)
• Little access to hydrolytic reagents and enzymes
• Very tough texture
• Not digestible by humans
• ?-1-4 glycosidic bonds
• Pass through digestive system
• Contributes no calories
• Dietary fiber
• Possibly lower cholesterol
• Improve bowel movements

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Cellulose

• Uses in foods
• Unmodified cellulose is made from wood pulp or
  cotton (dry powder) ? very cheap
• Minimal effect on viscosity
• Added as "fiber" (breads and cereals)
• Non-caloric bulk (no flavor, color etc)
• Very little effect in foods
• Can improve function slightly by heating
• Small number of H-bonds break
• Slight swelling, softening
• Only slightly soluble in water
• No change in digestibility

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Cellulose

• Cellulose can be modified to dramatically improve
  its function and use
• Microcrystalline cellulose (MCC)
• Prepared by partial acid hydrolysis
• Non-crystalline regions are penetrated by acid
  and cleaved to release the crystalline regions
• Crystalline regions combine to form microcrystals
  
• Still insoluble (all crystalline)
• Limited food uses
• Stabilizes emulsions
• Absorbs oils syrups
• Dry mixes - keeping them free-flowing

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Cellulose

• Two main products of MCC
• Powdered MCC
• Spray dried MCC
• Forms aggregated porous/sponge-like microcrystals
• Uses
• Flavor carrier
• Anticaking agent in powders and cheese

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Cellulose

• Colloidal MCC
• Mechanical energy applied after hydrolysis to rip
  microcrystals apart to form small
  micro-aggregates
• Water dispersible similar function as food gums
• Food uses
• Foam and emulsion stabilizer
• Pectin and starch stabilizer
• Fat and oil replacement

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Cellulose

• B) Methyl cellulose
• Cellulose treated with alkali to swell fibers and
  then methyl chloride is introduced
• Get methyl ether group

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Cellulose

• Unique results
• Soluble in cold water
• Methyl ether group breaks H-bonding
• Solubility ? as temperature ?
• Heating dehydrates the cellulose and hydrophobic
  methyl ether groups start to interact
• Viscosity increases and methyl cellulose forms a
  gel
• Becomes soluble again on cooling

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Cellulose

• Food uses
• Thermogelation properties
• Fat/oil barrier in batters for deep fried food
  applications
• The cellulose gels on heating and prevents fat
  uptake
• Holds moisture in food during thermal processing
• Acts as binder during thermal processing
• Fat replacer
• Methyl ether groups gives it fat-like properties
• Emulsion and foam stabilizer
• Due to increased viscosity (thickening effect)
• Film forming ability (e.g. water soluble bags)

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Cellulose

• C) Carboxymethyl cellulose (CMC)
• Cellulose treated with alkali
• to swell fibers and then
• chloroacetic acid is
• introduced
• Get carboxymethyl ether
• group

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Cellulose

• Food use
• Major use non-digestible fiber in dietetic foods
• Hot and cold water soluble
• Weak acid ? properties affected by pH due to
  carboxyl group
• COOH ? COO-
• Negative charge leads to repulsion between CMC
  making it a good thickening and stabilizing agent
• ?repulsion ?viscosity

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Cellulose

• Food uses (cont.)
• Common stabilizer in ice cream
• Retards ice crystal formation
• Foam stabilizer
• E.g. commercial meringues
• Tends to interact with proteins due to charge,
  increasing their viscosity solubility
• Used to stabilize milk proteins in milk
• Can form gels and films between pH 5-11

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Gums

• Plant polysaccharides (excluding unmodified
  starch, cellulose and pectin) that posses ability
  to contribute viscosity and gelling ability to
  food systems (also film forming)
• Obtained from
• Seaweeds
• Seeds
• Microbes
• Modified starch and cellulose
• All very hydrophilic
• Water soluble
• Highly hydrated
• High hydration leads to ?viscosity thickening
  and stabilizing effect
• Also good gel formers
• Some form gels on heating/cooling and in the
  presence of ions

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Gums

• Properties depend on
• 1) Size and shape
• Linear structures
• More viscous (occupy more space for same weight
  as branched)
• Lower gel stability ? get syneresis on storage
  (i.e. water squeezes out of the gel)
• Branched structures
• Less viscous
• Higher gel stability ? more interactions

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Gums

• 2) Ionization and pH
• Non-ionized gums little effect of pH and salts
• Negatively charged gums
• Low pH deionization aggregation ?
  precipitation
• Can modify by placing a strong acidic group on
  gum so it remains ionized at low pH (important in
  fruit juices)
• High pH highly ionized soluble ? viscous
• Ions (e.g. Ca2) salt bridges gels
• 3) Interactions with other components
• Proteins
• Sugars

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Gums

• Examples of gums and their applications
• A) Ionic gums
• Alginate
• From giant kelp
• Polymer of D-mannuronic
• acid and L-guluronic acid
• Properties depend on M/G ratio
• Highly viscous in absence of
• divalent cations
• pH 5-10
• Form gels when
• Ca2 or trivalent ions
• pH is at 3 or less
• Used as an ice cream and frozen dessert
  stabilizer
• Also used to stabilize salad dressings

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Gums

• A) Ionic gums
• Carrageenan
• From various seaweeds
• Seven different polymers
• ?-, ?- and ?-carrageenan most important
• Commercial carrageenan is a mixture of these
• Polymer is sulfated
• Stable above pH 7 (is charged)
• Function
• Depends on salt bound to the sulfate group
• Na cold water soluble and does not gel ?
  provides viscosity
• K produces firm gel
• Improves/modifies function of other gums
• Stabilizes proteins
• Interacts with milk/cheese proteins

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Gums

• B) Non-ionic gums
• Guar gum and Locust bean gum
• No effect of pH and ions (salts) since they are
  uncharged
• Guar gum has galactose side-groups on every other
  mannose unit (21) while locust bean gum does not
  have uniform distribution (41)

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Gums

• B) Non-ionic gums
• Guar gum and Locust bean gum
• Guar gum produces soluble in hot/cold water
  very viscous solutions at 1 and gels and films
  at 2-3 thixotropic
• Ground meats, salad dressings and sauces
• Locust bean gum Soluble at 80/90oC very viscous
  solutions Synergist with xanthan gum or
  carrageenan
• Binder in luncheon meat products and used in
  frozen desserts

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Guar gum uses

• Ice creams Smooth creamy texture
• Bakery products Texture, moisture retention
• Noodles Moisture retention, machine runnability
• Beverages Body, mouth feel
• Meat Binder, absorb water
• Dressings Thickener, emulsion stabilizer

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Gums

• Gum arabic
• One of the oldest known gums, from the bark of
  Acacia trees in the Middle-East and N-Africa
• Very large complex polymer
• Up to 3.500.000 Dalton (varies greatly with
  source)
• Glucuronic acid and galactose main building
  blocks
• Rhamnose and arabinose in minor amounts
• Very expensive compared to other gums but has
  unique properties

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Gums

• Properties of gum arabic
• Readily dissolves in water
• Colorless and tasteless solutions of relatively
  low viscosity
• Can go up to 50 w/w
• Newtonian behavior lt40
• Pseudoplastic behavior gt40
• Can manipulate solution viscosity of gum arabic
  by changing pH
• Low or high pH low viscosity
• pH 6-8 higher viscosity

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Gums

• Applications of gum arabic
• Gum candy (traditional hard wine gums) and
  pastilles
• Retards sugar crystallization
• Coating agent and binder
• Ice cream and sherbets
• induces and maintains small ice crystals
• Beverages
• foam and emulsion stabilizer
• used in beverage powders (e.g. citrus drink
  mixes) to maintain and stabilize flavor
  (encapsulates flavors)
• Bakery and snack products
• Lubricant and binder

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Gums

• C) Branched ionic gums
• Xanthan
• Produced by Xanthomonas a microbe that lives on
  leaves of cabbage plants
• Cellulose backbone with charged trisaccharide
  branches
• Branching prevents gelation
• Very viscous due to charged branches
• Expensive ingredient

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Gums

• Xanthan is widely used due to unique function
• Soluble in hot and cold water
• Very high viscosity at low concentrations
• Has pseudoplastic properties
• viscosity decreases when it is poured or agitated
  (shear-thinning)
• Viscosity is independent of temperature (10-95C)
  and pH (2-13)
• High freeze-thaw stability
• Compatible with most food grade salts

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Gums

• Xanthan is widely used due to unique function
• Ideal for emulsions ?excellent in fat-free
  dressings due to viscosity, pseudoplasticity and
  smooth mouth feel
• Excellent food stabilizer
• Good for thermally processed foods
• Expensive!