Transcript Polysaccharides - Food Science & Human Nutrition

Polysaccharides 11
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Pectin
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Pectic substances
◦ Middle lamellae of plant
cell walls
◦ Functions to move H2O
and cement materials for
the cellulose network
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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
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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
 < 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
O
C
O-
O
C
+ +
Ca
-O
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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
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Most abundant organic
compound on the planet
Plant cell wall component
CH OH
2
O
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
O
O
OH
OH
OH
O
O
O
OH
OH
OH
AMYLOSE
◦ Gives tensile strength to cell wall
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CH 2 OH
CH 2 OH
v.s.
CH OH
CH 2 OH
CH OH
2
2
O
O
O
O
OH
O
OH
OH
O
OH
OH
OH
CELLULOSE
AMORPHOUS
REGIONS
CRYSTALLINE REGIONS
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Cellulose
Hydrogen
Bond
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Cellulose
Properties
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Crystalline regions have very tight H-bonding
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Insoluble in water
No effect on viscosity (why?)
Little access to hydrolytic reagents and enzymes
Very tough texture
Not digestible by humans
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-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
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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:
A) Microcrystalline cellulose (MCC)
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Prepared by partial acid hydrolysis
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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:
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Stabilizes emulsions
Absorbs oils & syrups
Dry mixes - keeping them free-flowing
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Cellulose
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Two main products of MCC
1. Powdered MCC
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Spray dried MCC
Forms aggregated
porous/sponge-like
microcrystals
Uses:
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Flavor carrier
Anticaking agent in powders and
cheese
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Cellulose
2. Colloidal MCC
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Mechanical energy applied after hydrolysis to
rip microcrystals apart to form small microaggregates
Water dispersible – similar function as food
gums
Food uses:
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Foam and emulsion stabilizer
Pectin and starch stabilizer
Fat and oil replacement
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Cellulose
B) Methyl cellulose
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Cellulose treated with
alkali to swell fibers and
then methyl chloride is
introduced:
CH 2 OH
CH
2
CH
OH
O
2
OH
O
O
OH
O
O
OH
OH
O
OH
OH
OH
◦ Get methyl ether group
1] NaOH
2] CH 3 Cl
CH OCH
2
3
CH
2
O
OCH
CH
3
2
O
3
O
O
OH
OCH
O
OH
OH
O
OH
OH
OH
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Cellulose
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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
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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)
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Cellulose treated with
alkali
to swell fibers and then
chloroacetic acid is
introduced:
◦ Get carboxymethyl ether
group
CH OH
2
CH OH
2
CH 2 OH
O
O
O
O
OH
O
OH
OH
O
OH
OH
OH
1] NaOH
2] ClCH 2COOH
pH DEPENDENT
CH 2O CH 2 CO 2O
O
OH
CH O CH 2 CO -2
2
O
CH O CH 2 CO 22
O
O
OH
OH
O
OH
OH
OH
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Cellulose
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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
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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
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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
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Seaweeds
Seeds
Microbes
Modified starch and cellulose
All very hydrophilic
◦ Water soluble
◦ Highly hydrated
 High hydration leads to viscosity = thickening and stabilizing effect
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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
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Alginate
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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
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Form gels when:
1.
2.
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pH 5-10
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
 κ-, ι- and λ-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 (2:1)
while locust bean gum does not have
uniform distribution (4:1)
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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
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Ice creams: Smooth creamy texture
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Bakery products: Texture, moisture retention
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Noodles: Moisture retention, machine runnability
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Beverages: Body, mouth feel
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Meat: Binder, absorb water
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Dressings: Thickener, emulsion stabilizer
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Gums
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Gum arabic
◦ One of the oldest known gums, from the bark
of Acacia trees in the Middle-East and NAfrica
◦ 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 <40%
 Pseudoplastic behavior >40%
◦ 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
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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
1. Soluble in hot and cold water
2. Very high viscosity at low concentrations
3. Has pseudoplastic properties
 viscosity decreases when it is poured or agitated (shearthinning)
4. Viscosity is independent of temperature (10-95°C) and pH (213)
5. High freeze-thaw stability
6. Compatible with most food grade salts
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Gums
Xanthan is widely used due to unique function
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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!
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