FOOD CHEMISTRY
BY
DR BOOMINATHAN Ph.D.
M.Sc.,(Med. Bio, JIPMER), M.Sc.,(FGSWI, Israel), Ph.D (NUS, SINGAPORE), PDF (USA)
PONDICHERRY UNIVERSITY
IV lecture
10/August/2012
Goals
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Pectin structure
Pectin ingredients
Applications of Pectin in food industry
Different Gum structure
Physico-chemical properties
Applications of Gums in food industry
Plant cell wall
Pectin
Pectin
Pectin
Monomer:
D-galacturonic acid, L-rhamnose
Others: D-galactose, D-xylose,
D-arabinose short side chain)
Bonding:
-1,4
-gelling and thickening agents
-bound to calcium in the middle lamella
-bound to cellulose in the primary cell wall
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 etc. ) in acid
– Not a very well defined
material
– Pectins from different
sources may differ in
chemical and functional
details
Pectin contains:
~85% galacturonic acid
Some are esterified with methyl alcohol
DE = degree of esterification
10-15% galactopyranose, arabinofuranose &
rhamnose
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Pectin
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Most pectins have a DE of 50-80%
Young unripened plants/fruits have very
high degree of esterification  hard texture
Old ripened plants/fruits have
lower degree of esterification  softer texture
Food use
A. Thickener - some use, but less common than gums
B. Pectin gels are useful in making 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 ESTERIFIED
8
70%
SLOW SET –
50 - 70% ESTERIFIED
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-
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O
C
+ +
Ca
-O
Low methoxy pectin
High methoxy pectin
Pectin gel forming mechanism
Pectin
Pectin
Pectin and its characteristics:
Example: Citrus juices
• Normal juice - colloidal pectin - thickening
• Pectin esterase - demethoxylates pectin --loss of thickening-precipitation - due to H-bonding of COOH and Ca2+ bridging
• Must heat juice to inactivate enzyme - causes dramatic flavor
changes
Pectin esterase
Loss of precipitation
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High Methoxy Pectin
Partially De-esterified Pectin
at low pH
Partially De-esterified Pectin
Amidated Pectin
Pectin Esterase and Lyase
Polygalacturonase and Pectin Lyase
Pectins
• Unbranched polymers of 200 - 1,000 Galactose units, linked b 1-4
Glucosidic bonds
• Degree of esterification controls setting rate
• >50% High Ester Pectins (HM)
• <50% Low Ester Pectins (LM)
• 70 - 85% (DE) = Rapid Set
• 44 - 65% (DE) = Slow Set
• Calcium required to gel LM Pectins
• USES:
• Amidated LM Pectins used to gel natural fruit preserves
• High ester (HM) Pectins stabilize sour milk drinks - react with casein
• Low ester (LM) Pectins used for milk gels
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
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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
• 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
2) Ionization and pH
3) Interactions with other components
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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
Properties depend on:
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:
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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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Alginate
Alginate
G
M
G, M
Monomer: -mannuronic acid (M)
-L-guluronic acid (G)
Bonding:
-1,4/-1,4
Pectin-Alginate image
Algin and Alginate
• Polymers of Mannuronic and Galacturonic acids varying widely in ratios
of the two acids
• Viscosity of 1% solution ranges from 10 to 2,000 CP as a function of
molecular weight and calcium ion content
• Precipitates below pH 3.0
• Degrades above pH 6.5
• Forms gels with calcium ions - 0.5 to 1.0% calcium
• Propylene glycol derivative improves stability to calcium and acid
• Food functionality includes:
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Water binding
Gelling
Emulsifying
Stabilizing
Propylene Glycol Alginate
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Precipitate at low pH
Interaction with calcium ions
Some interaction with fat
"Slimy" mouthfeel can substitute for fat
Good foam stabilizer
Alginate Gels
– Extrude into calcium bath
– Use sodium alginate with a sparingly soluble
calcium salt
– Regulate calcium availability by regulating pH,
sequesterant
– Too much calcium gives grainy gels
– Too slow release gives weak gels
Carrageenan
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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Carrageenan: Properties
-Most important red seaweed polysaccharides used
by food industry.
-3 forms differ in sulfate ester
-commercial products contain a mixture of 3 fractions
-stabilize milk protein
-water gel in low-calorie jams and jellies
-thickeners/stabilizer (combine with other
hydrocolloid)
Carrageenan
Monomer: D-galactose (anhydro/sulfate)
Bonding:  -1,4/ -1,3
Kinds of Carrageenan
kappa
iota
Kinds of Carrageenan
lambda
Carageenan
• Source: Seaweed gum
• Structure: Linear D-galactopyranosyl chain with
alternating 1,3 and 1,4 links. Some residues have
one or two sulfate ester residues. Three broad types
of repeating structure (i, k, and l carageenan)
• Functional Properties: pH independent thickening.
Double helix formation in k or i carageenan can lead
to gelation.
– k-carageenan is used in dairy foods
Carrageenans
• Mixtures of nonhomogeneous polysaccharides
• Galactans having sulfate half-ester groups attached
to the sugar units
• Extracted from red seaweeds
• D-galactopyranosyl units joined with alternating (1
3)-a-D- and (1 4)-b-D-glycosidic linkages, with most
sugar
• units having one or two sulfate groups esterified to
a hydroxyl group at carbon atoms C-2 or C-6
Carrageenans
• Sulfate content-15 to 40%
• Carrageenan products dissolve in water to
form highly viscous solutions.
• The viscosity is quite stable over a wide range
of pH values because the sulfate half-ester
groups are always ionized, even under
strongly acidic conditions, giving the
molecules a negative charge.