pectin - college of health and human sciences - purdue ... occurs in plant middle lamella – they...
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Pectin
Pectin
Occurs in plant middle lamella– They “cement” the cells together
In food use– Pectin is a gel former, e.g., fruit jelly
Pectin comes from the Greek word meaning “to congeal”Pectin was discovered in 1790 by Vauquelin and later (1825) crudely characterized by Braconnot
Pectic Substance Nomenclature
Protopectin– Immature plant material
Pectinic acid– Mature plant material
Pectin– Mature plant material
Pectic Acid– Overripe plant material
Pectin content of plants
30-35Citrus albedo15-20Sugar beet pulp
25Sunflower heads10Carrot
15-20Apple pomace5-7Apple3Tomato
2.5PotatoPectin (%)Plant material
Pectin-Chemical Structure
O
OHH
H
HOH
H OH
H
OO
O
OH
H
HOH
H OH
H
OHO
O
OH
H
HOH
H OH
H
OHO
O
OH
HOH
HOH
H OH
H
OO
CH3
CH3
Methyl ester groups
Alpha-1,4-linked-D-galacturonic acid groups
Carboxyl Substitution in Pectin
Degree of Methoxylation (DM)– Protopectin 16%– Normal pectin 8%– Low Methoxyl Pectin 2-4%
Degree of Esterification– Protopectin 100%– Normal Pectin 50%– Low Methoxyl Pectin 12.5-25%
Pectic Substances and Gelation
Normal Pectin– Gels in the presence of acid and sugar
Low Methoxyl Pectin– Doesn’t need sugar, but does need calcium ion
Pectic Acid– Forms insoluble calcium pectate. This reaction is
responsible for the firming effect seen in certain plant tissues, e.g., canned tomatoes
Chemistry of pectic substances
O
OO
OO
O
O
H
H H
Me
O
OO
OO
OOH
H H
dilute acid
or base
dilute acidhigh temperature
depolymerization
O
OO
OO
O
O
H H
O
OO
OO
O
OH
H H
1) Protopectinase
Protopectin Pectinic acids(colloidal and soluble)(insoluble)
2) Pectinesterase
Me
MeOH+
Pectic enzymes
O
OO
O
OO
OH
H H
O
OO
OO
O
H H
H
O
OO
OO
O
OH
H
H H
O
OO
OO
O
OH
H
H H
3)Pectin polygalacturonase
+
In plants these enzymes operate (1) (2) (3) and are responsiblefor textural changes during ripening. The net effect of their action is to reducemolecular weight and degree of methylation of the pectic substances. (1), (2),
(3) may be exo or endo enzymes.
Pectic enzymes
Pectin Function in Plants
Immature Protopectin Firm
Pectinic acids
Pectin
Pectic acidsOverripe Soluble sugars Mushy
Texture
Commercial (Normal) Pectin
Liquid or powderedSource -- mostly lemon and lime peel– A new method has been developed utilizing
microwave technology that allows economical extraction of high quality pectin from orange peels, a common starting material in the US
Pectin grade = number of pounds of sugar that one pound of pectin can carry in a jelly
Commercial (Normal) Pectin
Grade is influenced by DE and molecular weightGel strength -- measured by ridgelimeter(measures % sag), penetrometer, Instron, or texture analyzer.
Low methoxyl pectin
Available commerciallyRequires no sugar to gelRequires calcium ion to gelPreparation– Pectin LM Pectin– Acid preparation is best, produces firmer gels– Enzyme preparation is inferior as demethylation is
not randomRange of pH for gel formation = 2.5-6.5
Enzyme, acid, alkali
Jelly making
Need -- pectin + acid + sugarPectin– 0.5-1.0%– If juice is low in pectin, may concentrate by
boiling or add more as commercial pectin– Peach -- poor gel, pectin contains acetyl
groups– Citrus and apple -- form good gels
Jelly making
Acid– Contributes flavor– pH optimum is 3.2– If juice is low in acid, add lemon juice
Jelly making
Sugar– Preservative– Micororganisms cannot grow due to the
jelly’s high osmotic pressure– Optimum sugar concentration is about 65%
soluble solids
Jelly making
Sugar– This is reflected in the final cooking
temperature (colligative properties) of 104.5o C
– During this cooking some of the sucrose is hydrolyzed to invert sugar which will discourage sugar crystallization in the jelly
Gel strength in normal pectin jellies
Jelly strength
Continuity ofstructure
Rigidity of structure
[Pectin]Optimum isabout 1%
Acidity [Sugar]
Optimum is 3.2Low-hard gelHigh-no gel
Optimum 65%Low-weak gelHigh-crystals
form
Theory of normal pectin gel formation
A pectin junction zone
Hydrogen bonds
Pectin junction zones
O
OHH
H
HOH
H OH
H
OH
O
O
HH
HOH
H OH
H
O
O
O
HH
HOH
H OH
H
O
O
O
HH
OH
HOH
H O
H
OHO
O
O
OCH3H
CH3
O
HH
OH
HOH
H O
H
O
O O
HH
HOH
H OH
H
O
O O
HH
HOH
H OH
H
O
O O
OHH
H
HOH
H OH
H
O
O
O
O
OHCH3
H
CH3 CH3
Theory of low methoxyl pectin gel formation
Calcium ion
Ionicbonds
LMP junction zones
O
OHH
H
HOH
H OH
H
O
O
O
HH
HOH
H OH
H
O-
O
O
HH
HOH
H OH
H
O
O
O
HH
OH
HOH
H O
H
O-O
O
O
OCH3H
CH3
O
HH
OH
HOH
H O
H
O-
O O
HH
HOH
H OH
H
OH
O O
HH
HOH
H OH
H
O-
O O
OHH
H
HOH
H OH
H
O
O
O
O
OH CH3
Ca2+Ca2+
Theory of low methoxyl pectin gel formation
– pH needs to be higher (3.2-4.0) because only carboxylate (COO-) groups can participate in these types of ionic bonds
RCOOHpH < pK
RCOO-
pH > pK
Theory of low methoxyl pectin gel formation
– No sugar is needed to form the gel, hence these types of pectins are used in dietetic products. In practice, a small amount of sugar is left in the dietetic products as a tenderizer/texturizer. When included in these preparations, the resulting jellies are not as brittle as they would be in the absence of the sugar.