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.