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Chapter 1 Pectin

Chapter 1 Pectin. 1-1 Structure and Terminology 1-2 Production 1-3 Characterization of pectin gel 1-4 Factors affecting gelation 1-5 Chemical properties 1-6 Pectic enzymes 1-7 Structure and mechanisms of gel formation 1-8 Application. 1-1 Structure and Terminology.

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Chapter 1 Pectin

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  1. Chapter 1 Pectin • 1-1 Structure and Terminology • 1-2 Production • 1-3 Characterization of pectin gel • 1-4 Factors affecting gelation • 1-5 Chemical properties • 1-6 Pectic enzymes • 1-7 Structure and mechanisms of gel formation • 1-8 Application

  2. 1-1 Structure and Terminology • Pectin is heterogeneous complex polysaccharide • Its composition varies with the source and the conditions applied during isolation • All pectin molecules contain linear segments of (14)-linked a-D-galactopyranosyluronic acid with some of the carboxyl groups esterified with methanol. • Some of the hydroxyl groups of the galacturonosyl unit (O-2 and O-3) are esterified with acetic acid.

  3. Pectin molecule with methyl esterified or nonesterified carboxyl groups

  4. Amidated pectin has commercial importance

  5. Terminology • Protopectin • Pectinic acids • Pectic acids • Pectins • Degree of esterification (DE) > 50 • High-methoxyl pectins (HM-pectins) • High concentration of soluble solids, low pH • DE < 50 • LM-pectins • Divalent cations

  6. 1-2 Production • 1-2-1 Raw materials • Citrus peel (20-30%), apple pomace(10-15%) • Sugarbeet waste, sunflower heads, mango waste • Sugarbeet pectin is inferior to citrus or apple pectin • Presence of acetate esterification • A relatively low molecular mass • Presence of large amount of neutral sugar side chain

  7. 1-2-2 Extration, Purification, Modification • Two general processes 1. Separating the pectin from most other water-soluble material by precipitation with an alcohol 2.Precipitating pectin as an insoluble salt with suitable multivalent metal ions

  8. 1-2-2 Extration, Purification, Modification • Extraction: 50-90 , pH 1-3, Time 0.5-24h

  9. 1-2-3 Standardization • Uncontrolled variations in the raw materials will affect their functional properties. • Reproducible performance from batch to batch of the final products is a must. • Unstandardized HM-pectins are usually ‘diluted’ to a uniform pectin grade (150 grade USA-SAG) • The grade USA-SAG is the number of parts of sucrose which, under standard conditions, can be turn into a gel of standard gel strength by one part of the pectin. • Standard conditions: refractometer soluble solids, 65%; pH 2.20-2.40; gel strength, 23.5% SAG in 2 min measured by Cox and Higby (1944)

  10. 1-3 Characterization of pectin gel1-3-1 gel strength and breaking strength • Some methods measure the gel strength within the elastic limits of the gel • Other methods measure breaking strength

  11. SAG determination method • The gel to be tested is prepared in a glass of standardized dimensions • After curing, the gel is carefully removed from the glass and allowed to stand without support • The height of the gel deformation by its own weight is measured after a specified time • % SAG = 100 x (loss of height/original height)

  12. Plunger methods • Strain is applied to the gel by means of a plunger--- compression strain • Corresponding values of deformation are measured • Strain-versus-distance curve can be obtained while the plunger is forced into the gel at a constant speed • Plunger methods are well suited for use in the jam and jelly industry

  13. 1-3-2 Gelling time and temperature • Commercial high-ester pectins are usually standardized to a certain gelling time or temperature under specified conditions • Gelation of high-ester pectins may begin later than instant when the gelling system was colled below the gelling temperature • Gelling time is often measured rather than the gelling temperature

  14. Gelling time measurement • The test gel is prepared in exactly the same way as for the SAG determination • The still liquid preparation is adjusted to 95 and poured into a standard glass in a 30 water bath • The setting time is then taken as the time span from the filling until visual signs of gelation appear • Setting time values are 50 sec for commercial ‘rapid-set’ pectins and 225 sec for ‘slow-set’ pectins

  15. 1-3-3 Factors affecting gelation • Temperature • Concentration of pectin • pH • Concentration of cosolutes • Concentration of ions • Molecular weight • Degree of esterification • Degree of amidation • Presence of acetyl groups • Heterogeneity and presence of neutral sugar residues

  16. Temperature • A pectin gel is in most cases prepared hot and then solidified by cooling • When cooled below the gelation temperature , systems containing LM-pectin will gel almost instantaneously whereas HM-pectin systems will gel after a time lag. • HM-pectin gel cannot be remelted • LM-pectin gel is thermoreversible • It is often desirable to fill commercial containers at a temperature close to the gelation temperature to prevent flotation of particles (berries)

  17. Concentration of pectin • Typical concentrations of pectin in jams and jellies range from 0.3% to 0.7% • 0.3% HM-pectin gelling at about 65% soluble solids • 0.7% amidated LM-pectin gelling at about 35% SS • The pectin concentration used is inversely related to the concentration of soluble solids • At fixed levels of all other parameters, increasing the amount of pectin causes the gel strength of the resulting gel to increase.

  18. pH • A pH of about 3.0-3.1 is typical for high-sugar jams (HM-pectin, 65% SS) • Low-sugar jams may be slightly less acidic for taste reasons. (pH 3.1-5.5) • Low pH values tent to increase the strength of both HM- and LM-pectin gels • Gels will generally not form above about pH 3.5 in the case of HM-pectin and about pH 6.5 in the case of LM-pectin • HM-pectin: lower DE need lower pH for gelation

  19. Concentration of cosolutes • HM-pectins will gel only in the presence of large concentration of materials that lower water concentration /activity • The soluble solids must be at least 55% (w/w) • Increasing the soluble solids content causes the gelation temp. and the gel strength of the resulting gel to increase • LM-pectins may be gelled at zero soluble solids, but increasing the soluble solids will also positively affect the gelation temp. and gel strength

  20. Concentration of ions • Gelation of LM-pectin will only happen in the presence of divalent cations • Except for pectates or very low ester pectins which may form gels with K inos under certain conditions • Most divalent cations may be effective, but only Ca2+ is used in food application • Increasing Ca2+ concentration results in increasing gel strength and gelling temp. • Divalent cations are not necessary for the formation of an HM-pectin gel

  21. Molecular weight • Gels made from either LM or HM-pectin with high molecular weights will be stronger than gels made with pectins of lower molecular weights

  22. Degree of esterification • DE values for commercial LM-pectins range from 20-40% • Those with the lowest DE-values show the highest gelling temperatures and the highest sensitivity to Ca2+ • In contrast, the highest gelling temp. and the fastest gelation of HM-pectins are found with those that have the higest DE • Rapid-set (70-75% DE) > medium-rapid-set (65-70% DE) > Slow-set (55-65% DE) • Gel strength: Slow-set + lower pH = rapid-set • Gel strength: R > M > S (at same pH)

  23. Degree of amidation (DA) • Most commercial LM-pectins are amidation • DA values range from 15-20% • Amidation causes the pectin to gel at higher temp. compared to a nonamidated pectin under the same conditions, and less Ca2+ is needed • Amidation has a positive effect on gel strength

  24. Presence of acetyl groups • If some of the galaturonic acid subunits contain acetyl group at O-2 or O-3, gelation will be hampered • Every eight units is esterified this way • The presence of acetyl esters may be a drawback to suggested alternative source of pectin such as sugar beet pulp and sunflower heads

  25. Heterogeneity and presence of neutral sugar residues • Two pectin batches may behave differently, even if they are similar with respect to molecular weight and DE • The distrubution of esterified and free carboxyl groups has received much attention because it is different in enzymicly deesterified pectins than it is in acid or alkali deesterified pectins. • Gel strength: enzymicly deesterified pectins < acid or alkali deesterified pectins • Heterogeneity has been reported to be advantageous to the gel-forming ability of a pectin

  26. Heterogeneity and presence of neutral sugar residues • The rhamnose content has impact on the flexibility of the molecules (rhamonse insertions in the backbone) • The side chains of neutral sugar may sterically hinder gelation or limit the size of junction zone

  27. 1-5 Chemical properties • Pectins are polyanions at neutral pH and approach zero charge at low pH • Dissociations of the individual –COOH groups are not independent: pK = 2.9-3.3 • The pH at 50% dissociation of the pectin ranges from 3.5 through 4.5 • React with positively charged polymers, such as protein at pH values less than their pI

  28. Breaking strength of pectin gels as a function of pH

  29. Decomposed of pectins • Dissolved pectins are decomposed spontaneously by deesterification as well as by depolymerization • Factors: pH, Aw, Temperature • HM-pectins: stable at about pH 3.5- 4, sugars or other agents that lower water activity reduces the rate of degradation • LM-pectins: stable at about pH 4-5, • In both acid- and base-catalyzed decomposition, the rate of DEster is faster than the rate of DPoly • Highly esterified pectins are more prone to depolymerization than are LM-pectins or pectic acids

  30. Decomposed of pectins • DPoly. At low pH-values is a hydrolysis reaction • DPloy. At alkaline conditions is a beta-elimination reaction • Glycosidic bonds to O-4 of an esterified galacturonic acid subunit eliminate much more easily than those to O-4 of an nonesteified subunit • The rate of beta-elimination is almost proportional to the amount of remaining methyl ester groups and slow down as they are saponified

  31. Decomposed of pectins • Powdered HM-pectins slowly lose their ability to form gels, especially if stored under humid or warm conditions • Stored at < 20 C • LM-pectins are more stable, loss should not be significant after 1 year storage at room temperature

  32. Analysis of pectins • Degree of esterification (DE) • Washing in 60% 2-propanol (isopropanol)/5% HCl • Several washing with 60% 2-propanol (isopropanol) • Titrate to the equivalence point with NaOH • Saponification

  33. Analysis of pectins • Degree of amidation • Heating the sample with excess of NaOH and trapping the evolved ammonia in a known amount of HCL • Acetyl content • Alkaline saponification • Acidification with dilute sulfuric acid and steam distillation • The evolved acetic acid is trapped in a known amount of NaOH and titrated

  34. Analysis of pectins • Average molecular weight • Intrinsic viscosity method • Membrane osmometry • LC method

  35. 1-6 Pectic enzymes • Pectin esterases (PEs) EC 3.1.1.11 • Polygalacturonases (PGs) • Exo-PGs EC 3.2.1.67 • Endo-PGs EC 3.2.1.15 • Pectate lyases (PALs) • Exo-PALs EC 4.2.2.9 • Endo-PALs EC 4.2.2.2 • Pectin lyases (PLs) EC 4.2.2.10

  36. Pectin esterases (PEs) • Catalyze hydrolysis of methyl ester bonds • Fungal PEs -- optimum pH about 4.5 • Bacterial PEs -- pH 6-9 • Attack prevailingly next to an unesterified galacturonic acid subunit

  37. Polygalacturonases (PGs) • Catalyze hydrolysis of glycosidic bonds • The rate of reaction is inversely related to the DE • Optimum pH 4.0-5.5 • Exo-PGs • Release mono- or di-saccharides from the nonreducing end • Endo-PGs • Attact at random

  38. Pectate lyases (PALs) • Catalyze depolymerization via b-elimination • Fig 5 273 • Only glycosidic bonds to O-4 of an unesterified galacturonic acid unit are attacked • Optimum pH 8-9.5 • Exo-PALs • Endo-PALs

  39. Pectin lyases (PLs) • Catalyze b-elimination at bonds to O-4 esterified galacturonic acid units • Only endo-PLS are known • Optimum pH 5-6 • Presence of Ca2+: optimum pH 8

  40. Pectin lyases (PLs) • Catalyze b-elimination at bonds to O-4 esterified galacturonic acid units • Only endo-PLS are known • Optimum pH 5-6 • Presence of Ca2+: optimum pH 8

  41. 1-7 Structure and mechanisms of gel formation • To take part in gel formation, a pectin module must aggregate with one or more other pectin molecules • The junction zones must be of limited size because the molecules would otherwise form a precipitate rather than a gel

  42. Citrus, apple and sunflower pectins preparations with acid under hydrolyzing conditions (Powell et al., 1982) • DP is about 25 • Because bonds to rhamnose were assumed to be more labile than ordinary glycosidic bonds in the galacturonan backbone • One rhamnose unit for every 25 galacturonic acid units(regularly distributed)

  43. Apple pectins fractionated by DEAE-cellulose • Rhamnose insertions are very unevenly distributed along the galacturonan backbone (Vries et al., 1982) • Pectin consists of smooth regions and hairy regions rich in neutral sugars predominantly present as side chain • Neutral sugar content tends to be higher if mild conditions have been employed for extraction • L-rhamnose, L-arabinose, D-xylose, D-galactose, D-glucose

  44. Pectins from spinach and sugar-beet • Ester-linked ferulic acid has been found in neutral sugar side chains • Formation of a covalent bond between ferulic acid by the action of hydrogen peroxide or peroxidase

  45. Model of a function zone in a high-solids pectin gel

  46. Model of a calcium pectate junction zone

  47. Egg box model of a junction zone in a calcium pectate gel

  48. 1-8 Application • Pectins are a constituent of all plants and is part of the natural diet of man • Pectins are generally recognized as safe (GRAS)

  49. 1-8-1 Jams and Jellies • Pectin has a dominant position as a gelling agent in jams and jellies because • The natural pectin content in fruits used for jam making is responsible for the gelation of traditional jam that has been produced for centuries • Pectin is compatible with a natural image of the product • Pectin has good stability at the pH of jams and jellies, even when hot

  50. 1-8-1 Jams and Jellies • The selection of suitable pectin for a particular application is dependent upon the desired texture and the desired gelling temperature • HM-pectins: rapid-set, medium-rapid-set, slow-set • The actural gelling rate is dependent on the application conditions • LM-pectins are the possibility if the pH of the product is above approximately 3.5 and/or the soluble solids (SS) concentration is below approximately 55%

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