Etiket Arşivleri: Braconnot


Pectin „ Occurs in plant middle lamella „ Gel former, e.g., fruit jelly „From the Greek word meaning tto congeal „Pectin was discovered in 1790 by Vauquelin and later (1825) crudely characterized by Braconnot O Pectic Substance Nomenclature „Protopectin-high methyl ester content „Pectinic acid-intermediate methyl ester contentcontent, solublesoluble – Salts are pectinates „Pectin-intermediate methyl ester content, colloidal „Pectic Acid-little methyl ester content – Salts are pectates

Pectin–Chemical structure Some other sugars, mainly L-rhamnose, are also present O Pectin structure „ Other monosaccharides (L-rhamnose, L- arabinose, D-xylose) in the structure may limit the size of junctions zones that can bbe fformedd andd, ththus, att lleastt partitialllly determine ultimate gel strength O Carboxyl Substitution in Pectin „Degree of Methoxylation (DM) – Protopectin 16% – NormalNormal pectinpectin 8%8% – Low Methoxyl Pectin 2-4% „Degree of Esterification – Protopectin 100% – Normal Pectin 50% – Low Methoxyl Pectin 12.5-25%

High and low methoxyl pectins „If DE is greater than 50%, it is a high methoxyl pectin (HM pectin) „If the DE is less than 50%, it is a low methoxyl pectin (LM pectin) O Degree of amidation (DA) „ The DA value indicates the percentage of carboxyl groups that nave been converted to the amide form by ammonia processiing „Amidated LM pectins may have a DA of 15-25% „Amidated pectins are especially sensitive to Ca+2 (see W&B, Figure 13.5) O Types of pectins HM LM Amidated LM -COOCH3 -COOCH3 -COOCH3 ((>> 50%)50%) ((<< 50%)50%) ((<< 50%)50%) -COOH -COOH -COOH – + – + – + -COO Na -COO Na -COO Na -CONH2 (15-25%)

Pectic Substances and Gelation „ Normal Pectin – Gels in the presence of acid and sugar „ Low Methoxyyl 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 O Pectin gels Atomic force microscopy image courtesy of Peter Cooke, ERRC, ARS, USDA O Chemistry of pectic substances Me O H O O O H O dilute acid O O O O O oror basebase O O H O O H H H dilute acid depolymerization high temperature

Commercial (Normal) Pectin „ Liquid or powdered „ Source — mostly lemon and lime peel (20-30% ppectin)). This is the higghest qqualityy. „ Some from apple pomace (10-15% pectin) „ Pectin grade = number of pounds of sugar that one pound of pectin can carry in a jelly O Pectin production „ Citrus peel is extracted at pH 1.5-3.0 and 60-100oC „Extract is filtered „Pectin is precipitated by addition of isopropanol O Commercial (Normal) Pectin „ Grade is influenced by DE and molecular weight „ Gel strength — measured by ridgelimeter (measures(measures %% sag)sag), penetrometerpenetrometer, InstronInstron, oror texture analyzer.

Low methoxyl pectin „Available commercially „Requires no sugar to gel „Reqquires calcium ion to ggel „Preparation Enzyme, acid, alkali – Pectin LM Pectin – Acid preparation is best, produces firmer gels – Enzyme preparation is inferior as demethylation is not random O Low methoxyl pectin „Range of pH for gel formation = 2.5-6.5 O Pectin uses „Principally used in jellies and jam „However, some is used in – Confections – Beverages – Acidified drinks „ See W&B, Table 13.2 for other uses of pectins

Jelly making „Need — pectin + acid + sugar „Pectin – 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 — forms a good gel O Jelly making „Acid – Contributes flavor – pH optimum is 3.2 – If juice is low in acid, add lemon juice O Jelly making „ Sugar – Preservative – Micororganisms cannot grow due to the jjelllly’’s hihighh osmotiic 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 CC – During this cooking some of the sucrose is hydrolyzed to invert sugar which will discourage sugar crystallization in the jelly O Gel strength in normal pectin jellies Jelly strength Continuity of Rigidity of structure structure [Pectin] Optimum is Acidity [Sugar] about 1% Optimum is 3.2 Optimum 65% Low-hard gel Low-weak gel High-no gel High-crystals form O Setting times HM pectins DE Setting time Rapid set 72-75% 20-70 sec Medium set 68-71% — Slow set 62-68% 180-250 sec

Setting times „Rapid set pectins are used in jams where quick gelling is desired to prevent flotation of the fruit „ Slow set pectins are used in jellies to provide time for bubbles to escape „High methoxyl pectin gels can not usually be melted and reformed O Theory of normal pectin gel formation A pectin junction zone Hydrogen bonds O Pectin junction zones O OH O O CH3 H OH H OH HO H O H H O O H H OH OH H OH H H OH H H OH H H O O H H O H O H H O CH 33 HH OOHH HH OO OO O OH H O O H O CH H OH HO O 3 H O H O H H O O H OH H OH H OH H H H H OH H HO O H H O O H H OH CH H OH CH O O 3 O O 3 H OH

Theory of low methoxyl pectin gel formation CCallciium iion Ionic bonds O LMP junction zones H CH O OH 3 H O H OH O O H O H O H H O O H OH H OH H OH H H H H OH H HO O H H O O H H OH H OH O O- O O- H OH 2+ Ca 2+ Ca – O O – O O H OH H OH HO H O H H O O H H OH OH H OHH H OHH H OH H H O O H H O H O H H O CH 3 H OH H O O O O CH 3 O 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

Methyl ester content and gelling ability 2 + a C h t i ww l e g o t y t i l i b A 0 20 40 60 80 100 DE O Characteristics and uses of low methoxyl pectin gels – These gels can be melted and reformed repeatedly – No sugar is needed to form the gel, hence thesethese typestypes ofof pectinspectins areare usedused inin dieteticdietetic 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. O Uses of low methoxyl pectin gels „Fat mimetic – From Hercules, this is a LM pectin gelled with Ca+2 and microparticulated (particle sizesize << 11 μμm)m) – Trade name is Slendid

9/23/2008 O Labeling „Both HM and LM pectin may be labeled pectin