Effect of Molecular Weight and Method of Deësterification on the Gelling Behavior of Pectin2

Vacuum infusion of plant or fungal pectinmethylesterase and calcium affects the texture and structure of eggplant

The effect of vacuum infusion on eggplant quality of a commercial fungal (Aspergillus niger) and citrus pectinmethylesterase (PME) with calcium chloride (4000 ppm) was investigated after processing and during storage. Firmness of infused eggplants using fungal or citrus PME was significantly increased compared to controls (fresh noninfused and water-infused control) after processing and during storage for 7 days at 4 degrees C. Activity of fungal PME-infused eggplant increased almost 32 times, whereas activity of eggplant infused with Marsh grapefruit PME increased 2-fold. Degree of esterification of pectin of eggplants infused with fungal or citrus PME decreased slightly. Cryo-SEM showed that samples treated with fungal PME/ CaCl2 displayed more integrity among cells as compared with water-infused control. The change of pectin in the cell wall was visualized using monoclonal antibodies JIM5 (low-esterified pectin) and JIM7 (high-esterified pectin). JIM5 showed more binding than JIM7 with the cell walls of eggplant tissues from fungal PME/ CaCl2 treatment.

Enzymatic modification of pectin to increase its calcium sensitivity while preserving its molecular weight

A commercial high-methoxy citrus pectin was treated with a purified salt-independent pectin methylesterase (PME) isozyme isolated from Valencia orange peel to prepare a series of deesterified pectins. A series of alkali-deesterified pectins was also prepared at pH 10 under conditions permitting beta-elimination. Analysis of these pectins using high-performance size exclusion chromatography (HPSEC) with on-line multiangle laser light-scattering, differential viscometer, and refractive index (RI) detectors revealed no reduction in weight-average molecular weight (M(w); 150000) in the PME-treated pectin series, whereas a 16% reduction in intrinsic viscosity (IV) occurred below a degree of esterification (DE) of 47%. In contrast, alkali deesterification rapidly reduced both M(w) and IV to less than half of that observed for untreated pectin. PME treatment of a non-calcium-sensitive citrus pectin introduced calcium sensitivity with only a 6% reduction in the DE. Triad blocks of unesterified galacturonic acid were observed in (1)H nuclear magnetic resonance spectra of this calcium-sensitive pectin (CSP). These results demonstrate that the orange salt-independent PME isozyme utilizes a blockwise mode of action. This is the first report of the preparation of a CSP by PME treatment without significant loss of the pectin's M(w) due to depolymerization.

Influence of the substituents of the carboxyl groups and of the rhamnose content on the solution properties and flexibility of pectins

Viscometric measurements were carried out on well-characterized apple, citrus, sugar-beet pectins in order to analyse the effect of the nature and the amount of substituents (methyl, amide, acetyl groups) and of the rhamnose content on the flexibility of the polymeric backbone. Through the dependence of the intrinsic viscosity with the ionic strength the flexibility parameter B was determined. B values between 0.072 and 0.017 indicate that pectins are relatively stiff molecules. However, an increase in flexibility is noticeable with the rise of the rhamnose content and of the amount of amide groups of the pectic acids. The flexibility is also sensitive to the degree of methylation.

[The distribution of free and esterified carboxyl groups within the pectin molecule after the action of pectin esterase from Aspergillus niger and oranges]

By reaction of pectin esterase (PE) from Aspergillus niger and oranges as well as lye, with 95% esterified citrus and apple pectin we prepared series of preparations with degrees of esterification between 35 and 77%. In these partial deesterified pectins the form of distribution of the free and esterified carboxyl groups has been determined from the activity coefficient gamma Ca2+ of the calcium counterions in the solutions of the corresponding calcium pectinates, from the electrostatic free enthalpy delta (Gel/N)KCa of the ion exchange Ca2+----2K+ in these systems as well as from the relative activity of the polygalacturonase reacting with sodium pectinate. The PE from A niger hydrolyzes the esterified carboxyl groups more or less randomly, in a manner similar to the effect of lye on pectin. On the other hand PE from oranges brings about block-like groupings of free carboxyl groups in the pectin molecule. The study revealed different reaction mechanisms of the pectin deesterification by pectin esterases from Aspergillus species and higher plants.