Pectic acid analysis

Note. Samples for pectic acid analysis require careful preservation to ensure that the pectin is not degraded or lost during sample handling and storage. Samples should be stored at — 20°C in sealed containers if they are not to be analyzed immediately, but some physical changes may occur that could interfere with the subsequent analysis. Analysis of fresh samples is undoubtedly the preferred approach. 

Pectin analysis also needs more refining and emphasis. Pectic substances are a group of related substances (protopectin, pectin, and pectic acids), each with specific chemical characteristics (9). Both the protopectin and the pectic acid, pectinic acid, form insoluble complexes with metals, especially calcium. These too, like the protein, are concentrated in the persistent middle lamella-primary wall complex. 

Pectin Most standard procedmres for pectin analysis in vegetables and legumes involve extraction of the pectin as pectic acid followed by precipitation. Aqueous extraction of the pectin is followed by saponification with cold alkali, acidification, and the quantification of the precipitate. The pectic acid can be precipitated as calcium pectate or, as in the AOAC procedure, using ethanol. 

Physical Methods. Infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopic methods have been applied to structural analysis of polysaccharides such as pectin. These applications have been reviewed ( ), and reference IR spectra of pectic substances have been published ( ). Quantitative IR has been used to estimate acid dissociation constants of polyuronides from the ratio of -CO2H to -CO2- as a function of pH ( ). Also, by... 

The analysis of acidic cell wall polymers (notably pectins) can also be complicated by their tendency to precipitate in acidic media. Jermyn and Isherwood (1956) found that aqueous solutions of pectic polymers from pear cell walls precipitated on the addition of nitric acid. If, however, the sample was warmed prior to the slow addition of acid, precipitation could be avoided. Ford (1982) has shown that quantitative analysis of water-soluble pectic polysaccharides can be improved by depolymerization with a poly-( 1,4-a-D-galacturonide)-glycanohydrolase (EC 3.2.1.15). This treatment can also be used prior to acid hydrolysis to prevent polymer aggregation. Although the recovery of neutral monosaccharides is increased, the stability of the glycosiduronic linkage still prevents its complete hydrolysis. 

These studies, coupled with information from the detailed analysis of pectins and pectic gums, give reasonably coherent general models for the structure of pectinic acids. 

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