Solubility of pectins

Commercial pectins are used in foods as thickeners, as gel formers, as emulsifiers, for suspending solids, and for a great variety of specific purposes. All of these uses depend upon the hydrophilic (or water-liking) nature of the pectin and, of course, upon the premise that the pectin is allowed to hydrate and go into solution properly. Considerable financial loss to users of pectin can result from a lack of understanding of the factors which effect the solubility of pectin. 

The rate of dehydration of pectin has not been studied, but recently Hinton (12) and Harvey (7) have developed a theory of the setting temperature of pectin gels based upon the solubility of pectin in the gelling medium. The evidence favors the postulation that as the solubility of pectin decreases, the temperature of set increases or the time of set decreases. 

It has been broadly reported that the solubility of pectin is attributed to the valency of the salt forming cations. Monovalent cationic salts of pectin are highly soluble in water whereas di- ortri-valent cationic salts of pectin are weakly soluble in water. Dilute solutions of pectin show Newtonian behavior but at moderate concentrations they exhibit Non-Newtonian behavior. Further, pectin tends to show a pseudo plastic behavior, which could be related to its concentration in a solution [49]. The viscosity of the pectin on the other hand, is influenced by the molecular weight, degree of esterification, concentration of the preparation and the pH. 

Pectins are readily soluble in water to give viscous stable solutions. However, the importance of pectin to industry, in particular the food industry, is the abihty of its solutions to form gels with sugar (ca 65% soHds) and acid or calcium ion under suitable conditions (51). 

Notwithstanding the chemical differences (alcohol groups in guaran, carboxyl groups in xanthan, and partially esterified carboxyl groups in pectin) these three polysaccharides in combination with chitosan in the microspheres appear to be able to bring chitosan into solution. This is particularly interesting if one considers the solubility of these three polysaccharides in water and their important applications in the food and pharmaceutical industries. 

When appreciable amounts of pectin, proteins, lipids, unwanted polyphenols, or other compounds are suspected to be present in anthocyanin-containing extracts, some of them can be precipitated or the anthocyanins may be crystalhzed and separated from the others. Pectin and proteins can be removed by organic solvents such as methanol and acetone in order to reduce their solubility, then precipitated and separated by centrifugation. Gelatin was used to remove proanthocyanidin due to its high molecular weight. Anthocyanins were reported to be precipitated early by lead acetate to achieve isolation from other materials. ... 

With endogeneous pectic polysaccharides as substrates, the pectin methyhransferase activity was measured as radioactivity linked to oxalate-soluble polys x harides after extensive washing of microsomes with IM ethanolic NaCL Figure 2 shows that the rate of methylesterification of pectic substances was maximal on days 4 and 6 these maximum activities were observed within this period in at least five independent ejqjeriments. On the other hand, little activity was noted in young cells before day 2, and in old cells after day 9. In other words during the stationary phase the newly synthesised pectins remained unesterified because of the lack of pectin methyltransferase activity. 

Fig. 5. Effect of PGl digestion on the ethylene synthesis-inducing activity of CDTA-soluble tomato pectin (a), Na2C03-soluble tomato pectin (b) and polygalacturonic acid (c). Controls were treated with solutions of heat-inactivated PGl. Treatment doses were 10 /tg of uronic acid equivalents. The line legends shown in panel a apply to all panels. Bars indicate SEs for the means of measurements of sets of 8 discs/teatment. fr wt, Fresh weight.

Extrusion-cooking increased very significantly the water-solubility of plant cell wall rich-materials. High amounts of pectins can be solubilised from sugar-beet pulp, citrus peels or apple pomace. 

Pectins from different tissue zones, namely epidermis, the outer parenchyma, the parenchyma of the Ccirpels zone, the carpels and the core line, were isolated firom alcohol-insoluble solids (AIS. In both zones of parenchyma, the cell-wall material represented about 80% of the total cell-wall material from the whole fruit. The pectins from the outer parenchyma accounted for 70% of the total. However, there was no change in galacturonic acid concentration. The enzymatic solubilisation of tissues or AIS was higher in the parenchyma zones than in the others. Nevertheless, the depolymerisation of the soluble pectins from parenchyma zones with an endopolygacturonase required the action of pectin methylesterase. The depoiymerisation of pectins from the other zones, however, did not. 

Experimental low-methoxyl pectin was obtained from dry heads (without seeds) of sunflower Helianthus annus L.). The extraction of pectin was carried out according to the method of Lin et al. (1975) with slight modifications. Only oxalate-soluble fraction which was submitted to consecutive treatments of purification as described previously was considered (Leitao et al., 1995). 

Laboratory studies on the extraction of pectin from orange peels, pretreated in an electromagnetic field of hyper frequency, were carried out. The influence of intensity of microwave treatment (P ) and time on pectin yield and pectin quality was investigated. It was established that the increase of Pw and time lead to increase in the pectin yield with 180-240 % in comparison with the control. Apparently, the microwave treatment leads to a considerable increase in the soluble form of pectin, characterized by increase in the jelly strenght and in the polyuronic content. 

Pectin exhibits solubility and gel strength peculiarities. The more soluble the pectin, the greater its gel-forming ability, because the pectin that does not actually disperse but remains in a submicroscopic condition cannot contribute to jelly formation. Hence, any interference with the complete solubility of a pectin inhibits its gel-forming ability. 

High levels of soluble carbohydrates may have an influence on the texture characteristics of products, too. Gelling of pectins, for example, depends on the level of sucrose, which has to be high to get the typical gel structure of jams and marmalades. 

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