Pectin, about

The radioactive method may be used when the titration method is not applicable, e.g., when the pH of the reaction is near the pK of pectin (about A.0), or when the reaction rate is low because of limited amount of enzyme or substrate. However, the automatic titration method when applicable, is advantageous because the reaction can be monitored continuously and any deviations from linearity readily recognized. 

Pectin [9000-69-5] Mr 25,000-100,000, amorphous. Dissolved in hot water to give a 1% soln, then cooled, and made about 0.05M in HCl by addition of cone HCl, and ppted by pouring slowly, with vigorous stirring into two volumes of 95% EtOH. After standing for several hours, the pectin is filtered onto nylon cloth, then redispersed in 95% EtOH and stood overnight. The ppte is filtered off, washed with EtOH/ElaO, then Et20 and air dried. 

The possible involvement of polygalacturonic acid-containing molecules in the defence reactions of tomato root cells against Fusarium oxysporum was suggested about 20 years ago by their accumulation at penetration sites. Since papillae are held to serve as a resistance mechanism to fungal penetration, it was assumed that the interrelation between pectin and other polymers - such as lignin - may contribute to enhancing the hardness of these newly formed structures (Benhamou et al., 1990). 

Most pectin solutions behave like Newtonian liquids below a pectin concentration of about 1 % (w/w). Onogi (1966) derived the critical concentration of polymer solutions from plotting the double logarithmic curves of viscosity (ii) against concentration at constant shear rates. Each curve consists of two straight lines intersecting at the critical concentration. At higher... 

This behaviour is thought to be due to the formation of a network structure caused by entanglement of the longchain molecules in solution. Plotting the data of viscosity measurements of pectin solutions of different concentrations reveals the same behaviour, confirming Onogi s observation, with a critical pectin concentration of about 1 % (w/w). 

Figure 3 gives an example of a typical force profile. The force is increased continuously and reaches the point - at the end of the first part of the force profile - where the pectin preparations start to flow. The so-called yield point is reached. The further increase leads to the continuous destruction of the internal structure and the proceeding shear thinning. The applied stress in part 3 of the stress profile destroys the structure of the fruit preparations completely. Now the stress is reduced linearly, see part 4 and 5, down to zero stress. The resulting flow curves 2, 3 and 4 and the enclosed calculated area from the hysteresis loop give important evidence about the time-dependent decrease of viscosity and a relative measure of its thixotropy. 

For more details about the sensory properties of the pectin preparations additional sizes such as thixotropy and viscosity have to be referred to (see figure 6). Fruit preparations with apple pectin Classic AY 901 have a relatively high yield point after shearing, a small thixotropic area, and a relatively high viscosity. This supports the statement, that their texture is weakly elastic and highly reversible. The apple pectin Classic AY 905 gives products with a sufficiently high yield point at 35 % soluble solids and a pronounced area of thixotropy. 

For commercial pectins, G increased rapidly up to 45 % of sucrose and remained constant. In contrast, the value of G for the other pectins increased more slowly and reached only a plateau for a concentration about 55 %. 

For apple maceration, the main activity is the endopectinlyase (PL) which decreases the viscosity due to the colloidal pectin by decreasing tlie molecular weight of this substrate methylated at about 90%. In association to this PL, exo B and endoarabanase (ARAs), pectinacetylesterase (PAE), rhamnogalacturonase (RG) and other hemicellulases allow to hydrolyse side chains of the pectin, decrease the steric hindrance and make the access of pectinases to the rhamnogalacturonic backbone more easy. 

Further research is directed to the determination of the fine-structure of the pectins and hemicelluloses isolated from soy meal, using chromatography and degradation with specific enzymes. With these results a model of the polysaccharides present in the cell wall of soy will be formulated. Furthermore, application directed experiments will be performed to obtain information about structure-function relationships. 

The data from Fig. 3 show that at all experiments with alkali and alkali-earth chlorides the amount of the formed amide groups is smaller than the content of the amide groups of the pectin from the control sample. Ca " " ions have the strongest retarding effect. Only at the experiments with NH4CI, higher values for the content of amide groups have been observed, but this increase is relatively small - about 10 %. 

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