Pectin, characteristics

Each fruit has specific quantities and ratio of pectin, hemicelluloses and cellulose. These polysaccharides are important concerning enzymes activities required to produce juices and concentrates. Moreover, even if molecular weight and methylation degree of the pectin are specific for each fruit, during the fruit maturation, endogenous pectinases depolymerases and esterase are changing the pectin characteristics This broad variability of raw material makes difficult the standardisation of fruits processing. 

This paper concerns the main properties of water soluble pectins in sol and gel states. First of all, the methods of purification and characterization are discussed. The method of steric exclusion chromatography equipped with different detectors is demonstrated as the most useful to determine the macromolecular characteristics of these polymers the role of aggregation is pointed out. 

At the moment, one recommends to determine the molecular characteristics of pectins using SEC chromatography equipped with a differential refractometer, a multiangle laser light scattering detector and a viscometer as previously described [25]. This technique needs no calibration with the usual molecular weight standards such dextrans and pullulans... 

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

The possibility that the initial degree of methyl-esterification might be controlled by the properties of the methyltransferase enzymes was examined partial characterisation of these enzymes in suspension-cultured cells of fiax. Pectin methyltransferases beii enzymes characteristic of the Golgi apparatus [22], microsomes were fiactionated daily for ten days from suspension-cultured flax cells and incubated in the presence of C-SAM, the universal donor of methyl groups. 

Recently, a gene coding for a novel pectin methylesterase, has been cloned (19). This gene, pemB, codes for a 433 amino add protein induding a N-terminal sequence of 21 amino adds which presents the characteristics of lipoprotein... 

Plastic flow pectins build up a yield point without giving the product a slimy characteristic. 

Characteristics of the pectins extracted from citrus fibres... 

In previous papers it was shown that the enzymatic pool of Pectolyase Y23 possesses high catalytic efficiency either as free or immobilized form in solution of pectins [28, 29] and of fresh vegetable tissues [30]. According to Baldwin and Pressor [31] the following enzymatic activities were detected in the preparation PL, PG and PE. The amount of the different enzyme detected per mg of Pectolyase Y23 and the main enzyme characteristic are quoted in Table 1... 

We used modifications of the standard solid-state CP-MAS (cross-polarisation, magic-angle spinning) experiment to allow the proton relaxation characteristics to be measured for each peak in the C spectrum. It is known that highly mobile, hydrated polymers can not be seen using either usual CP-MAS C spectrum or solution NMR (6). We found, however, that by a combination of a long-contact experiment and a delayed-contact experiment we could reconstruct a C spectrum of the cell-wall components that are normally too mobile to be visible. With these techniques we were able to determine the mobility of pectins and their approximate spatial location in comparison to cellulose. 

There are a number of possible locations within the cell wall for the pectin further away from cellulose. If there are covalent links between pectins and xyloglucans (16), then pectic chain segments close to these links would appear in the region sharing the same mean mobility characteristics as cellulose. The majority of the pectic molecule, diverging from the microfibrils would appear in the region with greater mean mobility. 

Characteristics of fraction I. The carbohydrate content of Fraction I was 84.4 %, in which the main component is D-galacturonic acid (71 %). Consequently, the polysaccharide is of a pectic type. The neutral sugars accounted for 13.4 % and according to their qualitative composition (Table 2) they correspond to the composition of pectin, isolated from sunflower heads (18). It is worth noting the high content of L-arabinose and D-galactose, compared with the other monosaccharides. The protein content was 7.8 %. 

Two characteristics of the lyase that we have purified may be significant. First, the small molecular size of the protein may confer it a high mobility that could be helpful to its movement through host cell walls. In second place, it is an endo-type enzyme, fact that has been considered essential for maceration of plant tissues [35]. In this sense it is noteworthy that between the battery of pectic enzymes produced by FORL, this pectin lyase is the only protein that behaves as an endo-type. 

The characteristics of the purified enzymes are summarised in Table 1. PAE was the only enzyme able to remove acetyl from beet pectin, FAE showed the highest activity towards... 

As previously described [2, 11], E. chrysanthemi possesses at least seven pectate lyases. Among them, PelZ seems to belong to a new family. However, its biochemical characteristics, basic optimal pH, calcium dependence and methylation sensitivity corroborate with those of the other pectate lyases of E. chrysanthemi. The weak expression of the pelZ gene and the weak activity of the PelZ protein seem to be correlated with specific environmental conditions. PelZ may act on pectin in synergy with the major pectate lyases. 

We described here the characterisation of the pemB gene and its product the second PME of E. chrysanthemi. The biochemical analysis of the purified protein indicated that PemB is actually an enzyme that demethylates pectin, leading to formation of methanol and PGA. However, PemB is more active on methylated oligogalacturonides than on polymeric pectin. The activating effect of non-ionic detergents on PemB was never pointed out for other pectinases and it is a characteristic of many membrane enzymes (21). 

Following earlier studies about physico-chemical characterization of sunflower pectin (Alarc o-Silva, 1990 Leitao et al, 1995) and technological utilization in the manufacture of low-caloric gels (Alarcao-Silva et al, 1992), we intend with this contribution to study the behaviour of this pectin in the confection of grape juice jellies and the evaluation of their organoleptic characteristics. 

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