Apple pectin, structure

Chemical classification of polysaccharides is the least ambiguous system of grouping these macromolecules. Polysaccharides of different origins can have similar structures, as Kravtchenko et al. (1992) discovered in lemon and apple pectin. Likewise, polysaccharides of the same origin can have different structures, as for example, the concentration of pyruvate and acetyl substituents in xanthan, depending on incubation conditions (Pettitt, 1982). 

Pectic Substances. Part 111. Composition of Apple Pectin and the Molecular Structure of the Araban Component of Apple Pectin, E. L. Hirst and J. K. N. Jones,/. Chem. Soc., (1939) 454-460. 

Figure 1. Schematic structure of apple pectin rhamno-galacturonan backbone with regions rich in neutral sugar side chains (hairy regions).

Sjoberg, A.M. 1987. The effects of gamma irradiation on the structure of apple pectin, Food Hydrocolloids, l(4) 271-276. 

Structural Studies of Apple Pectins with Pectolytic Enzymes... 

In recent years we have used these possibilities in our studies on the structural features of apple pectic substances (1-6). This work is reviewed here and complemented with results of other investigators. For detailed descriptions of experimental work the reader is referred to (1-4). The apple pectins studied were extracted from Golden Delicious apples as schematically shown in Fig. 1. In total less than 50% of the pectin present in the Alcohol Insoluble Solids (AIS) prepared from peeled and cored apples could be extracted under the mild conditions used. The amounts of pectin in the various extracts are indicated in Fig. 1. 

Figure 4.13 Hypothetical structure of apple pectin showing I xylogalacturonan region, II region with arabinan side-chains, III rhamnogalacturonan region making up the hairy region (HR). SR is the smooth region (from Ref. [41]).

Arabinogalactans (AGs) are widely spread throughout the plant kingdom. Many edible and inedible plants are rich sources of these polysaccharides. AGs occur in two structurally different forms described as type I and type II, associated with the pectin cell-wall component by physical bonds and some of them are covalently linked to the complex pectin molecule as neutral side chains. Commercial pectins always contain AG 10-15%). AG of type I has a linear (1 4)-y0-o-Galp backbone, bearing 20-40% of of-L-Ara/ residues (1 5)-linked in short chains, in general at position 3. It is commonly found in pectins from citrus, apple and potato [6]. Recently, this AG type has been isolated from the skin of Opuntia ficus indica pear fruits [372]. 

Schols HA, Posthumus MA, Voragen AGJ (1990b) Structural features of hairy regions of pectins isolated from apple juice produced by the liquefaction process. Carbohydr Res 206 117-129... 

Extrusion-cooking of cell-wall rich products (e.g. wheat bran, apple pomace, citrus peels, sugar-beet pulp, pea hulls.) led to an important solubilisation of polysaccharides of various types without extensive degradation of the polymeric structure. The possibility of obtaining gelled systems directly with the extruded pectin-rich materials was demonstrated. 

To date, the structural features of pectic polysaccharides and plant cell walls have been studied extensively using chemical analysis and enzymatic degradation. In addition, research on isolation and physicochemical characterisation of pectin from citrus peels, apple peels, sunflower head residues and sugar beet pulp has been reported (2). However, the pectic polysaccharides extracted from wheat straw have only previously been reported by Przeszlakowska (3). The author extracted 0.44% pectic substances from Author to whom correspondence should be addressed. 

Renard, C.M.G.C. and Thibault, J.-F. 1993. Structure and properties of apple and sugar-beet pectins extracted by chelating agents. Carbohydr. Res. 244 99-114. 

Apple cultivars have different textures due to their internal variability of structure and composition. Some apples resist boiling and do not readily sauce. Others may undergo ready cell separation. This wide range of textural behavior illustrates the complexity due to pectins and other cell-wall materials. Select apple cultivars according to the desired processing qualities. 

Figure 5. Tentative structure of pectin fragments released in apple juice by enzymic liquefaction (modified hairy regions MHR). (Reproduced with permission from reference 90, copyright 1987 Food Hvdrocolloids).

Pectin is obtained from unripe apples or citrus fruit. In both cases the residues left after pressing to extract juice are used for pectin production. In the fruit, pectin is responsible for the firm structure. Apples, for example, have a firm texture despite a water content of more than 85%. Pectin is sold almost exclusively in powder form. 

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