Extraction from plant cell walls

Pectins are probably the most complex polysaccharides known, in terms of their chemistry and are certainly so in terms of their biosynthesis. Classically they were regarded as al,4-galacturonans, with various degrees of methyl esterification, and the terms pectic acid and pectinic acid referred to the non-esterified and partially esterified forms respectively. A third term protopectin , was used of insoluble pectin that could not be extracted from plant cell walls by hot solutions of chelating agents. It was considered that these three classes of pectin constituted a pectic triad . This view is now known to be erroneous, but it is still frequently put forward, especially in botanical texts. Consequently any discussion of the synthesis of pectins must be prefaced by a description of their chemistry, as it is now understood. 

Xyloglucans are classified as gum when they are extractable with hot water from seed endosperm cell walls, such as the tamarind seed xyloglucan, and as hemicelluloses because they are alkali-extractable from the cell walls of vegetative plant tissues where they are closely associated with cellulose [2]. Also /3-glucans with mixed linkages appear under the name gum as well as hemicellulose in the literature. 

Milieu conditions in gastrointestinal tract can influence the pectin structure and properties. Under the acid conditions of the stomach (pH 2-4) extraction of pectin from plant cell walls and hydrolysis of side chains can occur. In small intestine (pH 5-6) -elimination of main chains or de-esterification seems to be possible. In caecum and colon (pH 6-8) a strong fermentation of pectin takes place causing depolymerization to oligomers and leading to formation of short chain fatty acids and gases. The presence of OligoGalA is not yet clarified. 

Pectins were extracted from isolated cell walls of 5-week-old wheat plants using different methods. Enzymic digestions of the cell walls involved pectinases such as a commercial pectolayse or recombinant endopolygalacturonase [Maness Mort, 1989]. Chemical extractions involved the chelating agent imidazole [Mort et al., 1991] or solvolysis with anhydrous HF at 0 °C in a closed teflon line [Mort et al., 1989] followed by imidazole extraction. 

In order to study the composition and structure of pectins and their changes during ripening, storage and processing, various procedures have been developed for fractional extraction of pectins. A typical laboratory scheme for sequential extraction of pectin from plant cell wall materials is summarized in Figure 9.3. (Selvendran and O Neill, 1987 Voragen et al., 1995, Vierhuis et al., 2000). Not every step in the scheme is used by all of the researchers, and decisions depend on the source materials. 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

Detergent Methods. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) methods (2), later modified for human foods (13), measure total insoluble plant cell wall material (NDF) and the cellulose—lignin complex (ADF). The easily solubilized pectins and some associated polysaccharides, galactomaimans of legume seeds, various plant gums, and seaweed polysaccharides are extracted away from the NDF. They caimot be recovered easily from the extract, and therefore the soluble fiber fraction is lost. 

These acidic molecules might result from either the lack of PMT activity or the action of PME, known to be present in most plant cell walls. Pectin methyltransferases and pectin methylesterases extracted from active and resting cells were therefore characterized. 

Some by-products from the food industry contain high proportions of plant cell walls which can be used in human nutrition to produce "dietary fibre" or "functional fibre", i.e. compounds which can be used for their water-holding/binding properties, oil-binding capacity,... or as a source of polysaccharides such as pectins which are suitable after extraction, as gelling or thickening agents. 

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. 

Cellulose is a polysaccharide that is also made up of C6H10O5 units. Molecules of cellulose are huge, with molecular weights of around 400,000. The cellulose structure (Figure 3.5) is similar to that of starch. Cellulose is produced by plants and forms the structural material of plant cell walls. Wood is about 60% cellulose, and cotton contains over 90% of this material. Fibers of cellulose are extracted from wood and pressed together to make paper. 

The short answer is no. Take (S)-alanine (in other words, alanine extracted from plants) and (R)-ala-n ine (the enantiomer found in bacterial cell walls) as examples. They both have identical NMR spectra, identical IR spectra, and identical physical properties, with a single important exception. If you shine plane-polarized light through a solution of (5)-alanine, you will find that the light is rotated to the right. A solution of (R)- alanine rotates plane-polarized light to the left. Racemic alanine, on the other hand, lets the light pass unrotated. 

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