ABSTRACT
Plant cells walls are complex in nature and are composed mostly of polysaccharides. The distribution of the components of a plant cell was proposed for the first time by Northcote (in 1958) and was confirmed by microscopy and electron micrography (Albersheim et al., 1960). This distribution includes a middle lamella, a primary wall, and a secondary wall. In the past three decades, the view of cell wall structure has been revised several times as new information about wall components, polymer structures, and interpolymer associations has become available (Ariel et al., 2007). The middle lamella forms one amorphous intercellular layer between the primary walls of adjacent cells and is considered the first material deposited
8.1 Cell Wall Structure and Polysaccharide Components in Fruits ................... 151 8.1.1 Cell Wall Structure ........................................................................... 151 8.1.2 Pectin ................................................................................................ 152 8.1.3 Cellulose and Hemicelluloses ........................................................... 153 8.1.4 Starch ................................................................................................ 154
8.2 Enzymes Used in Fruit Processing ............................................................... 155 8.2.1 Fruit Juice Problems Caused by Polysaccharides ............................. 155 8.2.2 Industrial Enzyme Preparations ....................................................... 155 8.2.3 Pectinases.......................................................................................... 158 8.2.4 Cellulases and Hemicellulases.......................................................... 161 8.2.5 Other Enzymes in Enzymatic Mixtures for Juice Treatment ........... 161 8.2.6 Enzymatic Activity Determination for Enzymatic Mixtures
for Juice Treatment ........................................................................... 162 8.2.7 Methods of Immobilized Pectinolytic Enzymes in Fruit Processing .... 164
8.2.7.1 Application of Native and Immobilized Pectinases in Ultrafiltration of Apple Juice ......................................... 165
8.2.7.2 Immobilized Pectinases in Mash Treatment and in Apple Juice Clarification ............................................... 166
References .............................................................................................................. 166
by the cytoplasm. It is a cellulose-free zone and is composed of pectic polymers in its free form and calcium salts. The primary wall is the structure that expands to accommodate cell growth. It is composed of three polymer networks in a mesh-like structure: the pectins, cellulose-xyloglucan framework, and structural glycoproteins (Cosgrove, 1997). Moreover, phenolic esters ionically and covalently bind minerals, and enzymes complete the structure of the wall. Pectin is formed with three components: homogalacturonan (HG), rhamnogalacturonan-I (RGI), and rhamnogalacturonan-II (RGII) (Zhan et al., 1998). Except for RGII, these galacturonans do not show a stable definite structure (O’Neill et al., 2004). Typically, the components of the pectin backbone are HG and the core of RGI, the latter being branched with neutral sugar side chains (some combination of arabinan and arabinogalactan I and II) (Coenen et al., 2007). Another model proposes RGI is the main pectin backbone and HG and RGII form the side chains (Vincken et al., 2003). These models are typically representations of the wall structural components, which collectively provide a static view of the overall architecture. One interesting aspect of these models is that they generally incorporate the results of research conducted on many divergent aspects of wall structure and composition. Different models have been proposed in the past (Talbott and Ray, 1992). However, while fundamental components are common to all these models, there is less agreement in terms of the bonding interactions and their distributions within the wall (Ariel et al., 2007). The most frequently cited models were presented by Carpita and Gibeaut (1993) and describe the cell wall as composed of two polysaccharide networks. One comprises cellulose microfibrils cross-linked by hemicelluloses (most often xyloglucans or xylans); a simple analogy of this network is that of the steel and wire grids in a reinforced concrete slab. The other network, the pectin polysaccharides, would be the concrete (Ariel et al., 2007). Turgor in the cell plant is produced by the water entrapped by different polysaccharides. Pressure is produced by a solution against the semipermeable membrane enclosing the cell due to osmotic pressure differences between the inside and outside of the cell. Cell turgor has a major function in determining tissue strength and changes in turgor are an integral part of fruit softening (Harker et al., 1997). A positive relationship between cell turgor and tissue puncture firmness was observed in cultivars with different softening rates (Tong et al., 1999). Fruit treated with a solution of calcium chloride had higher turgor and lower internal air space and was firmer than nontreated fruit after storage (Saftner et al., 1998). When turgor was modified by soaking the tissue in several solutions of mannitol, the fruit tissue failed by cell rupture at high turgor and by cell debonding at low turgor, and failure force under compression was reduced as turgor decreased (Lin and Pitt, 1986).
