574 Produce Degradation: Reaction Pathways and their Prevention
pulp that is less hydrophilic and less reactive to certain chemicals or crosslinking
agents. This might be a useful property to exploit in development of specialty
powders or tissues where strong fiber–fiber bonding is undesirable.
19.2.3.1 Pectins
Pectins constitute another major polysaccharide in cell walls of higher plants and
are commonly produced during the initial stages of primary cell wall growth in a
variety of dicotyledonous and some monocotyledonous plants [53–55]. In plants,
pectins function as a hydrating agent and cementing material for the cellulosic
network [56]. Pectins are found in relatively large amounts in soft plant tissues under
conditions of rapid growth and higher moisture contents. They appear to help with
the movement of fluids through the rapidly growing tissues [57]. Pectin is particularly
abundant in the middle lamella, the region that serves to cement together the cell
walls of adjacent cells. It is this layer that ruptures in some places, forming the
intercellular air spaces found in many tissues.
Pectin is a heteropolysaccharide with a main chain consisting of α-1,4 linked
poly-D-galactouronic acid units. The carboxylic groups are partially esterified with
methanol, and the hydroxyl groups (on C2 and C3) can be substituted with acetyl
groups. Pectins are heterogeneous, branched, and highly hydrated polysaccharides
that are negatively charged due to the presence of many galacturonic acid residues
in the polymer chain (Figure 19.11). Because of their negative charge, pectins form
strong ionic bonds in the presence of cations. For example, in the presence of divalent
cations such as Ca++, highly crosslinked pectin is produced. There is evidence that
similar Ca++ crosslinks play a role in holding the cell wall components together, but
the exact nature of the bonding that gives the wall its integrity is unknown. In plant
cell walls, pectin is linked together with other cell wall polymers such as proteins,
hemicellulose, and cellulose. The formation of covalent bonds between pectin and
hemicellulose have been reported in the plant cell wall [58–60], but apparently not
all pectin is bound in such a manner, as indicated by the presence of unbound pectin
recovered in preparations where cell walls have been subjected to extraction with
cold water [61–63].
Functionally, pectins are quite similar to collagen in animals, providing firmness
and integrity to plant tissues both as a part of the primary cell wall and as the main
FIGURE 19.11Structure of the pectin polymer.
OHOHOXOOC
O
OHOHOXOOCOH OOHOXOOC
OH
OHnX = H or CH 3