Polymerization amylopectin

FIGURE 16.19 Degree of polymerization distribution (m dpD d) of wild-type potatoe starch ( ), a nb/lcb fraction ( amylose -type A)- s b fraction ( amylopectin"-type ) of the native starch ... 

Amylopectin is the polymeric component of starch and consists mainly of glucose units joined at the 1,4-positions. Relative molar mass tends to be very high, e.g. between 7 and 70 million. A variety of modified starches are used commercially which are produced by derivatisation to give materials such as ethanoates (acetates), phosphates, and hydroxyalkyl ethers. Modified and unmodified starches are used in approximately equal tonnages, mainly in papermaking, paper coatings, paper adhesives, textile sizes, and food thickeners. 

FIGURE 5.1 Cluster model of amylopectin. A and B denote nomenclature of branch chains, 0=reducing end, c.l. = chain length in degree of polymerization. Reprinted from Carbohydrate Research, Vol. 147, Hizukuri (1986), Polymodal distribution of the chain lengths of amylopectin, and its significance, Pages 342-347, with permission from Elsevier. 

Potato starch exhibits different granular stmcture and composition, as opposed to cereal starches, which are responsible for the variation in functional behavior of these starches. Cereal starches exhibit the t5 ical A type X-ray crystalline pattern, whereas potato starch shows the B-form, andlegumesthe mixed state pattern C . The A, B, and C patterns are the different polymeric forms of starch that differ in the packing of amylopectin double helices. The structure of potato starch is discussed in more detail in Chapter 4. 

The amylose fraction from three different oat starches was characterized by high-performance size-exclusion chromatography (HP-SEC) and shown to be free of amylopectin.31 The degree of polymerization is given in Table 15.2. By treating amylose with isoamylase, it was found to be branched.31 The major fraction had a DP of 700, whereas a second fraction had a DP of 72. The amylose chain length was reported to decrease with increased amylose and starch-lipid content.30 The intrinsic viscosity of amylose has been reported to be 246-299 mL/g.7... 

Figure 2. Schematic of amylose and amylopectin polymeric structure before and after degradation by bacteria or enzymes...

Very early reports on these systems described them as polycondensates, consisting of broad molar-mass distributions with randomly branched topologies. The methods of synthesis included Friedel-Crafts coupling of benzyl alcohols [108] and the polymerization of 2,5,6-tribromophenol involving aryl ether formation [109], In addition, hyperbranched natural carbohydrate polymers, such as amylopectin, dextrin, and glycogen have been extensively studied [73-75]. 

Starch molecules are polymers of anhydroglucose and occur in both linear and branched form. The degree of polymerization and, accordingly, the molecular weight of the naturally occurring starch molecules vary radically. Furthermore, they vary in the ratio of branched-chain polymers (amylopectin) to linear-chain polymers (amylose), both within a given type of starch and from one type to another. These factors, in addition to any type of chemical modification used, affect the viscosity, texture, and stability of the starch sols significantly. 

Because of these new observations, we must conclude that the starch formulas discussed above apply to the amylopectin, whereas the corn amylose has the original Haworth formula but with a degree of polymerization of 200 to 300 instead of 25. The remaining question is the establishment of the pattern of ramification in amylopectin and in glycogen. 

Polysaccharides. Monosaccharides may link into an infinite number of polymeric units. The widely different properties of cellulose and amylopectin (both polymeric glucose) are dependent upon the linear, crystalline nature of the former and the highly branched, amorphous nature of the latter. 

Schardinger " himself started a trend in giving the early names crystallized amylose to a-dextrin and crystallized amylodextrin to /3-dextrin. Pringsheim reported relatively higher yields of /3-dextrin from glycogen and crude preparations of amylopectin, and postulated that amylose is polymerized a-diamylose amylopectin and glycogen are polymerized /3-triamylose. 

Eurther conversion (saccharification) to dextrose can be done using glucoamylase. Starch consists of polymeric linear a-1,4 linked dextrose units (amylose, 25 % of the starch) and polymeric mixed a-l,4/a-l,6 linked dextrose units (amylopectin, 75 % of the starch). Eor enzymatic degradation besides a-1,4 specific amylases also a-1,6 specific amylases (pullulanases) are required. A portion of the dextrose can be converted into fructose with a glucose isomerase yielding a high fructose com symp (MFCS). 

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