Starch is mainly composed of a mixture of two polymers: amylopectin and amylose, in variable proportions depending on the origin of the starch. They are polymers of α-D-glucopyranosyl. The monomers are in the most stable form, that is, to say the chair conformation 4C1; they are interconnected by α-(1,4) link and to a lesser measurement (4%-5%) of connections α-(1,6) (Schoch, 1945; French, 1984; Bule´on et al., 1998a). Each polymer has one free reductive end. Even if the basic unit is the same for both polymers, they are very different in size and degree of branching.
Amylopectin
Most often, amylopectin represents 70%-80% of the glucidic fraction, and 99% for the varieties called waxy (maize, sorghum, rice, and barley). The amylopectin glucose units are essentially linked by type α-(1,4), but 5%-6% of α-(1,6) links give amylopectin a highly branched structure. The first structural models of amylopectin exhibited a homogeneous branched molecule. Kainuma and French (1972) established a heterogeneous branched model more consistent with experimental results. In this model, modified by Robin et al. (1975), amylopectin consists of a set of clusters with short chains of an average degree of polymerization (DP) of 15-20 (S chains (short) or A chains). These are interconnected by longer chains with a DP of 40e45 and are called L chains (long or B chains). Burchard and Thurn (1985) and Thurn and Burchard (1985) had subsequently slightly modified this model by specifying that the only reductive end is carried by a DP chain greater than 60 (C chain) on which are grafted B chains.
The cluster structure gives to amylopectin some particular physicochemical properties:
- Low capacity to link iodine, less than 1 mg/100 mg of amylopectin with an absorption maximum λmax = 540 nm.
- The very low intrinsic viscosity of the order of 120-190 mL/g (at 22.5°C in KOH 1 mol/L), given its very high molecular weight (107-108 g/mol).
Amylose
Amylose represents around 20%-30% of the glucosidic portion of the starches. But this quantity can vary up to 80% (for certain varieties of maize (Zea mays), barley (Hordeum vulgare), rice (Oryza sativa), or peas (Pisum sativum)). Amylose is an essentially linear molecule, composed of 500-6000 units of α-D-glucopyranosyl, spread over 1-20 different chains with an average DP of 500. Banks and Greenwood (1975) showed that the number of binding α-(1,6) is low and randomly distributed (Takeda et al., 1992); however, it appears that they are frequently located near the reductive end. The branched character of amylose has been highlighted by the action of β-amylase, which hydrolyzes the α-(1,4) links to form maltose. During this hydrolysis, only 70%-80% of amylose is degraded because of the presence of links α-(1,6) which stop the enzymatic activity. The connection rate is even larger than the mass of amylose and is important (Takeda et al., 1992), but it also depends on the botanical origin of amylose (Hizukuri et al., 1981). The molar mass of amylose is generally between 2.105 and 2.106 g/mol and depends on the botanical origin (Takeda et al., 1984) and also on the fractionation method (Banks and Greenwood, 1975). An amylose population is heterogeneous with a polydispersity generally between 1.3 and 2.1 but may sometimes reach higher values (5-10). The binding capacity of amylose to iodine is about 20 mg/100 mg. Amylose has the particularity to trap many molecules like alcohols and lipids. Amylose can be extracted from starch grains dispersed in water by selective reprecipitation in the presence of alcohol (e.g., butanol) (Schoch, 1942, 1945). Amylose can also be synthesized in vitro in an enzymatic manner or from ADP-glucose with 1,4-α-D-glucan 4-α-D-glucosyltransferase (granule-bound starch synthase) (Ball et al., 1998) or sucrose with a recombinant amylosucrase, which permits to obtain highly crystalline amylose (DP35-58) (Potocki-Veronese et al., 2005).