Retrogradation

Under low energy input, as in freezing and chilling, further hydrogen bonding may occur, resulting in further tightening of structure with loss of water-holding capacity, known as retrogradation.

Starch retrogradation is a process which occurs when the molecules comprising gelatinized starch begin to reassociate in an ordered structure. In its initial phases, two or more starch chains may form a simple juncture point that may then develop into more extensively ordered regions. Ultimately, under favorable conditions, a crystalline order appears (Atwell et al. 1988).

Amylose has been thought to impart stiffness to food systems, especially after retrogradation. Amylose was reported to be the main factor in short-term

(several hours) development of the starch gel structure, while amylopectin was correlated with long-term (several days or weeks) development of the starch gel structure (Miles et al. 1983; Orford et al. 1987). Biliaderis and Zawistowski (1990) studied the time-dependent changes in network properties of aqueous starch gels. The storage modulus (G') time profiles revealed a two-phase gelation process: (a) an initial rapid rise due to amylose, and (b) a phase of slower G' development from amylopectin recrystallization (Biliaderis and Tonogal 1991). The G' of freshly prepared rice starch gels showed a linear relationship with the amylose content of the starch (IRRI 1991).

As shown earlier, retrograded amylose is classified as a resistant starch. It is believed that amylopectin does not substantially associate or retrograde upon standing because the outer branches are sufficiently long enough and therefore waxy starch pastes are nongelling (Pomeranz 1991) or form very weak gels. However, a role for amylopectin in retrogradation had been reported. The short amylopectin sidechains undergo a shift from coil to a helix transition (Winter and Kwak 1987). The WAXS technique has shown a slow development of crys-tallinity of the B form over time, which is closely related to the development of endothermic transition observed by DSC (Miles et al. 1985; Orford et al. 1987). These changes were due to a slow association of the double helices and were studied for potato and wheat starch by transmission electron microscopy, DSC, and rheology (Keetels et al. 1996). Ring et al. (1987) demonstrated that amylo-pectin staling within the gelatinized granule is the cause of increased firmness of the starch gel during storage. Varietal differences in rate and extent of amylo-pectin staling may help explain the variation in texture during storage of heat-processed rice products (Perez et al. 1993).

Shi and Seib (1992) reported that a decrease in the mole fraction of amylo-pectin chains between DP14 and 24 decreased the retrogradation tendency of waxy-type starches. Yuan and Thompson (1998) proposed that a greater portion of fraction chains between DP20-30 in du wx starch could explain the more rapid increase in G' and H during its storage. Yuan et al. (1993) reported that du wx starches from maize inbred lines had a greater tendency to retrograde than the wx starch at 10 and 30% concentrations. The rapid gelling behavior of du wx may be due the fact that commercial du wx has longer exterior chains (DP > 12) than the wx starches and has ~3% high-molecular-weight (DP ~ 320) material (Yuan and Thompson 1998).

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