Effect of sugars on retrogradation of waxy maize starch-sugar extrudates

The effect of sucrose, fructose, and xylose on the retrogradation of waxy maize starch extrudates at relatively low moisture contents (20-50 g of water/100 g of dry solid) at 277-353 K was investigated using X-ray diffraction and H-1 nuclear magnetic resonance (NMR) relaxometry. The role of the sugar depended on the type of sugar and its concentration, but most importantly on the retrogradation conditions (water content and storage temperature). For the isothermal retrogradation at 313 K, fructose considerably increased the rate of retrogradation over the range of water contents investigated, and the increase was proportional to the sugar concentration. The behavior of xylose and sucrose was more complex. At 10% sugar content, both sugars enhanced starch retrogradation, while at 30% sugar content, xylose accelerated the process at <35% (mc, dsb) but decreased the rate constant when more water was present. This crossover took place at 42% me for sucrose. Sugars enhanced the transformation of the A-type polymorph to a so-called pseudo-B with an X-ray diffractogram similar to that of B-type starch, with the exception of the 1.6-nm peak which was considerably depressed. The effects of these sugars on the retrogradation kinetics in variable storage temperature conditions mirrored those obtained when isothermal retrogradation was examined in the water content domain.