Structural characterisation of resistant starch fractions from extruded maize
 

Amparo Lopez Rubio and Elliot Gilbert (ANSTO)

Resistant starch (RS) is the fraction of starch that escapes digestion by a-amylases in the small intestine of healthy individuals. The interest aroused by RS comes from its ability to raise large bowel total-short-chain fatty acids, which are correlated with gut health and the prevention of several diseases like colorectal cancer[1].

Figure 1: TEM image of a starch granule showing the alternating crystalline and amorphous lamellae.

Figure 2: TEM image of a starch granule after digestion with a-amylase enzyme

The two components of starch are amylose, the linear a(1-4) linked glucan, and amylopectin, a a(1-4) linked glucan with a(1-6) branch points. It is now widely accepted that the amylopectin molecule is the crystalline component in granules, with the short, branched chains forming local organizations [2]. Small-angle X-ray scattering and electron microscopy have revealed a periodicity in the granule of about 10 nm (see Figure 1) that can be explained by stacks of thin crystalline lamellae alternating with amorphous zones in which the 1-6 branch points occur. 
 

In this study, USAXS, SAXS and XRD were used to characterise the structure of the fractions left after digestion of extruded maize starch. The combination of these techniques provided information in a scale ranging from a few angstroms to a few micrometers, i.e. screening over four orders of magnitude. Figure 3 summarises the type of information that can be obtained from each technique.

Digestion of native granules results in preferential attack of the softer parts, i.e. the amorphous layers as shown in Figure 2, but most of the starch is commercialised in processed products or it is cooked afterwards, leading to the process known as gelatinisation, which implies both the melting of the crystallites and loss of the original structure of the native granules into a continuous polymeric matrix. Understanding the mechanism of formation of resistant starch from processed samples is of outstanding importance as it will allow the design of specific food products with increased contents of these health-beneficial fractions.

From the SAXS experiments, a new type of molecular order was found in the resistant starch fractions from the extruded maize starch, reflected in a new peak at q~0.13?-1, corresponding to a characteristic length-scale of ~5nm (see Figure 4).

Extrusion leads to the gelatation of the starch, as observed from the disappearance of the crystalline reflections from XRD (see Figure 5), but the resistant-starch fraction shows a new type of crystal unit-cell (if compared with the native granule), highlighting that during digestion of the extruded starches, some molecular rearrangement takes place, probably promoted by the higher mobility of the shorter polymeric chains created during a-amylase hydrolysis[3]. 
 

Figure 3. Combined USAXS, SAXS and WAXS curve for native starch dry.
  

Figure 4. SAXS curves of starch. From above to below: native starch, extruded starch and resistant starch fraction

 
Figure 5. X-ray diffractograms of native, extruded and digested starch
 

References

1. Topping, D.L.; Clifton, P.M. (2001). Short-chain fatty acids and human colonic function: roles of resistant starch and non-starch polysaccharides. Physiological Reviews 81: 1031-1064.
2. French, D. In Starch: Chemistry and Technology. R.L. Whistler, J.N. Miller and E.F. Parshall (Eds.), Acad. Press Inc., Orlando 1984, pp. 183-247.
3. Lopez-Rubio, A.; Htoon, A.; Gilbert, E.P. The influence of extrusion and digestion on the nanostructure of high-amylose maize starch. Biomacromolecules, vol.8 (2007).