Simultaneous Measurement of Structure and Viscosity Changes during Starch Cooking
 

Authors

James Doutch, Ferdi Franceschini, Douglas Clowes and Elliot Gilbert (ANSTO-Bragg Institute), Mark Bason and Kevin James (Perten Instruments)

Starch is the key carbohydrate in the human diet and the major storage polysaccharide in plants. The structure of the starch granule is surprisingly complex and has a number of hierarchical levels extending from the micron- to nano-scale. These structural characteristics show considerable genetic variation and this, in turn causes considerable differences in the nutritional and industrial properties of different starches. This particularly manifests itself with differences in cooking properties. We set out to characterise this behaviour using simultaneous Rapid Visco analysis (RVA) and small-angle neutron scattering (1).

Starch granules and pasting

Starch is deposited by plants in granules that show considerable botanical variation in shape and size distribution; generally granules range from 2 mm to 100 mm in dimension. Most of the granule is composed of essentially linear amylose and highly branched amylopectin.

This ratio varies considerably between plant species. The granules are further subdivided into growth ring structures, which alternate between amorphous and semi-crystalline structures. The semi-crystalline rings have a repeating lamellar structure of periodicity 90 - 100Å. This is easily observed using small-angle scattering of neutrons and/or X-rays (SANS/SAXS) (2). Other sub-structures such as superhelices and blocklets are thought to exist within the granule (3).

The pasting properties of starch are readily studied using the RVA unit, in which slurries of starch are subjected to a defined program of heating and cooling cycles which allows the cooking properties of starches to be reproducibly tested, for example, prior to sale. This allows for grain quality control and can be used to assess viscosity changes during innovative processing techniques.

During a heating and cooling cycle in excess water, several changes in the RVA profile are observed. Firstly, a sufficient number of starch granules undergo rapid swelling and partial amylose leaching, particularly following gelatinisation, that a rapid rise in viscosity is observed. Under conditions of both heat and shear, the granules are partially disrupted and the leached amylose aligns leading to a reduction in viscosity in most cases.

On subsequent cooling, the hydrated polymers re-associate and the material undergoes a transition to a gel observed as a ‘setback’ or increase in viscosity. The corresponding changes in nanostructure had however not been characterised.

Figure 1. Modified Rapid Visco Analyser assembly for SANS study. B annotation indicates hollow impeller assembly through which neutrons pass

Simultaneous SANS and RVA

Neutrons have several key advantages for analysing the nanostructure of food systems, in particular their relatively high penetration through dense or concentrated samples and sample environments as well as the ability to avoid beam damage relative to synchrotron x-rays. An industrial RVA unit was modified to allow a neutron beam to pass through and is shown in Figure 1.

This was then used on the Quokka SANS instrument to collect simultaneous SANS and RVA using a typical industrial test cycle of heating and cooling and hydrated with deuterated water. The cycle used in this study was 13 minutes long and allowed the starch to be heated up to 95°C over the course of 5 minutes, held at that temperature for 2 minutes and then subjected to a gradual cooling for the remainder of the profile.

Figure 2 shows simultaneous scattering and RVA profiles for several starches: It can be seen that the scattering at low-q values increases quite dramatically for all starches at the point at which the viscosity first starts to rise. This implies the formation of large scale structures.

This occurs after approximately 4 minutes. The scattering for waxy (amylopectin only) maize is shown in detail in Figure 3. Here it can be seen that the periodic lamellar structure is destroyed after 4 minutes and replaced by power law scattering, in which the scattered intensity decays by some negative power with increasing q.

This power law scattering shows variation through the remaining time course of the experiment. Power law behaviour in small-angle scattering experiments can indicate a number of structural possibilities, for example, regular shapes like cylinders or spheres or fractal structures. We were able to differentiate between these possibilities by placing the data on an absolute scale, a task easily achieved with SANS. This clearly demonstrated the gel structures had the form of fractals on the nanometer scale; the data could be analysed using the method of Teixeira (4), as regular shapes did not yield the correct volume fractions expected for the systems studied.

This analysis yielded some interesting variations between different botanical starch varieties. In particular it was shown that potato, tapioca and waxy maize form aggregates which are relatively large (~200 Å) compared with wheat and normal maize (~90 Å). There were also interesting differences in fractal or Hausdorff dimension. This gives an indication of the morphological complexity of the system. We deduced that tapioca and potato gels appear to have quite linear and have relatively simple structure, whereas maize and wheat gels are much more complex and the aggregates quite polydisperse; waxy maize is intermediate between the two groups.

These nanoscale characteristics were found to show excellent correlation with the macroscopic properties of the gels and beg further study. The nanoscale parameters obtained from simultaneous SANS/RVA can be used to better understand structural transitions during starch gelation, across a wide variety of industrially relevant conditions.

References

1.   Doutch, J,Bason, M, Franceschini, F, James, K, Clowes, D, Gilbert, E.P., (2012) Structural changes during starch pasting using simultaneous Rapid Visco Analysis and small-angle neutron scattering, Carbohydrate Polymers, In Press, doi: 10.1016/j.carbpol.2012.01.066 
2. Blazek, J., & Gilbert, E. P. (2011). Application of small-angle X-ray and neutronscattering techniques to the characterisation of starch structure: A review. Carbohydrate Polymers 85, 281–293. 
3.  Perez, S, Bertoft, E, 2010, The molecular structures of starch components and their contribution to the architecture of starch granules: A comprehensive review, Starch, 62, 389-420 Teixeira, J, (1988) Small-Angle Scattering by Fractal Systems, Journal of Applied Crystallography 21, 781-785