RVA used in ground-breaking starch research

Mark Bason, mbason@perten.com Perten Instruments, Australia.

An RVA was specially modified to allow a beam of neutrons to shine through the starch during a standard pasting profile. Mark Bason, R&D Manager, Perten Instruments Australia and Elliot Gilbert, program leader in food science at the Australian Nuclear Science and Technology Organisation (ANSTO), have, for the first time, used neutron scattering to help unravel the changes that occur in starch structure at the molecular level during cooking.

Introduction

Starch is the key carbohydrate in the human diet and the major storage polysaccharide in plants. It also finds increasing use in the polymer, pharmaceutical and biofuels industries. The native starch granule has hierarchical structure from the micron- down to the nano-scale which affects properties such as pasting. The nano-scale structural changes that accompany pasting and other starch transitions are poorly understood.

Background

Starch is deposited in granules that show considerable botanical variation in shape and size distribution (2 – 100 μm). They are composed essentially of linear amylose and highly branched amylopectin, the exact ratio of which varies depending on genomic origin. Within the granules are alternating amorphous and semi-crystalline growth ring structures. The origin of these is not well understood, but may be related to diurnal deposition patterns. The semi-crystalline growth ring structures have alternating crystalline and amorphous structures repeating every 90 – 100 Å. At
the molecular level are various crystal lattice types. Cereal starches have so-called A-type, tuber starches have B-type with monoclinic unit cells and high amylose starches have B-type with hexagonal unit cells crystal structures. It is now accepted that there is an intermediate level of organisation between the lamellae and the growth rings.

A variety of granular, growth ring and molecular level mechanisms are thought to interplay when
starch is heated in excess water.

Method

An RVA was modified to enable the simultaneous measurement of viscosity and small-angle neutron scattering (SANS). Starches (waxy maize, regular maize, wheat, regular potato, tapioca and acid modified maize) (3.0g) were hydrated with D2O (47.8g) instead of H2O to reduce incoherent scattering background. A modified 13 minute RVA profile starting at 25°C and reaching a maximum of 95°C before cooling to 25°C was used to paste the starch. Scattering runs were taken simultaneously with the RVA profile.

Results and Discussion

The nanometer-scale structural changes accompanying starch pasting have been measured for the first time using simultaneous SANS/RVA. SANS data were found to be divided into two regimes for the starches tested.

The first corresponded to an initial principal lamellar structure up to approximately when the peak viscosity occurs. This is consistent with starch swelling affecting the amorphous growth rings, with little change in the semi-crystalline rings.

After the viscosity reaches a peak, the lamellar scattering is replaced by a scattering pattern indicating the abrupt formation of a networked, branched polymer structure with no apparent semi-crystalline properties. It can be analysed in terms of a fractal-like gel with basic building blocks about 1nm across. There appear to be interesting variations in the relative size and complexities of the structures formed. Aggregates are formed that are several times larger, and their sizes vary across the time course of the RVA test. In waxy maize, tapioca and potato pastes, the network structures detected by SANS are relatively large. These results are consistent with a less branched or less complex structure. RVA measurements indicate that these starches have the highest viscosity. Acid modified maize, conventional maize and wheat pastes have smaller aggregate sizes. This result may correlate to a more complex structure.

Changes throughout the RVA test hint at varying levels of disruption and re-association during the pasting curve, with interesting variations around the time the holding strength is reached.

It is likely that the networks can aggregate into the larger worm-like structures that are seen with electron microscopy during retrogradation.

Conclusion

The starch granule has a hierarchical structure from the micron- down to the nano-scale which influences a number of properties of relevance from both the physiological and industrial perspective. This work offers new insights into how starch-based products are formed. Better understanding of the structure–function relationship of starch during cooking offers opportunities to improve both the products and the processes used to make them.

Adapted from: Doutch, J. et al. Structural changes during starch pasting using simultaneous Rapid Visco Analysis and small-angle neutron scattering. Carbohydrate Polymers (2-12), doi: 10.1016/j.carbpol.2012.01.066