Rheology of starch gels

Rheological profiling offers an unparalleled insight into the sensory, handling, stability and appearance characteristics of starch-based products.  These insights are rapidly gaining the status of need-to-know information for product developers, process engineers and quality controllers needing to deliver winning products to a discerning public. Where rheological testing excels is in the analysis of gelatinized starches and starch-thickened formulations, delivering an easy-to-interpret yet revealing picture of product attributes that is not available from more established methods such as starch pasting viscometers.

Think Structure, not Viscosity

Starch functions by building structures in a formulation or recipe. It is the presence of such structures that imparts sensory, texture, handling, suspending and appearance attributes to a formulation. Rigid structures manifest as stiff products, soft structures as wobbly and gelatinous. Strong structures can maintain a suspension whilst weak structures pour easily. Rheological testing techniques are unique in that, in addition to viscosity, they directly measure strength, rigidity, brittleness and a host of other attributes that describe these characteristics.

Rheology testing: Gentle touch, not hatchet job.

Structures generated by starch gelatinization are soft and delicate.  Such structures necessitate equally soft and delicate measurement methods where the structure is gently stressed or deformed and its response is monitored. Contrast this to the relative wrecking ball of a rapidly-rotating paddle of a visco-analyzer or a spinning viscometer spindle, demolishing the very network interactions we need to characterize.

Gel rigidity and strength

In the rheology world rigidity is termed modulus and strength is yield stress.  Beaten egg whites and whipped cream create stiff but weak structures while jelly displays a soft (i.e. wobbly) but strong structure.  Gummy bears are both rigid and strong and chilli dipping sauce is soft and weak.

Starch gel rheology: Complex modulus and yield stress variations determine critical sensory attributes.
Complex modulus and yield stress variations determine critical sensory attributes.

Oscillatory stress sweeps are a great way to obtain a simple quantification of modulus and yield stress.  The test involves applying small but incrementing oscillating (clockwise then counter-clockwise in rotation) stresses to a sample whilst monitoring its resulting strain (i.e. deformation) response. Modulus is obtained from the ratio of applied stress amplitude to resulting strain amplitude.  A very useful parameter also obtained from the oscillation stress sweep is the phase angle: the phase lag between applied stress and resulting strain, which reveals the presence of structure by identifying elastic response. Phase angle ranges from 0° for an ideal elastic material (i.e. a perfect solid) to 90° for an ideal viscous material (a perfect liquid). In the oscillation stress sweep test the imposed oscillating stress is incremented and the complex modulus and phase angle observed.

The graphic below shows the result of oscillation stress  sweeps performed on a variety of freshly gelatinized cook-up starch gels (5g/100ml water) at 60°.  All the starches display complex modulus and phase angle plateaus at low stresses, leading to yielding as the incrementing applied stress disrupts structure.

Starch gel rheology: Complex modulus reveals soft-solid structure rigidity differences between the starch gels.
Complex modulus reveals soft-solid structure rigidity differences between the starch gels.

 

Starch gel rheology: Phase angle reveals extent (plateau value) and strength (yielding) of starch gel structures.
Phase angle reveals extent (plateau value) and strength (yielding) of starch gel structures.

However the test reveals marked differences between the gels: notably, waxy maize starch and tapioca starch gels are soft and delicate – signified by low modulus and yielding at low stresses – whereas, in contrast, the dent maize and potato starch gels display stiff and well-developed (low phase angle) structures.   An interesting nuance can be seen in the complex modulus behaviours for the dent maize and potato starches where, despite similar rigidities, the dent maize displays a more pronounced “snap” as the yield is reached whereas the potato starch gel shows a more gradual yielding.  These differences would manifest as differences in “cuttability”, the catastrophic yield of the dent maize correlating with a more cuttable texture.

Flow Profiling

Viscosity profiling across a range of defined shear rates demonstrates the continuing divergent behaviours of potato and dent maize starch gels beyond the yield point with the former manifesting viscosities at least five times greater than  the latter over the range of shear rates tested.  This would be expected to result in a heavier mouth feel in the potato starch.

Starch gel rheology: Viscosity profiles demonstrate texture aspects beyond the yield stress.
Viscosity profiles demonstrate texture aspects beyond the yield stress.

Yet more information can be gained from an analysis of the generation of normal stresses with increasing shear rate.  Normal stresses are stresses generated perpendicular to the direction of shearing (hence “normal”) when an elastic liquid is sheared.  A gel that generates high normal stresses tends to display  greater degrees of stringiness and sliminess, film-forming and lubricity.

Starch gel rheology: Normal stress generation reveals long, stringy, slimy behaviour most pronounced in potato starch gel
Normal stress generation reveals long, stringy, slimy behaviour most pronounced in potato starch gel

Here we can see a pronounced difference across the four gels with the long-textured potato starch gel generating normal stresses many times greater than the other much shorter-textured samples.

Here we have shown how the gentle touch of rheology profiling can capture and quantify nuanced behaviours that are picked up by the consumer yet are completely missed by the veritable hatchet job performed by viscometers and visco-analyzers.  With the advent of ultra-sensitive research rheometers running sophisticated oscillatory and viscoelastic analyses a world of hitherto hidden structure is revealed to the product developer, process designer and quality assurance technician.

If you would like the Centre for Industrial Rheology to perform the testing described in this article on your products please get in touch at the Contact Page.


Share this: