Beech, Daniel J.
(2024)
Impact of thermomechanical processes on maize starches containing differing amylose levels.
PhD thesis, University of Nottingham.
Abstract
Expansion of starchy ingredients to form a solid foam structure is a commonly used manufacturing technique across the food industry. The creation of an expanded product can be complex and requires bubble formation and biaxial extension. In order for bubble growth to occur, the bubble wall must be sufficiently flexible to accommodate bubble growth, yet sufficiently viscous to prevent collapse. For many aerated solid foods, starch is the essential ingredient forming the bubble wall. In its native form, starch is not suitable for bubble wall formation, so it is necessary for processing to cause changes to the native granules that allow the starchy material sufficient flexibility and integrity to support the bubble wall. Several factors have been reported in literature as influencing starch behaviour during processing and thus the extent of product expansion. These include shear level, water content, temperature and starch amylose/amylopectin ratio.
The work described in this thesis aimed to test the hypothesis that the extent of expansion observed in starches from the same botanical origin will differ substantially based on amylose/amylopectin ratio, hierarchical order and water content when processed using different shear regimes. This was based on the overall goal of improving upon current understanding of the properties of starchy materials in terms of their suitability for processing, using different shear regimes, to achieve an expanded product.
Maize starches with radically different amylose/amylopectin ratios were subjected to two processing methods with very different levels of shear. A rice-cake style popping machine was used for low shear processing, whilst thermomechanical extrusion was used for high shear processing. Maize flour was also processed under equivalent conditions in order to assess the influence of containment of starch within a flour matrix.
The potential for a novel ingredient screening method for extrusion was also investigated. This was based on rheological behaviour of starchy raw materials at moderate water content and shear levels, as measured by doughLAB.
Amylose/amylopectin ratio was found to influence starch properties and behaviour as a fluid. High amylose content restricted swelling and viscosity development at high water (>80%) low shear (Rapid Visco Analyser (RVA)) conditions, whereas at moderate water (~50%) and shear (doughLAB) conditions, differences in viscosity between the starches became less pronounced. The doughLAB was found to better approximate extrusion conditions of shear and water content than RVA. Specific mechanical energy (SME) during extrusion of the starches was found to be more strongly influenced by water content than amylose/amylopectin ratio, with lower water content increasing melt viscosity and in turn increasing SME.
Using both high and low shear processing techniques, amylose/amylopectin ratio had little influence on the final product expansion in maize starches. This was despite the fact that water absorption and solubility indices (WAI/WSI) of the expanded starch samples indicated a negative correlation between amylose content and starch conversion.
Thermomechanical extrusion generally caused a greater degree of starch conversion than popping as measured by WAI/WSI. Despite expectations that the high shear during extrusion would lead to an increase in bubble growth due to molecular weight reduction of the starch, this was not seen. Differences in water solubility, used as an indicator of relative starch molecular weights, did not correlate with product expansion.
Presence of non-starch components and physical location of starch within the native matrix of normal maize flour was found to reduce its viscosity, and hence SME during extrusion, when compared to isolated normal maize starch. However, this did not translate to clearly demonstrable differences in final product expansion.
The standard popping process used for this work, which was carried out at 12% sample water content, caused sufficient melting in native starch granules to create an expanded product from all of the maize starches used, despite them having substantially different amylose/amylopectin ratios. Popping water content correlated positively with expansion up to an optimum range of ~15-21% for all starches studied. Water was found to be a critical influencer of the starch behaviour, acting as a plasticiser and blowing agent, with sample hydration history also affecting subsequent starch-water interaction. The use of rice-cake style popping as a processing technique is currently underrepresented in literature and warrants further study.
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